vvERA
            United States
            Environmental Protection
            Agency
            Office Of Water
            (4204)
EPA 832-B-99-002
January 1999
Combined Sewer Overflows

Guidance For Monitoring
And Modeling

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                                                 EPA/832-B-99-002
                                                     January 1999
         COMBINED SEWER OVERFLOWS

GUIDANCE FOR MONITORING AND MODELING
                 Office of Wastewater Management
               U.S. Environmental Protection Agency
                     Washington, DC  20460
                        January 1999

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       \         UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
        I                       WASHINGTON, D.C. 20460
                                    pro
                                                                              OFFICE OF
                                                                               WATER
MEMORANDUM

SUBJECT:    Combined Sewer Overflows: Guidance for Monitoring and Modeling
                                           .
FROM:       Michael B. Cook, Direc
              Office of Wastewater MaMgferhenf

TO:           Interested Parties

       I am pleased to provide you with the Environmental Protection Agency's (EPA) guidance
document on the monitoring and modeling of combined sewer overflows (CSO) and their
impacts on receiving waters.  This is the seventh in a series of guidance manuals that EPA
prepared to support implementation of the 1994 Combined Sewer Overflow Control Policy.

       This manual presents a set of guidelines that provide flexibility for a municipality to
develop a site-specific strategy for characterizing its combined sewer system operations and
impacts and for developing and implementing a long-term CSO control plan. It is not a "how-
to" manual defining how many samples to collect or which flow metering technologies to use.

       EPA used a peer-review process and solicited comments from CSO stakeholders and the
general public. The EPA identified the Water Environment Research Foundation  (WERF) and
two technical experts to provide technical and scientific peer  review. WERF convened a panel of
its technical experts to review the document. The peer reviewers and the other reviewers
submitted detailed comments and recommendations. EPA will make available to  interested
parties a "response-to-comments" document detailing how it addressed comments received
during the peer review and the public  comment period. I am very grateful to the peer reviewers
and the other individuals and organizations who participated in preparation  and review. I believe
that this manual will assist municipalities as they develop  and implement long-term CSO control
plans to meet the requirements of the Clean Water Act and the objectives of the EPA's CSO
Control Policy.

       If you have any questions on the manual or its distribution, please contact Tim Dwyer in
the Office of Wastewater Management at (202) 260-6064. Mr. Dwyer's e-mail address is
dwver. tim(a),epa. sov.
             Recycled/Recyclable • Printed with Vegetable Oil Based Inks on 100% Recycled Paper (40% Postconsumer)

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                                ACKNOWLEDGMENTS
       The U.S. Environmental Protection Agency (EPA) wants to thank the Cities of
Columbus, Georgia; South Bend, Indiana; and Indianapolis, Indiana for allowing EPA to use
their experiences in monitoring and modeling as case studies for this manual. The experiences of
these cities provide excellent examples of the monitoring and modeling process associated with
developing and implementing combined sewer overflow (CSO) control programs.  EPA believes
that use of case studies greatly enhances the value of the document.

       EPA also acknowledges the peer reviewers who kindly donated their time and knowledge
to improving the technical and scientific discussions in this manual.  The peer reviewers were
David Dilks, Limno-Tech, Inc.; Raymond M. Wright, Ph.D., P.E., University of Rhode Island;
and John Marr, Limno-Tech, Inc., who reviewed it on behalf of the Water Environment Research
Foundation.

       Finally, EPA thanks  those individuals and organizations that took the time and energy to
review and submit comments as part of the public review process.  They are to be commended
for their perseverance and dedication to a long and arduous task.

       EPA believes that the peer review process and the public comments greatly improved the
technical and scientific aspects of the manual. We hope that users will find the information in
the manual useful as they develop and implement CSO control plans.

       Assistance in developing this manual was provided to EPA under contract number
68-C4-0034.

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                  NOTICE

The  statements in this  document  are intended
solely  as guidance.   This  document is  not
intended, nor can it be  relied on, to create  any
rights enforceable by any party in litigation with
the United States.  EPA and  State officials may
decide to follow the guidance  provided in  this
document, or to act at variance with the guidance,
based on an analysis of specific site circumstances.
This guidance  may be revised without public
notice  to reflect changes in EPA's strategy for
implementation of the Clean  Water Act and its
implementing  regulations,  or to  clarify  and
update the text.

Mention of trade names or commercial products
in this  document does  not constitute an
endorsement or recommendation for use.

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                             LIST OF ACRONYMS
BASINS   Better Assessment Science Integrating Point and Nonpoint Sources
BAT      Best Available Technology Economically Achievable
BCT      Best Conventional Pollutant Control Technology
BMP      Best Management Practice
BOD      Biochemical Oxygen Demand
BPJ       Best Professional Judgment
CAD      Computer Aided Design
COD      Chemical  Oxygen Demand
CSO      Combined Sewer Overflow
CSS      Combined Sewer System
CWA     Clean Water Act
DO       Dissolved Oxygen
EMAP    Environmental Monitoring and Assessment Program
EMC      Event Mean Concentration
EPA      U.S. Environmental Protection Agency
GIS      Geographic Information System
IDF       Intensity Duration Frequency
I/I        Infiltration/Inflow
LA       Load Allocation
LTCP     Long-Term Control Plan
MPN     Most Probable Number
NCDC    National Climatic Data Center
NMC     Nine Minimum Controls
NOAA   National Oceanic  and Atmospheric Administration
NPDES   National Pollutant Discharge Elimination System
NURP    Nationwide Urban Runoff Program
PDF      Probability Density Function
O&M     Operation and Maintenance
POTW   Publicly Owned Treatment Works
RBP      Rapid Bioassessment Protocol
QA        Quality Assurance
Q c       Quality Control
SCS       Soil Conservation Service
SSES      Sewer System Evaluation Survey
STORET   Storage and Retrieval of U.S. Waterways Parametric Data
SWMM    Storm Water Management Model
TDS       Total Dissolved Solids
TMDL    Total Maximum Daily Load
TSS       Total Suspended Solids
UAA      Use Attainability  Analysis
USGS     U.S. Geological Survey
VOC      Volatile Organic Compound
WLA      Wasteload Allocation
WQS      Water Quality  Standard(s)
                                                                         January 1999

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                         TABLE OF CONTENTS

                                                                        Page

1.    INTRODUCTION	 1-1

     1.1   BACKGROUND 	 1-1
     1.2  HISTORY OF 'THE CSO CONTROL POLICY  	  1-1
     1.3  KEY ELEMENTS OF THE CSO CONTROL POLICY	  1-3
     1.4  GUIDANCE TO SUPPORT IMPLEMENTATION OF THE CSO CONTROL
         POLICY 	 1-6
     1.5  PURPOSE OF  GUIDANCE	   1-7
     1.6  MANUAL  ORGANIZATION                                        1-8

2.    INTRODUCTION TO MONITORING AND MODELING 	2-1

     2.1   MONITORING AND MODELING FOR NINE MINIMUM CONTROLS
         AND LONG-TERM CONTROL PLAN	2-1
         2.1.1   Nine Minimum Controls	2-2
         2.1.2   Long-Term Control Plan Development	2-4
         2.1.3   Monitoring and Modeling During Phase I	2-9
         2.1.4   Monitoring and Modeling During Phase II	2-10
     2.2  MONITORING AND MODELING AND THE WATERSHED APPROACH . 2-11
     2.3   MEASURES OF SUCCESS  	2-14
     2.4  COORDINATION WITH OTHER WET WEATHER MONITORING AND
         MODELING PROGRAMS	2-15
     2.5   REVIEW AND REVISION OF WATER QUALITY STANDARDS	2-16
     2.6  OTHER ENTITIES INVOLVED IN DEVELOPING AND IMPLEMENTING
         THE MONITORING AND MODELING PROGRAM	2-18

3.    INITIAL SYSTEM CHARACTERIZATION - EXISTING DATA ANALYSIS  AND
     FIELD INVESTIGATION	3-1

     3.1   PHYSICAL CHARACTERIZATION OF CSS 	  3-2
         3.1.1   Review Historical Information	  3-2
         3.1.2   Study  Area Mapping  	   3-5
         3.1.3   System Field Investigation	 3-7
         3.1.4   Preliminary CSS Hydraulic Analysis 	  3-9
     3.2  CHARACTERIZATION OF COMBINED SEWAGE AND CSOS 	 3-11
         3.2.1   Historical Data Review  	  3-11
         3.2.2   Mapping 	 3-13
     3.3   CHARACTERIZATION OF RECEIVING WATERS  	  3-13
         3.3.1   Historical Data Review  	 3-13
         3.3.2   Mapping 	 3-17
     3.4  IDENTIFY DATA GAPS                                           3-18
                                                                 January 1999

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                    TABLE OF CONTENTS (Continued)

                                                                             Page


4.    MONITORING AND MODELING PLAN	4-1

     4.1   DEVELOPMENT OF A MONITORING AND MODELING PLAN           4-2
          4.1.1    Goals and Objectives  	   4-3
          4.1.2    Modeling  Strategy  	   4-5
          4.1.3    Monitoring Data Needs  	   4-8
     4.2   ELEMENTS OF A MONITORING AND MODELING PLAN                4-9
          4.2.1    Duration of Monitoring Program  	  4-10
          4.2.2    Sampling Protocols and Analytical Methods	  4-12
     4.3   CSS AND  CSO MONITORING                                        4-13
          4.3.1    CSS and CSO Monitoring Locations	  4-13
          4.3.2    Monitoring Frequency  	  4-17
          4.3.3    Combined Sewage and CSO Pollutant Parameters  	  4-21
     4.4   SEPARATE STORM SEWERS                                        4-23
     4.5   RECEIVING WATER MONITORING                                  4-24
          4.5.1    Monitoring Locations	  4-26
          4.5.2    Monitoring Frequency, Duration, and Timing	  4-27
          4.5.3    Pollutant Parameters	  4-30
     4.6   CRITERIA FOR INITIATING MONITORING OF WET WEATHER EVENTS 4-31
     4.7   CASE STUDY 	   4-34
     4.8   DATA MANAGEMENT AND ANALYSIS                              4-34
          4.8.1    Quality Assurance Programs  	  4-34
          4.8.2    Data Management	  4-39
     4.9   IMPLEMENTATION OF  MONITORING AND MODELING PLAN         4-41
          4.9.1    Recordkeeping and Reporting  	  4-42
          4.9.2    Personnel Responsible for Implementation 	  4-42
          4.9.3    Scheduling 	  4-42
          4.9.4    Resources 	4-43

5.   CSS MONITORING	5-1

     5.1   THE CSO  CONTROL POLICY AND CSS MONITORING                  5-1
     5.2   RAINFALL DATA FOR  CSS CHARACTERIZATION                      5-2
          5.2.1    Rainfall Monitoring 	    5-2
          5.2.2    Rainfall Data Analysis 	   5-4
     5.3   FLOW MONITORING IN THE CSS                                     5-12
          5.3.1    Flow Monitoring Techniques	   5-12
          5.3.2    Conducting the Flow Monitoring Program 	   5-19
          5.3.3    Analysis of CSS Flow Data 	   5-20
     5.4   WASTEWATER MONITORING IN THE CSS                            5-24
          5.4.1    Water Quality Sampling	  5-24
          5.4.2    Analysis of Wastewater Monitoring Data  	   5-31
     5.5   SAMPLING AND DATA USE CASE STUDY                            5-36
                                        iii                             January 1999

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                    TABLE OF CONTENTS  (Continued)

                                                                              Page

6.    RECEIVING WATER MONITORING	6-1

     6.1   THE CSO CONTROL POLICY AND RECEIVING WATER MONITORING .  6-1
     6.2   RECEIVING WATER HYDRAULIC MONITORING  	  6-2
          6.2.1    Hydraulic Monitoring	   6-3
          6.2.2    Analysis  of Hydraulic Data  	   6-6
     6.3   RECEIVING WATER QUALITY MONITORING 	  6-7
          6.3.1    Water Quality Monitoring  	   6-7
          6.3.2    Analysis  of Water Quality Data	  6-9
     6.4   RECEIVING WATER SEDIMENT AND BIOLOGICAL MONITORING .... 6-10
          6.4.1    Sediment Sampling Techniques	   6-10
          6.4.2    Analysis  of Sediment Data	   6-12
          6.4.3    Biological Sampling Techniques 	   6-12
          6.4.4    Analysis  of Biological Data	   6-16

7.    CSS MODELING	7-1

     7.1   THE CSO CONTROL POLICY AND CSS MODELING  	  7-1
     7.2   MODEL SELECTION STRATEGY 	   7-4
          7.2.1    Selecting Hydraulic Models	   7-7
          7.2.2    Selecting CSS Water Quality Models 	   7-11
     7.3   AVAILABLE MODELS  	   7-13
     7.4   USING A CSS MODEL	   7-17
          7.4.1    Developing the Model  	   7-17
          7.4.2    Calibrating and Validating the Model 	   7-18
          7.4.3    Performing the Modeling Analysis   	   7-22
          7.4.4    Modeling Results  	   7-23
     7.5   EXAMPLE SWMM MODEL APPLICATION	   7-27
          7.5.1    Data Requirements	   7-27
          7.5.2    SWMM Blocks	   7-31
          7.5.3    Model Calibration and Application  	   7-34
          7.5.4    SWMM Pollutant Modeling	   7-35
     7.6   CASESTUDY 	   7-38
                                        iv                             January 1999

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                   TABLE OF CONTENTS (Continued)

                                                                          Page

8.    RECEIVING WATER MODELNG	8-1

     8.1   THE CSO CONTROL POLICY AND RECEIVING WATER MODELING .... 8-1
     8.2   MODEL  SELECTION  STRATEGY 	   8-2
          8.2.1    Hydrodynamic Models	   8-3
          8.2.2    Receiving Water Quality Models	   8-4
     8.3   AVAILABLE MODELS  	   8-5
          8.3.1    Model Types	   8-8
          8.3.2    Computer Models Supported by EPA or Other Government Agencies . 8-17
     8.4   USING A RECEIVING WATER MODEL	  8-22
          8.4.1    Developing the Model  	  8-22
          8.4.2    Calibrating and Validating the Model 	  8-22
          8.4.3    Performing the Modeling Analysis  	  8-23
          8.4.4    Modeling Results 	  8-23

9.    ASSESSING RECEIVING WATER IMPACTS AND ATTAINMENT OF WATER
     QUALITY STANDARDS	9-1

     9.1   IDENTIFYING RELEVANT WATER QUALITY STANDARDS  	 9-2
     9.2   OPTIONS FOR DEMONSTRATING COMPLIANCE  	  9-5
     9.3   EXAMPLES  OF RECEIVING WATER ANALYSIS	  9-6
          9.3.1    Example 1: Bacterial Loads to a River  	  9-6
          9.3.2    Example 2: Bacterial Loads to an Estuary  	  9-23
          9.3.3    Example 3:  BOD Loads  	  9-29
     9.4   SUMMARY  	   9-35

REFERENCES  	R-l

APPENDIX A 	  A-l
APPENDIX B 	  B- 1
                                                                   January 1999

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   TABLE OF CONTENTS (Continued)
               LIST OF EXHIBITS
Exhibit  1-1

Exhibit 3-1.

Exhibit 4-1.
Exhibit 4-2.
Exhibit 4-3.

Exhibit 5-1.

Exhibit 5-2.
Exhibit 5-3.
Exhibit 5-4.
Exhibit 5-5.
Exhibit 5-6.
Exhibit 5-7.
Exhibit 5-8.
Exhibit 5-9.
Exhibit 5-10.
Exhibit  5-11.
Exhibit  5-12.
Exhibit 5-13.

Exhibit 6-1.

Exhibit 7-1.

Exhibit 7-2.

Exhibit 7-3.
Exhibit 7-4.
Exhibit 7-5.
Exhibit 7-6.
Exhibit 7-7.
Exhibit  7-8.
Exhibit 7-9.
Exhibit  7-10.
Exhibit  7-11.
Exhibit  7-12.
Exhibit  7-13.
                                                                Page
                                                                Page
Roles and Responsibilities  ....................................   1-5
Basic Flow Balance Diagram
                                                                3-10
Data for Example 4-1 .......................................   4-19
Receiving Water Monitoring Location Example  ..................  4-28
Decision Flowchart for Initiating a Wet Weather Monitoring Event . . .  4-33

Ranking of Yearly Runoff Characteristics as Simulated by the Storm
Model [[[ 5-6
Rainfall and Runoff Parameters for Typical and Extreme Years ....... 5-7
1993 Rainfall Data for a 5,305 Acre Drainage Area  ................  5-9
Rain Gage Map for Data Presented in Exhibit 5-3 .................  5-11
CSO Flow Monitoring Devices ...............................   5-13
Illustration of a Bottle Board Installation ........................   5-16
Example Outfall Bottle Rack Readings .........................   5-16
Total  Overflow Volume .....................................   5-21
Example CSS Plots of Flow and Head versus Time  ...............  5-23
Composite Sampling Data (mg/1)  ..............................   5-32
Pollutant Concentration Summary Statistics (mg/1) ................  5-32
Pollutant Loading Summary ..................................   5-33
Fecal Coliform Data for Outfall 1-Example Storm  ................  5-35

Overview of Field Biological Sampling Methods .................  6-14

Relevant CSS Hydraulic and Water  Quality Modeling
for EPA's CSO Control Policy .................................   7-9
Characteristics of RUNOFF, TRANSPORT, and EXTRAN Blocks of
the EPA Storm Water Management Model (SWMM)1  ..............  7-8
CSS Runoff and Hydraulic Models (Public Domain)  ..............  7-14
CSS Water Quality Models (Public Domain)  ....................  7-15
Selected Commercial CSS Models  ............................   7-16
Levels of Discretization .....................................  7-19
Drainage Area Map  ........................................  7-28
Sewer Network and Subareas  .................................   7-29
SWMM Runoff Block Input Parameters (SWMM HI Card)  ........  7-32
SWMM Transport Block Input Parameters (SWMM HI Card)  ......  7-33
Flow Hydrograph ..........................................  7-35
Pollutographs  .............................................  7-37
Predicted and Observed Pollutant Concentrations .................  7-37
                        VI

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                           LIST OF EXHIBITS (Continued)
                                                                                   Page
Exhibit 8-1.         Dissolved Oxygen Superposition Analysis  	  8-12
Exhibit 8-2.         EPA CEAM-Supported Receiving  Water Models	  8-18

Exhibit 9-1.         Map For Example 1  	 9-7
Exhibit 9-2.         CSO Events for Example 1  	 9-9
Exhibit 9-3.         Design Flow Analysis  	 9-13
Exhibit 9-4.         Flow Duration Curve  	   9-17
Exhibit 9-5.         Expected Exceedances of Water Quality Criterion 	  9-18
Exhibit 9-6.         Excursions of Water Quality Criterion by Month 	  9-21
Exhibit 9-7.         Receiving Water Flow During CSOs	  9-22
Exhibit 9-8.         Map for Example 2	   9-24
Exhibit 9-9.         Steady-State Predictions of Fecal Coliform Count (MPN/100 ml)  	 9-27
Exhibit 9-10.        Design Condition Prediction of DO Sag  	  9-32
Exhibit 9-11.        Relationship Between DO  Concentration and Upstream Flow 	 9-33


                                LIST OF EXAMPLES

Example 4-1.        One Approach to Selecting Discharge Monitoring Sites for a
                   Hypothetical CSS with 10  Outfalls  	  4-18
Example 4-2.        Monitoring Case Study	   4-35

Example 5-1.        Flow Monitoring  	   5-15
Example 5-2.        Sampling and Data Use Case Study	  5-36

Example 7-1.        Modeling Case Study	   7-39
                                          vn
January 1999

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                                       CHAPTER  1






                                    INTRODUCTION








1.1    BACKGROUND





       Combined  sewer systems (CSSs) are designed to carry  sanitary sewage (consisting of



domestic, commercial, and industrial wastewater) and storm water (surface drainage from rainfall



or snowmelt)  in a single pipe to a treatment facility.  CSSs serve about 43 million people in



approximately 950 communities nationwide, most of them located in the Northeast  and Great Lakes



regions.  During dry weather,  CSSs convey domestic, commercial, and industrial wastewater to a



publicly owned treatment works (POTW). In periods of rainfall or snowmelt, total  wastewater flows



can exceed the capacity of the CSS or the treatment facilities. When this occurs, the CSS is designed



to overflow directly to surface water bodies, such as lakes, rivers, estuaries, or coastal waters. These



overflows-called combined sewer overflows  (CSOs)-can be  a major source  of water  pollution.







       CSOs  contain  many  types of  contaminants,  including  pathogens,  oxygen-demanding



pollutants, suspended solids, nutrients, toxics,  and floatable matter. Their presence in CSOs and the



volume of the flows can cause a variety of adverse impacts on the physical characteristics of surface



water, impair the viability of aquatic habitats, and pose a potential threat to drinking water supplies.



CSOs have been shown to be  a major contributor to use impairment and aesthetic degradation of



many receiving waters and have contributed to shellfish harvesting restrictions, beach closures, and



even occasional fish kills.







1.2    HISTORY OF THE CSO CONTROL POLICY





       Historically, the control  of CSOs  has proven to  be extremely complex. This is partly due to



the difficulty in quantifying CSO impacts on receiving water quality and the site-specific variability



in the volume, frequency, and  characteristics of CSOs. In addition, the financial considerations for



communities with  CSOs can  be significant.  The U.S. Environmental Protection Agency  (EPA)
                                            1-1                                January 1999

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Chapter 1                                                                    Introduction


estimates the CSO abatement costs for the 950 communities served by CSSs to be approximately

$45 billion based on results from the 1996 Clean Water Needs Survey.


       To address these challenges, EPA issued a National Combined Sewer Overflow Control

Strategy on August 10, 1989 (54 Federal Register 37370). This Strategy reaffirmed that CSOs are

point source discharges  subject to National Pollutant Discharge Elimination  System  (NPDES)  permit

requirements and to Clean Water Act (CWA) requirements. The CSO Strategy recommended that

all  CSOs be identified and categorized according to  their status of  compliance with these

requirements. It also set forth three objectives:


          Ensure that if CSOs occur, they are only as a result of wet weather

          Bring all wet weather CSO discharge points into compliance with the technology-based
          and water quality-based requirements of the CWA

          Minimize the water quality, aquatic biota, and human health impacts of CSOs.


In addition, the CSO Strategy charged all States with developing state-wide permitting strategies

designed to reduce, eliminate, or control CSOs.


       Although the CSO Strategy was successful  in focusing attention, it failed to resolve many

fundamental  issues.  In mid-1991, EPA initiated a process to  accelerate implementation  of the

Strategy. The process included negotiations with representatives of the regulated community, State

regulatory agencies, and environmental groups. These negotiations were conducted through the

Office of Water Management Advisory Group.  The initiative resulted  in the development of a CSO

Control Policy,  published in the Federal Register on April 19, 1994 (59 Federal Register 18688).


       The intent of the CSO Control Policy is to:
       .  Provide guidance to permittees  with CSOs, NPDES  permitting  and enforcement
          authorities, and State water quality standards (WQS) authorities
                                           1-2                               January 1999

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Chapter 1                                                                    Introduction
          Ensure coordination  among the appropriate  parties in planning,  selecting,  designing, and
          implementing CSO  management practices and controls to meet the requirements of the
          CWA

          Ensure public involvement during the decision-making process.
       The CSO Control  Policy contains  provisions for developing appropriate,  site-specific NPDES

permit requirements for all CSSs that overflow due to wet weather events. It also announces an

enforcement initiative that requires the immediate elimination of overflows that occur during dry
weather and ensures that the remaining CWA requirements are complied with  as soon as possible.


1.3    KEY ELEMENTS OF THE CSO CONTROL POLICY

       The CSO Control  Policy contains four key principles to ensure that CSO controls are cost-

effective and meet the requirements of the CWA:
          Provide clear levels of control that would be presumed to meet appropriate health and
          environmental objectives

          Provide sufficient flexibility to municipalities, especially those that are  financially
          disadvantaged, to consider the site-specific nature of CSOs and to determine the most
          cost-effective means  of reducing pollutants and meeting  CWA objectives  and
          requirements

         Allow a phased  approach  for implementation of CSO  controls  considering a
          community's financial capability

          Review and revise, as appropriate, WQS and their  implementation procedures when
          developing long-term CSO control plans to reflect the site-specific wet weather impacts
          of CSOs.
                                            1-3                               January 1999

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Chapter 1                                                                     Introduction


       In addition, the CSO Control Policy clearly defines expectations for permittees, State WQS

authorities, and NPDES permitting and enforcement authorities. These expectations include the

following:
          Permittees should immediately implement the nine  minimum controls  (NMC), which are
          technology-based actions  or measures designed to reduce CSOs  and their effects on
          receiving water quality, as soon as practicable but no later than January 1, 1997.

          Permittees should give priority to environmentally sensitive areas

          Permittees should develop long-term control plans  (LTCPs) for controlling CSOs. A
          permittee may use one of two approaches:   1) demonstrate that its plan is adequate to
          meet the water quality-based requirements of the CWA ("demonstration approach"), or
          2) implement a minimum level  of treatment (e.g., primary clarification of at least
          85 percent of the collected combined sewage flows) that is presumed to meet the water
          quality-based requirements of the CWA, unless data indicate otherwise ("presumption
          approach').

          WQS authorities should review and revise,  as appropriate, State WQS during the CSO
          long-term planning process.

          NPDES  permitting authorities should consider the  financial capability of permittees
          when reviewing CSO control plans.
       Exhibit 1-1 illustrates the roles and responsibilities of permittees, NPDES permitting and

enforcement authorities, and State WQS authorities.


       In addition to these key elements and expectations, the CSO Control Policy also addresses

important issues such as ongoing or completed CSO control projects, public participation, small

communities, and watershed planning.
                                            1-4                                January 1999

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                                                     Exhibit 1-1.  Roles  and  Responsibilities
a
 Co
              Permittee
   Evaluate and implement NMC

    Submit documentation of NMC
    implementation by January 1, 1997

   Develop LTCP and submit for
    review to NPDES permitting
    authority

    Support the review of WQS in
    CSO-impacted receiving water
    bodies

   Comply with  permit conditions
    based on narrative WQS

   Implement selected CSO controls
    from LTCP

    Perform  post-construction
    compliance monitoring

   Reassess  overflows to sensitive
    areas

   Coordinate all  activities with
    NPDES permitting authority, State
    WQS authority, and State watershed
    personnel
  NPDES Permitting Authority
Reassess/revise  CSO permitting
 strategy

 Incorporate into Phase I permits
 CSO-related conditions (e.g., NMC
 implementation and documentation
 and LTCP development)

 Review documentation of NMC
 implementation

 Coordinate review of LTCP
 components throughout the LTCP
 development process and
 accept/approve permittee's  LTCP

 Coordinate the review and  revision
 of WQS as appropriate

 Incorporate  into Phase  II permits
 CSO-related conditions  (e.g.,
 continued NMC implementation
 and LTCP implementation)

 Incorporate  implementation
 schedule into an appropriate
 enforceable mechanism

 Review implementation activity
 reports (e.g., compliance schedule
 progress reports)
                                                                                           NPDES Enforcement Authority
                                                                                           Ensure that CSO requirements and
                                                                                           schedules for compliance are
                                                                                           incorporated into  appropriate
                                                                                           enforceable  mechanisms

                                                                                           Monitor adherence to January  1,
                                                                                            1997, deadline for NMC
                                                                                           implementation and documentation

                                                                                           Take appropriate  enforcement
                                                                                           action  against dry weather
                                                                                           overflows

                                                                                           Monitor compliance  with Phase I,
                                                                                           Phase II, and post-Phase II permits
                                                                                           and take enforcement action as
                                                                                           appropriate
                                                                                                                           s
                                                                                                                           I
                                                                                                                            State WQS Authorities
Review WQS  in CSO-impacted
 receiving  water bodies

Coordinate review  with  LTCP
 development

Revise WQS as appropriate:

 Development  of site-specific
 criteria

 Modification of designated use to

 -  Create  partial use reflecting
    specific  situations
 •   Define use more explicitly

 Temporary  variance from WQS
NMC = nine minimum  controls
LTCP = long-term control plan
WQS   = water  quality  standards
                                                                                                                           I
                                                                                                                           I
                                                                                                                           8

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Chapter 1	Introduction


1.4    GUIDANCE TO  SUPPORT IMPLEMENTATION OF THE CSO  CONTROL
       POLICY

       To help permittees and NPDES permitting and WQS authorities implement the provisions
of the CSO Control Policy, EPA has developed the following guidance documents:
          Combined Sewer Overflows - Guidance/or Long-Term Control Plan (U.S. EPA, 1995a)
          (EPA 832-B-95-002)

          Combined Sewer Overflows - Guidance for Nine Minimum Controls (U.S. EPA, 1995b)
          (EPA 832-B-95-003)

          Combined Sewer Overflows - Guidance for Screening and Ranking (U.S. EPA, 1995c)
          (EPA 832-B-95-004)

          Combined Sewer Overflows - Guidance for Funding Options (U.S. EPA, 1995d) (EPA
          832-B-95-007)

          Combined Sewer Overflows - Guidance for Permit Writers (U.S. EPA, 1995e) (EPA
          832-B-95-008)

          Combined Sewer Overflows - Guidance for Financial  Capability Assessment and
          Schedule Development (U.S. EPA, 1997) (EPA 832-B-97-004).
      EPA has printed a limited number of copies of each guidance document and has made them

available through several sources:


      .   EPA's Water Resource Center (202-260-7786)

          National Small Flows Clearinghouse (800-624-8301 or http://www.estd.wvu.edu/nsfc/)

          National Technical Information Service (NTIS) (800-553-6847 or http://www.ntis.gov)

      .   Educational   Resources    Information   Center (ERIC)  (800-276-0462  or
          http://www.aspensys.com/eric/catalog/)

      . State environmental offices

      . EPA Regional Offices.



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 Chapter 1                                                                     Introduction







        Electronic copies of some of the guidance documents are also available on EPA's Office of



 Water Internet site (http://www.epa.gov/ow/).







 1.5    PURPOSE OF GUIDANCE




        This manual explains the role  of monitoring and modeling in the  development and



 implementation of a CSO control program. It expands discussions of monitoring and modeling



 introduced in the CSO Control Policy and presents examples of data collection and  CSS simulation.







        This manual is not a "how-to" manual defining how many samples to collect or which flow



 metering technologies  to  use.   Rather, it is a set of guidelines that provides flexibility for a



 municipality to develop  a site-specific strategy for characterizing its CSS operation  and impacts and



for developing and implementing a comprehensive CSO  controlplan. CSSs  vary greatly  in their



 size, structure,  operation, and receiving water impacts.   A  monitoring and modeling strategy



 appropriate for a large city such as New York or San Francisco would generally not apply to a small



 CSS with only one or two flow regulators and  outfalls.  In addition,  communities have varying



 degrees of knowledge about how their CSSs react hydraulically to wet weather and how their CSOs



 affect receiving water quality. A municipality that does not know the location of its CSO outfalls



 has different information collection needs from a municipality that has already conducted CSS flow



 and water quality studies.







        This manual  provides guidance for communities  of all sizes. It  presents  low-cost monitoring



 and modeling techniques, which should prove particularly helpful to small communities. However,



 communities  with large  CSSs should note that inexpensive techniques often  prove useful  in



 extending monitoring resources and in verifying the performance of more sophisticated techniques



 and equipment.







        To use this manual,  a  municipality  should already be familiar with the basic functioning of



 its CSS, basic monitoring  procedures,  and the general purpose of modeling.  Since  basic monitoring



 and modeling techniques are already covered extensively in other technical literature, this  manual









                                             1-7                               January 1999

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Chapter 1	Introduction


focuses mainly on the process of characterization as described in the CSO Control Policy, referring
to other literature for more in-depth explanations of specific techniques or procedures.


1.6    MANUAL ORGANIZATION

       This manual begins with an overview of monitoring and modeling under the CSO Control
Policy, and then  provides  a detailed discussion  of the monitoring and modeling activities that should

be conducted for NMC implementation  and LTCP development and  implementation.  These
activities (and the chapters in which they are discussed) are as follows:


       .    Chapter 2 - Introduction To Monitoring and Modeling

       .    Chapter 3  - Initial  System Characterization-Existing  Data Analysis and
                      Field Investigation

       .    Chapter 4 - Monitoring and Modeling Plan

           Chapter 5 - CSS  Monitoring

       .    Chapter 6 - Receiving Water Monitoring

       .    Chapter 7 - CSS  Modeling

       .    Chapter 8 - Receiving Water Modeling

       .    Chapter 9 - Assessing  Receiving Water  Impacts and Attainment of Water Quality
                      Standards.
                                            1-8                                January 1999

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                                      CHAPTER 2

                 INTRODUCTION TO MONITORING AND MODELING


       Monitoring and modeling activities are central  to implementation of the CSO Control Policy.

Thoughtful development and implementation of a monitoring and modeling plan will support the

selection  and implementation of  cost-effective  CSO controls and  an assessment of their

improvements on receiving water quality.


       This chapter describes general expectations for monitoring and modeling activities as part

of a permittee's  CSO control program.  It also describes how monitoring and modeling efforts

conducted as part of CSO control program implementation can be coordinated with other key EPA

and State programs and efforts (e.g., watershed approach, other wet weather programs).


       While this chapter will describe general expectations, EPA encourages the permittee to take

advantage of the flexibility in the CSO Control Policy by developing a monitoring and modeling

program that is cost-effective and tailored to local  conditions,  providing adequate but not  duplicative

or unnecessary information.


2.1    MONITORING AND MODELING FOR NINE MINIMUM CONTROLS AND
       LONG-TERM CONTROL PLAN

       The  CSO Control  Policy urges permittees  to develop  a thorough understanding of the

hydraulic responses of their combined sewer systems  (CSSs) to wet weather events.  Permittees may

also need to estimate  pollutant  loadings from CSOs and the fate of pollutants in receiving water both

for existing  conditions and for various CSO control  options.  The CSO Control Policy states that

permittees  should immediately undertake  a  process to  characterize their CSSs, demonstrate

implementation of the nine minimum controls (NMC), and develop a  long-term CSO control plan.

Characterizing the CSS and its hydraulic response to wet weather events, implementing the NMC
and producing  related documentation, and developing  a long-term control  plan (LTCP) will involve

gathering  and reviewing existing data, and, in most cases, conducting  some field inspections,

monitoring,  and modeling. Since flexibility is a key principle of the CSO Control Policy, these

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Chapter 2                                        Introduction to Monitoring and Modeling


activities will be carried out to different degrees based on each permittee's situation. In particular,
the type and complexity of necessary modeling will vary from permittee to permittee.


2.1.1  Nine  Minimum Controls

       The  CSO Control Policy  recommends that  a Phase I permit require the permittee to
immediately implement technology-based requirements, which at a minimum include the NMC, as

determined on a best professional judgment (BPJ)  basis by the NPDES permitting authority. The

NMC are:


       1.  Proper operation and regular maintenance programs for the sewer system

       2.  Maximum use of the collection system for storage

       3.  Review  and modification of pretreatment requirements to  assure CSO impacts  are
          minimized

       4.  Maximization  of flow to the publicly owned treatment works (POTW) for treatment

       5.  Prohibition of CSOs during dry weather

       6.  Control of solid and floatable materials  in CSOs

       7.  Pollution  prevention

       8.  Public notification to ensure that the public  receives  adequate notification of CSO
          occurrences and CSO impacts

       9.  Monitoring to  effectively characterize CSO impacts and the efficacy of CSO controls.


       The  NMC are technology-based controls,  applied on a site-specific basis, to reduce the
magnitude, frequency, and duration  of CSOs and their impacts on receiving water bodies. NMC
measures typically do not require  significant engineering studies or major construction and thus
implementation was  expected by January 1, 1997. EPA's guidance document Combined Sewer
Overflows  - Guidance for Nine  Minimum Controls (U.S. EPA, 1995b) provides  a  detailed

description of the NMC, including example control measures and their advantages and limitations.



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Chapter 2                                         Introduction to Monitoring and Modeling


       Monitoring is specifically included as the ninth minimum control.  Implementation of this
control would typically involve the following activities:
           Mapping the drainage area for the CSS, including the locations of all CSO outfalls and
           receiving waters

           Identifying,  for each receiving water body,  designated and existing uses, applicable  water
           quality criteria, and whether water quality standards (WQS) are currently being attained
           for both wet weather and dry weather

           Developing  a record of overflow occurrences (number, volume, frequency, and duration)

           Compiling  existing information on water  quality impacts associated with CSOs  (e.g.,
           beach closings, evidence of floatables wash-up, fish kills, sediment accumulation, and
           the frequency, duration, and magnitude of instream WQS violations).
       Monitoring as part of the NMC is not intended to be extensive or costly. It should entail
collection of existing information from relevant agencies about the CSS, CSOs, the receiving water

body, and pollutant sources discharging to the same receiving  waters, as well as  preliminary

investigation activities such as field inspections and  simple  measurements  using  chalk boards, bottle

boards, and block tests. The  collected information and data will be used to establish a baseline of
existing conditions for evaluating the efficacy of the technology-based controls and to  develop the

LTCP (as described in Section 2.1.2).


       Data analysis and field inspection activities also support implementation of several other

NMC:
          Proper  operation  and  regular  maintenance programs for  the  sewer  system-
           Characterization of the CSS will support the evaluation of the effectiveness of current
           operation and maintenance (O&M)  programs and help identify areas within  the CSS that
           need repair.

           Maximum use of the collection  system for storage-Information gained during field
           inspections,  such as the system topography (e.g., location of any  steep slopes) and the
           need for regulator or pump adjustments, can assist in identifying locations where minor
           modifications to the CSS can increase in-system storage.
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Chapter 2                                         Introduction to Monitoring and Modeling
          Review and modification of pretreatment requirements to assure CSO impacts are
          minimized-Pretreatment program information and existing monitoring data will support
          assessment of the impacts of nondomestic discharges  on  CSOs and  identify opportunities
          to mitigate the impacts of nondomestic discharges during wet weather.

          Control of solid andfloatable materials in CSOs-Existing information about receiving
          water impacts and observations made during  field inspections of the  CSS will help
          determine the extent of solid and floatable materials present and the effectiveness of any
          controls installed.

          Dry weather  overflows-Field inspections will  assess the  presence of dry  weather
          overflows, the conditions under which they occur, and the effectiveness of any control
          measures  in place.
       Because specific NMC implementation requirements will be embodied in a permit or other
enforceable mechanism that is developed on a site-specific basis, the permittee should coordinate
NMC implementation with the NPDES permitting authority on an ongoing basis.


2.1.2   Long-Term Control Plan  Development

       The CSO Control Policy recommends that a Phase I permit require the permittee to develop
and submit an LTCP that, when implemented, will ultimately result  in compliance with  CWA

requirements. The permittee should use either the presumption approach  or the demonstration

approach in developing an LTCP that will provide for WQS attainment. The two approaches are
discussed in more detail below and in Chapters 7 through 9.


       The permittee should evaluate the data and information obtained through the initial system

characterization to determine which  approach is  more  appropriate  based  on  site-specific  conditions.
Generally, the demonstration approach would be selected when sufficient data are available,  or can

be collected, to "demonstrate" that a proposed LTCP is adequate to meet the water quality-based
requirements of the  CWA.   If sufficient data are not available and cannot be developed to allow use
of the demonstration approach,  and  the  permitting authority  believes it is likely that implementation
of a control program that meets certain performance criteria will result in attainment of CWA
requirements, the permittee would use the presumption approach.



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Chapter 2                                         Introduction to Monitoring and Modeling


       Demonstration Approach. Under the demonstration approach, the permittee demonstrates

the adequacy  of its CSO control program to meet the water quality-based requirements of the CWA.

As stated in the CSO Control Policy, the permittee should demonstrate each of the following:
        "i-     The planned control program is adequate to meet WQS andprotect designated uses,
              unless WQS or uses cannot be met as a result of natural background conditions or
              pollution sources other than CSOs;

       ii.      The CSO discharges remaining after implementation of the planned control program
              will not preclude the attainment of WQS or the receiving waters ' designated uses or
              contribute to their impairment. Where WQS and designated uses are not met in part
              because of natural background conditions or pollution sources other than CSOs, a
              total maximum daily load, including a wasteload allocation and a load allocation,
              or other means should be used to apportion pollutant loads;

       Hi-     The planned control program will provide  the maximum pollution reduction benefits
              reasonably attainable; and

       iv.     The planned control program is designed to allow cost effective expansion or cost
              effective  retrofitting  if additional controls are subsequently determined  to  be
              necessary to meet WQS or designated uses.  " (Section II.C.4.b of the CSO Control
              Policy)


Generally,  monitoring and modeling activities will be integral to successfully demonstrating that

these criteria have been met.
       Presumption  Approach.  This approach is based on the presumption that WQS will be
attained with implementation of an LTCP that meets certain performance-based criteria. For the
presumption approach, the CSO Control Policy states that:
        "A program that meets any of the criteria listed below would be presumed to provide an
       adequate level of control to meet the water quality-based requirements of the CWA, provided
       the permitting authority  determines that such presumption is reasonable in light of the data
       and analysis conducted in  the characterization, monitoring,  and modeling of the system and
       the consideration of sensitive areas described above.  These criteria are provided because
       data and modeling of wet weather events often do not give a  clear picture  of the level of CSO
       controls necessary to protect WQS.
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Chapter 2                                        Introduction to Monitoring and Modeling
       i.      No more than an average of four overflow events per year...

       ii.      The elimination or the capture for treatment of no less than 85% by volume of the
              combined sewage collected in the  CSS during precipitation  events  on a system-wide
              annual average basis...

       Hi.     The elimination or removal of no less  than the mass of pollutants, identified as
              causing water quality impairment..., for the volumes that  would be eliminated or
              capturedfor treatment under paragraph ii... " (Section II.CAa.)
Monitoring and modeling activities are also likely to be necessary in order to obtain the permitting

authority's approval for  using  the presumption approach.   Considerations for using both the

presumption approach and the demonstration approach are discussed in  Combined Sewer

Overflows - Guidance for Long Term Control Plan (U.S. EPA, 1995a).


       Whether the LTCP ultimately reflects  the demonstration  approach or the presumption

approach, it should contain the same elements, as identified in the CSO Control Policy:


          Characterization, monitoring, and modeling of the CSS

       .  Public participation

          Consideration of sensitive areas

          Evaluation of alternatives

       .   Cost/performance considerations

       .  Operational plan

          Maximization of treatment at the POTW

       .  Implementation schedule

          Post-construction compliance monitoring program.


       Of these elements, the first and last are directly linked to monitoring and modeling and are

described below.


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Chapter 2                                         Introduction to Monitoring and Modeling


       Characterization, monitoring, and modeling of the CSS

       The first step in developing an LTCP involves characterization, monitoring, and modeling

of the CSS. The CSO Control Policy states:
        "In order to design a CSO control plan adequate to meet the requirements of the
       CWA, a permittee should have a thorough understanding of its sewer system, the
       response of the system to various  precipitation events,  the characteristics of the
       overflows, and the water quality impacts that result from CSOs.  The permittee
       should adequately characterize through monitoring,  modeling, and other means as
       appropriate, for a range of storm  events,  the response of its sewer system to wet
       weather events including the number, location and frequency of CSOs,  volume,
       concentration and mass ofpollutants discharged and the impacts of the  CSOs on the
       receiving waters and their designated uses.  The permittee may need to  consider
       information on the contribution and importance of other pollution sources in order
       to develop a final plan designed to meet water  quality standards.  The purpose of the
       system characterization, monitoring and modeling program initially is to assist the
      permittee in  developing  appropriate measures to implement the  nine minimum
       controls and, if necessary,  to support development  of the long-term  CSO control
      plan.  The monitoring and modeling data also will be used to  evaluate the expected
       effectiveness  of both the nine minimum controls and, if necessary, the long-term CSO
       controls, to meet WQS. " (Section II.C.I)
       Characterization, monitoring, and modeling of the CSS can be broken into the following

elements:


       1.  Examination of existing  data

       2.  Characterization of the CSS
       3.  Monitoring of CSOs and receiving water
       4.  Modeling of the CSS and receiving water.


       Analysis of existing data should include an examination of rainfall records and available data

on flow, capacity, and water quality  for the collection system, treatment plant, and receiving water.
This analysis, as well as information from field inspections and simple measurements, provides the
basis  for  the preliminary system  characterization.   This initial characterization of the system

(described in more detail  in Chapter 3) should identify the number, location, and frequency of


                                           2-7                                January 1999

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Chapter 2                                         Introduction to Monitoring and Modeling







overflows and clarify their relationship to sensitive areas, pollution sources within the collection



system (e.g., indirect discharges from nondomestic sources), other pollution sources discharging to



the receiving  water (e.g.,  direct industrial  discharges, POTWs, storm  water discharges),  and



background/upstream pollution sources (e.g., agricultural or other nonpoint source runoff).







       Since  some of these activities are also conducted  as part of NMC implementation, the LTCP



should be developed in coordination with NMC implementation efforts. Ultimately, because the



LTCP is based on more detailed knowledge of the CSS and receiving waters than is necessary to



implement the NMC, the extent of monitoring and modeling for LTCP development is expected to



be more sophisticated.







       Examination of existing  data, field  inspections and  simple measurements,  and other



preliminary characterization activities will serve as the basis for the development of a cost-effective



monitoring and modeling plan (discussed in Chapter 4). The monitoring and modeling plan should



be designed to provide the information and  data needed to develop and evaluate CSO control



alternatives and to select cost-effective CSO controls.







       Chapter 4 provides an overview of the development of a monitoring and modeling plan.



Chapters 5 and 7 discuss CSS monitoring and modeling, and Chapters 6 and 8 discuss receiving



water monitoring and modeling, respectively.  It is important to remember that the monitoring and



modeling  plan should be based on the site-specific conditions of the CSS and receiving water.



Therefore the permittee should, on an ongoing basis, consult and coordinate these efforts with the



NPDES permitting  authority.







       Implementation of the monitoring and  modeling plan should enable the permittee to predict



the CSS's  response to various wet weather events  and evaluate CSO impacts on receiving waters for



alternative control strategies.  Evaluation of  CSO control alternatives is discussed in Combined



Sewer Overflows - Guidance for Long Term Control Plan (U.S. EPA, 1995a).
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Chapter 2                                         Introduction to Monitoring and Modeling


       Based on the evaluation of control strategies, the permittee, in coordination with the public,

the NPDES permitting authority,  and the  State WQS authority, should  select the cost-effective CSO
controls  needed to  provide for the attainment of WQS.   Specific conditions  relating to

implementation of these CSO controls will be incorporated into the NPDES permit as described in
Section  2.1.4.


       Post-construction compliance monitoring program

       Not only  should the LTCP contain a  characterization,  monitoring,  and  modeling plan

adequate to evaluate  CSO controls,  but it  should also contain a post-construction  compliance
monitoring plan to ascertain the effectiveness of long-term CSO controls in achieving compliance

with CWA requirements. Generally, post-construction compliance monitoring will not occur until
after development and at least partial implementation of the LTCP. Nevertheless, the permittee
should consider  its needs for post-construction monitoring as its monitoring and modeling plan

develops. The development of a post-construction compliance monitoring program is discussed in
Section 2.1.4 and Chapter 4.


2.1.3   Monitoring and Modeling During  Phase I

       The CSO Control Policy recommends that the Phase I permit require permittees to:
         Immediately implement  BAT/BCT  (best  available technology economically
           achievable/best conventional pollutant  control  technology), which  at a  minimum should
           include the NMC, as determined on a BPJ basis by the NPDES permitting authority

           Submit appropriate documentation on NMC implementation activities  within two years
           of permit issuance/modification but no later than January 1, 1997

           Comply with applicable WQS expressed as narrative limitations

           Develop and submit an LTCP as soon as  practicable, but generally within two years after
           permit  issuance/modification.
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Chapter 2                                        Introduction to Monitoring and Modeling


       The permittee should not view NMC  implementation and LTCP development as  independent
activities, but rather as related components in the CSO control planning process.  Implementation

of the NMC establishes the baseline conditions upon which the LTCP will be developed.


       In many cases, the LTCP will be developed concurrent  with NMC  implementation. As

described in Sections 2.1.1 and 2.1.2, both efforts require the permittee to  develop  a thorough
understanding  of  the CSS. For example, monitoring  done as part of the NMC to effectively
characterize CSO impacts and the efficacy of CSO controls should  provide a base of information and
data that the permittee can use in conducting  more thorough characterization, monitoring,  and

modeling activities for LTCP implementation.


       Therefore, the characterization activities needed to implement the NMC and develop the

LTCP should be a single coordinated effort.


2.1.4   Monitoring and Modeling During Phase II

       The CSO Control Policy recommends that a Phase II permit include:


          Requirements to implement technology-based controls including  the NMC on a BPJ
          basis

          A narrative requirement that selected CSO controls  be implemented, operated,  and
          maintained as described in the LTCP

          Water  quality-based effluent limits expressed in  the form of numeric performance
          standards

          Requirements to implement the post-construction compliance monitoring program

          Requirements to reassess CSOs to sensitive  areas

          Requirements for maximizing the treatment of wet weather flows at the treatment plant

          A reopener clause authorizing permit modifications if CSO controls fail to  meet WQS
          or protect designated uses.
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Chapter 2                                        Introduction to Monitoring and Modeling


The post-construction compliance monitoring program should provide sufficient data to determine

the effectiveness of CSO controls in attaining WQS.  The frequency and type of monitoring in the
program will  be site-specific.   In most cases,  some monitoring will  be  conducted during the

construction/implementation period to  evaluate  the  effectiveness  of the  long-term CSO controls.  In

some cases, however, it may be appropriate to delay  implementation of the post-construction
monitoring program until construction is well underway or completed.


       The post-construction compliance monitoring program may also include other appropriate

measures for determining the success of the CSO control program. Measures of success, which are
also discussed in Section 2.3, can address both CSO flow and quality issues. For example, flow-
related measures  could include the number of dry  weather overflows or CSO outfalls eliminated, and

reductions  in the frequency and volume  of CSOs.  Quality-related measures could include decreases
in loadings of conventional and toxic  pollutants in CSOs. Environmental measures focus on human
and ecosystem health trends such as reduced beach closures or fish kills, improved  biological

integrity indices, and the full support of designated uses  in receiving water bodies.


2.2    MONITORING AND MODELING AND THE  WATERSHED APPROACH

       The watershed approach represents a holistic approach to understanding and addressing all

surface water,  ground water, and habitat stressors within a geographically defined area, instead of
addressing individual pollutant  sources in  isolation.   It serves as  the basis for "place-based"

solutions to ecosystem protection.


       The watershed approach is based on a few main principles:


           Geographic Focus-Activities are  focused on specific drainage  areas

           Environmental Objectives and Strong Science/Data-Using strong scientific tools and
           sound  data, the  priority  problems are characterized, environmental objectives are
           determined, action plans are developed and implemented, and effectiveness is evaluated
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Chapter 2                                          Introduction to Monitoring and Modeling
          Establishment of Partnerships- Management teams representing various interests (e.g.,
          regulatory agencies, industry,  concerned citizens)  are  formed  to  jointly evaluate
          watershed management decisions

          Coordinated Priority Setting and Integrated Solutions- Using a coordinated approach
          across relevant organizations,  priorities can be set  and integrated actions  taken that
          consider all environmental issues in the context of various water programs and resource
          limitations.
       Point and nonpoint source programs, the drinking water program, and other surface and
ground  water programs  are  all integrated  into the watershed  approach.  Under  the  watershed
approach, these programs address watershed problems in an effective and cooperative fashion. The
CSO Control Policy encourages NPDES permitting authorities to evaluate CSO control needs on
a watershed basis and coordinate CSO control program efforts with the efforts of other point and
nonpoint source control activities within the watershed.


       The application of the watershed approach to a CSO control program is particularly timely
and appropriate since the ultimate goal of the CSO Control Policy is the development of long-term
CSO controls that will provide for the  attainment of WQS. Since pollution  sources  other than CSOs
are likely to  be  discharging to  the receiving water and  affecting whether WQS  are attained, the
permittee needs  to consider and understand these sources in developing its LTCP.  The permittee
should compile  existing information and  monitoring data  on these sources from the NPDES
permitting authority, State watershed personnel, or even other permittees or dischargers within the
watershed.  If other permittees within the watershed are also developing LTCPs, they may have an
opportunity to pursue a coordinated and cooperative approach to CSO control planning.


       The sources of watershed pollution and impairment, in addition to CSOs,  are varied and
include  other point source discharges, discharges from storm drains, overland  runoff,  habitat
destruction, land  use activities  (such as agriculture and construction), erosion,  and septic systems  and

landfills.   A watershed-based approach  to  LTCP  development  allows  for  the site-specific
determination  of  the relative impacts of CSOs and other pollution sources.  The flows  and  loads from
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Chapter 2                                         Introduction to Monitoring and Modeling


the pollutant sources are estimated using available site-specific data and modeling. In addition to
locally available data, potential data  sources include:
          BASINS  (Better Assessment Science Integrating Point and Nonpoint Sources) -
          Combines a geographic  information  system (GIS), national  watershed data,  and
          environmental assessment and modeling tools to facilitate watershed and water quality
          analysis.  Additional information is  available at http://www.epa.gov/OST/BASINS/.
          (U.S. EPA,  1997a)

          EMAP (Environmental Monitoring  and Assessment Program) - Contains data on a
          limited set of estuaries, surface waters, and coastal bays, as well as some information on
          landscape   characteristics   and  land  use.      EPA's  EMAP Internet site
          (http://www.epa.gov/emap/) also contains links to additional sources of environmental
          data.

          NAWQA (National  Water-Quality Assessment) Program  -  Contains information on  the
          status and trends in the quality of 60 U.S. river basins and aquifers. Information on  the
          NAWQA Program can be obtained from the U.S. Geological Survey (703-648-5716) or
          from the USGS Internet site (http://wwwrvares.er.usgs.gov/nawqa/).
       If the permittee determines during its LTCP development that WQS cannot be met because
of other pollution sources within the watershed, a total maximum daily load (TMDL), including
wasteload allocations for point sources and load allocations for nonpoint sources, may be necessary
to apportion loads among dischargers. Several publications provide  TMDL and wasteload allocation
guidance  (U.S.  EPA, 1995g; U.S. EPA, 1991b; Mills et al., 1986; Mancim et al., 1983; Martin et al.,
1990;  Mills et al.,  1985a,b). In many cases,  a  TMDL may  not have been developed for the
permittee's  watershed.  In these cases, the  monitoring and modeling  conducted  as part  of the
development and implementation of long-term  CSO  controls will support an assessment of water
quality and could support the development of a TMDL. BASINS (U.S. EPA, 1997a) also supports
the development of TMDLs.


       EPA's  Office of Water  is  committed to supporting  States that want  to implement a
comprehensive statewide  watershed management approach.  EPA has convened a  Watershed
Management Policy Committee, consisting of senior managers, to oversee the reorientation of all
EPA water programs to support watershed approaches.


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Chapter 2                                         Introduction to Monitoring and Modeling


       Of particular importance to CSO control planning and management is the NPDES Watershed
Strategy (U.S.  EPA,  1994e). This strategy  outlines national objectives  and implementation activities

to integrate the NPDES program into the broader watershed protection approach. The Strategy also
supports the  development  of statewide  basin  management as part of  an overall  watershed
management approach. Statewide basin management is an overall framework for integrating and

coordinating  water  resource management efforts basin-by-basin throughout an entire State.  This will

result  in  development  and  implementation  of basin  management plans that meet stated

environmental goals.


       The Clean  Water Action Plan, issued jointly by EPA and the U.S. Department of Agriculture,
calls for States to issue unified watershed assessments by October, 1998 (U.S. EPA/USD A, 1998).

Assessments  identify  degraded  watersheds  needing restoration,  watersheds needing  preventive  action
to sustain water  quality, and pristine or sensitive watersheds on Federal lands needing additional
protection. The Clean Water Action Plan identifies mechanisms for States and tribes to coordinate

with Federal  agencies  to prioritize watershed  restoration and protection efforts.  Additional

information  is available at http://www.cleanwater.gov/.


       Use  of the comprehensive watershed approach during  long-term CSO planning will promote
a more cost-effective program for achieving WQS in a watershed. LTCP development using the
watershed approach is discussed further in Combined Sewer  Overflows - Guidance for Long-Term

Control Plan (U.S. EPA, 1995a).


2.3     MEASURES  OF SUCCESS

       Before developing  a monitoring plan for characterizing the  CSS and determining post-

construction compliance, the permittee should identify appropriate measures  of success  based on
site-specific conditions. Measures of success are objective, measurable, and quantifiable indicators

that illustrate trends and results over time. Measures of success generally fall  into four categories:


       •  Administrative  measures that track  programmatic activities, such as  the number of
          inspections


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Chapter 2                                         Introduction to Monitoring and Modeling
          End-of-pipe measures that show trends in the discharge of CSS flows to the receiving
          water body, such as reduction of pollutant loadings, the frequency of CSOs, and the
          duration of CSOs

          Receiving water body measures that show trends of the conditions in the receiving water
          body, such as trends in dissolved oxygen levels, sediment oxygen demand, and solids
          and fecal coliform concentrations

          Ecological, human health, and use measures that show trends in conditions relating to
          the use of the water body, its effect on the health of the population that uses the water
          body, and the health of the organisms that reside in the water body, including beach
          closures,  attainment of designated uses, habitat improvements, and fish consumption
          advisories. Such measures would be coordinated on a watershed basis as appropriate.
Measures of success for a CSO control program should typically include a balanced mix of measures

from each of the four categories.


       As municipalities begin to collect data and information on CSOs and CSO impacts, they have

an important opportunity to establish a solid understanding of the "baseline" conditions and to

consider what information and data are necessary to evaluate and demonstrate the results of CSO

control. The permittee should choose measures of success that can be used to indicate reductions

in the occurrence and effects of CSOs. Municipalities and NPDES permitting authorities should
agree early in the planning stages on the data and information that will be used to measure success.
These measures of success may need to be adapted  as a municipality gains additional information
during its system characterization. (Measures of success for the CSO program  are discussed in

Combined  Sewer  Overflows-Guidance for Long-Term  Control Plan (U.S. EPA,  1995a)  and
Performance Measures for the National CSO Control Program  (AMSA, 1996)). The permittee

should consider these measures of success when determining which parameters to include in its
monitoring plan.


2.4    COORDINATION WITH OTHER WET WEATHER MONITORING AND
       MODELING PROGRAMS

       The permittee may be subject to monitoring requirements for other regulated wet weather

discharges, such as storm water, in addition to CSOs. Due to the unpredictability of wet weather


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Chapter 2                                          Introduction to Monitoring and Modeling


discharges, monitoring of such  discharges  presents  challenges similar to those  for monitoring CSOs.
The permittee should coordinate all wet weather monitoring efforts. Developing one  monitoring and

modeling program for all wet weather programs will enable the permittee to establish a clear  set of

priorities for monitoring and modeling activities.


2.5    REVIEW AND REVISION OF WATER QUALITY STANDARDS

       Section 301 of the CWA and NPDES regulations at 40  CFR 122.44 require the establishment
of both technology-based and water quality-based effluent limitations:
           Technology-based requirements. Section 301 of the CWA requires effluent reductions
           based on  various degrees of control technology for all discharges of pollutants.  NPDES
           regulations at 40 CFR 122.44(a) require that technology-based effluent limitations be
           established for pollutants of concern discharged by point sources that will be regulated
           under an NPDES permit. Under the  CSO Control Policy, permittees are expected to
           implement technology-based controls including, at a minimum, the NMC.

          Water quality-based  requirements.  Section 301(b)(l)(C) of the  CWA and NPDES
           regulations at 40 CFR 122.44(d) require  that NPDES permits contain water  quality-based
           effluent limitations for all discharges  that cause, contribute to,  or have the potential to
           cause an exceedance of a numeric or narrative WQS. As described in the CSO Control
           Policy,  Phase I permits should at least  require that the permittee immediately comply
           with applicable narrative WQS, while sufficient data may not be available at this point
           to evaluate the need for numeric effluent limits. For Phase II permits, the CSO Control
           Policy recommends that permits contain water quality-based effluent limits expressed
           as numeric performance standards  (e.g., number of overflow events per year) for the
           selected CSO  controls.  If sufficient  data  are available,  numeric  water quality-based
           effluent limitations should be developed and included in Phase II permits.
       The development of permit limits and conditions for CSO permittees is described in greater
detail in Combined Sewer Overflows - Guidance for Permit Writers (U.S. EPA, 1995e).


       Since CSO controls  must ultimately provide for the attainment of WQS, the  analysis of CSO
control alternatives should be tailored to the applicable WQS. A key principle of the CSO Control
Policy is the review and revision, as appropriate, of WQS and their implementation procedures to

reflect the site-specific wet weather impacts  of CSOs.  In identifying applicable WQS,  the permittee


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Chapter 2                                         Introduction to Monitoring and Modeling


and the permitting and WQS authorities should consider whether revisions to WQS are appropriate

for wet weather conditions in the receiving water.


       Review of WQS  should be conducted concurrent with the development of the LTCP to

ensure  that  the long-term  CSO controls will be sufficient to provide for the attainment of applicable

WQS.  The  information gained from LTCP development can then be used to support any efforts to

revise WQS.  (The identification of applicable WQS and methods for assessing attainment of WQS

are discussed in Chapter 9).


       The WQS program contains several types of mechanisms that could potentially be used to

address site-specific factors such as wet weather conditions.  These include the following:
          Adopting partial uses to reflect situations where a significant storm event precludes the
          use from occurring

          Adopting seasonal uses to reflect that certain uses do not occur during certain seasons
          (e.g., swimming does not occur in winter)

          Defining a use with greater specificity (e.g., warm-water fishery in place of aquatic life
          protection)

          Granting a  temporary variance to a specific discharger in cases where  maintaining
          existing standards for other dischargers is preferable to downgrading WQS.
       These potential revisions are described in detail in the Water Quality Standards Handbook,

Second Edition (U.S. EPA, 1994).


       Reviewing and revising WQS requires the collection of information and data to support the

proposed revision. In general, a use attainability analysis (UAA) is required to support a proposed

WQS revision. The process  for conducting UAAs  for receiving waters has been described in various

EPA publications (U.S. EPA, 1994; U.S. EPA, 1984a; U.S. EPA,  1984b; U.S. EPA, 1983b).
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Chapter 2                                         Introduction to Monitoring and Modeling


       The information and data collected during LTCP development could potentially be used to
support a UAA for a proposed revision to WQS to reflect wet weather conditions. Thus, it is
important for the permittee, NPDES permitting authority, State WQS authority, and EPA Regional
offices  to agree on the data, information,  and analyses that are necessary  to support the development
of long-term CSO controls  as well as the review  of applicable WQS  and implementation procedures,

if appropriate.
2.6    OTHER ENTITIES INVOLVED IN DEVELOPING AND IMPLEMENTING THE
       MONITORING AND MODELING PROGRAM

       Development and implementation of a CSO monitoring and modeling program should not

be solely the permittee's responsibility. Development of a successful and cost-effective monitoring

and modeling program should reflect the coordinated efforts of a team that includes the NPDES

permitting authority, State WQS authority, State watershed personnel, EPA or State monitoring
personnel, and any other appropriate entities.


       NPDES Permitting Authority
       The NPDES permitting authority should:

          Develop  appropriate system characterization, monitoring,  and modeling requirements for
          NMC implementation and LTCP development  (in a Phase I permit) and NMC  and LTCP
          implementation (in a Phase II permit)

       .  Determine,  in  coordination with  the permittee and appropriate  State  and Federal
          agencies, whether the permittee needs to consider any sensitive areas in developing a
          monitoring and modeling plan

          Coordinate with the permittee to ensure that the monitoring requirements in the permit
          are appropriately site-specific

          Assist in compiling relevant existing information, monitoring data, and studies at the
          State and/or EPA Regional level

          Decide if the presumption  approach is applicable  based on the data and analysis
          conducted in the characterization,  monitoring, and  modeling of the system and the
          consideration of any sensitive areas
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Chapter 2                                         Introduction to Monitoring and Modeling
          Coordinate the permittee's CSO monitoring and modeling efforts with monitoring and
          modeling efforts of other permittees within the watershed

          Coordinate the team  review of the  monitoring  and modeling  plan, monitoring and
          modeling data, and other components of the LTCP.  To ensure team review of the
          monitoring and modeling plan, the permitting authority could recommend that the plan
          include a signature page for endorsement by all the team members after their review.

         Develop  appropriate  monitoring  requirements  for  post-construction compliance
          monitoring to assess attainment of WQS and the effectiveness of CSO controls (in a
          Phase II  permit and ongoing).

          Assist in the review and possible revision of WQS.
       State WQS Authority
       The State WQS authority should:
          Provide input on the review and possible revision of WQS, including conduct of a use
          attainability analysis where necessary

          Assist in compiling existing  State information,  monitoring  data, and studies for the
          receiving water body

          Ensure that  the permittee's  monitoring and  modeling efforts  are  coordinated  and
          integrated with ongoing State monitoring programs

          Evaluate any special monitoring activities such as biological testing,  sediment testing,
          and whole  effluent toxicity testing.
       State Watershed Personnel
       State watershed personnel should:


          Ensure that the permittee's monitoring activities are coordinated with  ongoing  watershed
          monitoring programs

          Assist in compiling existing State information,  monitoring data, and studies for the
          receiving water body
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Chapter 2                                          Introduction to Monitoring and Modeling
          Ensure the permittee's monitoring and modeling efforts are integrated with  TMDL
          application or development.
       EPA/State Monitoring Personnel

       EPA and State monitoring personnel should:


       .  Provide  technical  support and reference material  on monitoring techniques  and
           equipment

           Assist in compiling relevant existing monitoring data and studies for the receiving water
           body

           Provide information on available models  and the monitoring  data needed as model inputs

           Assist in the evaluation and selection of appropriate models.


       The public should also  participate in  development  and implementation  of the  system
characterization activities and the monitoring and  modeling program.  Throughout the LTCP

development process, the public should have the opportunity to review and provide comments on
the results of the system characterization, monitoring, and modeling activities that lead to the

selection  of long-term CSO controls.  The public participation effort might involve public meetings
at key points during the system characterization phase  of LTCP development. Input from the public,

obtained during the early phases of the planning process, will enable a municipality to better develop
an  outreach  program that reaches a broad  base  of citizens.   In addition to public meetings,

municipalities  can  obtain input from telephone surveys,  community  leader interviews,  and
workshops.  Each of these activities can give the municipality a better understanding of the public
perspective on local water quality issues and sewer system problems, the amount of public concern
about CSOs in particular, and public willingness to participate in efforts to control CSOs.
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                                     CHAPTER  3

    INITIAL SYSTEM CHARACTERIZATION -  EXISTING DATA ANALYSIS AND
                               FIELD  INVESTIGATION
       As explained in Chapter 2, the development of a long-term control plan (LTCP) requires a
thorough characterization of the combined sewer system (CSS). Accurate information on CSS

design,  CSS responses to wet weather,  pollutant characteristics of CSOs, and biological and

chemical characteristics of receiving waters is critical in identifying CSO impacts and the projected
efficacy of proposed  CSO  controls.  Before in-depth monitoring and modeling efforts begin,

however, the permittee  should assemble as much  information as possible  from existing data sources
and preliminary  field  investigations.  Such preliminary activities will contribute to a baseline

characterization of the CSS and its receiving waters and help focus the monitoring and modeling

plan.


       The primary objectives of the existing data analysis and field investigation are:


          To determine the current level of understanding and knowledge of the CSS and receiving
          water

          To assess the design and current operating condition of the CSS

          To identify any known CSO impacts on receiving waters

          To identify the data that still need to be collected through the monitoring and modeling
          program

          To assist in implementation and documentation of the nine minimum controls (NMC).


       The activities required to meet these objectives will  vary widely from system to  system.

Many  permittees have  already  made significant  progress  in conducting  initial  system
characterizations. Implementation of the NMC, which was expected by January, 1997, should have
enabled permittees to  compile a substantial amount of information on their CSSs. In addition,



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Chapter 3                                                    Initial System Characterization


studies by EPA, State agencies, or other organizations may provide substantial information and data
for the receiving water characterization.


       This chapter describes the following activities in the initial system characterization:
           Physical Characterization  of CSS- identification  and description of all  functional
           elements of the CSS and sources discharging into the CSS,  delineation of the CSS
           drainage areas, analysis of rainfall data throughout the drainage  area,  identification of all
           CSO outfalls, and preliminary CSS hydraulic analyses.

           Characterization  of Combined Sewage and CSOs- analysis  of existing data to
           determine volume and pollutant characteristics of CSOs.

           Characterization of Receiving Waters-  identification of the designated uses and current
           status of the receiving waters affected by CSOs,  water quality  assessment of those
           receiving waters, and identification of biological receptors potentially impacted by  CSOs.
       The permittee should  consult with  the NPDES permitting authority and the review team (see
Section 2.6) when reviewing the results from the initial system characterization and in preparation
for developing the monitoring and modeling plan (Chapter 4). Performing and documenting initial

characterization activities  may  help satisfy certain  requirements for NMC implementation and
documentation. Thus, it is essential that the  permittee  coordinate with the NPDES  permitting

authority on an ongoing basis throughout the initial characterization process.


3.1    PHYSICAL CHARACTERIZATION OF  CSS

3.1.1 Review Historical  Information

       For the first part of the physical characterization, the permittee should compile, catalogue,
and  review  existing information on the design and construction of the CSS to evaluate how the CSS
operates, particularly in response to  wet weather events. The permittee should compile, for the entire
CSS, information on the contributing drainage areas, the location and capacity of the POTW and
interceptor network, the location and operation of flow regulating structures, the location of all

known or suspected CSO  outfalls, and the general hydraulic  characteristics of the  system (including



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Chapter 3                                                    Initial System Characterization


existing flow data for both wet weather and dry weather). Historical information is often available

from the following sources:
          Sewer Maps  of Suitable Scale- Sewer maps define the  pipe  network of the sewer
          system and may indicate the drainage areas that contribute to each CSO outfall. Ideally,
          they should include the combined, separate sanitary, and separate storm sewer systems,
          manhole locations for monitoring access,  catch basin locations, and pipe shapes and
          materials.  Sewer maps may also show curb/surface  drainage, roof connections,  pipe age,
          and ongoing roadway construction projects and their influence on storm flow. Many
          cities have also used Geographic Information Systems (GIS) to develop maps of their
          sewer systems. Data provided from these maps, such as the invert elevations, can be
          used to calculate individual pipe capacities and to  develop detailed hydraulic models.
          Sewer maps should be field checked because field conditions may differ significantly
          from the plans (see System Field Investigations, Section 3.1.3).

          Topographic Maps- The U.S. Geological Survey (USGS)  provides topographic maps,
          usually with lo-foot contour intervals. The local municipality or planning agency may
          have prepared topographic maps with finer contour intervals, which may be more useful
          in identifying drainage areas contributing to CSOs.

          Aerial Photograph-When  overlaid with sewer  maps and topographic  maps,  aerial
          photos may aid in identifying land uses in the drainage areas. Local planning agencies,
          past  land  use studies,  or State Departments of Transportation  may have aerial
          photographs suitable for the initial characterization.

          Diversion  Structure Drawings- Drawings of CSS structures, in plan and section view,
          indicate how the structures operate, how they should be monitored, and how they could
          be altered  to facilitate monitoring or improve flow control.

          Rainfall Data- Rainfall data are one of the most  important and useful types of data
          collected during the initial system characterization. Reliable rainfall data are necessary
          to understand the  hydraulic response of the CSS and, where applicable, to model this
          response.  Sources of data may include long-term precipitation data collected from a
          weather station within or outside the CSS drainage basin, or short-term, site-specific
          precipitation data from stations within the drainage basin or sub-basins. Wastewater
          treatment plants may also collect their own rainfall data or maintain records of rainfall
          data from  a local weather station.

          Long-term rainfall data collected within the drainage basin provide the best record of
          precipitation within the system and hence have the greatest value in correlating historic
          overflow events  with  precipitation events and in predicting the likelihood of wet weather
          events of  varying intensities. If such data are not  available, however, both  long-term
          regional and  short-term local data may be  used.   For calibration  and validation of
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Chapter 3                                                    Initial System Characterization


          hydraulic models (see Section 7.4), it is important to use rainfall data collected from
          within or in very close proximity to the drainage area.

          National rainfall data are available  from the National Weather Service, which operates
          thousands of weather monitoring stations throughout the country.  Rainfall data for some
          areas are available on the Internet (the National Weather Service home page can be found
          at http://www.nws.noaa.gov/). The local  municipality, airports,  universities, or other
          State or Federal facilities can also provide rainfall  data.  The National Oceanic and
          Atmospheric Administration (NOAA), National Climatic Data Center (NCDC), Climate
          Services Branch is  responsible for  collecting precipitation data. Data on hourly, daily,
          and monthly precipitation for each  monitoring station (with latitude and longitude) can
          be obtained on computer diskette, microfiche, or hard copy by calling (704) 259-0682,
          or by writing to NCDC, Climate Services Branch, The Federal Building, Asheville, NC
          28071-2733. Some NCDC data are also available on the Internet (NCDC's home page
          can be found  at http://www.ncdc.noaa.gov/).  The NCDC  also  provides a  computer
          program called SYNOP for data analysis.

          Additionally, permittees with few or no rain gages located within the system drainage
          basin may want to install one or more gages early in the CSO control planning process.
          Collection and analysis of rainfall data are discussed in Chapters 4 and 5.


       Other Sources of Data
       A variety  of other historical  data sources  may be used in completing the physical
characterization of a CSS.  As-built plans and documentation of system modifications can provide

reliable information on structure location and dimensions. Similarly, any recent surveys and studies
conducted on the system can verify or enhance  sewer map information.  Additional information may

also be available from:


       .  GIS  databases

          Treatment plant upgrade reports
          CSS flow records (for both dry weather and wet weather)

          Treatment plant and pump station flow and performance records
       .   Design  specifications

       .  Infiltration/inflow (I/I)  studies
           Sewer  system  evaluation surveys (SSES)
           Storm  water master plans



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Chapter 3
Initial System Characterization
           Storm water utility records and reports



           Section 208 areawide waste treatment plans



           Section 201 facility plans



           Local property taxation records



           Federal and State highway maps and plans



           County/city planning and zoning agencies.







The availability of these sources of information varies widely among permittees. Collection system



operation and maintenance personnel can be invaluable in determining the existence and location



of such data, as well as providing system knowledge and insight.







3.1.2  Study Area  Mapping





       Using the historical data,  the permittee  should develop a map of the CSS, including the



drainage basin of combined sewer areas and separate storm sewer areas. Larger systems will find



it useful to map sub-basins for each regulating structure and  CSO. This map will  be used for



analyzing system flow directions  and interconnections, analyzing land use and runoff parameters,



locating  monitoring networks, and developing model inputs. The map can also  be a valuable



planning tool in  identifying areas  of special concern in the CSS and planning further investigative



efforts and logistics.  The map should be modified as necessary to  reflect additional CSS  and



receiving water information (such as the locations of other point source discharges to the receiving



water, the  location  of sensitive areas, and planned or existing monitoring locations), when these



become available.







       The completed map should include the following information:







           Delineation of contributing CSS drainage areas (including topography)





           General  land uses  (e.g., residential, commercial,  industrial) and degree of imperviousness




           POTW and interceptor network
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                  January 1999

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Chapter 3                                                   Initial System Characterization


          Trunk sewer and interceptor sewer locations and sizes

          Diversion structures (e.g., gates, weirs)

          CSO outfalls (including the presence of backflow gates)

          Access points (e.g., manholes safely accessible considering traffic and pipe depth; flat,
          open areas accessible for sampling)

       .  Pump stations

       .  River crossings

       .  Rain gages

          Existing monitoring locations (CSS, CSO,  storm water, other  point and nonpoint sources,
          and receiving water)

       .  USGS gage  stations

          Receiving water bodies

       .  Soil  types

          Ground water flow

          Outlying separate sanitary sewer areas draining to the CSS (where applicable)

          Other point source discharges such as industrial discharges and separate storm water
          system discharges

          Existing industrial and municipal treatment facilities

          Existing non-domestic discharges to the CSS.


       It may be useful to generate two or more maps with different scales, such as a coarse-scale

map (e.g., 7.5-minute USGS map) for land uses and other watershed scale information and a finer-

scale map (e.g., 1" = 200' or 1" = 400') for sewer system details. In some cases, a Computer Aided

Design (CAD) or GIS approach can be used. Some advanced sewer models can draw information

directly from  CAD tiles, eliminating the  duplication of  entering  data  into  the  model. A



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Chapter 3                                                   Initial System Characterization


municipality's planning department may be a useful source for the hardware, software, and data

needed for such mapping efforts.


3.1.3 System  Field  Investigation

       Before  developing a monitoring and modeling program, the permittee should supplement

historical  CSS information with field observations of the system to verify findings or fill data gaps.

For example, visual inspection of regulator chambers and overflow structures during dry and wet

weather verifies information included in drawings and provides data on current conditions. Further,

it is necessary  to verify that gates or flow diversion structures operate correctly so that ensuing

monitoring programs collect information representative of the expected behavior of the system.

Field inspections should address all  areas of the CSS, including the pipe network, flow diversion

structures, CSO outfalls, pump stations, manholes, and catch basins.


       In general, field inspection activities may be used to:


           Verify the design and as-built drawings

           Locate and clarify portions of the system not shown on as-built drawings

           Identify dry weather  overflows and possible causes of the overflows (e.g., diversion
           structures set too low)

           Identify locations of CSO outfalls (and whether they are submerged)

           Identify non-standard engineering or construction practices (e.g., irregularly-designed
           regulators, use of atypical materials)

           Examine the general  conditions  and  operability of flow  regulating equipment (e.g., weirs,
           gates)

           Identify areas in need of maintenance, repair, or replacement

           Identify  areas that are curbed, areas  where roof downspouts  are directly  connected to the
           CSS, and impervious areas.
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Chapter 3                                                   Initial System Characterization


       Although generally beyond the scope of a small system characterization effort, in-line TV
cameras can be used to survey the system, locate connections, and identify needed repairs. WPCF
(1989) describes in-line inspection methods in detail and provides additional useful information for

system evaluations.


       The field investigation may also involve preliminary collection of both dry weather and wet

weather flow and  depth data, which can support the CSS flow monitoring and modeling activities
later in the CSO control planning process. Preliminary CSS flow and depth estimates can begin to
answer the following questions:


          How much rain causes an overflow at each outfall?

          How many dry weather overflows occur? How frequently and at which outfall(s)? How
          much flow is being discharged during dry weather?

          Do surcharging or backwater effects occur in intercepting devices or flow diversion
          structures?

          How deep are the maximum flows at the flow diversion structures? Would alteration of
          a diversion structure affect whether a CSO occurs?


       A variety of simple flow measurement techniques can help answer these questions prior to
development and  implementation  of a monitoring and modeling plan. These include:
           Chalk Board- A chalk board is a simple depth-measuring device, generally placed in
           a manhole.  It is a vertical board with a vertical chalk line drawn on it.  Sewer flow
           passing by the board washes away a portion of the chalk line, roughly indicating the
           maximum flow depth that occurred since the board was placed in the sewer.

           Chalk Spraying- A sprayer is used to blow chalk into a CSO structure. Passing sewer
           flow washes away the chalk, indicating approximate flow depth since spraying.

           Bottle Boards-A bottle board is a vertical board with a series of attached open bottles.
           As flow rises the bottles  with openings below the maximum flow are filled. When the
           flow recedes the  bottles  remain full indicating  the  height  of  maximum flow
           (see Exhibit 5-6).
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Chapter 3                                                   Initial System Characterization
          Block Tests-Block tests do not measure depth, but are used to detect the presence of an
          overflow. A block of wood  or other float is placed atop the overflow weir.  If an
          overflow occurs, it is washed off the weir  indicating that the event took place. The  block
          can be tethered to the weir for retrieval.
       These simple flow measurement techniques could be a useful component of the NMC for
monitoring to characterize CSO  impacts and the efficacy of CSO controls. The permittee should
discuss  this with the permitting authority.  In some limited  cases, automated  continuous flow

monitoring may be used. These techniques and other CSS monitoring techniques are discussed in

Chapter  5.


3.1.4   Preliminary  CSS Hydraulic Analysis

       The physical  characterization of the CSS should include a flow balance, using a schematic
diagram  of the  collection system.  Exhibit 3-1  provides an example of a basic flow balance diagram.

It shows expected  wet weather and dry  weather flows through each service area, and the likely flows
at each  CSO based  on sewer hydraulic capacities.   The diagram can be expanded to include

additional  detail,  such as breaking  down  the cumulative  flows  at each regulator  to  show
schematically where the flows are entering the system. This can sometimes reveal local bottlenecks
that may be resolved  by relocating the connection to  a downstream portion of the system where there

is greater capacity.


       The following steps can be used to develop a flow balance  diagram or conduct a similar flow

analysis:
           Section the  collection system into a series of basins of small  enough area to characterize
           the major collection system elements, differing land uses, receiving streams, and other
           characteristics that may become important during the development of a monitoring and
           modeling plan. These basins will likely be refined as work progresses.

           Establish the hydraulic  capacity  of each  element of the system. For a preliminary
           analysis, this can be done using the unsurcharged capacity of the system, based on pipe
           size and slope, pump station capacity, and a knowledge of bottlenecks in the system.
                                            3-9                               January 1999

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Chapter 3
                                                       Initial System Characterization
      N
    V
                            Exhibit 3-1. Basic Flow  Balance Diagram
                     River
                                                               CSO 004 = 3.47
                                                   Effluent
                                                                            Hydraulic
                                                                             Capacity = 50.0
                                                                 CSO 003 = 1.89
                          10.0
                                   CSO 001 = 2.07
                       Legend
       27.50
— Outlet Sewer Hydraulic Capacity (MGD)

— Sewer Service Area

— Cumulative Dry Weather Flow (MGD)

— Cumulative Wet Weather Flow (MGD)

— Total Flow (MGD)

— Inlet Sewer Hydraulic Capacity (MGD)
  * Cumulative flows = flows from the service area and service areas
   upstream in the collection system. Wet weather flow values are for
   the average of several sampled storm events.
                                                                          CSO 002 = 2.64
                                                                                               Total Flow
                                                                                               = 53.47
                                                                                               38.2
                                                                                               34.9
                                                                                27.5
                                                                                25.6
                                                                                               22.9
                                                                                               21.5
                                                  3-10
                                                                           January  1999

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Chapter 3                                                    Initial System Characterization
           For each basin, develop a dry weather estimate of flow delivered to the system. This can
           be done in a preliminary way by using total dry weather flow to the treatment plant,
           disaggregated to each basin using population. Care should be taken where significant
           differences in infiltration are suspected.

           For each basin, develop  an estimate of wet weather inflow and wet weather-induced
           infiltration. This estimate should be based on a consistent storm or return frequency in
           each basin. (Flow monitoring  in the CSS, including rainfall and runoff assessment, is
           discussed in Chapter 5.)

           Display these data in a manner that aids data analysis, such as in a flow balance diagram
           (Exhibit 3-1).
       The  schematic diagram, together with the historical  data review and supplemental field study,

should enable the permittee to assign typical flows and maximum capacities to various interceptors

for non-surcharged flow  conditions.  Flow capacities can be approximated from sewer maps or
calculated from invert elevations. The resulting values provide a preliminary estimate of system

flows at peak capacity. Calculations of flow within intercepting  devices or flow diversion structures

and flow records from the treatment plant help in locating  sections of the CSS that limit the overall

hydraulic capacity.


       The preliminary hydraulic analysis, together with other physical characterization activities,

will  be useful in designing the CSS monitoring program and identifying areas that should receive
greater attention in developing the monitoring and modeling plan. This preliminary analysis can

help  in identifying  likely CSOs,  the magnitude of rainfall that causes CSOs, estimated  CSO volumes,

and  potential control  points. A hydraulic model may be useful in conducting the analysis.


3.2    CHARACTERIZATION OF COMBINED SEWAGE AND CSOS

3.2.1 Historical Data Review

       As part of the initial system characterization, the  permittee should review existing data to

determine  the  pollutant characteristics  of combined  sewage during both dry and wet weather
conditions,  and, if possible, CSO pollutant loadings to the receiving water. The  purpose of this effort
is to identify pollutants of concern in CSOs, their concentrations, and  where  possible, likely sources


                                             3-11                                January 1999

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Chapter 3                                                   Initial System Characterization


of such pollutants. Together, these assessments will support decisions on what pollutants should be

monitored and where. This is discussed in detail in Chapter 4.


       The POTW's records can provide influent pollutant and flow data for both dry weather and

wet weather conditions. Such data can be analyzed to answer questions like:
          How do the influent volume, loads, and concentrations at the plant change during wet
          weather?

          What is the average concentration of parameters such as solids, BOD, and metals at the
          plant during wet weather flow?

          Which pollutants are discharged by industrial users, particularly significant industrial users?
For example,  data analysis could include  plotting a plant  inflow time  series by storm(s) and

comparing it to a rainfall time series plot for the same storm(s).   In some cases, the permittee may

also be able to use POTW data to identify which portions of the CSS are contributing significant

pollutant loadings.


       Potential sources of information for this analysis include:


       •   General treatment plant operating data

       •   POTW discharge monitoring reports (DMRs)

       •   Treatment plant optimization studies

       •   Special studies done as part of an NPDES permit application

       •   Pretreatment program data

       •   Collection system data gathered during NMC implementation

       •   Existing wet weather CSS sampling and analyses

       •   Facilities plans and designs.
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Chapter 3                                                     Initial System Characterization


        The permittee can potentially use national or regional storm water data (e.g., Nationwide

Urban Runoff Program (NURP) data1) (US. EPA, 1983a) to supplement its available data, although

more recent localized data are preferred. If approximate CSS flow volumes are known, approximate

CSS pollutant loads can be estimated  using POTW data, CSS flow volume,  and assumed storm water

concentration values.  However,  assumed constant or  event mean  concentration values for storm

water concentrations,  such as NURP data, should be used with some reservation for CSOs since

concentrations vary during a storm and from storm to  storm.


        In order to  obtain recent and reliable characterization data, the permittee may need to conduct

limited  sampling at locations within the CSS  as well as  at selected CSO outfalls as part of the initial

system  characterization.  Since this limited  sampling is usually less cost-effective than sampling

done as part  of the overall monitoring program, the permittee should  fully evaluate the  need  for such

data as  part of the initial characterization.  Chapter 5  provides details on CSS  monitoring procedures.


3.2.2  Mapping

        The permittee should  plot existing pollutant characterization data on  the study map for points

within the CSS as well as for CSO outfalls.  This will highlight areas where no data exist and areas

with high concentrations of pollutants.


3.3     CHARACTERIZATION OF RECEIVING  WATERS

3.3.1    Historical  Data Review

        The third part  of the  initial system characterization is to establish the status of  each  receiving

water body impacted by CSOs. Using existing data and information and working with the NPDES

and water  quality  standards (WQS) authorities, the permittee should  attempt to answer the following

types of questions:
   1 Some NURP data may no longer be useful due to changed conditions (e.g., lead data might not apply since control
programs have been in place for many years). The permittee should contact the permitting authority to determine the
applicability of NURP data.


                                             3-13                                 January 1999

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Chapter 3                                                   Initial System Characterization


          Does the receiving water body contain sensitive areas (as defined by the CSO Control
          Policy)?

          What are the applicable WQS? Is the receiving water body currently attaining WQS,
          including designated uses?

          Are there particular problems in the receiving water body attributable wholly or in part
          to CSOs?

          What are the hydraulic characteristics of the receiving water body (e.g., average flow,
          tidal characteristics, instream flow regulations for dams and withdrawals)?

          What other dry and wet weather sources of pollutants in the watershed are discharging
          to the receiving water body? What quantity of pollutants is being discharged by these
          sources?

          What is the receiving water quality upstream of the CSO outfalls?

          What are the ecologic and aesthetic conditions of the receiving water body?


The following types of receiving water data will help answer these questions:


       •   Applicable State WQS

       •   USGS and other flow  data (including tide charts)
       •   Physiographic and bathymetric data

       •   Water quality data

       •   Sediment data
       •   Fisheries  data
       •   Biomonitoring results

       •   Ecologic  data (habitat, species diversity)
       •   Operational data (hydropower records).


       The permittee may already have collected receiving water data as part of other programs or
studies. For example, the NPDES permit may require sampling upstream and downstream of the

treatment plant outfall or the permittee may have performed special receiving water studies as part

of its NPDES permit reissuance process.  Receiving water  data may also be obtained through


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Chapter 3                                                    Initial System Characterization


consultation with the NPDES permitting authority, EPA Regional staff, State WQS personnel, and
State watershed personnel. The CWA requires States to generate and maintain data on certain water
bodies within their jurisdictions.


        The following reports may provide information useful for characterizing a receiving water
body:
           State 303(d) Lists- Under CWA section 303(d), States and authorized Tribes identify,
           and establish total maximum daily loads (TMDLs) for, all waters that do not meet WQS
           even after implementation of  technology-based effluent  limitations and  any more
                                                                            o
           stringent effluent limitations or  other pollution control requirements.

           State 304(1) Lists- CWA section 304(1)  required  States  to  identify surface waters
           adversely affected by toxic and  conventional pollutants  from point and non-point
           sources,  with  priority given to waters adversely affected by point sources of toxic
           pollutants.  This  one-time effort was completed in 1990.  EPA recommends  that the
           permittee discuss with the permitting authority  data on  toxic "hot  spots" identified under
           this requirement.

           State 305(b) Reports- Under CWA section 305(b), States  must submit a water quality
           assessment report to EPA every two years.

           Section 319 State Assessment Reports- Under CWA section 319, States were required
           to identify surface waters adversely affected by nonpoint sources of pollution, in a one-
           time effort following  enactment of the 1987 CWA Amendments.
       Generally, permittees may retrieve this information at EPA or State offices, EPA's Storage
and Retrieval of U.S. Waterways Parametric Data (STORET) system, EPA's Water Quality System
resident within STORET, or EPA's Water Body  System (WBS).  Since these data bases might not
include the particular water  bodies being evaluated,  the permittee should contact State officials prior
to seeking the data.
   2 EPA recommends that the permittee discuss with the permitting authority the status of existing TMDL reports and
the schedule for doing new TMDLs for the CSO-impacted receiving water bodies.

    These lists are not complete for some locations, so the lists should be discussed with State WQS staff before they
are used extensively.


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Chapter  3                                                    Initial System Characterization


       In addition, studies  conducted under  enforcement actions, new permitting actions,  and special

programs and initiatives may provide relevant data on receiving water flow, quality, and uses.

BASINS  (Better Assessment Science Integrating  Point and Nonpoint Sources)  contains water quality

monitoring  data and  data on point  sources  and  land  use  (US. EPA, 1997a).  EPA's EMAP

(Environmental Monitoring and  Assessment Program) contains  data  on a  limited  number of

receiving waters and the EMAP Internet site (http://www.epa.gov/emap/)  provides links to other

sources of environmental data (including STORET).  EPA and State personnel  may have  information

on studies conducted by other Federal organizations,  such as the U.S. Fish and Wildlife Service, the

U.S. Army Corps of Engineers, USGS, and the National Biological Service, and other organizations

such as The Nature Conservancy and formalized volunteer groups. For example, USGS's  National

Water-Quality Assessment (NAWQA) Program contains water quality information on 60 U.S. river

basins and aquifers.  The permittee may save considerable time and expense by consulting directly

with these entities during the initial system characterization.


       The receiving  water characterization should also include an evaluation of whether CSOs

discharge to sensitive  areas, which are a high priority under the CSO Control Policy.  The LTCP

should prohibit new or significantly increased overflows to sensitive areas and eliminate or relocate

such overflows  wherever physically possible and economically achievable.  (This  is discussed in

more detail in Combined Sewer Overflows - Guidance for Long-Term  Control Plan,  U.S. EPA,

1995a). The permittee  should work with the NPDES permitting authority, the U.S. Fish and Wildlife

Service,  and relevant State agencies to determine whether particular receiving water segments may

be considered sensitive under the  CSO Control Policy.


       In addition to reviewing existing data, the permittee may wish to conduct an observational

study of the receiving water body, noting differences in depth or width, tributaries,  circulation (for
    4 Information on the NAWQA program is available from USGS (703-648-5716) and the USGS Internet site
(http://wwwrvares.er.usgs.gov/nawqa/).

    Sensitive areas, as discussed in the CSO Policy, are defined by the NPDES authority but include Outstanding
National Resource Waters, National Marine Sanctuaries, waters with threatened or endangered species and their habitat,
waters with primary contact recreation, public drinking water intakes or their designated protection areas, and shellfish
beds.


                                             3-16                                January 1999

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Chapter 3                                                    Initial System Characterization


estuaries), point sources,  suspected nonpoint  sources, plant growth,  riparian zones, and other

noticeable features.  This information can be used later to define segments for a  receiving water

model.


        To supplement the observational study, the permittee may consider limited chemical or

biological sampling of the  receiving  water. Biocriteria or indices may be used in States such as Ohio

that have systems in place. Biocriteria describe the biological integrity  of aquatic communities in

unimpaired waters for a particular designated aquatic life  use. Biocriteria can be numerical values

or narrative conditions  and  serve as a reference point since  biological communities in the  unimpaired

waters represent the best attainable conditions (U.S. EPA, 1991 a). A limitation of biocriteria is that

they normally do not take into account wet weather conditions unique to urban streams, such as

runoff from highly impervious areas.


3.3.2  Mapping

        The permittee should plot existing receiving water characterization data on the study map.

This will  permit visual identification  of areas  for which no  data exist, potential areas of concern,  and

potential monitoring locations. GIS mapping can be used as an aid in this process.  In addition to

the elements listed in Section 3.1.2 and 3.2.2, the map could include the following:
           WQS classifications for receiving waters at discharge locations and for upstream and
           downstream reaches,  and an indication of whether receiving waters  are tidal or non-tidal

           Location of sensitive  areas  such as downstream beaches,  other  public access areas,
           drinking water intakes, endangered species habitats, sensitive biological populations or
           habitats, and shellfishing areas

           Locations of structures, such as weirs and dams, that can affect pollutant concentrations
           in the receiving water

           Locations of access points, such as  bridges,  dams, and existing monitoring stations (such
           as USGS stations), that make convenient sampling sites.
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Chapter 3                                                   Initial System Characterization


3.4    IDENTIFY DATA GAPS

       The final task in the initial system characterization is to identify gaps in information that is

essential to a basic understanding of the CSS's response to rain events and the impact of CSOs on
the receiving water.  The following questions may help to  identify data gaps that need to be
addressed in the monitoring and modeling plan:


       Physical Characterization of CSS

          Have all CSO outfalls been  identified? (Has the permittee taken all reasonable steps to
          identify outfalls-e.g.,  reviewing maps,  conducting inspections,  looking at citizen
          complaints?)

          Are the drainage sub-areas delineated for each CSO outfall?

          Is sufficient information on the location, size, and characteristics of the sewers available
          to support more complex analysis, including hydraulic modeling (as needed)?

          Is sufficient information on the location, operation, and condition of regulating structures
          available to  construct at least  a  basic  hydraulic simulation? (Even  if a  hydraulic
          computer model is  not used, this  level of  knowledge is critical to understanding how the
          system works and  for implementing the NMC.)

          Are the minimum amount of rainfall and minimum rainfall intensity that cause CSOs at
          various outfalls known?

          Are the areas of chronic surcharging in the CSS known?

          Have potential monitoring locations in the CSS been identified?

          Are  there differences between POTW wet  weather and dry weather  operations? If so, are
          these clearly understood? (Improved wet weather operation can increase capture of CSS
          flows  significantly.)


       Characterization of Combined Sewage and CSOs
          Are the flow  and  pollutant concentrations of CSOs for a range of storm conditions
          known?

          Are the sources of CSS pollutants known?
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Chapter 3                                                    Initial System Characterization
          Is sufficient information available on pollutant loadings from CSOs and other sources
          to support an evaluation of long-term CSO control alternatives?
       Characterization of Receiving Waters

          Are the hydraulic  characteristics of receiving  waters  known,  such as the average/
          maximum/minimum (7Q10) flow of rivers and  streams or the freshwater component,
          circulation patterns, and mixing characteristics of estuaries?

          Are locations of sensitive areas and designated uses identified on a study map?

          Have  existing  monitoring locations  in the receiving  water been  identified?  Have
          potential monitoring locations (e.g., safe, accessible points) in the receiving water been
          identified for areas  of concern and areas where no data exist?

          Are sufficient data available to assess existing water quality problems and the potential
          for future water quality problems, including information on:

          -  Streambank  erosion
          -  Sediment  accumulation
          -  Dissolved  oxygen levels
              Bacterial problems, such as those leading to  beach closures
          -  Toxicity (metals)
              Nuisance algal or aquatic plant growths
              Damage to a fishery (e.g., shellfish beds)
              Damage to a biological community  (e.g., benthic organisms)
              Floatables or other aesthetic concerns?

          Is sufficient information available on natural background conditions that may preclude
          the attainment of WQS? (For example, a stream segment with a high natural organic
          load may have a naturally  low dissolved oxygen level.)

          Is sufficient information available on  other pollutant sources (e.g., agricultural sources,
          other nonpoint sources, and municipal and industrial point sources, including those
          upstream) that may preclude the attainment of WQS?
The answers to these types of questions will support the development of goals  and  objectives for the
monitoring plan, as described in Chapter 4.
                                            3-19                                January 1999

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                                      CHAPTER 4






                        MONITORING  AND  MODELING  PLAN








       Under the CSO Control Policy, the permittee should begin immediately to characterize its



combined sewer system (CSS), document implementation of the nine minimum controls (NMC),



and develop a long-term control plan (LTCP). The NMC and the LTCP both contain elements that



involve monitoring and modeling activities.  The NMC include monitoring to characterize CSO



impacts and the efficacy of CSO controls, while the LTCP includes elements for  characterization,



monitoring, and  modeling of the CSS and receiving  waters, evaluation and  selection of CSO control



alternatives, and development  of a post-construction monitoring  program. As discussed  in



Chapters 2 and 3, "monitoring" as part of the NMC involves gathering and analyzing existing data



and performing field investigations, but does not generally involve sampling or the use of complex



models. Thus the monitoring and modeling  elements discussed in this  chapter and subsequent



chapters primarily pertain to LTCP development and implementation.







       The NPDES permit is likely to contain requirements for monitoring  necessary to develop and



implement an LTCP. In  many cases, the permit will  first  require the permittee to submit a



monitoring and  modeling plan.  For example, the Phase I permit may  require  submission of a



monitoring and modeling  plan as an interim deliverable during LTCP development.







       A  well-developed  monitoring and modeling plan is essential throughout the CSO planning



process to provide useful  monitoring data for system characterization, evaluation and selection of



control alternatives,  and post-construction compliance monitoring. Development of the plan is  likely



to be  an iterative process, with changes made as more knowledge about the CSS and CSOs is gained.



The permittee should aggressively seek to involve the NPDES permitting authority, as well as State



water quality standards (WQS) personnel, State watershed personnel, and EPA Regional  staff,



throughout this process.







       This chapter describes how the permittee can develop a monitoring and modeling plan that



provides essential and accurate information about the CSS and CSOs, and the impact of CSOs on






                                           4-1                               January 1999

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Chapter 4                                                  Monitoring and Modeling Plan







the receiving  water.  The chapter discusses the identification of monitoring and modeling goals and



objectives and the development of a monitoring and modeling plan to achieve those goals  and



objectives. It provides  detailed discussions  and examples  on  identifying  sampling locations,



frequencies, and parameters to be assessed. In addition,  it briefly discusses certain monitoring and



modeling  plan elements that are common to all system components being monitored. Readers



should consult the appropriate EPA guidance documents (see References) for further information



on topics  such  as  chain-of-custody,  sample  handling, equipment,  resources,  and quality



assurance/quality control (QA/QC) procedures.







4.1    DEVELOPMENT OF A MONITORING AND MODELING PLAN




       A monitoring and modeling plan can be developed with the following steps:







       Step 1: Define the short-  and long-term objectives - In order to  identify  wet weather



impacts and make sound decisions on CSO controls, the permittee should first formulate the short-



and long-term objectives  of the  monitoring and modeling effort.  Every  activity proposed in the  plan



should contribute to attaining those objectives. (Step 1 is discussed in Section 4.1.1.)







       Step 2: Decide whether to  use a model - The permittee  should decide whether to use a



model during LTCP development (and, if so, which model to use). This decision should be  based



on site-specific considerations (e.g., CSS characteristics and complexity, type of receiving water)



and the information compiled in the initial system characterization. If a permittee decides to use a



model, the monitoring and modeling plan should include a modeling strategy. (Section 4.1.2)







       Step 3: Identify data needed - The permittee should identify the monitoring data needed



to meet the goals and objectives. If modeling is planned, the monitoring plan should include any



additional data needed for model inputs. (Section 4.1.3)







       Step 4: Identify sampling  criteria (e.g., locations, frequency)  - The permittee  should



identify monitoring locations within the CSS, which CSOs to monitor, and sampling points within









                                           4-2                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan


the receiving  water  body.   The permittee must  also determine the frequency and duration  of

sampling, parameters to be sampled, appropriate sample types to be collected (e.g., grab,  composite),

and proper sample handling and preservation procedures. If a model will be used, the monitoring

plan should include any additional sampling locations, sample types, and parameters necessary to

adequately support the proposed model. If this is not feasible, the permittee may need to reevaluate

the model choice and select a different or less-complex model. (Sections 4.2 to 4.7)


       Step 5: Develop data management and analysis procedures - A monitoring and modeling

plan also needs to specify QA/QC procedures and a data management program to facilitate storage,

use,  and analysis of the data. (Section 4.8)


       Step 6: Address implementation issues - Finally, the monitoring and modeling plan should

address implementation issues, such as record  keeping  and reporting, responsible  personnel,

scheduling,  and the equipment and  resources  necessary  to accomplish the  monitoring and modeling.

(Section 4.9)


       These  steps are described in detail in the remainder of this chapter.


4.1.1 Goals  and Objectives

       The ultimate goal of a CSO control program is to implement  cost-effective controls  to reduce

water quality impacts from CSOs and provide for compliance with CWA requirements, including

attainment of WQS. Monitoring and modeling will foster attainment of this goal by generating data

to support decisions for selecting CSO controls. The monitoring and modeling plan should identify

how data will  be collected and used to meet the following goals:


           Define the CSS's hydraulic response to rainfall.

              What level of rainfall causes CSOs?
           -   Where do the CSOs occur?
           -   How long do CSOs last?
              Which structures or facilities limit the hydraulic capacity of the CSS?


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Chapter 4                                                   Monitoring and Modeling Plan
          Determine CSO flows and pollutant concentrations/loadings.

              What volume of flow is discharged?
              What pollutants are discharged?
              Do the flows and  concentrations of pollutants vary greatly from event to event and
              outfall to outfall?
              How do pollutant  concentrations and loadings vary within a storm event?

          Evaluate the impacts of CSOs on receiving water quality.

              What is the baseline quality of the receiving water?
              What are the upstream background pollutant concentrations?
              What are the impacts of CSOs? Are applicable WQS being met?
              What is the contribution of pollutant loadings from other sources?
              Is biological, sediment, or whole effluent toxicity testing necessary?

          Support model input,  calibration, and verification.

          Support the review and revision, as appropriate, of WQS.

              What data are needed to support a use attainability analysis?
              What data are needed to support potential revision of WQS to reflect wet weather
              conditions?

          Evaluate the effectiveness of the NMC.

              Have any dry weather overflows been eliminated?
              Has wet weather  flow to the POTW increased (if  additional plant capacity  was
              available)?
              Has the level of rainfall needed to  cause CSOs increased?

          Evaluate and select long-term CSO control alternatives.

              What improvements in  water quality will result  from proposed  CSO  control
              alternatives in the LTCP?
              How will the CSS hydraulics and  CSO frequency and duration change under various
              control alternatives?
              What is the best combination of control technologies across the system?
              Can CSO flows to sensitive areas be eliminated? If not, can they be relocated to less
              sensitive areas?
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Chapter 4                                                   Monitoring and Modeling Plan


       In addition to selecting and implementing long-term CSO controls, the permittee will also

be required to develop  and implement a post-construction  compliance monitoring  program. For this

type of monitoring program, the goal will typically be to:


       .   Evaluate the effectiveness of the long-term CSO controls.

              Are applicable WQS being met?
              How much water quality improvement do environmental indicators show?
              Do the measures of success (see Section 2.3) indicate reductions in CSOs and their
              effects?


       Besides the broad goals,  a municipality may have  some  site-specific  objectives  for  its

monitoring  program. For example,  a permittee that is considering  sewer separation as  a CSO  control

alternative may wish  to assess the  likely impacts of  increased storm water loads on receiving  waters.


       The permittee should distinguish between short-term and long-term monitoring objectives.

Determining the length of short-term and long-term planning horizons will depend in part on how

much CSO control is already in place.


4.1.2 Modeling Strategy

       In developing  a monitoring and modeling  plan, the  permittee should consider up front

whether to use modeling. If a permittee has a relatively simple system with a limited number of

outfalls, the use of flow balance diagrams and similar analyses may be sufficient and modeling may

not be necessary. For more  complex systems,  modeling can help characterize and predict:


        .   Sewer system response to wet weather

        .   Pollutant loading to receiving waters

        .   Impacts within the receiving waters

        .   Relative impacts attributable to CSOs and other pollutant sources.
                                            4-5                                January 1999

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Chapter 4                                                   Monitoring and Modeling Plan


       Modeling also assists in formulating and testing the cause-effect relationships between wet
weather events and receiving water impacts. This knowledge can help the permittee evaluate control

alternatives and formulate an acceptable LTCP. Modeling enables the permittee to predict the
effectiveness of a range of potential control alternatives.  By assessing the expected outcomes of

control alternatives  before  their implementation, the permittee  can make more cost-effective

decisions. Modeling results may also be relevant to reviewing and revising State WQS. Since the
use of a model and its level of complexity affect the need for monitoring data, the permittee should
determine early on whether modeling is needed to provide sufficient information for making CSO

control decisions.


       Once a model is calibrated and verified, it can be used to:
           Predict CSO occurrence, volume, and in some cases, pollutant characteristics, for rain
           events other than those that occurred during the monitoring phase. These can include a
           storm event of large magnitude (with a long recurrence period)  or numerous  storm events
           over an extended period of time.

           Predict the wet weather performance of portions of the CSS  that have not been monitored
           extensively.

           Develop  CSO statistics such as annual number of CSOs and percent of combined sewage
           captured (particularly  useful for municipalities pursuing the  presumption approach under
           the CSO Control Policy).

           Optimize sewer  system performance as part of the NMC.  In particular, modeling can
           assist in locating storage opportunities and hydraulic bottlenecks and demonstrate that
           system storage and flow to the POTW are maximized.

           Evaluate and optimize control alternatives, from simple controls described under the
           NMC (such as raising weir heights to increase in-line storage)  to more complex controls
           proposed in the  LTCP.  The  model can be used to evaluate the resulting reductions in
           CSO volume and frequency.

           To predict the number and duration of WQS exceedances in  areas of interest (such as
           beaches or other sensitive areas).

           To evaluate water quality improvements likely to result from implementation of different
           CSO controls or combinations of CSO controls.
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Chapter 4                                                   Monitoring and Modeling Plan


       If the permittee decides to model, the monitoring and modeling  plan should  include a
modeling strategy. There are several considerations in developing an  appropriate modeling strategy:
          Meeting the expectations of the CSO Policy- The focus of modeling depends in part
          on whether the permittee adopts the presumption or demonstration approach under the
          CSO Policy. For some communities,  the demonstration  approach can  necessitate
          detailed simulation of receiving water impacts to show that CWA requirements will be
          met under selected CSO control measures. The presumption approach may not involve
          as much receiving water modeling since it presumes that CWA requirements are met
          based on certain performance criteria, such as the maximum number of CSO events or
          the percent capture of flows entering the system  during a wet weather event.

          Successfully simulating the physical  characteristics  of the CSS, pollutants, and
          receiving waters under study-  Models should be chosen to simulate the physical and
          hydraulic characteristics of the CSS and the receiving water body, characteristics of the
          pollutants of concern, and the time and distance scales necessary to evaluate attainment
          of WQS. Receiving waters should be modeled whenever there is significant uncertainty
          over the importance of CSO loads as compared to other sources. A model's governing
          equations  and boundary  conditions should match the characteristics of the  CSS, receiving
          water body, and pollutant fate and transport processes  under study. A model does not
          necessarily need to  describe the system completely in order  to  analyze CSO events
          satisfactorily. Different modeling strategies will  be necessary for the different physical
          domains being modeled: overland storm flow, pollutant buildup/washoff, and transport
          to the collection system; transport within  the CSS  to the  POTW, storage facility, or CSO;
          and  dilution and  transport in  receiving waters.  In most cases, simulation models
          appropriate for the sewer system also address pollutant buildup/washoff and overland
          flow. Receiving water models are typically  separate from  the storm water/sewer models,
          although in some  cases compatible interfaces are available.

          Meeting information needs at optimal cost- The  modeling strategy should  identify
          modeling  activities that provide  answers as detailed and  accurate  as needed at the lowest
          corresponding expense and effort.   Since more detailed, accurate  models are more
          difficult and expensive to use, the permittee needs to identify the point at which an
          increased modeling effort would provide diminishing returns. The permittee may use an
          incremental approach, initially  using simple screening models with limited data. These
          results  may then lead to refinements in the monitoring and modeling plan so that the
          appropriate data are generated  for more detailed modeling.  Another option is to use a
          simpler CSS model  for the whole  system and selectively apply a more complex sewer
          model to portions of the system to answer specific design questions.

       More  detailed discussions on modeling, including model selection, development, and
application,  are included in Chapters 7 (CSS Modeling)  and 8 (Receiving Water Modeling).
                                            4-7                               January  1999

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 Chapter 4                                                   Monitoring and Modeling Plan







4.1.3 Monitoring  Data  Needs




       The monitoring effort necessary to address each goal will depend on a number of factors:



the layout of the collection system; the quantity, quality,  and variability of the  existing historical data



and the necessary additional data; whether modeling will be done and, if so, the complexity of the



selected  model; and the available  budget. In some cases,  the  initial characterization will  yield



sufficient historical data so  that only limited additional monitoring will be necessary. In other  cases,



considerable effort may be necessary to fully investigate the characteristics of the CSS, CSOs, and



receiving waters.   Some  municipalities may choose to allocate a  relatively large portion  of the



available budget to monitoring, while others may allocate less. Because data needs may change as



additional knowledge is obtained, the monitoring program must be a dynamic program that evolves



to reflect any changes in data needs.







       In  identifying  goals  and objectives, developing a  modeling strategy,  and  identifying



monitoring  data needs, the permittee should work with  the team that will be reviewing  NMC



implementation and LTCP development and  implementation (e.g., NPDES permitting authorities,



State WQS  authorities, and State  watershed personnel). This coordination should begin in the  initial



planning stages so that appropriate goals and objectives  are identified and effective monitoring and



modeling approaches to meet these goals and objectives  are developed. Concurrence among the



review team participants  during the planning stages should ensure design  of a  monitoring and



modeling plan that  will support sound CSO control program decisions. The proposed plan should



be submitted to the review team and modified as necessary. The permittee should also  coordinate



the monitoring and modeling plan with  other Federal and State agencies, and with other point  source



dischargers, especially for effects on watersheds and ambient receiving waters.
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Chapter 4                                                  Monitoring and Modeling Plan
4.2    ELEMENTS OF A MONITORING AND MODELING PLAN

       In addition to identifying the goals and objectives, the monitoring  and  modeling plan should

generally contain the following major elements:


          Review of Existing Data and Information (discussed in Chapter 3)

              Summary of existing data and information
              Determination of how existing data meet goals and objectives
              Identification of data gaps and deficiencies

          Development of Sampling Program to Address Data Needs (discussed in Chapters 4-6)

              Duration of monitoring program
          -  Monitoring  locations
              Frequency of sampling and number of wet weather events to be sampled
              Criteria for  when the samples will be taken (e.g., greater than x days between events,
              rainfall events greater than 0.4 inches to be sampled)
              Strategy  for determining when to initiate wet weather monitoring
              Sampling protocols (e.g., sample types, sample containers, preservation methods)
              Flow measurement protocols
              Pollutants or parameters to be analyzed and/or recorded
              Sampling and safety equipment and personnel
              QA/QC procedures for sampling and analysis
              Procedures for validating, tracking, and reporting sampling results

          Discussion of Methods for Data Management and Analyses (discussed in Chapters 4-9)

              Data management  (e.g., type of data base)
              Statistical methods for data analysis
              Modeling strategy, including model(s) selected (discussed in Chapters 7 and 8)
              Use  of data to support NMC implementation and LTCP development

           Implementation Plan (discussed in Section 4.9, and Chapters 5 and 6)

              Recordkeeping and reporting
              Personnel responsible for implementation
              Scheduling
              Resources  (funding, personnel, and equipment)
              Health and safety issues.
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       The checklists in Appendix A, Tables A-l and A-2 list items that should be addressed in



formulating a monitoring program. Elements in the first checklist should be part of any monitoring



program and cover seven major areas:  sample and field data collection, laboratory analysis, data



management, data analysis, reporting, information use,  and general.  The second checklist applies



specifically to CSO monitoring and covers three areas: mapping of the CSS and identification of



monitoring locations, monitoring of CSO volume, and monitoring of CSO quality.







       As noted earlier, development of a monitoring and modeling plan is generally an iterative



process. The permittee should update the plan as a result of feedback from the NPDES permitting



authority and the rest of the CSO planning team, and as more knowledge about the CSS and CSOs



is gained.







       Because each permittee's CSS, CSOs,  and receiving water body are unique, it  is not possible



to  recommend  a generic, "one-size-fits-all"  monitoring and modeling plan in  this  document. Rather,



each permittee should design a cost-effective monitoring  and modeling  plan tailored to local



conditions and reflecting the size of the CSS, the impacts of CSOs, and whether modeling will be



performed.  It should balance the costs of monitoring against the amount of data and information



needed to develop, implement, and verify the effectiveness of CSO controls.







       While  a monitoring  and modeling  budget may initially seem large, it is  often a small



percentage of the total cost of CSO control.  Each municipality  should balance the  cost of monitoring



and modeling  against the risk of developing ineffective or  unnecessary CSO  controls  based on



insufficient or inaccurate data. The information obtained from additional monitoring and  modeling



may very well be  offset by the reduction in total CSO costs.







4.2.1   Duration  of Monitoring Program




       The duration of the monitoring program will vary from location to location and reflect the



number of storm events needed to provide the data for calibrating and validating the CSS hydraulic



model (if a model is used), and evaluating CSO control alternatives  and receiving water impacts.









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Chapter 4                                                    Monitoring and Modeling Plan







During that period (which generally may be a season or several months), the permittee should



monitor storms of varying intensity,  antecedent  dry days, and total  volume to ensure that calculations



and models represent the range of conditions experienced by the CSS.







        The monitoring program should span enough storm events to enable the permittee to fully



understand  the  pollutant loads from CSOs,  including the  means and  variations of pollutant



concentrations and the resulting effects on receiving water quality. If the permittee monitors only



a few storm events, the analysis should include appropriately conservative assumptions because of



the uncertainty associated with small sample  sizes.  For example, if monitoring data are collected



from a  few storms during spring, when CSOs are generally  larger  and more frequent, mean pollutant



concentrations may be lower due to dilution from snowmelt  and heavier rainfall and diminished first-



flush effects. When monitoring data are collected  for additional storms,  including those in  the



summer and fall when CSOs are less frequent, the mean pollution concentrations may  increase



significantly.  Additional samples  should reduce the level of uncertainty and allow the  use of a



smaller margin of safety in the analysis.







        The value of additional monitoring diminishes when  additional  data would  result in  a limited



change in the estimated mean and variance of a data set. The permittee should assess the value of



additional  data  as they are collected  by reviewing  how the estimated mean and variance of



contaminant concentrations changes over time. If estimated  values stabilize (i.e., the mean and



variance show almost no change  as  additional monitoring results are added to the data set), the need



for additional data should be reassessed.







        Pollutant loadings vary according to the number of days  since the last storm and the intensity



of previous  rainfalls.   Therefore, to better  represent the variability  of actual conditions,  the



monitoring program should be designed to sample storms with a variety of pre-storm conditions.
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Chapter 4                                                  Monitoring and Modeling Plan


4.2.2   Sampling Protocols and Analytical Methods

       The monitoring and modeling plan should describe the sampling and analytical procedures

that will be used.  Sample types depend on the parameter, site conditions, and the intended use of

the data. Flow-weighted composites may be most appropriate for determining average loadings of

pollutants to the receiving stream.  Grab samples may suffice if only approximate pollutant levels

are needed or if worst-case conditions (e.g., first 15 or 30 minutes of overflow) are being assessed.

In addition, grab samples should be collected for pollutant parameters that cannot be composited,

such as oil and grease, pH, and bacteria.   The monitoring plan should follow the sampling and

analytical procedures in 40 CFR Part 136, including the use of appropriate sample containers, sample

preservation methods, maximum allowable holding times, and analytical methods referencing one

or more of the following:


          Approved methods referenced in 40 CFR 136.3, Tables  1A through IE

          Test methods in Appendix A to  40 CFR Part 136 (Methods for Organic  Chemical
          Analysis of Municipal and Industrial Wastewater)

          Standard Methods for the Analysis of Water and Wastewater (use the most current, EPA-
          approved edition)

       . Methods  for  the Chemical Analysis of Water  and  Wastes  (U.S.  EPA,  1979.
          EPA 600/4-79-020).


       In some cases,  other well-documented analytical protocols  may be more appropriate for

assessing in-stream parameters.  For example,  in estuarine areas, a  protocol from NOAA's Status

and Trends Program may provide better accuracy and precision if it reduces saltwater interferences.


       These issues are discussed in further detail in Section 5.4.1.
                                           4-12                              January 1999

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Chapter 4                                                   Monitoring and Modeling Plan







4.3    CSS AND CSO MONITORING




       To  satisfy the objectives of the CSO Control Policy, the monitoring and modeling plan



should  specify how the  CSS  and CSOs will  be monitored, including monitoring  locations,



frequencies, and  pollutant parameters.   The plan should be coordinated with other concurrent



sampling efforts (e.g., ongoing  State water quality monitoring programs) to reduce sampling and



monitoring  costs and  maximize use  of available resources.  Careful selection of monitoring locations



can minimize the number of monitors and monitoring stations needed.







4.3.1   CSS and  CSO Monitoring Locations





       The monitoring  and modeling  plan  should specify how rainfall  data, flow data,  and pollutant



data will be collected to define the CSS's hydraulic response to wet weather events and to measure



CSO flows and pollutant loadings.  The monitoring program should also provide  background data



on conditions in the CSS during dry weather conditions, if this information is not already available



(see Chapter 3).  Dry weather monitoring of the CSS may help identify pollutants of concern in



CSOs during wet weather.







       Rainfall Gage  Locations



       The permittee should ascertain whether additional rainfall data are necessary to supplement



existing data. In general, rainfall should be monitored if CSO flow and quality are being measured



since areas often do not have routine rainfall monitoring data of sufficient detail.  In such cases the



monitoring and modeling plan should identify where rain gages  will be placed to provide data



representative of the entire CSS drainage area. Gages should be spaced closely enough that location



variation in storm tracking and storm intensity does not result in large errors in estimation of the



rainfall within the CSS area.
                                            4-13                                January 1999

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 Chapter 4                                                    Monitoring and Modeling Plan


       Recommended spacing is the subject of a variety of research papers. The CSO Pollution
Abatement Manual  of Practice (WPCF, 1989) provides the following summary of recommendations

 on rain gage spacing:
        "In Canada, rainfall and collection system modelers recommend one gauge every
        1  or 2 kilometers. In Britain, the Water Research Center has recommended only half
        that density, or one gauge every 2 to 5 kilometers. In the United States current
        spacing recommendations  are related to  thunderstorm size.    The  average
        thunderstorm is 6 to 8 kilometers in diameter,.. Therefore rain gauges are frequently
        spaced every 6 to  8 kilometers ..."
For small watersheds, rain gages may need to be placed more closely than every 6 to 8 kilometers

so  that sufficient  data are available for analysis and model calibration.  The monitoring  and modeling
plan should document the rationale  for rain gage spacing. Additional gages can provide valuable
information for CSS analysis and modeling and are usually a relatively inexpensive investment.


        CSS Monitoring Locations
        The monitoring and modeling plan will need to identify where in the  collection system flow

and pollutant loading  data  will  be collected.  To predict the likelihood  and  locations of CSOs during

wet weather, it is necessary  to assess general flow patterns and volume in the CSS and identify
which structures tend  to  limit the hydraulic capacity. This  may require sampling along various trunk
lines  of the collection system. Flow  data from existing monitors  and operating records for hydraulic
controls such as pump stations  and POTW headworks  can  also be used.  Some calculations may be
necessary to obtain flow data.  For example, pump station operating records may consist of pump

run times and capacities, which can  be used to calculate flow.


        To obtain complete flow and pollutant loading data, the plan should also target portions of
the collection  system that are likely  to receive significant pollutant loadings.  The plan  should
identify locations where industrial  users discharge  into the collection system, and specify any
additional monitoring that will be conducted to supplement data collected through the industrial
                                             4-14                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan







pretreatment program. The plan should give special consideration to these areas when they are



located near CSO outfalls.  Section 4.3.3 discusses the types of pollutants to be monitored.







       CSO Monitoring Locations



       The monitoring and modeling plan should provide for flow and pollutant monitoring for a



representative  range of land uses and basin sizes and at as many CSO outfalls as possible.  Small



systems may be able to monitor all  outfalls for each storm event studied, but large systems may need



a tiered approach in which only outfalls with higher flows or pollutant loadings receive the full range



of measurements. Discharges to sensitive areas would warrant continuous flow monitoring and the



use of composite samples for chemical analyses.  Lower-priority outfalls, meanwhile, would be



monitored  with simpler techniques such as visual observation, block tests, depth measurement,



overflow timers, or chalk boards (discussed in section 3.1.3) and limited chemical analyses. When



several outfalls are located along the same interceptor, flow monitoring of selected outfalls and at



one or two locations in the interceptor should suffice.







       Even if a monitoring program accounts for most of the total land area or estimated runoff,



monitoring other outfall locations, even with simple techniques, can provide information about



problem areas. For example, at an  overflow point with only 10 percent of the contributing drainage



area, a malfunctioning regulator may  result in discharges during dry weather or during small storms



when the interceptor has remaining capacity. As a result, this overflow point may become a major



contributor of flows. A simple technique such as a block test could identify this problem.







       Alternatively,  flow measurement equipment can be rotated between locations so that some



locations are monitored for a subset of the storms studied. For example, during one storm  the



permittee  could  monitor critical outfalls with automated  flow monitoring equipment, two less-



important outfalls  with portable flow meters, and the others using chalk boards. During a second



storm, the permittee could still monitor critical outfalls with automated flow equipment but rotate



the portable flow meters to two other outfalls of secondary importance. However, since variability



is  usually greater from storm to storm  than from site to site, it is generally preferable to measure



more storms at a set of representative sampling sites than to rotate between all CSO locations.






                                            4-15                               January 1999

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Chapter 4                                                    Monitoring and Modeling Plan


       If it is not feasible to monitor all outfalls, the  permittee  should identify a specific percentage
of the outfalls to be  monitored based on the size of the collection system, the total number of

outfalls,  the number of different receiving water bodies, and  potential and known impacts.  The
selected  locations should represent the system as a whole or represent the worst-case scenario (for
example,  where overflows  occur most frequently, have the largest  pollutant loading or flow  volume,

or discharge to sensitive areas). If a representative set of CSO locations is selected for monitoring,
the results can be more easily extrapolated to non-monitored areas in the system.


       In general, monitoring locations should be distributed to achieve optimal coverage of actual

overflows with a minimum number  of stations. The initial system characterization should have
already provided information useful in selecting and  prioritizing  monitoring locations, such as:
          Drainage Area  Flow Contribution- The relative flow contributions  from  different
          drainage areas  can be  used to prioritize flow and pollutant monitoring efforts.  There are
          several  methods for  estimating relative  flow contributions.  The  land area of each
          outfall's sub-basin provides only  an approximate  estimate of the  relative flow
          contribution because  regulator operation  and land use characteristics affect overflow
          volume. Other estimation methods, such as the rational method , account for the runoff
          characteristics of the upstream land area and produce relative peak flows of individual
          drainage areas.  Flow estimation  using Manning's  equation (see Section 5.3.1) may
          produce a better estimate of the relative flow contribution by drainage area.

          Land Use- During the initial sampling effort, the permittee should estimate the relative
          contribution of pollutant loadings from  individual drainage areas.  Maps developed
          during the initial system characterization should provide land use information that can
          be used to derive pollutant concentrations for the different land uses from localized data
          bases (based on measurements in the CSS). If local data are not available, the permittee
          may use regional land use-based National Urban Runoff  Program  (NURP)  studies,
          although NURP data reflect only storm water and must be adjusted for the presence of
          sanitary sewage flows and industrial wastewater. Pollutant concentration and drainage
          area flow data can then be used to  estimate loadings.  Since pollutant concentrations can
          vary greatly for different land uses, monitoring locations should represent subdivisions
          of the drainage area with differing land uses.
   1 The rational method is described in Schwab, et al, 1981.
                                            4-16                                January 1999

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Chapter 4                                                    Monitoring and Modeling Plan
          Location of Sensitive Areas-  Since the LTCP  should give the highest priority to
           controlling overflows to sensitive  areas, the monitoring and modeling plan should
           identify locations where CSOs to sensitive areas, and their impacts, will be monitored.

           Feasibility and Safety of Using the Location- After using the above criteria to identify
           which outfalls will provide the most useful data, the permittee should  determine whether
           the locations are  safe and accessible and identify which safety precautions  are necessary.
           If it is not feasible or practical to monitor at the point of discharge, the permittee should
           select the closest upstream or  downstream  location that is still representative of the
           overflow.
       Example  4-1  illustrates  one  approach to  selecting discharge  monitoring sites  for a

hypothetical CSS with ten outfalls. The selected outfalls-1,  4, 5, 7, and 9-  discharge flow from
more than 60 percent of the total drainage area and 70 percent of the industrial area.  Outfalls 1 and

5 are adjacent to sensitive areas. These five outfalls should provide sufficient in-depth coverage for
the city's monitoring program. Simplified flow and modeling techniques at outfalls 2, 3,6, 8, and
10 can supplement the collected monitoring data and allow estimation of total CSS flow.


       Combined Sewer  Overflows - Guidance for Screening and Ranking (U.S. EPA,  1995c)

provides  additional guidance  on prioritizing  monitoring  locations.  Although generally intended for

ranking CSSs with respect to one another, the techniques in this reference may prove useful for
ranking outfalls within a single system.


4.3.2 Monitoring Frequency

       The permittee should monitor a sufficient number of storms to accurately predict the CSS's

response to rainfall events and the characteristics of resulting CSOs.  The frequency of monitoring

should be based on site-specific considerations such as CSO  frequency and duration, which depend
on the rainfall pattern, antecedent  dry period, type of receiving water and circulation  pattern or flow,

ambient tide or stage of river or stream, and  diurnal flow to the treatment plant.
                                            4-17                                January 1999

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Chapter 4	Monitoring and Modeling Plan
           Example 4-1.  One Approach to Selecting Discharge Monitoring Sites for
                                 a Hypothetical CSS with 10 Outfalls
     A municipality has a combined sewer area with 4,800 acres and 10 outfalls discharging into a large river.
     Exhibit 4-1 shows the characteristics of the discharge points that are potentially useful in choosing which
     intercepting devices to monitor. Investigators used sewer and topographic maps to determine the size of the
     drainage areas. Aerial photographs and information from a previous study indicated land use. Sewer maps,
     spot checked in the field, verified the type of regulating structure. The sewer map and discussions with CSS
     personnel provided information about safely and ease of access.

     Outfalls 7 and 9 account for 33 percent of the total drainage area, and monitoring at outfall 7 would provide
     data on commercial and industrial land uses that may have relatively higher pollutant loadings. These sites
     pose no safety/accessibility concerns, making them desirable sampling locations.

     Outfall 5 discharges in an area that is predominantly residential  and includes one of the largest parks in the
     municipality.  This park has many recreational uses, including swimming during the warmer months. Since
     areas used for primary contact recreation are considered sensitive areas, they are given highest priority in the
     permittee's LTCP under the CSO Control Policy. This  outfall, which accounts for about 10 percent of the
     drainage area, should be monitored.

     Outfall 4,  which is  served by a pump station, accounts for 8 percent of the discharge area  and includes
     commercial areas. At this outfall, a counter  or timer on  the pump contacts  or the use of full pipe flow
     measurement  devices usually  provides  an accurate measure of flow.

     Outfall  1  discharges near the  north  edge of town, just before the  river curves at its  entrance to the
     municipality.  This outfall is located near a portion of the river that serves as a threatened species habitat and
     therefore is considered a sensitive area.  Since sensitive areas should be given the highest priority, this outfall
     will be monitored. Monitoring this outfall also accounts  for 13  percent of the total  drainage area and  a
     significant portion of the area with commercial land uses.

     In total, these five outfalls account for approximately  64 percent of the drainage  area and more  than 70
     percent of the industrial land  use.

     The remaining sites pose practical problems for monitoring. Outfall 3 is difficult to access  and poses safely
     concerns. Outfalls 2, 6,  8, and 10 all have backwater  effects,  and  access/safety  concerns  further limit
     monitoring opportunities.

          Outfall  2- Backwater effects,  difficult  access rating and safely concerns

          Outfall 3- Residential drainage area similar to Outfall 5, but difficult access rating and safely concerns

          Outfall  6-  Large residential drainage  area  but backwater  effects and  access/safety concerns limit
          monitoring  opportunities

          Outfall  8- Drainage area small, but includes  industrial and  commercial  land  uses. Backwater effects
          and access/safety concerns limit  monitoring opportunities

          Outfall  10-  Backwater and difficult  access limit monitoring opportunities.
                                                    4-18                                   January  1999

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Exhibit 4-1. Data for Example 4-1
Outfall
#
1
2
3
4
5
6
7
8
9
10
Total
Drainage
Area
(acres)
695
150
560
430
500
800
690
120
1,060
300
5,305
Land Use
Residential
80%
50%
75%
60%
90%
90%
20%
40%
80%
90%
71%
Industrial

20%

10%


60%
50%


10%
Commercial
20%
30%
5%
30%

10%
20%
10%


11%
Open/Park


20%

10%



20%
10%
8%
Flow Regulation Device
Weir
Gravity
/

/

/

/

/


Weir
Backflow





/

/



Orifice
Backwater

/







/

Pump
Station



/







Access/
Safety
Concerns

/



/

/

/

Sensitive
Area
/



/






Potential
Monitoring
Location
Yes
No
Yes
Yes
Yes
No
Yes
No
Yes
No


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Chapter 4                                                  Monitoring and Modeling Plan
       Monitoring frequency may be targeted to such factors as:
           Wet weather events that result in overflows

         A certain number  of precipitation events  (e.g.,  monitor until  five  storms  are
           sampled-each storm may  need to meet a certain minimum size)

           A certain size precipitation event (e.g., 3-month, 24-hour).
       A range of storm sizes should be sampled, if possible, to characterize the CSS response for
the variety of storm conditions that can  occur.  These data can be useful for long-term simulations.
Section 4.6 discusses a strategy for determining whether to monitor a particular wet weather event.

Overall, more frequency monitoring is  warranted where:


          CSOs discharge to sensitive or high-quality areas, such as waters with drinking water
          intakes or swimming, boating, and other recreational activities

          CSO flow volumes per inch of rainfall  vary  significantly from storm event to  storm
          event.

       The  number of samples  collected will also reflect the type  of sample collected. Where
possible, the permittee should collect flow-weighted composite samples to determine the average
pollutant concentration over a storm event (also known as the event mean concentration or EMC).
This approach  decreases the analytical cost of a program based on discrete samples. Certain
parameters,  such as oil and grease and  bacteria, however, have limited holding times and must be
collected by grab sample (see discussion in Section  5.4.1). Also, when the permittee needs to
determine whether a pattern of pollutant concentration, such as a first-flush phenomenon, occurs
during storms,  the monitoring program should collect several samples from the same  locations
throughout a storm.
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permittee  should carefully consider the tradeoffs involved in committing resources to a sampling
program.  A small number of samples may necessitate more conservative assumptions or result in
more uncertain  assumptions because of high sample variability.  A larger data set might better
determine pollutant concentrations and result in a more detailed analysis, enabling the permittee to
optimize any investment in  long-term CSO  controls. On the other hand, a permittee should avoid
spending large sums of money on monitoring when the additional data will not  significantly enhance
the permittee's understanding of CSOs, CSO impacts, and design of CSO controls.  The permittee
should  work  closely with  the NPDES permitting authority and  the  review team  to design  a
monitoring program that will adequately characterize the CSS, CSO impacts on the receiving water
body, and effectiveness of proposed CSO control alternatives.

4.3.3   Combined Sewage  and CSO Pollutant Parameters
        The monitoring and modeling  plan should state how the permittee will determine the
concentrations  of  pollutants  carried  in the  combined  sewage  and  the variability of these
concentrations during a  storm,  from  outfall  to outfall, and  from  storm to  storm.  Pollutant
concentration  data should be used with  flow  data to compute pollutant loadings to receiving  waters.
In some cases such data can also be used to  detect the sources of pollutants in the system.

        The monitoring and modeling  plan  should identify which parameters will be monitored.
These should include pollutants with water quality criteria for the specific designated use(s) of the
receiving  water.  The  NPDES permitting authority may have specific guidance  regarding parameters
for CSO monitoring.  Parameters of concern may include:

           Flow (volume and flow rate)
        .  Indicator bacteria
           Total suspended solids (TSS)
 2 Concentrations of bacteria in CSOs may be fairly consistent over time (around 106 MPN/100 ml for fecal coliform).
If  sampling yields consistent results over time, the permittee  may find that additional bacteria sampling is not
informative. Concentration data could be combined with flow data to determine bacteria loadings.

                                            4-21                               January  1999

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Chapter 4                                                     Monitoring and Modeling Plan


           Biochemical oxygen demand (BOD) and dissolved oxygen (DO)

       .   pH

       .  Settleable  solids

       .  Nutrients

           Toxic pollutants reasonably expected to be present in the  CSO based on an industrial
           survey or tributary land use, including metals typically present in storm water, such as
           zinc, lead, copper, and arsenic (U.S. EPA, 1983a).

       The monitoring and modeling plan should also include monitoring for any other pollutants

for which water quality criteria are being exceeded, as well as pollutants suspected to be present in
the combined  sewage and those discharged in significant quantities  by industrial users.  For example,
if the water quality criterion for zinc is  being  exceeded in the receiving water, zinc  should be
monitored in the portions of the CSS  where industrial users discharge zinc to the collection system.
POTW monitoring data and industrial pretreatment program data on  nondomestic discharges can
help identify  other pollutants that should be monitored.   In coastal systems,  measurements of
sodium,  chloride, total  dissolved solids, or conductivity can be used to detect the presence of sea
water in the CSS, which may be the result of intrusion through failed tide gates.


       Not all pollutants need to be  analyzed for each location sampled. For example:
           A larger list of pollutants should be analyzed for an industrial area suspected to have
           contaminated storm water or a large load of pollutants in its sanitary sewer.

           Bacteria should be analyzed in a CSO upstream of a beach or drinking water supply with
           past bacteriological problems, while it may not be necessary to analyze for metals or
           other toxics.
   3 The permittee should consider sampling both dissolved and total recoverable metals. The dissolved portion is more
immediately bioavailable, but does not account for metals that are held in solids.  Since CSOs generally contain elevated
levels of suspended solids, which can release metals over time, sampling for total metals is important for evaluating
CSOs and their impacts.


                                             4-22                                January 1999

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Chapter 4                                                   Monitoring and Modeling Plan

       The permittee should also ensure that monitored parameters correspond to the downstream
problem as well as the water quality criteria that apply in the receiving water body at the discharge
pipe.  For example, the downstream  beach may  have an Enterococcus standard while the water
quality criterion at the discharge point might be expressed in fecal coliforms. In this case, samples
should be analyzed for both parameters.

       The permittee should consider collecting composite data for certain parameters on as many
overflows as possible during the monitoring program.  This can help establish mean pollutant
concentrations  for  computing  pollutant loads.   For instance, TSS concentrations are  generally
important both because of potential habitat impacts and because they are associated with adsorbed
toxics. Collecting some discrete  TSS samples can also  be useful, particularly for evaluating the
existence  of first  flush.

       The permittee should  consider  initial  screening-level sampling  for a wide range  of pollutants
if sufficient  information  is  not  available to initially identify the parameters  of concern. The
permittee  can then analyze  subsequent  samples only for the subset of pollutants identified in the
screening. However, because pollutant  concentrations in CSO discharges are highly  variable, the
permittee  should  exercise caution in removing pollutants from the analysis list.

4.4     SEPARATE STORM SEWERS
       If separate storm sewers are significant contributors to the same receiving water as CSOs,
the permittee  should determine pollutant  loads from storm sewers as  well as CSOs. This information
is needed  to define the loadings from different wet weather sources and target CSO and storm water
controls appropriately.  If sufficient storm water  data are not available, the permittee  may need to
sample separate storm sewers and the monitoring and modeling plan should include storm water
sampling for  the pollutants being sampled in the CSS. Storm  water discharges from  areas suspected
of having high loadings,  such  as high-density commercial areas or industrial parks,  should have
priority.   Storm water discharges from highways can be  another major source of  pollutants,
   4 The potential significance of storm water discharges can often be assessed by looking at land uses and the relative
sizes of discharges.

                                            4-23                                January 1999

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Chapter 4                                                   Monitoring and Modeling Plan

particularly solids, oil and grease, and trace metals. For guidance on characterizing and monitoring
urban runoff, permittees can refer to EPA's NPDES Storm Water Sampling Guidance Document
(U.S. EPA, 1992) and the Guide for Collection, Analysis, and Use of Urban Storm  Water Data
(Alley,  1977).

       The monitoring and modeling plan should reflect storm water and other sampling programs
occurring concurrently and provide for coordination with them. This will ensure that  wet weather
discharges and their impacts are monitored and addressed in a cost-effective, targeted manner.  Many
communities operate their storm water programs  under  a different  department or authority  from their
sewer program.  Whenever possible, similar activities within these different organizations should
be coordinated on a watershed basis.

4.5    RECEIVING WATER MONITORING
       The goals of receiving water monitoring should include the following:

           Assess attainment of WQS (including designated uses)
           Define the baseline conditions in the receiving  water (chemical, biological,  and physical
           parameters)
           Assess the relative impacts of CSOs
           Gain sufficient understanding of the receiving water to support evaluation  of proposed
           CSO control alternatives, including  any receiving  water modeling that may be needed
           Support the review and revision, as appropriate, of WQS.

       The monitoring program should also provide background data  on conditions in the receiving
waters during dry weather conditions, if this information is not already available  (see Chapter 3).
Dry weather monitoring of the receiving water body helps define the background water quality and
will determine whether water quality criteria are being met or exceeded during dry weather.

       Where a  permittee intends to eliminate CSOs entirely (i.e.,  separate its system),  only limited
or short-term receiving water monitoring may be necessary (depending on how long elimination of

                                           4-24                                January 1999

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Chapter 4                                                   Monitoring and Modeling Plan


CSOs will take). It may be useful, however, to collect samples before separation to establish the
baseline as well as after separation to evaluate the impacts of CSO elimination.


       The permittee should coordinate monitoring activities closely with the NPDES permitting

authority.  In many  cases,  it may be appropriate to use a phased approach in which the receiving
water monitoring program focuses initially on determining the pollutant loads  from CSOs and

identifying  short-term water  quality  impacts. The  information obtained from the first phase  can then
be used to identify additional data and analytical needs in an efficient manner. Monitoring efforts

can be expanded as  circumstances dictate to provide additional  levels of detail, including evaluation

of downstream effects and longer term effects.


       The scope of the receiving water monitoring program will depend on several factors, such

as the identity of the pollutants of concern, whether the receiving water will be modeled, and the

relative size of the  CSO. For example:
           To  study  dissolved oxygen (DO) dynamics,  depth and flow velocity data must be
           collected well downstream of the CSO outfalls. DO modeling may require data on the
           plant and algae community,  the temperature, the sediment oxygen demand, and the
           shading of the river. Therefore, DO monitoring locations would likely  span  a larger  area
           than for some other pollutants of concern.
           When the volume of the overflow is small relative to the receiving water body, as in the
           case of a small CSO into a large, well mixed river, the overflow may have little impact.
           Such a situation generally would  not require extensive downstream sampling.
       In developing the monitoring and modeling plan, the permittee should consider the location

and impacts of other sources of pollutant loadings. As mentioned in Chapter 3,  information  on these
sources is generally compiled and reviewed during the initial system characterization. To  evaluate

the impacts of CSOs on the receiving water body, the permittee should try to select monitoring
locations that  have limited or known  effects from these  other sources, If the initial system
   5 In areas where the receiving water is used for swimming, the dilution needs to be at least 10,000 to 1 for bacteria.
                                            4-25                                January 1999

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Chapter 4                                                   Monitoring and Modeling Plan


characterization  did not provide sufficient information to adequately  determine the location of these
sources, the permittee may need to conduct some monitoring to better characterize them.


4.5.1 Monitoring Locations

       In planning where to sample, it is important to understand land uses in the drainage basin

(which affect what pollutants are likely to be present) and characteristics of the receiving water body

such as:


           Pollutants of concern (e.g., bacteria, dissolved oxygen, metals)

           Locations of sensitive areas

           Size of the water body
           Horizontal and vertical variability in the water body
           Degree of resolution necessary to assess attainment of WQS.


       Individual monitoring stations may be located to characterize:


       .  Flow patterns

           Pollutant concentrations and loadings from individual sources

           Concentrations  and  impacts at  specific locations, including sensitive areas such as
           shellfishing zones and recreational areas

           Differences  in concentrations between upstream and downstream sampling sites for
           rivers, or between inflows and outflows for lakes, reservoirs, or estuaries

           Changing conditions at individual  sampling stations before, during,  and after storm
           events

           Differences between baseline and current conditions in receiving water bodies

           Locations of point and nonpoint pollution sources.
                                            4-26                                January 1999

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Chapter 4                                                    Monitoring and Modeling Plan







       In selecting monitoring locations, the permittee needs to consider physical logistics (e.g.,



whether the water is navigable, if bridges are available from which to sample) and crew safety.







       Exhibit 4-2 illustrates how sampling locations might be distributed in a watershed to assess



the effect of other sources of pollution.  If monitoring  is conducted at the potential sampling



locations (labeled 1-6 in Exhibit 4-2), the results from the different locations could be compared to



provide a relative measure of the pollutant contributions from each source.







       The permittee should also consider making  cooperative sampling  arrangements  when



pollutants from multiple sources enter a receiving water or when several agencies share the cost of



the collection system and the POTW. The identification of new monitoring locations should account



for sites that may already be part of an  existing monitoring  system used by local or State government



agencies or research organizations.







4.5.2  Monitoring Frequency, Duration, and Timing




       In general, the monitoring and modeling plan should target receiving water monitoring to



those seasons, flow regimes, and other critical conditions  where CSOs have the greatest potential



for impacts, as identified in an initial system characterization (see Chapter  3). It should specify



additional monitoring as necessary to fill data gaps  and to support receiving water modeling and



analysis  (see Tables  B-2 through B-5 in Appendix B for potential modeling parameters), or to



determine the relative contribution of other sources to  water quality impairment.







       In establishing the frequency, duration, and timing of receiving water monitoring in the



monitoring  and modeling plan, the permittee  should consider  seasonal variations  to  determine



whether  measurable and significant changes occur  in the receiving water body and uses during
                                            4-27                                January 1999

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Chapter 4
Monitoring and Modeling Plan
                Exhibit 4-2. Receiving Water Monitoring Location Example
                                      Industrial
                                     Discharge"   *~~
        Sampling Location Key
        Upstream of Study Area
        Downstream of industrial Point Sources
        Upstream of Tributary (at bridge)
        Mouth of Tributary
        Downstream of CSO
        Downstream End of Study Area
                                            4-28
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Chapter 4                                                   Monitoring and Modeling Plan


different times  of year.   The monitoring and modeling plan should also enable the permittee to

address issues regarding attainment of WQS, such as:
          Assessing  attainment of WQS for recreation:   This may require determination of a
          maximum or geometric mean coliform concentration at the point of discharge into a river
          or mixing  zone boundary. This requires grab  samples during and immediately after
          discharge events in sufficient number (possibly specified in the WQS) to reasonably
          approximate actual in-stream conditions.

          Assessing  attainment of WQS for nutrients:   This may  call for samples  collected
          throughout the water body and timed to examine long-term average conditions over the
          growing season.

          Assessing  attainment of WQS for aquatic life support: This may call for biological
          assessment in potentially affected locations and a comparison of the data to reference
          sites.
       Receiving water sampling designs include the following:
          Point-in-time single-event samples to obtain estimates where variation in time is not a
          large concern.

          Short-term intensive sampling for a predetermined period  of time in order to detail
          patterns of change during particular events, such as CSOs.  Sample collections for  such
          studies may occur at intervals such as five minutes, one hour, or daily.

         Long-term less-intensive samples collected at regular intervals-such  as weekly,
          monthly,  quarterly, or annually-to establish  ambient or background conditions or to
          assess seasonal patterns or general trends  occurring over years.

          Reference site samples collected at separate locations for comparison with the  CSO
          study site to determine relative changes between the locations.

          Near-field studies to sample and assess receiving waters within the immediate  mixing
          zone of CSOs. These studies can examine possible short-term toxicity impacts or long-
          term habitat alterations near the CSO.

          Far-field studies to sample and assess receiving waters outside the immediate vicinity
          of the CSO.  These  studies  typically  examine  delayed impacts, including  oxygen
          demand,  nutrient-induced  eutrophication,  and changes in macroinvertebrate  assemblages.
                                           4-29                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan

Section 4.6 discusses a strategy for determining whether to initiate monitoring for a particular wet
weather event.

4.5.3 Pollutant  Parameters

       The monitoring and modeling plan should identify parameters of concern in the receiving
water, including pollutants with water quality criteria for the designated use(s)  of the receiving
water. The NPDES authority may have specific requirements or guidance regarding parameters for
CSO-related  receiving water  monitoring.  These parameters  may include the  ones previously
identified for combined sewage (see Section 4.3.3):

       .  Indicator  bacteria
       .  TSS
       .  BOD and DO
       .  pH
       .  Settleable  solids
       .  Nutrients
          Metals (dissolved and total recoverable) and other toxics.

In addition, the permittee should  consider the  following types  of monitoring  prior to or concurrently
with the other analyses:

       .  Flow  monitoring
          Biological assessment (including  habitat assessment)
           Sediment monitoring (including metals and other toxics)
          Monitoring other pollutants known or expected to be present.

       Monitoring should  focus on  the parameters  of concern. In many cases, the  principal concern
will be pathogens, represented by fecal coliform.
                                            4-30                                January 1999

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Chapter 4                                                    Monitoring and Modeling Plan


       Depending on the complexity of the receiving water and the analyses to be performed, the

monitoring and modeling  plan may need to reflect a larger list of parameters.   Measuring

temperature, flow, depth, and velocity, and more complex parameters such as solar radiation, light

extinction, and sediment oxygen demand, can enable investigators to simulate the dynamics of the

receiving water that  affect basic  parameters  such as bacteria, BOD,  and TSS.  Table B-l in

Appendix B  lists the data needed to perform the calculations for  several dissolved oxygen, ammonia,

and algal studies. Indirect indicators, such as beach closings, fish advisories, stream bank erosion,

and the appearance of floatables, may also  provide a relative measure of the impacts of CSOs.


4.6    CRITERIA FOR INITIATING MONITORING OF WET WEATHER EVENTS

       The monitoring program should include enough storm events to enable the permittee to

predict the CSS's response to rainfall events, the characteristics of resulting CSOs, and the extent

of impacts on receiving waters (as discussed in Sections 4.2.1,  4.3.2, and 4.5.2). By developing a

strategy for determining which storm events are most appropriate for wet weather monitoring, the

permittee can collect the needed data while limiting the number  of times the sampling crew is

mobilized and the number of sampling events. This can result in significant savings in personnel,

equipment, and laboratory costs.


       The following list (ORSANCO, 1998) contains key elements to consider in determining

whether to initiate monitoring for a wet weather event:


          Identifying local site conditions

               Establish the amount and intensity of precipitation needed to initiate CSOs
               Characterize seasonal stream conditions (flow, stage, and velocity)
               Characterize historical climatic patterns

           Setting criteria for monitoring activities

               Establish minimum  amount  of precipitation and duration to  trigger event monitoring
               Focus on frontal storms instead of thunderstorms
    For example, a Streeter-Phelps DO analysis requires temperature, flow rate, reach length, and sediment oxygen
demand.
                                            4-31                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan
              Identify time periods contained within the monitoring  schedule that may  not be
              representative of the system (holiday weekends) and avoid monitoring during those
              periods

          Identify local rain gage networks

          - Airports
              Municipalities

          Identify monitoring contact personnel

              Laboratory  managers
          - Consultant crew leaders
              Municipality crew leaders

          Identify weather sources

              Local meteorologist
              National Weather Service
              -  Contact at regional forecast office
              --  NOAA weather radio broadcast
          - Cable  TV broadcasts
              -  Local radar
              -  Weather Channel
          - Internet sites
              -  Local television network sites
              -  National weather information sites

       .  Storm tracking

              The monitoring leader tracks weather conditions and stream conditions
              The monitoring leader notifies all monitoring contact personnel of potential events
              when:
              -  Stream conditions are acceptable
              "  Monitoring criteria may be met
              The monitoring leader initiates monitoring following the flowchart.

       The  flowchart  in Exhibit  4-3  provides an example of how  to  apply these  elements

(ORSANCO, 1998).
                                           4-32                               January 1999

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Chapter 4
Monitoring and Modeling Plan
     Exhibit 4-3. Decision Flowchart for Initiating a Wet Weather Monitoring Event
Establish
Wet Weather Monitoring Cr teria
(Site Specific Based on CSO)*

Monitoring Leader
Monitors Stream Conditions

Safe for Deployment
of Crews?



1
Continue to Monitor Antecedant D
Stream Conditions Criteria t

Ml 1 .,_ 	


Continue to Monitor
Stream Conditions


1

ry Per od
/et?






Start to Monitor
Weather Patterns

| Internet Sources (— -| Cable TV Broadcasts |

| No

Return to M
Stream Co

1 \ Alert/Contact
j Won toring Crews & Laboratories
i 1
i 1 ., 	 .
Field Crew Leaders Laboratory Managers |
'> 1
I I \ \ I
Stream Source Laboratory Laboratc
Crew Members Crew Members Runners Analys


Sampling Efforts Can Isolate
an Individual Storm Front?




Dnitoring
iditions

Yes

Track Storm Front

Alert Contact
Monitoring Crews — Local Meterologists
and Laboratories National Weather Service

No
1

Rainfall and Duration
Criteria Met?
(Check Rain Gages)
1
MH

Suspend Alert Trigger Monitoring
1
i
Contact Contact
Monitoring Crews & Laboratories Monitoring Crews & Laboratories
i
i
Return to 1 nitiate
Monitoring Stream Conditions | Monitoring Program

>ry
s

* Since the amount and intensity of precipitation needed to initiate an overflow and the physical conditions may vary significantly from CSO to CSO,
the permittee may need to establish different monitoring criteria for different CSOs.
                                          4-33
                 January 1999

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Chapter 4                                                   Monitoring and Modeling Plan







4.7    CASE STUDY





       The case study in Example 4-2 outlines the monitoring aspects of a comprehensive effort to



determine CSO  impacts on a river and evaluate possible control  alternatives. The city of South



Bend, Indiana developed and implemented a monitoring program to characterize flows and pollutant



loads in the CSOs and receiving water. The city then used a model to evaluate possible control



alternatives.







       In developing its monitoring plan, South Bend carefully selected monitoring locations that



included roughly  74 percent of the area within the  CSS and represented the  most  characteristic land



uses.   The city  conducted its complete monitoring program at 6 of the 42 CSO outfalls and



performed simpler chalking measurements at the remaining outfalls to give some basic information



on the occurrence of CSOs across the system. By using existing flow monitoring stations in the



CSS, the city was able to limit the need to establish new monitoring stations.







4.8    DATA MANAGEMENT AND ANALYSIS




4.8.1  Quality Assurance Programs





       Since inaccurate or unreliable data may lead to faulty decisions in evaluating, selecting, and



implementing CSO controls, the monitoring and modeling plan must provide for quality assurance



and quality control to ensure that the  data collected have the required  precision and accuracy.



Quality assurance and quality control (QA/QC) procedures are necessary both in the field (during



sampling) and in the laboratory to ensure that data collected in environmental monitoring programs



are of known quality, useful, and reliable.  The implementation of a vigorous QA/QC program can



also reduce monitoring expenses. For example, a QA/QC program for flow monitoring  may help



prevent the need for resampling due to meter fouling or loss of calibration.
                                           4-34                               January 1999

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Chapter 4                                                             Monitoring and Modeling Plan
                                Example 4-2. Monitoring  Case Study
                                            South Bend, Indiana

     The City of South Bend, population of 109,000, has 42 combined sewer service areas covering over 14,000
     acres.

     Monitoring  Goals

     The ultimate goal of the CSO control effort was to reduce or eliminate  impacts on uses of the receiving
     water, the St.  Joseph River.  The more immediate goal consisted of  quantifying CSO impacts to the St.
     Joseph River and  evaluating alternatives for cost-effective CSO control.  To  achieve these  goals, the City
     reviewed its existing data to determine what additionaldata were needed to characterize  CSO impacts.  The
     City  then developed  and implemented a sampling and flow monitoring plan to fill  in these data gaps.
     Objectives of  the monitoring plan included quantifying  overflow volumes and  pollutant  loads  in the
     overflows and  flows  and pollutant loads in the receiving water.  After  evaluating various analytical and
     modeling tools, the City decided to use the SWMM model  to assist in  predicting the benefits of alternative
     control strategies  and  defining problems caused by CSOs.

     Monitoring Plan Design and Implementation

     The monitoring plan was designed to focus on the 6 largest drainage areas, which were most characteristic
     of land uses within the CSS  area and included 74 percent of that area,  Monitoring  all 42  outfalls was
    judged  to be unnecessarily  costly. The  monitoring  plan  specified 8 temporary and  9  permanent  flow
     monitoring locations along the main interceptor and in the  influent  and  outfall  structures of the 6  largest
     CSOs.  The interior surface  of each  non-monitored CSO  diversion structure was chalked  to determine
     which storms caused overflows; after each storm, the depth to which the chalk disappeared was recorded.
     Although the plan included  monitoring only 14 percent of the outfalls, it measured  flow and water quality
     for most of the CSS area and covered a representative range of land uses and basins.  Flow monitoring data
     were used to calibrate the  SWMM model.

     The monitoring plan described water  quality  sampling procedures  for both dry  weather and  wet weather
     periods.  The plan specified sample collection from four CSO structures during at  least five storm events
     representing  a range of storm sizes.  For the CSOs,  monitored water  quality  parameters included nine
     metals, total suspended solids (TSS), BOD, CBOD (carbonaceous biochemical oxygen  demand),  total
     Kjeldahl nitrogen  (TKN), ammonia, total phosphorus, total and fecal  coliform bacteria, conductivity, and
     hardness.  Periodic dry-weather grab sample collections at the  interceptors  were also  planned.

     During storm events, water quality samples were collected using 24-bottle automatic samplers at the four
     CSO points.  To  quantify "fist-flush" concentrations,  the automatic samplers began collecting  samples at
     the start of an overflow event and continued collecting samples every  five minutes for the first two hours
     of the monitored  events. A  two-person  crew drove  between sites during each monitored event to check
     equipment operation and the  adequacy of sample collection.

     River samples were taken from eight bridges along the St. Joseph River during and after three storms.   Six
     bridges are located within South Bend, and two are located just downstream in  Michigan.  River samples
     were analyzed to  determine the impacts of CSOs on the St. Joseph River and to calibrate and verify the
     river model for dissolved oxygen, E.  coli, and  fecal coliform.
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Chapter 4	Monitoring and Modeling Plan


                       Example 4-2. Monitoring; Case Study (Continued)
    River samples were collected concurrently from the eight bridges every four hours. Four people sampled the
    eight bridges. One person collected samples from two adjacent bridges within 30 minutes, Samples were
    collected at the center of each bridge at the same location where the City collects its monthly river samples.
    At least two sets of samples were collected before the storm to establish the baseline condition and the river
    was sampled for at least 48 hours after onset of the storm to allow the river to return to its baseline condition.

    Hourly rainfall data were collected from a network of five  rain gages located in the drainage basins.

    Results of the Sampling and Flow Monitoring Program

    Results from the sampling and monitoring program for three storms during summer and early fall of 1991
    indicated little or no impact on dissolved oxygen in the St. Joseph River,  Large pulses in river bacteria counts
    (E. coli and fecal coliform) were observed during the storms.  Bacteria counts returned to baseline values
    within 48 hours after the onset of each storm,  Wet weather CSO sampling results showed a "first flush"
    effect in three of the four sampled CSO structures. The fourth structure did not exhibit a "first flush" effect,
    probably because of a high biochemical oxygen demand (BOD) loading at the upstream end of the trunk
    sewer to the structure.  Wet weather CSO sampling results also showed that the soluble metal concentrations
    were much lower than the particulate  metal concentrations.

    The objective of the CSO control program is to solve real  pollution problems and improve the river water
    quality for specific uses.  Based on the results of the monitoring program, bacteria reduction  in the river
    during wet weather has been the primary  focus; A cost-performance curve was developed, using bacteria
    reduction as the performance measure; to select the most cost-effective alternative and level of CSO control.

    For an additional case study on CSO and receiving water monitoring, see Chapter 2 of Combined Sewer
    Overflows -  Guidance for Long-Term Control Plan (EPA, 1995a).
        Quality assurance refers to programmatic efforts to ensure the quality of monitoring and
measurement  data.  QA  programs increase  confidence in the validity  of the reported analytical data.
Quality control, which  is a  subset of quality  assurance, refers to the application of procedures
designed  to obtain  prescribed  standards  of performance  in monitoring  and  measurement. For
QC.
        QA/QC procedures  can be divided  into  two  categories:
procedures. Both types of QA/QC are described in the following subsections.
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Chapter 4                                                   Monitoring and Modeling Plan


       Field QA/QC. QA programs for sampling equipment and for field measurement procedures

(for such parameters as temperature, dissolved oxygen, and pH) are necessary to ensure data are of

the appropriate quality. A field QA program should contain the following documented elements:
           The sampling  and analytical method;  special sample  handling  procedures; and  the
           precision, accuracy, and detection limits of all analytical methods used.

           The basis for selection of sampling and analytical methods.  Where methods do not exist,
           the  QA plan  should state how the new  method  will be documented, justified, and
           approved for use.

           Sample tracking procedures (labeling, transport, and chain of custody).

           Procedures for calibration and maintenance of field instruments and automatic samplers
           during both dry and wet weather flows.

          The  organization  structure,  including assignment of decision-making and other
           responsibilities for field operations.

           Training of all personnel involved in any function affecting data quality.

           A performance evaluation system assessing the performance of field sampling personnel
           in the following areas:

               Qualifications of field personnel for a particular sampling situation

               Determination of the best representative sampling site

               Sampling technique including monitoring locations, the choice  of grab or  composite
               sampling,  the  type of automatic sampler, special handling procedures,  sample
               preservation, and sample identification and tracking procedures

           - Flow measurement

               Completeness of data, data recording, processing, and reporting

               Calibration and maintenance of field instruments and equipment

               The use of QC samples such as duplicate, split, or spiked samples and blanks as
               appropriate to assess the validity of data.
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Chapter 4                                                   Monitoring and Modeling Plan
           Procedures for recording, processing,  and reporting data; procedures for use of non-
           detects/results-below-detection  in  averaging  or  other statistical  summaries  (e.g.,
           substituting one-half the detection level for results of non-detect at the lowest standard
           used); procedures for review of data and invalidation of data based upon QC results.

           The amount of analyses for QC, expressed as a percentage of overall analyses, to assess
           the validity of data.
       Sampling QC includes calibration and preventative maintenance procedures for sampling

equipment, training of sampling personnel, and collection and  analysis of QC samples.  QC samples

are used to determine the performance of sample collection techniques and the homogeneity of the

water and should be collected when the other sampling is performed. The following sample types

should be part of field QC:
          Duplicate Samples  (Field) - Duplicate field samples collected at selected locations
          provide a check for precision in sampling equipment and techniques.

          Equipment Blank - An aliquot of distilled water which is taken to and opened in the
          field, its contents poured over or through the sample collection device, collected in a
          sample container, and returned to the laboratory for analysis to check sampling device
          cleanliness.

           Trip Blank - An aliquout of deionized/distilled water or solvent that is brought to the
          field in  a sealed container and transported back to the laboratory with the sample
          containers for analysis in order to check for contamination from transport, shipping, or
          site conditions.

         Preservation Blank - Adding  a known amount of  preservative  to an aliquot of
          deionized/distilled  water  and  analyzing the  substance  to  determine  whether  the
          preservative is contaminated.
       The permittee should also consider analyzing a sample of blank water to ensure that the water

is free of contaminants.


       Laboratory  QA/QC.  Laboratory  QA/QC procedures ensure analyses of known  and
documented quality through  instrument calibration and the processing of samples. Precision of
laboratory findings refers to the reproducibility of results. In a laboratory QC program, a sample is


                                           4-38                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan


independently analyzed more than once, using the same  methods and  set of  conditions. The
precision is estimated by the variability between repeated measurements.  Accuracy refers to the
degree  of difference between  observed values and known or true values.  The accuracy of a method

may be determined by analyzing samples to which known amounts of reference standards have been

added.


       The following techniques are useful in determining confidence in the validity of analytical

data:
          Duplicate Samples (Laboratory) - Samples received by the laboratory and divided into
          two or more portions at the laboratory, with each portion then separately and identically
          prepared  and  analyzed.   These  samples  assess precision and  evaluate  sampling
          techniques and equipment.

          Split Samples (Field) - Single samples split in the field and analyzed separately check
          for variation in laboratory method or between  laboratories.  Samples can be split and
          submitted to a single laboratory or to several laboratories.

          Spiked Samples (Laboratory) -  Introducing a known quantity of a  substance into
          separate aliquots of the sample or into a volume  of distilled water and analyzing for that
          substance provides a check of the accuracy of laboratory and analytic procedures.

          Reagent Blanks - Preserving and analyzing a quantity of laboratory blank water in the
          same manner as environmental water samples  can indicate contamination caused by
          sampling and laboratory procedures.
QA/QC programs  are discussed in greater detail in EPA Requirements for Quality Assurance Project
Plans for Environmental Data Operations (U.S. EPA, 1994d) and Industrial User Inspection And

Sampling Manual For POTWs (U.S. EPA 1994c).


4.8.2 Data Management

       Although a permittee may collect accurate and representative data through its monitoring
efforts and verify the reliability of the data through QA/QC procedures, these data are of limited

usefulness if they are not stored in an organized  manner and analyzed properly. The  permittee



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 Chapter 4                                                   Monitoring and Modeling Plan







 should develop  a  data management program to provide ready access to data, prevent data loss,



prevent introduction of data errors, and facilitate data review and analysis. Even if a permittee



intends to use a "complex" model to evaluate the impacts of CSOs and proposed CSO control



alternatives, the model still  requires  appropriate data for input parameters, as a basis for assumptions



made in  the modeling process, and  for model calibration and verification.  Thus, the permittee needs



to properly manage monitoring data and perform some review and analysis of the data regardless of



the analytical tools selected.







       All monitoring data should  be organized and stored in a form that allows for ready access.



Effective data management is necessary because the voluminous and diverse nature of the data, and



the variety of individuals who can be involved in collecting, recording and entering data, can easily



lead to data loss or error and severely damage the  quality of monitoring programs.







       Data  management  systems must  address both  managerial and technical  issues.  The



managerial issues  include data storage, data validation and verification, and data access. First, the



permittee should  determine  if a  computerized data management system will be used.  The permittee



should consider factors such as the volume of monitoring data  (number of sampling stations, samples



taken at  each station, and  pollutant parameters), complexity  of data analysis, resources available



(personnel,  computer equipment, and software), and whether modeling will  be  performed. To  enable



efficient and accurate data analysis, a computerized system  may  be necessary for effective data



management  in all but the  smallest  watersheds. Computerized data management systems may also



facilitate modeling if the data can be uploaded directly into the model rather than being reentered.



Thus, when modeling will be performed, the permittee  should  consider compatibility with the  model



when selecting  any computerized  data  management system.   Technical issues  related to data



management systems involve the selection  of appropriate computer  equipment  and  software and  the



design of the data system,  including data definition,  data standardization, and a data dictionary.







       Data quality must be rigidly controlled from the point of collection to the point of entry into



the data management system. Field and laboratory personnel  must carefully enter data  into proper



spaces on data sheets and avoid transposing numbers. To avoid transcription errors when using a






                                            4-40                               January 1999

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Chapter 4                                                    Monitoring and Modeling Plan







computerized data management system, entries into a preliminary data base  should be made from



original  data  sheets  or photocopies.   As a preliminary  screen for data  quality,  the data



base/spreadsheet design should include automatic range-checking of all parameters, where values



outside defined ranges are flagged and either immediately corrected or included in a follow-up



review.  For some parameters, it  might be appropriate to include  automatic checks to disallow



duplicate values. Preliminary data base/spreadsheet files should be printed and verified against the



original data to identify errors.







       Additional data validation can include expert review of the verified data to identify possible



suspicious  values. In some cases,  consultation with the individuals responsible for collecting or



entering original data may be necessary to resolve problems.  After all data are  verified and



validated, they can be merged into the monitoring program's master data files.  For computerized



systems, to prevent loss of data from computer failure at least one set  of duplicate  (backup) data files



should be maintained.







       Data analysis is discussed in Chapters  5  (CSS  Monitoring)  and  6  (Receiving  Water



Monitoring).  The use of models  for more  complex data  analysis and simulation is discussed in



Chapters 7 (CSS Modeling) and 8  (Receiving Water Modeling).







4.9    IMPLEMENTATION OF MONITORING  AND MODELING PLAN





       During development of the monitoring and modeling plan, the permittee needs to consider



implementation issues such as recordkeeping and reporting requirements, personnel responsible for



carrying out each  element  of the plan, scheduling, and resources. Although  some implementation



issues  cannot be fully addressed in  the monitoring  and modeling plan until other  plan elements have



evolved, they should be considered on a preliminary basis  in order to ensure  that the resulting plan



will satisfy reporting requirements and be feasible with available resources.
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Chapter 4                                                  Monitoring and Modeling Plan







4.9.1  Recordkeeping  and  Reporting




       The monitoring  and modeling  plan includes a recordkeeping and reporting  plan, since future



permits will contain recordkeeping and reporting requirements such as progress  reports on NMC and



LTCP implementation  and submittal of monitoring and modeling results. The recordkeeping and



reporting  plan  addresses the post-compliance monitoring program  the permittee will develop as part



of the LTCP.







4.9.2   Personnel  Responsible for Implementation





       The monitoring and modeling plan identities the personnel that will implement the plan. In



some cases, particularly in a city with a small CSS, the appropriately trained personnel available for



performing the tasks specified in  the monitoring and modeling  plan may be very limited. By



reviewing personnel and  assigning tasks,  the permittee  will be prepared to develop an



implementation schedule that  will be attainable and will be  able to identify resource limitations and



needs (including training) early in the process.







4.9.3 Scheduling





       The monitoring and modeling plan has a tentative implementation schedule to ensure that



elements of the plan are implemented continuously and efficiently. The schedule can be revised as



necessary to reflect the review team's assessment of the plan and the evaluation of monitoring and



modeling results. The schedule should address:







          Reporting and compliance dates included in the NPDES permit



       .  Monitoring frequencies



           Seasonal sampling schedules and dependency on rainfall patterns



          Implementation schedule  for the NMC



           Coordination  with other ongoing sampling programs



          Availability of resources (equipment and personnel).
                                           4-42                               January 1999

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Chapter 4                                                   Monitoring and Modeling Plan







4.9.4 Resources




       The monitoring  and modeling  plan identifies equipment, personnel, and other resource needs.



If modeling will be conducted, resource needs include a copy of the model and the equipment and



technical expertise to  use the model.   The plan may need  to be  modified after assessing  the



availability  of these resources. For  example, if the monitoring and modeling plan identifies  complex



modeling strategies, resource limitations  may  require  the permittee to  consider  modeling techniques



that  have more moderate data requirements.  Alternatively,  if the permittee does  not have  the



resources to purchase the hardware or software needed to run a detailed model, the permittee may



be able to make arrangements to use the equipment at another facility (e.g., another municipality



developing a CSO control program) or at a State or Federal agency. However, if such arrangements



are not possible, the permittee may need to choose  a less detailed model which could lead to reduced



monitoring costs.







       Through a review of resources, the permittee may identify monitoring equipment needed to



implement the  monitoring and modeling plan. By obtaining needed equipment such as automatic



samplers, flow  measuring equipment,  rain  gages,  and safety equipment before the date when



monitoring is scheduled to begin, the permittee can prevent some potential delays.
                                            4-43                                January 1999

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                                     CHAPTER 5

                                 CSS MONITORING


       This chapter describes how to monitor rainfall, combined sewer system (CSS) flow, and

CSS water quality, and describes procedures for organizing and analyzing the data collected. It
discusses  a range of monitoring and  analysis options and  provides  criteria for identifying

appropriate options.


5.1     THE CSO CONTROL POLICY AND CSS MONITORING

       The CSO Control Policy identifies several possible objectives of a CSS monitoring program,
including:


           To gain a thorough understanding of the sewer system

           To adequately characterize the system's  response to wet weather events, such as the
           volume, frequency, and duration of CSOs and the concentration and mass of pollutants
           discharged

           To support a mathematical model to characterize the CSS

           To support development of the long-term control plan (LTCP)

           To evaluate the expected effectiveness of a range of CSO control options.


       CSS monitoring also directly supports implementation of  the following elements of the nine
minimum controls (NMC):


           Maximum use of the collection system for storage

           Maximization of flow to the POTW for treatment

           Control of solids and floatable materials in CSOs

           Monitoring to effectively characterize CSO impacts  and the efficacy of CSO controls.
                                           5-1                                January 1999

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Chapter 5                                                                  CSS Monitoring







       CSS monitoring  will also  support the in-depth system characterization  and post-construction



compliance monitoring  that are central elements in the LTCP.







       This chapter outlines the steps that are critical to collection and analysis of rainfall, flow,



and water quality data in accordance with the CSO Control Policy.







5.2    RAINFALL DATA FOR CSS  CHARACTERIZATION




       Rainfall data are a vital part of a CSS monitoring program. This information is necessary



to analyze the CSS, calibrate and  validate CSO models,  and develop  design  conditions  for



predicting current  and future CSOs. Rainfall  data should include long-term rainfall records and data



gathered at specific sites throughout the CSS.







       This section describes how to install and use rainfall monitoring equipment and how to



analyze the data gathered.







5.2.1 Rainfall Monitoring





       The permittee's  rainfall data will probably include both national and local data. National



rainfall  data are available from a number of Federal  and local  sources, including the National



Weather Service,  the National  Climatic Data  Center (NCDC), airports, and universities (see



Chapter 3). Because rainfall conditions vary over short distances, the permittee will probably need



to supplement  national data  with data from  local rainfall monitoring stations. Wastewater treatment



plants may already collect and maintain local rainfall data. If sufficient local rainfall data are not



available, the permittee  may need to install rain gages. Where possible, the permittee should place



gages in every monitored CSO basin  because of the  high spatial variability of rainfall.







       Equipment



       Two types of gages  are used to measure the amount and intensity of rainfall. A standard



rain  gage collects  the rainfall  directly in a marked container and the  amount of rain is measured
                                             5-2                                 January 1999

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Chapter 5                                                                  CSS Monitoring


visually. Although inexpensive, standard gages do not provide a way to record changes in storm
intensity unless frequent observations are made during the storm.


       Because wet weather flows vary with rainfall intensity, CSS monitoring programs typically

use recording gages, which provide a permanent record  of the rainfall  amount over time. The  three

most common types of recording  gages are:
           Tipping Bucket Gage - Water caught in a collector is funneled into a two-compartment
           bucket. Once a known quantity of rain is collected, it is emptied into a reservoir, and
           the event is recorded electronically.

           Weighing Type  Gage - Water is weighed when it falls into a bucket placed on the
           platform of a spring or lever balance.  The weight of the  contents is recorded on a chart,
           showing the accumulation of precipitation.

           Float Recording Gage - Rainfall is measured by the rise of a float that is placed in the
           collector.
       It is possible to save money by using a combination of standard and recording gages.

Placing recording gages  strategically amid standard gages makes  it possible to compare spatial

variations in total rainfall at each recording gage with the surrounding standard gages.


       Equipment Installation and Operation

       Rain gages are fairly easy to operate and provide accurate data when installed and used
properly.  Some installation recommendations are as follows:
           Gages should be located in open spaces away from the immediate shielding effects of
           trees or buildings.

           Gages should  be installed at ground level (if vandalism is not a problem)  or on a
           rooftop.

           Police, fire, public works, or other public buildings are desirable installation sites.
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Chapter 5                                                                  CSS Monitoring


5.2.2 Rainfall Data Analysis

       The permittee should synchronize rainfall monitoring with CSS flow monitoring, so that

rainfall characteristics can be related to the amount of runoff and CSO volume and a CSS model

can be calibrated and validated. In addition, long-term rainfall data gathered from existing gages

are necessary to develop appropriate design conditions for determining existing and future CSO
impacts on receiving water bodies.  Because precipitation  can vary considerably within short
distances, it is usually  necessary to  use data from several  rain gages to estimate the average
precipitation for an area.


       Development of Design Conditions

       Using rainfall data for  planning purposes involves development of a "design storm." A
design storm is a precipitation  event with a specific characteristic that can be used to estimate a

volume of runoff or discharge of specific recurrence interval.  Design conditions can be estimated
if historic rainfall data (such as data from NOAA's National Climatic Data Center) exist that:


           Extend over a sufficient period of time (30 or more  years is preferable; 10 is usually
           acceptable);  and

           Were collected close enough to the CSS's service area to reflect conditions within that
           area.

Common  methods for characterizing rainfall include total volumes, event statistics,  return

period/volume curves, and intensity-duration-frequency curves.  These are described below.


       Total Volumes.  The National  Weather  Service publishes annual, monthly, and daily rainfall
totals, as well as averages and deviations from the average, for each rain gage in its network. The

time period for detailed simulation modeling can be selected by:
           Identifying wet- and dry-year rainfalls by comparing a particular year's rainfall to the
           long-term average; and

           Identifying seasonal  differences by calculating monthly totals and averages.
                                             5-4                                 January 1999

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Chapter 5                                                                 CSS Monitoring







       Simple hydraulic models can be used to predict total volumes of runoff, which can be used



to identify typical rainfall years and the variations across years. For example, 38 years of rainfall



records, 1955-1992, were collected at a NOAA gage near (but not within) a CSS drainage area.



These records indicate an average of 44 storm events per year, with a wide variation from year to



year. To generate runoff predictions for the CSS drainage area, the STORM runoff model (HEC,



1977) was calibrated and run using the 38 years of hourly rainfall data.  The model predicted the



number of runoff events per year, the total annual runoff, and the average overflow volume per



event in inches/land area.  Exhibit 5-1 ranks the years based on the number of events, inches of



runoff, and average runoff per event predicted by the model. Results showed the year 1969 had



both the highest number of runoff events (68) and largest total runoff volume (15.1 inches). The



year 1967 had the highest predicted average overflow per event (0.33 inches).







       Exhibit  5-2  lists minimum, maximum, mean, and median values  for the modeled runoff



predictions based on the data in Exhibit 5-1 for the example site. These statistics identify typical



and extreme years to select for modeling or predicting the frequency of overflows under various



control  alternatives. Long-term computer simulations  of the CSS using a multi-year continuous



rainfall  record,  or  one-year  simulations using typical or wet years, are  useful for assessing



alternative long-term control strategies.







       The data generated by the STORM model can be reviewed for typical or extreme years to



determine  the uniformity of the monthly distribution of runoff.  The years 1969 and 1956 represent



extreme high flows. The year 1956 had the  most severe event over the 38-year evaluation period,



with 6.0 inches  of runoff in 30 hours.  The years  1970 and 1985 were selected  as typical years,



having the most uniform distribution of rainfall throughout the year.







       For some systems, the permittee  may be able  to identify typical years and analyze variations



by reviewing the rainfall record manually.  In these cases, it may not be necessary to use a simple



hydraulic  model to analyze rainfall data.
                                            5-5                                January 1999

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Chapter 5
CSS Monitoring
 Exhibit 5-1. Ranking of Yearly Runoff Characteristics as Simulated by the Storm Model
Rank
i
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Mean
Median
Year
1969
1984
1987
1983
1976
1989
1974
1966
1980
1956
1988
1975
1972
1957
1960
1962
1971
1970
1955
1985
1979
1968
1959
1992
1982
1965
1964
1991
1990
1978
1967
1958
1981
1977
1963
1986
1973
1961

No. of Events
68
58
57
56
56
54
54
54
53
53
52
52
52
52
50
49
47
47
47
45
43
43
43
41
40
40
40
34
34
33
33
32
31
30
30
29
29
28
44
46
Year
1969
1987
1984
1975
1974
1956
1960
1980
1983
1955
1966
1962
1992
1976
1965
1957
1970
1967
1988
1971
1979
1991
1985
1989
1982
1959
1990
1968
1981
1972
1973
1964
1977
1963
1978
1986
1961
1958

Total Runoff (in)
15.1
14.9
14.7
14.2
13.1
13.1
12.8
12.6
12.5
12.5
12.4
12.1
12.1
12.0
12.0
11.9
11.7
11.0
10.9
10.9
10.7
10.6
10.4
9.7
9.1
8.2
8.1
7.9
7.6
7.3
7.2
7.1
6.7
6.3
6.0
5.8
4.8
4.6
10.3
10.9
Year
1967
1991
1992
1965
1975
1955
1987
1960
1984
1979
1973
1970
1962
--.' 1936: !: .i;;:;
1989
1981
1980
1974
1985
1982
1971
1966
1957
1983
1977
1969
1988
1976
1963
1986
1959
1989
1978
1968
1964
1961
1972
1958

Avg Overflow
(in./event)
0.33
0.31
0.30
0.30
0.27
0.27
0.26
0.26
0.25
0.25
0.25
0.25
0.25
0.25
0.24
0.24
0.24
0.24
0.23
0.23
0.23
0.23
0.23
0.22
0.22
0.22
0.21
0.21
0.21
0.20
0.19
0.18
0.18
0.18
0.18
0.17
0.14
0.14
0.23
0.23
              Extreme Year = 1969
                                  Typical Year = 1970
                                           5-6
   January 1999

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 Chapter 5
CSS Monitoring
       Exhibit 5-2. Rainfall and Runoff Parameters for Typical and Extreme Years

Maximum (all years)
1969
1956
1970
Mean (all years)
Median (all years)
1971
198S
1985
1979
Minimum (all years)
No. of
Events
68
68
53
47
44
46
47
52
45
43
28
Total
Runoff
(inches)
15.1
15.1
13.1
11.7
10.3
10.9
10.9
10.9
10.4
10.7
4.6
Average
Overflow
(in./event)
0.33
0.22
0.25
0.25
0.23
0.23
0.23
0.21
0.23
0.25
0.14
       Event Statistics. Information may also be developed on the characteristics of individual
storm  events for a site. If the sequence of hourly rainfall volumes from the existing gages is
grouped into separate events (i.e., each period of volume greater than zero that is preceded and
followed by at least one period of zero volume would mark a separate event), then each storm event
may be characterized by its duration, volume, average intensity, and  the time interval between
successive  events.  The event data can be analyzed using standard statistical procedures to determine
the mean and standard deviation for each  storm event, as well as probability distributions and
recurrence intervals. The computer program SYNOP (Driscoll,  et al, 1990) can be used to group
the hourly rainfall  values into independent rainfall events and calculate the storm characteristics and
interval since the preceding storm.

       Return Period/Volume Curves. The "return period" is  the frequency of occurrence for a
parameter (such as rainfall volume) of a given magnitude. The return period for a storm with a
specific rainfall volume may be plotted as a probability  distribution indicating the percent of storms
with a total volume less than or equal to a given volume. For example, if approximately ten percent
of the  storm events historically deposit 1.5 inches of rain or more, and  there are an average  of 60
                                             5-7
   January 1999

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Chapter 5                                                                  CSS Monitoring







storm events per year, an average of 6 storm events per year would have a total volume of 1.5 inches



or more, and the 1.5-inch rain event could be characterized as the "two-month storm." Return



periods are discussed in Hydrology and Floodplain Analysis (Bedient and Huber, 1992).







       Intensity-Duration-Frequency Curves. Duration can be plotted against average intensity



for several constant storm return frequencies, in order to design hydraulic structures where short



duration peak flows must be considered to avoid local flooding. For example, when maximizing



in-system storage (under the NMC), the selected design event should ensure that backups in the



collection system, which cause flooding, are avoided. Intensity-duration-frequency (IDF) curves



are developed by analyzing an hourly rainfall record so as as to compute a running sum of volumes



for consecutive hours  equal to the  duration of interest. The volumes for that duration are then



ranked, and based on the length in years of the record, the recurrence interval for any rank  is



determined. This procedure is used to calculate the local value for design storms such as a 1 -year,



6-hour  design  condition.  Development and use of IDF curves is discussed in Hydrology and



Floodplain Analysis (Bedient and Huber, 1992) and the  Water Resources Handbook  (Mays, 1996).







       Local Rain Gage Data



       In order to calibrate and verify runoff and water quality models, it is also necessary to



analyze rainfall data for specific storm events in which CSO quality and flow are sampled.







       Local rain gage data can be used to assess the applicability of the long-term record of the



site. For example, Exhibit 5-3 presents six weeks of local rainfall data from three tipping bucket



gages (labeled A,  B, and C in Exhibit 5-4). Comparison with regional rainfall records indicates that



the average value of the three gages was close to the regional record with only slight variations



among gages.
                                            5-8                                January 1999

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Chapter 5
CSS Monitoring
             Exhibit 5-3. 1993 Rainfall Data for a  5,305 Acre Drainage Area
Storm
Event
1
2
3
4
5
6
7
8
9

Date
4/6
4/14 M
4/21
4/28 M
5/5
5/8
5/11
5/13 M
5/14
Total
Gage A
(inches)
0.58
0.22
0.11
0.87
0.12
0.47
0.50
0.44
0.48
3.79
GageB
(inches)
0.58
0.17
0.12
1.20
0.18
0.40
0.45
0.31
0.43
3.84
GageC
(inches)
0.62
0.19
0.08
1.05
0.12
0.42
0.45
0.22
0.52
3.67
Regional Record
of Rainfall
(inches)
0.59
0.19
0.10
1.04
0.14
0.43
0.47
0.32
0.48
3.70
Duration
(hours)
4.8
1.5
1.4
2.5
1.5
9.4
4.5
0.8
4.3
30.7
Intensity
(in/hr)
0.12
0.13
0.07
0.42
0.09
0.05
0.10
0.40
0.11
0.12
M = event selected for detailed water quality monitoring









       Storm events 2, 4,  and 8 were selected for detailed water quality sampling and analysis.



Subsequent analyses of CSS flow and CSS water quality data for this example are discussed in



Sections 5.3.3 and 5.4.2, respectively.







       In cases where local rain gages are placed near but not exactly at the locations where CSS



flow and quality is being monitored, rainfall data from several nearby rain gage locations can be



interpolated to estimate  the  rainfall at the  sampling location.  The inverse distance weighting



method (see box on next page) can be used to calculate the rainfall over a CSS  sampling location



in watershed 4 in Exhibit 5-4.







       It may also be possible to use radar imaging data to estimate rainfall intensities at multiple



locations throughout the rainfall event.
                                            5-9
   January  1999

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Chapter 5
                                                                               CSS Monitoring
                               Inverse Distance Weighting Method

   Using this method, the estimated precipitation at the sampling location is calculated as the weighted
   average of the precipitation at the surrounding rain gages.  The weights are the reciprocals of the squares of
   the distances between the sampling location and the rain gages. The estimated rainfall at the sampling
   location is calculated by summing the precipitation times the weight for each rain gage and dividing by the
   sum of the u-eights.  For example, if the distance between the sampling location in watershed 4 and ram
   gage A is X, rain gage B is Y, and rain gage C is Z and the precipitation at each rain gage is PA, PB, and Pc,
   then the precipitation at the sampling location in watershed 4 can be estimated by:
                                    1
                                  'x2
                                              (PcXZ2>] 7  (X2+Y2+Z2)
   If PA, PB, and Pc are 0.87, 1.20, and 1.05 inches, respectively, and X, Y, and Z are 1.5,  1.0, and 2.5 miles,
   respectively, then
P4 = [(0.87 x
                           + (1.20 x—L_) + (1.05 x— I—)]  / (_
                                                                1
                                                                       1
                                    (1.0)'
                                            (2.5)2
(1.0)*  (2.5)2
           -) =  1.09 inches
                                                 5-10
                                                                                   January 1999

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Exhibit 5-4. Rain Gage Map for Data Presented in Exhibit 5-3
                                                                    4   CSO Outfall Drainage
                                                                        Area
                                                                    •   CSO Outfall

                                                                   I   I Separate Sewer Area
                           Not to Scale

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Chapter 5	CSS Monitoring
5.3    FLOW MONITORING IN THE CSS
       Accurate flow monitoring is critical to understanding the hydraulic characteristics of a CSS
and predicting the magnitude, frequency, and duration of CSOs. Monitoring flows in CSSs can be
difficult because of surcharging, backflow, tidal flows, and the intermittent nature of overflows.
Selecting the most appropriate flow monitoring technique depends on site characteristics, budget
constraints,  and availability of personnel. This section outlines  options  for measuring  CSS flow and
discusses how to organize and analyze the data collected.

5.3.1   Flow Monitoring Techniques

       Flow measurement techniques vary greatly in complexity,  expense, and accuracy. This
section describes a range of manual and automated  flow monitoring techniques. Exhibit 5-5
summarizes their advantages and disadvantages.

       Manual Methods
       The simplest flow monitoring techniques include  manual measurement of velocity  and  depth,
use of bottle boards and chalking (see Example 5-1), and dye testing. Manual methods are difficult
during wet weather, however, since they rely extensively on labor-intensive  field  efforts during
storm events and do not  provide an accurate,  continuous flow record. Manual methods are most
useful for instantaneous  flow measurement, calibration of other flow measurements, and flow
measurements in small systems. They are difficult to  use for measuring rapidly changing flows
because numerous  instantaneous measurements must be taken at the proper position to correctly
estimate the total flow.

       Measuring Flow Depth
       Primary flow devices, such as weirs, flumes, and orifice plates, control flow  in a portion of
pipe such that the  flow's depth is proportional to its flow rate. They enable the flow rate to be
determined by manually or automatically measuring the depth of flow. Measurements taken with
these devices are  accurate in the appropriate hydraulic conditions  but  are not accurate where
surcharging or backflow  occur.  Also, the accuracy of flow calculations depends on the reliability
of depth-sensing equipment,  since small errors in depth measurement can result in large errors  in

                                           5-12                               January 1999

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Chapter 5
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                      Exhibit 5-5. CSO Flow Monitoring Devices
Monitoring Method
Manual Methods
Timed Flow
Dilution Method
Direct Measurement
Chalking and
Chalking Boards
Bottle Boards
Description

Timing how long it takes to fill a
container of a known size
Injection of dye or saline solution in
the system and measuring the
dilution
Use of a flow meter and surveying
rod to manually measure flow and
depth
Blowing chalk into a CSO structure,
or installation of a board with a
chalk line. The chalk is erased to
the level of highest flow
Installation of multiple bottles at
different heights where the highest
filled bottle indicates the depth of
flow
Advantages

• Simple to implement
• Little equipment needed
• Accurate for instantaneous
flows
• Easy to collect data
• Easy to implement
• Easy to implement
Disadvantages

• Labor-intensive
• Suitable only for low
flows
• Not appropriate for
continuous flow
• Outside contaminants
could affect results
• Labor-intensive
• Multiple measurements
may be needed at a single
location
• Provides only a rough
estimate of depth
• Provides only a rough
estimate of depth
Primary Flow
Weir
Flume
Orifice Plate
Device placed across the flow such
that overflow occurs through a
notch. Flow is determined by the
depth behind the weir
Chute-like structure that allows for
controlled flow
A plate with a circular or oval
opening designed to control flow
• Many CSOs have an existing
weir
• More accurate than other
manual measurements
• Accurate estimate of flow
• Less prone to clogging than
weirs
• Can measure flow in full
pipes
• Portable and inexpensive to
operate
• Cannot be used in full or
nearly full pipes
• Somewhat prone to
clogging and silting
• Not appropriate for
backflow conditions
• More expensive than
weirs
• Prone to solids
accumulation
Depth Sensing
Ultrasonic Sensor
Pressure Sensor
Bubbler Sensor
Float Sensor
Sensor mounted above the flow that
measures depth with an ultrasonic
signal
Sensor mounted below the flow
which measures the pressure
exerted by the flow
Sensor that emits a stream of
bubbles and measures the resistance
to bubble formation
Sensors using a mechanical float to
measure depth
• Generally provide accurate
measures
• Generally provide accurate
measures
• Generally provide accurate
measures
• Generally provide accurate
measures
• May be impacted by
solids or foam on flow
surface
• Require frequent cleaning
and calibration
• Require frequent cleaning
to prevent clogging
• Must be accurately
calibrated prior to use and
regularly checked for
fouling
Velocity Meters
Ultrasonic
Electromagnetic
Meter designed to measure velocity
through a continuous pulse
Meter designed to measure velocity
through an electromagnetic process
• Instrument does not interfere
with flow
• Can be used in full pipes
• Instrument does not interfere
with flow
• Can be used in full pipes
• More expensive than
other equipment
• More expensive than
other equipment
                                          5-13
    January 1999

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Chapter 5                                                                 CSS Monitoring


flow rate calculation. Monitoring devices need to be resistant to fouling and clogging because of

the large amounts of grit and debris in a CSS.


       Depth-sensing devices can be used with pipe  equations or primary flow and velocity-sensing

devices to determine flow rates.  They include:
           Ultrasonic Sensors, which are typically mounted above the flow in a pipe or  open
           channel and send an ultrasonic signal toward the flow. Depth computations are based
           on the time the reflected signal takes  to return to the sensor. These sensors provide
           accurate depth measurements but can be affected by high suspended solid  loads or
           foaming on the water surface.

          Pressure Sensors, which use transducers to sense the pressure of the water above them.
           They are used  with a  flow monitor that  converts  the  pressure value  to  a depth
           measurement.

          Bubbler Sensors, which emit a continuous  stream of fine bubbles. A pressure transducer
           senses resistance to bubble formation, converting it to a depth value.  These devices
          provide accurate measurements.  The  bubble tube can clog, however,  and the device
           itself requires frequent calibration.

          Float Sensors, which sense depth using a mechanical float, often within a  chamber
           designed to damp out surface waves. Floats can clog with grease and solid materials and
           are, therefore, not commonly used to sense flow in sewers.
                                           5-14                               January 1999

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Chapter 5                                                                                CSS Monitoring
                                    Example  5-1. Flow Monitoring
     A bottle rack is used to determine the approximate depth of overflows from a 36-inch combined sewer in an
     overflow manhole (Exhibit 5-6). The overflow weir for this outfall is 12 inches above the invert of the sewer,
     and flows  below this level are routed out the bottom of the structure to the interceptor and the wastewater
     treatment plant. Any flow overflowing the 12-inch weir is routed to the 42-inch outfall sewer. Attached to
     the manhole steps, the bottle rack approximates the flow level in the  manhole by the height of the bottles that
     are filled.  This outfall has potential for surcharging because of flow restrictions leading to the interceptor.
     Consequently,  the bottle rack extends well  above the crown of the outfall  sewer. After each rainfall, a
     member of the monitoring team pulls the rack from the manhole, records the highest bottle filled, and returns
     the rack to the manhole,  Exhibit 5-7 presents depth data for the nine storms listed in Exhibit 5-3.

     Storm 3, which had 0.1 inch of rain in 85 minutes, was contained at the outfall with no overflow, although
     it did overflow at other locations. Storm 5, with an average volume of 0.14 inches and an average intensity
     of 0.09 in/hour, had a peak flow depth of approximately six inches above the weir crest.

     It is instructive to examine the individual rain gages (located as indicated in Exhibit 5-4) and  compare them
     to the flow depths. Rain gage A indicated that Storms 3 and 5 had similar depths and that 3 was slightly more
     intense.  Why, then,  did Storm 5 cause an overflow, while  Storm 3  did not? Rain gage B, which lies nearer
     to the outfall, indicates 50 percent more volume and 50 percent higher intensity for storm 5.  Using only rain
     gage A  in calibrating a hydraulic  model to the outfall  for storms 3 and 5 could have posed a problem.
     Because a bottle board indicates approximate maximum flow depth, not duration or flow volume, it is not
     sufficient to calibrate most models.

     Storms 4  and  8 caused flow depth to  surcharge, or increase above the crown of the pipe. Both  storms
     occurred during late afternoon when sanitary sewer flows are typically highest, potentially  exacerbating the
     overflow,  The surcharging pipe indicates that flow measurements  will  be difficult for large storms at this
     location. Further field investigations will be necessary to define the hydraulics of this particular outfall and
     intercepting  device,  Because of safely considerations in gaming access to this location, the  monitoring team
     used only the bottle board during the early monitoring period.  Later, the team installed a velocity meter and
     a series  of depth probes to determine a surface profile.
                                                     5-15                                      January  1999

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Chapter 5
CSS Monitoring
                 Exhibit 5-6. Illustration of a Bottle Board Installation

                                            Section
     42"CSO
     to River
                                                                             36'
                                     To Wastewater
                                     Treatment Plant
                   Exhibit 5-7. Example Outfall Bottle Rack Readings
Storm Event
1
2
3
4
5
6
7
8
9
Manhole Flow Level
(inches)
21
18
12
48
18
18
30
42
24
Height of Overflow
(inches)
9
6
none
36 (surcharge)
6
6
18
30 (surcharge)
12
                                          5-16
   January 1999

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Chapter 5                                                                   CSS Monitoring


       Using depth measurement  data,  pipe equations can be  applied to develop flow estimates.  The
Hazen-Williams equation, Manning equation,  and similar equations can be useful for estimating flow
capacity of the system and performing a preliminary flow analysis of the CSS.  The Hazen-Williams
equation  is generally used for pressure conduits, while the Manning equation is  usually used in open-
channel situations (Viessman, 1993). The Hazen-Williams equation is:
                                   V = 1.318 C(R)ฐ'63 (S)0'54
where:
       V = mean flow velocity
       C = Hazen-Williams coefficient, based on material and age of the conduit
       R = hydraulic  radius
       S  = slope of energy gradeline (ratio of rise to run).
The Manning Equation is:
where:
                                   V = (1.49/n) (R)0'666 (Sf5
       V = mean flow velocity
       n = Manning roughness coefficient, based on type and condition of conduit
       R = hydraulic radius
       S = slope of energy gradeline (ratio of rise to run).

The volumetric flow rate (Q) is computed by:

                                           Q = VA

where:

       V = mean flow velocity
       A =  cross-sectional  area.


        Since the calculations are based on the average upstream characteristics of the pipe, personnel
should measure depth at a point in the sewer where there are no bends,  sudden changes in invert
elevation, or manholes immediately upstream. These features can introduce large errors into the
flow estimate. Anomalies  in sewer slope, shape,  or roughness  also can cause large errors (50 percent
and greater) in flow  measurement.  However, in  uniform pipes, a careful application  of these



                                             5-17                                January 1999

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 Chapter 5                                                                CSS Monitoring

 formulas can measure flows with an error as low as 10 to 20 percent (ISCO, 1989). The permittee
 can improve the accuracy of the equation somewhat by calibrating it initially, using measurements
 of velocity and depth to adjust slope and roughness values.

       Velocity Meters
       Velocity meters use ultrasonic or electromagnetic technology to sense flow velocity at a
 point, or in a cross section of the flow. The velocity measurement is combined with a depth value
 (from a depth sensor attached to the velocity meter) to compute flow volume. Velocity meters can
 measure flows in a wider range of locations and flow regimes than depth-sensing devices used with
 primary flow devices,  and they  are  less prone to clogging. They  are  comparatively expensive,
 however, and can be  inaccurate at low flows and when suspended solid  loads vary rapidly.  One type
 of meter combines an electromagnetic velocity sensor with a depth  sensing pressure transducer in
 a single probe. It  is useful for CSO applications because it can sense flow in surcharging and
 backflow conditions. This device is available as a portable model or for permanent installation.

       Measuring Pressurized Flow
       Although sewage typically flows by gravity, many CSSs use pumping stations or other
 means  to  pressurize their flow. Monitoring pressurized flow requires  different techniques from those
 used to monitor gravity flows.  If a  station is designed to pump at a constant rate, the flow rate
 through the station can be estimated from the length of time the pumps are on.  If a pump empties
 a wet well or cavern, the pumping rate can  be determined by measuring the change in water level
 in the  wet well. If the  pump rate is variable, or pump monitoring time  is insufficient to measure
 flow, then full-pipe metering is required.

       Measuring Flow in Full Pipes
       Full pipes can be monitored using orifices, Venturis, flow nozzles, turbines,  and ultrasonic,
 electromagnetic, and vortex shedding meters.   Although most of these technologies  require
 disassembling the piping and inserting a meter, several types of meters strap to the outside of a pipe
 and can be moved easily to different locations. Another measurement technique involves using two
 pressure transducers, one at the bottom of the pipe, and one at the top of the pipe or in the manhole
just above the  pipe crown.  Closed pipe  metering  principles  are discussed  fully in The Flow
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Chapter 5                                                                 CSS Monitoring

Measurement Engineering Handbook (Miller, 1983). Manufacturers' literature should be consulted
for installation requirements.

5.3.2  Conducting the Flow Monitoring Program
       Most flow monitoring involves the use of portable, battery-operated depth and  velocity
sensors,  which are left in place for several storm events and then moved elsewhere. For some
systems, particularly small CSSs, the monitoring program may involve manual methods.  In such
cases, it  is important to allocate the available personnel and prepare in advance for the wet weather
events.

       Although temporary metering installations are designed to operate automatically,  they are
subject to clogging in CSSs and should be checked as often as possible for debris.

       Some systems  use permanent flow  monitoring installations to collect data continuously at
critical points.  Permanent installations also  can allow centralized control of transport system
facilities  to maximize storage of wastewater  in the system and maximize  flow to the treatment plant.
The flow data recorded at the site may be recovered manually or telemetered to  a central location.

       To be of use in monitoring CSSs, flow metering installations should be able to measure all
possible flow situations, based on local conditions. In a pipe with smooth flow characteristics, a
weir or  flume in combination with a depth  sensor or a calibrated Manning  equation  may be
sufficient. Difficult locations might warrant redundant metering and frequent calibration.  The key
to successful monitoring is combining good design  and judgment with field observations, the
appropriate metering technology, and a thorough meter maintenance and calibration schedule.
                                            5-19                               January 1999

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Chapter 5                                                                 CSS Monitoring

5.3.3   Analysis of CSS Flow Data
       The CSS flow data can be evaluated to develop an understanding of the hydraulic response
of the system to wet weather events  and to answer the following questions for the monitored outfalls:

          Which CSO outfalls contribute the majority of the overflow volume?
          What size storm can be contained by the regulator serving each outfall? What rainfall
          amount is needed to initiate overflow? Does this containment capacity vary from storm
          to storm?
          Approximately how many overflows  would occur and what would be their volume,
          based on a rainfall record from a different year? How many occur per year, on average,
          based on the long-term rainfall record?

       Extrapolating from the monitored period to other periods, such as a rainfall record for a year
with more storms or larger volumes, requires professional judgment and familiarity with the data.
For example, as shown in Exhibit  5-8, the flow regulator serving Outfall  4 prevented overflows
during Storm 3, which had 0.10 inch of rain  in 1.4 hours.  However, approximately half of the
rainfall volume overflowed from Storm 5, which had 0.14 inch in 1.5 hours. From these data, the
investigator  might conclude  that,  depending on the  short-term intensity of the storm or  the
antecedent moisture conditions, Outfall 4 would contain a future storm of 0.10  inches but that even
slightly larger storms would cause  an overflow. Also, Exhibit 5-8 indicates that a storm even as
small as Storm 3 can cause overflows at  the other outfalls.
                                           5-20                              January 1999

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                                                     Exhibit 5-8. Total  Overflow Volume
Storm
1
2
3
4
5
6
7
8
9
Average
Rainfall
Depth (R)
(inches)
0.59
0.19
0.10
1.04
0.14
0.43
0.47
0.32
0.48
0.42
Duration
(hours)
4.8
1.5
1.4
2.5
1.5
9.4
4.5
0.8
4.3
3.41
Outfall (and service area size, in acres)
#1 (659 acres)
V
0.24
0.07
na
0.62
0.06
0.19
0.26
na
0.26
0.24
V/R
0.41
0.37
na
0.60
0.43
0.44
0.55
na
0.54
0.48
#4 (430 acres)
V
0.39
0.085
0.00
0.832
0.071
0.195
0.32
0.252
0.32
0.27
V/R
0.65
0.45
0.00
0.80
0.51
0.45
0.68
0.79
0.66
0.55
#5(500 acres)
V
0.27
na
0.04
0.39
0.05
0.18
0.16
0.15
0.14
0.17
V/R
0.46
na
0.41
0.73
0.37
0.43
0.34
0.46
0.29
0.43
#7 (690 acres)
V
0.50
0.14
0.06
0.81
0.102
0.361
0.334
0.25
0.29
0.32
V/R
0.85
0.72
0.56
0.77
0.73
0.84
0.71
0.78
0.60
0.73
#9 (1,060 acres)
V
na
0.072
0.045
0.44
0.051
0.23
0.2
0.141
0.17
0.17
V/R
na
0.38
0.45
0.67
0.36
0.53
0.42
0.44
0.35
0.45
               V  =  overflow volume (inches depth when inches of overflow is spread over drainage area)
               R = rainfall depth (inches)
               na = no measurement available
vo

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Chapter 5                                                                   CSS Monitoring

       Comparing the  overflow volumes of different outfalls  indicates which  outfalls contribute the
bulk of the  overflow volume  and, depending on loading measurements, may contribute most heavily
to water quality problems. To compare the hydraulic performance of different outfalls, flows should
be normalized against the drainage area and rainfall. Provided that rainfall data are representative
of the area's rainfall, inches  of overflow (spread over the discharge subarea) per inch of rainfall
constitutes a useful statistic.  Exhibit 5-8 presents the overflow volumes in inches and the ratio of
depth of overflow to depth of rain (V/R).

       For each outfall, V/R  varies  with the storm depending  on the number of antecedent dry days,
the time of the storm, and  the  maximum rainfall intensity.   V/R also varies with the outfall
depending on land characteristics such as its impervious portion, the hydraulic capacity upstream
and downstream of the flow regulator, the operation  of the flow regulator, and features  that  limit the
rate at which water can enter the  system draining to that overflow point. Because of the  large
number of factors affecting variations in V/R, small differences generally provide little information
about overflow patterns. However, certain patterns, such as  an increase in V/R over time or large
differences in V/R between  storms or between outfalls, may indicate  design  flaws, operational
problems, maintenance problems, or erroneous flow measurements, or a rainfall gage that does not
represent the average depth of rain  falling on the discharge subarea.

       In addition to supporting an analysis of CSO volume,  flow data can be used to create a plot
of flow and head for a selected conduit during a storm event,  as shown in Exhibit 5-9. These plots
can be used to illustrate the conditions under which  overflows occur at a specific outfall.  They can
also be used during CSS model calibration and verification (see Chapter 7).

       Exhibits 5-8 and 5-9  (representing  different  CSS  monitoring  programs) illustrate some of the
numerous methods available  for analyzing CSO flow monitoring data. Additional methods include
plotting  regressions  of overflow volume and rainfall  to  interpret monitoring data  and identify
locations that will cause difficulty in calibrating a model.  For this type of regression, the y-intercept
defines the  rainfall needed to cause  an overflow and the slope roughly defines the gross runoff
coefficient for the basin. Flow data can also be used to tabulate  CSO  volumes and frequencies
during the monitored time period and to compare the relative  volumes and frequencies from different
                                            5-22                                January 1999

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                     20

                     18

                     16

                     14

                     12


                     1ฐ
                     8

                     6

                     4

                     2

                     0
                                                 Exhibit 5-9. Example CSS  Plots of Flow and Head  versus Time
                                 MAY 31 - JUNE 1, 1992 STORM
                         Hyetograph     Extran Flow     Meier B19A
                                       12    16    20
                                          Time, Hours
                                                       24
                                                            28
                                                                         0.0
                                                                          0.1
                                                                  32
                                                                       36
                                                                                                                SEPTEMBER 26 - 27, 1992 STORM

                                                                                                        Hyetograph     Extran Flow    Meter b19a
20

18

16

14

12

10

8

6

4

2

0
                                                                                                                            "•*-   >T
                                                                                                                         12    16    20    24
                                                                                                                          Time, Hours
                                                                                                                                                28
                                                                                                                                                         0.0
                                                                                                                                                         0.1
                                                                                                                                                      32
NJ
 c
^ง
                                  MAY 31 - JUNE 1, 1992 STORM
                         Hyetograph      Extran HGL 10729   Meier Bos019a.REO
                                                                      l9-
                    120

                    118

                    116

                    114

                    112

                  ^- 110
                  ซ
                  I 108

                    106

                    104

                    102

                    100
                                       12   16   20
                                          Time, Hours
                                                       24
                                                            28
                                                                 32
                                                                          0.1
                                                                       36
           SEPTEMBER 26 - 27, 1992 STORM
    Hyetograph     Extran HGL 10729   Meter Bos019a,RE019-
120
118
116
114
112
T," 110
fO
I 108

106
104
102
100
(

: "T -
-
-
/~x
jป * \
/> \ \
___j\___^__j \ \ 	
r~
•
> 4 8 12 16 20 24 28 3
Time, Hours
                                                                                                                                                         0.0

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Chapter 5                                                                 CSS Monitoring

monitoring sites in the CSS. Data  are  plotted,  tabulated,  and analyzed prior  to a modeling
assessment (described in Chapter 7).

5.4    WASTEWATER MONITORING IN THE CSS
       Collecting and analyzing CSS wastewater samples is essential to characterizing an overflow
and determining its impact on a receiving  water body. Wastewater monitoring information can be
used to:

          Indicate potential exceedances of water quality criteria
          Indicate potential human health and aquatic life impacts
          Develop CSO quality models
          Assess pretreatment and pollution prevention programs as part of the NMC.

This section  outlines various methods for collecting, organizing, and analyzing CSS wastewater data.
Sampling during wet weather events involves some factors that are not a significant concern during
dry weather. These additional considerations  are discussed in the section  on sample program
organization for receiving water quality monitoring (Section 6.3.1).

5.4.1  Water  Quality Sampling
       In general, wastewater  sample types fall into the following two categories:

       . Grab samples
       . Composite  samples.

       Grab Sampling. A grab sample is a discrete, individual sample collected over a maximum
of 15 minutes. Grab  samples represent the conditions at  the time the sample is taken and do not
account for variations in quality throughout a storm  event. Multiple grab samples can be gathered
at a station to define such variations, although costs increase due to additional labor and laboratory
expenses.
                                           5-24                               January 1999

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Chapter 5                                                                 CSS Monitoring

       Composite Sampling. A composite sample is formed by combining samples collected over
a period of time, or representing more than one specific location or depth.  Composite sampling
provides data representing the overall  quality of combined sewage averaged over a storm  event.  The
composited sample can be collected by continuously filling a container throughout the time period,
collecting a series of separate aliquots, or combining individual grab samples from separate times,
depths,  or locations. Common types of composite samples include:
           Time composite samples - Composed of discrete sample aliquots, of constant volume,
           collected at constant time intervals.
          How-weighted composite samples - Composed of samples combined in relation to the
           amount of flow observed in the period between the samples.
       Flow-weighted compositing can be done in two ways:
          Collect samples at equal time intervals at a volume proportional to the flow rate (e.g.,
          collect 100 ml of sample for every 100 gallons of flow that passed during a lo-minute
          interval).
          Collect samples of equal volume at varying times proportional to the flow (e.g., collect
          a 100 ml sample for each 100 gallons of flow irrespective of time).
       The second method is preferable for sampling wet weather flows, since it results in the
greatest number of samples when the flow rate is the highest. More  detailed information on  methods
of flow weighting  is presented in the NPDES Storm  Water Sampling Guidance Document (U.S.
EPA, 1992).

       Grab and composite samples can be collected using either of two sample methods:  manual
and automatic.

       Manual Sampling. Manual samples  are usually collected by an individual using a hand-held
container. This method requires minimal equipment and allows field personnel to record additional
observations while the sample is collected.  Because of their special characteristics, certain pollutants
should be collected manually. For example, fecal streptococcus, fecal coliform, and chlorine have


                                           5-25                              January 1999

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Chapter 5                                                                 CSS Monitoring

very short holding times (i.e., 6 hours), pH and temperature need to be analyzed immediately, and
oil and grease can adhere to the sampling equipment and cause inaccurate measurements. Volatile
compounds must be collected manually according to standard procedures since these compounds will
likely volatilize as a result of agitation during automatic sampler collection (APHA, 1992).

       Manual sampling can be labor-intensive and expensive when the sampling program is long-
term and involves many locations. Personnel must be available around the clock to sample storm
events. Safety issues  or hazardous conditions may affect sampling at certain locations.

       Automated Sampling. Automated samplers are useful for CSS sampling because they can
be programmed to collect multiple discrete samples as  well  as  single or multiple composited
samples.  They can collect samples on a timed basis or in proportion to flow measurement signals
from a flow meter. Although automated samplers require a large investment, they can reduce the
amount of labor required in a sampling program and increase the  reliability of flow-weighted
compositing.

       Automated  samplers have  a  lower  compartment, which  holds  glass or plastic sample
containers and an ice well to cool samples, and an upper part,  containing a microprocessor-based
controller, a pump assembly, and a filling mechanism.  The samplers can operate off of a battery,
power pack,  or electrical supply.  More expensive samplers have  refrigeration  equipment and require
a 120-volt power supply.  Many samplers can be connected to flow meters that will activate flow-
weighted compositing programs, and some samplers are activated by  inputs from rain gages.

       Automated samplers also have limitations:

          Some pollutants (e.g., oil and grease) cannot be sampled by automated equipment  unless
          only approximate results are desired.
          The self-cleaning capability of most samplers provides reasonably separate samples, but
          some cross-contamination is unavoidable because water droplets usually remain in the
          tubing.
          Batteries may  run down or the power supply may fail.
                                           5-26                               January 1999

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Chapter 5                                                                 CSS Monitoring
          Debris in the sewer, such as rags and plastic bags, can block the end of the sampling line,
          preventing sample collection. When the sampling line is located near a flow meter, this
          clogging can also cause erroneous flow measurements.  Samplers and meters should be
          checked during  storms and must be tested and serviced regularly. If no field checks are
          made during a storm event, data for the entire event may be lost.
          The sample nozzles of many automatic samplers do not have the velocity capabilities
          necessary for picking up the sand and gravel in untreated CSO flows.
       Sampling Strategies
       In developing a sampling strategy, the permittee should consider the timing of samples and
sampling intervals (i.e., duration of time between the collection of samples).   Since pollutant
concentrations can vary widely during  a storm  event, the permittee should  consider sampling
strategies that include pre-storm,  first flush, peak flow, recovery, and post-storm  samples. For
example, the permittee could take individual grab samples  at each site during the different storm
stages. Another sampling regime the permittee can use is  taking a series of samples during the
stages at each site:

       . Pre-storm grab  sample
          Composite samples collected during first flush
          Composite samples collected during peak flow
          Composite samples collected after peak flows
       . Post-storm grab  sample.

       A third possible sampling regime could include a first flush composite taken over the first
30 minutes of discharge, followed by a second composite over the next hour of discharge, followed
by a third composite for the remainder of the storm. To characterize first flush, a sample should be
collected as  close to the beginning of the CSO event as feasible. Appropriate  sampling intervals
depend on such factors as drainage area sizes, slopes, land uses, and percent imperviousness.
                                           5-27                              January 1999

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Chapter 5	CSS Monitoring

       Contaminants Requiring Special Collection Techniques
       The above discussion focuses on CSS sampling for contaminants with no special collection
requirements. The following  contaminants  have special handling requirements (as identified in
40CFRPart 136):
          Bacteria - Because samples collected for bacteria analysis cannot be held for more than
          six hours, they must be collected manually. Bacteria samples are collected directly into
          a sterile container or plastic bag, and it is important not to contaminate the sample by
          touching it.  Often the samples are preserved with sodium thiosulfate.
           Volatile Organic Compounds (VOCs) -  Samples analyzed for VOCs  are collected
          directly into special glass vials. Each vial must be filled so that there is no  air space into
          which the VOCs can volatilize and be lost.
          Oil and Grease -  Samples analyzed for oil and grease must be collected by grab sample
          using a glass jar with a Teflon-coated lid.  Samples are preserved by lowering the pH
          below 2.0 using a strong acid.
          Dissolved Metals  - Samples collected  for dissolved metals analysis must be filtered
          immediately after sample collection and before preservation.
       The monitoring program may also include toxicity testing, in which the acute and chronic
impacts to aquatic life are determined. Toxicity testing procedures for wet weather discharges are
in Technical Support Document for Water Quality-based Toxics Control (U.S. EPA, 1991 a).

       Sample Preparation and Handling
       Sample bottles  are typically  supplied by the laboratory that will perform  the  analysis.
Laboratories may provide properly cleaned sampling containers with appropriate preservatives. For
most parameters, preservatives should be added to the container after the sample. To avoid hazards
from fumes and spills, acids and bases should not be in containers without a sample. If preservation
involves adjusting sample pH, the preserved sample should always be checked to make sure it is at
the proper pH level. The maximum allowed holding period for each analysis is specified in Table II
of 40 CFR Part 136. Acceptable procedures for cleaning sample bottles, preserving their  contents,
and analyzing for appropriate chemicals are detailed in various methods manuals, including APHA
(1992) and U.S. EPA (1979).
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Chapter 5                                                                 CSS Monitoring

       Water samplers, sampling hoses, and sample storage bottles should always be made of
materials compatible with the pollutants being sampled. For example, when sampling for metals,
bottles  should not have  metal components that can contaminate the samples. Similarly, bottles and
caps used for organic samples should be made of materials not likely to leach into the sample.

       Sample Volume, Preservation, and Storage. Sample volumes,  preservation  techniques,
and maximum holding times for most parameters are specified in 40 CFR Part 136. Refrigeration
of samples during and after collection at a temperature of 4ฐC is required for most  analyses.  Manual
samples are usually placed in a cooler containing ice or an ice substitute. Most automated samplers
have a well next  to the sample bottles to hold either ice or ice substitutes.  Some expensive samplers
have mechanical  refrigeration equipment. Other  preservation  techniques  include pH  adjustment and
chemical fixation. pH adjustment usually requires strong acids and bases,  which should be handled
with extreme caution.

       Sample Labeling.  Samples should be identified by waterproof labels containing enough
information to ensure that each is unique. The information on the label should also be recorded in
a sampling notebook. The label typically includes the following information:

       •  Name of project
       •  Date  and time of sample collection
       •  Sample location
       •  Name or initials of sampler
       •  Analysis to  be performed
       •  Sample ID number
       •  Preservative used
       •  Type of sample (grab, composite).

       Sample  Packaging  and Shipping.  Sometimes it is necessary to  ship samples  to the
laboratory.  Holding times should be checked prior to shipment to ensure that they will not be
exceeded. While wastewater samples generally are not considered hazardous, some samples, such
as those with extreme pH, require special procedures. Samples shipped through a common  carrier
or the U.S. Postal Service must comply with Department  of Transportation Hazardous Material

                                           5-29                               January 1999

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Chapter 5	CSS Monitoring

Regulations (49 CFR Parts 171 - 177). Air shipment of samples classified as hazardous may also be
covered by the Dangerous Goods Regulations (International Air Transport Association, 1996).

       Samples should be sealed with chain-of-custody form  seals in leak-proof bags and padded
against  jarring  and breakage.   Samples must be packed with  an  ice substitute to maintain a
temperature of 4ฐC during shipment.  Plastic  or metal  coolers make ideal shipping  containers
because they protect and insulate the samples. Accompanying paperwork such as the chain-of-
custody documentation should be sealed in a waterproof bag in the shipping container.

       Chain of Custody. The  chain-of-custody  form documents the changes of possession of a
sample between time of collection and time of  analysis.  At each transfer of possession,  both the
relinquisher and  the receiver sign  and date the form in order to  document transfer of the samples and
to minimize opportunities for tampering.  The container holding the samples  can also be  sealed with
a signed tape or seal to document that the samples are uncompromised.

       The sampler and the laboratory should retain copies of the chain-of-custody form.  Contract
laboratories often supply chain-of-custody forms  with sample containers.  The form is also useful
for documenting which analyses will be performed on the samples.  Forms typically contain the
following information:

       •   Name of project and sampling locations
       •   Date and time that each sample  was  collected
       •   Names  of sampling personnel
       •   Sample identification names and numbers
       •   Types  of sample containers
       •   Analyses to be performed on each sample
       •   Additional comments on each sample
       •   Names  of all personnel transporting  the samples.
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Chapter 5                                                                   CSS Monitoring

5.4.2   Analysis of Wastewater Monitoring Data
       Since monitoring  programs can generate large amounts  of information, effective management
and analysis of the data are essential. Even small-scale programs,  such as those involving  only a few
CSS and receiving water monitoring locations, can generate an extensive amount of data. This
section discusses  tools  for  data analysis including spreadsheets, graphical  presentations,  and
statistical analysis. (Data management is discussed in Section 4.8.2. Chapters 7 and 8 discuss more
detailed  data analysis during modeling.)

       This section outlines an example analysis of data collected during three storms, where flow-
weighted composite samples were collected and analyzed for BOD and TSS. Exhibit 5-10 shows
average  concentrations for each  storm at the monitored outfalls; the small sample size does not
provide statistically reliable information on the expected  variability of these concentrations for other
events. To estimate pollutant concentrations for a large set of storm events, expected values can be
calculated by assuming a lognormal distribution. (The lognormal distribution has been shown to be
applicable to CSO quality (Driscoll, 1986).) Exhibit 5-11 shows that the mean  and median for the
data are similar and are within  typical ranges for CSO quality. The  mean and median  for the
sampling data  can be used with a lognormal  distribution to compute the expected mean, median, and
90th-percentile value for a large  data set of many storm events.  If used as a basis for estimating
impacts, the 90th-percentile values would be more conservative than the means for BOD and TSS
since only 10 percent of the actual concentrations  for these pollutants should exceed the 90th-
percentile values.

       Multiplying  flow  measurements (or  estimates) by pollutant concentration values drawn from
monitoring data gives the total pollutant  load  discharged during each storm at each  outfall.
Exhibit 5-12 lists pollutant loads  for the three storms at each monitored outfall. As with flow data,
these brief statistical summaries  provide insight into the response  of the system before any more
involved computer modeling is  performed.   For example, the  load in pounds of BOD and TSS
discharged at each  outfall, normalized to account for differences in rainfall depth  or land area at each
outfall, helps  to identify differences in loading  rates across outfalls over the long term. These
loading factors can provide rough estimates of the loads from unmonitored outfalls that have land
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                      Exhibit 5-10. Composite Sampling Data (mg/1)
Outfall
1
4
5
7
9
Average
Storm #2
BOD
115
96
128
92
110
108
TSS
340
442
356
552
402
418
Storm #4
BOD
80
94
88
82
120
93
TSS
200
324
274
410
96
261
Storm #8
BOD
110
120
92
71
55
90
TSS
240
350
288
383
522
357
Average
BOD
102
103
103
82
95
97
TSS
260
372
306
448
340
345
             Exhibit 5-11. Pollutant Concentration Summary  Statistics (mg/1)

Mean
Median
Expected Mean*
Expected Median*
Expected 90th Percentile Value*
Typical CSO Characteristics1
BOD
96.87
94.00
97.16
94.70
126.64
60 - 220
TSS
345.27
350.00
352.53
321.29
558.03
270 - 550
              *Projected statistic from sampling population (i.e., very large data set)
              'Metcarf & Eddy, Inc.,  1991.
uses or impervious areas similar to the monitored area. Finally, the total load per storm helps in
comparing storms and projecting storm characteristics  that would produce higher or lower loads.
Pollutant loads are affected by the number of dry days  and the number of days without a flushing
storm because these factors represent a period when no severe scour activity occurred in the sewer
system.

       Three storms can indicate trends but do not provide enough data to characterize the load of
the CSS or its individual source areas. As additional data are  collected during the monitoring
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Chapter 5
                       CSS Monitoring
                          Exhibit 5-12. Pollutant Loading Summary

STORM 2
composite
composite
load
load
STORM 4
composite
composite
load
load
STORMS
composite
composite
load
load
Total Load*

Area Load**
(Ib/acre/storm)
Loading Rate
(Ib/inch rain)
Flow (MG)
BOD (mg/1)
TSS (mg/1)
BOD (Ibs)
TSS (Ibs)
Flow (MG)
BOD (mg/1)
TSS (mg/1)
BOD (Ibs)
TSS (Ibs)
Flow (MG)
BOD (mg/1)
TSS (mg/1)
BOD (Ibs)
TSS (Ibs)
BOD (Ibs)
TSS (Ibs)
BOD
TSS
BOD
TSS
OUTFALL
1
1.39
115
340
1,333
3,941
11.67
80
200
7,786
19,466
na
110
240
0
0
9,119
23,407
7
18
7,417
19,038
4
0.99
96
442
793
3,649
9.72
94
324
7,620
26,265
2.95
120
350
2,952
8,611
11,365
38,525
9
30
7,329
24,843
5
na
128
356
0
0
5.31
88
274
3,897
12,134
2.00
92
288
1,535
4,804
5,432
16,938
5
17
3,997
12,465
7 .
2.55
92
552
1,957
11,739
15.09
82
410
10,320
51,599
4.68
71
686
2,771
26,775
15,048
90,113
7
44
9,709
58,144
9
2.07
110
402
1,899
6,940
12.64
120
96
12,650
10,120
4.07
55
522
1,867
17,719
16,416
34,779
5
11
10,595
22,440
TOTAL
7.00
-
-
5,982
26,269
54.43
-
-
42,273
119,584
13.70
-
-
9,125
57,909
57,380
203,762
7
24
7,809
27,386
na = No flow data available. MG = millions of gallons.
load (Ibs) = composite concentration (mg/1) x flow (MG) x 8.34
* For monitored storms
** Acreage data taken from Exhibit 5-8; for monitored storms (i.e
(conversion factor)

, either 2 or 3)
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                           January 1999

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Chapter 5                                                                  CSS Monitoring

program, estimates based on the data set become statistically more reliable because the size of the
data sets  increases.  The additional information allows  continual refinement of the permittee's
knowledge of the system.

       The example shown in Exhibit  5-13, involving bacteria sampling,  illustrates  the value of
correlating flow and concentration data.   Because  automated samplers are not appropriate for
collecting bacterial samples, manual grab  samples  were collected and analyzed for fecal coliform
bacteria. During a single storm event, samples were collected from Outfall 1  at 30 minute intervals,
beginning shortly after the storm started and ending with  sample #6 approximately 21A hours later.
Peak flow occurred within the first 90 minutes. The fecal coliform concentration peaked in the first
half hour and declined nearly one-hundredfold to the  last  sample, exhibiting a "first flush" pattern.
The average concentration was 3.14 x 10  MPN/100  ml. To calculate total fecal coliform loading,
flow measurements were multiplied by the corresponding grab sample concentrations at each half-
hour interval, as shown in the right-hand  column.  The average concentration was also multiplied
by  the  total flow  for comparative  purposes.    This  second  calculation (1.79 x  1014  MPN)
overestimates the total loading, primarily because it fails to correlate the decreasing bacteria level
to the changing flows.

       In many cases background conditions  or upstream wet weather sources, such as separate
storm sewer systems,  may provide  significant pollutant loads. Where possible, the permittee  should
try  to assess loadings from non-CSO sources in order to fully characterize the receiving water
impacts  from CSOs.  In some cases, these  other sources may be outside the  permittee's jurisdiction.
If the permittee cannot obtain existing monitoring  data on these sources, the permittee should
consider monitoring these sources or entering into  an agreement to have the  appropriate party
conduct the monitoring. The  data analysis techniques discussed in this section apply equally well
to other wet weather sources,  although the pollutant concentrations  in such sources may differ
significantly.
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              Exhibit 5-13. Fecal Coliform Data for Outfall 1-Example Storm
Sample
l
2
3
4
5
6
Fecal Coliform Concentration
(No./lOO ml)*
9.20 x 106
6.44 x 106
1.80 x 10"
8.90 x 105
4.20 x 105
l.OOxlO5

CSO Flow 30 Minute
Avg (cfs)
9.6
20.4
28.8
24.4
18.7
10.2
Total Load
Load for 30 Minute Interval**
(No. of Fecal Coliforms)
4.50xl013
6.70 x 1013
2.64 x 1013
l.lOx 1013
4.00 x 1012
5.20 x 1011
1.54 x 1014
Average Concentration
3.14 x 106
Total Flow
112.1
Estimated Total Load***
1.79 x 1014
* For CSOs, fecal coliform concentrations typically range from 2.0 x 105 - 1.1 x 106 colonies/100 ml (Metcalf &
Eddy, 1991).
** Load = [Concentration (No./lOO ml) x Total Flow (ml)] / 100 (since concentration is for 100 ml)
  Total Flow (in ml) = cfs x 1800 (# of seconds in one 30-minute interval) x 28,321 (# of ml in one cf)
*** Load estimated by multiplying the average bacteria concentration by the total flow
       Single composite samples or average data may be sufficient for a preliminary estimate of
pollutant  loadings  from  CSOs.   Establishing  an upper-bound estimate for such loads may  be
necessary  in order to analyze  short-term impacts based on short-term pollutant concentrations in the
receiving water and to develop estimates for rarer  events  that have not been measured. A statistical
distribution, such as normal  or lognormal, can be developed  for the data and mean values and
variations can be estimated.  These  concentrations can  be  multiplied  by measured flows or  an
assumed design flow to generate storm loads in order to predict rare or extreme impacts.  Chapters 8
and 9 discusses further how to predict receiving water impacts.
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5.5     SAMPLING AND DATA USE CASE STUDY

        The case study in Example 5-2 presents an approach for sampling and data analysis used by
Columbus, Georgia. The City found this approach useful  in assessing CSO control options.


                        Example 5-2. Sampling and Data Use Case Study
                                          Columbus, Georgia

  The City  of Columbus, Georgia, in a CSO technology demonstration project, found  significant correlation
  between the timing and volume of CSO pollutant loadings and the pre-storm dry weather conditions. These
  relationships can be used for:

           1.  quantifying annual and event loads to assess water quality impact,
          2.  developing  alternatives and evaluating treatment controls, and
          3.   operating the disinfection process.

  The Approach

  The approach involves conducting discrete sampling (for flow and water quality) and using these sampling
  results and historical rainfall data to establish annual load and design rate relationships (% of annual quantity vs.
  design flow for volume  and pollutants). The discrete sampling is timed to obtain more samples at the beginning
  of the storm event and fewer samples  as the event progresses (pollutant weighted sampling}. Using this
  sampling plan in Columbus has resulted in data that show a significant correlation between the cumulative
  volume and pollutant mass for different pre-storm conditions.

  Flow measurements can be correlated  with rain rate measurements to establish a rainfall/runoff relationship for
  the total event and rainfall intensity. These pollutant and runoff correlations are used with the historical rainfall
  data to quantify annual pollutant loads and to  define a relationship between design rate and annual quantity for
  control or treatment.

  Using the Data

  These relationships can be used to evaluate any  specific  control or various combinations  of controls and define
  annual  pollutant quantities for each control level. Types of controls include collection system maximization of
  flows and attenuation, storage, and direct treatment.

  The entire procedure  can be applied using simple spreadsheet methods or can be incorporated into more
  sophisticated  modeling  efforts.

  The methodology  can be used in either the presumption  or demonstration approaches.  In the presumption
  approach, where the objective is to treat the mass from 85% of the annual volume using primary clarification,
  the Columbus method can show that the objective can be reached with facilities at much smaller flow rates by
  applying  better treatment to the more  polluted, more frequent rainfall events.  The net result can be less costly to
  facilities.
     The specific approach used by Columbus, GA, may not be appropriate for all CSO communities.
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Chapter 5	CSS Monitoring


                 Example 5-2.  Sampling  and Data Use  Case Study (Continued)
   Cost-benefit levels of control can be determined from "knee-of-the curve" analyses using design rate
   relationships, and may represent different annual objectives for different pollutants to be reduced. For
   example:

           •   Treatment rate versus percent annual pollutant treated can be used to define the design storm
               criteria

           •   Treatment rate versus percent annual CSO volume treated can be used to define the level of high
               rate disinfection.

   Alternatively, different levels of control can be evaluated to estimate the end-of-pipe loads  and resulting in-
   stream concentrations for various flows. This provides a historical distribution of in-stream concentrations
   that can be compared to a waste load allocation to define statistical exceedances  in a wet weather permit.

   Finally, the evaluated treatment options can be compared using life-cycle costs and pollutant removal results,
   Fur chemical disinfection, the TSS loading relationship can be used in controlling the rate of disinfection.
   The disinfectant feed is varied according to the variation of incoming solids to accomplish  the disinfection
   objective while minimizing the potential for overdosing.
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                                      CHAPTER  6

                         RECEIVING WATER  MONITORING


       This chapter discusses techniques and equipment for receiving water monitoring, including

hydraulic, water quality,  sediment, and biological sampling procedures.  The techniques vary in
applicability and complexity, but all are generally applicable to CSO-impacted receiving waters.
In collecting and analyzing receiving water monitoring data, the permittee needs to implement a

quality assurance and quality control (QA/QC) program to ensure that accurate and reliable data are
used for CSO planning  decisions  (see  Section 4.8.1).  For  purposes  of the post-construction

compliance  monitoring program, all sampling  and analysis needs  to be done in accordance with EPA

regulations.


6.1    THE CSO CONTROL POLICY AND RECEIVING WATER MONITORING

       The CSO Control Policy  discusses characterization and  monitoring  of receiving water

impacts as  follows:
          In order to design a CSO controlplan adequate to meet the requirements of the CWA,
          a permittee should have a thorough understanding of its sewer system, the response of
          the system to various precipitation events, the characteristics of the overflows, and the
          water quality impacts that result from CSOs.

          The permittee should adequately characterize... the impacts of the CSOs on the receiving
          waters and their designated uses.  The permittee may need to consider information on
          the contribution and importance of other pollution sources in order to develop a final
          plan designed to meet water quality standards.

          The permittee  should  develop a comprehensive, representative monitoring program that
          ,,, assesses the impact of the CSOs on the receiving waters.  The monitoring program
          should include necessary CSO effluent and ambient in-stream monitoring and, where
          appropriate, other monitoring protocols such as biological assessment, toxicity testing
          and sediment sampling.  Monitoring parameters should include, for example, oxygen
          demanding pollutants, nutrients, toxic pollutants, sediment contaminants, pathogens,
          bacteriological indicators (e.g., Enterococcus, E. Coli), and toxicity. A representative
          sample of overflow points can be selected that is sufficient to allow characterization of
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Chapter 6                                                     Receiving Water Monitoring
           CSO discharges and their water quality impacts and to facilitate evaluation of control
          plan  alternatives.  (Section II.C.I)
       As discussed in Chapter 2, the CSO Policy intends for the permittee to use either the
presumption approach or the demonstration approach in identifying controls that will provide for

attainment of water quality standards (WQS). Under the demonstration approach, the permittee

demonstrates the adequacy of its  proposed CSO  control  program to attain  WQS. Generally,
permittees selecting the demonstration approach will need to monitor receiving waters to show that

their control programs are adequate.


       The presumption  approach is  so named because it is based on the presumption that WQS will
be attained when certain performance-based criteria identified in the CSO Policy are achieved, as

shown by the permittee in its long-term control plan (LTCP). The regulatory agency is likely to
request some  validation of the presumption,  such  as receiving water quality sampling or end-of-pipe

sampling of overflows combined with flow information and dilution calculations. Chapters 7 (CSS

Modeling) and 8 (Receiving Water Modeling) discuss the different modeling considerations related
to the demonstration and presumption approaches.


6.2     RECEIVING WATER HYDRAULIC MONITORING

       When a CSO enters a receiving water body, it is subject to fate and transport processes that
modify pollutant concentrations in  the receiving water  body. The impact of CSOs on receiving
waters  is largely  determined by the hydraulics of the receiving water body and the relative magnitude
of the  CSO loading. Assessing receiving water hydraulics is an important first step in a receiving

water  study,  since  an  understanding of how CSOs are transported  and diluted  is  essential to

characterizing their impacts on  receiving  waters.  Awareness  of large-scale and small-scale
hydrodynamics can help the permittee determine where to sample in the receiving water for the

effects  of CSOs. Large-scale water  movement  largely determines  the  overall  transport and
transformation of pollutants. Small-scale hydraulics, such as water movement near a discharge point

(often  called near-field), determine  the initial dilution and mixing of the discharge, For example,
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Chapter 6                                                       Receiving Water Monitoring







a discharge into a wide, fast-flowing river might not mix across the river for a long distance since



it will quickly be transported downstream.







6.2.1  Hydraulic Monitoring





       Hydraulic monitoring  involves measuring the depth  and velocity of the  receiving water  body



and its other physical characteristics (e.g., elevation, bathymetry,  cross section) in order to assess



transport and dilution characteristics. This may include temporary or permanent installation of gages



to determine depth and velocity  variations during wet weather events. In all cases, the permittee will



need to use existing mapping information or perform a new survey of the physical characteristics



of the receiving  water in order to  interpret the hydraulic  data  and understand the hydraulic dynamics



of the receiving water. (Section 4.5 discusses receiving water sampling designs and the selection



of monitoring locations.)







       Identifying a  suitable hydraulic monitoring method depends  largely  on the type  and



characteristics of receiving water.







       Rivers and Streams




       In rivers and streams, flow rate is generally a factor of the depth, width, cross-sectional area,



and  hydraulic geometry of  the river or stream channel.   Flow in rivers and streams is usually



determined by measuring the stage (elevation of water above a certain base level) and relating stage



to discharge with a rating  curve. This relationship is developed by measuring flow velocity in the



stream or river at different stages, and using velocity and the area of the stream or river channel to



determine the total discharge  for each stage (Bedient and Huber, 1992).  For large rivers and streams,



long-term  flow and geometry data are often available for specific gaging stations from USGS and



the U.S. Army Corps of Engineers.
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Chapter 6                                                       Receiving Water Monitoring


       For a CSO outfall located near a USGS gage, the monitoring team can use the nearest USGS
gage watershed areas to estimate flow at the discharge site.  Flow information may also be available
from stage measurements at bridge crossings and dams, and from studies performed by other State

and  Federal agencies.  In the absence of such flow data, the permittee may need to  install stage
indicators or use current meters to collect  stream flow measurements.  Some  of the CSO flow

monitoring devices described in Exhibit 5-5 of Chapter 5 also may be used to measure  open channel

flow in rivers and streams. The USGS (1982) and USDI (1984) have published detailed manuals

on stream gaging and measurement techniques.


       Estuaries

       Estuaries connect rivers and oceans and thus represent a complex system of tides, salinity,

and  upstream  drainage. Tidal variations and density effects from the varying levels of salinity need

to be defined to determine how pollutants from CSOs are transported.


       Tidal variations affect estuarine circulation patterns which,  along with salinity patterns,

determine how pollutant loadings entering the estuary are dispersed. Based on velocity and salinity
patterns, estuaries can be classified as one of the following types:
           Stratified estuaries have a large fresh water inflow over a more dense salt water layer.
           Tidal currents are not sufficient to mix the separate layers. Transport of pollutants is
           largely dependent on the difference in the densities of the pollutants and the receiving
           water.

           Well-mixed estuaries have a tidal  flow much greater than the river outflow,  with mixing
           and flow reversal sufficient to create a well-mixed water column  at all depths. Pollutants
           tend to move with the motion of the tides and are slowly carried seaward.

           Partially-mixed estuaries have flow and stratification characteristics between the other
           two types and have tide-related flows much greater than river flows. Pollutant transport
           depends somewhat on density, but also involves significant vertical mixing.
         For example, the 5,000-square mile Merrimack River watershed in New Hampshire and Massachusetts
has 46 USGS gages that monitor most of the larger tributaries and the main stem in several locations. Using flow
data from the one or two gages closest to a CSO outfall, flow at the outfall can be estimated based on the relative
watershed areas between the gages and the outfall.


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Chapter 6                                                     Receiving Water Monitoring







Classification  depends on  the river outflow. Rivers with  large flows generally lead to more stratified



estuaries (U.S. EPA, 1985b).







       Tidal height data and current predictions,  published  annually  by NOAA,  may provide



sufficient information, or it may be necessary to install a new tide gage (stage monitor) to develop



data closer to the CSO-impacted area. Due to the variation of tides and winds, estuarine and coastal



currents often change  rapidly.   It is necessary, therefore,  to  measure tides  and  currents



simultaneously using continuously recording depth and velocity meters. Tidal currents can be



measured with meters similar to those used for measurement of river currents, but the direction of



the currents must also be recorded. Information on monitoring  methods for such areas may also be



found in USGS (1982) and USDI (1984).







       Lakes




       The hydraulic characteristics of lakes  depend on several factors, including the  depth, length,



width,  surface area, volume, basin material,  surrounding ground cover, typical wind  patterns, and



surface inflows  (including CSOs) and outflows. Lakes tend to have relatively low  flow-through



velocities and significant vertical temperature gradients,  and thus are usually  not well-mixed



(Thomann and Mueller, 1987). To determine how quickly pollutants are likely to be removed from



a lake, it is necessary to define the flushing rate. The flushing rate depends on water inputs (inflows



and  precipitation) and outputs (outflows,  evaporation, transpiration, and  withdrawal),  pollutants and



their characteristics, and the degree of mixing in the lake. Mixing in lakes is primarily due to wind,



temperature changes, and atmospheric pressure.







       Analysis of pollutant fate and transport in lakes is often complex and generally requires the



use  of detailed  simulation models.  (Some  less-complex  analysis can  be done, however, when



simplifying  assumptions, such as complete  mixing in the lake, are  made.) Pollutant  fate and



transport analysis requires definition of parameters such as lake volume, surface area, mean depth,



and mean outflow and inflow rates. Analytical and  modeling methods for lakes  and  the  data
                                             6-5                                January 1999

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Chapter 6                                                        Receiving Water Monitoring


necessary to use the methods are discussed in greater detail in Section 8.3.2 and in Thomann and

Mueller (1987) and Viessman, et al. (1977).
6.2.2       Analysis of Hydraulic Data

       Receiving water hydraulic data can be analyzed to characterize the relationship between

depth, velocity, and flow in the receiving water. This analysis may involve:


           Developing stage-discharge,  area-depth,  or volume-depth  curves  for  specific monitoring
           locations, using measured velocities to calibrate the stage-discharge relationship

           Pre-processing the data for input into hydraulic models

           Plotting and review of the hydraulic data

           Evaluating the data to define hydraulic characteristics, such as initial dilution, mixing,
           travel time, and residence time.

       Plotting programs such as spreadsheets and  graphics programs are useful for presenting

hydraulic data. A data base and a plotting and statistical analysis package will typically be necessary

to analyze the data and generate such information as:


           Plots of depth, velocity, and flow vs. time

           Plots of depth, velocity, and flow vs. distance from the outfall

           Frequency distributions of velocities and  flows

           Vector components of velocities and flows

           Means, standard deviations, and other important statistical measures for depth, velocity,
           and flow data.
         Stage-discharge curves, also referred to as rating curves, are plots of water level (stage) vs. discharge.
Development of these curves is discussed in Bedient and Huber (1992). USGS (1982) and USDI (1984)) discuss
methods for developing hydraulic curves for various types of flow monitoring stations.


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Chapter 6                                                      Receiving Water Monitoring







       As presented later in Chapter 8, receiving water models need physical system and hydraulic



data as input. Processing of input data is specific to each model. In general, however, the physical



characteristics of the receiving water (slopes, locations, and temperatures) are used to develop the



model computational grid.  The  measured hydraulic data (depths,  velocities, and flows)  are



compared with model calculations  in order to validate the model.







6.3    RECEIVING WATER QUALITY MONITORING




       Collection and analysis of receiving water quality data is necessary when available data are



not sufficient to describe water quality impacts from  CSOs. This section discusses  some approaches



for conducting receiving water sampling and for analyzing the  collected samples. (Chapters 3  and



4  discuss how to identify sampling locations, sampling parameters,  and  sampling frequency. Section



6.4 discusses biological and sediment sampling and  analysis.)







6.3.1       Water Quality Monitoring




       Receiving water monitoring  involves many techniques similar to  CSS  monitoring (see



Section  5.4.1) and many of  the same decisions, such as whether to collect grab or composite samples



and whether to use manual  or automated methods. Receiving water quality monitoring involves the



parameters  discussed in Section  4.5.3 as well  as field  measurement  of parameters such as



temperature and conductivity.







        Sample Program Organization





        Sampling receiving waters, especially large  water bodies,  requires careful planning and a



sizable  resource commitment. For  example, a dye study of a large river requires careful planning



regarding travel time, placement of sampling crews, points of access,  safety concerns, and  use of



boats.  Sampling during wet weather events is typically more complicated than sampling during dry



weather, since it often requires rapid  mobilization of several sampling teams  on short notice,



sampling throughout the night, and sampling in rainy conditions with higher-than-normal flows in



the receiving water  body. Time of travel between the various sampling stations may necessitate the



use of additional crews when sample collection must occur at predetermined times.






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Chapter 6                                                      Receiving Water Monitoring


       Wet weather sampling requires specific and accurate weather information. Local offices of
the American Meteorological  Society can provide  a list  of Certified Consulting Meteorologists  who
can provide  forecasting services specific to the needs of a sampling program. Many weather services

also have Internet sites that provide real-time radar updates across the U.S. Radar coverage can also
be arranged in some areas for real-time observation of rainfall conditions. These efforts represent

an additional cost to the program, but they can be invaluable for planning wet weather surveys and

can result in significant savings in costs associated with false starts and unnecessary  laboratory
charges.  Section 4.6  discusses a  strategy  for determining whether to  initiate monitoring for a

particular wet weather event.


       The rainfall,  darkness, and  cold temperatures  that often accompany wet weather field

investigations can make even small tasks difficult and sometimes unsafe.  Contingency planning and
extensive preparation can, however, minimize mishaps and  help  ensure safety. Prior to field
sampling, the permittee should ensure that:
           Sampling personnel are well trained and familiar with their responsibilities, as defined
           in the sampling plan

           Personnel use appropriate safety procedures and equipment

         A  health  and safety plan identifies the necessary emergency  procedures, safety
           equipment, and nearby rescue organizations and emergency medical services

           Sample containers are assembled and bottle labels are filled out to the extent possible

           All necessary equipment  is inventoried, field monitoring equipment is calibrated and
           tested, and equipment such as boats, motors, automobiles, and batteries are checked
           Boat crews are used when landside and bridge sampling are infeasible or unsafe.
        Sample Preparation and Handling

        As discussed in Section 5.4.1, sample collection, preparation and handling, preservation, and

storage  should  minimize  changes  in the condition of sample constituents. The standard procedures
for collecting, preserving, and storing receiving water samples are the same as those for combined


                                             6-8                                 January 1999

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Chapter 6                                                         Receiving Water Monitoring


sewage samples and are described in 40 CFR Part 136.  Procedures for cleaning sample bottles,

preserving water quality samples, and analyzing for appropriate chemicals are detailed in various

methods manuals,  including APHA  (1992) and  U.S. EPA (1979). NOAA's Status and Trends

Program also provides information on standard protocols for sampling and analysis. Collection and

analytical methods  depend on the constituents in  the  receiving water (e.g.,  salinity,  suspended

sediments, ionic strength) as well  as the required precision  and accuracy.  Samples should be labeled

with unique identifying information and should have chain-of-custody  forms documenting  the

changes of possession between time of collection and time of analysis (discussed in Section 5.4.1).

The use of sample  bar  code labels and  recorders can be  particularly helpful  during wet weather

sampling when paper records are often infeasible.


6.3.2      Analysis of  Water Quality Data

       As was the case for hydraulic data, water quality data for receiving waters  are analyzed by

plotting and reviewing the raw data to define water quality  characteristics  and  by processing the data

for input to  water quality models.  Data can be analyzed  and  displayed using spreadsheets, databases,

graphics  software,  and  statistical packages, such as  Statistical Analysis  Software  (SAS) and

Statistical Package for Social Sciences (SPSS).


        Simple receiving water analyses  could include:
           Comparing receiving water quality with applicable water quality criteria to determine
           whether criteria are being exceeded

           Comparing sampling results from before, during, and after a wet weather event to assess
           whether water quality problems are attributable to CSOs and other wet weather events
         Use of these statistical packages generally requires solid statistical capabilities, so they should be used
cautiously by someone who is not experienced in statistical data evaluations and survey design. For information on
SAS, contact: SAS Institute, Inc., SAS Campus Drive, Gary, NC 27513 or (919) 677-8000. For information on
SPSS, contact: SPSS Incorporated, 444 N. Michigan Avenue, Chicago, IL 60611 or (800) 543-2185.

       4 An alternative approach is to stratify the analysis by those samples that time travel analyses (e.g.,
Lagrangian analysis) indicate are impacted by CSO discharges. Many instream samples collected during a wet
weather event represent times either before or after the CSO "slug" has passed.


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Chapter 6                                                      Receiving Water Monitoring


           Comparing data from downstream of CSOs to data collected upstream of CSOs (or to a
           reference point) to distinguish CSO impacts.

       Since CSO controls  must ultimately provide for attainment of WQS,  receiving water analyses
should be tailored to the applicable WQS. If the WQS for a pollutant contain numeric criteria

specifying  frequency, magnitude,  and duration, receiving water analyses should use the same

frequency,  magnitude, and  duration (see Sections 4.5, 9.1, and 9.2 for additional discussion.)


       Water quality data are also used to calibrate receiving water models (see Chapter 8). This

is  generally  facilitated  by plotting the data vs.  time  and/or distance to compare  with  model

simulations.  Special studies may be required to  determine rate constants, such as  decay rates,
bacteria die-off rates, or suspended solids settling rates, if these values are used in the model.


6.4    RECEIVING WATER SEDIMENT AND  BIOLOGICAL MONITORING

       It is often difficult and expensive to identify  CSO impacts during wet weather using only
hydraulic and water quality sampling.  Sediment and biological monitoring may be cost-effective

supplements or, in  limited cases,  alternatives to water  quality sampling.  Since sediment  and

biological monitoring do not address public health risks, they can only be used as alternatives when

bacterial contamination is not a CSO concern.  The following  sections discuss sediment  and

biological sampling techniques  and data analysis.


6.4.1   Sediment Sampling Techniques

       Sediments  are sinks for a wide variety  of materials. Nutrients,  metals,  and organic
compounds bind to suspended solids and settle to the  bottom of a water body when flow velocity is
insufficient to keep them in suspension. Once re-suspended through flood scouring, bioturbation,

desorption,  or biological  uptake,  free contaminants can dissolve in the water column, enter sediment-
dwelling organisms, or accumulate  or  concentrate in fish  and other aquatic organisms  and
subsequently be ingested by humans and other terrestrial animals.
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Chapter 6                                                      Receiving Water Monitoring


       Typically, CSOs contain suspended material that can settle out in slower-moving sections

of receiving waters. Sediments can release  accumulated contaminants  for years after overflows  have

been eliminated.


       Sediment samples are collected using hand or winch-operated dredges as follows:


           The sampling device is lowered through the water column by a hand line or a winch

           The device is then activated either by the attached line or by a weighted messenger sent
           down the line

           The scoops or jaws of the device close either by weight or spring action

           The device is brought back to the surface.
       Ideally,  dredging should disturb the  bottom as little as possible and collect all fine particles.
       In cases where sediments are physically amendable to coring, core samples can be collected

to determine how pollutant types,  concentrations, and accumulation rates have varied over time.


       To avoid sample  contamination, sediments should be removed from the dredge or core

sampler by scraping back  layers  in contact with the  device and extracting sediments from  the central

mass of the sample. In many cases the upper-most layer of sediment will be the most contaminated

and, therefore, of most interest. Sediment samples for toxicological  and chemical  examination

should be collected  following Method E 1391  detailed  in Standard Guide for  Conducting Sediment

Toxicity Tests with Freshwater Invertebrates  (ASTM, 1991).
         Commonly used sediment samplers include the Ponar, Eckman, Peterson, Orange-peel, and Van Veen
dredges.  Macroinvertebrate Field and Laboratory Methods for Evaluating the Biological Integrity of Surface
Waters (Klemm, 1990) has detailed descriptions of such devices.


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Chapter 6                                                     Receiving Water Monitoring







6.4.2   Analysis of Sediment Data




       CSO  investigations  will benefit from  analysis of a  range  of sediment  characteristics,



including  physical characteristics (grain size distribution, type of sediment), chemical composition



(toxics, metals, total solids), and benthic makeup (discussed in Section 6.4.3). Sediment sampling



locations for CSO investigations should include the depositional zone below the outfall. The same



sediment characteristics should  also be evaluated  in sediments  from upstream reference stations and



sediments from non-CSO sources to facilitate comparison with sediments near the CSO outfall. In



comparing the chemical composition and biological communities of multiple sites, it is important



to select sites that have similar physical characteristics.







       Sediment data are typically analyzed by developing grain size distributions and plotting



concentrations of chemicals vs.  distance. If the area of interest is two-dimensional horizontally,



isopleths can be plotted showing contours of constant concentration from the CSO outfall. If vertical



variations from core samples are available, concentration  contours can also be plotted vs.  depth.



Sediment  chemistry data may be statistically analyzed to compare areas that are affected by  CSOs,



non-CSO  sources, and unaffected (background) areas. These analyses can give a longer-term view



of CSO impacts than water quality monitoring.







       Additional information on sediment monitoring is  available in  EPA's Guidance for Sampling



of and Analyzingfor Organic Contaminants in Sediments (U.S. EPA, 1987).







6.4.3  Biological  Sampling Techniques




       Evaluation  of  aquatic organisms is another way to obtain information  on cumulative impacts



of CSOs,  since resident communities of  aquatic  organisms integrate over time  all the environmental



changes that affect them. Biological sampling  should be accompanied by habitat assessment since



it is necessary to separate out the effects of habitat condition when determining the presence and



degree of biological impairment due to  CSOs. It may be difficult to trace any impacts to CSOs



unless there are no other significant pollutant sources present. Biological sampling results may be
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Chapter 6                                                        Receiving Water Monitoring


more useful for determining the overall impacts from all pollution sources on the biological health

of the receiving water.


       Collection and Handling of Biological Samples

       This  section  describes  collection techniques  for fish,  phytoplankton, zooplankton,  and

benthic macroinvertebrates. Additional information on sampling methods for these species, as well

as for riparian and aquatic macrophytes, is in Exhibit 6-1.


       Fish. Although other aquatic organisms may be more sensitive to pollutants, fish generate

the greatest  public  concern.   Observable  adverse effects  from  pollutants include  declines  of

populations and tumor growth on individuals. Fish monitoring programs can identify the relative

and absolute numbers of individuals of each species; the size distributions within species; growth

rates;  reproduction or recruitment success; the incidence of disease,  parasitism, and tumors; changes

in behavior; and the bioaccumulation of toxic constituents.


       Common  fish sampling methods  include  angling, seines,  gill and trap  nets,  and

electrofishing.  The references in Exhibit 6-1 provide guidance on methods used  for collection,

measurement, preservation, and analysis  offish samples.'


       Phytoplankton. Phytoplankton are free-floating,  one-celled algae.  They  are useful  in

monitoring receiving water quality because many species are highly sensitive to specific chemicals.

Because phytoplankton have relatively  rapid  rates  of growth  and population turnover (approximately

3 to 5 days during the summer season), only short-term CSO impacts can be analyzed. Laboratory

analyses can provide information on the  abundance of each taxon, the presence of,  or changes in,

populations of indicator species, and the total biomass of phytoplankton present.  Lowe (1974) and
         Two reference works published by the American Fisheries Society are especially informative. Fisheries
Techniques (Nielsen and Johnson, 1983) focuses mainly on field work considerations, discussing most of the
sampling techniques currently practiced. The companion volume, Methods for Fish Biology (Schreck and Moyle,
1990) focuses primarily on methods used to analyze and assess collected fish samples. It includes material on fish
growth, stress and acclimation, reproduction, behavior, population ecology, and community ecology.


                                             6-13                                January 1999

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Chapter 6
Receiving Water Monitoring
                Exhibit 6-1. Overview of Field Biological Sampling Methods
Sample Parameter
Fish
Limitations:
Phytoplankton
Algae)
Limitations:
Zooplankton
Limitations:
Benthic invertebrates
Limitations:
Riparian and aquatic
macrophytes
Limitations:
Information Gained
• Community
structure
• Distributions (depth
& basin wide)
• Biomass
• Density
• Bioconcentration
• Fecundity
Method of Collection
• Electroshocking
• Seines
. Gill nets
• Trawls
• Angling
• Traps
References ?
APHA, 1992; ASTM, 1991;
Everhart et al, 1975; Nielsen and
Johnson, 1983; Plafkin et al., 1989;
Schreck and Moyle, 1990; Ricker,
1 975; Weber etal, 1989
Each method is biased to some degree as to the kind and size of fish collected. Some methods are
designed for use in relatively shallow water.
• Chlorophyll a
• Community
structure
• Primary
productivity
• Biomass
• Density
• Plankton buckets attached to
vertical or horizontal tow net
(e.g., Wisconsin style net)
• Discreet depth samples using
VanDorn or Kemmer bottles
• Periphytometer
American Public Health
Association-(APHA), 1992;
American Society for Testing and
Materials-(ASTM), 1991; Lind,
1985; Vollenweider, 1969;
Weber etal., 1989;
Wetzel and Likens, 1979
Small organisms can pass through the net, and periphytometers can only be used for algae that
attach to a substrate.
• Community
structure
• Distributions
• Biomass
• Sensitivity
• Density
• Plankton buckets attached to
vertical or horizontal tow net
(e.g., Wisconsin style net)
• Discreet depth samples using
VanDorn or Kemmer bottles
APHA, 1992; ASTM, 1991; Lind,
1985; Pennak, 1989; Weber et al.,
1989; Wetzel and Likens, 1979
Small organisms can pass through the net; some zooplankton migrate vertically in the water
column, so it is possible to miss some species.
• Community
structure
• Biomass
• Density
• Distributions
• Tissue analysis
• Ponar grab sampler
• Eckman dredge sampler
• Surber sampler
• Hess sampler
• Kick net or D-ring net
• Artificial substrates
APHA, 1992; ASTM, 1991; Lind,
1985; Memtt and Cummins, 1984;
Pennak, 1989; Weber et al., 1989;
Klemm, 1990; Wetzel and Likens,
1979; Plafkin etal., 1989
Some methods are time-consuming and labor-intensive; some methods can only be used in shallow
waters.
• Community
structure
• Distributions (depth
& basin wide)
• Biomass
• Density
• Tissue analysis
• Usually qualitative visual
assessments
• Quantitative assessments
using quadrant or line point
methods
APHA, 1992; ASTM, 1991; Dennis
and Isom, 1984; Vollenweider,
1969; Weber et al., 1989; Wetzel
and Likens, 1979; Plafkin et al.,
1989
Limited to the growing season for many species.
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               January 1999

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Chapter 6                                                        Receiving Water Monitoring


VanLandingham (1982)  provide useful guides to the environmental requirements and pollution

tolerances of diatoms and blue-green algae, respectively.


       Zooplankton. Zooplankton  are free-floating  aquatic  protozoa and  small animals. Many

species are sensitive indicators of pollution. Particularly in lakes and reservoirs,  zooplankton can

provide information on the presence of specific toxics.  Zooplankton are often collected by towing

a plankton net through a measured or estimated volume of water.  To calculate population density

it is necessary to determine the volume of the sampling area, using a flow meter set in the mouth of

the net or calculations based on the area of the net opening  and the distance towed.  Laboratory

analyses can provide information similar to that for phytoplankton.


       Benthic Macroinvertebrates. Benthic macroinvertebrates are organisms such as plecoptera

(stoneflies), ephemeroptera (mayflies), and trichoptera (caddisflies) that live in and on sediments.

Like  plankton,  benthic  macroinvertebrates include useful  indicator species  that can provide valuable

information about the degree of organic enrichment, local dissolved oxygen conditions, and the

presence and nature of toxics in the  sediments of lakes and reservoirs.


       Monitoring  teams generally use  dredges, artificial  substrates, and  kicknets  to  sample benthic

macroinvertebrates,  depending  on  the bottom substrate  and  water depth. Samples are either

preserved in their entirety in polyethylene bags or other suitable containers or are washed through

a fine sieve and then preserved in a suitable container (Klemm, 1990). The sample can  be analyzed

for taxa present, the total density of each taxon,  relative abundance by numbers or biomass of these

taxa,  changes in  major and indicator species populations, and the  total biomass  of  benthic

macroinvertebrates present.
         Three manuals (U.S. EPA, 1983b, 1984a, 1984b) discuss the interpretation of biological monitoring data
for larger bottom-living invertebrates. The Rapid Bioassessment Protocols (Plafkin et al,  1989) manual discusses
the use of fish and macroinvertebrates as a screening method in assessing environmental integrity.
Macroinvertebrate Field and Laboratory Methods for Evaluating the Biological Integrity of Surface Waters
(Klemm,  1990) discusses analysis of qualitative and quantitative data,  community metrics  and pollution indicators,
pollution  tolerance of selected macroinvertebrates,  and Hilsenhoffs family-level pollution  tolerance values for
aquatic arthropods.


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Chapter 6                                                        Receiving Water Monitoring







6.4.4  Analysis of Biological Data




       Community structure  can  be described in terms of species  diversity, richness,  and evenness.



Diversity is affected  by colonization rates, the presence  of suitable  habitats,  extinction rates,



competition, predation, physical disturbance, pollution, and other factors (Crowder,  1990).







       A qualitative data assessment can help determine which factors have caused  measured



variation  in species  diversity.   In such an  assessment, the collected  species and their  relative



population sizes are compared with their known sensitivities to contaminants present.  The tendency



of species to be abundant, present,  or  absent relative to their tolerances or sensitivities to sediments,



temperature regimes, or various chemical pollutants can indicate the most likely cause of variation



in species diversity at the sampled sites.







       Two cautions should be noted regarding qualitative analysis. First, different strains of the



same species can sometimes  have differing  sensitivities to a  stressor, particularly where species  have



undergone  extensive  hatchery breeding programs.   Second, because  listed  characteristics of



organisms can vary from region to region, it is important when using lists of indicator species to note



whether the data were collected in  the same region  as the CSO study.  Investigators should generally



limit the use of diversity  indices as  general  indicators  of environmental  effects to comparisons within



the study  where sampling  and sample analysis  methods are  consistent.  Before  conducting  a



biological assessment,  investigators  should contact  local  authorities  to  determine whether biological



reference data can be obtained to use in the CSO study. Data should be from  biological reference



sites that have similar physical characteristics (e.g., comparable habitat).







       Rapid Bioassessment Protocols




       Rapid  biological assessments, using techniques such  as rapid bioassessment protocols



(RBPs),  are a valuable and cost-effective approach to evaluating the status of aquatic  systems



(Plafkin et  al., 1989). RBPs integrate information on biological communities  with information on



physical and chemical characteristics of aquatic habitats.
                                              6-16                                January 1999

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Chapter 6                                                       Receiving Water Monitoring

RBPs have been used successfully to:

           Evaluate whether a stream supports designated aquatic life uses
           Characterize the existence and severity of use impairments
           Identify sources and causes of any use impairments
           Evaluate the effectiveness of control actions
           Support use attainability analyses
           Characterize regional biotic components within ecosystems.

       Typically, RBPs  provide integrated evaluations  that compare habitat and biological measures
for studied  systems to empirically-defined reference conditions  (Plafkin et al,  1989). Reference
conditions are defined based  on data from systematic monitoring of either a site-specific control
station or several comparable sites in  the same region.  A site-specific control is  generally  considered
to be representative of the "best attainable" conditions  for a particular  waterbody.  When using data
from several regional sites, the sites are selected to represent the natural range of variation in "least
disturbed" water chemistry, physical habitat, and  biological conditions.  A percent similarity is
computed for each biological, chemical, or physical parameter measured at the study sites relative
to the conditions found at the reference site(s). These percentages may be computed based on the
total number of taxa found, dissolved oxygen saturation, or the embeddedness of bottom material.

       Generally, where  the computed percent similarity  is greater than 75-80 percent of the
corresponding reference condition (depending on the parameter compared), the results can indicate
that conditions at the study sites are  sufficiently similar to those  occurring at the reference site(s).
For such cases it is reasonable to conclude that the study sites' conditions are "non-impaired." In
contrast,  where the computed percent similarity of conditions at the study sites is  less than  50 percent
of the reference conditions (depending on the  parameter compared), it  is reasonable to conclude  that
conditions at those study sites are "severely  impaired," relative to the reference  site(s).  For those
sites with a percent similarity falling between these ranges, the results can indicate that conditions
at the study sites are "moderately impaired" (Plafkin  et al.,  1989).   An application of the use of RBPs
                                             6-17                                January 1999

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Chapter 6                                                    Receiving Water Monitoring







in two case studies is presented in Combined Sewer Overflows and the Multimetric Evaluation of



Their Biological Effects:  Case Studies in Ohio and New York (U.S. EPA, 1996).
                                          6-18                              January  1999

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                                      CHAPTER 7

                                    CSS MODELING


       This  chapter discusses the use of modeling to  characterize the combined sewer system (CSS)

and  evaluate CSO control  alternatives.   It  discusses different approaches  to  identifying  the
appropriate level  of modeling, based  on site-specific considerations, and describes the various types

of available  models. Because of the site-specific nature of CSSs, the varying information needs of

municipalities, and the numerous available models,  it does not recommend a specific model or
modeling approach.


7.1    THE CSO CONTROL POLICY AND CSS MODELING

       The  CSO Control Policy refers to  modeling as a tool for characterizing  a CSS and  the

impacts  of CSOs on receiving waters.  Although not every CSS needs to be analyzed using complex

computer models, EPA anticipates  that most permittees will need to  perform some degree of

modeling to support CSO control decisions.


       The CSO Control Policy describes the  use of modeling as follows:
       Modeling - Modeling  of a sewer system  is recognized as a valuable tool for predicting sewer
       system response to various wet weather events and assessing water quality impacts when
       evaluating different  control strategies and alternatives.  EPA supports  the proper and
       effective  use of models, where appropriate, in the evaluation of the nine minimum controls
       and the development  of the long-term CSO controlplan. It is also recognized that there are
       many models which may be used to do this.  These models range from simple to complex.
       Having decided to use a model,  the permittee should base its choice of a model on the
       characteristics of its sewer system,  the number and location of overflow  points, and the
       sensitivity of the receiving water body to the CSO discharges...  The sophistication of the
       model should relate to the complexity  of the system to be modeled and to  the information
       needs associated with evaluation  of CSO control options  and water quality impacts.
       (Section  Il.C.l.d)


       The Policy also states that:
       The permittee should adequately characterize through monitoring, modeling, and other
       means as appropriate, for a range of storm events, the response of its sewer system to wet

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Chapter 7                                                                   CSS Modeling
       weather events including  the  number,  location and frequency of CSOs, volume,
       concentration and mass of pollutants discharged and the impacts of the CSOs on the
       receiving waters and their designated uses. (Section II.C.I)
       Finally, the CSO Control Policy also states:
       EPA believes that continuous simulation models, using historical rainfall data, may be the
       best way to model sewer systems, CSOs, and their impacts.  Because of the iterative nature
       of modeling sewer systems, CSOs, and their impacts, monitoring and modeling efforts are
       complementary and should be coordinated.  (Section II.C. 1. d)
       The CSO  Policy supports continuous simulation modeling (use of long-term rainfall records
rather than records for individual storms) for several reasons. Long-term continuous rainfall records

enable simulations  to be based on a sequence of storms so that the additive effect of storms occurring
close together can be examined. They also enable  storms  with a range of characteristics to be
included.   When a municipality  uses the  presumption approach, long-term  simulations are
appropriate because the performance criteria are based on long-term averages, which are not readily
determined from design storm simulations. Continuous simulations do not require highly complex

models.   Models that simulate runoff without  complex  simulation of sewer  hydraulics (e.g.,
STORM,  SWMM RUNOFF) may be appropriate where the  basic hydraulics of the system are

simple  or  have been analyzed using  a more complex model.  In  the second case, the results  from the
more complex model can be used  to enable  proper characterization of system hydraulics  in the

simple model.


       Running a model in both continuous mode and single  event mode can be useful for some
systems.  When only long-term hourly rainfall  data are available, it may be desirable to calibrate the
model using more refined single event rainfall data before running the model  in continuous mode.

For instance, if a CSS is extremely responsive  to  brief periods of high-intensity  rainfall, this may not
be adequately depicted using hourly rainfall data.
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Chapter 7	CSS Modeling


       The CSO  Control Policy also states that  after instituting the nine minimum controls (NMC),

the permittee should assess their effectiveness and should
       submit any information or data on the degree to which the nine minimum controls achieve
       compliance with water quality standards  (WQS). These data and information should include
       results made available through monitoring and modeling activities done in conjunction with
       the development of the long-term CSO control plan described in this Policy.  (Section II.B)

       The purpose of the system characterization, monitoring and modeling program initially is
       to assist the permittee in developing appropriate measures to implement the nine minimum
       controls and, if necessary,  to support development of the long-term CSO  control plan.  The
       monitoring and modeling data also will be used to  evaluate  the expected effectiveness of both
       the nine minimum controls, and, if necessary, the long-term CSO controls, to meet WQS.
       (Section II.C.I)
       The long-term control plan (LTCP)  should be based on more detailed knowledge of the CSS
and its receiving waters than is necessary to implement the NMC. The LTCP should consider a
reasonable range of alternatives, including various levels of controls. Hydraulic modeling may be
necessary to predict how a CSS will respond to various control scenarios.  A computerized model
may be  necessary for a  complex  CSS, especially  one with looped  networks or sections  that
surcharge. In simpler  systems, however, basic equations (e.g., Hazen-Williams or Manning equation

- see  Section 5.3.1) and spreadsheet  programs  can be  used to compute hydraulic profiles and predict
the hydraulic effects  of different control measures. (Verification using monitoring data becomes

more important in these latter situations.)


       Finally, modeling can support either the presumption or demonstration approaches of the

CSO  Control Policy.  The demonstration approach requires demonstration that a proposed LTCP

is adequate  to meet  CWA requirements.  Meeting this requirement can necessitate detailed CSS
modeling as an input to receiving  water  impact analyses.  On the other hand, the presumption

approach involves performance-based limits on the number or volumes of CSOs. This approach
may require less modeling of  receiving water impacts, but is acceptable  only if "thepermitting
authority determines that such presumption is reasonable in light of the data and analysis  conducted
in the characterization, monitoring, and modeling of the system and the consideration of sensitive


                                            7-3                                January 1999

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Chapter 7	CSS Modeling


areas .  . . ." (Section II.CAa) Therefore, the presumption approach does not eliminate the need to
consider receiving water impacts.
7.2    MODEL SELECTION STRATEGY

       This section discusses how to select a CSS model. Generally, the permittee should use the

simplest model that meets the objectives of the modeling effort. Although complex models usually

provide greater precision than simpler models, they also require greater expense and effort. This
section does not describe all of the available CSS-related models,  since other documents provide  this
information (see Shoemaker et al, 1992; Donigian and Huber, 1991; WPCF, 1989).


       CSS modeling involves hydrology, hydraulics, and water quality:
          Hydrology is the key factor in determining runoff in CSS drainage basins. Hydrologic
          modeling is generally done using runoff models to estimate flows influent to the sewer
          system. These models provide input data for hydraulic modeling of the CSS.
          CSS hydraulic modeling predicts the pipe flow characteristics  in the CSS. These
          characteristics include the different flow rate  components (sanitary, infiltration, inflow,
          and runoff), the flow velocity and depth in the interceptors, and the CSO flow rate and
          duration.

          CSS water quality modeling consists of predicting the pollutant characteristics of the
          combined sewage in the system, particularly at CSO outfalls and at the treatment plant.
          CSS water quality is measured in  terms of bacterial counts and concentrations of
          important  constituents  such as BOD,  suspended solids, nutrients,  and toxic  contaminants.
       Since hydraulic models are usually used together with a runoff model or have a built-in

runoff component, runoff models  are discussed as part of hydraulic modeling in the following

sections.


       Some models include both hydraulic and water quality  components, while others are limited
to one or the other. Although CSO projects typically involve hydraulic modeling, water quality
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Chapter 7	CSS Modeling







modeling in the CSS is less common, and a community may decide to rely on CSS water quality



monitoring data instead.







       Several factors will dictate whether  CSS  water  quality modeling is appropriate. WPCF



(1989) concludes that "simulation of quality parameters should only be performed when necessary



and only when requisite calibration and verification data are available[...]  Another option is to couple



modeled hydrologic and hydraulic processes  with measured quality data to simulate time  series of



loads and overflows." Modeling might not be justified in  cases where measured CSS water quality



variations are difficult to relate to parameters such as land use, rainfall intensity,  and pollutant



accumulation rates. For these cases, using statistics (such as mean and standard deviation) of CSS



water quality parameters measured in the sewer system can be a valid approach. One limitation of



this approach, however,  is that  it cannot account for the  implementation  of best management



practices (BMPs) such as street sweeping or the use of detention basins.







       Exhibit 7-1  shows how model selection can be affected by the status of NMC implementation



and  LTCP  development, and  by whether  the  LTCP  will  be based on  the presumption  or



demonstration approach.  To avoid duplication of effort, the permittee should always  consider



modeling needs that will arise during later stages of LTCP development or implementation.







       Nine Minimum Controls (NMC)



       In this initial phase of CSO control, hydraulic modeling can be used to  estimate existing CSO



volume and  frequency and  the  impacts of  implementing alternative controls under the NMC.



Typically,  in this stage of analysis,  modeling focuses more on reductions in CSO magnitude,



frequency, and duration than  on contaminant transport.







       Long-Term Control  Plan (LTCP)



       EPA anticipates that  hydraulic modeling will be necessary  for most CSSs regardless of



whether the community uses  the  presumption approach or demonstration approach.  Both approaches



require accurate predictions  of the number and volume  of CSO events; under the demonstration
                                            7-5                                January 1999

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Chapter 7
                                          CSS Modeling
approach, this information will help determine the amount and timing of pollutant loadings to the

receiving water.
            Exhibit 7-1. Relevant
                              for
CSS Hydraulic and Water Quality Modeling
EPA's CSO Control Policy

CSS Hydraulic Modeling
CSS Water Quality Modeling
Nine Minimum Controls
Demonstrate implementation of the nine
minimum controls
Simple to complex models of
duration and peak flows
Limited - Not usually performed
LTCP "Presumption Approach"
Limit average number of overflow
events per year
Capture at least 85% of wet weather
volume per year
Eliminate or reduce mass of pollutants
equivalent to 85% capture requirement
Long-term continuous
simulations (preferred) or
design storm simulation
Same
Same
Limited - Not usually performed
Limited - Not usually performed
Use measured concentrations or
simplified transport modeling
LTCP "Demonstration Approach"
Demonstrate that a selected control
program ... is adequate to meet the water
quality -based requirements of the CWA
Design storm simulations
and/or
Long-term continuous
simulations
Use measured concentrations
or, in limited cases, contaminant
transport simulations
       Presumption Approach. The presumption approach is likely to require hydraulic modeling

to develop accurate predictions of the number and volume of CSOs.  Some level of contaminant

transport modeling may also be necessary to ensure that the presumption approach will not result

in exceedances of water quality criteria in light of available data.  In such cases, loading estimates

can be developed using measured concentrations or simplified screening methods, coupled with

hydraulic modeling.


       Demonstration Approach.  Under the  demonstration  approach, the permittee needs  to  show

that the planned controls will provide for attainment of WQS  unless WQS cannot be attained as a

result of  natural background conditions or pollution sources other than CSOs.
                                           7-6
                                            January 1999

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Chapter 7	CSS Modeling


       Therefore,  CSS  modeling under  the  demonstration  approach  should describe pollutant

loadings to the receiving water body. Since water quality modeling in the CSS is directly linked to

water quality modeling in the receiving water,  the CSS model must generate sufficient data to drive

the receiving water model. Further, the resolution needed for the CSS pollutant transport estimates

will depend on the time  resolution called for in the receiving  water model, which is in turn driven

by  WQS.  For pollutants with long response times in the  receiving  water (such as BOD and

nutrients), the appropriate level of loading information is usually the total load introduced by the

CSO  event.  For  pollutants  with shorter response  times  (such as bacteria  and acutely  toxic

contaminants), it may be necessary to consider the timing of the pollutant load during the course of

the CSO event.


7.2.1   Selecting Hydraulic Models

       Hydraulic models used for CSS simulations can be divided into three main categories:


           Runoff models based  on  Soil  Conservation Service (SCS) runoff  curve  numbers,  runoff
           coefficients,  or other similar methods for the generation of flow.  These models can
           estimate runoff flows  influent to  the sewer system  and, to a lesser degree, flows at
           different points in the  system. Runoff models do  not simulate flow in  the CSS, however,
           and therefore do  not predict  such parameters as the flow depth, which frequently control
           the occurrence of CSOs.  (The RUNOFF block of EPA's  Storm Water  Management
           Model (SWMM) is an example.2)

           Models based  on  the kinematic wave approximation  of the full hydrodynamic
           equations. These models can predict flow depths, and  therefore flow and discharge
           volumes, in systems that are not subject to surcharging or back-ups (backwater effects).
     SCS runoff curves were developed based on field studies measuring runoff amounts from different soil cover
combinations. The appropriate runoff curve is determined from antecedent moisture condition and the type of soil.
(Viessman et al, 1977)

   2 The SWMM RUNOFF model also has limited capabilities for flow routing in the CSS.

   3 Flow, which is caused by the motion of waves, can be described by the hydraulic routing technique. This technique
is based on the simultaneous solution of the fully hydrodynamic equations (the continuity equation and the momentum
equation for varying flow). Under certain conditions, these hydrodynamic equations  can be simplified to  a one-
dimensional continuity equation and a uniform flow equation (in place of the full momentum equation).  This is referred
to as the kinematic wave approximation (discharge is  simply a function of depth). (Bedient and Huber, 1992)


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Chapter 7	CSS Modeling
           These models require the user to input hydrographs from runoff model results.  (The
           TRANSPORT block of SWMM is an example.)

           Complex, dynamic models  based on the  full hydrodynamic equations.  They  can
           simulate surcharging, backwater effects, or looped systems, and represent all pertinent
           processes.  These models require the user to input hydrographs from runoff model
           results. (The EXTRAN block of SWMM  is an example.)
Exhibit 7-2 compares the flow routing capabilities of the three SWMM blocks. Section 7.3 discusses
available hydraulic models.


       The simpler models were developed to support rapid evaluations  of CSSs.  They require little
input data, are relatively easy to use, and require less computer time than complex models. These
features, however, are becoming less significant because  complex models with user-friendly pre- and
post-processors are now  widely available.  Advances  in  computer technology render run-time a
secondary issue for all but the largest of applications.


       Criteria for the selection of a CSS hydraulic model include:
       1.      Ability to accurately represent CSS's hydraulic behavior. The hydraulic model
              should be selected with the characteristics of the above three model categories in
              mind. For example, a complex, dynamic model may be appropriate when CSOs are
              caused by back-ups or surcharging.  Since models differ in their ability to account
              for such factors as conduit cross-section shapes, special structures, pump station
              controls,  tide simulation, and automatic regulators, these features  in a CSS may
              guide the choice of one model over another.

       2.      Ability to accurately represent runoff in the CSS  drainage basin. The runoff
              component of the hydraulic model (or the runoff model, if a separate hydrologic
              model is used) should adequately estimate runoff flows  influent to the sewer system.
              It should  adequately  characterize rainfall characteristics  as well as hydrologic factors
              such as watershed size, slope, soil types, and imperviousness.

       3.      Extent of monitoring. Monitoring usually cannot cover an entire CSS, particularly
              a large CSS. A  dynamic  model  is more reliable for predicting the behavior of
              unmonitored overflows, since it can simulate all the hydraulic features controlling
              the overflow, but  it often requires extensive resources for its application. In addition,
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Chapter 7
CSS Modeling
     Exhibit 7-2. Characteristics of RUNOFF, TRANSPORT, and EXTRAN Blocks of
                    the EPA Storm Water Management Model (SWMM)1
Characteristics
1. Hydraulic simulation method
2. Relative computational expense for
identical network schematizations
3. Attenuation of hydrograph peaks
4. Time displacement of hydrograph
peaks
5. In-conduit storage
6. Backwater or downstream control
effects
7. Flow reversal
8. Surcharge
9. Pressure flow
10. Branching tree network
11. Network with looped connections
12. Number of preprogrammed conduit
shapes
13. Alternative hydraulic elements (e.g.,
pumps, weirs, regulators)
14. Dry -weather flow and infiltration
generation (base flow)
15. Pollution simulation method
16. Solids scour-deposition
17. User input of hydrographs/
pollutographs3
Blocks
RUNOFF
Nonlinear reservoir,
cascade of conduits
Low
Yes
Weak
Yes
No
No
Weak
No
Yes
No
3
No
No
Yes
No
No
TRANSPORT
Kinematic wave,
cascade of conduits
Moderate
Yes
Yes
Yes
No
No
Weak
No
Yes
No
16
Yes
Yes
Yes
Yes
Yes
EXTRAN
Complete equations,
conduit networks
High
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
8
Yes
Yes
No
No
Yes
1 After Huber and Dickinson, 1988.
  Backwater may be simulated as a horizontal water surface behind a storage element.
3 The RUNOFF block sub-model is primarily intended to calculate surface runoff, but includes the capability to simulate
simple  channel conveyance of flows. The TRANSPORT and EXTRAN blocks are sewer conveyance models with no
runoff components and thus require user input of hydrographs.
                                               7-9
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Chapter 7                                                                    CSS Modeling
              most of these models use a complex finite-difference technique to solve for the
              governing equations.   Sound  simulation of hydraulic behavior requires that the
              modeler achieve numeric stability of the solution technique through the selection of
              appropriate time and space intervals. In some cases, however, estimates of overflow
              at unmonitored locations can be made based on monitoring in areas with similar
              geographic features (like slope, degree of imperviousness, or soil conditions), based
              on V/R ratios  and drainage basin characteristics (see Section 5.3.3).

       4.      Need for long-term simulations. Long-term simulations are desirable to predict
              CSO frequency, volume, and pollutant loadings over certain time periods, like one
              year.  This  information can help support the  presumption approach. For  large
              systems,  long-term simulations  using a complex dynamic model often require
              lengthy computer run times and may be impractical.

       5.      Need to assess water quality in CSS. If CSS water quality simulations are needed,
              the permittee should consider the model's capability to simulate water quality. To
              simulate  CSS water quality, it is often better to use actual pollutant concentrations
              from monitoring results together with modeled CSS flows.

       6.      Need to  assess water quality in receiving waters. The pollutants  of concern and
              the nature of the receiving water affect the resolution of the CSO data needed for the
              water quality  analyses.  For  example, bacteria analysis  typically requires hourly
              rather than daily loading data,  and the hydraulic model must be capable  of providing
              this resolution.

       7.      Ability to assess the effects of control alternatives. If control alternatives involve
              assessing downstream back-ups  or surcharging and the effects  of relieving them,
              correct simulation may require use of a dynamic model, since other models do not
              simulate  surcharging or back-ups.

       8.      Use of the presumption or demonstration approach.  Some permittees using the
              first presumption approach option-no more than four untreated overflow events per
              year—can estimate the number of overflow events fairly  accurately by calculating
              the probability of exceeding storage and treatment capacity.  Other permittees may
              need to account for transient flow peaks, which requires accurate flow routing. The
              other two presumption approach options-percent volume capture and pollutant load
              capture-generally require some analysis of the timing  and peaking of flows, so a
              hydraulic simulation approach may be needed.

              If a permittee  is using the  demonstration approach,  receiving water monitoring
              and/or modeling is  necessary.  The time intervals for pollutant transport in a
              receiving water model may influence the time intervals for CSS quality modeling.
  4 V/R is the ratio of the overflow volume to the rainfall depth.
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Chapter 7                                                                   CSS Modeling
              This in turn will constrain the time resolution for CSS hydraulic modeling. The
              permittee should consider the  level of time resolution derived when selecting a
              model.

              Ease of use and cost. As mentioned above,  simple models tend to be easier to use
              than complete dynamic models. Although user-friendly dynamic models now exist,
              they are generally commercial models that cost more than public domain models and
              can be  used incorrectly by inexperienced users.  Another option is to use commercial
              pre- and post-processors (or shells) designed to facilitate the use of public domain
              models such as SWMM. They  can provide graphically-oriented, menu-driven data
              entry and extensive results plotting capabilities at a cost lower than that of complete
              dynamic models.

              Another issue related to ease of use and accuracy is robustness, which is a model's
              lack of propensity to become unstable. Instabilities are uncontrolled  oscillations of
              the model's results due to the approximations made in the numerical solution of the
              basic differential equations. Instabilities tend to occur primarily in fully dynamic
              models, and are caused by many factors, including incomplete sewer information and
              short conduits. Resolving model instabilities can be time-consuming and requires
              extensive experience with the model.
7.2.2       Selecting CSS Water Quality Models

       CSS water quality models can be divided into the following categories:
          Land Use Loading Models - These models provide pollutant loadings as a function of
          the distribution of land uses in the watershed. Generally, these models attribute to each
          land use a concentration for each water quality parameter, and calculate overall  runoff
          quality as a weighted sum of these concentrations.  Pollutant concentrations for  the
          different land uses can be derived from localized data bases or the Nationwide Urban
          Runoff Program (NURP), a five-year study initiated in 1978 (U.S. EPA, 1983a).  Local
          data are usually preferable to NURP data since local data are generally more recent and
          site-specific.

          Statistical Methods - A more sophisticated version of the previous method, statistical
          methods  attempt to formulate a derived frequency distribution  for  event  mean
          concentrations (EMCs). The EMC is the total mass of a pollutant discharged during an
          event  divided by the total discharge volume. NURP  documents discuss the use of
          statistical methods to characterize CSO quality in detail (Hydroscience, Inc., 1979) and
          in summary form (U.S. EPA, 1983a).
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Chapter 7                                                                    CSS Modeling
           Build-Up/Washoff Models - These models simulate the  basic processes that  control
           runoff quality,  accounting for  such factors as time periods between events,  rainfall
           intensity, and BMPs. They require calibration and are not regularly used due to the
           expense and difficulty of defining site-specific rates.
       Many  models  do not address the potentially important role of chemical reactions  and
transformations  within the CSS.  Calibration may be difficult because pollutant loading into the CSS

is often uncertain.


       The permittee should consider the following criteria when selecting a CSS water quality

model:
       1. Needs of the  receiving  water quality simulation.  The time scale of the pollutant
           concentration  simulation in the CSS, and the degree of sophistication of the model,
           depends partly on the needs of the receiving water quality simulation (if used) and,
           ultimately, on the  level of detail required to demonstrate attainment of WQS. If it is only
           necessary to estimate the average annual loading to the receiving water, then detailed
           hourly or sub-hourly simulation  of combined sewage quality generally will not be
           necessary. As noted above, in many  cases it is appropriate to combine sophisticated
           hydraulic modeling with approximate CSS water quality modeling.

       2.  Ability to assess  control and BMP alternatives. When the control alternatives under
           assessment include specific  BMPs  or control  technologies, the CSS water quality model
           should be sophisticated enough to estimate the effects of these alternatives.

       3.  Ability to accurately represent significant characteristics of pollutants of concern.
           The pollutants involved in  CSS quality simulation can be roughly grouped as bacteria,
           BOD, nutrients, sediments  and sediment-associated pollutants, and toxic contaminants.
           Most water quality models are designed to handle sediments and nutrients, but not all can
           model additional pollutants.  In  some cases, this limitation can  be circumvented  by using
           a sediment potency factor, which relates the mass of a given pollutant to sediment
           transport. However, this alternate  approach has limited usefulness for CSO concerns
           since it is generally not appropriate for bacteria and dissolved metals. As noted earlier,
           another alternate  approach is to  combine  the results of hydrologic and hydraulic
           modeling of the CSS with  bacteria and dissolved metals concentrations from sampling
           results to estimate pollutant loads.

       4.  Capability  for  pollutant  routing.  Another concern is the model's capability  for
           pollutant routing-i.e., its  capacity to account for variability in pollutant concentrations
           during storm events. Most models translate pollutant concentrations from sources and


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Chapter 7                                                                     CSS Modeling
           CSO quantity to pollutant loading without taking separate account of the timing of
           pollutant delivery due to transport through the CSS.  Some basins deliver the highest
           concentrations of pollutants in the rising  limb of the storm flow (the "first flush" effect).
           If the CSO loading for such  systems is modeled using  overflow quantity and average
           concentrations, inaccuracies  may result, particularly  if the "first flush" is effectively
           captured by the POTW or storage.
       5. Expense and ease  of use.   Sophisticated water quality models can be expensive to
           calibrate and generally are more difficult to use. If a simpler model is applicable to the
           situation and can be properly calibrated, it may be sufficient and can be more accurate.
7.3    AVAILABLE MODELS

       Exhibit 7-3 summarizes several runoff and hydraulic models and Exhibit 7-4 summarizes
several water quality models.  These models have been developed by EPA and the Army Corps of
Engineers and are available in the public domain.   Some of the models in Exhibit 7-3 are runoff
models (such as STORM); others have a runoff component but also simulate flow in the CSS (such
as SWMM and Auto-Q-ILLUDAS).

       An increasing number of high-quality commercial models and pre-/post-processors are also
available. Commercial models can be either custom-developed software or enhanced, more user-
friendly versions of popular public domain models.  In exchange for the cost of a commercial model,
users generally  receive additional  pre- and/or post-processing  capabilities and technical support
services.  Several  of the  available commercial models are listed in Exhibit 7-5.   Commercial
pre/post-processors exist for use with some of the public domain  models. Pre-processors can help
users prepare their input files for a model.  Post-processors provide additional capabilities for
analyzing and displaying the  model output through graphing, mapping, and other techniques. For
    The commercial packages have not been reviewed by EPA and they are subject to continued evolution and change,
like all commercial software. This listing is provided to assist potential users; it is not meant to endorse any particular
model or imply that models not listed are not acceptable. A recent listing of some available models is found in Mao
(1992). Recent developments in sewer and runoff models include linking models to geographic information systems
(GIS), computer-aided design (CAD) systems, and receiving water models such as WASP.

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Chapter 7
                                                               CSS Modeling
              Exhibit 7-3. CSS Runoff and Hydraulic Models (Public Domain)
Model Name
EPA Statistical'
The Simple Method
USGS Regression
Method
SLAMM
P8-UCM
Auto-Q-ILLUDAS
STORM
DR3M-QUAL
HSPF
SWMM
Characteristics
Hydraulic Time
Scales
Annual, Event
Annual, Event
Annual, Event
Continuous-
Daily
Continuous-
Hourly
Continuous-
Hourly
Continuous-
Hourly
Continuous-
Sub-hourly
Continuous-
Sub-hourly
Continuous-
Sub-hourly
Hydraulic
Simulation Type
Runoff
Coefficient
Runoff
Coefficient
Regression
Water Balance
Curve Number
Water Balance
Runoff Coeff./
Curve Number
Kinematic Wave
Kinematic Wave
Kinematic &
Dynamic Wave
Assess Control
Alternatives
No
No
No
Limited
Advanced
Limited
Limited
Advanced
Moderate'
Advanced
Key to
Reviews
1,2,3
1
1,2
1
1
1,3
1,2,3
1,2,3
1,2,3
1,2,3
Major
References
Hydroscience, 1979
Driscoll et al, 1990
Schueler, 1987
Driver & Tasker,
1988
Pitt, 1986
Palmstrom &
Walker, 1990
Terstnep et al., 1990
HEC, 1977
Alley & Smith,
1982a & 1982b
Johanson et al., 1984
Huber & Dickinson,
1988;Roesneretal,
1988
    Notes:
1   Reviewed as "FHWA" by Shoemaker et al., 1992.
2   Can be used for assessment of control alternatives, but not designed for that purpose.
    Key to Reviews:    1   Shoemaker et al., 1992.
                     2   Donigian and Huber, 1991.
                     3  WPCF,  1989.
            Some of the public domain models listed above are available from EPA's Center for
            Exposure Assessment Modeling (CEAM). CEAM can be contacted at:

                CEAM
                National Exposure Research Laboratory-Ecosystems Research Division
                Office of Research and Development
                USEPA
                960 College Station Road
                Athens, GA 30605-2700
                Voice:     (706) 355-8400
                Fax:       (706) 355-8302
                e-mail:    ceam@epamail.epa.gov
                CEAM also has an Internet site at http://www.epa.gov/CEAM/
                                               7-14
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Chapter 7
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                   Exhibit 7-4.  CSS Water Quality Models (Public Domain)
Model Name
EPA Statistical1
The Simple Method
USGS Regression Method
Watershed
GWLF
SLAMM
PB-UCM
Auto-Q-ILLUDAS
STORM
DR3M-QUAL
HSPF
SWMM
Characteristics
Quality
Time Scales
Annual
Annual
Annual
Annual
Continuous -
Daily
Continuous -
Daily
Event
Continuous -
Hourly
Continuous -
Hourly
Continuous -
Sub-hourly
Continuous -
Sub-hourly
Continuous -
Sub-hourly
Pollutant
Types
S, N, O
S,N, 0
S,NO
S, N,0
S, N
S, N,0
N, O
S,N, 0
S, N, O
S, N, O2
S, N, O
S, N, O2
Pollutant
Routing-
Transport
Capability
no
no
no
no
low
medium
low
medium
no
high
high
	 3
Pollutant
Routing -
Transformation
Capability
no
no
no
no
no
no
no
no
no
no
high
low
BMP
Evaluation
Capability
low
low
no
medium
low
medium
high
medium
medium
medium
high
high
        Notes:   1   Reviewed as "FHWA" by Shoemaker et al,  1992.
                2   Other constituents can be modeled by  assumption of a sediment potency fraction.
                3   SWMM received a low rating from Shoemaker  et al. for "weak" quality simulations. This
                   rating may not be justified when SWMM's pollutant  routing-transport capabilities are
                   compared to those of other models.

        Key to Pollutant  Type:   S  - Sediment N - Nutrients O - Other.
             Some of the public domain models listed above are available from EPA's Center for
             Exposure Assessment Modeling (CEAM). CEAM can be contacted at:
                 CEAM
                 National Exposure Research Laboratory-Ecosystems Research Division
                 U.S. EPA Office of Research and Development
                 960 College Station Road
                 Athens, GA 30605-2700
                 Voice:    (706)355-8400   Fax:     (706)  355-8302
                 e-mail:    ceam@epamail.epa.gov
                 CEAM  also has an Internet site  at http://www.epa.gov/CEAM/
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Chapter 7
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                    Exhibit 7-5. Selected Commercial CSS Models
Package Name
Hydra/Hydra Graphics
Eagle Point Hydrology Series
Mouse
HydroWorks
XP-SWMM32
Type of Hydraulic
Simulation
Dynamic
Dynamic
Dynamic
Dynamic
Dynamic
Water Quality
Capability
No
No
Yes
Yes
Yes
Contact
PIZER Incorporated
4422 Meridian Avenue N
Seattle, Washington 98103
(800) 222-5332
www.pizer.com
Eagle Point Software
4131 Westmark Drive
Dubuque, Iowa 52002-2627
(800) 678-6565
www. eaglepoint. com
Danish Hydraulic Institute
Agem Alle 5
DK-2970 H0rrsholm, Denmark
011-4545 179 100
www.dhi.dk
HR Wallingford, Wallmgford Software
Howbery Park
Wallingford
Oxfordshire OX10 8BA, UK
01 1-44(0)1491 835381
www.hrwallingford. co .uk
BOSS International
66 12 Mineral Point Rd.
Madison, Wisconsin 53705-4200
(800) 488-4775
www.bossintl.com
example, SWMMDuet  allows the integration of SWMM and Arc/INFO for database management
and GIS analysis.

       These exhibits summarize some  important technical criteria, and can  be used as a preliminary
guide. However,  to evaluate the  use of  a specific model in  a particular situation the permittee should
refer to the more detailed  reviews and major references  listed  in Exhibits 7-3 and  7-4. Both
Shoemaker et al. (1992) and Donigian and Huber (1991)  provide preliminary evaluations of the
functional  criteria, including cost and  data requirements. The Water Resources Handbook (Mays,
1996) discusses both hydraulic and water quality models and compares their attributes.
    SWMMDuet is a SWMM/GIS Interface. Further information can be obtained from the Delaware Department of
Natural Resources at (302) 739-3451.
                                            7-16
  January 1999

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Chapter 7	CSS Modeling

7.4    USING A CSS MODEL
7.4.1  Developing  the Model
       In developing the model, the modeler establishes initial  conditions for various model
components (such as the level  of discretization) and  input data parameters (such as percent
imperviousness of subcatchments). These elements are  then adjusted through model calibration,
which is discussed in the next section.

       Until  recently the modeler had to compromise between the level  of detail  in a model
(temporal and spatial precision), the mode in which it was run (complex vs. simple), and the time
period for the simulation (event vs. continuous).  As computer technology continues  to improve,
limitations in computing power are becoming less of a factor in determining the appropriate level
of modeling complexity. However, for increased model complexity  to lead to greater  accuracy,
complex models  should be used by knowledgeable,  qualified modelers  who have  sufficient
supporting data.  In  some cases,  where  detail is not  required, a simplified model may save time  spent
filling the data  requirements of the model, preparing tiles, and doing the model runs. Shoemaker
et al.  (1992, Tables 7 to 9) provides a tabular summary of the main input and output data for each
of the models presented in Exhibits 7-3 and 7-4.

       The level of discretization (i.e., coarse vs. fine scale) determines how  precisely the geometry
of the CSS and the land characteristics of the drainage basin are described in the model.  At a very
coarse level of discretization, the CSS is  a black box with lumped parameters and the model (e.g.,
STORM) primarily simulates CSOs.  A more complex approach might be to simulate the larger
pipes of the CSS,  but to lump the characteristics of the smaller portions of the CSS.   Another
intermediate level of complexity is to simulate the interceptor when it is the limiting component in
the CSS for controlling overflows.   Much can be learned about system behavior by simulating
interceptor hydraulics in response to surface runoff. More complex simulations would include
increasing levels of detail about the system.
                                           7-17                              January 1999

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Chapter 7	CSS Modeling







       In determining the appropriate level of discretization, the modeler must ask:







           What is the benefit of a finer level of detail?



           What is the penalty (in accuracy) in not modeling a portion of the system?







For systems that are controlled hydraulically at their downstream ends, it may only be necessary to



model the larger downstream portion of the CSS. If flows are limited due to surcharging in upstream



areas, however, a simulation neglecting the upstream portion of the CSS would over-estimate flows



in the system. In some cases it is difficult to determine ahead of time what the appropriate level of



detail is. In these cases, the modeler can take an incremental approach, determining the value of



additional complexity or data added at each step. Exhibit 7-6, for example, compares a simulation



based on five  subcatchments (coarse discretization) and  a simulation based on  twelve subcatchments



(finer discretization)  with observed values.   Only marginal improvement is  observable when



subcatchments are increased from five to twelve. The modeler should probably conclude that even



finer discretization (say, 15 subcatchments) would provide little additional value.







7.4.2   Calibrating and Validating the Model





       A model general enough to tit a variety of situations typically needs to be adjusted to the



characteristics of a particular site and situation. Model calibration and validation are used to "fine-



tune" a model to  better match  the observed conditions  and demonstrate the credibility of the



simulation results. An uncalibrated model may be acceptable for screening  purposes, but without



supporting evidence the uncalibrated result may not be accurate.  To use model simulation results



for evaluating control alternatives, the model must be reliable.







       Calibration is the  process of running a model using a set of input data and then comparing



the results to actual measurements of the system. If the  model results do not reasonably approximate



actual measurements,  the modeler reviews the components of the model to  determine if adjustments
                                            7-18                               January 1999

-------
                                         Exhibit 7-6. Levels of Discretization
-4
            60
           50
           40
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                                                                     ~"— 12 Subcatahment System



                                                                     	•• 5 - Subcatehment System




                                                                     ~""~ Observed
VO
                       10       20       30       40      50

                                               Time (Min.)
                                                               60
70
80
90

-------
Chapter 7	CSS Modeling


should be made so that the model better reflects the system it represents. For example, a CSS

hydraulic model used to simulate overflows is calibrated by running the  model using measured

rainfall  data to simulate the volume, timing, and depth of CSOs.  The model results are then

compared to actual measurements of the overflows. The modeler then adjusts parameters such as

the Manning roughness  coefficient or the percent  imperviousness  of  subcatchments  within

scientifically credible ranges and runs the model a second time, again comparing the results to

observations.  Initial calibration runs often point to features of the system, such as a connection or

bypass,  which may not  have been evident based on the available maps. The modeler repeats this

procedure until satisfied that the model produces reasonable simulations of the overflows. Models

are usually calibrated for more than one storm, to ensure appropriate performance for a range of

conditions. Exhibit  5-9 shows some example  model calibration plots of flow  and depth  during storm

events. For calibration, the most important comparisons are total volumes, peak flows, and shapes

of the hydrographs.


       Validation  is the process of testing the calibrated model using one or more independent  data

sets. In the case of the hydraulic simulation, the model is run without any further adjustment using

independent  set(s)  of rainfall  data.   Then  the results  are  compared to the field  measurements

collected concurrently with these rainfall  data.   If the results are suitably close, the  model is

considered to be validated.  The modeler can then use the model with other sets of rainfall data or

at other outfalls.  If validation  fails, the modeler must recalibrate the model and validate it again

using a  third independent data set.  If the model fails a validation test,  the next test must use a new

data set.  (Re-using a data set from a previous validation test  does not constitute a fair test, because

the modeler has already adjusted model parameters to better fit the model to the data.) Validation

is important because it assesses whether the model retains its generality; that  is, a model that has

been adjusted extensively to match a particular storm might lose its ability  to predict the effects of

other storms.
    Model calibration is not simply "curve fitting" to meet the data. Model adjustments should make the modeled
elements of the system better reflect the actual system.


                                            7-20                               January  1999

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Chapter 7	CSS Modeling


       The  availability of adequate  calibration  data  places  constraints  on which models  are

appropriate.  When identifying the time period for conducting CSS flow monitoring, the permittee

should consider the effect of using larger data sets.  The Combined Sewer Overflow  Control Manual
(U.S. EPA,  1993) states that "an adequate number of storm events (usually 5  to 10) should be

monitored and used in the calibration." The monitoring period should indeed cover at least that
many storms, but calibration and validation are frequently done with 2 to 3 storms each.


       EPA's  Compendium of Watershed-Scale Models for TMDL Development  (Shoemaker et al.,

1992) includes the following  comments on calibration and validation:
       Most models are more accurate when applied in a relative rather than an absolute manner.
       Model  output data concerning the  relative contribution... to overall pollutant loads is more
       reliable than an absolute prediction of the impacts of one control alternative viewed alone.
       When examining model output. . . it is important to note three factors that may influence the
       model  output and produce  unreasonable data.   First, suspect data  may result from
       calibration or verification data that are insufficient or inappropriately applied.  Second,  any
       given model, including detailed models, may  not represent enough detail to adequately
       describe existing  conditions and generate  reliable output.  Finally,  modelers should
       remember that all models have limitations and the selected model may not be capable of
       simulating desired conditions.   Model results must therefore be interpreted within the
       limitations of their  testing and their range of application.  Inadequate model calibration  and
       verification  can result in spurious model results, particularly when  used for absolute
       predictions.   Data limitations may require  that model results be used only for relative
       comparisons.
       Common practice employs both judgment and graphical analysis  to  assess a model's
adequacy. However, statistical evaluation can provide a more rigorous and less subjective approach
to validation (see Reckhow et al.,  1990, for a discussion of statistical evaluation of water quality
models).
                                            7-21                                January  1999

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Chapter 7                                                                   CSS Modeling
       Nix ( 1990) suggests the following general sequence for calibrating a CSS model:
       1.  Identify the  important model algorithms and parameters. A combination of
          sensitivity analysis and study of model algorithms can determine which parameters are
          most important for calibration of a given model-site pairing.

       2.  Classify model parameters to determine the degree to which they  can be directly
          measured,  or, alternatively, are conceptual  parameters  not amenable to direct
          measurement.  For instance, a parameter such as area is usually easily defined, and thus
          not varied in calibration, while parameters that are  both important to model performance
          and not amenable to direct measurement (e.g.,  percent imperviousness) will be the
          primary adjustment factors for calibration.

       3.  Calibrate the model first for the representation (prediction)  of overflow volume.

       4.  After obtaining a reasonable representation of event overflow volume,  calibrate to
          reproduce the timing and peak flow (hydrograph shape) of overflows.

       5.  Finally, calibrate the pollutant parameters only after an acceptable  flow simulation
          has been obtained.
        Section 7.5 describes an example of CSS modeling, including commentary on calibration

and simulation accuracy.


7.4.3   Performing the Modeling Analysis

       Once a model has been calibrated and validated, it can be run for long-term simulations

and/or for single events (usually a set of design storms).
          Long-term simulations can account for the sequencing of the rainfall in the record and
          the effect of having storms immediately follow each other. They are therefore useful for
          assessing the long-term performance  of the system under the presumption approach.
          Long-term simulations  also  assess  receiving  water quality accurately under the
          demonstration approach. Water quality criteria need to be evaluated with the frequency
          and duration of exceedance in order to be relevant. This is best done using long-term
          continuous simulations  or  skillfully  done probabilistic  simulations.  Although continuous
          simulation models should be calibrated using continuous data where possible, they may
          be calibrated with single events if antecedent conditions are taken into account. As the
                                           7-22                               January 1999

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Chapter 7                                                                    CSS Modeling
           speed of desktop computers increases, modelers may be able to perform long-term
           continuous simulations with higher and higher levels of detail.

           Single event simulations are useful  for developing an  understanding of the system
           (including the causes of CSOs) and formulating control measures, and can be used for
           calibrating models.
       Although increased computer capabilities enable continuous simulations with greater levels

of detail, continuous simulation of very large systems can have some drawbacks:


           The model may generate so much data that analysis and interpretation are difficult

           Limitations  in the accuracy  of hydrologic input data (due to the inability to continuously
           simulate spatially variable rainfall over a large catchment area) may lead  to an inaccurate
           time series of hydraulic conditions within the interceptor

           The more  storms that are simulated,  the greater the chance that instabilities will  occur in
           complex models. Correctly  identifying and resolving these instabilities requires  capable,
           experienced modelers.


7.4.4 Modeling Results

       Model Output
       The most basic type of model output is text files in which the model input is repeated and

the results are tabulated. These can include flow and depth versus time in selected conduits and
junctions, as well as other information, such as which conduits are surcharging.  The model output

may include an overall system mass balance with such measures as the runoff volume entering the
system, the volume leaving the system at the downstream boundaries,  the volume lost due to

flooding, and the change  of volume  in storage.  The model output can also measure the mass balance
accuracy of  the  model run, which  may indicate that problems, such as  instabilities (see
Section 7.2.1) occurred.


       Most models also produce plot tiles, which are easier to evaluate than text files. Output data
from plot files can be plotted using spreadsheet software or commercial post-processors, which are
available for  several public domain models (particularly SWMM).  Commercial models typically


                                             7-23                                January 1999

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Chapter 7	CSS Modeling







include extensive post-processing capabilities, allowing the user to plot flow or depth versus time



at any point in the system or to plot hydraulic profiles versus time along any set of conduits.







       Interpretation of Results



       Simulation models predict CSO volumes, pollutant concentrations, and other variables at a



resolution that depends on the model structure, model implementation, and the resolution of the



input data.  Because the ultimate purpose of modeling is generally to  assess the CSO controls needed



to provide for the attainment of WQS, the model's  space and time resolution should match that of



the applicable WQS.  For instance,  a State WQS may include a criterion that a one-hour average



concentration not exceed a given concentration more than once every 3 years on average. Spatial



averaging may be represented by a concentration averaged over a receiving water mixing zone, or



implicitly by the specification  of monitoring locations to establish whether the instream criteria can



be met.  In any case, the permittee should note whether the  model predictions use the same averaging



scales as the relevant water quality criteria. When used for continuous rather than event simulation,



as suggested by the CSO Control Policy, simulation models provide output that can be analyzed to



predict the occurrence and frequency of water quality criteria exceedances.







       In interpreting  model results, the permittee needs to  be aware that modeling  usually will not



provide  exact predictions  of system  performance measures  such as overflow volumes  or exceedances



of water quality criteria.  With sufficient effort, the permittee often can obtain a high degree of



accuracy  in  modeling  the hydraulic  response of a CSS, but results of  modeling  pollutant



buildup/washoff, transport in the CSS, and fate in receiving waters are considerably less accurate.



Achieving a  high degree of accuracy may be more  difficult in a continuous simulation because of



the difficulty of specifying continually changing boundary conditions for the model parameters.
                                            7-24                                January 1999

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Chapter 7                                                                    CSS Modeling
       In interpreting model results, the permittee should remember the following:
           Model predictions  are only as accurate as the user's understanding and knowledge of the
           system being modeled and the model being used

           Model predictions  are no better than the quality of the  calibration  and validation exercise
           and the quality of the data used in the exercise

           Model predictions are only estimates of the response of the system to rainfall events.
       Model Accuracy and Reliability

       Since significant CSO control decisions may be based on model predictions, the permittee

must understand the uncertainty (caused by model parameters that cannot be explicitly estimated)

and  environmental  variability  (day-to-day variations in explicitly measurable model inputs)

associated with  the  model  prediction. For instance, a model for a CSO event of a given volume may

predict a coliform count of 350 MPN/100 ml in the overflow, well below the hypothetical water

quality criterion of 400 MPN/100 ml. However, the model prediction is not exact, as observation

of an event of that volume  would readily show. Consequently, additional information specifying

how much  variability to expect around the  "most  likely" prediction of 350 is useful. Obviously, the

interpretation of this  prediction differs, depending  on whether the  answer is "likely  between 340 and

360" or "likely between 200 and 2000."


       Evaluating  these issues involves the closely related  concepts of model accuracy  and

reliability.    Accuracy is  a measure  of the  agreement between  the  model  predictions  and

observations. Reliability is a measure of confidence in model predictions for  a specific  set of

conditions and for  a specified confidence level. For example, for a simple mean estimation, the

accuracy could  be measured by the sample standard deviation, while the  reliability of the  prediction

(the sample mean in this case) could be evaluated at the 95 percent confidence level as plus or minus

approximately two  standard deviations around the mean.


       Modeling as part of LTCP development enables the permittee to demonstrate that a given

control option is "likely" to result in compliance with the requirements of the CWA and attainment


                                            7-25                               January 1999

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Chapter 7	CSS Modeling


of applicable WQS. During LTCP development, the permittee will justify that a proposed level of
control will be adequate to provide for the attainment of WQS. Therefore, the permittee should be
prepared to estimate and document the accuracy and reliability of model predictions.


       Evaluating model  accuracy and  reliability is particularly important for the analysis of
wet-weather  episodic loading,  such as CSOs.  Such  analysis invariably  involves  estimation of
duration (averaging period) and frequency of excursion above a water quality criterion, regardless
of whether the criterion is  expressed as  average monthly and  maximum daily values, or as a

maximum  concentration for a given  design stream flow (e.g.,  7Q10). Estimating duration and

frequency of excursion requires knowledge of model reliability, and the duration and frequency of

the storm events serving as a basis for the model.


       Available techniques for quantifying uncertainties in  modeling studies include sensitivity

analysis for continuous simulations, and first-order error analysis and Monte Carlo simulations for
non-continuous simulations:
           Sensitivity analysis  is the  simplest and most commonly used technique in water quality
           modeling (U.S. EPA, 1995g). Sensitivity analysis assesses the impact of the uncertainty
           of one or more input variables on the simulated output variables.

           First-order  analysis is  used in  a manner similar to sensitivity  analysis where input
           variables are assumed to be independent, and the model is assumed to respond linearly
           to the input  variables. In addition to estimating the change of an output variable with
           respect to an input variable, first-order error analysis also estimates the output variance.

           Monte Carlo simulation,  a more complex technique, is a numerical procedure where
           an input variable is  defined to have a certain probability density function (pdf). Before
           each model  run, an input variable is randomly selected from each  predefined pdf. By
           combining the results of several model runs,  a  pdf  can be developed for the output
           variable which is useful in  predicting overall model results.  The number  of model runs
           is extremely large compared to the number of runs typically done for sensitivity or first-
           order error analysis. Monte Carlo analysis can be used to define uncertainty (due to
           uncertain model coefficients) and environmental  variability (using historical records to
           characterize the variability of inputs such as stream flow).
                                            7-26                               January 1999

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Chapter 7	CSS Modeling







       The main input variables for simulating the impact of CSO loadings are properties of the



mean rainfall event (storm event depth,  duration, intensity, and interval between events), CSO



concentrations of specific pollutants, design flow of the receiving water body, and its background



concentrations.8  The output consists of an assessment of the water quality impact in terms of



duration and frequency of exceedances of water quality criteria.  CSO pollutant concentrations are



the main "uncertain" (sensitive) input variables and can be varied over a range of reasonable values



to assess their impact on the resulting water quality. Uncertainty analysis can improve management



decisions and indicate the need for any additional data collection to refine the estimated loads. For



instance, if a small change in CSO pollutant concentrations results in an extremely large variation



in the prediction of water quality, it may be appropriate to allocate  resources to more accurately



estimate the CSO pollutant concentrations used in the model.







7.5    EXAMPLE SWMM MODEL APPLICATION





       This  section  applies the Storm Water Management Model (SWMM) to a single drainage area



from the example CSS drainage area presented in Chapters 4 and 5. While some of the details of



the application  are particular to the SWMM model, most of the explanation applies to a range of



hydraulic  models. The TRANSPORT block of the SWMM model was chosen for the flow routing



because the system hydraulics  did not include extensive surcharging, and the system engineers felt



that a dynamic hydraulic model such as SWMM EXTRAN was not needed to accurately predict the



number and volume of CSOs.







7.5.1 Data Requirements





       The first step in model  application is defining the limits of the combined sewer service area



and delineating subareas draining to each outfall (see  Exhibit 7-7). This can be done using a sewer



system map, a topographic map, and aerial photographs as necessary. The  modeler next must decide



what portions of the  system to model  based on their  contributions  to CSOs (as  illustrated  in



Example 4-1).  The modeler then  divides selected portions of the  CSS and drainage area into



segments and translates drainage area and sewer data  into model parameters. This process, referred
    Continuous simulations do not require use of the "mean" rainfall event or "design" flow data.
                                           7-27                               January 1999

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                                                        Exhibit 7-7. Drainage Area Map
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oo
                                                                                                                 CSO Outfe.ll Drainage

                                                                                                                 Area

                                                                                                             *  CSO Outfell
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                                                                    Not to Scale

-------
Chapter 7	CSS Modeling







to as discretization, begins with the identification of drainage boundaries, the location of major sewer



inlets using sewer maps, and the selection of channels and pipes to be represented in the model. The



drainage area is then further divided into subareas, each of which contributes to  the nodes of the



simulated network.







       The modeler must consider the tradeoff between a coarse model that simulates only the



largest structures in the CSS, and a fine-scale model that considers nearly every portion of the CSS.



A coarse model requires less detailed knowledge of the system, less model development time, and



less computer time. The  coarse model, however, leaves out details of the system such as small  pipes



and structures  in the upstream end of the CSS. Flow in systems that are limited by upstream



structures and flow capacities will not be simulated accurately.







       Where pipe capacities limit the amount of flow leaving a drainage area or delivered to the



wastewater  treatment plant,  the modeler should use the flow routing features  of the model to



simulate channels and pipes in those areas of concern. The level of detail should be consistent with



the minimum desired level of flow routing resolution. For example, information cannot be obtained



about upstream  storage unless the upstream conduits and their subcatchments  are simulated. Further,



sufficient detail needs to be provided to allow control options  within the system to be evaluated for



different areas.







       In this example, the modeled network is  carried to points where the  sewers  branch into pipes



smaller than 21 inches. The system is not directly modeled upstream of these points. Instead, runoff



from the  upstream area is estimated and routed into the 21-inch pipes. Exhibit 7-8 shows the



modeled sewer lines and the subareas tributary  to those lines  for Service Area 1.
                                            7-29                               January  1999

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Chapter 7
                                                                 CSS Modeling
                      Exhibit 7-8. Sewer Network and Subareas
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           Service Area ttl
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                                        7-30
                                                                  January 1999

-------
Chapter 7	CSS Modeling

7.5.2  SWMM Blocks
       RUNOFF block. The RUNOFF block of SWMM generates surface runoff and pollutant
loads in response to precipitation input and modeled surface pollutant accumulations.  The main data
inputs for the RUNOFF block are:

          subcatchment width
          subcatchment area
          subcatchment imperviousness
          subcatchment ground slope
          Manning's roughness coefficient for impervious and pervious areas
          impervious and pervious area depression storage
          infiltration parameters.

Exhibit 7-9 shows the main RUNOFF block  data inputs  (by subcatchment area number) for the
example. The subcatchment area is measured directly from maps. Subcatchment width is generally
measured from the map, but is more subjective when the subcatchment is not roughly rectangular,
symmetrical  and uniform.   Slopes are taken from  topographic maps, and determinations  of
imperviousness,  infiltration parameters,  ground  slope, Manning's  roughness coefficients, and
depression storage parameters are based on field observations and aerial photographs.

       The RUNOFF block data file is  set up  to generate an interface file that transfers hydrographs
generated by the RUNOFF block to subsequent SWMM blocks  for further processing. In this
example, the  data generated in the RUNOFF block are processed by the TRANSPORT block.

       TRANSPORT block.  The TRANSPORT block  is  typically used to route flows and
pollutant loads through the sewer system. TRANSPORT also allows for the introduction of dry
weather sanitary and infiltration flow to the system. Exhibit 7-10 presents the main TRANSPORT
block  inputs  by element number. It lists the number and type of each element (including upstream
elements), the element length (for pipe elements), and inflow (for manholes).
                                          7-31                              January 1999

-------
Chapter 7
CSS Modeling
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0
0
0
0
0
0
0
0
0
0
0
0
0
0
Element
Tvne
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
sewer pipe
manhole
Inflow (cfs)
[for manhole]
or Length (ft)
[for pipe element]
0.087
1000
0.188
840
0
390
0.097
651
0.163
733
0.076
841
0.176
620
0.136
727
0.103
771
0.221
1110
0.258
1007
0.131
Pipe
Dimension
(ft)
NA
.45

2.75

1.75

2.0

4.5

4.0

4.0

3.5

3

2.75

1.75

Pipe Slope
(ft/10 ft)
NA1
0.5

0.28

0.39

0.34

0.07

0.16

0.09

0.12

0.16

0.13

0.4

Manning Pipe
Roughness
(n)
NA1
0.014

0.014

0.014

0.014

0.014

0.014

0.014

0.014

0.014

0.014

0.014

is not applicable for manholes.

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Chapter 7	CSS Modeling

       The inflow parameter allows for introduction of dry-weather (sanitary) flow to the system.
Dry-weather flow is typically distributed proportional to area served. Here it is set to 0.0035 cfs per
acre.  If the  records  are available, this  parameter can be refined by multiplying the per-capita
wastewater flow (typically available from the wastewater treatment plant or latest facilities plan) by
the average population density calculated from census figures and sewer service area maps.

7.5.3  SWMM Hydraulic  Modeling
       Exhibit  7-11  shows the output  hydrograph for  element (manhole)  125  from  the
TRANSPORT block,  with the measured flow for the event plotted for comparison. The peak flow,
shape of the hydrograph, and the total volume of overflow for thiscalibration run are very close to
the measured values.

       The SWMM model is applied to monitored drainage  areas within the CSS using available
monitoring data to calibrate the hydraulic portions of the program to monitored areas.  For outfalls
that are  not monitored, parameters are adjusted  based on  similar monitored areas and  on  flow  depths
or flow determinations obtained from the initial system characterization (see Chapter 3). Once the
entire CSS drainage area is modeled and the SWMM model calibrated, the model then needs to be
validated.  It can then be used to predict the performance of the system for single events (actual or
design)  and/or for a continuous rainfall record. Recall that it is desirable to  calibrate the model to
a continuous sequence  of storms  if is to be applied to a continuous rainfall record.  Individual storms
related to monitored events can be run to calculate the total  volume of overflow for the system. Peak
flow values from the SWMM hydrographs can be used for preliminary  sizing of conveyance
facilities that may be  needed to alleviate restrictions.

       To  predict the  number of overflows per  year, the  calibrated model  can  be run in a continuous
mode and/or for design storm events. In the continuous mode the model can  be run using the long-
term rainfall record (preferable where the data are available), or for a shorter period of time (e.g., for
a typical or extreme year from the example discussed throughout Chapter 5).  While the event mode
is useful for some design tasks and for estimating hourly loading for a fine-scale receiving water
model,  the continuous  mode is  preferable for evaluating the  number of overflows under  the
presumption approach. In this example, the model was  run in continuous mode, using data from the


                                            7-34                               January 1999

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Chapter 7	CSS Modeling

                                Exhibit 7-11. Flow Hydrograph
            90
            80
            70
          CO
          o40
            20 +
            10 +
               ooooooooooooooooo
               owpwowocopcqocoocoocoo
                                     TIME OF DAY (hours)
                                         Predicted  	I  Measured
38-year rainfall record. The model predicted that between 12 and 32 overflow events would occur
per year. The average-22 overflow events per year-is used for comparison with the 4-event-per-
year criterion in the presumption approach.  (Note that only one outfall  in the system needs to
overflow to trigger the definition of "CSO event" under the presumption approach.)

       Based  on model results, system modifications were recommended  as  part  of  NMC
implementation. After the NMC are in place, the model will be rerun to assess improvement and
the need for additional controls.

7.5.4 SWMM Pollutant Modeling
       Once the SWMM model has been hydraulically calibrated, it can be used  to predict pollutant
concentrations in the overflow.  The summary of the  flow-weighted concentrations generated by the
model can then be compared to composite values of actual samples taken during the course of the
                                           7-35                               January 1999

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Chapter 7	CSS Modeling

overflow. Plots of individual concentrations versus time (pollutographs) can also be used to match
the variation in concentration of a pollutant during the course of the  overflow. First flush effects can
also be observed from the model output if buildup/washoff is used.

       Model Results
       Exhibit 7-12 presents the BOD and total  solids output  of the SWMM model for the example
storm. Note that the modeled concentrations of both pollutants follow a similar pattern throughout
the overflow event with little if any first  flush concentration predicted in the  early part  of the
overflow. The initial loads assigned within the  model for this calibrated example were 70 pounds
per acre for BOD and 1,000 pounds per acre for total solids.  This model was previously calibrated
using monitoring data.

       Exhibit 7-13 presents predicted and observed values for BOD and total solids concentrations.
The observed concentrations are from analyses of composite samples collected in  an automated  field
sampler for this storm.  The modeled values give an approximate,  but not precise, estimate of the
parameters. While some  studies have  resulted in  closer predictions, this discrepancy between
predicted and observed pollutant values is not uncommon.

       The modeling in this example could be  useful for evaluating the CSS performance against
the four-overflow-event-per-year criterion  in the presumption approach. It could also be used to
evaluate  the performance of simple controls.
                                            7-36                               January 1999

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Chapter 7
                                        CSS Modeling
                               Exhibit 7-12. Pollutographs
   700 -
          CM   -V   O
                    S  S
8Sง88ง8|S||ง88



  TIME OF DAY (Hours)
                                                                                9  8
                                          - TS -•— BOD i
             Exhibit 7-13. Predicted and  Observed Pollutant Concentrations

Flow-weighted concentration (mg/1)
Predicted
BOD
31.4
TS
420
Observed
BOD
94
TS
300
                                           7-37
                                         January 1999

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Chapter 7	CSS Modeling

7.6    CASE STUDY
       Example 7-1 is  a case  study illustrating the CSS and CSO modeling strategy that was
developed and implemented by the City  of Indianapolis, Indiana. The  City,  after carefully evaluating
available options and regulatory requirements, developed this modeling strategy to characterize
system hydraulics and estimate average  annual  CSO  characteristics  (i.e.,  volume, frequency,  percent
capture, and pollutant loads). The City  used the CSS and CSO models to determine CSO impacts
on the receiving  streams  (the White River and  its tributaries within the City's combined sewer area),
and is now using the models to  evaluate various CSO controls and develop an LTCP.

       Recognizing  that  the interceptor sewers  and  regulators, not the  combined sewers, control wet-
weather system conveyance capacity to the wastewater treatment plants (and therefore control the
occurrences of CSOs), the City used SWMM/EXTRAN to develop a detailed model of interceptor
sewers  and regulators  that included  approximately 82 miles  of sewer,  173  regulators, and
134  outfalls.     The City  used SWMM/RUNOFF to generate runoff flows  from drainage
subcatchments and to calibrate wet-weather flow to the EXTRAN model. The City then used the
linked RUNOFF/EXTRAN models to establish  critical input data for the STORM model of the  CSS,
specifically the regulator/interceptor  capacities  (STORM "treatment  rates")  and the  impervious area
estimates (STORM "C" coefficients).   The  City  performed long-term  (44-year)  continuous
simulations using STORM to compute average annual CSO characteristics. The selected modeling
strategy enabled the City of Indianapolis to accurately determine interceptor sewer conveyance and
system storage capacities, identify system optimization projects, characterize overflows and pollutant
loads to receiving streams, and  evaluate various CSO control strategies.
                                           7-38                               January 1999

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   Chapter 7	CSS Modeling


                      Example 7-1. Modeling Case Study — Indianapolis, Indiana
The City of Indianapolis has a population of 741,952 (1990 census) and is the largest city in Indiana,  As reported in the City's
CSO Operational Plan (December, 1995), the service area includes a combined sewer area of approximately 41 square miles with
approximately 82 miles of interceptor sewer, 173 regulators, and 134 outfalls. Pipe sizes in the interceptor sewer system range
from 12 inches to 120 inches.

Model Development Strategy

The City used a two-phased modeling strategy to characterize its CSS. Phase 1 focused on the system of interceptor sewers and
regulators that deliver flow to the City's advanced WWTPs for treatment, since this part of the system controls the occurrence of
CSOs.  Phase 1 modeling analysis using SWMM/EXTRAN supported the characterization of the system (required under the CSO
Operational Plan), determined the hydraulic capacity of the interceptor sewer system to capture combined sewer flows  for
treatment, and identified low-cost capital improvement projects to maximize flows to the WWTPs.

For initial analysis and hydraulic characterization of CSOs, the City generated inflow hydrographs to the interceptor model using
a ramped hydrography function, which is a synthetic approximation of the rising limb of the actual inflow hydrographs associated
with most ram events. The site-specific inflow rates are defined as a function of the ramp slope and the impervious area within
the watershed tributary to each model inflow node. Ramped inflow hydrographs are more effective than observed or design event
hydrographs for analyzing and evaluating interceptor sewer system capacities and identifying constraints in the system.  However,
ramp hydrographs cannot be used to calibrate the EXTRAN and STORM models since the response to precipitation must be
simulated for calibration. For these reasons, the City used ramped inflow hydrographs in Phase 1 to estimate interceptor system
capacities and the SWMM/RUNOFF model for model calibration, and for more detailed analysis, in Phase 2. This let the City
efficiently perform initial analysis and hydraulic characterization of CSOs and identify low-cost capital improvement projects to
maximize the capture of combined sewer flows in Phase 1, even before model calibration was complete,

In Phase 2, modeling focused on characterization of sewersheds using a more detailed hydrologic (rainfall/runoff)  model
(RUNOFF) and linking this model directly to the EXTRAN interceptor model developed hi  Phase 1.  The City used the linked
models with flow monitoring data from a network of eight flow monitors in the interceptor system and rainfall data for calibration
and verification of the interceptor model. Phase 2 modeling also focused on developing and calibrating the CSO model (STORM),
using flow monitoring and sampling data at four representative outfalls and simulations to characterize the volume, frequency,
and pollutant loads of CSOs. Using regulator/interceptor capacities ("T") from the EXTRAN model, and impervious area estimates
("C") from RUNOFF, the City performed continuous simulations using STORM and the available historical (44-year) hourly
precipitation data to generate average annual CSO statistics. STORM can efficiently perform long-term simulations because it
uses constant values for "T" and "C", which in the prototype system may vary during long-term simulation.  For example, "C"
values may vary due to changes in soil moisture conditions in the subcatchments.  Therefore a range of values for the CSO
characteristics were obtained to reflect these variations in system behavior.

CSS and CSO Characterization Results

As a result of Phase 1 modeling, the City developed its CSO Operational Plan to implement the NMC. The plan used the system
conveyance capacity and in-system storage analyses to define a program of hydraulic modifications to the system at 28 individual
locations. These modifications enhanced the capture of combined flows during wet weather and reduced overflows to the area's
smaller and most sensitive CSO receiving streams. During Phase 2, the City determined average annual CSO characteristics for
each CSO outfall, for each major drainage system, and on a system-wide basis using the STORM model of the CSS.

STORM simulations determined that an average annual CSO volume of 4,000 to 5,500 million gallons is discharged from the
CSS^ CSOs occur at an average frequency of 24 per year; and the interceptor system captures  59 to 66 percent of average annual
wet weather combined sewage flow.  STORM simulations were also used to estimate that the CSS discharges  1.8 to 2.5 million
pounds of BOD and 6.3 to 8.5 million pounds of TSS to the receiving streams on an average annual basis.  Based on Phase 2
modeling, the City identified five initial CSO facility projects to demonstrate the effectiveness of various CSO control alternatives.
These facilities are now under construction and STORM simulations have been used to estimate that untreated CSO volumes will
be reduced by over 80 percent at these five locations.
                                                   7-39                                  January 1999

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                                      CHAPTER 8






                           RECEIVING  WATER  MODELING








       This chapter discusses the use of receiving water modeling to evaluate CSO impacts to



receiving  waters.  It  uses the term "modeling" broadly to refer to a range  of receiving water



simulation techniques. This chapter introduces simplified techniques, such as dilution and decay



equations, and more complex computer models, such as QUAL2EU and WASP.







8.1     THE CSO CONTROL POLICY AND RECEIVING WATER MODELING





       Under the CSO Control Policy a permittee should develop a long-term control plan (LTCP)



that  provides for attainment  of water  quality standards  (WQS)  using either the demonstration



approach or presumption approach. Under the demonstration approach, the permittee documents



that the selected CSO  control measures will provide for  the attainment of WQS,  including designated



uses in the receiving water. Receiving water modeling may be necessary to characterize the impact



of CSOs on receiving  water quality and  to predict the improvements that would  result from different



CSO  control  measures. The presumption approach does not explicitly call for analysis of receiving



water impacts.







       In many cases,  CSOs discharge to receiving waters that  are water quality-limited and receive



pollutant loadings  from other sources, including  nonpoint sources and other point sources. The CSO



Control Policy states  that the permittee should characterize the  impacts of the CSOs and other



pollution sources on the receiving waters and their designated uses (Section II.C.I). Under the



demonstration approach, "[w]here WQS and designated uses are not met in part because of natural



background conditions  or  pollution sources other than CSOs,  a  total  maximum daily load, including



a wasteload allocation and a load allocation, or other means should be used to apportion pollutant



loads."  (Section II.CAb)
                                            8-1                                January 1999

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Chapter  8                                                        Receiving Water Modeling


       Established under Section 303(d) of the CWA, the  total maximum daily  load (TMDL)

process assesses point and nonpoint pollution sources that together  may contribute to a water body's
impairment. This process relies on receiving water models.


       An important initial decision-which water  quality parameters to model-should be based

on data from receiving water monitoring.  CSOs affect several receiving water quality parameters.
Since the impact on one parameter is frequently much  greater than on others, relieving this main

impact will likely also relieve  the others. For example, if a  CSO causes exceedances of bacteria
WQS by several hundredfold, as well as moderate dissolved oxygen (DO) depressions, solving the
bacterial problem will likely solve the DO problem  and so it may be sufficient to monitor bacteria

only. Reducing the scope of modeling in this fashion may substantially reduce costs.


8.2    MODEL SELECTION STRATEGY

       A receiving water model should be selected according to the following factors:
           The type and physical characteristics of the receiving water body. Rivers, estuaries,
           coastal areas, and lakes typically require different models.

         The water  quality parameters  to  be modeled.  These  may  include bacteria,  DO,
           suspended solids, toxics,  and nutrients.   These parameters are affected by different
           processes (e.g., die-off for  bacteria,  settling for solids, biodegradation for DO, adsorption
           for  metals)  with different  time  scales  (e.g.,  hours for bacterial die-off,  days for
           biodegradation) and different kinetics. The time scale in  turn affects the  distance over
           which the receiving water is modeled (e.g., a few hundred feet for bacteria to a few- miles
           for DO).

           The number and  geographical distribution of CSO outfalls and the need to simulate
           sources other than CSOs.
       This section discusses some important considerations for hydrodynamic and water quality

modeling of receiving waters, and how these considerations affect the selection and use of a model.
                                             8-2                                 January 1999

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Chapter 8                                                        Receiving Water Modeling


       The purpose of receiving water modeling is  primarily to predict receiving water quality under

different CSO pollutant loadings and flow conditions in the receiving water. The flow conditions,

or hydrodynamics, of the receiving water are an important factor in determining the effects of CSOs
on receiving water quality. For simple cases, hydrodynamic conditions can be determined from the
receiving water monitoring program; elsewhere a hydrodynamic model may be necessary.


       Hydrodynamic and water quality models are either steady-state or transient. Steady-state

models assume that conditions do not change  over time, while transient models  can simulate
conditions that vary over time. Flexibility exists in the choice of model  types; generally, either a

steady-state or transient water quality simulation can be done regardless of whether flow conditions

are steady-state or transient.


8.2.1 Hydrodynamic Models

       A hydrodynamic model provides the flow conditions,  characterized by the water depth and
velocity, for  which receiving water quality must  be predicted.  The  following factors should be

considered for different water body types:
          Rivers- Rivers generally flow in one direction (except for localized eddies or other flow
           features) and the stream velocity and depth are a function of the flow rate. The flow rate
           in relatively large rivers may not increase significantly due to wet weather discharges,
           and a constant flow can be used as a first approximation.  This  constant flow can be a
           specified low flow, the flow  observed during  model calibration surveys,  or a flow typical
           of a season or month. When the increase of river flow is important, it can be estimated
           by adding together all upstream flow inputs or by doing a transient flow simulation. The
           degree of refinement required also depends on  the time scale  of the water quality
           parameters  of interest. For example, assuming a constant river flow may suffice for
           bioaccumulative toxicants  (e.g., pesticides) because long-term  exposure is  ofimportance.
           For DO, however, the time variations in river flow rate may  be need to be considered.

          Estuaries-  CSO impacts in  estuaries  are affected by tidal  variations of velocity and
           depth (including reversal  of current direction) and by  possible salinity  stratification.
           Tidal fluctuations can be assessed by measuring velocity  and  depth variations over a tide
           cycle or by using a one- or two-dimensional model. Toxics with relatively small mixing
           zones can be analyzed using steady currents corresponding to different times during the
           tidal cycle, but this may require using a computed circulation pattern from a model.
                                             8-3                                 January 1999

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Chapter 8                                                       Receiving Water Modeling
           Coastal Areas- CSO impacts in coastal areas are also affected by tidal fluctuations. The
           discussion on estuaries generally applies to coastal areas, but, because the areas are not
           channelized, two-dimensional or even three-dimensional models may be necessary.

           Lakes- CSO impacts in lakes  are affected  by wind and thermal stratification. Wind-
           driven currents can be monitored directly or simulated using a hydrodynamic model
           (which may need to cover the entire lake to simulate wind-driven currents properly).
           Thermal stratification can generally be measured directly.
       Because the same basic hydrodynamic equations apply,  some of the  major models for
receiving waters can be used to simulate more than one type of receiving water body.  Ultimately,

three factors dictate whether a model can be used for a particular hydraulic regime.  One factor is

whether it provides a one-, two-, or three-dimensional simulation.  A second is its ability to handle

specific boundary conditions, such as tidal boundaries.


       A  third factor is whether the model  assumes  steady-state  conditions  or allows for
time-varying pollutant loading.  In general, models  that assume steady-state  conditions cannot

accurately  model  CSO problems that require analysis  of far-field effects. However,  in  some

instances a steady-load model can estimate the maximum potential effect, particularly in systems

where the transport of  constituents is  dominated by the main flow of the water body,  rather than local
velocity gradients. For example, by assuming a constant  source and following the peak discharge
plug of water downstream, the steady-load model  QUAL2EU  can  determine  the maximum
downstream effects of conventional pollutants. The result is a compromise that approximates the
expected  impact but  neglects  the  moderating effects  of longitudinal  dispersion.  However,
QUAL2EU cannot give an accurate estimate of the duration of excursions above WQS.


8.2.2  Receiving Water Quality Models

       The frequency and duration of CSOs are important determinants of receiving water impacts
and need  to be considered  in determining appropriate time scales for modeling. CSO  loads are
     The basic hydrodynamic equations are for momentum and continuity. The momentum equation describes the
motion of the receiving water, while the continuity equation is a flow balance relationship (i.e., total inflows to the
receiving water less total outflows is equal to the change in receiving water volume).


                                            8-4                                January 1999

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Chapter  8                                                       Receiving Water Modeling







typically delivered in pulses during storm events. Selection of appropriate time scales for modeling



receiving water impacts resulting from a pulsed CSO loading depends upon the time and space scales



necessary to evaluate the WQS. If analysis requires determining the concentration of a toxic at the



edge of a relatively  small mixing zone, a steady-state mixing zone model may be  satisfactory.  When



using a  steady-state mixing zone model in this way, the modeler should apply appropriately



conservative but  characteristic assumptions about instream flows  during CSO events. For pollutants



such as oxygen demand, which can have impacts lasting several days and extending several miles



downstream of the discharge point,  it may  be warranted to incorporate  the pulsed nature of the



loading.  Assuming  a constant loading is much simpler (and less costly)  to model;  however,  it  is



conservative (i.e., leads to impacts larger than expected). For pollutants such as nutrients where the



response time of the receiving water body may be slow, simulating only the average loading rate,



usually over a period of days (e.g., 21 days)  depending on the nutrient, may suffice.







       Receiving water models vary from simple estimations to complex software  packages. The



choice  of model  should reflect site conditions. If the  pulsed load and  receiving water characteristics



are adequately represented, simple estimations may be appropriate for the analysis of CSO impacts.



To demonstrate compliance with the CWA, the permittee may not need to know  precisely where  in



the receiving water excursions above WQS  will occur. Rather, the permittee needs to know the



maximum pollutant concentrations and the likelihood that excursions above the  WQS can occur  at



any point within  the water body.  However, since CSOs  to sensitive  areas are  given a higher priority



under the CSO Policy, simulation models for receiving  waters with sensitive areas may need to use



short time scales (e.g.,  hourly pollutant  loads), and have high resolution (e.g., several hundred yards



or less) to specifically  assess impacts to sensitive areas.







8.3    AVAILABLE MODELS





       Receiving water models cover a wide variety of physical and chemical situations and, like



combined sewer system  (CSS)  models, vary  in  complexity.  EPA has produced guidance on



receiving water  modeling  as part of the Waste Load  Allocation (WLA) guidance series. These



models, however, tend to concentrate on continuous sources and thus may not be the most suitable









                                            8-5                                January 1999

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Chapter 8                                                       Receiving Water Modeling







for CSOs. Ambrose et al. (1988a) summarizes EPA-supported models, including receiving water



models.







       This guidance does not provide a complete catalogue of available receiving water models.



Rather, it describes simplified techniques and provides a brief overview of relevant receiving water



models supported by EPA or other government agencies. In many cases, detailed receiving water



simulation may not be  necessary.  Use of dilution and mixing zone calculations or simulation with



simple spreadsheet  models may be sufficient to  assess  the magnitude of potential impacts or evaluate



the relative merits  of various control options.







       Types of Simulation



       Water quality parameters can be simulated using either single-event, steady-state modeling



or continuous, dynamic modeling.  Many  systems  may  find it beneficial  to  use  both types  of



modeling.







       Many of the simpler  approaches to receiving water evaluation assume steady flow and steady



or gradually varying  loading. These assumptions may be appropriate  if an order-of-magnitude



estimate or an upper bound  of the impacts is required. The latter is obtained by using conservative



parameters such as peak loading and low current speed.  If WQS  attainment is  predicted under



realistic worst-case assumptions, more complex simulations may not be needed.







       Due to the random nature of CSOs, the  use  of dynamic simulation may be preferable to



single-event, worst-case, steady-state modeling.  Dynamic techniques allow the modeler to derive



the fraction of time during which a concentration was  exceeded and water  quality  was  impaired.  For



instance, when using  daily simulated results, specific concentrations are first ranked with the



corresponding  number  of occurrences during the  simulation  period.  Frequency  distribution  plots  are



then developed and used to determine how often the  1-day-acute water quality criteria are likely to



be exceeded. The same approach can be used to develop frequency distributions for longer periods



such as 4-day or 30-day average  concentrations. EPA  (1991a) recommends three dynamic  modeling



techniques:  continuous simulation, Monte Carlo simulation, and lognormal probability modeling.






                                            8-6                                January 1999

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Chapter 8                                                        Receiving Water Modeling







       Continuous  simulation models solve  time-dependent differential equations to  simulate  flow



volume and water quality in receiving waters. These deterministic models incorporate the manner



in which flow and toxic pollutant concentrations change over time in a continuous manner rather



than relying on simplified terms for rates of change. They use daily effluent flow and concentration



data with daily receiving water flow and concentration data to estimate downstream  receiving water



concentrations. If properly calibrated and verified, a continuous  simulation model can predict



variable flow and water quality accurately-although at  a  considerable time  and resource



expenditure, however.







       Monte Carlo simulation is generally  used for  complex  systems  that have  random



components. Input variables are sampled at random from pre-determined probability distributions



and used  in a toxic fate and transport model. The distribution of output variables from repeated



simulations is analyzed statistically to derive a frequency distribution.  However, unlike continuous



simulation models, the temporal frequency  distribution of the output depends on the temporal



frequency distribution of the input data.  For instance, if the water quality criterion is based on a 4-



day average, the input variables  must use the probability distributions based on a 4-day average.







       Lognormal  probability  modeling  estimates  the same output  variable probability



distributions as continuous and Monte Carlo simulations but with less  effort. However, like Monte



Carlo simulation, the input must be probability distributions based on input data for the  specific



temporal frequency distribution desired. The theoretical basis  of the technique permits the stochastic



nature of the CSO process to be explicitly considered. This method assumes that each of the four



variables that affect downstream receiving water quality (rainfall, runoff, event mean concentration



of contaminant in the  runoff (EMC), and streamflow) can be adequately represented by a lognormal



probability   distribution. When the EMC is coupled with  a  lognormal distribution of runoff volume,



the distribution of runoff loads can be derived.  The storm water load frequency is then coupled with



a  lognormal  distribution of streamflow to derive the  probability distribution of in-stream



concentrations. The main advantage of lognormal probability modeling is that  the probability



distributions can be derived using only  the median and the coefficient of variation for each input



variable.






                                             8-7                                January  1999

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Chapter 8	Receiving Water Modeling


8.3.1 Model  Types

       The following sections discuss techniques for simulating different water quality parameters

in rivers, lakes and estuaries.



       RIVERS

       Bacteria and Toxics. Bacteria and  toxic contaminants  are primarily a concern  in  the

immediate vicinity of CSO  outfalls. They are controlled by lateral mixing, advection, and decay

processes such as die-off (for bacteria), vaporization (for toxics), and settling and resuspension (for

bacteria and toxics).  When stream flow is small relative to CSO flow,  lateral mixing may occur

rapidly and a  one-dimensional model may be appropriate. Initial estimates can be  made using a

steady-state approach  that neglects the time-varying nature of the CSO. In this case, concentrations

downstream of a CSO are given  by:

                                        n  C +Q C   —
                                   r _ *^u u ^e e   u
                                   L/              c-
       where:     Cx =   max pollutant concentration at distance X from the outfall (M/L )
                  Ce =   pollutant concentration in effluent (M/L )
                  Cu =   pollutant concentration upstream from discharge (M/L )
                  Qe =   effluent flow (L3/T)
                  Qu =   stream flow upstream of discharge (L /T)
                  QS =   stream flow downstream of discharge, Qu + Qe (L /T)
                 X  =   distance from outfall (L)
                 u  =   stream flow velocity (L/T)
                 K  =   net  decay rate  (die-off rate for bacteria,  settling velocity divided by
                         stream depth for settling, resuspension velocity divided by stream depth
                         for  resuspension,  vaporization  rate  divided  by stream  depth  for
                         vaporization) (1/T)
                  e  =   2.71828...

       Since bacteria and toxics can settle out of the  water column  and attach  to sediments,

sediments can  contain significant amounts of these  pollutants. Resuspension of sediments and

subsequent desorption of bacteria and toxics into the water column can  be an important source of

receiving  water  contaminants.    Modeling of  sediment resuspension  requires  estimation  of
   M=unit of mass, L=unit of length, and T=unit of time.
                                                                               January 1999

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Chapter 8                                                        Receiving Water Modeling


resuspension velocities and knowledge of sediment transport processes.  Thomann and Mueller
(1987) discusses how to determine the solids balance in a river and estimate sediment resuspension
velocities.  Modeling of sediment transport is complex and is often done using computer models
such as WASPS and HSPF.


       In large rivers, lateral mixing may occur over large distances and bacterial counts or toxics
concentrations on the same  shore as  the discharge  can be calculated using the following expression,
as a conservative estimate (U.S. EPA,  199la):
                                             C Q W
                                      s-t       e e
                                      c*= —
                                           Q,
                                             \
                                                   X
                                                   y
                                                      9 	
       where:    Dy =   lateral dispersion coefficient (L /T)
                  W =  stream width (L)
                  n  =  3.14159...
       This equation is conservative because it neglects any discharge-induced mixing. Simulating
over the correlated probability distributions of Ce, Qe, Qs, and Qu can provide an estimate of the
frequency of WQS exceedances at a specific  distance from the outfall. The method requires the
estimation of a lateral dispersion coefficient, which can be measured in dye studies or by methods
described in Mixing in Inland and Coastal Waters (Fischer et al., 1979). Fischer's methods calculate
the lateral dispersion coefficient Dy as follows:

                                    Dy = 0.6 du* ฑ 50%

       where:     d  =   water depth at the specified flow (L)
                  u* =  shear velocity  (L/T).


       In turn, the following equation estimates shear velocity:

                                         u*  =  (gdsf2
                                             8-9                                January 1999

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Chapter 8                                                       Receiving Water Modeling
       where:    g  =  acceleration due to gravity (L/T )
                 s   =  slope of channel (L/L)
                 d = water depth (L).
           The model DYNTOX (LimnoTech, 1985) is specially designed for analysis of toxics in

rivers and can handle all three dynamic modeling techniques. U.S. EPA (1991 a) and the WLA series

by Delos et al. (1984) address the transport of toxics and heavy metals in rivers.


           Oxygen Demand/Dissolved Oxygen.  The time scales  and distances affecting  DO

processes are  greater than for bacteria and toxics. Lateral mixing therefore results in approximately

uniform conditions over the river cross section and one-dimensional models are usually appropriate

for simulation. The WLA guidance (U.S. EPA, 1995g)  discusses the effects of steady and dynamic

DO loads, and provides guidelines for modeling impacts  of steady-state sources.  Simple spreadsheet

models such as STREAMDO IV (Zander and Love, 1990) have recently become available for  DO

analysis.


       In general, screening analyses using  classical steady-state  equations  can examine DO impacts

to rivers  as a result of episodic loads. This approach assumes plug flow, which in turn allows an

assumption of constant loading averaged over the volume of the plug (Freedman and Marr, 1990).

This approach does not consider longitudinal diffusion from the plug,  making  it a  conservative

approach. The plug flow analysis should correlate with the duration of the CSO. For example, a

plug flow simulation of a 2-hour CSO event would result in a downstream DO sag that would also

last for 2 hours. Given the plug flow assumption, the classic Streeter-Phelps equation can estimate

the DO concentration downstream:
                                           8-10                               January 1999

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Chapter 8                                                        Receiving Water Modeling
       where:    D  =  DO deficit downstream (M/V)
                 D0 =  initial DO deficit (M/V)
                 Ka =  atmospheric re-aeration rate (1/T)
                  t  =  time of passage from source to downstream location (T)
                  W =  total pollutant loading rate (M/T)
                  Q =  total river flow (V/T)
                 Kd =  biochemical oxygen demand (BOD) deoxygenation rate (1/T)
                 Kr =  BOD loss rate (1/T).
       This method can address the joint effects of multiple steady sources through the technique
of superposition (Exhibit  8-1).  Superposition  is used when linear  differential equations,  such  as the

Streeter-Phelps equation, govern pollutant concentrations along a receiving stream. For such linear

systems, the concentration of a pollutant in a river due to multiple steady-state sources is the linear
summation of the responses due to the individual sources.  Superposition techniques are also used

to estimate  pollutant concentrations due to multiple  steady-state sources of toxic pollutants.

However, it cannot address multiple sources that change over time, nor can it address the effects of
river morphology. When such issues are important,  more sophisticated modeling  techniques are

necessary.


       More sophisticated modeling techniques are also necessary to assess the effects of sediment
oxygen demand (SOD) and plant respiration (which remove oxygen from the receiving water), and

photosynthesis  by aquatic plants (which adds oxygen to the water). The Streeter-Phelps equation

makes  the  simplifying  assumption that there  are only point sources of CBOD, so  SOD,
photosynthesis, and respiration are assumed to be zero. If photosynthesis, respiration, and SOD are
significant, more complex analysis is  needed to evaluate these factors.  These distributed  sources and
sinks of DO and BOD are addressed by Thomann and Mueller (1987) and by several computer
models, including QUAL2EU and WASPS.


       Nutrients/Eutrophication. Nutrient discharges affect river eutrophication over time scales
of several days to several weeks. Nutrient/eutrophication analysis  considers  the  relationship between
                                            8-11                                January 1999

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                              Exhibit 8-1. Dissolved Oxygen Superposition Analysis
CO
 I


bo
     x
     O
         7 -R
                           Other Sources of BOD
         6 --
9 3

o
CO
02

Q o
                           Point Sources of BOD
                           CSO BOD
ง
I
                                      0               15



                                         River Mile
                                                                           20
25

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Chapter 8                                                        Receiving Water Modeling
nutrients and algal growth. Analysis of nutrient impacts in rivers is complex because nutrients and

planktonic algae,  which are  free-floating one-celled algae,  usually move through the system rapidly.
       The current WLA guidance (U.S. EPA, 1995g) considers only planktonic algae (rather than

all  aquatic plants) and discusses nutrient loadings  and eutrophication in rivers primarily as a

component in computing DO. The guidance applies  to narrative criteria that limit nuisance plant

growth in large, slowly flowing rivers.


        LAKES

       Bacteria and Toxics. Mixing zone analysis can often be used to assess attainment of WQS

for bacteria  and toxics in  lakes.   For a small lake  in which the effluent mixes  rapidly,  the

concentration response is given by the following equation (Freedman and Marr, 1990):
       where:     C  = concentration  (M/L )
                 M = mass  loading  (M)
                  Q  = flow (L3/T)
                 K  =  net decay rate (bacteria die-off, settling and resuspension, volatilization,
                        photolysis, and other chemical reactions) (1/T)
                  V =  lake volume (L )
                  t  =  time  (T).
       For  an incompletely-mixed lake,  however, a  complex simulation model is  generally

necessary to estimate transient impacts from slug loads. The EPA WLA guidance series contains

a manual on chemical models for lakes and impoundments (Hydroqual, Inc., 1986). This guidance,

which also  applies to bacteria, describes  simple and complex models  and presents criteria for

selecting models and model parameters.
   3 Aquatic plants can be divided into those that move freely with the water (planktonic aquatic plants) and those that
are attached or rooted in place.


                                            8-13                               January 1999

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Chapter 8                                                       Receiving Water Modeling


       Oxygen Demand/Dissolved Oxygen. Simple analytical approximations can model oxygen

demand and DO in cases where DO mixing occurs quickly relative to depletion by COD/BOD.

Where lateral mixing occurs rapidly but vertical temperature stratification exists, DO concentration

can be addressed for a two-layer stratified lake under the following simplifying assumptions (from

Thomann and Mueller, 1987):


          The horizontal area is constant with depth

          Inflow occurs only to the surface layer

          Photosynthesis occurs only in the surface layer

          Respiration occurs at the same rate throughout the lake

          The lake is at steady-state.


       With these severe restrictions, the solution is given by:
                     ,   q  .   ^,   L  .
                 c =( — - — )c +( - )c
                     KL+q  ฐ   KL+q
                                  KL  .    pHl -RH -SB   KdlH^L -K  H L
and
                                c =c -
                                     1
                                               ฃ///.
where the subscripts 1 and 2 refer to the epilimnion (top layer) and hypolimnion (lower layer),

respectively, and variables without subscripts refer to the whole lake, and where:
          q      =   Outflow rate (L/T)
          KL     =   DO transfer rate at lake surface (L/T)
          c      =   DO concentration (M/L )
          GO,  cs =   Initial and saturation dissolved oxygen concentrations (M/L )
          p      =   Gross photosynthetic production of DO (m/L -T)
          H     =   Depth (L)
          Ht     =   H/2 when H2 = H2 and H2 when H2 ป H2 (L)
          R      =   Phytoplankton DO respiration (M/L3-T)
                                           8-14                               January 1999

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Chapter 8                                                       Receiving Water Modeling
           SB    =  Sediment oxygen demand (M/L -T)
          K     = Deoxygenation coefficient (1/T)
          L      =  Steady-state CBOD concentration in water column (M/L3), =
                     where W is the mass loading rate, Q is the rate of flow through the lake, V is
                     the volume, and K,. is the net loss rate.
                 = Dispersion coefficient  (L /T).
       Because this analysis assumes  steady-state loading and because measuring some  of the

parameters proves difficult, the method may only have limited application to CSOs. A modeler able

to define all of the above parameters may choose to apply a more spatially resolved model.


       In many cases, complex simulation models are necessary to analyze DO in lakes. These are

either specialized lake models or flexible models, such as EUTROWASP, that are designed to

address issues specific to lakes.   Some experienced modelers have been successful in modeling

thermally stratified lakes with one or two dimensional river models (e.g., QUAL2EU) that assume

the river bottom is the thermocline.


       Nutrient/Eutrophication Impacts. For lakes, simple analytic equations often can analyze

end-of-pipe impacts and whole-lake impacts, but evaluating mixing phenomena frequently requires

a complex computer model (Freedman and Marr, 1990). Simple analytical methods can be applied

to lake nutrient/eutrophication impacts  in  situations  where the CSOs mix  across the lake  area within

the time scale required to obtain a significant response  in the algal population.  In most lakes,

phosphorus is considered to be the limiting nutrient for nuisance algal impacts and eutrophication.

Mancini et al. (1983) and Thomann and Mueller (1987) have developed a procedure for calculating

the allowable surface loading rate. The following steps are drawn from this procedure:


       Step 1.     Estimate the lake volume, surface area,  and mean depth.

       Step 2.     Estimate the mean annual inflow and outflow rates. Where urban areas draining
                  to the  lake constitute a significant fraction of the total drainage  area,  flow
   4 Such techniques should not be used by inexperienced modelers as they can lead to inaccuracies if they are not used
with caution.
                                            8-15                               January 1999

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Chapter 8                                                         Receiving Water Modeling
                  estimates from urban runoff and CSOs should be included in the hydrologic
                  balance around the lake. For lakes with large surface areas, the estimate should
                  include surface precipitation and evaporation.

       Step 3.     Determine the average  annual total phosphorus loading due to  all sources,
                  including all tributary inflows,  municipal  and industrial sources, distributed urban
                  and rural runoff,  and atmospheric  inputs.  Technical Guidance  Manual for
                  Performing Waste Load Allocation (Mancini et al., 1983) discusses techniques
                  for estimating these loadings.

       Step 4.     For total phosphorus, assign a net sedimentation loss rate that is consistent with
                  a local data base.

       Step 5.     Select trophic state objectives  of  either total  phosphorus  or chlorophyll-a
                  consistent with local experience. Calculate the  value of the  allowable phosphorus
                  areal  loading,  W ' ,  from:
                                                           s


                  where:     W is the allowable areal surface loading rate (M/L -T)
                             a  is the  trophic state objective concentration of total phosphorus or
                                chlorophyll-a (M/L3),
                             Q  is outflow (L3/T),
                             V  is lake volume (L ),
                             z  is mean depth (L), and
                             v,  is the net sedimentation velocity (L/T).

       Step 6.     Compare the total areal loading determined in Step 3 to the value of W1 obtained
                  in Step 5.
       Additional approaches are discussed in Reckhow and Chapra (1983b).


       ESTUARIES

       Unlike most rivers,  estuaries are tidal (i.e., water moves upstream during portions of the tidal

cycle and downstream during other parts of the cycle).  When averaged on the basis of tidal cycles,

pollutant transport in narrow, vertically mixed estuaries with dominant longitudinal flow is similar

to that in rivers.  However, due to tidal reversals of flow, a narrow estuary may have a much larger

effective dispersion  coefficient since shifting  tides may cause greater lateral dispersion. In such a

system, the modeler can apply approximate or screening models used for rivers, provided that an


                                            8-16                                January 1999

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Chapter 8                                                        Receiving  Water Modeling







appropriate  tidal dispersion coefficient has been calculated. In wider  estuaries, tides and winds  often



result in complex flow patterns and river-based models would be inappropriate. WLA guidance for



estuaries is  provided in several EPA manuals (Ambrose et al., 1990;  Martin et al., 1990; Jirka, 1992;



Freedman et al., 1992).







       In addition to their tidal component, many estuaries are characterized by salinity-based



stratification.  Stratified estuaries have the horizontal mixing due to advection and dispersion that



is associated with rivers and the vertical stratification characteristic of lakes.







       In  complex  estuaries,  accurate  analysis  of  far-field  CSO  impacts-such  as  nutrients/



eutrophication, DO, and impacts on particular sensitive areas-typically requires complex  simulation



models.  Simpler analyses are sometimes possible by treating the averaged effects of tidal and



wind-induced  circulation  and mixing as temporally  constant  parameters. This approach may require



extensive site-specific calibration.







       Near-field mixing zone analysis in estuaries also presents special problems, because of the



role of buoyancy differences in mixing. Jirka (1992) discusses mixing-zone modeling for estuaries.







8.3.2   Computer Models Supported by EPA or Other Government Agencies




       This section describes some computer models relevant to receiving water modeling. Most



of these models are supported by EPA's Center  for Exposure  Assessment  Modeling (CEAM).



CEAM maintains  a distribution center for water  quality  models  and related data bases.



CEAM-supported models relevant to modeling  impacts on receiving water  include QUAL2EU,



WASPS, HSPF, EXAMSII,  CORMTX,  MTNTEQ, and SMPTOX3.  The applicability and key



characteristics of the  CEAM-supported  models are  summarized in Exhibit  8-2.
    See Section 7.3 for information on obtaining models from CEAM.
                                            8-17                               January 1999

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                                          Exhibit 8-2. EPA CEAM-Supported  Receiving Water Models
                                                   Applicability to Hydraulic Regimes and Pollutant Type

Model
QUAL2EU
WASPS
HSPF
EXAMSII
CORMIX
MINTEQ
SMPTOX3
Rivers & Streams
Nutrients
/
/
/

Oxygen
/
/
/

Other
/
/
/
/
Lakes & Impoundments
Nutrients Oxygen Other

/ / /
/ / /
/
Estuaries
Nutrients

/


Oxygen Other

/ /

/
Near-field mixing model for all water body types
Equilibrium metal speciation model


/



Near Field Mixing





/


Key Characteristics and References
Model
QUAL2EU
WASPS
HSPF
EXAMSII
CORMIX
MINTEQ
SMPTOX3
Pollutant Loading Type
Steady
Dynamic
Dynamic (integrated)
Dynamic
Steady (near field) '
Steady
Steady
Transport Dimensionality
1-D
Quasi-2/3-D (link-node)
1-D
User input (quasi 3-D)
Quasi-3-D (zonal)
None
1-D
Current
Version
3.22
5.10
10.11
2.96
2.10
3.11
2.01
Key References
Brown & Barnwell, 1987
Ambrose, et al., 1988
Johanson, et al., 1984
Burns, etal., 1982
Doneker & Jirka, 1990
Brown & Allison, 1987
LimnoTech, 1992
oo
 i
oo
        CORMIX was originally developed assuming steady ambient conditions; Version 3 allows for application to some unsteady environments (e.g., tidal reversal
       conditions) where transient recirculation and pollutant build-up can occur (CEAM, 1998).

-------
Chapter 8                                                        Receiving Water Modeling


       QUAL2EU is a one-dimensional model for rivers. It assumes steady-state flow and loading

but allows simulation of diurnal variations in temperature or algal photosynthesis and respiration.

QUAL2EU  simulates temperature, bacteria, BOD, DO, ammonia, nitrate, nitrite, organic nitrogen,

phosphate, organic phosphorus, algae, and additional conservative substances. Because it assumes

steady flow and pollutant loading, its applicability to CSOs is limited. QUAL2EU can, however,

use steady loading rates to generate worst-case projections for CSOs to rivers. The model has pre-

and post-processors for performing uncertainty and sensitivity analyses.


       Additionally, in certain cases, experienced users may be able to use the  model to simulate

non-steady pollutant loadings  under  steady  flow  conditions by establishing certain initial  conditions

or by  dynamically varying climatic conditions.  If used in this way,  QUAL2EU should be considered

a screening  tool since the model was not designed to simulate dynamic quality conditions.


       WASPS is a  quasi-two-dimensional or quasi-three-dimensional water  quality model  for

rivers, estuaries, and  many  lakes. It has a link-node  formulation, which simulates storage at  the

nodes  and transport along the links. The  links  represent a  one-dimensional solution of  the  advection

dispersion equation, although quasi-two-dimensional  or  quasi-three-dimensional simulations  are

possible if nodes are connected to  multiple links.  The model also  simulates limited sediment

processes. It includes the time-varying processes of advection, dispersion,  point and nonpoint mass

loading, and boundary exchanges. WASPS can be used in two  modes: EUTRO5 for nutrient and

eutrophication analysis and  TOXI5 for analysis  of toxic pollutants and metals.


       WASPS is essentially a pollutant fate and transport model. Transport can be driven by

another hydrodynamic model such as DYNHYD5. DYNHYD5 is  a  one-dimensional/quasi-two-

dimensional model that simulates transient hydrodynamics  (including tidal  estuaries).
   6 A conservative substance is one that does not undergo any chemical or biological transformation or degradation
in a given ecosystem. (U.S. EPA, 1995g)


                                             8-19                                January 1999

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Chapter 8                                                        Receiving Water Modeling







       HSPF  is a one-dimensional,  comprehensive hydrologic  and  water quality simulation package



which can simulate both receiving waters and runoff to CSSs for conventional and toxic organic



pollutants. HSPF simulates the transport and fate of pollutants in rivers and reservoirs. It simulates



three  sediment types:  sand, silt, and clay.







       EXAMSII can rapidly evaluate the fate, transport, and exposure concentrations of steady



discharges  of synthetic organic chemicals to aquatic  systems. A recent upgrade of the model



considers seasonal variations in  transport and  time-varying  chemical loadings, making  it



quasi-dynamic. The user must specify transport fields to the model.







       CORMIX is an expert system for  mixing zone analysis. It can simulate submerged or



surface, buoyant or non-buoyant discharges into stratified or unstratified receiving waters, with



emphasis on the geometry and dilution characteristics of the initial mixing zone.  The model uses



a zone  approach, in  which  a  flow classification scheme determines which near-field mixing



processes to calculate.  The CORMIX model  cannot be calibrated in the classic  sense since rates are



fixed based on the built-in logic of the  expert system.







       MINTEQ determines  geochemical  equilibrium for priority pollutant metals.  Not  a transport



model, MINTEQ provides a means for modeling metal partitioning in discharges.  It provides only



steady-state predictions.  The  model  usually must  be run in connection with another fate and



transport model, such as those described above.  A number of  assumptions (e.g., equilibrium



conditions at the point of mixing between a CSO and the receiving water) must be made to link



MINTEQ predictions to another fate  and transport model,  so it should be  used cautiously in



evaluating wet weather impacts.







       SMPTOX3 is  a one-dimensional  steady-state  model for  simulating the  transport  of



contaminants in the water column and bed sediments in streams and non-tidal rivers. SMPTOX3



is an interactive computer program that  uses an EPA technique for calculating concentrations  of
   7 In some applications CORMIX has proven inaccurate for single port discharges.
                                            8-20                               January 1999

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Chapter 8                                                        Receiving Water Modeling




toxic substances in the water column and stream bed as a result of point source discharges to streams

and rivers. The model predicts pollutant concentrations in dissolved and particulate phases for the

water column and bed sediments, as well as total suspended solids.  SMPTOX3 can be run at three

different levels of complexity:  as  described above (highest complexity),  to calculate toxic water

column concentrations but no interactions with bed sediments (medium complexity), or as a total

pollutant toxics model (low complexity) (LimnoTech,  1992).



       	                                                                               o
       The following additional models are supported by EPA or  other government agencies:




       DYNTOX is a one-dimensional, probabilistic toxicity dilution model for transport in rivers.

It provides continuous, Monte Carlo, or  lognormal probability simulations that  can  be used to

analyze the frequency and duration  of ambient toxic  concentrations resulting from a waste discharge.

The  model considers dilution  and net first-order loss,  but  not sorption and benthic  exchange.

DYNTOX Version 2.1 and the draft manual are available from the Office of Science  and  Technology

in EPA's Office of Water (202-260-7012).




       CE-QUAL-W2 is a reservoir and narrow estuary hydrodynamics and water quality model

developed by the Waterways Experiment Station of the U.S. Army Corps of Engineers.  The model

provides dynamic two-dimensional (longitudinal and vertical) simulations. It accounts for density

effects on flow as a function of the water temperature, salinity and  suspended solids concentration.

CE-QUAL-W2 can  simulate up  to  21 water quality parameters in addition  to temperature, including

one passive tracer (e.g., dye), total dissolved solids, coliform bacteria, inorganic suspended solids,

algal/nutrient/DO dynamics (11 parameters),  alkalinity, pH and carbonate species  (4 parameters).
   8 McKeon and Segna (1987), Ambrose et al. (1988a) and Hinson and Basta (1982) have reviewed some of these
models.
                                            8-21                                January 1999

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Chapter 8                                                        Receiving Water Modeling


8.4    USING A RECEIVING WATER MODEL

       As  was the case for CSS  models (see Section 7.4), receiving water modeling  involves
developing the model, calibrating and  validating the model,  performing the simulation,  and

interpreting the results.


8.4.1  Developing the  Model

       The input  data needs for  a  specific receiving water model  depend upon the hydraulic regime
and model used. The permittee should refer to the model's documentation, the relevant sections of
the WLA guidance, or to texts such as Principles of Surface Water Quality Modeling and Control
(Thomann and Mueller, 1987).  Tables B-2 through B-5  in Appendix B contain general tables of data

inputs.


8.4.2   Calibrating and Validating the  Model

       Like CSS models, receiving water models need to be calibrated and validated. The model

should be run to simulate events for which receiving water hydraulic and quality monitoring were

actually conducted, and the model results should  be  compared  to the measurements. Generally,
receiving water models are calibrated and validated first for receiving water hydraulics  and then for
water quality. Achieving a high degree of accuracy in calibration can be difficult because:


           Pollutant loading inputs typically are estimates rather than precisely known values.

           Three-dimensional receiving water models are  still  not  commonly used for CSO projects,
           so receiving water models  involve  spatial averaging (over  the depth, width or cross-
           section).  Thus, model results are not directly  comparable  with  measurements, unless the
           measurements also have sufficient spacial  resolution to allow comparable averaging.

           Loadings from non-CSO sources, such as storm water, upstream boundaries, point
           sources, and atmospheric deposition, often are not accurately known.

           Receiving water hydrodynamics are  affected by numerous factors which are difficult to
           account for. Those include  fluctuating winds, large-scale eddies, and density  effects.
                                            8-22                               January 1999

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Chapter 8                                                        Receiving Water Modeling







       Although these factors make model calibration challenging, they also underscore the need



for calibration to ensure that the model reasonably reflects receiving water data.







8.4.3   Performing the Modeling Analysis




       Receiving water  modeling can involve  single  events or long-term simulations. Single event



simulations are usually favored when  using complex models, which require more input data and take



significantly longer to run (although advances in computer technology keep pushing the limits of



what can practically be achieved.) Long-term  simulations can predict water quality impacts on an



annual basis.







       Although a general goal is to predict the number of water quality criteria exceedances,



models can evaluate exceedances using different measures, such as hours of exceedance at beaches



or other critical points, acre-hours of exceedance, and mile-hours of exceedance along a shore.



These  provide a more refined measure of the  water quality impacts of CSOs and of the expected



effectiveness of different control measures.







       CSO loadings commonly are simulated separately from other loadings in order to assess the



relative impacts of CSOs.  This is appropriate because the equations that best approximate receiving



water quality are usually linear and so effects are additive (one exception, however, is the non-linear



algal growth response to nutrient loadings).







8.4.4 Using Modeling  Results





       By calculating averages over space and time, simulation models predict CSO volumes,



pollutant  concentrations,  and other variables of interest. The extent of this  averaging  depends  on  the



model structure, how the model is applied, and the resolution of the input data. The model's space



and time resolution should match that of the necessary analysis. For instance, the applicable WQS



may be expressed as a 1-hour average concentration not to exceed a given concentration more than



once every three years on average.  Spatial averaging may be represented by a concentration



averaged  over a receiving water mixing zone, or implicitly by the  specification of monitoring







                                            8-23                                January 1999

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Chapter 8                                                         Receiving Water Modeling







locations to establish compliance with instream criteria. In any case, the permittee should note



whether the model predictions use the same averaging scales required in the permit or relevant WQS.







       When used for continuous rather than event simulation, as suggested by the CSO Control



Policy,  simulation models  can predict the frequency of exceedances  of water quality  criteria.



Probabilistic models, such  as  the Monte Carlo simulation, also can make  such predictions. In



probabilistic models, the simulation is made over the probability distribution of precipitation and



other forcing functions such as temperature, point sources, and flow.  In either case, modelers can



analyze the output for the frequency of water quality criteria exceedances.







       The key result of receiving water modeling is the  prediction of future conditions due to



implementation of CSO control alternatives. In most cases, CSO control decisions will have to be



supported by model predictions of the pollutant load reductions necessary to achieve WQS. In the



receiving waters, critical or design water quality conditions might be periods of low flows and high



temperature that are established based on a review of available data. Flow, temperature, and other



variables for these periods then form the basis for analysis of future conditions.







       It is useful  to assess the sensitivity of model results to variations in parameters,  rate



constants, and coefficients. A sensitivity analysis can determine which parameters, rate constants,



and  coefficients merit particular attention in evaluating CSO control alternatives.  The modeling



approach should accurately represent features that are fully  understood,  and sensitivity  analysis



should  be used  to  evaluate the significance of factors  that are  not  as clearly  defined.  (See



Section 7.4.4 for additional discussion of sensitivity analysis.)
                                             8-24                               January 1999

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                                     CHAPTER 9

         ASSESSING RECEIVING WATER IMPACTS AND ATTAINMENT OF
                           WATER  QUALITY  STANDARDS


       This chapter focuses on the link between CSOs and the attainment of water quality  standards
(WQS). As discussed in previous chapters, permittees can consider a variety of methods to analyze
the  performance of the combined sewer system  (CSS) and the response of a water body to pollutant
loads. Permittees can use these methods to estimate  the water quality impacts of a proposed CSO
control program and evaluate whether it is adequate  to meet CWA requirements.


       Under the  CSO  Control  Policy, permittees  need to develop long-term control  plans (LTCPs)
that provide for WQS  attainment using either the  presumption approach or the  demonstration
approach.  This chapter  focuses  primarily on issues related to the demonstration approach since this
approach requires the permittee to demonstrate that the selected CSO controls will provide for the
attainment of WQS. As mentioned in Chapter 8, the presumption approach does not explicitly call
for analysis of receiving water impacts and thus generally involves less complex modeling.


       Modeling time-varying wet weather sources such as CSOs is more complex than modeling
more traditional point sources. Typically,  point-source modeling  assumes  constant pollutant loading
to a receiving water body  under critical, steady-state conditions-such as the minimum seven-
consecutive-day average  stream flow occurring once every ten years (i.e.,  7Q10). Wet weather loads
occur in pulses, however, and often have their  peak  impacts under conditions other than low-flow
situations.  This makes modeling the in-stream impact of CSOs more complicated  than modeling
the impacts of steady-state point source discharges such as POTWs. A receiving water model must
therefore accommodate the short-term variability of pollutant concentrations  and flow volume in the
discharge as well as the dynamic conditions in the receiving water body. Notwithstanding these
limitations, however, properly-applied modeling techniques can be useful in analyzing the impact
of CSOs on receiving waters.
                                           9-1                                January 1999

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Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS


       CSO pollutant loads can be incorporated into receiving water models using either a steady-

state or a dynamic approach,  as  discussed in Chapter 8.  A steady-state model can provide an

approximate solution using, for example, average loads for a design storm. A dynamic approach

incorporates time-varying loads and simulates  the time-varying response of the  water body. The

steady-state approximation uses some average conditions that do not account for the time-varying

nature of flows and loads.  Thus a steady-state model may provide less exact results, but typically

requires less cost and effort. A dynamic model requires more resources but may result in a more

cost-effective CSO control plan, since it does not use some of these simplifying assumptions.


       Generally,  the modeler should use the simplest approach that is appropriate  for local

conditions.  A steady-state model  may be appropriate in a  receiving water that  is relatively

insensitive to short-term variations  in load rate.  For instance, the response time of lakes and coastal

embayments to some pollutant  loadings may be measured in weeks to years, and the response time

of large rivers  to oxygen demand  may be measured in  days (Donigian and Huber, 1991). Steady-

state models  are also useful for  estimating the dilution of pollutants, such as acute toxins or bacteria,

close to the point of release.


9.1    IDENTIFYING RELEVANT WATER QUALITY STANDARDS

       The demonstration approach requires the permittee  to show that its selected CSO controls

will provide for attainment of WQS. The CSO Control Policy states that:


       The permittee should demonstrate...

          i. the planned control program is adequate to meet  WQS and protect designated uses,
          unless WQS or uses cannot be met as a result of natural background conditions or
          pollution sources other than CSOs;

          ii.  the CSO discharges remaining after implementation of the planned control program
          will not preclude the attainment of WQS or the receiving waters' designated uses or
          contribute to their  impairment.  Where WQS and designated uses are not met in part
          because of natural background conditions or pollution sources other than  CSOs, a total
          maximum daily load, including a wasteload allocation and a load allocation, or other
          means should be used to apportion pollutant loads... (Section II.CAb)


                                            9-2                               January 1999

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Chapter  9                       Assessing Receiving Water Impacts and Attainment of WQS


       The first step in analyzing CSO impacts on receiving water is to identify the pollutants or

stressors  of concern and the corresponding WQS. CSOs are distinguished from storm  water loadings
by the increased levels  of such pollutants  as  bacteria,  oxygen-demanding wastes,  and certain
nutrients. In some cases, toxic pollutants entering the CSS from industrial sources also may be of

concern.


       State WQS include designated uses and both numerical and narrative water quality criteria.

Since CSO  controls must ultimately provide for attainment of WQS, the analysis of CSO control

alternatives should be  tailored to the applicable WQS. For example,  if the  water quality criterion
of concern is expressed as a daily average concentration, the analysis should address daily averages.

Many water bodies have narrative criteria such as a requirement to limit nutrient loads to an amount
that does not produce a "nuisance" growth of algae, or a requirement to prevent solids and floatables

build-up. In such cases, the permittee could consider developing  a site-specific, interim numeric

performance standard that would result in attainment of the narrative criterion.


       As noted in Chapter 2, a key principle of the CSO  Control Policy is the review and revision,
as appropriate, of WQS and their implementation procedures.  In identifying applicable WQS, the
permittee and the permitting and WQS authorities should consider whether revisions to WQS are
appropriate for wet weather conditions in the receiving water.


       EPA's water quality criteria assist States in developing numerical standards and interpreting
narrative standards (U.S. EPA, 1991a).  EPA recommends that water  quality criteria for protection

of aquatic life have a magnitude-duration-frequency format, which requires that the concentration
of a given constituent  not exceed a critical value more than once in a given return period:
           Magnitude- The concentration of a pollutant, or pollutant parameter such as toxicity,
           that is allowable.

           Duration- The averaging period, which is the period of time over which the in-stream
           concentration is  averaged  for comparison  with  criteria concentrations. This specification
           limits the duration of concentrations above the criteria.
                                             9-3                                January  1999

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS







          Frequency- How often criteria can be exceeded.





       A  magnitude-duration-frequency  criteria  statement directly addresses protection  of the water



body by expressing  the acceptable likelihood of excursions  above the WQS.  Although  this approach



appears useful, it requires estimation of long-term average rates of excursion above WQS.







       Many  States rely instead  on the  concept  of design flows,  such as 7Q10. Evaluating



compliance at a design low flow of specified recurrence is a simple way to approximate the average



duration and frequency of excursions above the WQS. A single critical low flow, however, is not



necessarily the best  choice for wet-weather flows, which may  not occur simultaneously  with drought



conditions. Consequently, a design flow-based control strategy may be overly conservative,  and



suitable mainly for  situations where monitoring data are very limited or areas are highly sensitive.







       Some water quality criteria are expressed in formats that vary from the magnitude-duration-



frequency format. In some cases, such as State WQS for indicator bacteria, water quality criteria are



expressed as an  instantaneous maximum  and a long-term  average  component. The  long-term



average component of water quality  criteria  for  fecal  coliforms typically specifies  a 30-day



geometric mean or median, and  a certain small  percentage of tests performed within a 30-day period



that may exceed a particular upper value. For dissolved oxygen (DO) and pH, State criteria may be



expressed as fixed minimum concentrations, rather than as magnitude-duration-frequency.







       The  statistical form of the relevant WQS is important in determining an appropriate model



framework.  Does the permittee need to calculate  a  long-term average, a worst case maximum,  or



an actual  time sequence of the number  of water quality excursions? An  approach  that gives a



reasonable estimate of the average may not prove useful for  estimating an upper bound.
                                            9-4                                January 1999

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS


9.2    OPTIONS FOR DEMONSTRATING  COMPLIANCE

       Receiving water impacts can be analyzed at varying levels of complexity, but all approaches
attempt to answer the same question:  Using a prediction of the frequency and volume of CSO events
and the pollutant loads delivered by these events, can WQS in the receiving water body be attained

with a reasonable level of assurance?


       Any of the following types of analyses, arranged in order of increasing complexity, can be
used to answer this question:
         Design Flow Analysis-  This approach analyzes  the  impacts of  CSOs under  the
           assumption that they occur at a design condition (e.g., 7Q10 low flow prior to addition
           of the CSO flow).  The CSO is added as a steady-state load. If WQS can be attained
           under such a design condition, with the CSO treated as a steady source, WQS are likely
           to be attained for the actual wet weather conditions. This  approach is conservative in  two
           respects:  (1) it does not account for the short-term pulsed nature  of CSOs, and (2)  it does
           not account for increased receiving water flow during wet weather.

           Design Flow Frequency Analysis- Where the WQS is expressed in terms of frequency
           and  duration, the frequency of occurrence of CSOs can be  included  in the  analysis. The
           design flow approach can then be refined by determining critical design conditions that
           can  reasonably be expected to take place concurrently with  CSOs. For instance, if CSO
           events occur primarily in one season, the analysis can include critical flows and other
           conditions appropriate to that season, rather than the 7Q10.

           Statistical Analysis- Whereas the previous two approaches rely on conservative design
           conditions, a statistical analysis can be used to  consider the range of flows that may occur
           together with CSO events. This analysis more accurately reflects the frequency of WQS
           excursions.

           Watershed Simulation- A statistical analysis does not consider the  dynamic relationship
           between CSOs and receiving water flows. For example,  both the CSO and  receiving
           water flows increase  during wet weather.   Demonstrating  the  availability of  this
           additional capacity, however, requires a model that includes the responses of both the
           sewershed and its receiving water to the rainfall events. Dynamic watershed simulations
           may be carried out for single storm events or continuously for multiple storm events.
       The permittee should consider the tradeoffs between simpler and more complex types of
receiving water analysis. A more complex approach, although more costly, can generally provide


                                            9-5                                January  1999

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 Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS







 more precise analysis using less conservative assumptions. This may result in a more tailored, cost-



 effective CSO control strategy.







        Additional discussion on data assessment for determining WQS attainment is in Guidelines



for the Preparation of the 1996 State  Water Quality Assessments (305(b) Reports) (U.S. EPA,



 1995f).







 9.3     EXAMPLES OF RECEIVING WATER ANALYSIS





        This section presents three examples to illustrate key points for analyzing CSO impacts on



 receiving waters. The  examples focus on  (1) establishing the link  between model results  and



 demonstrating the attainment of WQS, and (2) the uses of receiving water models at different levels



 of complexity, from design flow analysis to dynamic continuous simulation.







        The first example shows  how design flow analysis or more sophisticated methods can be



 used to analyze bacteria loads to a river from a single CSO event. The second example, which is



 more complex, involves bacterial loads to  an estuary. The third example illustrates how biochemical



 oxygen demand (BOD) loads from a CSS contribute to DO depletion.







 9.3.1   Example 1: Bacterial  Loads to a River




        This example involves a CSS in a small northeastern city that overflows relatively  frequently



 and contributes to WQS excursions. CSOs are the only pollutant source, and only a single water



 quality  criterion—for fecal coliforn-applies. The  use classification  for this receiving water body



 is primary and secondary  contact  recreation.  The city has planned several engineering improvements



 to its CSS and wishes to assess the water quality impacts of those improvements.







        Exhibit 9-1 is a map of key features in this example.
                                             9-6                                January 1999

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                            Exhibit 9-1. Map For Example 1
Boat Launch
7Q10 = 313.3 cfs
                    CSO
                                                                        Beach
                              River  Flow

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Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS


       In this example,  dilution calculations  may suffice to predict whether  the water  quality

criterion is likely to be attained during a given CSO event. This is because:


       (1) The State allows mixing zones, so the water quality criterion must be met at the edge of
           the mixing zone.  If the criterion is met there, it will also be met at points farther away.

       (2) Die-off will reduce the numbers of bacteria as distance from the discharge increases.

       (3) Since the river  flows constantly  in  one direction,  bacterial concentrations  do  not
           accumulate or combine loads from several days of release.


       To illustrate the various levels of receiving water analysis, this example assumes that the

magnitude and  timing of CSOs  can be predicted  precisely and  that the long-term  average

characteristics of the CSS will remain constant.  In the absence of additional CSO controls, the

predictions for the next 31 years include the  following (Exhibit 9-2):


       (1) The system should experience a total of 238 overflow events, an average of 7.7 per year.

       (2) The largest discharge is approximately 1.1  million cubic feet, but most of the CSOs are
           less than 200,000 cubic feet.

       (3) The maximum number of overflow events  in any one month is 18.

       (4) During that month, the maximum receiving water concentration  resulting from CSOs
           exceeds 6,000 MPN/100 ml. Even in this "worst-case" month, however, the geometric
           mean is 400  MPN/100 ml, based  on 30  daily samples  and assuming a background
           concentration of 100.

       At least one CSO event occurs in each calendar month, although 69  percent of the events

occur in March and April when snowmelt increases flow in the CSS. Because river flow is lower

in summer and fall, the rarer  summer and fall CSOs may cause greater  impact in the receiving water.
   1 An overflow event is the discharge from one or more CSO outfalls as the result of a single wet weather event.  In
this example, the number and volume of CSOs pertains to the discharges from the single outfall.
                                            9-8                                January 1999

-------
                               Exhibit 9-2. CSO Events for Example 1
p
P
                1,200,000
                1,000,000
             CO
                 800,000
             (D

             ^  600,000
O
CO
O
                 400,000
                 200,000
                                        I
                                r.  •.*rซ*t   :.:.^.>.
                                ^ft r+f, f i ?| fr-n fffiift, rfh*
                                                         I t  I I
                         10     15     20
                                   Year
                                                            25
30
35

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS


       Water Quality Standards

       The applicable water quality criterion for fecal coliforms specifies that:

       (1)  The geometric  mean  for any  30-day period not exceed 400 MPN ("most probable
          number") per 100 ml,  and

       (2) Not more than 10  percent of samples taken during any 30-day period exceed 1,000 MPN
          per 100 ml.2


The  water quality  criterion  does not specify an instantaneous  maximum count  for this  use

classification.


       It is comparatively simple to assess  how the first component-the geometric mean  of

400  MPN/100 mi-applies.  In the worst-case  month, which had  18 overflow  events,  the geometric

mean is still only 400 MPN/100 ml based on 30 daily samples. It  is therefore extremely unlikely

that the geometric mean concentration WQS  of 400 MPN/100  ml will  be violated in any other

month.


       In general, the second component of the water quality criterion-a percentile (or maximum)

standard-will prove more restrictive for CSOs. A CSS that overflows less than 10 percent of the

time  (fewer than 3 days  per  month) could  be  expected to meet a  not-more-than-10-percent

requirement,  on  average,  but probably  only if loads from other sources  were well  below

1000 MPN/100 ml and the  CSS discharged  to a flowing river system, where bacteria do not

accumulate from day to day. It is possible  that an actual  overflow event might not result in an

excursion above the 1000 MPN/100 ml criterion //the flow in the receiving water were sufficiently

large. The  permittee, however, must demonstrate that  the likelihood  of a  30-day period when CSOs

result in non-attainment of the WQS more than 10 percent of the time is extremely low. This means

that the analysis  must  consider both the likelihood  of occurrence of  overflow events and the  dilution
    Most Probable Number (MPN) of organisms present is an estimate of the average density of fecal coliforms in a
sample, based on certain probability formulas.

    The geometric mean, which is defined as the antilog of the average of the logs of the data, typically approximates
the median or midpoint of the data.


                                            9-10                               January 1999

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS


capacity of the receiving water at the time of an overflow.  The following sections demonstrate

various  ways to make this determination.


       Design  Flow Analysis

       Design flow analysis is the simplest but not necessarily the most appropriate approach. It

uses  conservatively low receiving water flow to  represent  the minimum reasonable dilution capacity.

If the effects of all CSO events would not prevent the attainment of WQS under these stringent

conditions, the  permittee has clearly demonstrated that the applicable WQS should be attained. In

cases where nonattainment is indicated, however, the necessary reductions to reach attainment may

be unreasonably high since CSOs are unlikely to occur at the same time as design low flows.



       The CSO outfall in this example is at a bend in the river where mixing is rapid. Therefore,

the loads are considered fully  mixed through the cross-section of flow. The concentration in the

receiving water is determined by a simple mass balance equation,
cso^cso
        +Q
         ^
                                   RW
where C represents concentration and Q flow (in any consistent units). The subscripts RW, CSO,

and U refer to "receiving water," "combined sewer overflow," and "upstream," respectively.



       For the design flow analysis, upstream volume Qu is set to a low flow of specified recurrence

and receiving water concentration CRW is set equal to the water quality criterion. In this example,

upstream volume Qu is set at the 7Q10 flow. The 7Q10 flow is commonly used for steady-state

wasteload analyses; although it has a lo-year recurrence  and is  much more  stringent than the

not-more-than- 10-percent requirement of the standard, this conservatism ensures that excursions of

the standard will indeed occur only rarely.


       The 7Q10 flow in this  river is 313.3 cfs,  so upstream volume QU5 is set to 313.3. The

background (upstream) fecal coliform concentration is 100 MPN/lOOml, so Qj is set to 100. The



                                           9-11                               January 1999

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 Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS

 WQS stipulates that not more than 10 percent of samples taken during any 30-day period exceed
 1,000 MPN/100 ml; thus receiving  water concentration CRW is set at 1000. Given 7Q10 flow in the
 receiving water, the  mass balance equation may be rearranged to express the CSO concentration  that
just meets the standard, in terms of the CSO flow volume:
                     CRw(Qcso +Qu) -CuQu _ 100ฐ(QcSo +313-3) - 10ฐ x 313-3
                              QCSO                        QCSO

The equation treats both the concentration and flow from the CSO as variables, unlike a standard
wasteload  allocation for a point source, where flow is usually considered constant. For a given CSO
concentration, the capacity of the receiving water increases as  increased CSO volume provides
additional dilution capacity. Therefore, the relationship between allowable concentration and CSO
flow is not linear.  The necessary levels of control on CSOs are not represented by a single point,
but rather by a set of combinations of concentration and flow that meet the water quality criterion.

       Exhibit 9-3 shows combinations of  CSO concentration and CSO flow that just meet the WQS
at 7Q10 flow. The region below the line represents potential control strategies. For instance, for
CSO flows below 1 cfs, the WQS would be met at the design low flow of 313.3 cfs in the receiving
water when the concentration in the CSO remained below 0.28 x 10 MPN/100 ml. At a CSO flow
of 6 cfs,  however, the concentration must be below 0.048 x 106  MPN/100 ml for WQS to be
attained.

       Since the  typical  concentration of fecal  coliforms in CSOs is approximately 2  x  10
MPN/100 ml, demonstrating attainment of the water quality criterion via a design low flow analysis
would be  difficult.
                                           9-12                              January 1999

-------
                        Exhibit 9-3. Design Flow Analysis
P



P
                 DESIGN FLOW ANALYSIS

                       Bacterial Loads to a River
          10000000
           1000000
         o

        I
        •e
         8
100000
        O
        CO
        o
10000
 1000
                              CSO Flow (cfs)
                                        9.0
10.0

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Chapter 9
Assessing Receiving Water Impacts and Attainment of WQS
       A design low flow analysis is often conservative because CSOs typically occur when the



receiving water  is responding to precipitation  and higher-than-normal dilution capability is  available.



Further,  while CSOs may  occur during design  low flows, this will be much  rarer than the occurrence



of the low flows themselves. Therefore, the use of the design low flow protects to a more  stringent



level  than  indicated since  dilution  effects are likely to be greater. Dilution effects can be



considerable in  areas  of multiple sources  of storm water discharge.  Design flow analysis is usually



not sufficient in circumstances involving multiple storm water discharges, highly sensitive habitats,



and river areas  particularly prone to sediment deposition.
       Design Flow Frequency Analysis



       A design flow frequency analysis differs from design



flow  analysis in that it  also  considers  the  probability of



exceeding WQS at a given flow.  Although still  simple, the



design flow frequency approach better tailors the level of CSO



control to the WQS. The major difference between CSOs and



steady-state  sources is  that  CSOs occur intermittently,



providing no load on  most days but large  loads  on  an



occasional basis.
       Over the 31 years, 238 CSO events occur, giving an



average of 0.64 events per month. However, CSO events are



unevenly distributed throughout  the year:  over 31  years, only one CSO has occurred in August but



96 have occurred in April. Box 9-1 shows the average numbers by month.
Box 9-1. Average
CSOs per Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Get
Nov
Dec
Number of
in Example
0.32
0.16
2.23
3.10
0.52
0.13
0.19
0.03
0.13
0.13
032
0.42
       Since most CSOs occur in spring, the probability of a water quality criterion exceedance



needs to be calculated on a month-by-month rather than annual average basis. Here, reducing the



relatively high number of overflows in April should result in attainment of the criterion in other



months.
                                           9-14
                                             January 1999

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS







       Additional refinements can focus more specifically on eliminating only those CSO  events



predicted to exceed WQS at actual receiving water flow. Not all of the April events result in such



excursions; many are very small. Further, the dilution capacity of the receiving water tends to be



high during the spring.   Therefore, the analysis  can be refined by considering a design flow



appropriate to the month in question and then counting only those CSO events predicted to result



in excursions above WQS  at  this flow.   The  resulting table of predicted  receiving  water



concentrations can be analyzed to determine the percentage  reduction in CSO volume needed to meet



the WQS.







       The design flow frequency analysis can give results that are overly conservative, because  the



analysis assumes low flow at the same time that it imposes  a low probability of exceeding  the



standard at that low flow.  This approach, then, pays a price for its simplicity,  by requiring  highly



conservative  assumptions. A less restrictive analysis would need information on the probability



distribution of receiving water flows likely to occur during CSO events.







       Statistical Analysis



       The next level considers not only design low flows,  but the whole range of flows experienced



during  a month.  Although CSOs are more likely when receiving water flow is high, CSO events do



not always have increased dilution capacity available. Clearly, however, CSOs will experience at



least the typical range of dilution capacities. Therefore, holding the probability of excursions to a



specified low frequency entails analyzing the impacts of CSOs  across the  possible range of receiving



water flows, and not only design low flows.







       This example assumes that the permittee has a  predictive model of CSO volumes and



concentrations and adequate knowledge of the expected distribution of flows based on 20 or more



years of daily gage data. In short, the permittee knows the loads and the range of available dilution



capacity but not the frequency with which a particular load will correspond to a particular dilution



capacity. A Monte Carlo simulation can readily address this type of problem, and is used with data
                                           9-15                               January 1999

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Chapter 9                         Assessing Receiving Water Impacts and Attainment of WQS


on CSOs in April, since this is the month with the highest average number of CSOs and is the only

month in which overflows occur more than 10 percent of the time, on average.


       Exhibit 9-4  summarizes the April  receiving  water flows in a flow-duration curve, which

indicates the percent of time a given flow  is exceeded. The distribution of flows is asymmetrical,

with a few  large  outliers.   An analysis  of flow  data  indicates that daily flows  typically are

lognormally distributed. April's  flows are lognormal with mean natural log  of 7.09, which  is

In (1,200 cfs) , and  standard deviation of 0.46.


       The 31 years of CSS data include 96 overflow events in April. In the Monte  Carlo simulation

these 96 events were matched with randomly selected receiving water flows from the April flow

distribution,  for a total of 342 "Aprils" of simulated  data.  The number of events in which the

1,000  MPN/100 ml standard would be exceeded was then calculated, and the count for the month

tabulated.


       Exhibit 9-5 shows  the results. Of the 342 Aprils simulated, 122 had zero excursions of the

standard attributable to the CSS.  The  maximum number of predicted excursions in any April was

17. The average number for the month was 2.45.


        This analysis more closely approaches the actual pattern of water quality excursions caused

by  the CSS.  The  objective  implied by the  WQS  is three or fewer excursions per  month.  In

Exhibit 9-5, the right-hand axis gives the cumulative frequency of excursions,  expressed on a
    The Monte Carlo approach describes statistically the components of the calculation procedure or model that are
subject to uncertainty. The model (in this case, the simple dilution calculation) is run repeatedly, and each time the
uncertain parameter, such as the receiving water flow, is randomly drawn from an appropriate statistical distribution.
As more and more random trials are run, the resulting predictions build up an empirical approximation of the distribution
of receiving water concentrations that would result if the CSO series were repeated over a very long series of natural
flows. Monte Carlo analysis can often be performed using a spreadsheet. The resulting distribution can then be used
for analyzing control strategies. Also see discussion in Section 8.3.

    For a lognormal distribution, the mean is equal to the natural log of the  median of the data (7.09 = In (median)).
Therefore, the median April flow = e709 = 1,200 cfs.


                                              9-16                                January 1999

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                           Exhibit 9-4. Flow Duration Curve
           t

           JD
           LL
              12000
              10000-
              8000
6000
              4000
              2000
                 0
                   0
             20       40       60       80
              Percent of Time Exceeded
100
VO

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          Exhibit 9-5. Expected Exceedances of Water Quality Criterion
140 =1=
                                      H—I—I—h
     01   2  3 4  5  6  7  8  9  10 11 12 13 14 15 16 17
                   Number of Events

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS


zero-to-one scale.  Of the 342 simulated Aprils, over 75 percent were predicted to have three or

fewer excursions,  leaving 25 percent predicted to have four or more. Note that the 11 simulated

Aprils with either  16 or 17 excursions all result from the same month of CSS  data, corresponding

to an abnormally wet period.


       Once set up, the Monte Carlo simulation readily evaluates potential control strategies. For

instance, to evaluate a control strategy with the goal of a 20-percent reduction in CSO flow and a

30-percent reduction in coliform levels, the  Monte Carlo simulation is rerun for these reduced  CSO

flows and  coliform levels. The results  show that of the 342 simulated Aprils, 82  percent were

predicted to meet the water quality criterion.  Although the Monte Carlo analysis introduces a

realistic distribution of flows, it may still result in an overly conservative analysis for how CSOs

correlate with receiving water flows, since it involves using a distribution,  such  as lognormal, which

at best  approximates  the true  distribution  of flows.   A more exact analysis  needs accurate

information about the relationship between CSO flows  and loads  and receiving water  dilution

capacity.


       Continuous Watershed Simulation

       The most precise approach may be a dynamic simulation of both the CSS and the receiving

water.  This approach uses the same time series of precipitation to drive both the CSS/CSO model

and  the receiving water model.  In cases where a dynamic simulation of the entire watershed would

be prohibitively expensive, and where sufficient flow and precipitation records are available, the

permittee  may combine measured upstream flows and a  simulation of  local  rainfall-runoff to

represent the receiving water portion of the simulation.
       As above,  receiving water  modeling  entails  an extremely  simple dilution calculation.

Determining the data for the dilution calculation by simulating dilution capacity or flows, and the
     An analysis of flow distribution must be made so that the appropriate Monte Carlo distribution and range are
calculated.


                                            9-19                                January 1999

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Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS







analysis of the data, introduces complexity. This analysis uses a model that accurately predicts the



available dilution capacity corresponding to each CSO event. Such a model accurately represents



the actual  coliform counts in  the receiving water  and enables the permittee to determine which events



exceed the standard of 1,000 MPN/100 ml.







       Exhibit 9-6 presents the results as the count of CSO  events by  month  which result in



receiving water concentrations greater than or equal to 1,000 MPN/100 ml. For 31 years of data,



only three individual months  are predicted to have more than three days (i.e., greater than 10 percent



of the days in a month) in excess  of the standard.  Consequently, excursions  above the monthly



percentile goal occur only about 0.8 percent of the time. Further, the return period for years with



exceedances  of  this standard is 10.3 years  (3  occurrences  over 31 years).  Although the CSS



produces  relatively frequent  overflows,  the rate  of actual  WQS  exceedances is quite low.



Exhibit 9-7, which plots CSO volumes versus receiving water flow volume,  illustrates why WQS



exceedances  remain rare. This figure shows that all the CSO events have  occurred  when the



receiving  water  is  at flow  above 7Q10. Furthermore,  most of the large  CSO discharges are



associated with receiving water flows well above low flow. Although this excess dilution capacity



reduces the effect of the CSO pollutant  loads, demonstrating compliance also necessitates careful



documentation of the degree of correlation.







       Of course,  no  simulation  represents reality perfectly. Further, the model  is  based on



precipitation  series  or rainfall-runoff relations that are likely to change with time.  Therefore, an



analysis  of the uncertainty  present  in predictions should accompany any predictions  based on



continuous  simulation modeling. An LTCP justified by  the demonstration  approach should include



a margin of safety that reflects the  degree of uncertainty in the modeling effort.
                                            9-20                               January 1999

-------
                          Exhibit 9-6. Excursions of Water Quality Criterion by Month

                  10
                   8
to
               O
              O  4
                   0
p

p
4   6   8  10 12  14  16  18 20 22 24 26  28  30
                     Year

-------
                             Exhibit 9-7. Receiving Water Flow During CSOs
K)
K)
              1200000
              1000000
               800000
               600000
   I p f I *l (* I  I ป  I I  > I  I t
                          7Q 10 Flow
      20000
RW Flow (cfs)
                                                               30000
111
 40000

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS
9.3.2   Example 2: Bacterial Loads to an Estuary

       The second example involves bacterial WQS in a tidal estuary. Like the previous example,

it attempts to estimate the frequency  of excursions of the WQS.  However, the fate and transport of

bacteria in an estuarine system is more complex than the transport in freshwater systems.  Estuaries

are  both dispersive and advective in nature which creates considerable variations in the water quality.

Dispersion is caused by the effects of tidal motion, which is the result of upstream and downstream

currents. Advection is the result of the freshwater flow-through in the estuary. Exhibit 9-8 is a map

of the estuary with the  locations of the CSO outfall,  mixing zone, and two sensitive areas (beach and

shellfish bed) with more-restrictive  bacterial standards.


       As in the previous example, WQS for fecal coliform are expressed as a geometric mean of

400 MPN/100  ml and not  more  than  10  percent of  samples in  a 30-day  period above

1,000 MPN/100 ml. The shell fishing and bathing areas have more restrictive WQS, specifying that

the  30-day geometric mean of fecal coliform counts not exceed 200 MPN/100 ml on a minimum of

five samples and that no more than 20 percent of samples exceed 400 MPN/100 ml.


       Design  Condition Analysis

       The use of a "design-condition" approach in an estuary requires the use of a model which

includes several simplifications to the overall transport. The simplifications can be summarized

through the  following  assumptions:
        1. The estuary is one-dimensional.  It is  not strongly stratified near the source and the
           longitudinal gradient of bacterial concentration is dominant.

        2.  The bacterial  concentration is described as a type of average condition over a number of
           tidal  cycles. In other words, the model does not describe the variations in bacterial counts
           within the tidal cycle, but from one tidal cycle to the next.

        3.  The estuary is in a steady-state condition and area, flow, and reaction rate are constant
           with distance.
                                           9-23                                January 1999

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                               Exhibit 9-8. Map for Example 2
                                                                             Shellfish  Bed
            Beach
Upstream
freshwater
inflow
7010 = 900 cfs
30Q10 = 1,500cfs
                                             E = 2 - 3 midday
                        •1.5 mi
                                                                                 Bay
                                      CSO
                                                     5.5 mi
               Negative x
Positive x
Distance
   (x)
                                   Estuary

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Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS



Under these assumptions, the following mass balance equation can be derived for an infinitely long

estuary  with a waste input at x = 0.  This differential equation is often referred to as the one-

dimensional advection-dispersion equation.
                                       (1)
                 dx2
for           n = n0     at x= 0      (2)

              n = 0      at x=  +/- ฐฐ  (3)




where E is the tidal dispersion (mi /day), U = Q/A the net non-tidal velocity, K is the bacteria die-off

rate (/day), and n is the bacterial concentration (MPN/100 ml).



The solutions to equation (1) with conditions (2) and (3) are:



           n = n0 expfjjx)        for x <  0

           n = n0 exp(j2x)        for x >  0
where     j. =—(1 +cc)      the coefficient j, is associated with negative values of x
               2E



           j2 = —(1 -a)      the coefficient j2 is associated with positive values of x
               2E


                W
and        n0 =	          n0 is the concentration at x = 0, the point of the CSO input
                             and W is the CSO input load to the estuary
where     a = y 1 +4KE/U 2  a is a coefficient that accounts for the dispersive nature

                             of the estuary.
       The ratio KE/U , referred to as the Estuary Number, strongly controls the character of the

solution. As KE/U approaches zero, advection predominates and the concentrations in the estuary
                                            9-25                                January 1999

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Chapter 9
Assessing Receiving Water Impacts and Attainment of WQS
become increasingly similar to the transport in a stream and, as KE/U  becomes large, the

concentrations approach those in a purely dispersive system. Note that in a well-mixed river with

no tides, a is equal to 1, and n0 is given by the input CSO load divided by the flow. In an estuary,

the concentration is reduced by the coefficient a due to the transport of the substance upstream and

downstream because of tidal mixing.
       Selected data for the  example are

presented  in Box 9-2.  A mixing zone of

0.5 mile up- and down-estuary is allowed.

The beach location (1.5 miles up-estuary of

the outfall) and the shellfish bed (5.5 miles

down-estuary of the outfall)  are of

particular interest.  The geometric mean

requirement of the water quality criterion is

taken as an average condition over time for

scoping; that is, the 30-day time frame for

this analysis is assumed sufficiently long to

allow the variability in the load, as well

tidal cycles, to be averaged out. The model

was applied to a  variety of conditions,

including freshwater flow at 7Q10  and
                    Box 9-2. Assumptions for
                     Estuarine CSO Example

          Upstream Flows
                7Q10      =  900 cfs
                U(7Q10)   =  1.5 mi/day
                30Q10     = ,1,500 cfs
                U(30Q10)  =  2.5 mi/day
          Estuary
               A
               E
               T
               K
=  10,000ft2
=  2~3mi2/day
-  27ฐC
=  1.1 I/day
               Unstratified
          CSO
                           ป  2 x 106coliforms/100ml
                           =  0,1 MGD as maximum
                              average per month, 2 MGD
                              as daily maximum
30Q10 levels and bacteria loads at the estimated event maximum daily average load and expected

maximum 30-day average load. Because the result depends on the value assigned to the dispersion

coefficient, sensitivity  of the answer  to  dispersion coefficients of 2 mi /day  and  3 mi  /day,

representing the expected range for the part of the estuary near the outfall, was examined.



       Exhibit 9-9 displays the results of this analysis. It predicts fecal coliform counts at different

locations in the estuary under different assumptions for tidal dispersion and non-tidal velocity.
                                          9-26
                                             January 1999

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Chapter 9
Assessing Receiving Water Impacts and Attainment of WQS
       Exhibit 9-9. Steady-State Predictions of Fecal Coliform Count (MPN/100 ml)
Upstream Flow
Load
Dispersion (miVday)
Upstream Mixing Zone
(x=- 0.5 mile)
Downstream Mixing Zone
(x = 0.5 mile)
Beach
(x=- 1.5 mile)
Shellfish Bed
(x = +5. 5 mile)
Applicable WQS
(MPN/100 ml)
- shellfish/bathing areas
- other
7Q10: 900 cfs
30Q10: 1,500 cfs
Event Maximum Load
E = 2
1672
2,420
504
238
400
1,000
E = 3
1640
2,096
666
268
400
1,000
E = 2
1192
2,200
246
378
400
1,000
E = 3
1302
1,960
414
386
400
1,000
Average Load
E = 2
60
110
12
18
200
400
E = 3
66
98
20
18
200
400
       It is most appropriate to compare the geometric mean criteria to the 30Q10 upstream flow



and average load (since the standard is written as a 30-day average), and the percentile standards to



the 7Q10 upstream flow and event maximum load.  Scoping indicates that the CSOs may cause the



short-term criterion to be exceeded at the mixing zone boundaries and may cause impairment at the



up-estuary beach.  Increasing  the estimate of the dispersion coefficient increases the estimated



concentration at the beach, reflecting increased up-estuary "smearing" of the contaminant plume,



which illustrates that the minimum mixing power may  not be a reasonable design condition for



evaluating maximum impacts at points away  from the  outfall. Potential WQS excursions at the



beach are a concern only at low upstream flows, since the combination of average loads and 30Q10



freshwater  flows is not predicted to  cause impairment. In evaluating impacts at the  beach, recall that



scoping was conducted using a one-dimensional model, which averages a cross-section. If the



average is correctly estimated, impacts at a specific point (e.g., the beach) may still differ from the



average.   Concentrations at the beach may be higher or lower than the cross-sectional average,



depending on tidal circulation patterns.
                                           9-27
                                             January  1999

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS







       The design condition analysis identifies instantaneous concentrations at the down-estuary



boundary of the mixing zone and the beach as potential compliance problems. In this example,



sensitivity analysis was performed on the dispersion coefficient, which varied within an expected



range.   Similar analysis can be made using other sensitive design variables such as temperature,



which influences the coliform die-off rate and ultimately the predicted coliform count. Numerical



experiments with the design condition scoping model suggest that a target 25-percent reduction in



CSO flow volume would provide for the attainment of WQS.







       Design Flow Frequency Analysis



       The design condition analysis addresses the question of whether there is a potential for



excursions of WQS.  It does not address the frequency of excursions, which depends on (1) the



frequency and magnitude of CSO events and (2) the dilution capacity of the receiving water body



at the time of discharge. Note that, in the estuary, the range of dilution capacities (on a daily basis)



is less extreme than  in the river, because the tidal influence is always present, regardless of the level



of upstream flows. To obtain  an upper-bound (conservative) estimate of  the frequency of excursions,



an analysis of the monthly or seasonal frequency of CSO events should be combined with a design



dilution capacity appropriate to that month.







       Statistical Analysis



       The design flow analyses of the previous two sections contain a number of conservative



simplifying assumptions:







       (1) They assume a steady (rather than intermittent) source




       (2) They assume a design minimum dilution capability for the  estuary





       (3) They do not account for many of the real-world complexities of estuarine mixing




       (4) They do not account for the effects of temperature and salinity on bacterial die-off.
                                            9-28                               January 1999

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 Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS







       The  scoping analysis  can be improved by considering a full  distribution of probable upstream



 flows in a Monte Carlo simulation. The expected range of hydrodynamic dispersion coefficients



 could also be incorporated into the analysis.







       Watershed  Simulation



       Building a realistic model of contaminant distribution and transport in estuaries is typically



 resource-intensive and  demanding.  A watershed  simulation may, however, be needed  to demonstrate



 compliance  for some  systems  where the results of conservative design flow analyses are unclear.



 Detailed guidance on the  selection and use of estuarine models is provided in EPA's Wasteload



Allocation series, Book III (Ambrose et al,  1990; Martin et al, 1990).







 9.3.3 Example 3:  BOD Loads





       The  third example concerns BOD and depletion of DO, another important water quality



 concern for  many CSSs. Unlike bacterial loads, BOD impacts are usually highest downstream of



 the discharge and occur some time after the discharge has occurred.







       The  CSS in an older industrial city has experienced frequent overflow events. The CSOs



 discharge to a moderate-sized river on a coastal plain. In the  reach below the CSS  discharge, the



 river's 7Q10 flow is  194  cfs,  with a depth of 5  feet and a velocity of 0.17 ft/s. Above the city,



 velocities range from 0.2  to 0.3 ft/s at  7Q10 flow.  A major industrial point source of BOD lies



 18 miles upstream.  A POTW with advanced secondary treatment discharges three miles upstream



 of the CSO  (Box 9-3).







       The  river reach below the city has a designated use of supporting a warm water fishery.  For



 this designation, State criteria for DO  are a 30-day mean of 7.0 mg/1 and a 1-day minimum of



 5.0  mg/1. The  State also  requires that  WLAs for BOD be calculated on the basis of the 1-day



 minimum DO standard calculated at 7Q10 flow and the maximum average monthly temperature.



 The 5.0 mg/1 criterion is not expressed in a  frequency-duration format; the 1-day minimum is a  fixed



 value, but evaluation  in terms  of an extreme low flow of specified recurrence implicitly  assigns an









                                            9-29                               January  1999

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Chapter 9
Assessing Receiving Water Impacts and Attainment of WQS
acceptable frequency of recurrence to DO

1-day  average concentrations less  than

5.0 mg/1. (The State criterion for DO is

thus hydrologically-based and is roughly

equivalent to  maintaining an acceptable

frequency of biologically-based excursions

of the water quality criteria  for ambient

DO.)


       Design Condition Analysis

       A  conservative  assessment of

impacts from the CSS  can be established

by  combining a reasonable  worst-case  load

(the maximum design storm with a 10-year

recurrence interval) with extreme  receiving

water design conditions.      Limited

monitoring data and studies of other CSO

problems  suggested  that a reasonable

worst-case   estimate was  a  1-day  CSO

volume of 4 MGD, with an average BOD5

concentration of 200 mg/1.
            Box 9-3. Assumptions for BOD Example

         CSO Discharge (at maximum load)
             BOD5 = 200 mg/1
             CBODU/BOD5 = 2.0
             NBOD = 0 mg/1
             Qe= 4 MGD

         Point Source Effluent Upstream
             Distance Upstream = 18 mi
             BOD5 = 93 mg/1
             CBODU/BOD5 = 2.5
             NBOD = 0 mg/1
             Qe = 5 MGD

         POTW
             Distance Upstream = 3 mi
             BOD5 = 11.5 mg/1
             Qe = 10 MGD

         Reaction Parameters

             Ka  = [12.9 x U1/2/H3/2]  x (1.024)(T-20)
             where U = avg stream velocity (ft/s)
             and  H = average depth (ft)
             Kd = Kr = 0.3x(1.047)(T"20)
             SOD (below CSS) = 0.3 mg/l-day
             SOD (elsewhere) = 0

         Upstream Background
             BODU =  1 mg/1
             DOD = 1  mg/1
       As described in Chapter 8, initial
scoping was carried out using a simple, steady-state DO model (see Section 8.3.1, Rivers-Oxygen
Demand/Dissolved Oxygen subsection) . The initial scoping assumes the presence of the upstream
industrial point source and the POTW,  and  the estimated worst-case CSO load. All BOD5 was
initially assumed to be CBOD and  fully available to the dissolved phase.  Sediment oxygen demand
(SOD), known to play a role in the reach below the CSS, was estimated at 0.3 mg/l-day. No SOD
   7  c
     Similar DO  analysis  is discussed in  Thomann and Mueller (1987).
                                           9-30
                                             January 1999

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Chapter 9                       Assessing Receiving Water Impacts and Attainment of WQS


was  assumed for other reaches upstream of the CSO.  This is a simplifying assumption that is

sufficient for the scoping analysis described here.  SOD in the river reach below the CSO has been

included in the analysis since this is the reach of concern.  Since there are many sources of SOD

other than CSOs, contributions of SOD from other sources should be considered at the next level of

analysis.


       Results of the scoping model application are shown in Exhibit 9-10, which shows  the

interaction of the point source, POTW,  and CSO.  The exhibit combines two worst-case conditions:

high flow from the episodic source and low  (7410) flow in  the receiving water. Under  these

conditions, the maximum DO deficit is expected to occur 7.5 miles downstream of the CSO, with

predicted  DO  concentrations as low as 3.9 mg/1.  Under such conditions, the CSO flow is

approximately 25 percent of total flow in the river.


       Design Flow Frequency Analysis

       The State criterion called for a one-day minimum DO concentration of 5 mg/1, calculated at

design low flow conditions for steady sources.  Use of the 7Q10 design flow was interpreted as

implying  that an approximately once-in-three-year excursion  of the  standard,  on average, was
                              o
acceptable (U.S.  EPA, 1991a).  As in  the  previous examples, the rate of occurrence  of  CSOs

provides an upper bound  on the frequency of WQS excursions  attributable to CSOs.  In this case,

however, the once-in-three-year  excursion  frequency  cannot be attained  through CSO control  alone.

Instead, the co-occurrence of CSOs and receiving water flows must be examined.


       To accommodate  this relationship, the design flow model can be  modified to assess the

dependence of DO concentrations on upstream flow during maximum likely loading from the CSO.

Design flow was simulated using the worst-case CSO flow over a variety of concurrent upstream
     The average frequency of excursions is intended to provide an average period of time during which aquatic
communities recover from the effects of the excursion and function normally before another excursion.  Based on case
studies, a three-year return interval was determined to be appropriate. The three-year return interval was linked to the
7Q10 flow since this flow is generally used as a critical low flow condition.


                                            9-31                               January  1999

-------
                              Exhibit 9-10. Design Condition Prediction of DO Sag
NJ
                               Minimum DO
                               Standard
                   0.3.5
                                                 River Mile

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS







flows, since upstream flows  affect both the dilution capacity of the river and the velocity of flow and



reaeration rate. As  shown  in Exhibit 9-11, the estimated DO concentrations depend strongly on



upstream flow. Note that WQS are predicted to be attained if the upstream  flow is greater than about



510 cfs. A flow less than 510 cfs occurs about five times per year, on average, in this segment of



the river.







       The target rate of WQS excursions is one in three years. An upper bound  for the actual



long-term average rate of excursions can be established as the probability  that flow is less than



510 cfs in the river  multiplied by the probability that a CSO occurs:
                                   P   =p(Q<510cfs)f.
                                                     CSO
where Pexo is the probability of a WQS excursion on any given day and foso is the fraction of days in



the year on  which CSO discharges occur, on average. Since the goal for excursions is once every



three years, Pexcis set at l/(3 x 365), or .000913. Since a flow less than 510 cfs occurs five times



per year, p(Q<510) is  5/365, or  .0137.   Substituting  these  values  into  the  equation  yields



fcso = .000913/.0137 = 0.067. This implies that up to 24 CSOs  per year will meet the long-term



average goal for DO WQS excursions, even under the highly conservative assumption that all  CSOs



provide the  reasonable maximum BOD  load.







       An important caveat, however, is that no other significant wet weather sources  are assumed



to be present in the river. In  most real rivers, major precipitation events also produce BOD loads



from storm water, agriculture, etc.  Where such loads are present,  conservative  assumptions



regarding these additional sources need to be incorporated into the scoping  level  frequency analysis.
                                            9-33                               January 1999

-------
                           Exhibit 9-11. Relationship Between DO Concentration and Upstream Flow
                                                                                                                !
                       100     200     300     400     500      600     700
                                                    Upstream Flow (cfs)
800
900
1000
p
p

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Chapter 9                        Assessing Receiving Water Impacts and Attainment of WQS







       As with the other examples, further refinement in the analysis can be attained by examining



the statistical behavior of the CSO and receiving water flows in more detail. For example, the use



of a constant CSO load is a conservative, simplifying assumption that is appropriate for the scoping



level analysis presented here. Dynamic continuous simulation models could be used  to provide a



more realistic estimate  of the actual time series of DO concentrations resulting from CSOs.







9.4    SUMMARY





       As illustrated in the preceding examples, no one  method is appropriate for a particular CSS



or for all CSSs, and a complex dynamic simulation is not always necessary. The method should be



appropriate  for the receiving water problem. The municipality (in cooperation with the NPDES



authority) needs to balance effort spent in analysis with the level of accuracy required. However,



as the first example illustrated, as additional effort is invested assumptions can usually be refined



to better reflect the actual situation.
                                           9-35                               January 1999

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                                           R-3                               January 1999

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References
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                                          R-6                              January 1999

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                                           R-7                              January 1999

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                                         R-8                              January 1999

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                                         R-9                             January 1999

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                                         R-10                              January 1999

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       EPA Region VTII, Water Management Division, Denver, CO.
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References
A.     Annotated References on Monitoring

       In addition to the monitoring references listed above, many documents contain
information useful in designing a monitoring program for CSO controls. This section briefly
highlights information from these documents, as well as from some of the documents listed
above.

            The Water Environment Federation's Combined Sewer Overflow Pollution
           Abatement Manual of Practice No. FD-17 (WPCF, 1989) includes discussions on
            establishing planning objectives  for characterizing receiving waters, their aquatic
            life, and meteorologic conditions; identifying critical events; evaluating system load
            characteristics; selecting analytic methods; mapping the system; developing the
            sampling plan; selecting field sampling procedures; monitoring CSS and
            environmental flow;  and modeling.

           Design of Water-Quality Monitoring Systems (Ward et al, 1990) includes insightful
            discussions on the design of monitoring plans, the essential role of statistics,
            frameworks for designing water-quality information systems, quantification of
            information, data analysis, and the documentation of monitoring plans. This
            reference also includes four case studies of large-scale and long-term monitoring
            programs.

           NPDES Storm  Water Sampling Guidance Document, EPA 833-B-92-001, (EPA,
            1992) details EPA's requirements  for monitoring storm water discharges. When
            such monitoring is required as a  condition of a CSS's NPDES permit, monitoring
            efforts for CSO control should be coordinated with this required monitoring effort in
            order to maximize data collection efficiencies and minimize monitoring costs.

           A Statistical Method for Assessment of Urban Stormwater Loads, Impacts,  and
            Controls, EPA 440/3-79-023, (Driscoll et al., 1979) discusses approaches for
            defining the purpose of monitoring programs; monitoring rainfall; using rainfall data
            to project and evaluate impacts;  selecting monitoring sites;  characterizing drainage
            basins; determining study periods,  sampling frequencies, and sampling intervals
            during storms;  selecting sampling procedures and sampling parameters;
            understanding special considerations for monitoring receiving waters; and using
            continuous monitoring. It also provides an extensive literature compilation regarding
            storm water and CSO monitoring.

           Data Collection and Instrumentation in Urban Stormwater Hydrology (Jennings,
            1982) reviews  data and instrumentation needs for urban storm water hydrology. This
            reference considers monitoring strategy design and the collection and use of data to
            characterize rainfall, other meteorological  characteristics, streamflows, receiving
            water biologies and chemistries,  and land use.
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References
            Use of Field Data in Urban Drainage Planning (Geiger, 1986) describes rainfall-
           runoff processes and data collection constraints, the need to match data collection to
           study objectives, the use of data in urban drainage planning, the application and
           verification of models used in urban drainage planning, the validity of the design
           storm concept, the reliability of storm water simulations, and the real-time use of
           monitoring data in control and sewer system operation.

           "Water Body Survey and Assessment Guidance For Conducting Use Attainability
           Analyses (UAA)." In Water Quality Standards Handbook (EPA, 1994).  The UAA
           concepts discussed in this Handbook include useful field sampling methods,
           modeling, and interpretation approaches in three Technical Support Documents for
           flowing waters, estuaries, and lakes (EPA,  1983b,  1984a, and 1984b).

           Several guidance documents that discuss or pertain to EPA's Waste Load Allocation
           (WLA) process also provide useful information on a wide range of topics that are
           potentially valuable when planning monitoring programs for CSO control:

                Guidance for State Water Monitoring and Waste Load Allocation Programs
                (EPA, 1985) includes a chapter on monitoring for water-quality-based controls.
                It discusses the process of collecting and analyzing effluent and ambient
                monitoring data in establishing water quality standards and EPA's
                responsibilities in this process.

                Handbook - Stream Sampling for Waste Load Allocation Applications  (Mills et
                al, 1986) addresses sampling considerations for acquiring data on stream
                geometry, hydrology, meteorology, water quality,  and plug flows.  It also
                reviews sampling considerations for gathering data to meet various modeling
                needs.

           -  "Nutrient/Eutrophication Impacts," Chapter 2 of Technical Guidance Manual
               for Performing Waste Load Allocations,  Book IV: Lakes and Impoundments,
                (Mancini et al.,  1983) primarily emphasizes modeling considerations. However,
                this chapter also provides  useful introductions to approaches for estimating
                loading rates to standing water systems and needs  for monitoring data to support
                modeling efforts.

                Technical Guidance Manual for Performing Waste Load Allocations, Book III:
                Estuaries, Part 2: Application of Estuarine Waste Load Allocation Models
                (Martin et al., 1990) includes a chapter on monitoring protocols for calibrating
                and validating estuarine WLA models.  It reviews the types of data needed,
                frequency of collection, spatial coverage, and quality assurance.

                Water Quality Assessment: A Screening Procedure for Toxic and Conventional
                Pollutants in Surface and Ground Water (Mills et al., 1985a, b) presents a broad
                array of modeling  and data management approaches for assessing aquatic fates
                of toxic  organic substances, waste-load calculations, rivers and streams,
                impoundments, estuaries, and ground waters.
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                                           APPENDIX A
                                               Table  A-l
                    Checklist of Considerations for Documenting Monitoring
           Program Designs and Implementation (expanded from Ward et al., 1990)
Sample and Field Data Collection

        Pre-Sampling Preparations
        —    Selecting personnel and identifying responsibilities
        —    Training personnel in safety and confined space entry; verifying first aid and wet-weather training,
              CPR, currency of vaccinations etc.)
        —    Preparing site access and obtaining legal consents
        —    Acquiring necessary  scientific sampling or collecting permits
        —    Developing formats for field sampling logs and diaries
        —    Training personnel in pre-sampling procedures (e.g., purging sample lines, instrument calibration)
        —    Checking  equipment availability, acquisition, and maintenance
        —    Scheduling sample collection (random? regular? same-time-of-day?)
        —    Preparing pre-sampling  checklist

        Sampling Procedures
        	    Procedures  documentation
        	    Staff qualifications and  training
        	    Sampling protocols
        	    Quality-control procedures (equipment checks, replicates, splits, etc.)
        	    Required  sample containers
        	    Sample numbers and labeling
        	    Sample preservation (e.g,  "on ice" or chemical preservative)
        	    Sample transport (delivery to laboratory)
        —    Sample storage  requirements
        —    Sample tracking and chain-of-custody procedures
        	    Quality control or quality  assurance
        	    Field measurements
        —    Field log and diary entries
        —    Sample custody and audit records

        Post-Sampling Follow Up
        —    Filing sample logs and diaries
        —    Cleaning and maintaining equipment
        —    Disposing of chemical wastes properly
        —    Reviewing documentation and audit reports
                                                   A-l                                    January 1999

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Appendix A
                                         Table A-l (continued)
                    Checklist of Considerations for Documenting Monitoring
           Program Designs and  Implementation (expanded from Ward et al., 1990)
Laboratory Analysis
        Preparations Prior to Sample Analysis
        —   Verifying use of proper analytical methods
        —   Scheduling analyses
        —   Verifying sample number
        —   Defining a recording system for sample results
        	   Applying a system to track each sample through the lab
        	   Maintaining and calibrating equipment
        —   Preparing quality  control solutions

        Sample Analysis
        	   Sample analysis methods and protocols
        	   Use of reference samples, duplicates, blanks, etc.
        	   Quality control and quality assurance compliance
        	   Sample archiving
        	   Proper disposal of chemical wastes
        	   Full documentation in bench sheets

        Data Record Verification
        —   Coding sheets, data loggers
        —   Data verification procedures and compliance with project plan
        —   Verifying analysis of splits within data quality objectives
        —   Assigning data-quality indicators and explanations
Data Management
              Selecting appropriate hardware and software
              Documenting data entry practices and data validation (e.g., entry-range limits, duplicate entry
              checking)
              Data tracking
              Developing data-exchange protocols
              Formatting data for  general availability
Data Analysis
              Selecting software
              Handling missing data and non-detects
              Identifying and using data outliers
              Planning graphical procedures (e.g., scatter plots, notched-box and whisker)
              Parametric  statistical procedures
              Non-parametric statistical procedures
              Trend analysis procedures
              Multivariate procedures
              Quality  control checks on statistical analyses
                                                   A-2                                     January 1999

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Appendix A
                                       Table A-l (continued)
                   Checklist  of Considerations for Documenting Monitoring
          Program Designs  and Implementation (expanded from Ward et al., 1990)
Reporting
             Scheduling reports  - timing, frequency, and lag times following sampling
             Designing report contents and formats
             Designing planned tables and graphics
             Assigning report sign-off responsibility(ies)
             Determining report distribution recipients and availability
             Planning use of paper and electronic formats
             Presentations
Information Use
        	    Identifying and applying decision or trigger values, resulting action
        	    Implementing construction, control, and/or monitoring design  alternatives
        	    Planning public-release procedures
 General
        	    Contingencies
        	    Follow-up  procedures
        	    Data  management
        	    Data  analysis
        	    Reporting
        	    Information use
                                                  A-3                                   January 1999

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Appendix A
                                               Table A-2
                          Checklist for Reviewing CSO Monitoring Plans
CSO Drainage and Sewer System Map
              Up-to-date
              Shows "as-built" sewer system
              Shows drainage areas with land use information
              Shows location of major industrial sewer users
              Shows location of all direct discharge points, including all related CSO, POTW, storm water, and
              industrial discharges
              Distinguishes bypass points from CSOs points and shows locations
              Shows locations of CSO quantity  and quality monitoring sites
              Identifies receiving waters
              Identifies designated and existing  uses of receiving waters
              Shows areas of historical use impairment
CSO Volume
CSO Quality
              Identifies number of storms to be monitored
              Identifies number of CSO outfalls to be monitored
              Ensures that sampling points include major CSOs
              Provides for monitoring of POTW influent flow
              Ensures  adequacy of method  of flow measurement
              Identifies frequency of flow measurement during each storm event
              Identifies storm statistics  to be  reported-mean,  maximum, duration
              Identifies storm statistics to be reported for all storms during the study period
              Identifies number of storms to be monitored
              Identifies number of CSO outfalls to be monitored
              Ensures that sampling points include major CSOs
              Provides for monitoring of POTW influent quality
              Provides for monitoring of drainage areas representative of land use and sewer users
              Identifies method and frequency of sampling
              Identities parameters to be analyzed
              Ensures  adequacy of detection limits
              Identifies toxicity test(s) to be conducted
              Identifies receiving  water(s) to be sampled
              Provides for monitoring of aesthetics
                                                   A-4                                    January  1999

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                                                 APPENDIX B

                                                   Table B-l

           Documents and Screening Manual (Mills et al.) for Analysis of Conventional Pollutants
     Data Requirements
 Streeter-
Phelps DO    NH3 Toxiciry
Analyses-11    Calculations-
                                                            Algal
                                                          Predictions
                                                           Without
                                                           Nutrient
                                                         Limitations-0
    Algal
 Predictions    Algal Effects  Algal Effects
With Nutrient    on Daily      on Diurnal
 Limitations-0    Average DO-C      DO-C
Hydraulic and Geometric Data
Flow Rates-

Velocity

Depth

Cross-sectional area

Reach length
Constituent Concentrations-0
DO x
CBOD, NBOD x
NH3 *
Temperature * x
Inorganic P
Inorganic NPDES
Chlorophyll a f
PH



X X X X
X X X X
X X X X
X X X X

DO/BOD Parameters
Restoration rate coefficient

Sediment Oxygen Demand

CBOD decay rate

CBOD removal rate

NBOD decay rate

NH3 oxidation rate

Oxygen per unit chlorophyll a

Algal oxygen production rate

Algal oxygen respiration rate
                                                       B-l
                                                                      January  1999

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Appendix  B
                                       Table B-l  (continued)
     Data Requirements for Hand-Calculation  Techniques Described in WLA Guidance
  Documents  and  Screening Manual  (Mills et  al.) for Analysis of Conventional  Pollutants

                                                      Algal         Algal
                                                   Predictions    Predictions  Algal  Effects
                          Streeter-                   Without        With       on Daily   Algal Effects
                         Phelps DO NH3 Toxicity    Nutrient      Nutrient     Average    on Diurnal
   Data Requirements    Analyses-3  Calculations-1"  Limitations-0 Limitations-0     DO-0        DO-0

Phvtoplankton Parameters	
Maximum growth rate                                    x            x             x            x

Respiration rate                                          x            x             x            x

Settling velocity                                         x            x             x            x

Saturated light intensity                                   x            x             x            x

Phosphorous  half-                                                     x             x            x
saturation constant

Nitrogen half-saturation                                                x             x            x
Phosphorous to
chlorophyll ratio

ratio
x x x x
x x x x

Light Parameters
Daily solar radiation

Light extinction
coefficient
x x x x
x x x x
x x x x
a) Streeter-Phelps DO calculations are described in Chapter 1 of Book II of the WLA guidance documents
        (Table 1- 1) and the Screening Manual (Mills et. al.).

b) Ammonia toxicity  calculations are described in Chapter 1  of Book II of the WLA guidance documents.

c) Algal predictions and their effects on DO are discussed in Chapter 2 of Book II of the WLA guidance documents.

d) Flow rates are needed for the river and all point sources at various points to define nonpoint flow,

  Constituent concentrations are needed at the upstream boundary and all point sources.

  Chlorophyll a concentrations are also needed at the downstream end of the reach to estimate net growth rates,
                                                  B-2                                    January  1999

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Appendix B
                                         Table B-2
                           Model Input Parameters for Qual-2E
          Input Parameter
Variable
by Reach
Input Parameter
Variable  by  Variable
  Reach    with Time
Dissolved Oxygen Parameters
Reservation rate coefficients
O2 consumption per unit of NH3 oxidation
O2 consumption per unit of NO2 oxidation
O2 production per unit photosynthesis
O2 consumption per unit respiration
Sediment oxygen demand

Carbonaceous BOD Parameters
CBOD decay rate
CBOD settling rate

Organic Nitrogen
Hydrolize to ammonia

Ammonia Parameters
Ammonia oxidation rate
Benthic source rate

Nitrite Parameters
Nitrite oxidation rate

Nitrate Parameters
None

Organic Phosphorous
Transformed to diss. p

Phosphate Parameters
Benthic source rate


Yes




Yes


Yes
Yes


Yes


Yes
Yes


Yes





Yes


Yes

Nonconservative Constituent Parameters
Decay rate

Meteorological Data
Solar radiation
Cloud cover
Dry bulb temperature
Wet bulb temperature
Wind speed
Barometric pressure
Elevation
Dust attenuation coefficient
Evaporation coefficient

Stream Geometry Data
Cross-sectional area vs. depth
Reach length

Hydraulic Data (Stage-flow Curve Option)
Coefficient for stage-flow equation
Exponent for stage-flow equation
Coefficient for velocity-flow equation
Exponent for velocity-flow equation

Hydraulic Data (Manning 's Equation
Option)
Manning's n
Bottom width of channel
Side slopes of channel
Channel slope

















Yes
Yes


Yes
Yes
Yes
Yes


Yes
Yes
Yes
Yes






Yes
Yes
Yes
Yes
Yes
Yes





















                                            B-3
                                              January 1999

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Appendix B
                                   Table B-2 (continued)
                           Model Input Parameters for Qual-2E
          Input Parameter
Variable
by Reach
Input Parameter
                                                                         Variable by Variable
                                                                           Reach   with Time
Phytoplankton Parameters
Maximum growth rate
Respiration rate
Settling rate
Nitrogen half-saturation constant
Phosphorous half-saturation constant
Light half-saturation constant
Light extinction coefficient
Ratio of chlorophyll a to algal biomass
Nitrogen fraction of algal biomass
Phosphorous fraction of algal biomass

Coliform Parameters
Die-off rate



Yes



Yes
Yes




Yes
Flow Data
Upstream boundaries
Tributary inflows
Point sources
Vonpoint sources
Diversions

Constituent Concentrations
Initial conditions
Upstream boundaries
Tributary inflows
Point sources
Nonpoint sources


Yes
Yes
Yes
Yes
Yes


Yes

Yes
Yes
Yes










Yes




                                             B-4
                                              January 1999

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                                                               Table B-3
              Comparison of Qual-II With Other Conventional Pollutant Models Used in Waste Load Allocations
                                                                                                            •a
                                                                                                            •a
                                                                                                             n
                                                                                                             a
                                                                                                             a
             Temporal Variability
             Water
   Model    Quality    Hydraulics
                                                                                                                 Process Simulated
  Variable                      Spatial
Loading Rated  Types of Loads   Dimensions
Water Body
             Water Quality
               Parameters
                Modeled
Chemical/Biological
Physical
DOSAG-I


SNSIM




QUAL-II









RECEIV-II







Steady-
state

Steady-
state



Steady-
state or
dynamic







Dynamic







Steady- state


Steady- state




Steady- state









Dynamic







No


No




No









Yes







multiple point
source

multiple point
sources &
nonpoint sources


multiple point
sources &
nonpoint sources







multiple point
sources






I-D


I-D




I-D









l-Dor2-D







stream network


stream network




stream network









stream network
or well-mixed
estuary





DO, CBOD,
NBOD,
conservative
DO, CBOD,
NBOD,
conservative


DO, CBOD,
temperature,
ammonia, nitrate,
nitrite, algae,
phosphate,
coliforms, non-
conservative
substances, three
conservative
substances
DO, CBOD,
ammonia, nitrate,
nitrite, total
nitrogen,
phosphate,
coliforms, algae,
salinity, one
metal ion
Ist-order decay of
NBOD, CBOD,
coupled DO
Ist-order decay of
NBOD, CBOD,
coupled DO, benthic
demand (s),
photosynthesis (s)
Ist-order decay of
NBOD, CBOD,
coupled DO, benthic
demand (s), CBOD
settling (s), nutrient-
algal cycle




Ist-order decay of
NBOD, CBOD,
coupled DO, benthic
demand (s), CBOD
settling (s), nutrient-
algal cycle


dilution,
advection,
reservation
dilution,
advection,
reservation


dilution
advection,
reservation, heat
balance






dilution,
advection,
reservation





(s) = specified.

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Appendix B
                                      Table B-4
            Methods for Determining Coefficient Values in Dissolved Oxygen
                              and Eutrophication Models
Model Parameter
Dissolved Oxygen Parameters
Reaeration rate coefficient
O2 consumption per unit of NH3 oxidation
O2 consumption per unit NO2 oxidation
O2 production per unit photosynthesis
O2consumption per unit respiration
Sediment oxygen demand
Carbonaceous BOD Parameters
CBOD decay rate
CBOD settling rate
Ammonia Parameters
Ammonia oxidation rate
Benthic source rate
Nitrite Parameters
Nitrite oxidation rate
Phosphate Parameters
Benthic source rate

Symbol

Kss
al
a2
a3
a4
KSOD

Kd
Ks

KM
KBEN

K.N2

KBEP

Method Determination

Compute as a function of depth and velocity using an
appropriate formula, or measure in field using tracer
techniques.
Constant fixed by biochemical stoichiometry
Constant fixed by biochemical stoichiometry
Literature values, model calibration and measurement
by light to dark bottles and chambers.
Literature values and model calibration.
In situ measurement and model calibration.

Plot CBOD measurements on semi-log paper or
measure in laboratory.
Plot CBOD measurements on semi-log paper and
estimate from steep part of curve.

Plot TKN measurements and NO3+NO2 measurements
on semi-log paper.
Model calibration.

Use literature values and calibration, since this rate is
much faster than the ammonia oxidation rate.

Model calibration.

                                         B-6
January 1999

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Appendix B
                                Table B-4 (continued)
            Methods for Determining Coefficient Values in Dissolved Oxygen
                              and Eutrophication Models
Model Parameter
Phytoplankton Parameters
Growth rate
Respiration rate
Settling rate
Nitrogen fraction of algal biomass
Phosphorous fraction of algal biomass
Half-saturation constants for nutrients
Saturating light intensity or half-saturation
constant for Tight
Symbol

V-
r
Vs
a5, a6,
a7
a8, a9
J\.n, JVp
L or Kx
Method Determination

Literature values and model calibration, or
field using light-dark bottle techniques.
Literature values and model calibration, or
field using light-dark bottle techniques.
measure in
measure in
Literature and model calibration.
Literature values and model calibration or
determinations from field samples.
Literature values and model calibration or
determinations from field samples.
laboratory
laboratory
^iterature values and model calibration.
Literature values and model calibration.
                                         B-7
January 1999

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Appendix B
                                               Table B-5
        Summary of Data Requirements for Screening Approach for Metals in Rivers
            Data
 Calculation
Methodology
Remarks
Hydraulic Data

1.   Rivers:

    River flow rate, Q         D, R, S, L


    Cross-sectional area, A    D, R, S

    Water depth, h            D, R, S, L


    Reach lengths, x          R, S

    Stream velocity, U        R; S
              An accurate estimation of flow rate is very important because of
              dilution considerations. Measure or obtain from USGS gage.
              Average water depth is cross-sectional area divided by surface
              width.
              Required velocity is distance divided by travel time. It can be
              approximated by Q/A only when A is representative of the reach
              being studied.
2. Lakes:

. Hydraulic residence time,
    T

    Mean depth, H
              Hydraulic residence times of lakes can vary seasonally as the flow
              rates through the lakes change.

              Lake residence times and depths are used to predict settling of
              absorbed metals in lakes.
Source data

1. Background

    Metal concentrations, Ct   D, R, S, L


    Boundary flow rates, Qu   D, R, S, L

.  Boundary  suspended      D, R, S, L
    solids, Su
    Silt, clay fraction of       L
    suspended solids

 .  Locations                 D, R S, L
              Background concentrations should generally not be set to zero
              without justification.
               One important reason for determining suspended solids
               concentrations is to determine the dissolved concentration, C, of
               metals based on CT, S, and Kp  However, if C is known along
               with CT and S, this information can be used to find Kp.
                                                   B-8
                                                             January 1999

-------
Appendix B



                                         Table B-5 (continued)
        Summary of Data Requirements for Screening Approach for Metals in Rivers


2. Point sources

     Locations                 D, R, S, L

•   Flow rate, Qw             D, R, S, L

    Metal concentration, Ctw   D, R, S, L

    Suspended solids, Sw      D, R, S, L
Bed Data

    Depth of contamination
               For the screening analysis, the depth of contamination is most
               useful during a period of prolonged scour when metal is being
               input into the water column from the bed.
    Porosity  of sediments, n

    Density of solids in sediments (e.g., 2.7 for sand) us

    Metal concentration in bed during prolonged scour period, Ct2
Derived Parameters

    Partition coefficient, Kn
All
     Settling velocity, ws         S,L

 .  Resuspension velocity, Wrs R
Partition coefficient is  a very important parameter. Site-specific
determination is preferable.

Parameter derived based on suspended solids vs. distance profile.

Parameter derived based on suspended solids vs. distance profile.
Equilibrium Modeling

 .  Water quality
    characterization of river:
 .   pH

 .  Suspended  solids

 .  Conductivity

 .  Temperature

    Hardness


    Total organic carbon

    Other major cations and anions
               Equilibrium modeling is required only if predominant metal
               species and estimated solubility controls are needed.
               Water quality criteria for many metals are keyed to hardness, and
               allowable concentrations increase with increasing  hardness.
 *D - Dilution (Includes total dissolved and adsorbed phase concentration predictions)

 R - dilution and resuspension.

 S - dilution and settling.

 L - lake.
                                                     B-9
                                                              January  1999

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