EPA/600/R-01/034
December 2000
Exfiltration in Sewer Systems
by
Robert S. Amick, P.E.
Environmental Quality Management, Inc.
Cincinnati, Ohio 45240
and
Edward H. Burgess, P.E.
Camp, Dresser & McKee
Cincinnati, Ohio 45249
Order No. 8C-R551-NASX
Project Officer
Ariamalar Selvakumar, Ph.D., P.E.
National Risk Management Research Laboratory
Edison, New Jersey 08837
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
-------
Notice
The U.S. Environmental Protection Agency through its Office of Research and
Development funded and managed the research described here under contract 8C-R551-
NASX to Environmental Quality Management, Inc. It has been subjected to the Agency's
peer and administrative review and has been approved for publication as an EPA
document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
-------
Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the
Nation's land, air, and water resources. Under a mandate of national environmental laws,
the Agency strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural systems to support and nurture life. To
meet this mandate, EPA's research program is providing data and technical support for
solving environmental problems today and building a science knowledge base necessary
to manage our ecological resources wisely, understand how pollutants affect our health,
and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for preventing and reducing
risks from pollution that threatens human health and the environment. The focus of the
Laboratory's research program is on methods and their cost-effectiveness for prevention
and control of pollution to air, land, water, and subsurface resources; protection of water
quality in public water systems; remediation of contaminated sites, sediments and ground
water; prevention and control of indoor air pollution; and restoration of ecosystems.
NRMRL collaborates with both public and private sector partners to foster technologies that
reduce the cost of compliance and to anticipate emerging problems. NRMRL's research
provides solutions to environmental problems by: developing and promoting technologies
that protect and improve the environment; advancing scientific and engineering information
to support regulatory and policy decisions; and providing the technical support and
information transfer to ensure implementation of environmental regulations and strategies
at the national, state, and community levels.
This publication has been produced as part of the Laboratory's strategic long-term
research plan. It is published and made available by EPA's Office of Research and
Development to assist the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
-------
Abstract
This report was submitted in fulfillment of Order No. 8C-R551-NASX by Environmental
Quality Management, Inc. and Camp, Dresser & McKee of Cincinnati, Ohio under the
sponsorship of the United States Environmental Protection Agency. This report covers the
period from September 1998 to February 2000 and work was completed in April 2000.
The study focused on the quantification of leakage of sanitary and industrial sewage from
sanitary sewer pipes on a national basis. The method for estimating exfiltration amounts
utilized groundwater table information to identify areas of the country where the hydraulic
gradients of the sewage are typically positive, i.e., the sewage flow surface (within
pipelines) is above the groundwater table. An examination of groundwater table elevations
on a national basis reveals that the contiguous United States is comprised of groundwater
regions (established by the U.S. Geological Survey) which are markedly different. Much
of the northeastern, southeastern, and midwestern United States has relatively high
groundwater tables that are higher than the sewage flow surface, resulting in inflow or
infiltration. Conversely, a combination of relatively low groundwater tables and shallow
sewers creates the potential for widespread exfiltration in communities located in the
western United States.
This report presents information on typical sewer systems, identifies and assesses the
factors that cause or probably cause exfiltration, presents commonly used and advanced
corrective measures and their costs for dealing with exfiltration, identifies technology gaps,
and recommends associated research needs and priorities. This report also examines
urban exfiltration, including a case study of Albuquerque, New Mexico.
IV
-------
Contents
Foreword iii
Abstract iv
Figures vi
Tables vi
Acknowledgement vii
Chapter 1 Introduction 1
1.1 Background 1
1.2 Objectives 1
Chapter 2 Identification and Assessment of Causative Factors and Health/
Environmental Impacts 3
2.1 Causative Factors 3
2.2 Health and Environmental Impacts 4
2.2.1 Ground Water 4
2.2.2 Water Supply Distribution System 5
2.2.3 Surface Water Pollution 6
Chapter 3 Methodology for Determining the Magnitude of Exfiltration on a
National Scale 7
3.1 Identification of Exfiltration Susceptible Sewer Systems 7
3.2 Estimating National Exfiltration Rates 8
Chapter 4 Corrective Measures 10
4.1 External Sewer Rehabilitation Methods 10
4.2 Internal Sewer Rehabilitation Methods 11
4.3 Issues Related to the Limitations of Existing Technologies 13
Chapter 5 Results 14
5.1 National Scale Quantification 14
5.1.1 Estimates Based on Direct Measurements
(U.S. EPA Study) 15
5.1.2 Estimates Based on Darcy's Law and Related Theory
(European Studies) 16
5.1.3 Estimates Based on Drinking Water - Wastewater
Balance 18
5.1.4 Comparison of the Various Methodologies -
Albuquerque Case Study 20
5.2 National Depth to Groundwater Mapping 22
5.3 Conclusions 24
5.4 Corrective Measure Costs 26
Chapter 6 Recommendations 28
References 30
Glossary of Terms 32
-------
Figures
2-1 Sanitary sewer system components and exfiltration sources 4
5.1 National depth-to-groundwater map 24
Tables
5-1 Summary of Exfiltration Rates from Continuous Flow Monitoring
and Hydrostatic Testing 15
5-2 Summary of Exfiltration Measurements 16
5-3 Calculated Exfiltration Rates Using United States EPA Study Results 20
5-4 Estimates of Sewer Exfiltration Quantities forthe Albuquerque Sewer System Based
on Published European Exfiltration Rates 22
5-5 Example Sewer Rehabilitation Costs for Exfiltration Corrective
Action 27
VI
-------
Acknowledgement
The contributions of several individuals are acknowledged and appreciated for the
preparation of the project report. Dr.AriamalarSelvakumarand Mr. Richard Field were the
Project Officer and Technical Advisor, respectively for EPA's National Risk Management
Research Laboratory (NRMRL), and contributed toward the review and preparation of the
project report. Mr. Robert Amick, P.E., was the Project Manager for Environmental Quality
Management, Inc. Mr. Edward Burgess, the Technical Manager for Camp, Dresser &
McKee, was responsible for the quantification of exfiltration on a national level.
VII
-------
Chapter 1
Introduction
1.1 Background
Many municipalities throughout the United States have sewerage systems (separate and
combined) that may experience exfiltration of untreated wastewaterfrom both sanitary and
combined sewers. Sanitary sewer systems are designed to collect and transport to
wastewater treatment facilities the municipal and industrial wastewaters from residences,
commercial buildings, industrial plants, and institutions, together with minor or insignificant
quantities of ground water, storm water, and surface waters that inadvertently enter the
system. Over the years, many of these systems have experienced major infrastructure
deterioration due to inadequate preventive maintenance programs and insufficient planned
system rehabilitation and replacement programs. These conditions have resulted in
deteriorated pipes, manholes, and pump stations that allow sewage to exit the systems
(exfiltration) and contaminate adjacent ground and surface waters, and/or enter storm
sewers. Exfiltration is different from sanitary sewer overflows (SSOs). SSOs are overflows
from sanitary sewer systems usually caused by infiltration and inflow (I/I) leading to
surcharged pipe conditions. SSOs can be in the form of direct overflows to receiving
water, street flooding, and basement flooding; whereas exfiltration is not necessarily
caused by excess I/I and is merely caused by a leaking sewer from its inside to its
surrounding outside.
Untreated sewage from exfiltration often contains high levels of suspended solids,
pathogenic microorganisms, toxic pollutants, floatables, nutrients, oxygen-demanding
organic compounds, oil and grease, and other pollutants. Exfiltration can result in
discharges of pathogens into residential areas; cause exceedances of water quality
standards (WQS) and/or pose risks to the health of the people living adjacent to the
impacted streams, lakes, ground water, sanitary sewers, and storm sewers; threaten
aquatic life and its habitat; and impair the use and enjoyment of the Nation's waterways.
1.2 Objectives
Although it is suspected that significant exfiltration of sewage from wastewater collection
systems occurs nationally, there is little published evidence of the problem and no known
attempts to quantify or evaluate it on a national basis. Accordingly, the objectives of this
-------
study were to quantify through desk-top estimates the magnitude of the exfiltration problem
in wastewater collection systems on a national basis; identify the factors that cause and
contribute to the problem; and document the current approaches for correcting the
problem, including costs. The resulting information was used to identify information and
technology gaps and research priorities.
Chapter 2 identifies and qualitatively assesses the causative factors and health impacts
of exfiltration; the methodology employed for quantification of exfiltration on a national
scale is presented in Chapter 3; Chapter 4 presents corrective measures applicable to
exfiltration; national magnitude of exfiltration and corrective measure costs results are
presented in Chapter 5; and Chapter 6 identifies existing information/data gaps and makes
recommendations for further research.
-------
Chapter 2
Identification and Assessment of Causative Factors
and Health/Environmental Impacts
2.1 Causative Factors
A search for publications regarding exfiltration of sewage from wastewater collection
systems did not locate any exfiltration-specific discussion of unique/causative factors
because most factors which cause inflow/infiltration are identical to those associated with
exfiltration (i.e., they both occur through leaks in pipes, depending on the relative depth of
the ground water).
Factors that contribute to exfiltration include:
size of sewer lines
age of sewer lines
materials of construction (sewer pipe, point/fitting material, etc.)
type and quality of construction (joints, fittings, bedding, backfill)
depth of flow in the sewer
Geological conditions that contribute to exfiltration include:
groundwater depth (in relation to sewer line/depth of flow of sewage)
type of soil
faults
Climate conditions that influence exfiltration include:
average frost line in relation to sewer depth
average rainfall, which helps determine groundwater depth
In a typical exfiltrating sanitary sewer system, with the groundwater level below the sewage
flow surface, exfiltration can occur in several areas. Figure 2-1 schematically represents
these exfiltration sources, including defective joints and cracks in the service laterals, local
mains, and trunk/interceptor sewers. The level of ground water and the depth of flow in
the sewer will influence the extent of exfiltration rates, since the pressure differential
-------
Defective Manhole
Connection
Broken/cracked pipe
Defective Manhole
Casing
Service Lateral
Defective Lateral
Connection
Local Sewer Main
^^^^^H
Trunk/Interceptor Sewer
Defective Manhole Connection
Broken/cracked pipe
Figure 2-1. Sanitary sewer system components and exfiltration sources.
between the hydraulic head in the sewer and the groundwater hydraulic head will force
water out of the sewer apertures into the surrounding soil material.
2.2 Health and Environmental Impacts
This section addresses the potential health impacts of exfiltration on ground water, drinking
water distribution systems, and surface water.
2.2.1 Ground Water
Little published data is available on specific incidents of groundwater pollution and
associated health/environmental impacts arising from leaking sewers, despite the
widespread acknowledgment that these incidents occur. Several studies have indicated
widespread pollution of ground water in urban areas arising from the general leakiness of
sewers, including bacteria and ammonium reported from Wisconsin and general pollution
in the San Joaquin Valley in California.1
-------
Transport of the sewage and pollutants leaking into the subsurface/ground water depends
on a variety of factors, including but not limited to: the difference in hydraulic head between
the sewage surface and the groundwater table level, the substrate physical/chemical/
biological characteristics (which determines attenuation potential), and the sewage
pollutants and their concentrations. Fecal bacteria contamination is the most serious
health risk associated with domestic sewage exfiltration. Contamination by viruses,
protozoa, and other microorganisms is also a concern. Increased concentrations of total
organic carbon, nitrate, chloride, and sulfate, however, can also make the water unfit for
consumption. Phosphate and boron are good indicators of sewage pollution since they are
not naturally occurring in ground water.2
The solids present in sewage can plug the porous media beneath the pipe and rapidly
decrease the exfiltration rate. In an experiment completed to examine this effect, the
leakage was reduced to a steady state within an hour.3
As evidence of pollution from sewage, chloride and nitrate have been found to travel
together. A California study indicated that ammonium disappeared within 4 feet, probably
by adsorption and bacteriological activity. Bicarbonate and nitrate increased several
hundred percent and nitrite disappeared.4
2.2.2 Water Supply Distribution Systems
Because of minimum separation requirements for potable water supply distribution systems
and sanitary sewers and vigilant application of cross-connection control programs, the
opportunity for sewer exfiltration to contaminate drinking water supplies is theoretically
rather limited. Only one such potential documented case was found in a comprehensive
data/information search.5 Sewage from exfiltration can enter a distribution system through
a broken water main or, under reduced pressure conditions, through a hole which leaks
drinking water out under normal positive pressure conditions. Situations which could allow
infiltration of the sewage through a lowering of water main pressure primarily involve
backflow and surges.
Main Breaks
Despite the best efforts of utilities to repair water main breaks using good sanitary
procedures, these breaks represent an opportunity for contamination from exfiltration to
enter the distribution system. When a main breaks, utilities typically isolate the affected
section, superchlorinate, and flush the repaired pipe. Flushing velocities may not always
remove all contaminated debris, however, and microbiological testing of the final water
quality may not detect contaminating microorganisms. In 1989, Cabool, Missouri5
experienced a suspected cross-connection between sewage overflow and two major
distribution system line breaks (backflow may have occurred during simultaneous repair
of numerous water meters) caused by freezing temperatures, resulting in 243 cases of
diarrhea, 32 hospitalizations, and four deaths due to E. co//O157:H7 strain. This town of
-------
2000 was on an untreated groundwater system and did not superchlorinate during repairs
of the water main breaks.
Backflow
Backflow devices to prevent the entry of contaminated water constitute an important
distribution system barrier. Because of cost considerations, backflow-prevention devices
are primarily installed on commercial service lines at facilities that use potentially
hazardous substances. Such facilities include hospitals, mortuaries, dry cleaners, and
industrial users. It is uncommon for all service connections to have backflow prevention
devices; thus, back siphonage can occur at these unprotected points. Furthermore,
installation of backflow devices at all service connections would make routine checking of
the devices nearly impossible. Without routine inspection, proper functioning of the units
cannot be determined.
Surges
Recent research is focusing on transient pressure waves that can result in hydraulic surges
in the distribution system. These waves, having both a positive and negative amplitude,
can draw transient negative pressures that last for only seconds and may not be observed
by conventional pressure monitoring. Because these waves travel through the distribution
system, at any point where water is leaking out of the system, the transient negative
pressure wave can momentarily draw water and sewage (if present) back into the pipe.6
2.2.3 Surface Water Pollution
No data or narrative information in the literature demonstrate, or even suggest, that sewer
exfiltration has directly contaminated surface waters. Several factors that control the
occurrence of sewer exfiltration may explain the absence of a linkage between exfiltration
and surface water pollution.
The occurrence of exfiltration is limited to those areas where sewer elevations lie above
the groundwater table. Since groundwater elevations near surface water bodies are
typically near the ground surface, sewers near surface water bodies generally are below
the groundwater table, and infiltration (rather than exfiltration) will dominate the mode of
sewer leakage in these areas. In areas of steep topographic conditions, where sewers are
located near surface waters and at elevations that lie above the surface water, exfiltration
impacts may be possible. However, these situations are assumed to be sufficiently rare
that exfiltration impacts on surface waters are not observed.
-------
Chapter 3
Methodology for Determining the Magnitude of Exfiltration on a National Scale
The process of estimating the magnitude of the exfiltration problem on a national scale has
been performed as a series of two independent steps:
Qualitatively assessing the portion of the nation's sewer systems that are
susceptible to exfiltration;
Applying assumptions about exfiltration rates (percent of base sewer flow)
to the exfiltration susceptible sewer systems to provide an assessment of the
extent of sewer exfiltration on a national scale.
3.1 Identification of Exfiltration Susceptible Sewer Systems
The key factor influencing the occurrence of exfiltration is the direction of the hydraulic
gradient between the sewer flow surface and the groundwater table (GWT) external to the
sewer. Where (and/or when) the direction is toward the sewer, exfiltration will be <0 (i.e.,
the hydraulic gradient will cause infiltration, rather than exfiltration). This situation is
probably best analyzed by evaluating the depth of the sewers (and service laterals) relative
to the groundwater table. In much of the northeastern, southeastern, and midwestern
United States, relatively high groundwater tables typically result in infiltration conditions.
Exceptions include shallow sewers, service laterals, and seasonal variation in GWTs that
can significantly change the spatial extent of the sewer system that lies above the GWT
(i.e., that can be considered to be "exfiltration susceptible"). To a lesser degree, short-term
reversals in the gradient that may occur during wet weather (e.g., surcharged sewers which
temporarily experience high sewage flow surface above the GWT, and may therefore
briefly exfiltrate) may also need to be considered.
Given the importance of first screening out those areas that are not "exfiltration
susceptible," the initial desktop analysis task was to perform spatial analysis of sewer
depth relative to regional GWT elevations. Existing national-scale groundwater information
was examined, such as that provided by the U.S. Geological Survey (e.g., USGS
Groundwater Regions of the United States). As the various national groundwater data
sources were reviewed, however, it was determined that mapping in support of the
purposes of this study was not readily available. For this reason, a national depth-to-
groundwater map was prepared under this project from groundwater level data available
-------
in the national databases (U.S. EPA STORE! and USGS WATSTORE) and presented in
Section 5 of this report.
It is recognized that there may be seasonal variability in the portion of sewer systems
susceptible to exfiltration in some areas, as GWTs can vary seasonally. The extent to
which seasonal differences must be accounted for was assessed in reviewing the
correlations to sewer depth.
National-scale sewer depth data does not exist, but for purposes of the desktop analysis
some assumptions aboutthis parameter can be made. For example, typical service lateral
depth can be assumed to be 8 feet for buildings with basements, and 2 to 4 feet for houses
built on slabs. Typical sewer main depth can be assumed to be 6 to 10 feet; it may be
possible for more detailed assessments to develop a typical depth distribution (i.e., x % 4-
10 ft deep, y% 11-15 ft deep, z% > 15 ft deep). Regional differences should be
considered; for example, sewer depths typically are shallower in the western United States
than in other areas of the country. Sewer system density (miles/acre) can be correlated
with readily available national population density data to create a CIS coverage of sewer
system density.
CIS processing incorporating the general spatial (mapped) relationships between sewer
depth and groundwater elevations allowed the development of a characterization of the
"exfiltration susceptibility" of various areas. This was attempted at the national level, but
the data required to support this analysis are unavailable; thus, a representative area
(Albuquerque, New Mexico) for which a recent exfiltration study had been completed, was
selected on which to perform the analysis. National exfiltration rate assessments can be
extrapolated from this analysis. However, more detailed identification and inventory of
exfiltration susceptible areas is required to support a meaningful quantification of national
exfiltration rates.
3.2 Estimating National Exfiltration Rates
Estimation of the extent of exfiltration that actually occurs was addressed with the same
set of parameters that are applied to characterize and quantify the infiltration problem:
sewer condition, joint type, pipe material, age, etc. Similarly, correcting the problem can
be assumed to involve the same technologies as are applied to infiltration (various lining
approaches, etc.). For purposes of this project, however, it was necessary to make
simplifying assumptions about exfiltration rates and corrective actions. More detailed
investigations in the future can examine the spatial variability in exfiltration rates that can
be correlated to the sewer condition, joint type, pipe material, and sewer age parameters.
Corrective action costs can also be refined later with more detailed assessments of
required actions.
For purposes of this study, unit rates for exfiltration (gallons/day/inch/mile) available from
the 1989 EPA study7 were used to generate the assessment of the magnitude of the
8
-------
national exfiltration problem. These unit rates were applied to the "exfiltration susceptible"
areas (together with assumptions about the inch-miles of sewers/service laterals in those
areas) to generate exfiltration rates in the Albuquerque case study. The unit rates based
on gallons/day/inch/mile were compared with estimates based on percent of base sewer
flow. Comparisons of the two methods proved useful in developing the final estimates.
-------
Chapter 4
Corrective Measures
The proper selection of corrective or rehabilitation methods and materials depends on a
complete understanding of the problems to be corrected, as well as the potential impacts
associated with the selection of each rehabilitation method. Pipe rehabilitation methods
to reduce exfiltration (and simultaneously infiltration) fall into one of the two following
categories:
External Rehabilitation Methods
Internal Rehabilitation Methods
Certain conditions of the host pipeline influence the selection of the rehabilitation method.
It is therefore necessary to assess these factors to prepare the pipe for rehabilitation.
Rehabilitation is proceeded by surface preparation by cleaning the pipe to remove scale,
tuberculation, corrosion, and other foreign matters.
4.1 External Sewer Rehabilitation Methods
External rehabilitation methods are performed from the aboveground surface by excavating
adjacent to the pipe, or the external region of the pipe is treated from inside the pipe
through the wall. Some of the methods used include:
External Point Repairs
Chemical Grouting
Acrylamide Base Gel
Acrylic Base Gel
Cement Grouting
Cement
Microfine Cement
Compaction
10
-------
4.2 Internal Sewer Rehabilitation Methods8 9 10
The basic internal sewer rehabilitation methods include:
Chemical Grouting - Internal grouting is the most commonly used method for
sealing leaking joints in structurally sound sewer pipes. Chemical grouts do not stop
leaks by filling cracks; they are forced through cracks and joints, and gel with
surrounding soil, forming a waterproof collar around leaking pipes. This method is
accomplished by sealing off an area with a "packer," air testing the segment, and
pressure injecting a chemical grout for all segments which fail the airtest. The three
major types of chemical grout are:
Acrylic
Acrylate
Urethane
Sliplining - In this method, pipes are inserted into an existing line by pulling or
pushing pipes into a sewer. The space between the existing pipe and liner pipe is
grouted. Sliplining can be segmental or continuous. Small pipes including service
laterals are usually continuous, with the larger sizes being segmental. Major types
of Sliplining are:
Continuous Pipe - insertion of a continuous pipe through the existing pipe
Polyethylene
Polypropylene
Segmental- Short segments of new pipe are assembled to form a continuous line,
and forced into the host pipe. Generally, this method is used on larger sized pipe
and forced into the host pipe.
Polyethylene
Polyvinyl Chloride
Reinforced Plastic Mortar
Fiberglass Reinforced Plastic
Ductile Iron
Steel
Cured-in-Place Pipe (CIPP) - The CIPP process involves the insertion of a flexible
lining impregnated with a thermosetting resin into a cleaned host pipe using an
inversion process (hot water or steam). The lining is inserted using existing
manholes.
Because the liner initially is flexible, the pressurized steam or water also serves to
form it in the shape of the existing pipe. The resin hardens with the application of
11
-------
heat and with the passage of time (generally a few hours) to form a pipe within the
existing pipe.
Closed-fit Pipe - This involves pulling a continuous lining pipe that has been
deformed temporarily so that its profile is smaller than the inner diameter of the host
pipe. After installation, the new pipe expands to its original size and shape to
provide a close fit with the existing pipe. Most lining pipe is deformed in the
manufacturing plant.
Fold and Form Pipe - This is similar to sliplining, except that the liner pipe is
deformed in some manner to aid insertion into the existing pipe. Depending on the
specific manufacturer, the liner pipe may be made of PVC or HOPE. One method
of deforming the liner is to fold it into a "U" shape before insertion into the existing
pipe. The pipe is then returned to its original circular shape using heated air or
water, or using a rounded shaping device or mandrel. Ideally, there will be no void
between the existing pipe and the liner pipe after expansion of the liner pipe with the
shaping device. For the "U" shape liner, the resulting pipe liner is seamless and
jointless.
Spiral Wound Pipe - This involves winding strips of PVC in a helical pattern to form
a continuous liner on the inside of the existing pipe. The liner is then strengthened
and supported with grout that is injected into the annular void between the existing
pipe and the liner. A modified spiral method is also available that winds the liner
pipe into a smaller diameter than the existing pipe, and then by slippage of the
seams, the liner expands outward.
Pipe Bursting - Pipe Bursting is a method of replacing existing sewers by
fragmenting the existing pipe and replacing the pipe in the void.
1. Hydraulic Method - In this method a solid rod is inserted into the existing pipe
and a bursting head is attached to the rod, which is then attached to a new
replacement pipe. Hydraulic power is used to retract the rod and bursting
head, and draw in new pipes. Existing sewer pipe is broken into fragments,
which are driven into the surrounding soil.
2. Pneumatic Method - This system consists of a pneumatic burster unit that
splits the existing pipe while simultaneously installing a new polyethylene
pipe of the same size or larger. Over 90 percent of the bursting is done by
this method.
3. Static Head - Static heads have no moving parts. The head is simply pulled
through the old pipe by a heavy-duty pulling device.
Spot (Point) Repair - Point repairs are used to correct isolated problems in a pipe.
Sometimes they are used as the initial step in the use of other rehabilitation
12
-------
methods. Point repairs include:
1. Robotic Repair
2. Grouting/Sealing
3. Special Sleeves
4. Point CIPP
4.3 Issues Related to the Limitations of Existing Technologies
The City of Houston, Texas recently completed model simulations and determined that
comprehensive rehabilitation was not cost-effective.11 It was found cheaper to relieve
Houston's collection system bottlenecks for the short duration. This study noted that many
types of rehabilitation and varying levels of rehabilitation, however, were not tested and
could prove to be cost-effective. Soil characteristics and climatology vary from region to
region, as do sewer system conditions and available system capacity, and the conclusions
found in Houston may not be applicable to other parts of the country.
Thousands of communities have rehabilitated portions of their collection systems; yet very
few know whether or not they have been successful. The problem is that no one can
forecast how effective the rehabilitation will be. A recent literature search found that only
91 sewer sheds worldwide have post rehabilitation infiltration/inflow (I/I) reduction
information available.12 Average reported reduction is 49 percent of peak I/I rate. No data
was found on the amount of exfiltration reduction from rehabilitation.
Pipe bursting may be limited in use where the pipe has sags. This technology's use is
limited to cast or ductile iron pipe or concrete encasement. Pipe bursting may not be
applicable where other existing utilities are close to the pipe.
Some sliplining applications require a round host pipe. Clearance should be checked
before this method is employed.
13
-------
Chapter 5
Results
This section describes the results of using various methods to estimate exfiltration from
sewers. These methods have been developed and used in several locations in the United
States and Europe. Some of these methods have been applied to calculate potential
exfiltration in Albuquerque's sewer system, for which one of the most extensive exfiltration
studies in the United States to-date has been completed.12 For this reason, Albuquerque
has been selected as a case study, from which the national extent of sewer exfiltration can
be assessed.
The results of the 1998 exfiltration study from Albuquerque are extrapolated qualitatively
by evaluating the exfiltration susceptibility of sewer systems throughout the United States.
Susceptibility is defined by the relative depths of the sewers and groundwater table. In
cases where sewer depths are generally shallower than the surrounding ground water, the
potential to exfiltrate exists (because the direction of the hydraulic gradient is toward the
exterior of the sewer) and these sewers can therefore be considered exfiltration
susceptible. A national depth-to-groundwater map has been prepared for use in this
assessment of the national extent of exfiltration susceptible sewer systems.
The findings of the Albuquerque case study were combined with the national depth-to-
groundwater mapping to present a qualitative assessment of the extent to which sewer
exfiltration represents a risk to water quality and human health on a national scale. Much
of the information presented in Section 5.1 is taken from the 1998 Albuquerque study.12
5.1 National Scale Quantification
Although exfiltration is not a widely studied phenomenon, several exfiltration studies and
investigations have been completed throughout the world. These include work completed
in the United States for the U.S. EPA and several studies in Europe, the majority of which
are focused on Germany. Some of the more applicable previous studies are discussed
below.
Three basic approaches have been used to quantify sewer exfiltration rates: 1) direct
measurement of flow in isolated sewer segments, 2) theoretical estimates using Darcy's
Law and related hydraulic theory, and 3) water balance between drinking water
14
-------
produced/delivered and wastewater collected/treated. Each of these approaches has been
applied to the Albuquerque case study and is described below.
5.1.1 Estimates Based on Direct Measurements (U.S. EPA Study)
An EPA study entitled "Evaluation of Groundwater Impacts of Sewer Exfiltration" was
completed in the late 1980's.7 The work estimated exfiltration in two California city sewer
systems to develop a correlation between exfiltration and infiltration. The tests were
conducted in areas of vitrified clay pipe (VCP) predominance, where older pipe of known
or suspected poor condition existed. Only those pipe segments located above
groundwater levels were tested. Water consumption was metered for all sewer service
connections corresponding with each measured sewer line to determine the actual quantity
of wastewater flow entering the system. It was assumed that all internal household water
entered the sewer system. Measurements of sewage flow in the sewer lines were made
by continuous flow monitoring and hydrostatic testing. Calculated sewer exfiltration was
reported in units of gallons per inch diameter per mile length per day (gpimd). Table 5-1
presents a summary of the exfiltration rates.
Table 5-1. Summary of Exfiltration Rates from Continuous Flow Monitoring and
Hydrostatic Testing (Engineering Science, Inc., 1989)
Location
Berkeley, CA
Pardee Street
Berkeley, CA
7th Street
Santa Cruz, CA
Beach Street
Santa Cruz, CA
Riverside Parking Lot
Pipe Information
320 linear feet (If)
of 8-in. - diameter
VCP
298lfof6-in. -
diameter VCP
260 If of 8-in. -
diameter VCP
124lfof6-in. -
diameter VCP
Exfiltration Rate
Cont. Flow
Monitoring
(gpimd)3
5,649
(34% of flow)
5,283
(56% of flow)
6,557
77,745
Exfiltration Rate
Hydrostatic
Testing
(gpimd)
6,327
5,649
2,417
8,324
a gallons per inch diameter per mile length per day
This table shows that a large discrepancy exists between the results from the continuous
flow monitoring and the hydrostatic testing at one Santa Cruz location. The study
concludes that the continuous flow monitoring achieved reliable data and that the
hydrostatic test data was influenced by the tidal cycle. A correlation model between
exfiltration and infiltration was developed, but not field tested.
A second evaluation was performed using field measurements at another location to verify
the correlation model. This evaluation used similar methodologies as the first task.
15
-------
Exfiltration measurements were made in the Washington Suburban Sanitary Commission
(WSSC) sewer system near Washington, D.C., and in Lexington, Kentucky. Table 5-2
presents a summary of the measurement results from the evaluation.
Table 5-2. Summary of Exfiltration Measurements (Engineering Science, Inc., 1989)
Location
WSSC John Hanson
Highway
WSSC
University of MD
Lexington, KY
Lumber Yard
Lexington, KY
Car Lot
Lexington, KY
Various Shops
Lexington, KY
Various Shops
Pipe Information
1,400lfof8-in. -
diameter VCP
832 If of 10-in. -
diameter VCP
455lfof8-in. -
diameter VCP
1,029lfof8-in. -
diameter VCP
586 If of 10-in. -
diameter VCP
586 If of 10-in. -
diameter VCP
Average
Exfiltration Rate
(gpimd)3
16,248
63,312
17,103
9,061
5,664
15,689
Exfiltration as
Percentage flow
(%)
16.6
49.1
22.6
31.3
11.9
34.5
a gallons per inch diameter per mile length per day; If = linear feet
Several problems with the measurement methodologies were noted, and overall the
hydrostatic test method was judged to be not successful. It was resolved that the flow
monitoring procedure worked well and should be applied to areas with a minimum of 400-
500 linear feet of pipe with little or no service connections.
5.1.2 Estimates Based on Darcy's Law and Related Theory (European Studies)
The study of exfiltration has been of great interest in Germany. This country has a very
old, deteriorated infrastructure. The cost to complete the necessary repairs to Germany's
sewer systems is estimated to be nearly $100 billion (U.S.). Therefore, several exfiltration
studies have been conducted to prioritize repair work. These studies have both applied
theoretical (Darcy's Law) approaches and direct measurements to estimate sewer
exfiltration. Excerpts from some of the studies are summarized below.
A report from England13 provided an estimate of 300 x 106 m3/yr (793 x 108gal/yr)
or approximately 1 liter/day/m (397 gal/day/mile) for the exfiltration of the 880,000
km (547,000 miles) of sewer lines in Germany, although the basis of the estimate
is not clear. This very low sewer leakage rate is actually riet exfiltration, which is the
difference between exfiltration and infiltration. The study indicates that total
exfiltration and infiltration in Germany are nearly equal, but the amounts are not
provided.
16
-------
To better understand the mechanics of exfiltration, sewage migration from leaking
pipes to ground water was correlated in a study using Darcy's Law (see Equation
1 ).3 The rate of exfiltration is linearly dependent on the area of the pipe exfiltrating
and the pressure head:
(1) Q = LAdh
where Q is the exfiltration rate (ftYs) through a pipe leak area A (ft2) at a pressure
head of dh (ft), and L is leakage factor (s"1).
The leakage factor is defined in Equation 2:
(2) L = K/dL
where K is the permeability of the surrounding soil (ft/s) and dl_ is the thickness of
the settleable soil layer (ft).
This study found that the settleable solids in the wastewater act to reduce the
permeability of the bedding material and lower the exfiltration rate rapidly at low
flows and velocities. This clogging reduces the rate of exfiltration immediately. In
fact, a steady-state rate of exfiltration was reached after one hour, even with large
area of joint damage.
A research project undertaken by the Institute of Environmental Engineering (ISA)
at the University of Technology of Aachen, Germany, studied the water pollution
hazard of leaking sewers.14'15'16 The ISA developed and used a special exfiltration
measuring device at every joint in several sections of sewer pipe on several tests
conducted throughout Germany. This study determined that the most significant
VCP sewer damages which permit exfiltration are leaking service junctions, leaking
sewer joints, pipe cracks, and pipe fractures. At a pressure head below the sewer
crown, which is typically the case in gravity flow sewer lines, exfiltration rates were
minimal. At a pressure head of one pipe diameter, the exfiltration rate increased
dramatically, to more than 26 gal/hour (gph) per joint in some segments. This high
leakage rate can, in part, be attributed to the generally poor condition of the old
sewer systems. A linear correlation between pressure head and exfiltration rate for
several types of sewer defects was noted for pressure heads greater than 500 mm
(20 inches). It was also noted that at lower flows and pressure heads, the
exfiltration rate decreases exponentially, most likely from self-sealing from sewer
film and settleable solids in the sewage. If the flow and pressure head increases,
however, this self-sealing property is broken and the exfiltration rate increases
rapidly.
17
-------
5.1.3 Estimates Based on Drinking Water- Wastewater Balance
In this section, exfiltration from Albuquerque's sewer system is estimated using a
water/sewage balance calculation, backed up by some previous local studies on infiltration.
The results are then compared with leakage rates calculated from the other methodologies
and unit rates derived from the EPA and European studies presented above.
A direct method for estimating exfiltration is to compare water pumpage and usage with
wastewater received at Albuquerque's Southside Water Reclamation Plant (SWRP). To
make this comparison, it is necessary to identify the base water demand, which is the
indoor component of the total household use. Demands during mid-winter (January and
February) are assumed to be near base flow because no or very minimal outdoor water
usage occurs. Water and wastewater data obtained from the City for January 1998
revealed the following:
Average daily influent flow at the SWRP: 51.4 mgd
Average daily water pumpage into transmission/distribution system: 61.2
mgd (this is then considered to be the daily base flow for that month)
Subtracting wastewater flow from the pumpage rate yields a difference of 9.8 mgd, which
is the first approximation of sewage leakage. However, several other factors also impact
the water balance in the water and wastewater systems. These are:
Sewer infiltration
In-house water consumption
Water distribution system leakage
Sewer exfiltration
City of Albuquerque staff, using a range of available information (including meterand billing
records, pumpage records, and other data), have estimated losses in the water system at
about 11 percent of the total amount pumped. A 1997 study17 found water system losses
ranging from 8 percent in Hong Kong, which is considered to have a relatively "tight" and
high-quality system, to the 20-25 percent range in England, which has many very old
distribution systems. An 11 percent loss in the system would account for a daily average
loss of about 6.73 mgd.
In-house consumption is the portion of the water entering the house that does not leave
as sewage, but is consumed in cooking, drinking, watering plants, cleaning, etc. National
experience indicates that about 3 percent of water entering the home is consumed on an
average day in January 1998. With negligible non-domestic consumption, the remaining
amount of water, about 1.4 mgd, represents the net difference between the two other
factors in the water balance: sewer infiltration and exfiltration. The net amount is positive,
indicating that exfiltration exceeds infiltration by 1.4 mgd, which is plausible given that the
18
-------
great majority of Albuquerque's sewers, and particularly those most susceptible to
exfiltration (older VCP), are in exfiltration areas (well above groundwater levels).
In order to estimate the exfiltration volume, previous studies addressing infiltration in the
Albuquerque sewer system were reviewed. One of the studies18 utilized several
approaches to gain an approximation of inflow and infiltration in the Albuquerque system,
most of which was attributed to infiltration in the valley of the Rio Grande. Some of these
methodologies are described below:
A flow comparison between winter water use and sewage flow. This
methodology resulted in an infiltration flow of 3.7 mgd. However, the report
stated that "this estimation is probably within + 50 (percent) of the actual
value..."
Early morning sewage flow versus water use. This methodology resulted in
an infiltration flow of nearly zero.
Sewage flow versus population. Using a 100-gallons-per-capita-per-day
wastewaterflow and a population of 300,000, infiltration was estimated at 5
mgd. It was also noted that the average sewage flow for Albuquerque at this
time was actually 117 gpcd.
Influent BOD versus domestic wastewater BOD. The expected BOD
concentration in the wastewater was calculated based upon a generally
accepted BOD loading of 0.17 Ib/cap/day. This BOD concentration was
compared with the average influent concentration to calculate an infiltration
flow of 5.9 mgd. However, this was thought to be a high estimate based
upon the relatively small industrial component and the high institutional
contribution.
In addition, the study field-verified the areas subject to infiltration. Based upon the above
calculations and results of the field tests, infiltration was thought to be somewhat less than
3 mgd, or 9 percent of the wastewater flow in 1975. Nine percent of today's wastewater
flow would be in the 5 mgd range.
Another infiltration analysis was completed as part of the Albuquerque ASAM Model
Loading and Verification Task.19 Interceptor manholes that were within 2 feet of ground
water were identified. Flow monitoring was completed in a sewer subbasin, and the
resulting flows were compared with the predicted flows to determine infiltration. The
infiltration rate for Albuquerque was calculated at 0.925 mgd, but, again, the impact of
exfiltration was not included. Therefore, the work revealed a net infiltration rate, indicating
that actual infiltration is about 1 mgd greater than total exfiltration.
From the foregoing investigations, it is estimated that the total average infiltration rate for
the Albuquerque system is in the vicinity of 3.5 mgd. The 9 percent field-verified rate
19
-------
reported in the Molzen-Corbin report is probably high, given the repair and replacement of
major interceptors in the valley that have occurred since 1975, as well as the use of better
quality materials and construction techniques for new pipelines since then. On the other
hand, repairs have generally not been made to the sewers most susceptible to exfiltration
-- old vitrified clay pipes (VCP).
The total exfiltration rate is obtained by adding the 1.4 mgd remaining in the water balance
to the infiltration rate, for a total of 4.9 mgd, or approximately 5 mgd.
5.1.4 Comparison of the Various Methodologies - Albuquerque Case Study
Unit Rates from U.S. EPA Study
The 1989 U.S. EPA exfiltration study is discussed in Section 5.1.1 above, and some of the
results are summarized in Tables 5-1 and 5-2. Application of measured exfiltration rates
from this study (in gpimd)to the 66.5 miles of Albuquerque VCP sewers (average diameter
of 8.57 inches) that are potentially in condition C (major cracks) or D (severe cracks)
results in total exfiltration rates ranging from 1.38 mgd to 44.1 mgd (504 Mg/yr to 16,907
Mg/yr). These calculated quantities are listed in Table 5-3. Although there is a very wide
range in calculated rates, many of them are in the 3 to 4 mgd range calculated above using
a water balance.
Table 5-3. Calculated Exfiltration Rates Using United States EPA Study Results
Location
Berkeley, CA, Pardee Street
Berkeley, CA, 7th Street
Santa Cruz, CA, Beach Street
Santa Cruz, CA, Riverside
Parking Lot
WSSC, John Hanson Highway
WSSC, University of MD
Lexington, KY, Lumber Yard
Lexington, KY, Car Lot
Lexington, KY, Various Shops
Measured Unit Rates
(gpimd)
5,649; 6,327
5,283; 5,649
6,557; 2,417
77,745; 8,324
16,248
63,312
17,103
9,061
5,664; 15,689
Equivalent Albuquerque
Quantities3
(mgd)
3.2; 3.6
3.0; 3.2
3.7; 1.4
44.3; 4.7
9.3
36.1
9.8
5.2
3.2; 8.9
For 66.5 miles of suspected Class C and D pipe, average diameter 8.57 inches.
20
-------
European Methods
Section 5.1 discusses the results of several exfiltration studies carried out in Germany.
Applying these methods and unit rates to the Albuquerque sewer system yields several
estimates as follows:
The study by Lerner and Halliday13 presented an estimated net exfiltration rate of
397 gal/day/mile for the whole of Germany. Applying this figure to the entire length
of clay and concrete sewers in Albuquerque's system yields a total net exfiltration
rate (net leakage) of about 0.46 mgd. This is reasonably close to the net exfiltration
rate of 1.4 mgd calculated by the water balance in Section 5.1.3. It is expected that,
on average, a greater percentage of Germany's sewers are in infiltration areas than
is the case in Albuquerque. On the other hand, Germany's sewers are also older
and undoubtedly in overall worse condition, therefore more susceptible to
exfiltration. Thus, a near balance in exfiltration and infiltration is possible.
Albuquerque has a greater percentage of sewers above groundwater level, but a
smaller portion that is likely to heavily exfiltrate.
The study completed by Rauch and Stegner3 determined that exfiltration could be
correlated by Darcy's Law. A leakage factor dependent upon the bedding grain size
and permeability affects the exfiltration rate (refer to Equations 1 and 2 in Section
5.1.2). For this study, the leakage factor was back-calculated using Darcy's
Equation with the data presented in Rauch's report. This calculated leakage factor
was then used in Darcy's Equation to calculate the exfiltration rate for 8-inch-
diameter pipes flowing half full, with every joint separated one-quarter inch to
approximate conditions for Albuquerque. The exfiltration rate was calculated as 7.9
mgd (2,900 Mg/yr). However, not every joint will have a quarter-inch separation.
The ISA German studies discussed above14 1516 summarized the sewer damage
noted in the project. About 30 percent of the VCP sewers have leaking sewer joints.
The infrastructure in Albuquerque is not as old as that of Germany and therefore is
in better condition. If we assume every fourth joint (25 percent) will be separated
one-quarter inch, the exfiltration quantity is 2 mgd or 725 Mg/yr.
The German ISA project determined that at a 4-inch head (equivalent to an 8-inch
pipe flowing half full), the exfiltration rate was nearly zero. However, a storm sewer
was found to have an exfiltration rate, dependent upon the type of damage, ranging
from 4 to 10.5 gallons per hour per joint. This rate yields an exfiltration quantity of
8.2 to 21.9 mgd (3,000 to 8000 Mg/yr) for the Albuquerque sewer system. It is
probable, however, that not every joint is leaking even in pipe of condition C or D.
Assuming every fourth joint is leaking (25 percent as discussed above) presents an
estimate of 2 to 5.5 mgd (769 to 2,000 Mg/yr).
Table 5-4 presents a summary of the estimates of sewer exfiltration for the Albuquerque
area based on data from the European studies.
21
-------
Table 5-4. Estimates of Sewer Exfiltration Quantities for the Albuquerque Sewer System
Based on Published European Exfiltration Rates
Source/Study Location
Munich, Germany measurement of 24,600
gpmd
Darcy's Equation, every joint offset 0.25 inch
Darcy's Equation, every 4th joint offset 0.25
inch
ISA Study - every joint leaking 4 g/hr
ISA Study - every joint leaking 10.5 g/hr
ISA Study - every 4th joint leaking 4 g/hr
ISA Study - every 4th joint leaking 10.5 g/hr
Daily Quantity
1.65 mgd
7.9 mgd
2 mgd
8.2 mgd
22 mgd
2 mgd
5.5 mgd
Annual Quantity
600 Mg/yr
2,900 Mg/yr
730 Mg/yr
3,000 Mg/yr
8,000 Mg/yr
730 Mg/yr
2,000 Mg/yr
Based on a review of the above exfiltration rates for Albuquerque as calculated with the
various EPA and European unit figures and methodologies, it can be seen that the rate of
5 mgd determined in Section 5.1.3 is very much within the range that would be expected.
Although the calculated rates vary widely, the majority are within the 2 to 10 mgd range.
Therefore, the rate of 5 mgd, as determined by the water balance described in Section
5.1.3, is presented as the best estimate of the average daily wastewater exfiltration rate
from Albuquerque's sewer system.
It is further concluded that the majority of this leakage will occur in those areas most
susceptible to exfiltration, as approximately 15 percent of the sewer system in Albuquerque
is estimated to be below the groundwater table and therefore not exfiltration susceptible.
5.2 National Depth to Groundwater Mapping
In order to extrapolate the Albuquerque findings to a national scale, a qualitative
assessment of exfiltration susceptibility has been made using depth-to-groundwater
information. Since no such mapping at a national scale suitable for this purpose was
readily available, an initial mapping effort was undertaken as part of this study.
The development of a nationwide depth-to-groundwater atlas is difficult at best due to the
lack of easily obtainable data for most of the country. Data to determine the depth to the
shallowest water table may be gathered from local, state, federal, and private sources
through well logs, water level measurements, location of wetlands and seeps,
characterization of streams and rivers, and locations of lakes and other water bodies. A
thorough characterization of the U.S. water table is a long and exacting process.
Within the context of this study, the depth-to-groundwater map presented in Figure 5-1 is
a generalized view created using readily available data from the EPA STORET and USGS
WATSTORE databases of depth-to-groundwater parameters. The data were downloaded
22
-------
from CD ROM databases resident at the COM Hydrodata Center in Denver, Colorado. The
data were screened to eliminate missing depth-to-water values, missing latitude and
longitude, duplicate data, and easily recognized anomalous data. The resultant set
contained approximately 93,000 data points in the coterminous United States, Alaska, and
Hawaii (only the coterminous U.S. is shown below). Since the data retrieved from
STORET and WATSTORE is dependent upon the data owner for accuracy, there is no
comprehensive method of quality control. USGS data are continually reviewed, however,
and these data may be deemed reasonably accurate. The STORET and WATSTORE
databases, while certainly robust, do not contain all data available; therefore, data gaps
exist which are labeled (in the data tables) as insufficient data.
Despite the large dataset applied to build the map, many regions of the United States have
relatively limited data; these areas are unshaded on the map. Areas with the greatest
concentration of valid data points within the deep groudwater range are generally west of
the Mississippi River and along the Appalachian Mountains.
The data set was plotted upon a map of the United States using ESRI Arcview 3.1 CIS
application with a Spatial Analyst extension. A grid was produced with a cell size of 10000
for the coterminous U.S. and Alaska and 1000 for Hawaii. An inverse distance weighted
interpolation method (IDW) was used based on the 12 closest points. The IDW
interpolator assumes that each point has a local influence that diminishes with distance.
23
-------
Depth to water
^B < 5 Feet
5-15 Feet
| | > 15 Feet
Figure 5-1. National depth-to-groundwater map.
Note: It is important to read Section 5.2 for a detailed explanation of background data
basis.
5.3 Conclusions
Most of the urban areas in the northeastern, southeastern, and coastal areas of the U.S.
have relatively shallow groundwater tables (<15 feet). In these areas, where a significant
portion of the population (and therefore sewer systems) exists, relatively few exfiltration-
susceptible sewer systems are expected. One caveat is exfiltration from service laterals.
Even in the areas mentioned, many shallow service laterals may exist above groundwater
tables. However, the hydraulic head available to drive exfiltration in these service lines is
generally very low (typically only one or two inches, and intermittent). Further study in this
area may be warranted to assess the extent of service lateral exfiltration.
24
-------
Based on a review of the depth-to-groundwater map, it is expected that widespread
exfiltration is probably limited to a relatively small portion of the total U.S. population, as
relatively few large urban areas in the U.S. are located in these deeper groundwater areas.
Cities such as Albuquerque, Phoenix, Tucson, and others, are among the larger urban
areas where significant exfiltration potential exists. Further study of exfiltration conditions
in cities such as these, with relatively large areas with sewers above the groundwater table,
may be warranted on a case-by-case basis where evidence of exfiltration (e.g.,
groundwater contamination) has been observed, or is revealed by more detailed
evaluations. Areas with extremely deep groundwater tables probably experience relatively
less risk associated with exfiltration due to the long subsurface travel times and distances
of the exfiltrated sewage from the sewer to the groundwater table. Areas with significant
portions of the system above, but in close proximity to, the groundwater table are probably
at greatest risk. There is an increased risk in the relatively few areas with significant
exfiltration potential when there is, for example, a thin soil and fractured rock hydrogeologic
setting which allows pathogens and other contaminants from the sewage to reach the
ground water quickly and with minimal attenuation. However, since public water supplies
are treated with chlorination, ozonation, or other systems to kill fecal bacterial
contamination, an added measure of protection is provided.
A greater potential problem, albeit isolated, may be exfiltration from sewers carrying
industrial wastewater. Organic and inorganic constituents of industrial sewage can be
much more persistent than those of domestic sewage, and therefore much more likely to
reach the ground water in areas of significant exfiltration potential. The disposition of
industrial sewage contaminants which reach ground water used for drinking water supplies
may not be the same as that of fecal bacteria from domestic sewage [i.e., the treatment
processes (flocculation, filtration, chlorination, activated carbon filtration, etc.) may not
eliminate or reduce these contaminants to render them harmless]. Untreated well water
in some rural, small community, commercial, and private-owner drinking water systems
does not enjoy this added protection. However, these systems are not typically in close
proximity to large municipalities and associated sewer systems/exfiltration potential.
The Albuquerque Case Study concluded that the rate of exfiltration from that sewer
system, expressed as a percentage of base flow, is on the order of 10% of average daily
base wastewater flow - in absolute terms, roughly 5 mgd. This rate, expressed as an
average annual rate, is 1,825 Mg/yr. Another relevant conclusion of the Albuquerque study
was that there is a greater impact on ground water from septic tank usage than from sewer
exfiltration. As the foregoing depth-to-groundwater analysis indicates, however, exfiltration
is expected to vary significantly on a regional basis. Further study should expand the initial
depth-to-groundwater analysis performed here and identify more precisely the "exfiltration
susceptible" sewer systems throughout the U.S. and the extent to which exfiltration impacts
ground water in these systems.
In summary, exfiltration appears to be a problem in certain cities in the United States
(mainly located west of the Mississippi River and along the Appalachian Mountains) based
on an evaluation of: 1) available groundwater table data to nationally assess the extent to
25
-------
which sewer systems are susceptible to exfiltration, 2) past studies of measured and
estimated exfiltration rates, and 3) protective mechanisms, particularly natural
soil/hydrogeological setting attenuation and drinking water treatment plants. Exfiltration
may be a regional, or more likely, local problem where the GWT lies closely under the
sewage flow surface. Situations where the exfiltrate can reach even deep ground water
through a thin soil/fractured rock hydrogeologic setting, especially where persistent,
potentially toxic contaminants are present (such as those often associated with industrial
sewage) also pose a problem.
5.4 Corrective Measure Costs
Given the relatively high rates of exfiltration that potentially discharge from exfiltration-
susceptible sewer systems in the U.S., corrective measures may be required to adequately
protect groundwater resources, and in some limited instances surface waters, in these
areas. The site-specific nature of exfiltration problems, however, requires a more detailed
assessment of the larger urban areas in the exfiltration-susceptible western U.S. be
completed before a meaningful estimate of corrective costs can be developed.
Corrective actions to address exfiltration in those situations where local-level evaluation
calls for such action will generally be accomplished with similartechnologies as those used
to address infiltration. These technologies are described in Section 4. Although an
estimate of national-scale costs to address exfiltration must follow more detailed evaluation
of exfiltration-susceptible sewer systems, it is possible to identify corrective action costs
on a unit basis (i.e., cost ($) per linear foot of sewer) in this study. The following table
provides an example of those costs assuming the use of cured-in-place lining as the
method of sewer rehabilitation.20
26
-------
Table 5-5. Example Sewer Rehabilitation Costs for Exfiltration Corrective Action
Sewer Diameter (inches)
8
10
12
15
18
21
24
27
30
36
Cost ($) per linear foot
60
71
77
130
160
225
295
310
535
590
27
-------
Chapter 6
Recommendations
This study identified the following data/technology gaps associated with exfiltration.
Recommendations for research and development to fill these gaps were developed for
each data/technology gap identified.
1. Data Gap - comprehensive national depth-to-groundwater maps: Although a large
portion of the U.S. has readily available, accurate depth-to-groundwater data, many
regions of the United States have relatively limited data.
Recommendation:
An effort to refine the initial depth-to-groundwater mapping produced in this study
with an expanded and updated database would support a more detailed national
estimate of exfiltration and the cost of associated corrective measures.
2. Data Gap - extent of exfiltration in municipalities: There are relatively few large
urban areas in the U.S. which have the potential for widespread exfiltration.
Western arid U.S. cities such as Albuquerque, Phoenix, and Tucson are among the
larger metropolitan areas where significant exfiltration potential exists and little is
known about it. Albuquerque's exfiltration has recently been studied extensively.
Recommendation
Further study of localized exfiltration conditions in cities with high exfiltration
potential may be warranted on a case-by-case basis where evidence of exfiltration
has been observed, or is revealed by more detailed groundwater study. This study
should be preceded by assessment using the refined depth-to-groundwater
mapping recommended above to produce a national inventory of exfiltration
susceptible areas. This localized study will be of greater value than an attempt to
quantify the problem nationally, due to the localized nature of the problem.
3. Data Gap - exfiltrate fate and transport: No information is available regarding the
biological disposition of sewage exfiltrate. Also, it would be useful to determine if
a biological crust forms in the bedding below an exfiltrating sewer that would serve
to insulate/protect groundwater and/or water supply distribution systems.
28
-------
Recommendation:
Research to fill the exfiltration disposition data gap could involve the use of existing
sewage systems known or determined to be leaking in significant amounts (using
carefully excavated examination of the bedding beneath and adjacentto the leaking
sewerjoints), or by construction of an experimental leaking sewer system (artificially
introducing sewage into the sewer systems bedding). An analysis of bedding
samples from points at increasing depths and horizontal distances from the leak
would help to reveal the extent of exfiltrate transport.
4. Combined/Separate Sewer Considerations for Detailed Urban Study
Recommendation
The sewer systems to be considered in future exfiltration assessments should
include both combined and separate sewer areas, since combined sewers are often
located in highly urbanized areas where imperviousness is high. The result is a
decreased rainfall infiltration into the soil and lowering of the GWTs, making these
sewers potentially more susceptible to exfiltration. Additionally, combined sewers
are often shallower than separate sewers, older than separate sewers, and
constructed with less-watertight pipe joints - all factors that can contribute to higher
exfiltration rates. Another special case that must be considered in more detailed
studies is force mains. Although they are often constructed with tighter pipe joints
and more durable pipe material, they nonetheless operate under pressure and may
therefore be more exfiltration susceptible.
5. Inclusion of Service Laterals
Recommendation
It will be important to more detailed exfiltration assessments of urban areas to
consider service laterals together with public sewers in identifying and evaluating
the exfiltration susceptible sewers. Service laterals are the shallowest portion of the
sewer system (largest hydraulic gradient difference with GWT) and typically of the
poorest construction.
29
-------
References
No.
1. Hoffman, M., and D. Lerner. "Leak Free Sewers." Water and Waste Treatment,
Vol.35, No. 8, 1992.
2. "The Impact of Sewers on Groundwater Quality." Groundwater Problems in Urban
Areas, 1994.
3. Rauch, W., and T. Stegner. "The Culmination of Leaks in Sewer Systems During
Dry Weather Flow." Water Science and Technology, pp. 205-210, Vol. 30, No. 1,
1994.
4. "Nitrogen Compounds in Natural Water - A Review." Waste Resources Research.
Vol.2, No. 1, 1966.
5. Geldreich, E.E. Microbial Quality of Water Supply in Distribution Systems. Lewis
Publishers, Boca Raton, FL. 1996.
6. LeChevallier, M.S. "The Case for Maintaining a Disinfectant Residual." Journal of
the American Water Works Association. Vol. 91, Issue 1.
7. U.S. Environmental Protection Agency. Results of the Evaluation of Groundwater
Impacts of Sewer Exfiltration - Engineering-Science Pb95-158358. February 1989.
8. Bell, R.E., Jr., and G.G.Williams. "Sewer Rehabilitation: Developing Cost-Effective
Alternatives." Construction Congress V, Engineered Construction in Expanding
Global Markets: Proceedings of the Congress, 1997.
9. Sewer Maintenance for Rehabilitation. Chapter D-9 from Public Works Manual,
1996.
10. Gwaltney, T. "The Total System Solution" In Proceedings of the 1995
Construction Congress.
11. Camp, Dresser & McKee, Greater Houston Wastewater Program, 1993.
12. Camp, Dresser & McKee Inc. City of Albuquerque - Water Construction Program
Evaluation - Sanitary Sewer Exfiltration Analysis, September 1998.
13. Lerner, D.N., and D. Halliday. "The Impact of Sewers on Groundwater Quality,"
Groundwater Problems in Urban Areas, Thomas Telford, London, 1994.
30
-------
14. Decker, J. "Environmental Hazard by Leaking Sewers," In Proceedings of Water
Quality International, IAWQ 17th Biennial International Conference, Budapest
Hungary, July 24-29, 1994.
15. Decker, J. "Pollution Load of Subsoil, Groundwater, and Surface Water by Leaky
Sewer," In Proceedings from Hydrotop 94, Marseille France, April 12-16, 1994.
16. Decker, J. "Investigations about Quantitative and Qualitative Pollution Load of
Subsoil, Ground - and Surface Water by Leaky Sewer," In Proceedings of the Sixth
International Conference on Urban Storm Drainage, Niagara Falls, Ontario, Canada,
September 12-17, 1993.
17. Lerner, D. "The Use of Marker Species to Establish the Impact of the City of
Nottingham, UK on the Quantity and Quality of Its Underlying Groundwater,"
Groundwater in the Urban Environmental: Problems, Processes and Management,
Balkema, Rotterdam 1997.
18. Moltzen-Corbin & Associates. City of Albuquerque, New Mexico, Wastewater
Facilities Plan, Infiltration/Inflow Analysis, 1976.
19. CH2M Hill. Wastewater System Modeling and Analysis - Model Loading and
Verification, Task 205 and 206. City of Albuquerque, January 1991.
20. Camp, Dresser & McKee. Internal project cost records. 1999.
31
-------
Glossary Of Terms1
1. Combined Sewer
A sewer intended to serve as a sanitary sewer and a storm sewer, or as an
industrial sewer and a storm sewer.
2. Excessive Infiltration/Inflow
The quantities of infiltration/inflow which can be economically eliminated from a
sewer system by rehabilitation, as determined by cost-effectiveness analysis that
compares the costs for correcting the infiltration/inflow conditions with the total costs
for transportation and treatment for the infiltration/inflow.
3. Exfiltration
Exfiltration is the leaking of wastewater from a sanitary or combined sewer into the
surrounding soil, and potentially, into the groundwater. Exfiltration occurs when the
sewer condition degrades to an extent where pipe defects (cracks, joint separation,
etc.) allow wastewater to leak out of the sewer. Exfiltration can cause groundwater
pollution if the rate and/or volume of wastewater leakage exceeds the ability of the
subsurface soil to filter, absorb or immobilize certain pollutant constituents that may
be present in the wastewater. Exfiltration is distinguished from infiltration (see
below) by the direction of the hydraulic gradient across the sewer wall boundary.
For exfiltration to occur, the hydraulic gradient must drive flow external to the sewer;
with infiltration, groundwater depths above the flow line in the sewer drive flow into
the sewer.
4. Infiltration
The water entering a sewer system and service connections from the ground,
through such means as, but not limited to, defective pipes, pipe joints, connections
or manhole walls. Infiltration does not include, and is distinguished from, inflow.
5. Infiltration/Inflow
The total quantity of water from both infiltration and inflow without distinguishing the
source.
6. Infiltration/Inflow Analysis
U.S. Environmental Protection Agency, Office of Water Program Operations, Handbook
for Sewer System Evaluation and Rehabilitation, December 1975.
32
-------
An engineering and, if appropriate, an economic analysis demonstrating possibly
excessive or nonexcessive infiltration/inflow.
7. Inflow
The water discharged into a sewersystem, including service connections, from such
sources as, but not limited to, roof leaders, cellar, yard and area drains, foundation
drains, cooling water discharges, drains from springs and swampy areas, manhole
covers, cross connections from storm sewers and combined sewers, catch basins,
storm waters, surface run-off, street wash waters, or drainage. Inflow does not
include, and is distinguished from, infiltration.
8. Internal Inspection
An activity of the Sewer System Evaluation Survey. This activity involves inspecting
sewer lines that have previously been cleaned. Inspection may be accomplished
by physical, photographic and/or television methods.
9. Physical Survey
An activity of the Sewer System Evaluation Survey. This activity involves
determining specific flow characteristics, groundwater levels and physical conditions
of the sewer system that had previously been determined to contain possibly
excessive infiltration/inflow.
10. Preparatory Cleaning
An activity of the Sewer System Evaluation Survey. This activity involves adequate
cleaning of sewer lines prior to inspection. These sewers were previously identified
as potential sections of excessive infiltration/inflow.
11. Rainfall Simulation
An activity of the Sewer System Evaluation Survey. This activity involves
determining the impact of rainfall and/or runoff on the sewer system. Rainfall
simulation may include dyed water or water flooding the storm sewer sections,
ponding areas, stream sections and ditches. In addition, other techniques such as
smoke testing and water sprinkling may be utilized.
33
-------
12. Rehabilitation
Repair work on sewer lines, manholes and other sewer system appurtenances that
have been determined to contain excessive infiltration/inflow. The repair work may
involve grouting of sewer pipe joints or defects, sewer pipe relining, sewer pipe
replacement and various repairs or replacement of other sewer system
appurtenances.
13. Sanitary Sewer
A sewer intended to carry only sanitary and industrial wastewaters from residences,
commercial buildings, industrial plants and institutions.
14. Sewer System Evaluation Survey
A systematic examination of the tributary sewer systems or subsections of the
tributary sewer systems that have demonstrated possibly excessive
infiltration/inflow. The examination will determine the location, flow rate and cost of
correction for each definable element of the total infiltration/inflow problem.
15. Storm Sewer
A sewer intended to carry only storm waters, surface run-off, street wash waters,
and drainage.
34
------- |