EPA/600/2-86/023
February 1986
LARGE SOIL ABSORPTION SYSTEMS FOR WASTEWATERS
FROM MULTIPLE-HOME DEVELOPMENTS
by
Robert L. Siegrist
Damann L. Anderson
David L. Hargett
RSE Group
Madison, Wisconsin 53704
Contract No. 68-03-3057
Project Officer
James F. Kreissl
Wastewater Research Division
Water Engineering Research Laboratory
Cincinnati, Ohio 4268
WATER ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
The information in this document has been funded wholly by the United
States Environmental Protection Agency under Contract No. 68-03-3057 with
Urban Systems Research and Engineering, Inc., and the RSE Group. It has
been subject to the Agency's peer and administrative review, and it has
been approved for publication as an EPA document. Mention of trade names
or commercial products does not constitute endorsement or recommendation
for use.
ii
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with
protecting the Nation's land, air, and water systems. 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. The Clean Water
Act, the Safe Drinking Water Act, and the Toxics Substances Control Act
are three of the major congressional laws that provide the framework for
restoring and maintaining the integrity of our Nation's water, for preser-
ing and enhancing the water we drink, and for protecting the environment
from toxic substances. These laws direct the EPA to perform research to
define our environmental problems, measure the impacts, and search for
solutions.
The Water Engineering Research Laboratory is that component of EPA's
Research and Development program concerned with preventing, treating, and
managing municipal and industrial wastewater discharges; establishing
practices to control and remove contaminants from drinking water and to
prevent Its deterioration during storage and distribution; and assessing
the nature and controllability of releases of toxic substances to the air,
water, and land from manufacturing processes and subsequent product uses.
This publication is one of the products of that research and provides a
vital communication link between the researcher and the user community.
This study of large soil absorption systems for domestic wastewater
treatment and disposal revealed that their design is not based on entirely
rational criteria at this time. Although many design criteria have not
yet been addressed this report emphasizes the need to greatly improve site
evaluation procedures to more thoroughly characterize the potential move-
ment of effluent with the groundwater, predict mounding and maximize the
available landscape. Several design improvements noted include the use
of trenches wherever possible in lieu of beds, resting cycles with multiple
cells and shallow trench use wherever feasible.
Francis T. Mayo, Director
Water Engineering Research Laboratory
111
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ABSTRACT
Large subsurface soil absorption systems are being promoted as low-
cost, effective alternatives for treatment and disposal of wastewaters
from subdivisions, small communities and similar applications. When a
large system was constructed for the community of Westboro, Wisconsin in
1977, it was one of the first of its kind in the country. Since that
time, however, many similar systems are in design, under construction or
in operation.
The design and operation practices applied and the performance ex-
pected from large systems have largely evolved from experience gained with
small, single-home systems. However, lacking field experience regarding
the performance of community-scale systems, the suitability of this prac-
tice remains in question.
An investigation was conducted to provide insight into the design and
performance of large soil absorption systems for treatment and disposal of
wastewaters from multiple-home developments. The objectives were to investi-
gate absorption system performance and identify potential deficiencies in
presently used design criteria. Where possible, recommendations regarding
more appropriate design and operation practices were to be made.
The study was conducted in three parts. A survey of state regulatory
agencies was conducted to enable characterization of the distribution,
regulatory structures, design restrictions and state attitudes associated
with multiple-home systems. An in-depth field investigation of the community
wastewater absorption system at Westboro, Wisconsin was conducted between
June 1981 and May 1983. This work included delineation of the soil and site
characteristics and monitoring of the applied wastewater, absorption system
soil infiltrability, subsystem soil moisture and pore gas, and groundwater
Impacts. The final part of this study included a more casual field investi-
gation of six multiple home systems located in the State of Washington. The
basic characteristics of each system were determined including the facility
design, soil conditions, applied wastewater character and the absorption
system performance.
This report was submitted in partial fulfillment of Contract No.
68-03-3057 by RSE under the sponsorship of the U. S. Environmental Protec-
tion Agency. This report covers the period from 1980 to 1983, and work
was completed as of September, 1983.
iv
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CONTENTS Page
Disclaimer ii
Foreword iii
Abstract - iv"
Table of Contents . v
List of Figures v—"
List of Tables ix
Abbreviations/Symbols xl
Ac know led gni e n t.s ; x i i
Section 1: Introduction 1
Section 2: Conclusions 3
Section 3: Recommendations 4
Section 4: Current Practices 6
Distribution 6
Regulatory Structures 6
Design and Operation Requirements 9
State Attitudes and Policies 10
General Experience 10
Section 5: Performance Evaluation of the Community Soil Absorption
System at Westboro, Wisconsin 11
Wastewater System Characteristics 11
Soil and Site Characteristics 13
Applied Wastewater Characteristics 13
Soil Infi 1 trabi 1 ity 16
Subsystem Characteristics 18
Groundwater Characteristics 20
Surface Water Characteristics 24
Performance Assessment 24
Section 6: Field Evaluation of Multiple-Home Soil Absorption
Systems in Washington State 26
Soil and Site Characteristics 28
Applied Wastewater Characteristics 28
Performance Assessment 31
Section 7: References 32
Appendix A: Performance Characteristics of the Community Soil
Absorption System at Westboro, Wisconsin 38
Materials and Methods 39
Description of Westboro and its Wastewater Facility 39
Review of Previous Monitoring Efforts 44
Monitoring Procedures of This Study 45
Results and Discussion 54;
Soil and Site Characteristics 54
Applied Wastewater Characteristics 60
Soil Infiltrabi 1 ity 6G
Subsystem Soil Moisture and Gas Characteristics 64
Groundwater Characteristics 67
v
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Surface Water>Quality
Operation and Maintenance
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FIGURES
Number Page
1 Example Variables Potentially Affecting the
Performance of Community Soil Absorption Systems 1
2 Location of Soil Absorption Beds at Westboro,
Wisconsin 12
3 Location of Soil Evaluation Points and Monitoring
Instruments at Westboro, Wisconsin 14
4 Computer-Generated contour Map of the Groundwater
Surface Elevation on August 27, 1981 21
APPENDIX
A1 Sanitary District #1 of the Town of Westboro, Wisconsin.. 40
A2 Pressure Distribution Network Schematic 42
A3 Profile Schematic of the Siphon Chamber at Westboro 42
A4 Subsurface Monitoring Instruments Under and Beside Bed 2. 47
A5 Well Monitoring Apparatus 51
A6 Typical Hydraulic Conductivity Curves (Bouma, 1975) :"66
A7 Groundwater Surface Elevations Measured at Wells 1, 2, 5,
12 and 15 Versus Time 68
A8 Groundwater Surface Elevations Measured at Wells 3, 4 and
14 Versus Time .69
A9 Groundwater Surface Elevations Measured at Wells 13, 18,
19 and 21 Versus Time 70
A10 Computer-Generated Contour Map of the Groundwater Surface
on October 22, 1981 72
All Computer-Generated Contour Map of the Groundwater Surface
on April 28, 1982 73...
A12 Approximate Velocities Using Darcy's Law and a Soil
Permeability of 500 cm/d (Based on Contours 8-27-81) 76..
A13 Profile of the Groundwater Surface Under Bed 2 73.
vi i
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A14 Computer-Generated Contour Map of the Total Dissolved Solid
Levels on August 27, 1981 85
A15 Computer-Generaged Contour Map of Arranonia Nitrogen Levels
on August 27, 1981 36
A16 Ammonia and Nitrate Nitrogen Concentrations at Well 13-3
Versus Time 88
viii
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TABLES
Number Page
1 Results of a 1981 State Survey of Current Practices
Associated With Large Soil Absorption Systems 7
2 Percolation Test Results 15
3 Saturated Hydraulic Conductivity of Soil Samples 15
4 Comparison of Septic Tank Effluent Composition As
Determined by Various Investigators 17
5 Soil Moisture Tensions (SMT) beneath Bed 2 19
6 Soil Gas Composition Beneath Bed 2 19
7 Mean Concentration of Conservative Parameters in
the Groundwater Across and Downgradient from the
Absorption Bed Site 23
8 Mean Concentrations of Potential Pollutants in the
Groundwater Across the Absorption Bed Site 23
9 Characteristics of Six Systems Studied in Washington 27
10 Particle Size Characteristics of Soil Samples and
Field Percolation Rates 29
11 Estimated Current Hydraulic Loading Rates 30
12 Mean Septic Tank Effluent Concentrations 30
APPENDIX
A1 Westboro Construction Cost Summary 43
A2 Facility Maintenance Schedule 43
A3 Elevation of Subsystem Monitoring Instruments at Bed 2... 48
A4 Elevations of Groundwater Monitoring Wells 50
A5 Particle Size Distribution of Soil Samples 53
A6 Daily Wastewater Flow Volume 52
A7 Septic Tank Effluent Composition 5!
" - i x
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A8 Per Capita Mass Loadings g2
A9 Estimated Pollutant Removal Afforded by Septic Tank
Pretreatment g2
A10 Estimated Horizontal Groundwater Velocities 75
All Estimated Horizontal and Verticle Hydraulic Gradients at
Nested Wells 75
A12 Groundwater Composition Near the Wastewater Absorption
Beds - Dissolved Solids, Chlorides and Conductivity gg
A13 Groundwater Composition Near the Wastewater Absorption
Beds - Nitrogen gl
A14 Groundwater Composition near the Wastewater
Absorption Beds - Phosphorus, Chemical Oxygen Demand
and Fecal Coliforms 32
A15 Qualitative Characteristics of the Shallow Groundwater
Upgradient of the Absorption Beds g3
A16 Surface Water Quality Near the Wastewater Absorption
Beds , gg
• X
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ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
BOD
--5-day biochemical oxygen
kg
--Kilogram
D
demand
L
— 1 iter
CC
--cubic centimeter
lb
—pound
CL"
--chloride
Log
—Logrithm
cm
--centimeter
L/min
—liters per
COD
—chemical oxygen demand
minute
Col i
—coliform organisms
m,
--meter
Cond.
--conductivity
m
--square
d
—day
meter
D.O.
--dissolved oxygen
m
--cubic
Fecal Coli.
--fecal coliform
meter
ft,
--feet
max
—maximum
ft2
--square feet
mbar
--millibar
gm
--gram
mm
--mi 11imeter
gal.
—gallon
mg
--mi 11igram
gpd
--gallons per day
mg/L
--milligrams
gpm
--gallons per minute
per liter
h, hr
--hour
min
--minute,
H,0
--water
minimum
ll
--infiltration capacity
mL
--milliliter
i.d.
--inside diameter
TSS
--total
in
--inch
suspended
IR
--infiltration rates
sol ids
pH
--negative log of the
USDA
--U.S. Dept.
hydrogen ion activity
Agriculture
SD
--Standard Deviation
n
—number
SCS
--soil conversation service
N
—Nitrogen
p
--phosphorus
NH.-N
--Ammonia
SMT
--soil moisture tension
Nitrogen
t
--time
NOo-N
--Nitrate
temp
--temperature
•J
Nitrogen
TDS
--total dissolved solids
o.d.
--outside
TKN
--Total Kjeldahl Nitrogen
diameter
TS
--total solids
TVS
--total volatile solids
TVSS
--total volatile
suspended solids
SYMBOLS
SYMBOLS (continued)
°C
—degrees celsius
Rb
--hydraulic
(centigrade)
IJ
resistance
K
--hydraulic conductivity
--micro,
of barrier
or crust
micron
Z
—height
#
--number
above
c/
—percentage
xi
datum
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ACKNOWLEDGMENTS
The performance of this study was made possible through funding provided
by U.S. Environmental Protection Agency, Contract No. 68-03-3057, Study Area
#2. The efforts of several other parties also contributed to the completion
of this study. Past efforts by staff of the Small Scale Waste Management
Project at the University of Wisconsin resulted in much background information
on the facilities at Westboro and the establishment of a network of
groundwater monitoring wells, all of which were utilized in the present study.
Carl C. Crane and Associates of Madison, Wisconsin shared their knowledge of
the system design, construction and operation and provided valuable surveying
and drafting services. The Town of Westboro allowed the monitoring described
in this report to occur and residents were patient with the project staff
throughout the study. Finally, the efforts of others not specifically
recognized here are gratefully appreciated.
xi i
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SECTION 1
INTRODUCTION
Large subsurface soil absorption systems for treatment and disposal of
wastewaters from subdivisions and small communities are becoming increasingly
popular. When the large soil absorption system was constructed for the
community of Westboro, Wisconsin in 1977, it was one of the first of its kind
in the country. Since that time many similar systems are under design,
construction or in operation. These systems are being designed not as interim
solutions until conventional sewerage arrives, but as permanent means of
wastewater management in previously unsewered areas.
The design and operation practices utilized for large multiple-home soil
absorption systems appear to have simply evolved from the laboratory and field
experience gained with small, single-home systems. However, lacking field
experience regarding the performance of community-scale systems, the
suitability of this practice remains in question. As the size of a subsurface
soil absorption system increases to handle the wastewater from a small
community the design, construction and management practices necessary to
ensure acceptable performance become less clear. Figure 1 presents numerous
fundamental factors which interact dynamically to influence the performance of
large soil absorption systems.
Absorption
• Condition of Infiltrative
Surface
Compaction/Smearing
Gravel Masking
Soil Clogging
• Condition of Subsystem
Soil Permeability
-L Unsaturated Flow Regime
Groundwater Mounding
Purification
• Wastewater Composition
• Loading Pattern
• Soil Characteristics
- Texture
Structure
Unsaturated Depth
• Subsystem Aeration Status
• Climate
• System Age
Figure 1. Example Variables Potentially Affecting the Performance of
Community Soil Absorption Systems.
The objectives of this study were to investigate the performance of
community-scale soil absorption systems, to identify potential deficiencies in
presently used design criteria and to develop recommendations regarding design
and operation practices. More specifically, the project endeavored to
accomplish the following:
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1. Determine the current attitudes, policies and level of use of
community-scale subsurface wastewater absorption systems.
2. Investigate in detail, the performance characteristics of the
community wastewater absorption system in Westboro, Wisconsin, and
3. Characterize, generally, a number of multiple-home wastewater
absorption systems in the state of Washington.
This study was accomplished between June 1981 and December 1983. The
overall study was coordinated by RSE while each of the study objectives was
accomplished through the efforts of staff from several organizations:
0 Urban Systems Research and Engineering, Inc.
2067 Massachusetts Avenue
Cambridge, MA 02140
(Objective 1)
0 RSE Group
Scientists/Engineers
2445 Darwin Drive
Madison, WI 53704
(Project Coordination, Objective 2)
0 Pac-Tech Engineering, Inc.
615 South 9th
Tacoma, WA 98405
(Objective 3)
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S ECU ON 2
CONCLUSIONS
The use of large soil absorption systems for subdivisions and small
communities has grown in recent years. The level of use varies widely from
state to state as do the regulatory structures and policies. In general,
siting and design requirements imposed on large systems are not markedly
different than those used for small systems. Despite their increased use,
regulatory agencies appear to have little sound experience regarding large
system performance.
Soil and site characteristics significantly influence the design and
performance of Large soil absorption systems. It is inappropriate to use
small diameter boreholes and simple percolation tests to assess site
suitability and to design wastewater absorption systems. Inspection of stem
cuttings from a flight auger or even an undisturbed core from a split spoon
sampler may not adequately reveal important soil morphological features.
Natural soil percolation rates determined through standard procedures likely
reflect horizontal permeabilities and not the vertical permeability of
restrictive horizon(s).
Design infiltration rates to yield successful long-term operation of
large subsurface wastewater absorption systems are poorly defined.
Appropriate rates for various effluent compositions, loading schedules,
infiltrative surface geometries and soil conditions have not been delineated
as yet.
The contaminant load in domestic septic tank effluent may be too great to
enable long-term operation of some large subsurface wastewater absorption
systems at the infiltration rates currently prescribed. Higher levels of
pretreatment to reduce the levels of oxygen-consuming materials, suspended
solids and other constituents may be necessary to sustain high design
infiltration rates (e.g. 5 cm/d), and to achieve adequate purification prior
to groundwater recharge.
Anaerobic conditions may predominate under large mature soil absorption
systems, even those installed in sandy soils. Alternative system types such
as narrow, trenches may be necessary to facilitate aerobic soil environments.
[_ The continuous application of large volumes of wastewater to subsurface
soils may raise the groundwater surface under such systems in some cases" to an
extent which significantly reduces the depth of unsaturated soil beneath the
system. This could diminish renovation of the applied wastewater, especially
in soils with significant capillary fringe above the saturated zone.
Analytical models for predicting groundwater mounding beneath soil absorption
systems appear difficult to apply and imprecise in their predictions due to
the input data required and the heterogenous, anisotropic character of many
soil systems.
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SECTION 3
RECOMMENDATIONS
The results of this investigation and related work have raised several
basic questions regarding appropriate practices for siting, design,
construction and operation of subsurface wastewater absorption systems serving
community-scale developments. It is clear that the practices used for
community systems cannot be as arbitrary or casual as those used for
individual home systems. Pending the results of further research and
experience, the following modifications to small-system practices are
suggested for community-scale wastewater absorption systems on a single site:
Site Evaluation
The site evaluation should be performed by a trained professional (e.g.,
soil scientist experienced in siting and conceptual design of large wastewater
absorption facilities). Detailed soil morphological inspections should be
conducted to a depth of at least 2 m below the proposed infiltrative surface,
with special attention devoted to vertical permeability characteristics. The
groundwater hydrology should be addressed to enable estimation of any
hydraulic impacts due to different wastewater absorption system design
concepts and to ensure that excessive groundwater mounding will not occur.
The local groundwater hydrology is also important as it may affect the
purification requirements established for a given wastewater absorption
system. The depth to flow restricting or limiting conditions can affect both
infiltration/percolation and purification. Sufficient unsaturated soil depth
should be available to allow for groundwater mounding and facilitate soil
aeration.
Design
The design flow should be based upon a population projection with
adequate provisions for infiltration/inflow. Since hydraulic load is critical
to system performance, wherever possible, flow monitoring should be conducted
to adequately characterize the wastewater flows. Design infiltration rates
should be cautiously selected based on performance requirements, soil
morphology and the hydraulic capacity of the entire site. The length to width
ratio of the absorption area should be maximized with trench configurations as
opposed to bed systems used wherever possible. The absorption system
infiltrative surface should be installed as shallow as feasible to take
advantage of biologically and chemically active soil zones and to maximize
aeration. The wastewater absorption system should be sized for at least 150
percent of the design flow to enable routine long-term resting. At least
three separate absorption cells should be provided to enable alternating
service and prolonged resting of individual cells.
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System Installation
Installation should be accomplished in as short a time period as
feasible. Extended exposure of the subsoil infiltrative surface should be
avoided. The driving of any construction machinery (tired or tracked) over
the soil infiltrative area, even if covered by'a layer "of" gravel or'sand,
should be prohibited.
System Operation and Monitoring
The absorption units should be periodically rotated out of and back into
service. The resting periods should be at least six months long and rotation
should be avoided during cold weather seasons. All absorption units should be
inspected at least monthly to determine their operational status and the
occurrence and magnitude of continuous ponding. The wastewater hydraulic
loading to each absorption unit should be monitored at least monthly to
determine the average daily loading. The composition of the applied
wastewater should be characterized during initial operation and at least
annually thereafter. The constituents analyzed for should be carefully
selected to provide useful data at least cost.
The elevation of the shallow groundwater beneath the absorption system
should be monitored to determine the hydraulic impacts of the system and
ensure that there is sufficient unsaturated soil depth. In light of the
applied wastewater characteristics and the local soil and groundwater
conditions, a ground water monitoring program should be established to ensure
that acceptable purification is achieved. Appropriate groundwater monitoring
programs may vary widely depending on site specific factors as will the
judgement regarding acceptable purification.
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SECTION 4
CURRENT PRACTICES
A survey of regulatory agencies was conducted to obtain information about
large soil absorption systems. State and local regulatory agencies in 33
states were contacted and information was solicited to enable characterization
of the distribution, regulatory structures, design restrictions, state
attitudes and policies and the general experience associated with large soil
absorption systems. The results of this survey are summarized in Table 1.
DISTRIBUTION
As Table 1 indicates, every state contacted had at least a few large soil
absorption systems. However, these were mostly for single structures such as
schools, hospitals or other large institutions. Some large systems served
many hook-ups, most frequently trailer parks, campgrounds or recreational
vehicle parks. In all cases, the number of large systems was relatively
small. States usually claimed to have a limited number of institutional or
commercial systems and maybe ten to twenty trailer parks. Seven states were
exceptions to this generalization. Washington, Missouri, Maryland, Virginia,
Utah, West Virginia and Wisconsin reported they had many systems serving
institutions, commercial buildings, or mobile home parks. Systems serving
institutions and mobile home parks tended to be conventional disposal fields
or trenches, with common septic tanks and gravity sewers.
Seventeen states were identified to have large soil absorption systems
serving multiple-home developments. An additional nine states were
considering applications for such systems, or had already approved the
designs. These systems did not usually serve an entire community but rather
subdivisions or vacation areas. Vermont, South Dakota, Kentucky, Washington
and Wisconsin all had community systems in the ground, designed or proposed.
Some had received EPA construction grants funding. Some residential cluster
systems had common septic tanks while others employed individual tanks. While
conventional disposal fields or trenches were most common, some systems were
mounds or evapotranspiration beds.
REGULATORY STRUCTURES
Regulatory structures varied from state to state. For the most part,
regulation of large soil absorption systems was shared between state and local
agencies. At the state level, jurisdiction was frequently shared by health
and environmental agencies. With the exception of two or three states, most
states had codes or guidelines for subsurface disposal. The most common
regulatory vehicle was a permit, issued either by the state or local agency.
Individual residential systems and smaller institutional or commercial
systems were usually regulated at the local level and the design was usually
based on gallons per day, number of hook-ups, or, number of people served.
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Table 1. Results of a 1981 Survey of Practices
Associated with Large Soil Absorption System.
SPECIAL LSAS
CODE
INSTITUTIONAL
LSAS
MOBILE HOME
PA2KS LSAS
multiple
HOME
LSAS
;
PERMIT j
REQUIREMENTS
1
A1 aba'na
no
f
0
1
Ari zcna
no
f
t
0
>1 house
i
Cali fornia
Colorado
no
no
s
S
S
s
s
1
Counties and |
regional water j
quality boards i
permit I
> 2,000 gal. \
Connecticut
no
s
s
'
m
1
> 5,000 gpd. and |
engineer design
Florida
Georgi a
yes
no
s
7
1
0
i
(monitoring |
requi red) 1
i
Idaho
yes
Iowa
no
s
s
0
1
>15 houses ana |
engineer design |
1
Kansas
no
5
36
0
1
Public sewer |
district
i
Kentucky
yes
f
f
1
>1 house i
Mai ne
Maryland
yes
no
s
IT,
s
m
0
s
> 2,000 gal and
engineer design
Massachusetts
yes
f
f
(<10,000 gpd)
Minnesota
Mi ssi ssippi
no
no
f
15
f
S
0
Engineer design
if >5,000 gpd
and >15 units
Missouri
Montana
yes
no
IT
m
s
10
>10 houses and
en aineer design
i
Nebraska
no
>50 people
Nevada
no
S
s
2
>5,000 gpd and
engineer design
(state ground
water discharge
permit if >10,000
9Pd)
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-8-
Table 1. Results of 3,1981 Survey of Practices
Associated with Large Soil Absorption Systems
(Continued).
r
SPECIAL LSAS
CODE
INSTITUTIONAL
LSA5
MOBILE 'HOME
PARKS LSA.S
MULTIPLE
- -HOME.
LSa.S
PERMIT
requirements
New Hampshire
f
f
New Jersey
>50 homes
engineer design
if >8,000 gpd)
New Mexico
yes
s
f
f
>2,000 spd and
engineer design
New York
yes
f
N. Dakota
no
¦p
f
Ohio
no
s
0
Pennsylvani a
no .
'
f
yes
Rhode Island
0
>5,000 cpd
(<10,000 gpd)
¦Si Dakota
no
.
f
1
>1 house and
engineer design
1
Tennessee
no
f
1
Utah
yes
m
0
yes
Vermont .
s
S
3
Certifi ed operator
state permits,
engineered sewage
district
Vi rgi ni a
no
s
n
f
>5,000 gpc and
and engineer design
Wisconsin
no
s
m
s
yes
Wyomi ng
s
5
>1 house and
engineer design
LSAS = Large soil
LEGEND: f = few
s = some
m = many
absorption systems
i
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States frequently maintained the authority to regulate large systems. The
minimum size for this shift of jurisdiction ranged from 2,000 to 8,000 gaL/d
(7.6 to 30.3 m3/d) on a flow criteria, from two to fifty households on a
number of connections basis or, in one state, over fifty persons on a
population served criteria.
Some states split their regulatory authority between environmental and
health agencies with the basis being system size or ownership. State health
departments generally regulated the smaller, private systems while
environmental agencies handled large private and community systems. Five
states had no legal authority over subsurface disposal unless it was a
community system. At least one state, Missouri, theoretically regulated all
systems with more than 10 individually owned residences, but lacked the
personnel to enforce the regulation. Consequently, the state only reviewed
the plans when developers chose to submit them.
Most states had codes and guidelines governing the design and
installation of subsurface soil absorption systems. However, relatively few
had separate regulations for large systems. Usually developers had to simply
scale up the guidelines for single dwellings. While states generally didn't
have large system codes, they often required that larger systems be
engineered, and that designs be carefully reviewed by their agency. In
contrast, smaller systems were often installed by a contractor, with no
engineering plans, and little or no regulatory review. Community-scale
systems were reported to be carefully engineered and closely scrutinized. In
some cases the regulating agency was intimately involved in the design.
Community systems were often required to obtain NPDES-type permits and some
states required larger systems to obtain groundwater discharge permits.
DESIGN AND OPERATION REQUIREMENTS
States did not normally impose siting or design requirements on large
systems other than those defined in their general subsurface disposal codes.
Engineers were usually allowed to propose what they deemed to be good
engineering practice for state review and approval. There were a few
regulations directed specifically at large systems. Several states required
that the design include the identification of a site for replacement of the
initial system should it fail or that the initial system be built to ac-
commodate twice the anticipated flow. Some states and counties imposed both
these rules, while others required alternating fields, or dosing. Only one
state required that systems have common as opposed to individual septic tanks,
although states usually preferred one method or the other.
Other limitations concerned use or size. A few states limited size by
policy to 20,000 gaL/d (75.7 m3/d). Ohio prohibited cluster systems unless
they were funded through the construction grants programs, and one regional
water quality agency in California prohibited all commercial subsurface
absorption systems. Opposition was often based on the agency's concern about
the problems of multiple ownership.
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To ensure proper operation and maintenance some states imposed an
institutional restriction on cluster systems. Often developers were required
to form homeowner's associations or get the county or city to form a sewer or
special improvement district in order to get state approval. In some cases,
bonds were required to be posted. In others, special districts had to have
the power to tax, or assess homeowners for maintenance costs. Similarly, some
states insisted that systems have licensed or certified operators, and others
required groundwater monitoring programs.
STATE ATTITUDES AND POLICIES
State reactions to the use of large soil absorption systems were neither
markedly positive or negative. Only Connecticut, Minnesota, Maine and Vermont
encouraged such systems while twelve, or a little over a third of the states
contacted were discouraging them. Some states did not think large systems
were technically feasible. A number of states reported that large systems
were impractical due to local soil or groundwater conditions. Mississippi,
Kansas, and North Dakota fell into this category. Community systems were
considered unnecessary in Nebraska, where their zoning commonly required a
minimum lot size of 3 acres. Nevada discouraged large systems because the
state was concerned that failures would lead to public health problems.
GENERAL EXPERIENCE
The survey revealed that state agencies have little experience with
subdivision or community soil absorption systems. Most of these systems have
not been in place long enough to enable adequate evaluation. Those that have
been designed and installed in recent years "appear" to be working well.
However, since most of the states where clusters are located lack both the
funding and the manpower to monitor such systems, evaluation of their
performance is impossible using existing data.
States have had years of experience with large soil absorption systems
serving institutions and trailer parks, some as old as twenty years. These
systems have reportedly performed well. The failures that have occurred to
date tend to be systems that were improperly designed, not designed at all,
improperly sited, or poorly maintained. Most frequently noted problems were
associated with the past. Many states did not require engineering designs for
systems of this type. Developers simply installed off-the-shelf designs,
without adequate evaluation of soil and site conditions. Another common
problem was overloading. For example, trailer park owners would install
systems for a specific number of pads, then expand their facility, without
making alterations to the wastewater handling facilities.
-------
-11-
SECTION 5
PERFORMANCE EVALUATION OF THE COMMUNITY SOIL
ABSORPTION SYSTEM AT WESTBORO, WISCONSIN
The field monitoring efforts of the overall study were concentrated on
the community soil absorption system at Westboro, Wisconsin (Otis, 1978; Otis
and Fey, 1981). Westboro is a community of approximately 205 persons located
in northern Wisconsin (Figure 2). Until 1977, all buildings in the community
were served by private septic tank systems. With over 80 percent of these
systems malfunctioning, the Town of Westboro investigated alternative
wastewater management options. During the Summer of 1977, public wastewater
facility was constructed. It included a system of small diameter gravity
sewers for conveying the effluent from septic tanks serving each building to a
community subsurface wastewater absorption system.
Monitoring of the wastewater absorption system occurred over a two-year
period between June 1981 and May 1983. Data were collected regarding system
design and performance:
0 Wastewater facilities design
0 Soil and site characteristics
0 Applied wastewater characteristics
0 Soil infiltrability
0 Subsystem soil moisture conditions and gas composition
0 Groundwater characteristics
0 Surface water characteristics
The results of this work ave summarized in this section. Additional details
may be found in Appendix A of this report.
WASTEWATER SYSTEM CHARACTERISTICS
The wastewater facilities were comprised of individual septic tanks,
small diameter gravity sewers, a siphon-dosing system and three large
subsurface wastewater absorption beds. The absorption beds were each 100 ft
(30.5 m) by 130 ft (39.6 m) in area (Figure 2). They were sized for a 1.2
gal/ft2/d (5 cm/day) hydraulic loading rate which was determined from an
examination of soil morphology and the results of standard percolation tests.
Each bed was designed to receive 50 percent of the design daily flow or 15,000
gal/d (56.8 m3/d) per bed, so that at any time, only two beds would be needed
and the third could be rested. The beds were installed such that their bottom
infiltrative surfaces were at a depth of 42 - 49 in (1.1 to 1.4 m). Each bed
was filled with 18 in (0.5 m) of stone and then backfilled with natural soil
materials from the site. A siphon-dosing system and pressure-distribution
networks were utilized to automatically distribute septic tank effluent
throughout the beds. Twelve observation vents (4-in diameter) extended from
the bottom of each bed to roughly 1 m above the final ground surface. The
wastewater absorption beds were constructed during the Summer of 1977 and put
-------
-12-
SIPHONX
\ \
cth 0
"STTR 3=
IrT STATION,
Figure 2.
Location of Soii Absorption Eeds at
Wes^boro, Wisconsin (Otis, 1973).
-------
-13-
into operation in September, 1977. During the course of this study, Beds 2
and 3 (Figure 2) were in service.
SOIL AND SITE CHARACTERISTICS
The wastewater absorption beds were located on a gently sloping glacial
outwash terrace adjacent to a small stream (Figure 2 and 3). Inspection of
backhoe pits showed the soils to be highly variable verticallyand laterally.
Soil profiles in the northern one-third to one-half of thefsite^erq_..1
dominantly very fine sands and silts which were complexly interbedded with
layers of coarser sands and gravels. These finer deposits consisted of very
well-sorted particles mostly in the 0.02 to 0.20 mm size range, often compact
and resistant to vertical flow. Laboratory measurement confirmed their low
permeabilities with saturated hydraulic conductivity (k .) values as low as
10 cm/d. By contrast, the medium to coarse sands and gravels were estimated
to have K . values of 500 to 1000 + cm/d. ' The soils of the southern half of
the site wire distinctively more sandy and less interbedded. Although some
stringers of very fine sands and silts were observed, they appeared to be thin
and discontinuous. Vertical permeabilities of these materials were judged to
be much greater than that in the northern half of the site due to the vertical
continuity of the coarser materials.
Particle size distribution data were determined for selected soil samples
representing the range of deposits present. Samples of the finer, more
hydraulically restrictive deposits were classified texturally (USDA) as fine
sands, sandy loams and loams. Three of these samples contained content of
particles smaller than 0.5 mm (medium sand) of greater than 98 percent,
although clay content in each case was less than 1 to 15 percent.
The permeability of the soil system was estimated according to standard
percolation test procedures (U.S. EPA, 1980; Wis. Adm. Code, 1980).
Percolation rates ranging from 25 to 8 min/in (140 to 470 cm/d) were measured
in the northern half of the site at three locations with considerable
interbedding of fine and medium sands. In contrast, percolation rates
measured prior to system construction at three locations in more permeable
materials in the southern half of the site were all less than ,2 min/in "(2i5Q
cm/d) (Table 2). Saturated conductivity measurements were conducted on
selected soil samples in the laboratory utilizing concentric-ring permeameters
(Hargett, 1982). A loam sample displayed a low K . of 9.5 cm/d while a very
fine sandy loam sample had a K . of 86 cm/d. Two fine sand samples yielded
K , values of 247 and 366 cm/a [Table 3). These conductivity data confirmed
the relatively low permeabilities of the fine, well-sorted, compact materials
observed in the field during the soils investigations.
WASTEWATER FLOW AND COMPOSITION
The average daily wastewater flow at Westboro was determined by recording
the discharge cycles of the siphons delivering STE to the soil absorption
beds. Based upon measurements made between June 1981 and October 1982,
-------
-14-
e» 89 ¦
• SCO 2
» •
_HS£!£
¦ borings
• Percolator Tcsls
NOPiH • GrountUa'sr Wells
ip Su&*y*l*m Morilt>rif>g Ft.
• Su*1»c« rt'Jl*' Simpl* Pt.
s"" cT ~—'x7o'
Figure 3. Location of Soil Evaluation Points and Monitoring
Instruments at Westboro, Wisconsin.
-------
Table
2r'ercc
;lation Test Resul
ts. '
rest
Depth 1
3s1ow**
Soil
Strctu::i Percolation Rate
Nc. *
Grade •
- m (ft)
Sidewal1
Underlying nr.in/in
cm/day
Date
PI
1 .07
(3.5)
ms
hsil-vfsl 25
145
10/21/01
P2
1 .07
(3.5)
cs hsil (dry) 16
(near
saturation)
229
10/21/81
P3
1 .22
(4)
cs
v f s 1 -1 7.8
469
1C/21/31
?4
1 .07
(3.5)
si
si 1.7
2150
8/05/76
P5
; .07
(3.5)
fs
fs 1.1
3325
8/05/76
P6
; .07
(3.5)
si
si 1 .2
3050
8/05/76
* See site plan (Figure 3) for test locations.
** Bottom of all percolation test holes at or slightly above
elevation of systems' infiltrative surface (i.e. 453 m
[1502.-7.'-trf , except. F6 aoca: lO.'j rn [1..B ft]/ below) .•
Table 3. Saturated Hydraulic Conductivity
of Soil Samoles.
Boring Depth USDA
-------
-16-
approximately one siphon discharge occurred daily representing an average
daily flow of 8,500 gal/d (32.2 m3/d). During this study, Beds 2 and 3 were
used exclusively while Bed 1 was rested. With two absorption beds in service,
the average daily flow resulted in a STE application rate of approximately 1.4
cm/d (0.33 gpd/ft2) of bottom area. Under normal operation, the siphon would
alternately discharge 2.8 cm (0.65 g/ft2) to each operating bed, but only
every other day.
The composition of the STE was determined from grab samples collected
from the siphon chamber between September 1981 and May 1983 (Table 4).
Wastewater composition included substantial concentrations of biochemical
oxygen demand (168 mg/L), suspended solids (85 mg/L), ammonia nitrogen (44
mg-N/L) and fecal coliforms (2.0 x 10 org./L). The measured composition was
substantially similar to that reported by earlier investigators (Table 4).
SOIL INFILTRABILITY
The infiltrability of the wastewater absorption beds was indirectly
measured through monitoring the occurrence and magnitude of STE ponding.
Records indicated that ponding was noted in all three beds as early as June
1979, less than two years after start up. The first measurements of this
study were made in June 1981. These revealed high levels of ponding. Bed 2
and 3 were in service at the time and both were found to have approximately 46
cm (18 in) of STE ponded above their bottom infiltrative surfaces. During the
course of the study, the level of ponding fluctuated somewhat, but both
operating beds remained nearly fully ponded.
The average daily STE application rate was calculated at 1.4 cm/d based
upon bottom area alone. Assuming all of the STE applied to the beds under
normal operation infiltrated into the bottom of the beds, the 1.4 cm/d can be
viewed as roughly equal to the infiltrability of the absorption system. This
1.4 cm/d rate was less than 50 percent of an apparently conservative design
loading rate of 3.0 cm/d for this site based upon the slowest percolation rate
measured in this study.
During the installation of subsurface monitoring instruments in Bed 2,
the infiltrative surface was visually inspected and sampled. At both
locations within the bed (Figure 3, Point A, C), the gravel was found to be
embedded with the natural sand surface. The infiltrative surface did not
exhibit a surface clogging mat as such. The sandy soil was black in color to
a depth of approximately 5 to 8 cm and was quite wet. At 15 to 20 cm depth,
however, the sand had graded to natural coloration and was only moist. These
observations were consistent with those of previous investigators (Bouma,
1975; Anderson et al, 1982).
Soil samples were collected with depth during installation of a
groundwater observation well at Location A (Figure 3). Very high volumetric
moisture contents (33.9 and 37.8%) were measured in the upper 10 cm of soil
immediately beneath the bottom of the bed. With the total soil porosity
-------
Tabic A. Comparison of .Septic Tank Effluent Composition As
Determined by Various Investigator:-;*
Multiple Homes Ij^djvidual Homes
Wcstboro,
Bend,
Glide,
Manila,
Col lege
Wi scon-
Pennsyl-
Parameter
IJni ts
WI
OR
OR
CA
S ta., TX
si n
v a n i a
Oregon
bod5
mg/L
168
157
118
189
132
217
COD
ir.q/L
338
276
228
284
266
445
483
-
TS
CKJ/L
663
-
376
355
-
895
-
-
TSS
mg/L
85
36
52
75
-
87
108
146
TKN
mgN/L
57
41
50
-
29.5
81 .5
74 .4
57.1
r.'H4
rny M/L
<14
-
32
-
24.7
53.5
-
40.6
r:03
mgN/L
6.4
-
0.5
-
0.2
0.95
<0.33
0.42
TP
ngP/L
8.1
-
-
-
8.2
21 ,'d
18.2
-
oil
-
6.9-7.4
6.4-7.2
6.4-7.2
6.5-7.8
7.36
7.3
-
-
Cl.-
mg/L
62
-
-
-
1 .83
164
230
-
lC
umhos/cm
1073
-
-
-
3204
909.8
-
-
Grease
mg/L
65
16
22
-
-
-
-
F. Col i-
f orris
Log if/L
7.3
-
-
-
6.04
6.45
-
6.41
K. Strep-
tococci
Log#/L
5.7
-
-
-
-
5.40
-
-
F1 ow
l.pcd
136
151-227
182
151-216
166
-
-
' Description
Wcstboro, HI
Bend, OR
Glide, OR
Manila, CA
Col 1ege Sla.,
Texas
Wi sconsin
Pennsylvania
Oregon
Small diameter gravity sewer collected STE from a snail community.
Pressure sewer collected STE from 11 single family homes (Bowne, 19fi?).
Pressure sewer collected STE from a small community (Bowne, 1982)
- Pressure sewer collected STE from 330 connection (Bowne, 1982).
- Ste from one septic tank serving 9 homes (Brown, et al., 1977)
- 33 single family homes in Wisconsin (Harkin, et al., 1979)
- 10 single family homes in Pennsylvania (Cole and Sharpc, 1981)
- 8 single family homes in Oregon (Ronayne, et al., 1982)
-------
-18-
estimate dt 40 percent, these data suggest that nearly all of the soil pores
were waterfilled. Below this, at 15 to 20 cm, a much lower moisture content
was measured (24.8%). The accumulations of volatile matter (TVS) and nitrogen
(TKN) were most pronounced in the soil near the infiltrative surface and
decreased with depth. Similar findings have been reported previously
(Kristiansen, 1981).
Infiltration of septic tank effluent into soil materials is known to
result in soil clogging which leads to large reductions in soil
infiltrability. In sands, the primary cause appears to be accumulation of
wastewater suspended solids and microbial cell mass and metabolic by-products.
Long-term infiltration rates are supposedly used in system design to account
for these reductions and represent the infiltrability of a mature, clogged
soil system.
The work of Bouma (1975) led to the widespread acceptance of design
infiltration rates for subsurface wastewater absorption systems based upon
four major soil types; I - sands, II - sandy loams, loams, III - silt loams,
some si 1ty clay loams, and IV - clays, some silty clay loams (Bouma, 1975;
EPA, 1980; Anderson et al., 1982). Review of Bouma's results raised questions
regarding the design infiltration rate for systems installed in sandy soils
which contain substantial percentages of finer sands, silt or clay matter.
The results of Bouma (1975) for the coarse sands studied support a recommended
design infiltration rate of 5 cm/d. However, the results regarding sandy
loams were variable and do not support a 3 cm/d recommended rate. The work of
Simons and Magdoff (1979) supports lower design infiltration rates for sandy
soils of other than coarse texture. These investigators recommended a maximum
safe infiltration rate of 2 cm STE per day for sands based upon column studies
with a sand containing 33 percent medium and 54 percent fine particles. Otis
and Hargett (1983) have also advocated conservative loading rates. The
results of the study at Westboro suggest that even 2 cm/d may be too high for
large bed systems installed in fine sands.
SUBSYSTEM CHARACTERISTICS
Monitoring instruments were installed at four locations within or near
Bed 2: 5,1 m outside the west edge (Pt. D), near the center (Pt. A), at the
east edge (Pt. C) and 5.1 m outside the east edge (Pt. B) (Figure 3). These
included groundwater observation wells, and soil moisture tensiometers and
pore gas samplers at approximately 30 cm and 75 cm below the bottom of Bed 2.
Soil moisture tensions (SWT's) beneath Bed 2 were similar at both depths
below the infiltrative surface suggesting that flow in the soil was at unit
gradient. Under the Bed, the SMT's ranged from 21 to 44 cm (Table 5). The
resistance to infiltration, R, , was calculated according to the method of
Bouma (1975) to equal approximately 60 days. These SMT and R. values are
similar to those measured by Bouma.
-------
-19-
Table 5. S>
oil Moisture Tension
3 (Sr-L ) Beneath
Bed 2*.
Da te/Parameter
Jrii ts
Loca ti or;
I'en ter
Kdge
Outside (B)
Shal1ow
Deep
S/'.al 1 cw
Deep
Shallow Deep
Auq-jst 198?
*
SMT f
Crr.H^O '
Crn 91
35
tL 0
37
43 39
Distance Above
Water Tables
42
120
73
208 158
Auqust 24, 1982
SMT
CmH.20 34
4C
36
44
51 61
Distance Above
Water Table
Cm 105
53
135
88
220 170
October 12, "983
SMT
CmH^O 21
21
21
21
0 39
Distance Above
Water Table
Cm 83
35
105
58
212 162
* Refer to Figure
3 for instrume
nt lcc
:ations.
M St-r = Soil moisture tension.
Distance above w
¦ater tab_e = 1
r.struj.ent elevaticn -
• groundwater
elevaticn.
Table 6
Soi. Gas Coir.posit
ion Beneath Bed
2*.
Parameter
Unit
Distance Below
Infi1t
rative Surface
30 cm
75 cu
Soil Moisture Tension
CmH^O
21
21
Pore Gas - CH^
- C02
c/
/o
19.1
17.9
%
3.8
4.1
- o2
" N2
c/
1.3
0.2
%
70.8
72.4
* Data shown were collected on October 12, 1983 beneath center
of Bed 2 (Figure 3, Location A).. Distance to water table was
1 07 cm.
-------
-20-
Samples of soil pore gas were collected at the same locations at which
the SMTs were measured. Concentrations of CH. gas were 19.1 and 17.9 percent
at 30 and 75 cm depths beneath the infiltrative surface of the bed,
respectively. Concentrations of C0„ gas were 3.8 and 4.1 percent at these
same depths (Table 6}. Both of these gases are produced as a result of
microbial fermentation under anaerobic conditions. Concentrations of CH. and
C0? measured at the edge of the bed were similar to those measured beneath the
beo, suggesting that little horizontal diffusion of 02 was occurring from the
natural soil surrounding the bed or that it was insufficient compared to the
0? demand exerted. These results were unexpected based upon the results of
earlier investigators (Walker et al., 1973; Sikora and Corey, 1976) which
indicated aerobic conditions beneath subsurface wastewater absorption systems.
However, previous work had addressed only small single-home systems. Siegrist
et al. (1984) suggested that subsystem anaerobiosis may characterize large
bed systems.
GROUNDWATER CHARACTERISTICS
Surface Elevations and Flow Patterns
The groundwater in the vicinity of the absorption beds was thoroughly
characterized using 27 existing monitoring wells (Otis, 1978; Appendix A).
These included wells at 16 locations around the site with nests of two to
three wells at different depths at seven of the locations (Figure 3). Sixteen
shallow observation wells (2.4 m maximum penetration) located across the site
were used to measure groundwater surface elevations. To delineate the
groundwater surface and to estimate groundwater flow patterns a
multi-dimensional spline smoothing routine was coupled with a computer contour
plotting routine and graphics system to obtain computer drawn groundwater
contour maps (MACC, 1981; Anderson et al., 1984).
Representative groundwater surface elevations near the system are shown
in Figure 4. Although elevations fluctuated seasonably about 60 cm, contour
patterns stayed very much the same and indicated multi-directional groundwater
flow under the absorption beds. The general flow pattern was as expected
based upon surface topography which slopes from northwest to southeast (Figure
2). In addition, the groundwater surface was elevated in an area extending
from west of Bed 3 to under it with groundwater flow radiating out to the
northeast, east and southeast (Figure 4). This pattern suggested that a small
recharge area existed west of bed 3. A low-lying marshy area was located
approximately 61 m (200 ft) west of Bed 3 and could have been partly
responsible for the pattern indicated. This could also have been caused by
surface water flowing downslope over restrictive horizons until encountering
vertically permeable soil conditions near Bed 3.
Mounding of groundwater beneath the absorption beds was estimated to be
18 cm using a model developed by Finnemore and Hantsche (1983). In contrast
to this estimate, inspection of the groundwater contour plots suggested that
groundwater mounding may have been higher. Groundwater elevations measured
-------
-21-
SILVER Cr
-------
-22-
along a west to east transect under Bed 2 were interpreted to suggest a
potential maximum mound of approximately 60 cm under the center of Bed 2. The
discrepancy between the field results and the model predictions are believed
to be due to the heterogenous, anisotropic character of the subsystem soil
conditions at Westboro and the potential site variability of the estimated
model parameters.
Groundwater Composition
The composition of the groundwater in the vicinity of the wastewater
absorption beds was determined through sampling of 25 groundwater monitoring
wells (Figure 3). The multi-dimensional spline smoothing routine was applied
to an extensive groundwater database to map contour levels for the parameters
measured. Details of this analysis may be found elsewhere (Anderson et al.,
1984; Appendix A). A synopsis of the groundwater quality impacts is presented
below. Throughout the groundwater monitoring period, the water table surface
varied in elevation but remained 75 to 200 cm beneath the bottom of the
absorption beds.
The monitoring wells located closest to the absorption beds revealed
significant increases in the concentrations of measured parameters in the
groundwater relative to background levels (Tables 7-8). In particular,
samples from the shallow wells located near and downgradient from Beds 2 and 3
exhibited mean concentrations of total dissolved solids, chlorides and
conductivity not significantly different (« = 0.05) from the concentrations in
the applied STE. These concentrations of conservative parameters indicated
that the monitoring wells were installed so that samples were withdrawn within
the effluent plume.
The mean ammonia nitrogen concentrations measured near the beds were
approximately 100 times greater than background levels and not significantly
different from the NH, concentrations of the applied STE (Table 8). These
data indicated that little nitrification was occurring presumably as a result
of anaerobic conditions. The mean concentrations of phosphorus (P) measured
near the beds were not significantly different than background levels (Table
8). Mass balance calculations indicated a reasonable P immobilization of
about 100 jug/g of soil (Sikora and Corey, 1976). Mean concentrations of
chemical oxygen demand (COD) varied from 37 to 86 mg/L compared to 338 mg/L in
the applied STE and 5 mg/L in the background groundwater (Table 8). The
maximum concentration of fecal coliform bacteria measured near the beds was 8
organisms/100 mL, a value equal to or lower than that measured in the
background wells.
The groundwater quality degradation exhibited in the immediate vicinity
of the beds appeared to be limited to the upper zone of the groundwater. For
example, samples collected from nested wells at location 13 exhibited
decreasing levels of the measured parameters with increasing depth. The
groundwater composition measured in the deepest well (6.4 m deeper than the
shallow well) approached that of the background groundwater.
-------
-23-
Table 7. Mean Ccr.centratior; of Conservative Parameters in the
Groundwater Across and Downgracien: from the Absorption
Bed Site.
Lccati on
Li stance**
TDS
mg/L
CL- .
mg/L
Cond.
ui;iho/crc
Well 1-3
53.0
170
14.3
150
Applied
Wastewater
-
576
62
1073
Well 13-3
< 7."
495
65
1015
Well 18
9.i
467
63
1025
Well 19
0.8
480
66
960
Well 12
32. C
407
25
551
Well 11
82.3
216
3.9
2C7
* Pefer to Figure 3 for well locations.
** Approximate horizontal distance fron closest edge of Bed 3.
Table 8. t--ean Concentrations of Potential Pollutants in the
Groundwater Across the Absorption Red Site.
Location*
Di stance**
n
TKN
ir.g-N/L
nh4
mg -N/L
N03
mg-N/l.
TP
mg-P/L
COC
ng/L
Well 1-3
53.0
C. 62
0.21
2.74
0.08
5
Applied
Wastewater
-
57
44
6.4
8.1
338
Well 13-3
17.1
50.1
45.7
2.50
0.11
69,5
Well 18
9.1
49.3
41 .5
4.28
0.12
47.5
Well 19
0.8
40.6
33.1
1 .64
0.10
75
Well 12
32.0
1.2
0.72
1 .70
C.13
28.5
Well 11
82.3
0.68
0.15
1.61
0.07
<5
* Pefer to Figure 3 for locations of wells.
** Approximate horizontal distance frem closest edge of Bed 3.
-------
-24-
Approximately 32 m (105 ft) downgradient from the edge of Bed 3 (Figure
3, Well 12), the concentrations of conservative parameters were lower than
those measured next to the beds but still significantly higher than background
levels (Table 7). The mean concentration of ammonia nitrogen measured at Well
12 was only 0.72 mg-N/L, but still significantly higher than the mean level at
Well 1-3 of 0.21 mg-N/L. The mean concentration of phosphorus was not
significantly different from background levels. The chemical oxygen demand
(COD) measured in two samples from Well 12 were 19 and 38 mg/L compared to the
5 mg/L or less determined in two samples from Well 1-3, a background well.
Fecal coliform bacteria were not detected in the groundwater samples extracted
from Well 12.
The groundwater quality data from Well 12 revealed reductions in the
elevated levels of potential pollutants as measured in the groundwater near
the beds (Table 8). These data suggest that pollutant attenuation through
dilution and dispersion or removal and degradation may have been achieved
within the saturated zone. The groundwater as characterized at Well 11,
approximately 82 m downgradient and adjacent to Silver Creek, exhibited a
composition essentially equal to that of the background groundwater (Tables
7-8).
SURFACE WATER CHARACTERISTICS
Water quality monitoring was conducted in a nearby stream located within
approximately 47.2 m (155 ft) of the southeast corner of Bed 3 (Figure 3).
Monitoring both upstream and downstream revealed no degradation attributable
to the wastewater absorption system.
PERFORMANCE ASSESSMENT
The site evaluation and design of the wastewater absorption beds at
Westboro were judged adequate and accurate according to Wis. Code and the
state of knowledge available at the time of their implementation (i.e.
1976-1977). According to these criteria, the majority of the two operating
absorption beds (Beds 2 and 3) were located in areas well-suited for the
systems as designed for a 5 cm/d long-term hydraulic loading rate, with regard
to both infiltration and purification of STE.
The results of the soils investigations indicated the inappropriateness
of using small diameter boreholes and simple percolation tests to assess site
suitability and size large wastewater absorption systems. Inspection of stem
cuttings from a flight auger or even an undisturbed core from a split spoon
sampler may not have adequately revealed the hydraulically important but
relatively thin interbedded layers of very fine sands and silts. Percolation
rates determined through standard procedures likely reflect horizontal
permeability and not the vertical permeability of the restrictive horizon(s).
By example, the slowest percolation rate measured was 146 cm/d (25 min/in)
compared to the < 10 cm/d K measured by the CRP technique in the finer
soils. sat
-------
-25-
The performance of the wastewater absorption beds at Westboro was
different than expected with regards to both infiltration and purification.
Conventional practice, based primarily on experience with single-home systems,
would have predicted an operating infiltration rate of 5 cm/d and aerobic
conditions for wastewater renovation. At Westboro, the operating infiltration
rate was estimated to be only 1.4 cm/d and anaerobic conditions were present
under the system. Some localized degradation of the shallow groundwater
beneath the beds has occurred. The reasons for this performance are believed
to be associated with the bed geometry, stratified sandy soil materials, and
shallow groundwater conditions.
The absorption beds at Westboro were large and square in geometry (30.5 m
by 39.6 m each). As a result, minimal sidewall area was available for
infiltration causing mostly vertical flow under a unit hydraulic gradient.
The horizontal soil infiltrative surface consisted of gravel from the
absorption beds intermingled with the natural sandy soil. This effectively
reduced the available pore space for infiltration and percolation. Thus the
apparent 1.4 cm/d infiltration rate may have been substantially higher if
expressed per unit area of soil matrix only.
Subsystem anaerobiosis may also have contributed to intensified
wastewater induced soil clogging and the low infiltration rates observed. The
anaerobiosis results from a wastewater-induced oxygen demand exceeding the
oxygen transfer capacity of the system. Besides the ponding over the soil
surface, the oxygen transfer capacity was probably limited by the bed
geometry, the 1.4 m infiltrative surface depth and- the-shallow groundwater beneath
the beds (75 to 200 cm). All of these inhibit tie _supp-ry::of oxygen .to. :t-ne".
wastewater infiltrative surface.
The squarish bed geometry and shallow groundwater conditions probably
contributed to the measured groundwater impacts. Aside from the lack of
nitrification, the actual wastewater renovation achieved through soil
treatment was similar to that anticipated. However the comparatively
concentrated loading of adjacent beds and the shallow groundwater conditions
no doubt led to the measurable impacts observed in this study. Regardless,
more than 32 m (105 ft) downgradient from the absorption beds, the groundwater
quality approached that of the groundwater upgradient from the system.
-------
SECTION 6
FIELD EVALUATION OF MULTIPLE-HOME SOIL ABSORPTION
SYSTEMS IN WASHINGTON STATE
In Washington during the 1970's, large subsurface soil absorption systems
were increasingly utilized to treat and dispose of the wastewater from
clusters of residential dwellings, typically single-family homes,
condominiums, apartments or mobile-home trailers. Extensive use of these
systems occurred in Spokane County in eastern Washington and Pierce County in
western Washington. To gain further insight into the performance
characteristics of large soil absorption systems, a number of these systems
were investigated.
The selection of systems to be studied was made utilizing the following
criteria:
0 The system served a subdivision or small community and included a
septic tank and subsurface soil absorption system (trench, bed, or
mound).
0 The system was designed and installed within the past five years
according to accepted criteria.
0 The system was intended as a permanent wastewater facility for the
service area and not an interim solution pending sewers becoming
available.
0 The system was located in soils of sand or finer texture with the
depth to limiting layer (e.g., groundwater, bedrock . . .) less than
10 feet.
0 The current flow was estimated to be at least 70% of the design
flow.
0 The physical characteristics of the system enabled data collection
and the owners and operators were willing to cooperate.
Approximately 22 systems serving multiple residential units were identified
and of these, six systems were selected for study. The six study systems
possessed the characteristics shown in Table 9.
The field investigation of these systems was not intended to be anywhere
near as thorough as that conducted at Westboro, Wisconsin (Section 5).
Rather, efforts were made at each site to determine the'basic characteristics
of the wastewater-faci Vity,^-the soil conditions, the applied wastewater and
the absorption system performance. The results of this field study are
summarized in this section.
-------
-27-
Table 9. Characteristics of Six Systems Studied in Washington.
Na~e
•larbar
Nesika Bay'
Tclmie Park
Carve' 1
Gal en
Riegel
Coi-nfy
[states
Estates
Parr;
Hi>ichU
Estates
County
P i erce
K' tsap
ThLC ten
Spo'-iane
Spckane
Spokane
Orvel 1 "inq Urits
Type
Apartme". ts
Cor.dos/Houses
Hcuses
hojses
Holsss
Houses
Njmtei-
j'v
0/5
17
1"
11
10
Design
Flow (epe!)
16,100
2,437
7,650
3, BSC
3.B53
3,50C
Coilecticn
Type
(irv.vi ty
Grovity
Grav i ty
Gimv • ty
Gr.jv l ty
Gravity
5'.ze (in.
dian. )
6, 3
3
7
1
?
Freatreatnent
Type
Septic Terk
Septic Tank
Ind. Septic
Septic Tank.
Septic Tank
Ind. Septic
TaiA
T ankJ
Size (gal)
23,700
8.0C3
1125/home
4.50C
4.1C3
1 ,OOC,"ic-s
(2:15,£30 -
(3)
(2)
(3:1,50C +
(3:", ,5C3 +
7,530 )~
",:0C ~ ',503)
1,503 + 1 ,100)
Crainf i e1d
Type
Trenches
Trenches
Deep
Trencies
Trenches
Trercnes
Trer.cr-.es**
Width (ft)
3
2
2
2
O
L
2
Lergth (ft)
1C3
100
ICO
130
IOC, 85
100
NumSer
54
IS
15
7
4; 2
14
A-ea (ft!)
16.20C
3,630
3,00C
l.iCC'
1 .143
2.ao:
Scil*
Sand
Grave!lv Sand
Icamy Sane
loa~y Coarse
'.ce-y Coarse
Gravelly Lea-
Sord
Sand
Sard
Ccsts
Ccnstrjc-
64,125
, O
28,900
'.2,COO
6,250
13,730
tion i$)
0/C. ($/Yr/
90
60
78
43
19
0
'Jni t)
System Age
4
4-5
4
5
2.5
4
(Yr.)
CAM
Ccur ty Util-
Ho-ecwners
Private
homeowne-s
Homeowners
Homeowners
Resporsi b"i I i ty
ities
Assn.
'Jtil ity
Assn.
Assn.
Assn.
District
Manager
* Systems originally designed for 6 CDndcs in 1973; d^ain'ield expanded tc size shuwr in 1979.
~ Indicates compartr.entalizaticn number and size (if knowr)
*" 5' of grave' provicsd beneath distribution pipe.
* + Indicates rumher nf laterals of lergth speeded.
Classification acco*"dinq to SCb.
-------
-28-
SOIL AND SITE CHARACTERISTICS
All systems were installed on landscapes of 0 to 20 percent slope. The
systems at Tolmie Park, Nesika Bay arid Farwell Estates were installed on
nearly level terrain. The remaining systems were installed on slopes of 10 to
20 percent.
Sieve analyses were performed on samples (0.9 - 1.8 kg) of soil materials
obtained from the site of each of the six systems, generally from the
approximate depth of the wastewater infiltration surface (i.e. trench bottom).
As indicated in Table 10, the soil materials at all sites were generally of
coarse texture. The percentage of finer particles (<0.07 mm) was less than
13 percent at all sites except Riegel Heights where it was less than 20
percent (Table 10).
Soil permeability was estimated using standard percolation test
procedures (U.S. EPA, 1980). Measured rates were typically less than 10
min/in (365 cm/d) (Table 10). These rates were consistent with those expected
based upon the sand texture of the soil materials at the sites.
Restrictive layers (hardpan) were encountered below the Harbor Country,
Nesika Bay and Galen Park systems at a depth of 0.9 to 2.4 m (3 to 8 ft).
Based upon available resource materials, it was believed that Riegel Heights
and Tolmie Park also had a hardpan or impermeable layer within 3.0 m (10 ft)
of the surface.
APPLIED WASTEWATER CHARACTERISTICS
Average daily wastewater flows were determined at each site using water
meters on the individual residences and/or elapsed time indicators on the
pumps discharging effluent to the soil absorption systems. Due to
uncertainties regarding the true discharge rates of the pumping systems, the
elapsed time indicators generally did not yield useful data. At two sites,
wastewater flows had to be estimated using representative residency and per
capita flow information. As shown in Table 11, current wastewater flows were
estimated to be approximately 1,375 to 7,000 gal/d (5.2 to 26.5 m3/d), or 43
to 61 percent of the respective design flows.
The compositions of the septic tank effluent (STE) determined for each
system are shown in Table 12. The compositions measured were generally within
the ranqe of values determined at Westboro and by other investigations (see
Table 4j. The total suspended solid (TSS) concentrations measured, however,
were atypically low, less than 40 mg/l for five of the six sites. The TSS
concentration at Galen Park was substantially higher at 102 mg/L, yet within
the range of values reported elsewhere (Table 4).
-------
Table 10. Particle Size Characteristics of Soil Samples and Field Percolation Rates.
PardmL' ler
Harbor
Country
Nesika Bay
Tolmie Park
Estates
Farwel1
Fstates
f>a 1 e n
Park
Ri egel
Hei ghts
1
2
]
2
1
2
1
2
1
1
2
Particle Si7e
(% by wt.)
¦
> 2.00 mm
35.4
20.4
76.9
15.4
49.2
55.5
39.4
36.0
14.1
14.4
12.0
> 0.-12 - 2.00
mm
15.9
(24.4)**
12.7
(16.0)
12.3
(53.2)
18.7
(22.1)
21 .2
(41.7)
18.3
(41.1)
52.9
(91.8)
57.7
(90.2)
37.8
(44.4)
28.9
(33.8)
28.8
(37.7)
> 0.25 - 0.42
mm
17.7
(2/.3)
26.2
(32.9)
6.9
(29.9)
27.1
(32.0)
10.9
(21.5)
10.0
(22.5)
1 .0
( 1-/)
3.3
( 5.2)
31 .2
(36.6)
17.0
(19:9)
18.6
(21.1)
> 0.07 - 0.25
nm
23.0
33.4
2.4
31 .6
12.3
10.9
2.0
2.3
15.1
23.3
26.0
> 0.07
8.3
(12.8)
7.3
( 9.2)
1.5
( 0.06)
7.2
( 8.5)
6.4
(12.6)
5.3
(11.9)
1.7
( 3.0)
0.7
( 1-1)
1.1
( 1.3)
16.4
(19.2)
14.6
(16.6)
Pnrcolati on
Rate
(nin/in)
5.5
4.0
8.0
5.1
4.7
6.6
9A
11A
10.5
11.5
9.0
* Results of analyses at the sites of six multiple-hone soil absorption systems in Washington
+ Results of analyses of two separate locations on site.
** Number in parenthesis equals percentage by weight of fine earth (< 2.00 rm
-------
-30-
lable 11. Estimated Current Hydraulic Loading Rates*.
Parameter Unit Harbor Nesika To'.mie Farwell Galen Riegel
Country Bay Park Estates Park Heights
Design Flew
cpd
16,100
2,437
7,650
3,85C
3,850
3,503
Trench Bottom
Area
ft?
6 , 2C0
3,600
3,000
1 ,400
1 ,140
2,800
Design I.R.+
cpd/ft2
1 .0
C. 7
2.6
2.8
3.4
i . 2
Current Flow
DPd
7,000**
1 ,375?
4,260 'A
2,320 *1
2,35Ct
• . 84G ¦-
Current I.R.
gpd/f t2
0.4
0.4
1 .4
1.7
2.0
C. 7
Pondi ng
-
No
Yes
7
No
Yes
No
* For six mul ti pi e-hone soil absorption systens in Washington.
+ Design infiltration rate (r.R.J in cpd per fts of trench bottom.
** Estimate based upon 56 dwellings x 2.5 persons/dwellings x 50 gpd/person
* Estimate based up or, 11 cwel lings x 2.5 persons/dwellings x 50 god/person
a Based on water meter readings. Groundwater infiltration suspected eq'jal to
2 to 3 times the measured water use shown.
Based upon water meter readings.
. cb.
e 12. M
ear. Sept
ic Tank
Effluent
Concent ra t.i ons*.
Parameter
U n t
Harbor
Nesi ka
Tolmie
Farwel1
Gal en
Rieqel
Country
Bay
Park+
Estates
Park
Hei ghts
bgd5
rc/L
164
91
46
139
165
87
COD
ir.g/L
359
231
102
232
341
179
TVS
mg/L
175
144
75
32
72
22
TS5
mg/L
40
34
24
34
102
26
TKN
rrg-N/L
49
-j o
21
/ J
23
33
NH4M
ing-N/L
-
-
-
23
25
¦1) •
p
m g - P / L
1 ; . 1
10.3
5.2
10.4
13.4
11 .9
pH
-
6.9-7.1
6.8-7.1
CI
-
-
-
* Resul ts cf
analyses
of crab
samples
(5-6 samples typical)
col 1ec
ted dctween
January 20, 1S83 and March 15, 19S3 at six mul tipie-home systems in Washington.
+ Groundwater infiltration into the punp chamber was believed responsible for
tie relatively lower concentrations measured.
-------
-31-
PERFORMANCE ASSESSMENT
The performance of the multiple-hove soil absorption systems can only be
assessed in regards to infiltration as no purification data (i.e. groundwater
quality) were collected.
The occurrence of STE ponding in the wastewater absorption system was
utilized as an indirect measure of the infiltrability of the soil system.
Excavations were made into five of the six soil absorption systems, in each
case at two different locations. Ponding was only observed in two of the
systems, Nesika Bay and Galen Park.
The Nesika Bay system had been in operation for approximately 5 years
(Table 9). The applied wastewater was typical STE in composition and the
bottom area loading rate was estimated to be approximately 2 cm/d (0.4
gpd/ft2). This rate was less than the 3 to 5 cm/d long-term infiltration rate
one might have predicted based on the sand texture of the soil materials at
the site. However, a hardpan layer located about 0.6 m (2 ft) below the
bottom of this system resulted in perched groundwater and this may have
adversely impacted the infiltrative capacity of the system.
The Galen Park system had been in operation for only 2.5 years (Table 9).
The applied wastewater contained substantial .concentrations of BODr (165
mg/L) and TSS (102 mg/1) and the estimated hydraulic loading rate was
approximately 8.1 cm/d (2.0 gpd/ftz) of trench bottom (Tables 11-12). Despite
the very permeable soil materials (Table 10), these loading rates were much
higher than commonly recommended rates (U.S. EPA, 1980). The presence of
ponding is not surprising, but it is uncertain as to how long the system will
continue to function under the present loading conditions before effluent
begins to surface or backup.
-------
-32-
SECTION 7
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Walker, W.G., J. Bouma, D.R. Keeney and and F.R. Magdoff. 1973.
Nitrogen Transformations During Subsurface Disposal of Septic Tank
Effluent in Sands: I. Soil Transformations. J. Environ. Qua!.,
Vol. 2, No. 4.
Weber, B.A. and C.E.R. Lawson. 1972. Subsurface Investigation, Silver
Creek Dam - North of Taylor County Trunk Highway "D," Westboro,
Wisconsin, Warzyn Engineering and Service Company, Inc., Madison.
Wendelberger, James G. 1981. The Computation of Laplacian Smoothing
Splines With Examples. University of Wisconsin-Madison, Department
of Statistics, Technical Report No. 648, 66 pp.
-------
-37-
60. Wisconsin Administrative Code. 1972. Private Domestic Sewage Treatment
and Disposal Systems, Section H62.20, effective Nov., 1972.
61. Wisconsin Administrative Code. 1980. Private Sewage Systems, Section
H63, effective June, 1980.
-------
-38-
APPENDIX A
Performance Evaluation of the Community
Soil Absorption System at Westboro, Wisconsin
-------
-39-
MATERIALS AND METHODS
Description of Westboro And Its Wastewater Facility
Westboro is a small rural community located in Taylor County, Wisconsin,
adjacent to Silver Creek within the upper Chippewa River drainage basin
(Figure Al). In 1976, this community of 205 people had 69 occupied buildings
including private residences, a school, four churches and several commercial
establishments. All buildings in the community were served by private septic
tank systems. A survey by the Wisconsin Department of Natural Resources (DNR)
during the mid 19601 s revealed that 80 percent of these systems were
discharging wastes above ground with many discharging directly into Silver
Creek. Consequently, DNR issued an order to the Town of Westboro to upgrade
the existing septic tank systems or construct a public wastewater collection
and treatment facility.
As a result, the Town Sanitary District No. 1 of the Town of Westboro was
formed. Initial facility planning efforts resulted in a recommendation for
conventional gravity sewers and two-cell lagoon. The estimated construction
costs for this facility were $234,800 in 1967, excluding engineering fees and
contingencies. The district residents were unable to afford this facility, so
the plan was never implemented. Facility planning efforts were not resumed
until the mid 1970's. A variety of conventional and alternative options were
investigated by researchers at the University of Wisconsin-Madison and as part
of a demonstration grant from the Upper Great Lakes Regional Commission (Otis,
1978). The facilities plan was prepared by Carl C, Crane, Inc. of Madison,
Wisconsin (Carl C. Crane, Inc., 1976; Otis and Fey, 1981). The recommended
option included a system of small diameter gravity sewers for the conveying
the effluent from the existing septic tanks to a community subsurface soil
absorption system.
Wastewater pretreatment was accomplished at each building in the
community using individual septic tanks. Existing septic tanks were inspected
during construction and replaced as needed with prefabricated single-chambered
concrete tanks. The tank volume used for individual homes was 1000 gal (3785
L) while for commercial establishments it varied depending upon the type of
facility. Four-inch (10 cm) diameter gravity sewers were installed at a
minimum gradient of 0.67 percent. Manholes were placed at the upstream end of
each line, at junctions and at spacings up to 600 ft (182.9 m). Curvilinear
alignments in both the horizontal and vertical planes were employed. Where
homes had building drains below the proposed sewer invert, small lift stations
following the septic tanks were used. Community lift stations were also
necessary at three points in the collection system.
Preliminary site investigations in and around Westboro disclosed the
predominant soils to have loam or silt loam surfaces overlying sandy glacial
till (Otis, 1978). These soils are deep and well to somewhat poorly drained.
On an outwash terrace just west of Silver Creek (Figure 2), deposits of well
drained, well graded sand were identified. This location, approximately 25 ft
-------
-40-
tMi.XU LAMt
|l V
¦H.
CKS>»«S
AQOI'ION
• 1 "
I *
..Tl
J i
'/¦ |;!.j
iCi-oc. I *—3im '
"?'• ic-.
5r //
J OOllicw
L
n
' ^ciuk 1^* — - i"*• > j,v- " * * M
r . ?!Vf ?T7« i •.'"¦! m -
\ ..-v..
.-'C jf":
vL
¦=4i
•euiE»s*o»M
"V. I
^ j'i
''1\ !
i ,$
igure A1 . Sanitary District irl of trie Town of Westcoro
V.'isconsi r (C v! s, ' 973 !.
-------
-41-
(7.6m) above the stream elevation, was selected as the site for construction
of a network of large soil absorption beds.
The wastewater application rate chosen for the absorption beds was based
upon site soil characteristics. Soil borings characterized the subsurface
soils as sand and loamy sand textures. An application rate of 1.2 qpd/ft (5
cm/d) was selected based on work with individual systems (Bouma, 1975). The
design flow, based udqh 250 qpd (0.9 m /d) per home, was projected to be
3Q,000 qpd (113.6 m ' . The necessary infiltration area of 25,000 ft (2322
M ) was provided in three, 100 ft (30.5 m) by 130 ft (39.6 m) beds (Figure
A2). Each bed was designed to handle 15,000 gal/d so that at any one time,
two of the beds could handle the design flow with the third acting as a
standby. Annually, the standby bed was to be rotated into service so that
each bed would receive wastewater for two years and then rest for one year.
The resting period was intended to restore the bed's infiltrative capacity
through drainage, aeration and biochemical oxidation of the clogging zone.
Pressure distribution networks were designed to distribute septic tank
effluent (STE) uniformly over the infiltrative surface of each bed (Figure
A2). Two manifolds, 8-inch (30 cm) telescoping down to 4-inch (10 cm)
diameter were used to feed 4-inch (10 cm) laterals spaced every 5.25 ft
(1.6 m). The inverts of the laterals were perforated with 15/32 inch (1.2 cm)
holes located at 6.5 ft (1.9 m) spacings. STE was delivered to each bed by a
separate 12-inch (30 cm) diameter force main served by a 10-inch siphon,
capable of discharging an average of 1000 gpm (3.8 m /m) at the design head
(Figure A3). Two siphons were to be in service at any time, automatically
alternating operation and discharging approximately 8,000 gal per cycle. At
design capacity, each bed was to receive 2 doses per day. The siphon serving
the resting bed could be deactivated by closing a ball valve installed in the
siphon's blow-off vent.
Construction began in April, 1977 and was completed in September, 1977.
The construction costs are delineated in Table Al. Construction funding was
provided through financial assistance from the Farmer's Home Administration
and the Wisconsin DNR as well as local assessments and hook-up charges. At
least a 12 percent savings in construction costs was realized compared to
conventional sewerage. Moreover, the facility reduced operating costs over
conventional gravity sewers and a stabilization pond facility by 65 percent
(Otis and Fey, 1981). User charges for each residence during 1981 were only
$10.00/month.
The facility has been in operation since the Fall of 1977. The Westboro
Sanitary District has been responsible for the operation and maintenance of
all components of the system beginning at the Inlet to each septic tank. The
facility was designed to require very little attention. All duties are
performed by a local operator in an average of 2 to 4 hours per week. The
proposed facility maintenance schedule is shown in Table A2.
-------
-42-
\ ,
-12" PVC CELIVER*
RP[ FROM
S I p HON
I l
i i I
8" PVC manipclC:
P'PE I |
ORIclCES | 6'-6"0 CJN
INVCR~ or 4'
PVC LATERALS
igure A2. Pressure Distribution System Schematic (Otis, 1978
-SCLSOE MANHOLE
LtGHT ourr c*s' iron p »e bell
BALL VALVE
¦ LEVEL
BLOW Off- I ' J
VENT
*" CAST IRON *E«r
Dc/
•' ccw-'icitD ohavI:i
PRC • A5SEKJ-.E5 P.v.c lO' '"siPmOk vi ih
10 DRAW
U'" TO A&SORM 3*
BfOS
Figure A3. Profi.e Schematic of the Siphon Charter
at Westboro iOtis, 1973;.
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-43-
Table A1. Westboro Construction Cost Summary (Otis ar.c
Fey, 1981 ).
;-7iD0".erit
i i.em
Cost
(1977 Col If. r
Co11ccti on
(House lateral, septic
tanks, sev.'cs and lift
station)
"reatment
(Siphon c'-aTber and
sc'l absorption fields]
Cons true t.. on
Land and Rinhts
Legal Services
Engineering and Inspection
Bond Council
Subtotal
Construction
Land Rigvts
Legal Services-
Engineering and Inspection
Bond Council
Subtotal
T o tal
S25?,535
jOC
325
,CC0
S 305,J 60
98,865
5, CCD
2 ,6 C 0
20.750
6CKJ
SI 28,715
$435,175
.able A2. Facility Maintenance Schedule (Otis anc Fey, 1981).
Daily
Weekly
Month! v
Annually
Check lift station alarm lights.
Open lift stations for visual inspection of pump operation float, control
operation and debris accumulation.
Record total weekly flow from pump running time meters as per
WPDES* permit requirement
Check siphon operation.
Inspect observation vents in each bed fcr depth of ponded water;
switch cells if necessary.
Each spring, alternate resting bed into service and drain manifold of
bed taken out of service.
Inspect the surface of the absorption field and fill any holes and
depressions.
Pump i/3 of septic tanks each year according to schedule.
Pump lift stations and siphon chamber to remove sludge.
Flush sewer lines as necessary.
* Wisconsin Pollution Discharge Elimination System.
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-44-
Review of Previous Monitoring Efforts
- - - - — ^
Monitoring at Westboro was initiated prior to construction of the
wastewater facilities in 1977 and during their first year of operation. The
monitoring was performed by staff from the University of Wisconsin-Madison
with funds provided by the Upper Great Lakes Regional Commission. Detailed
information may be found elsewhere and only a brief summary follows (Otis,
1978).
Water use was metered in 25 homes and businesses between April, 1977 and
November, 1978, to determine average daily flows and identify any increase in
water use after the public facility became operational. Based upon monthly
water meter readings, the average residential flow measured at 21 homes ranged
from 25 to 546 gal/d (94.6 to 2067 Lpd) while the average for all homes
combined was 169 gal/d (640 Lpd.) The average number of occupants was 4.7
persons and per capita usage averaged only 36 gpcd (136.3 Lpcd), with a range
of 8 to 71 gpcd (30.3 to 269 Lpcd). No change in water use occurred as a
result of the new wastewater facilities (Otis, 1978).
The average daily wastewater flow was determined from elapsed time meters
on the pumps in the main lift station and the measured discharge rate of the
pumps. The average daily flow determined between February and October, 1978
was 7,345 gal/d (27.8 m /d) (Otis, 1978). With two beds in service, this
daily flow represented only 25 percent of the design flow.
Groundwater observation wells were installed in and around the soil
absorption beds to determine the response of the groundwater to the
percolating wastewater. In June, 1977, 28 observation wells were installed
prior to the facility becoming operational. Subsequently, certain of these
wells were abandoned due to well screen clogging or other problems.
Additional wells were installed in close proximaty to the wastewater
absorption beds to facilitate the assessment of any groundwater impacts. As
of September, 1979, 27 wells were installed at 16 locations around the site
with nests of two to three wells at different depths at eight of the locations
(see Figure 3). Each well consisted of lengths of 1 1/4-inch (3.2 cm)
diameter plastic pipe with a 3 ft (91.4 cm) slotted well point. They were set
in an augered 4*-inch (11.4 cm) diameter hole and then backfilled with natural
soil materials. A chalked tape and hand bailer were used initially to measure
water potentials and to evacuate the wells prior to sampling. Due to the
potential for well contamination and the difficulty in monitoring during the
winter, a new well head was designed. The new head utilized compressed air
from a hand tire pump to enable measurement of water potentials using a
bubbler tube and to evacuate the wells for sampling.
Observed groundwater elevations prior to any wastewater loading revealed
the groundwater was at least four feet (1.2 m) below the bottom of the
absorption beds and the flow was both downward and southeasterly towards
Silver Creek. Groundwater samples collected during the first year of
operation were analyzed for total solids, nitrogen forms, total phosphorus,
-------
-45-
chloride, calcium, magnesium, total and fecal coliforms and fecal
streptococcus. Compared to a single sampling prior to startup of the
facility, water quality changes were only noted at well 13-3 (Otis, 1978).
This well extracted water from approximately 15 ft (4.6 m) below the
wastewater infiltrative surface and 40 ft (12.2 m) down slope from the
southeast edge of bed 3 (Figure 3). Water samples from this well exhibited an
increase in total nitrogen concentrations from less then 0.5 mg-N/L to over 15
mg-N/L. Soon after the beds began receiving wastewater, the increase in
nitrogen was in the form of nitrate, but later changed to ammonium while the
total nitrogen remained constant.
Water quality sampling in Silver Creek began in July, 1975. To measure
stream flows, a gaging station was established at the CTH D bridge just
downstream of the absorption beds (Figure 2 and Al). The result of analyses
of stream samples collected upstream and downstream of the wastewater
absorption beds revealed no effects on stream water quality.
Monitoring Procedures of This Study
Soil and Site Characteristics-
Soil and site conditions were investigated in this study during October,
1981. Based upon operational and monitoring data available at that time,
questions had arisen as to the siting of the absorption beds, the wastewater
loading rate, and soil conditions affecting the system's performance.
Detailed soil evaluations included the inspection of soil profiles from seven
backhoe pits located around the perimeter of the absorption beds (B1-B7) in
addition to hand borings (B8-B10) (Figure 3). The profiles were described in
detail, substantially according to the method of SCS (1981) and the Wisconsin
Administrative Code (1980). Particular attention was directed to morphologic
features which might impact wastewater absorption and treatment. Soil samples
were collected from selected representative horizon's of some pits. Particle
size analysis of the samples was conducted according to the methods of Day
(1965).
Three percolation tests (P1-P3) were conducted along a north-south
transect just east of Bed 1 where fine materials (vfs-sij were apparent in
adjacent pits B1-B2. These tests conducted according to the method prescribed
by Wisconsin Adm. Code (1980), provided some estimate of percolation rates in
the more hydraulically restrictive materials of the site. Saturated hydraulic
conductivity was determined for re-packed grab samples (maximum dry density)
of selected soil horizons using concentric-ring permeaters (CRP's) (Hargett,
1982).
-------
-46-
Applied Wastewater Characteristics--
The hydraulic loading to the absorption system was monitored using a
float-operated mechanical counter installed in the siphon chamber. Each time
wastewater was discharged from the siphon chamber, the counter recorded the
event. The counter was read periodically and the average number of
applications per day determined. These data, coupled with the known volume of
wastewater per siphon discharge cycle, were used to calculate the average
hydraulic loading per day. Grab samples of septic tank effluent were
periodically collected from the siphon chamber. Analyses were performed
according to U.S. EPA approved procedures for biochemical oxygen demand,
chemical oxygen demand, solids, nitrogen forms, phosphorus, chlorides,
conductivity, total and fecal coliforms and fecal streptococci.
Soil Infiltrability--
Ponding of the applied septic tank effluent above the bottom infiltrative
surface of each operating bed was utilized as an indirect measure of the soil
infiltrability. Ponding depths were monitored through existing observation
ports installed during construction of the facility in 1977 (Otis, 1978). The
4-in (10 cm) diameter ports were placed with the bottoms resting on the
infiltrative surface and the top extending several feet above the final ground
surface and topped with a vent cap. The bottom section of each observation
port was perforated to allow the free movement of septic tank effluent in and
out of the port. At least three observation ports in each bed were inspected
periodically to determine the depth of ponding, if any.
Subsystem Soil Moisture and Gas Characteristics--
As part of this project, monitoring instruments were installed at four
locations within or near Bed 2: 20 ft (5.1 m) outside the west edge, near the
center, at the east edge and 20 ft (5.1 m) outside the east edge (Figure 3,
A4; Points D, A, B, C). These instruments included groundwater observation
wells as well as soil moisture tensiometers and gas samplers at approximately
12 inches (30.5 cm) and 30 inches (76.2 cm) below the bottom of Bed 2 (Table
A3).
The groundwater observation wells consisted of lengths of 1 1/4-inch (3.2
cm) diameter plastic pipe with a 3 ft (91.4 cm) slotted well screen. The
groundwater level in each well was measured periodically using a sounding
device (popper) attached to a fiberglass measuring tape.
The soil moisture tensiometers were constructed of 7/8 inch (2.2 cm)
outside diameter (o.d.) porous ceramic cups (Soil Moisture Equipment Corp.,
Santa Barbara, CA., Model 2600A) attached to the bottom of a length of clear
acrylic tubing. Nylon tubing with 1/8 inch (3.2 mm) o.d. was used as the lead
conduit from the tensiometer to a mercury manometer. Manometers were
assembled adjacent to the instrument location early on the day that
measurements were to be made. After tensiometer equilibration, several
-------
-47-
^
-------
-48-
Table A3. Fievations cf Subsystem Monitoring Instruments a" Bee 2.
PI ccenent Irstrurr.snt Ground Infitrctive Instrument
Location* Surface Surface Botton
( t: *¦ \ i £ *¦ \ ( C *- ^
\ ¦ <-1 \ ¦ -! I; - J
Groundwater
Obsorveticn
V!el 1
1507.5
1502.8
14S5.4
Tensi ornater
1
1501 - 9
""ens krveter
9
500.3
Gcs Saxpler
1
1501.9
Gas Sampler
2
150C.4
Groundwater
Observat;on
\Jsl 1
1507.4
1502.8
K-98.0
Tensiomster
1
1502.5
Tensiometer
2
1501.0
Gas Sampler
T
1
1502.5
Gas Sampler
2
1501.0
Grour,
-------
-fig-
measurements were made to assure instrument stability. The manometers were
disassembled between measurements. Further details regarding procedures for
the measurement of soil moisture potential may be found in Richards (1965) and
Bouina, et. al. (1974).
Soil pore gas samples were obtained using equilibrium diffusion chamber
samplers similar to the bottomless chamber type described by Smith (1977a).
These samplers consisted of 1-inch (2.5 cm) i.d. plastic pipe with an
air-tight end piece through which a glass capillary protruded on the access
end. The gas reservoir end of the sampler was recessed to provide a 24-cc
volume of soil atmosphere in continuity with the adjacent soil and sampleable
via the glass capillary. The access end of the capillary was capped with an
air-tight rubber septum through which gas samples were collected using 1-cc
(0.16 inch ) disposable nylon syringes with 27 ga. needles. The volume of the
glass capillary was first evacuated and discarded and then three sequential
1-cc samples were collected from each instrument and labled. Immediately upon
sampling, the needle syringe assembly was stuck into a rubber stopper and
completely immersed in oil for transport stability. Gas samples were analyzed
within 24 hours on a gas chromatograph (Fisher Model 1200), equipped with dual
columns and dual thermal, detectors. A strip-chart recorder was used to
record chromatographic response. A standard gas mixture was used as a
reference to determine the percent composition of O2, CO2 and CH^ in each
sample.
Installation of monitoring instruments within Bed 2 was accomplished as
follows (refer to Figure 3 and A4). THe soil material over the absorption bed
was excavated and as the gravel was removed, a galvanized metal cylinder (0.6
m diam.) was pressed into the soil infiltrative surface. Any septic tank
effluent within the interior of the cylinder was then removed using a
hand-operated diaphragm pump. A 3-inch (7.6 cm) diameter bucket auger was
used to excavate three separate holes: one for a groundwater observation
well; a second for a tensiometer and gas sampler at 12 in. (30.5 cm) depth;
and a third for a tensiometer and gas sampler at 30 in. (76.2 cm) depth (Table
A3). After instrument placement, the holes were backfilled with natural
materials and a layer of bentonite. The metal cylinders were then perforated
to allow septic tank effluent to pond over the location of the instruments.
To facilitate access to the instruments for servicing and measurements, the
large metal cylinder was extended up to the ground surface.
Groundwater Characteristics--
The groundwater at the site was characterized using 27 existing
monitoring wells (Otis, 1978). These included wells at 16 locations around
the site with nests of two to three wells at different depths at seven of the
locations (Figure 3, Table A4). Each well had been previously outfitted with
a specially designed pneumatic appartus that enabled the measurement of
groundwater potentials and the collection of water samples (Figure A5; Otis,
1978).
-------
-50-
Table A4. Elevations of Groundwater Monitoring Wells.
Mo"
tori nq
Surfsee
Wei 1
lis: 1 Bcttc~
Bubble Tube
1:811
Elev.
Depth**
El ev.
Bettor.: Elev.
Locat"
on' Point
(ft)
(•"ti
(t ;¦
(ft)
-
1
1503.5
34.4
1469.1
1471.0
2
11
12.0
1491.5
1492.0
3
M
8.6
1494.9
K95.9
2
1
1505.2
35.0
1470.2
14 72.5
2
"
22.8
1482.4
1483.3
3
"
15.1
1450.1
14 9:.0
3
1
1503.6
11 .5
1492.;
1492 .5
C\
1
1504.6
21 .S
1482.7
1483.0
2
ir
12.4
1492.2
1493.1
5
1
1497.4
21 .6
1475.3
1477.5
2
il
11.7
1485.7
1488.7
11
1
14S3.1
9.7
1473.4
1476.3
12
1497.7
17.c
1480.1
148C.4
13
1
1503.1
37.5
1465.6
1469.2
2
"
26.7
1476.4
1477.5
3
11
16.5
1486.5
1489.0
14
i
15GG.5
44.7
1461.9
1467.9
2
II
31 .1
1475.5
1476.5
li
19.1
1487.5
1490.5
15
1
1502.8
27.0
1475.7
1477.4
2
n
16.3
1485.0
1488.2
17
1
1507.6
15.7
1491.9
1492.1
1G
1
1506.2
17.4
1488.9
1489.3
19
1
1507.4
15.6
1491.9
1494.9 '
20
1
1507.4
18.9
1488.5
1489.1
21
1
1505.3
14.3
1490.9
1494.4
22
1
1507.3
17.5
1*83.8
1490.7
* Refer to Figure 3 for locations.
** Distance fron ground surface to bottorr. of well screen.
-------
_c - _
¦„oppsr
3/16 x 1/16
Tvgon Tub in
r.^ir'.g - 5/16"
PVC Sheet
2"-l/£" Scr <-0
?VC PiDe
i l/f-siip x r.;PT
(Set*. iG fvc: -
1/2" Slip Coupling
2ch "-0 PVC
1/i" 31 i 9 X f'MPI'
(Sell « PVC)
1/2" Scr hO
i 1/=.- set- rvc.
1 l/i" PVC Slo-tC'd
Well Poir.t —
i/k" O-D. Plexiglass
lUDiHg
1/2" 3cf. iC PVC
Cap —
1/4'' £ Ball
Bearing
© o
Figure A5. Weil Monitoring Apparatus (Otis, R. Unpublished
Drawings).
-------
-52-
The following procedures were used to measure groundwater potentials and
collect samples during each site visit. During the first day of a site visit,
any groundwater within each well was evacuated. The wells were then allowed
to recover for approximately 18 to 24 hrs. After this recovery period,
groundwater samples were collected. First, approximately 100 mL of
groundwater were discarded. Then a 500 to 1000 mL sample was collected in
two, polyethylene bottles. One sample volume was preserved with concentrated
sulfuric acid. These two samples were subsequently analyzed for
chemical/physical parameters. Subsequently, a 100 to 200 ml sample was
collected in each of two sterile, plastic bags for subsequent analyses for
microbiological parameters. All samples were refrigerated during transport
back to Madison, Wisconsin where analyses were performed by a private testing
lab according to U.S. EPA approved procedures for total dissolved solids,
nitrogen forms, phosphorus, chlorides, conductivity, fecal coliforms and fecal
streptococci.
The groundwater data collected in this study were analyzed by several
means. Routine statistical methods and hand contour plotting techniques were
initially used. Further analyses were performed using a computer statistical
analysis program known as multi-dimensional spline smoothing in conjunction
with a contour plotting routine. Details of the theory of this method can be
found in Wendelberger, 1981 and MACC, 1981, while the application of this
technique to groundwater monitoring has been described by Fine, 1983.
Briefly, multi-dimensional spline smoothing consists of using a laplacian
smoothing spline to model a smooth but otherwise unknown function. The method
of generalized cross validation is used to choose and optimize the smoothing
parameter. Use of this model for groundwater applications assumes a
relatively smooth groundwater surface with no sharp peaks and one that can be
expressed by some function, 1n this case, Darcy's Law.
Surface Water Characteristics--
The flow volume of Silver Creek, a stream adjacent to the soil absorption
beds, was determined periodically via a gaging station located just downstream
of the soil absorption beds. Grab samples were collected from the stream,
both upstream and downsteam of the soil absorption beds and handled and
analyzed the same as for the groundwater samples (Figure 3).
Analytical Quality Control--
To insure quality control of analytical laboratory procedures
approximately 10 percent of the samples collected in the field were split with
one fraction of each submitted to the laboratory as an unknown. Blank and
spiked samples were prepared and analyzed by laboratory personnel along with
each round of field samples. In addition, U.S. EPA reference samples were
analyzed periodically.
-------
-53-
Operation arid Maintenance—
The operation and maintenance history of the wastewater facilities was
investigated. Of concern was the period between system start up in the Fall
of 1977 and the beginning of this study in June of 1981. Information was
gathered from published reports (Otis, 1978) and conversations with the Town
of Westboro Sanitary District commissioners and facility operator.
-------
-54-
RESULTS AND DISCUSSION
Soil and Site Characteristics
General Description —
The wastewater absorption beds at Westboro are located on a glacial
outwash terrace adjacent to Silver Creek (Figure 3). The site of the beds is
nearly level, at an elevation of 457 - 461 m (1500 - 1510 ft). The landscape
surrounding the system includes agricultural fields sloping toward it from the
north and undeveloped fields sloping gently to Silver Creek 75 m east, and a
drainage ditch 30 m south which feeds into the creek. The soils of this
setting formed in 0.5 - 1.0 m or less of silty loess overlying loose sandy and
gravelly outwash. Soils of the Antigo, Brill, (both Typic Glossoboralfs,
fine-silty over sandy or sandy skeletal, mixed frigid) Poskin (Aquic
Glossoboralf, same family) and Onamia (Typic Eutroboralf, fine-loamy over
sandy or sandy skeletal, mixed, frigid) association typify this landscape
position and geologic setting (SCS, 1966; Hole, 1976).
Thin bands of finer textured fluvial deposits are commonly interbedded in
the sand and gravel. This banding may be locally more dominant in some
profiles or absent in other profiles. Because of the highly variable deposi-
tional environment, soil profiles may vary markedly over a short distance,
especially in sections perpendicular to the general drainage.
The glacio-fluvial deposits vary in depth, with the underlying glacial
till generally at an elevation below 445 m (1460 ft) (Weber and Lawson, 1972).
This Wisconsin Age till is typically sandy loam to loam and may be gravelly or
cobbly. Precambrian granite bedrock underlies the till at approximately 425
to 430 m elevation (30-37 m depth).
Review of Pre-Construction Site Evaluation --
The site was evaluated for its suitability for wastewater absorption
during the summer of 1976, Staff of Carl C. Crane, Inc., of Madison,
Wisconsin and the Small Scale Waste Management Project at the University of
Wisconsin-Madison were involved. The evaluation included examination of soil
morphology and horizonation from numerous borings and open pits, and
estimation of saturated hydraulic conductivity based upon morphology and
percolation tests. The results of this evaluation as determined from personal
communication (Otis, 1983; Fey, 1983) and previous publications (Otis, 1978)
were reviewed as part of this study.
The soil profiles observed in pits Bll-15 (Figure 3) were reported as
dominantly very sandy in texture with considerable fines (silts) in some
strata. Groundwater was observed at 1.8 m (72 in.) in pits Bll and B12
although no soil mottling was associated. Shallow, seasonally perched con-
ditions, above very silty horizons, were reported for areas to the west of the
proposed system. The saturated conditions observed in pits Bll and B12 may
also have been due to perched water above flow-restricting horizons.
-------
Elsewhere on the site groundwater was estimated at depths greater than 2.4 m
(96 in.).
Saturated hydraulic conductivities (K .) were estimated based upon soil
morphology to be of the order of 100 cm/d f8r fine sands to 400 cm/d or more
for medium and coarse sands. Silt loam to silty clay loam materials perching
groundwater in the area west of the absorption bed site were estimated to have
Ksat values of 2 to 20 cm/d. Percolation tests conducted on the site (P4-P6,
Figure 3) at a depth of 1.07 m (42 in.) yielded values greater than 200 cm/d
( 2 min/in) (see Table 2).
Based upon the groundwater conditions, finer textures and interbeddlng
observed in the area of borings Bll and B22 and west thereof, this area was
rejected as a suitable site for the system. The preferred area was located
along a transect from southwest to northeast, parallel to Silver Creek and
centered along the terrace. The soils in this area were judged to be more
uniformly sandy and permeable with greater depth to groundwater. Given these
site conditions, an installation depth for the infiltrative surface of the
system was chosen to be 1.07 m (42 in.).
Unfortunately, the entire preferred area for locating the system was not
available and Bed 1 had to be constructed further west and in finer soils than
desired. During excavation of the beds, very silty deposits were encountered
in the northwestern one-third of Bed 1. These very well sorted materials
(very fine sands and silt) were about 1.5 m (60 in.) thick at their deepest
point and were perching shallow groundwater. This material was removed expos-
ing very permeable coarse sands, and local sandy fill was put in its place.
After excavation of the vertically restrictive finer material, the laterally
flowing groundwater readily infiltrated into the sandy strata below.
The absorption beds were sized based upon a maximum design infiltration
rate of 5 cm/d (1.2 gpd/ft ) for sandy soils with percolation rates less than
10 min/in. (Bouma, 1975). This application rate seems in retrospect, very
reasonable considering the soil conditions and percolation rates reported, and
the state of knowledge available at the time (i.e. 1976).
Review of Groundwater Monitoring Well Logs --
During the summer and fall of 1977, groundwater monitoring wells (nests
1-16) were installed across the site (Figure 3). General descriptive logs
were prepared and Karnauskas and Eisen (1977) developed a report regarding the
hydrogeological conditions at the site. Soil samples (hollow stem auger
cuttings) of selected strata were sieved and permeabilities were estimated
according to the methods of Masch and Denny (1966). The sands, mostly medium
(0.25-Q.50 mm) were estimated to have a mean permeability of 1330 cm/d (325
gpd/ft ) with a range of 370 to 3890 cm/d (90 to 950 gpd/ft ).
Additional wells were installed during Summer 1979 (Wells 17-22, Figure
3). Descriptive logs provided limited information on the homogenized cuttings
of finely interbedded strata extracted from flights of a hollow-stem augar.
Descriptions were very imprecise in textural character and depth of origin.
-------
-56-
It is important to note that the clayey materials described in some logs were
listed at depths generally greater than 6 m (20 ft), deeper than the
background water table. It was believed that these materials would not impact
the site hydrogeology, at least for the purpose of this study.
Detailed Soil Description --
Detailed descriptions of soil pits B1-B7 were prepared as part of this
study (Figure 3). The soil descriptions evidence considerable variability
across the site, especially from north to south. Pit B2 represents a rather
finely interbedded soil profile, with several abrupt textural breaks judged
capable of impeding vertical water movement. The stratum at 137 to 155 cm
(54-61 in.) in Pit B2 was exemplary of very well sorted, very fine sands and
silts which effectively perched water in coarser sandy horizons above.
Considerable free water was observed skirting through the overlying sandy
zone, down the side of the pit and into the coarse sands below. Most of the
strata underlying the site appeared to dip roughly 5 to 20 degrees toward the
south-southeast.
These interbedded materials are characteristic of inwash-outwash deposits
which are alternately subjected to high and low energy depositional environ-
ments, resulting a wide range of sediments. The lateral continuity of finer
textured deposits in these strata was highly variable, although in some cases
it was extensive enough to intercept vertical flow and confine it to more
permeable layers.
Although no continuous free standing water was observed in pits B1-B3
located east of Beds 1 and 2, soil mottling was observed as shallow as 25 to
46 cm (10-18 in.) in each pit. This indicated some possible seasonal satu-
ration or near saturation conditions (Vepraskas, 1974). In each case, the
same horizon displayed platy (horizontally oriented) structure, likely some-
what resistant to vertical water movement. In pits B1 and B2, the strata
underlying the zones was quite mottled, interbedded and varied in texture.
To the south, the pits revealed soils much coarser in general texture
with fewer and thicker layers. Pit B3 was transitional to the more dominant
sands and gravels found in pits B4 and B5 adjacent to Bed 3. However, even in
the B4 and B5 profiles, some bands or stringers of silts and very fine sands
very found which were similar to the thicker zones found in Pit B2.
Pit B6> west of Best 1 and opposite of Pit Bl, displayed interbedded
sands, gravels and silt strata with free water, possibly perched, standing at
180 cm (70 in.). Pit B7 showed similar conditions both with a thick zone of
well-sorted silts and very fine sands at 117 to 168 cm (46-66 in.), and
standing water at 168 cm. Both B6 and B7 were highly mottled below a depth of
25 cm (10 in.).
-------
-57-
Particle Size Distribution --
Particle size distribution data for selected representative soil horizons
are presented in Table A5. Five of the eight samples analyzed were classified
as sands (USDA), two as sandy loams and one as loam. Three of the five sands
qualified as fine sands and had fine plus very fine sand contents of 57 to 76
percent. Seven of the samples had more than 64 percent sand and six had more
than 66 percent material finer than medium sand (0.25 mm). The three "loamy"
samples are deposits of very well sorted and densely packed particles of the
silt to very fine sand size. In these three samples the summation percentage
of silt plus very fine sand ranged from 71 to 77 percent. These same deposits
were observed to very effectively impeded vertical water movement in soils
pits B1-B3 and B6-B7.
Percolation Rates --
The results of percolation tests performed as part of this study are
shown in Table 2 along with those conducted prior to construction of the
wastewater absorption beds. The percolation rates measured in this study
(P1-P3) were for soils in the less permeable and somewhat impermeable strata
east of Bed 1. Rates ranging from 140 to 470 cm/d (25 to 8 min/in) were
determined. Percolation rate measured prior to construction were in excess of
2150 cm/d (£2 min/in).
The soil materials -at. the installatiqn depth of 1.07 m (42 in.) in Pits
B1 and B2 (near P1-P3), contained some sandy strata inter-layers with finer
horizons. Because of the likelihood of considerable horizontal flow in these
materials, the percolation test procedure would be a very poor indicator of
the vertical permeability required of large, rectangular absorption beds.
Saturated Conductivity --
Table 3 presents the results of saturated hydraulic conductivity (K .)
measurements conducted on selected samples. These samples were judged to
represent the most hydraulically restrictive materials encountered (Pit B2,
loam horizon at 81-91 cm depth) and the typical fine sand material (Pit B2,
112-125 cm and 163-203 cm depth). The loam sample (Pit B2) displayed a K t
of 9.5 cm/d while a very sandy loam sample had a K . of 86 cm/d. The two
fine sand samples resulted in K ^ values of 366 ana 247 cm/d.
Discussion of Soil and Site Characteristics --
The 1976 pre-construction site investigations were carried out in accor-
dance with Wisconsin Adm. Code (WAC H62.20, 1972) and Department of Health and
Social Services policies. Numerous soil borings and pits were described and
percolation tests were conducted according to Code requirements. Less desir-
able soil areas were delineated through these evaluations, although some of
these areas had to be used for construction. Elsewhere, the site soil con-
ditions were properly judged, based on the criteria used and data available,
to be well-suited for installation of a network of large soil absorption beds.
-------
Table A5. Particle Size Distribution of Soil Samples.
Percent of Fine Earth (Mass %)
. % Whole
Boring Total Sand Fractions** USDA Unified Soil
No.* Depth Sand Silt Clay vcs cs ms fs vfs ^i~s"**»Texture Texture Gravel
B1 52-62 in 61.1 3C.1 5.8 0.3 0.0 0.1 19.1 44.2 90.6 vfsl SM 0.0
(132-157 cm)
32 32-36 41.4 44.3 14.3 0.1 0.5 1.8 11.1 27.8 97.6 1 ML 0.1
(81-91)
B2 44-49 95.5 2.5 2.0 0.2 0.2 19.3 68.7 7.2 80.4 fs SP 0.3
(112-124)
B2 64-80 90.8 7.3 1.9 4.4 7.0 21.7 43.8 13.9 66.9 fs SP 2.0
(163-203)
B3 40-75 96.7 2.0 1.3 8.1 16.2 42.2 26.8 3.4 33.5 s SP 16.8
(102-193)
B4 16-42 90.9 6.3 2.8 3.6 5.7 23.5 47.9 9.6 67.3 fs SP 3.9
(«ll-107)
42-54 97.6 l.0 1.1 18.9 25.9 34.6 15.1 2.1 19.6 cs GP 61.9
(107-137)
B7 46-61 65.3 33.9 0.8 0.0 0.2 0.5 21.1 43.5 99.2 vfsl SH 0.0
(117-163)
* Refer to Figure 3 for boring locations.
** vcs = very coarse sand (1.0-2.0 mm)
cs = coarse sand (0.50-1.0 mn)
ms - medium sand (0.25-0.50 mm)
fs = fine sand (0.10-0.25 min)
vfs - very fine sand (0.05-0.10 mm)
*** Percent of fine earth with a particle diameter equal to or smaller than fine sand
(0.10-0.25 mm)
-------
-59-
Based on soil conditions and percolation tests, a design loading rate of 5
cm/d (1.2 gpd/ft ) was applied.
A more rigorous re-evaluation of soil conditions at the site revealed
considerable additional Information regarding site and soil suitability. A
detailed assessment of backhoe pits spaced around the system beds showed that
the soils across the absorption bed area are highly variable vertically and
laterally and certainly more complex than previously thought. In fact, the
stratigraphy of the area was too complex to be accurately traceable, beyond
speculation, for even 30 m (100 ft).
Moreover, the north one-third to one-half of the site contained a solum
of highly interbedded very fine sands and silts with truncating layers of
coarser sands and gravels. These very fine sandy loams, loams, and silt loams
contained highly uniform particles, mostly in the 0.02 to 0.20 mm size range.
These materials were often compact and very resistant to vertical flow as
observed in the field. Laboratory measurement of saturated hydraulic
conductivity confirmed their low permeabilities with K . less than 10 cm/d.
By contrast, the medium to coarse sands and gravels 11lery have K . values of
500-1000+ cm/d. s"
The soils to the south half of the installation were distinctively more
sandy and less bedded. Although some stringers or bands of fines (vfs-si)
existed, they appeared to be thin and possibly discontinuous. Vertical
permeabilities of these materials were likely much greater than those of the
north area, due to the vertical continuity of the coarser materials.
The 1976 percolation tests, P4-P6, were confined to soils in the area of
the south bed. Thus, the very rapid rates observed would be expected, given
the soil materials present. The 1981 percolation tests, P1-P3, were inten-
tionally placed in a more hydraulically restrictive soil profile and produced
slower rates (Table 2). However, the rates measured likely reflect horizontal
permeability and not the vertical permeability of the restrictive horizon(s).
By example, the slowest percolation rate measured was 145 cm/d (25 min/in)
compared to the 10 cm/d K . measured by the CRP technique in the finer soil
present (Tables 2 and 3). Tnus, the percolation test is clearly a very faulty
method upon which to site and size a system, especially in soil conditions
such as those present in the north half of the site.
If the system area were considered for subsurface wastewater absorption
under current State policies or generally accepted site evaluation practices
(U.S. EPA, 1980), the northern half might not be found suitable due to soil
mottling, characteristic of seasonal saturation. However, based on the
textural character of the profile (in general sands and sandy loams) and
loading rate similar to that used previously. By comparison, the south half
of the site would very likely be judged as excellent considering the medium or
coarser sandy conditions. Based on the state of the art of site evaluation
these soils suggest no obstacle to system siting and loading at a high rate
(i.e. 5 cm/d). Further, based on the current state of practice these soils
would likely be assumed to provide excellent wastewater renovation.
-------
-60-
Applied Wastewater Characteristics
Wastewater Flow —
The average daily wastewater flow at Westboro was determined by recording
the discharge cycles of the siphons deliverying wastewater to the soil absorp-
tion system. Based upon measurements made between June 24, 1981 and October
5, 1982, approximately one siphon discharge occurred daily representing an
average daily flow of 32.2m /d (8,506 gal/d) (Table A6).
During this study, Beds 2 and 3 were used exclusively while Bed 1 was
rested. With two absorption beds in service, the average daily flow resulted
in a wastewater application rate of approximately 1.4 cm/d (0.33 gpd/ft ) of
bottom area. Under normal operation, the siphon would alternately discharge
2.8 cm/d (0.56 gpd/ft ) to each operating bed, but only every other day.
The daily flow volume measured in this study was roughly 16 percent
higher than the 27.8 m /d (7,345 gal/d) flow determined during 1978 based upon
monitoring of the pumps in the main lift station feeding the siphon chamber
(Otis, 1978). This increase is probably due in large part to the increased
number of connections now contributing to the daily flow. The actual daily
flow of 32.2 m /d measured during this study was equal to only 28 percent of
the design capacity of the soil absorption system.
Wastewater Composition --
The composition of the septic tank effluent (STE) delivered to the soil
absorption system was determined from grab samples collected from the siphon
chamber between September 18, 1981 and May 5, 1983. The STE contained sub-
stantial concentrations of organic matter (168 mg/L BOD,-). suspended7solids
(85 mg/L), total nitrogen (63 mgN/L) and fecal coliform bacteria (10 orga-
nisms/L) (Table A7). The composition of the STE at Westboro was generally
within the range of values determined in previous, studies of individual homes
and multiple-home developments (Tables 4 and A8). Comparison of the STE mass
loadings calculated at Westboro with typical values for raw household wastewa-
ters enabled a crude estimate of the maximum pollutant removals achieved with
septic tank pretreatment: B0Dc - 54%, TSS - 71%, N - 49%, and P - 78% (Table
A9). b
Soil Infiltrability
The infiltrability of the wastewater absorption beds was indirectly
measured through monitoring of the occurrence and magnitude of wastewater
ponding within each bed. Records indicated that ponding was noted in all
three beds as early as June 1979, less than two years after startup. This is
in itself not surprising as subsurface wastewater absorption systems commonly
operate in a continuously ponded state after as little as nine months of
service (Bouma, 1975).
-------
-61-
Table An.
"ail y
Wastswater ?
lev; Volume.
F rein
Period
Tc
Jays
Si piicn
Cycles
Cycl2S/day*
Flow**
(*J/d)
G/2C/81
7/28/81
34. c
58
1 . 70
54.1
7/28/81
0/9^ rOl
W / (.» '
27.7
IS
0. 65
20.7
8/25/81
8/2 7/81
2.05
2
0.98
31.2
10/01/31
1C/20/81
i 0 ?
20
1 .04
33.1
10/22/81
11/18/81
26.5
19
0.71
22.6
11/18/81
12/01/81
13.1
10
0.75
24.2
12/01/81
¦1/27/82
147.5
1 58
. 1.14
36.3
4/28/82
6/OS/8?
40.8
41
1 .00
31 .8
5/08/82
6/1C/82
2.2
2
0.91
29.0
6/10/82
7/13/82
32.8
24
0.73
23.2
7/13/82
7/15/82
2.2
2
0.S1
29.0
7/15/82
8/03/82
19.1
13
0.68
21 .7
(\l
CO
oo
CO
10/5/82
41 .8
37
0.89
28.3
TOTAL
409.6
414
-
-
AVERAGE
(Weighted)
-
-
1 .01
32.2
* 1 eye:
Le = 31 . 8r; t. 3 (
8-15 g)
** £?d = '
:6A.2 x r.'/a.
Table AT. S
eptic 7;
ank Effluent
Compos i tier..
Parameter Urrits
Sanples
Mean S.D.
Mi niinum
Maximum
BCD.
:ng/L
4
168 37
120
204
COD
;ng/L
5
338 42
291
414
TDS
nr,g/L
4
576 84
478
658
TSS
mg/L
4
85 46
31
142
TKN
mcN/L
6
57 10
42
70
NH4M
rcicN/L
6
44 19
11
6C
no3-n
-a g K /_
6
6.4 4.5
0.9
13.0
T?
mcP/L
6
8.1 1.9
6.4
10.8
pH
-
3
-
6.9
7.4
CI"
trrg/L
6
62 28
19
104
CCND.
j* mhos/cm
6
1073 104
968
1245
F. Coli.
Log#/L
4
7.3 0.35
6.92
7.64
F. Strep-
tococci
Log#/L
4
5.7 0.30
5.32
6.00
-------
-62-
Table A3.
Fer Capita Mass
L,c ao i rigs,
gmcd (lb/cap
/d) .
Location
Pcraneto
r l-Jsstborc
WI.
Bend,
OR.
[-11 i de
03.
Mam 1 a,
OA.
Ranue
BCD5
22.8
(3.050)
23.7-35.6
(0.052-0.C73)
21 .
(0.047)
2S .5-40.8
(0.063-0.090)
2" ./•-/iO.C
(0.047-0.090)
TSS
'.1.6
(0.025)
5.4-8.2
(0.012-0.013)
Q 5
(0.021;
11.3-16.2
(0.025-0.035)
5.-1-16.2
(0.0"2-0.0361
TKri
7.8
(0.017)
6.2-9.3
(0.014-0.02C)
9.1
(0.020)
-
6.2-9.3
(0.014-0.020)
* Based upon the data presented ir. Table A7 and Table A.
Table AS.
Estimated Pollutant Remc.
Tank Pretreatment at Wes
'als Afforded by
tboro.
Septic
Parar-.eter
Raw Wastewater*
Septic Tank
Effluent**
Estimated
% Removal
B0Dr
5
35-50 gr.icd
22.8 crr.cd
35-54%
TSS
35-50 gncd
11.6 gmcd
67-772
K
6-17 cncd
8.6 gmcd
0-4 9X
P
3-5 gncc!
1 .1 gmcd
63-78%
* Based upon data presentee in U.S. F.rA, 1980, Table 4-3.
** Mean results from Table A8.
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The first measurements of this study were made on June 23, 1981. These
revealed significant levels of ponding. Beds 2 and 3 were in service at the
time and both were found to have approximately 18 in. (46 cm) of STE ponded
above their bottom infiltrative surfaces. The design of the beds called for
12 in. (30 cm) of gravel above the soil infiltrative surface followed by a
4-in. (10 cm) distribution lateral and 2 in. (5 cm) of gravel on top of that.
This yielded a total bed depth of about 18 in. (46 cm). During the course of
the study, the level of ponding fluctuated somewhat, but both operating beds
remained basically fully ponded.
The average daily wastewater application rate was measured at 1.4 cm/d
based upon bottom area alone. Assuming all of the wastewater applied to the
beds under normal operation infiltrated into the bottom of the beds, the 1.4
cm/d can be viewed as roughly equal to the infiltration capacity of the ponded
absorption system. Even at 1.4 cm/d the infiltration capacity of the beds was
less than 50 percent of an apparently conservative design loading rate of 3.0
cm/d for this site based upon the slowest percolation rate measured in this
study (see Soil and Site Characteristics section). The results of efforts to
determine possible cause(s) for the lower than expected infiltration capacity
are described below.
During installation of the subsurface monitoring instruments, in Bed 2,
the infiltrative surface was visually inspected and sampled. At both lo-
cations within the bed (Figure 3, Point A,C), the gravel was found to be
intimately meshed with the natural sand surface. The gravel was relatively
large (2 in.) and contained some platy fragments. The infiltrative surface in
Bed 2 did not exhibit a surface clogging mat as such, but rather a clogging
zone. The sandy soil was black in color to a depth of approximately 2 to 3 in
(5-8 cm) and was quite wet. By 6 to 8 in., (15-20 cm), however, the sand
had graded to natural coloration and felt only moist. These observations were
consistent with previous work, regarding the visual characteristics of subsys-
tem clogging zones (Bouma, 1975; Anderson, et al., 1982).
Soil samples were collected with depth during installation of the
groundwater observation well at Location A (Figure 3) on November 18, 1981.
Very high volumetric moisture contents (33.9 and 37.8%) were measured in the
upper 4 in. (10 cm) of soil immediately beneath the bottom of the bed. With
the total soil porosity estimated at 40 percent, these data suggest that
nearly all of the soil pores were water-filled. Below this, at 6 to 8 in
(15-20 cm), a much lower moisture content was measured (24.8%). The accumu-
lation of volatile matter (TVS) and nitrogen (TKN) was most pronounced in the
soil near the infiltrative surface and decreased with depth. Similar findings
have been reported previously (Kristiansen, 1981). These results appear to
confirm the visual observations regarding the existence of a zone rather than
a surface mat as responsible for reducing infiltration into the subsoil.
Infiltration of septic tank effluent into soil materials has been shown
to result in soil clogging which leads to large reductions in infiltrative
capacity compared to the natural soil (Avnimelech and Nevo, 1964; McGauhey and
Krone, 1967; Laak, 1970; DeVries, 1972; Rice, 1974; Hargett et al, 1982). The
reduction of infiltration capacity has been attributed to both physical and
-------
-64-
chemical causes. In sands, the primary cause appears to be accumulation of
wastewater suspended solids and biological organisms and their waste products.
Long-term infiltration rates are used to design systems and are supposed to
account for these reductions and represent the infiltrative capacity of a
mature, clogged soil system.
The work of Bouma (1975) led to the widespread acceptance of design
infiltration rates for subsurface wastewater absorption systems based upon
major soil types, usually four: Type I - sands, Type II - sandy loams, loams,
Type III - silt loams, some silty clay loams, and Type IV - clays, some silty
clay loams (U.S. EPA, 1980, Anderson, et al., 1982). Typically the soil type
is determined based upon a percolation test or simple morphological inspection
and a design infiltration rate chosen accordingly. Careful review of the
published results of Bouma (1975) raised questions, particularly regarding the
design infiltration rate adopted for systems installed in sandy soils which
contain substantial percentages of finer sands, silt or clay matter. The
results of Bouma (1975) for the coarse sands studied support his recommended
design infiltration rate of 5 cm/d. However, Bouma's results regarding sandy
loams were variable and do not support the 3 cm/d recommended design infiltra-
tion rate. This recommendation was based upon the measured Soil Moisture
Tensions (SMT's) under three ponded but operating individual home systems
located in gravelly sandy loams (percolation rates of 6, 6 and 30 min/inch).
SMT's of 80, 120 and 65 cm of water, respectively, were measured and combined
with hydraulic conductivity curves for each site to yield calculated infiltra-
tion rates of only 0.4, 0.03 and 1.9 cm/d, respectively. The relatively
higher value for the third system (1.9 cm/d) may have been due, in part, to
the fact that it has been in operation only two years compared to six years
for the other two systems (Bouma, 1975).
The work of Simons and Magdoff (1979) supports lower design infiltration
rates for sandy soils of other than coarse texture. These investigators
recommended a maximum safe infiltration rate of 2 cm septic tank effluent per
day for sands based upon column studies with a sand containing 33 percent
medium and 54 percent fine particles. Otis and Hargett (1983) have also
advocated conservative loading rates. The results of this study at Westboro
suggest that even 2 cm/d may be too high for large bed systems installed in
fine sands.
Subsystem Soil Moisture and Gas Characteristics
Soil Moisture Conditions --
Soil moisture tensions (SMT) were measured at two depths at each of three
locations under and beside Bed 2 (Table 5). All SMT values in this discussion
are expressed as cm of water. The SMT's measured under Bed 2 were similar at
both depths below the infiltrative surface suggesting that flow in the soil
was under unit gradient, due only to gravitational forces. Under the Bed, the
SMT's ranged from 21 to 44 cm. The resistance to infiltration, R, , was
calculated according to the method of Bouma (1975) to equal approximately 60
-------
-65-
days (using an SMT = 34 cm, a ponding head = 46 cm, a clogging zone thickness
= 4 cm, and an infiltration rate of 1.4 cm/d).
The SMT's measured outside of the Bed (Point B) fluctuated from 0 to 61
cm (Table 5). Since no liquid ponding was present over this location and
steady infiltration was not occurring, soil moisture conditions and the
resulting SMT's were no doubt affected by antecedent precipitation and other
climatic influences.
The SMT's measured under Bed 2 at Westboro were within the range of
values measured previously by Bouma (1975). Bouma found that the SMT's under
four ponded but operating individual home systems located in coarse sands
ranged from 23 to 25 cm water while those measured under three ponded systems
located in gravelly sandy loams ranged from 65 to 120 cm water. The latter
SMT values indicated dryer subsurface soil conditions under wastewater absorp-
tion systems installed in finer textured soils. Bouma (1975) suggested that
the dryer subsystem soil moisture conditions were induced by a more restric-
tive clogging zone (Bouma, 1975).
Curves relating soil moisture tension data and hydraulic conductivity can
be derived for a given soil system utilizing the crust-test procedure (Bouma,
et al., 1972; Bouma, et a!., 1974). Typical hydraulic conductivity curves for
four major soil types have been proposed by Bouma (1975) as shown in Figure
A5. Also shown are the approximate hydraulic conductivity and soil moisture
tension data measured in this study. With a knowledge of the soil materials
at the site, the measurements made at Westboro appear similar to those
predicted by the generalized curve of hydraulic conductivity for a sandy soil
material (Figure A6). This is a Type I soil according to Bouma's classifica-
tion. Interestingly, the design loading rate often recommended for wastewater
absorption systems installed in Type I soils is 5 cm/d (Bouma, 1975). The
operating infiltration capacity of the Westboro system, estimated at only 1.4
cm/d, suggests that hydraulic conductivity curves may not be sensitive enough
to predict long-term wastewater acceptance rates.
Soil Pore Gas —
Samples of soil pore gas were collected at the same locations at which
the SMTs were measured (Table 6). Concentrations of CH» gas were 19.1 and
17.9 percent at 30 and 75 cm depths beneath the infiltrative surface of the
bed, respectively. Concentrations of C0? gas were 3.8 and 4.1 percent at
these same depths. Both of these gases are produced as a result of microbial
respiration under anaerobic conditions. Concentrations of CH. and C0?
measured at the edge of the bed were similar to those measured beneatn the
bed, suggesting that little horizontal diffusion of 0, was occurring from the
natural soil surrounding the bed or that it was insufficient compared to the
Op demand exerted. The low concentrations of Op measured were believed to be
dae to atmospheric contamination of field samples during sampling or trans-
port. Previous researchers have reported similar contamination problems
(Lance, et al., 1973). These gas data indicated that the wastewater
percolating through the unsaturated soil beneath the system was very likely
undergoing anaerobic treatment.
-------
-66-
\t)-pe I
Westboro
type 'I isor.dy
H. ; ICO
Rh s 100
type ( I -y
iiilt learn) ^
tciay)
I «0 6 0 sc ICO
Oil MOISTURE TENSICf. (CM WATER)
SOU. MOISTURE POTENTIAL (-CM WATER)
Figure A6. Typical Hydraulic Conductivity Curves (Bouma, 1975)
and the Measurements Made at Westboro, Wisconsin.
-------
-67-
The soil gas composition measured in this study was unexpected based upon
the known results of earlier investigators (e.g., Walker et al., 1973; Sikora
and Corey, 1976). However, previous work had addressed only small single-home
systems. If anaerobic conditions characterize the soil system beneath large
bed subsurface wastewater absorption systems, questions arise concerning the
wastewater infiltration and purification characteristics of these systems.
Much of the data characterizing the performance capabilities of subsurface
wastewater absorption systems has been obtained from laboratory experiments
involving soil columns or small lysimeters (e.g., Robeck, et al., 1963, 1964;
Schwartz and Bendixen, 1970; DeVries, 1972; Lance, 1977; and Tare and Bokil,
1982). The physical and environmental characteristics of the soil systems in
th labs experiments may produce soil aeration dramatically different from that
experienced in the field under operating systems. This may be particularly
true for large rectangular or square bed systems.
Siegrist et al. (1984) suggested that subsystem anaerobiosis may charac-
terize large bed systems since the wastewater-induced oxygen demands can
exceed the oxygen transfer capacity of the system. Anaerobiosis may not occur
initially but may develop after some period of operation when oxygen diffusion
through the infiltrative surface has been reduced to a critical level due to
the effects of soil clogging. Based upon a preliminary review of the
literature, Siegrist et al. (1984) reported that subsystem aeration may
significantly affect the performance of a subsurface wastewater absorption
system, but the nature and magnitude of the effects are known only
qualitatively. The loss of infiltration capacity as a result of soil clogging
was reported to be more rapid under anaerobic conditions, although it was
unclear if long-term infiltration capacity or system longevity are diminished.
Percolation through an anaerobic as compared to an aerobic vadose zone was
found to diminish the degradation of certain pollutants such as organic matter
and ammonia nitrogen, increase that of other pollutants such as phosphorus and
certain complex organics, while having variable effects on the others such as
coliform bacteria.
Groundwater Characteristics
Groundwater Potentials and Flow --
Groundwater potentials were measured over a 22-month period between
August 27, 1981 and October 6, 1982. During this period, absorption beds 2
and 3 were in operation. The shallow observation wells (2.4 m maximum pene-
tration) at sixteen locations across the site were used to determine
groundwater surface elevations (Figure 3, Table A4). The groundwater surface
measured at representative wells over the course of this study are shown in
Figures A7 to A9. As shown, the groundwater surface elevation fluctuated over
the monitoring period. All wells showed approximately the same pattern,
indicating that the variations were seasonal or due to precipitation. Previ-
ous monitoring at Westboro showed similar variations (Otis, 1978).
Only wells 1-15 were installed at Westboro before startup of the system
in September 1977. The groundwater surface elevation at these wells in June,
-------
1500
U95
cz
o
<11
6/
Da t e
Figure A7. Groundwater Surface Elevations Measured at Welly 1, ?, 5, 1 ? and 1'3.
-------
1500
*V e f I 14-3
o
ro
CJ
1490
1185
Da t e
Figure A3. Groundwater Surface Elevations Measured at. Wells 3, 4 and 14 Versus Time-
-------
c
Date
Figure A9.
-------
-71-
1977 is shown on the elevation axis in Figures A7 to A9. These pre-startup
elevations lie within the ranges of elevations measured during this study for
all the shallow wells except wells 13-3 and 14-3. The pre-startup levels of
these two wells are lower than the lowest value measured since startup. These
are also the only two original wells in the immediate vicinity of the beds.
This suggests that the water table has been affected by the continuous appli-
cation of wastewater. Unfortunately, wells 17-22 and A-D were not installed
until well after system startup, and cannot be used to confirm this observa-
tion.
To delineate the groundwater surface and to estimate groundwater flow
patterns the multi-dimensional spline smoothing routine was coupled with a
computer contour plotting routine and graphics system to obtain computer drawn
groundwater contour maps. This computer-mapping technique was compared to
manual mapping methods and judged to be acceptable for the purposes of this
study (Anderson et a!., 1984).
Contour maps of the groundwater surface in the vicinity of the absorption
beds for three dates during the period of this study are shown in Figures 4,
A10 and All. Although elevations fluctuated with time as discussed earlier,
contour patterns stayed very much the same. These patterns indicated multidi-
rectional groundwater flow under the absorption beds. The general flow
pattern was as expected based upon surface topography which slopes from
northwest to southeast and downward to Silver Creek. However, other minor
patterns were also noted. The groundwater surface was elevated in an area
extending from west to Bed 3 to under Bed 3 with groundwater flow radiating
out to the northeast, east and southeast (Figures 4, A10 and All). This
pattern suggested that a small recharge area existed west of Bed 3. A
low-lying marshy area was located approximately 61 m (200 ft) west of Bed 3
(Figure 3) and could have been partly responsible for the pattern indicated.
The groundwater maps also showed flow patterns associated with the drainage
ditch located south of the absorption beds (Figure 3).
In general, groundwater flow patterns measured in this study did not
differ substantially from those reported to exist before and shortly after
system startup (Otis, 1978). However, insufficient monitoring facilities at
that time did not allow delineation of groundwater levels close to the absorp-
tion beds.
Using the groundwater contour maps, Darcy's Law, and the
Dupuit-Forchheimer assumption, the direction and velocity of groundwater flow
at various locations was estimated. Darcy's Law states that:
V=K dH (1)
iT
where, V=velocity, cm/d
K=soil permeability, cm/d
dH=hydraulic gradient, cm/cm
-------
-72-
SILVER Cr
1494.
BED 2
493
1495
8ED 3
1497-
1498.
J L
J I i I I I I I 1 L
I I
I I I* I I I
I I I
I'll
Figure A10. Computer-Generated Contour Map of the Groundwater
Surface on October 22, 1981 (See Figure 3 for
Site Plan; Scale: 1 unit = 3.75 ft).
-------
-73-
I I I I | 1 I I I | I I I I
«•••«• « t
S<+ 99 M — O 9 e
9 9 9 01 9 49 «
+ •+ *r <+ •+ +
-------
-74-
Soil permeability values determined at Westboro varied widely, but a K , of
500 cm/day was assumed to be representative of most of the sandy maten^rs at
the site, based on soil stratigraphy and permeability data (see soil and site
characteristics). Table A10 compares the velocity values estimated for A12
various permeabilities at three locations near the site while Figure 17 shows
the location and direction. These estimates were derived from the groundwater
contour maps for August 17, 1981 (Figure 4 and A12). Although elevations
fluctuated, the hydraulic gradient did not change significantly on other
dates.
The highest velocity occurred directly to the east of Bed 3, the area of
the most permeable soils and greatest hydraulic gradient. Assuming a K . of
500 cm/day (16.4 ft/day) a velocity of approximately 40 cm/d (1.3 ft/day; was
calculated in this area. This value was within the normal range for
groundwater flow given by Todd (1966). At this rate it would take approxi-
mately 115 days for groundwater from under the beds to reach Silver Creek.
The vertical and horizontal hydraulic gradients estimated at three of the
nested well sites are given in Table All. These estimates were based on the
differences in groundwater potentials measured at the nested wells and on the
contour plots. The vertical hydraulic gradient was approximately 7 to 10
times the horizontal gradient based on these estimates. Despite these rela-
tively high vertical gradients relative to the horizontal gradient, the
vertical groundwater flow may be insignificant compared to horizontal flow due
to the fluvial character of the aquifer sediments (Karnauskas and Eisen,
1977).
Mounding of groundwater under wastewater absorption systems has become of
greater concern as the size of such systems has increased to include subdi-
visions and small communities. The critical concern regarding groundwater
mounding is that the water table may rise and reduce the depth of unsaturated
soil to the point where adequate treatment may not occur prior to groundwater
recharge.
Several models have been developed to estimate the groundwater mounding
which may occur due to wastewater application (Allen, 1980; Parker, 1982;
Fielding, 1982; EPA, 1981; Finnemore and Hantzsche, 1983). Most commonly
these models use the Hantush procedure for calculating groundwater rise
(Hantush, 1967). A simplified procedure based on the Hantush method and
applied specifically to on-site wastewater treatment systems has been devel-
oped by Finnemore and Hantsche (1983). This method was used to estimate the
magnitude of groundwater mounding at Westboro as follows.
For groundwater recharge over long time periods (i.e., years) the maximum
water table rise, Zrn, can be estimated by:
Zm = IC r L n r 1 i 0.5n
£ 4
where,
I=wastewater loading rate, ft/day
Ny]
1-0.5n
-------
-75-
Ta'ole A'O. Estimated Horizontal Groundwater Velocities.
Location ** Direction __ K Estimated Velocity
cm/day ft/day cm/cay ft/day
103
"V -i
J . Ji
8
0.3
A
ESE
500
16.4
41
1.4
1000
32.8
82
2.1
100
3.3
4
0.1
3
SE
500
16. 4
19
0.6
1000
32.8
38
1 .2
100
3.3
2
0.1
C
NE
500
16.4
12
C. 4
1C00
32.8
24
0.8
* 3asec upon August 27, 1981 groundwater surface contours.
** Refer to Figure Al2 for locations.
Table ATI. Estimated Horizontal anc Vertical
Hydraulic Gradients at Nested Wells*.
Wei 1 **
m/m
Verti cal
m/m Horizontal
13-1,2,3
0.36
0.05
1-1,2,3
0.17
0.02
2,1,2,3
0.36
0.03
* Based on
** Refer to
potentia
Figure 3
is measured
for* locatic
June 23, "931.
:ns.
-------
-76-
B€
P1
BED 1
P2
R2
PQ
lr,t«i'C«3*0f Oram
B16
BED 2
A ~
BED 3
P6
12
¦ BO
,¦13
Figure A12.
Approximate Velocities Using Darcy's Law and a Soil
Permeability of 500 cm/cay (Based cn contour maps for
August 27, 19Q1 - Figure 4).
-------
-77-
C,n=constants dependent on disposal field geometry, from Table 1 in
Finnemore and Hantzsche (1983)
L=length of disposal field, ft
K=saturated horizontal conductivity of aquifer, ft/d
h =depth of saturated zone. Can assume h=h for approximation
t-time since beginning of wastewater applicStion.
Sy=specific yield of aquifer (0-1.0)
Application of this model at Westboro was based upon the following site data.
1=0.0437 ft/day (1.4 cm/d)
L=260 ft (79.2 m) (both beds)
K=16.4 ft/d (500 cm/day)
h =90 ft (27.4 m)
t-2190 days
Sy=0.20
C=1.7928
n=l.76012
Using Eg. 2, the groundwater mounding under the absorption beds was calculated
to be approximately 18 cm (0.60 ft). This is consistent with the data pre-
sented by Finnemore and Hantzche (1983) according to which groundwater mound-
ing would not be anticipated due to the high permeability (5 m/d) and thick-
ness (27.4 m) of the underlying aquifer. In general, where aquifer thickness
exceeds 12.2 m (40 ft) and permeabilities exceed 1.5 m/d (5 ft/d), groundwater
mounding should be less than 30 cm (1 ft), regardless of system size or
geometry.
Inspection of the groundwater contour plots (Figures 4, A10 and All)
suggested that some groundwater mounding may have been present under the
absorption beds. In an attempt to quantify the occurrence and magnitude of
any mounding, groundwater elevations were measured along a west to east
transect under Bed 2. A profile of the groundwater surface along this
transect is shown in Figure A13. This profile runs from observation well 1-3
to well B. Under Bed 2, the groundwater surface was measured at 90 to 130 cm
(3 to 4.3 ft) below the bottom of the bed. Normally one might expect a
relatively uniform curve of the groundwater surface following that of the site
topography. In contrast, the profile revealed a rapid drop-off in the
groundwater surface just east of the absorption bed (Figure A13). Assuming a
uniform elevation change with distance under the bed if wastewater application
was not occurring, an estimate of the apparent groundwater mounding can be
made by connecting the elevations measured at Well D and B and comparing the
interpolated value with that measured at Wells A and C. Data for July 5, 1983
indicated mounding of 18 cm (0.6 ft) near the center of Bed 2 (Well A) and 64
cm (2.1 ft) near the east edge (Well C).
The discrepancy between the field results and the model predictions are
believed to be due to the non-homogeneous character of the site conditions at
Westboro and the potential site variability of the estimated model parameters.
-------
-78-
Figure A13
Profile of the Grcu mi water Surface Under Bed 2
(Note: Exaggerated Vertical Scale).
-------
-79-
Finnemore and Nantzche (1983) emphasized that their model was only applicable
to flow systems involving a single permeable layer with a horizontal
impermeable boundary underlying it. They also stated that available models to
predict groundwater mounding were theoretical and in need of field verifica-
tion.
Groundwater Composition --
The qualitative characteristics of the groundwater in the vicinity of the
wastewater absorption beds were determined through sampling of 25 groundwater
monitoring wells (Figure 3). The results of this monitoring are tabulated in
Tables A12 to A14.
The multi-dimensional spline smoothing routine was also applied to the
groundwater quality data to map contour levels for the parameters measured.
Figures A14 and A15 show contour maps for total dissolved solids (TDS) and
ammonia nitrogen (NH.) on August 27, 1981. The computer maps all showed peak
levels of the plotted parameters at the southeast corner of Bed 3, decreasing
radially from there. This agrees with the well data and groundwater flow
results discussed earlier.
Based on the flow patterns developed from the groundwater elevation
analysis, several wells were chosen to characterize the upgradient groundwater
duality. Wells 1-3, 3, and 21 were most suitably located for this purpose
(Figure 3}. The mean concentrations of the parameters monitored are shown in
Table A15. The values are generally consistent between wells and indicate the
background water quality in the top 1.8 to 2.4 m (6 to 8 ft) of the
groundwater. Differences in concentrations of chloride and conductivity may
be due to well spatial location or penetration into the groundwater. All
parameters were generally below the recommended values for drinking water
(Bouwer, 1978).
The monitoring wells located close to the absorption beds revealed
significant increases in the concentrations of measured parameters in the
groundwater relative to background levels (Table 7). In particular, samples
from the shallow wells 13-3, 18 and 19 located near and downgradient from Beds
2 and 3 exhibited mean concentrations of total dissolved solids, chlorides and
conductivity not significantly different ( = 0.05) from the concentrations
in the applied wastewater (Table A7 and A15). These are conservative
parameters which should not be attenuated or removed in the soil treatment
system. The fact that the concentrations in the shallow groundwater were
equal to that of the applied wastewater indicated that the monitoring wells
were installed so that samples were withdrawn within the effluent plume.
The mean ammonia nitrogen concentrations measured at wells 13-3, 18 and
19 were 45.7, 41.5 and 33.1 mg N/L, respectively (Tables 8 and A13). These
values were approximately 100 times greater than background levels and not
significantly different from the NH.+ concentrations of the applied wastewater
(Table 8). These data indicated that little nitrification of the applied NH.+
was occurring.
-------
-80-
Table A12. Qualitative Characteristics of the Groundwater in the
Vicinity of the Wastewater Absorption Bees - Dissolved
Solids, Chlorides and Conductivity*.
TOS CL" Conductivity
Well"* Sample ; ng/L) (ng/L) (pmhos/cn)
Elevation"*" n 7 s.d. —n x s. d. ~~n 7 s. d.
1-3
1496.4
7
170
30.7
9
14.3
4.0
6
150
19
-2
1493.0
7
180
39
a
15.6
4.4
6
161
15.7
-1
147C.6
7
295
no
9
2.4
1 .0
6
234
11
2-3
1491.6
7
406
101
2
51 .0
22
6
437
162
-2
14S3.9
7
471
53
c
50.4
9.0
6
6S6
37
-1
1471.7
7
273
41
c
2.6
0.6
6
238
18.7
3-1
1493.6
1
(
157
34.4
o
2.1
1.0
6
95
26
5-2
1467.2
6
343
119
8
5.2
1.5
5
476
106
-1
1477.2
6
435
93
8
20.2
8.6
5
576
1 66
11-1
1474.9
6
216
81 .6
0
u
3.9
0.9
5
2C7
87.7
12-1
1481.5
7
4C7
64
9
25
12
6
551
123
13-3
14S8.1
7
495
73
8
65
6.2
6
1015
104
-2
1477.9
7
341
90
8
14
5.5
6
470
107
_ i
1467.1
7
173
S 1
9
3.4
2.3
6
219
14.3
14-3
1489.0
7
245
114
9
24
9.9
5
271
3C
-2
1477.0
6
244
99
9
13
1.1
6
303
20
-1
1463.4
7
235
114
8
5.3
4.2
6
221
12.7
15-2
1487.5
6
380
134
8
20
16.4
5
602
149
-1
1477.2
5
218
9C-
8
4.3
2.6
4
286
15
17
1493.4
2
3C5
-
4
41 .4
4.4
2
555
-
18
14S0.4
7
467
90
9
63.1
10.6
6
1025
115
19
1493.4
7
480
27
9
66
5.5
6
960
76
20
1490.0
7
404
46.8
9
56.1
17.5
6
538
114
21
1492.4
6
155
36
6
10.7
3.6
?
105
9.2
22
1491 .3
6
251
69
7
25.5
13.2
4
363
5c.4
* Based upon grab saT.oles collected between August 27, '931 and
October 6, 1982.
** Refer to Figure 3 for well locations.
+ Taken as the elevation of the middle of the well screen.
-------
Table
A1 3. Gualitat
IV s
C'r.ar
acterist
ics
of the
Groundwater ir.
the
Vicinity
of
the
Wastewat
e p
A'osorpti
.on Bed
• Mi trogen*.
Sample
TKN
NH4+
NO3-
Wei 1**
El evat":on+
l
(:i;g-N/L)
^rrg-N'/Lj
(r.c-N/
l;
Ir't)
A
s. d.
n
X
s.a.
n
J
3.d.
1-3
K95.4
5
0'. 62
0.19
3
0.21
0.17
5
2.74
0 . 4 c '
-2
K93.0
6
0.62
0.19
9
0.5
0.07
9
2.30
0.93
-1
1170.6
6
0.66
0.21
Q
0.24
0.24
9
1.67
0.58
2-3
U91 .6
6
2. CO
0.38
9
0.58
0.25
9
16.09
9.92
-2
U83.9
6
1 .05
0.85
9
0.17
0.08
9
: .70
0.74
-1
1471.7
6
3.21
0.88
9
1 .58
1.13
9
1 .27
0.65
3-1
1493.6
6
0.78
0.30
9
0.30-
0.11
9
2.01
1 .23
5-2
1487.2
5
2.75
0.30
8
2.10
0.58
8
1 .55
0.74
-1
1477.2
5
3.50
0.50
8-
3.29
0.72
8
1.31
0.73
11-1
1474-9*
5
0.68.
0.25
8
0.15
0.08
8
1 .51
" .01
12-1
1481'.6
6
1 .23
0.34
Q-
0.72
0.21
9
1 . 7C ¦
: .10
13-3
1488.1
6
50.08
i 5.53
9
45.7
6.51
9
2.52
¦ .55
-2
1477-9*
6
2.02
0.29
Q
1.27
-.71 *
9
1 .:o
0.74
:1
1467-1*
6
1.13
0.26
Q
0.88'
0.54
n
0.92
0.62
14-3
1489.0
6
2.15
1.17
G
1 .28
0.85
9
4.48
2.81
-2
1477.0
6
0.72
0.35
9
0.31
0.29
Q
1 .52
0.73
1463.4
5
0.94
0.43
8
0.91
1 .28
8
1 .36
0.92
15-2
1487.5
5
0.78
0.32
8
0.25
0.21
8
1 .40
C-. 7 5
1477.2
5
0.56
0.18
8
0.40
0.44
8
1 .54
C. 86
17
1493.4
2
0.78
-
5
1 .02
1 .05
5
7.54
-
18
1490.4
6
49.3
5.44
9
41 .5
4.1
9
4.28
4.50
19
1493.4
6
40.6
8.0
9
33.1
7.6
9
1 .64
C.ol
20
1490.0
6
2.72
1 .69
9
3.0:
2.08
9
1 .22
C. 37
21
1492.4
5
0.62
0.13
8
0.30
0.37
7
2.5
C .55
22
1491.3
V
0.52
0.20
8
0.20
0.15
8
4.13
3.42
* Based upon grab samples collected between August 27, 198" and
October 6, 1?S2.
** Refer to Figure 3 for well locations.
+ Taken as the elevation of the middle of the well screen.
-------
Table A"A. Qualitative Characteristics of the Groundwater in the
Vicinity of the Wastewater Absorption Beds - Phosphorus,
Chemical Oxygen Demand and Fecal Conforms*.
We 11**
Sanple
Elevation
TP
(r.g-P/i
; \
i
COD
(mg/L)
(
eca". Coli
*/100 r.L)
n
A
s.d.
n
r. i n.
max.
n
["in.
max
1-3
1496.4
5
0.08
C.05
2
<5
5
6
0
100
- 2
1493.0
5
0.09
0.05
n
L
<5
<5
6
0
10
-1
1470.6
5
0.?1
0.21
1
15
-
5
0
36
2-3
1491.6
5
0.07
C. 04
2
12
35
7
0
8
-2
1483.9
5
0.05
0.C3
2
17
19
7
0
2
-1
147; .7
5
0.42
0.33
1
57
-
0
2
3-1
1493.6
5
0.07
0.C4
2
11
35
7
0
10
cr_?
1487.2
c,
0.11
0.10
2
26
35
5
0
<1
-1
1477.2
5
0.58
0.43
2
16
19
5
0
4
11-1
1474.9
5
0.07
0.05
1
<5
-
4
0
58
12-1
1481.6
5
0.13
c.r
2
19
38
6
0
<1
13-3
1488.1
5
0.11
G. 07
2
55
84
6
0
8
-2
1477.S
5
0.30
0.08
2
<5
49
6
0
<1
-1
1467.1
5
0.15
C-.01
2
<5
14
6
0
Q
14-3
1489.C
5
0.24
0.10
2
12
38
7
0
3
-2
1477.0
5
0.34
0.25
2
<5
52
6
0
<1
-1
1453.4
4
0.32
0.29
2
<5
35
5
0
<1
15-2
1487.5
t
0.05
0.04
2
22
44
5
0
<1
-1
1477.2
c
0.19
9.06
2
13
24
5
0
J
1?
1493.4
2
C. 23
-
1
26
-
3
0
n
u
18
1490.4
5
C.12
0.16
2
37
58
0
3
19
1493.4
5
0.10
0.14
2
64
86
6
0
3
20
1490.0
5
O.C-3
0.01
?
24
29
7
G
28
21
1492.4
5
0.03
0.02
2
<5
17
6
0
640
22
1491.3
4
C. 03
0.02
1
20
_
6
0
24
* Based upon grab samples collected between August 27, 1981 and
May 5, 1933.
** Refer to Figure 3 for well locations.
+ Taken as the elevation of the middle of the well screen.
-------
Table A"5. Qualitative Characteristics of the Shallow Groundwater
Upgradient of the Absorption Beds*.
Background .DS CL" Cond. TKM NO. TP COD Fecal Col-.
Wel hc/l xg/L urcho rng-N/L mc-N/L nc-N/L mg-P/L ir.g/L «/100 nL
] -3
1 70
14.3
150
0.62
0.21
2.7
0.03
5
0-3CC
3
157
2.1
95
0. 78
0.30
2.0
0.Q7
23
0-10
21
155
10.7
106
0.62
0.3C
2.5
0.03
11
0-64C
* Based upon data presented ir. "able?. A*2-A1A.
** Refer to Figure 3 for well locations.
-------
-84-
The mean concentrations of phosphorus (P) measured at wells 13-3, 18 and
19 were not significantly different than background levels at well 1-3 (Tables
8 and A14). Mass balance calculations were made to determine if the apparent
immobilization of P was reasonable. Assuming an influent P concentration of
8.1 mg/L (Table A7) and a daily loading of 32.2 m3/d (Table A6), approximately
381 kg of P would have been applied as of Fall 1981. gThe unsaturated soil
volume beneath beds 2 and 3 was estimated at 3.5 x 10 grams (79.2 m x 30.5 m
x 0.9 m = 2174 m3; Bulk Density = 1.6 g/cm3). Thus, the apparent P
immobilization was estimated to equal approximately 109 ug/g soil. This value
is well within the range of values presented previously by Sikora and Corey
(1976).
A limited number of samples (two from each well) were analyzed for
chemical oxygen demand (COD). Mean concentrations measured varied from 37 to
86 mg/L compared to 338 mg/L in the applied wastewater and 5 mg/L in the
background groundwater at well 1-3 (Tables 8 and A14). These data suggested
that significant attenuation of COD occurred.
The maximum concentration of fecal coliform bacteria measured at wells
13-3, 18 and 19 was 8 organisms/100 mL (Table A14). This maximum concen-
tration was lower than that measured in the background wells (wells 3-1, 21,
3).
The groundwater quality degradation exhibited in the immediate vicinity
of the beds appeared to be limited to the upper layer of the groundwater. For
example, samples collected from nested wells at well location 13 exhibited
decreasing levels of the measured parameters with increasing depth. Levels
measured in the deepest wells (6.4 m deeper than the shallow well) approached
background quality (Tables A12-A14).
At well 12, approximately 32 m (105 ft) downgradient from the edge of Bed
3 (Figure 3), the concentration of various measured conservative parameters
were lower than those measured at wells 13-3, 18 and 19 but still significant-
ly higher ( * = 0.05) than background levels (Table A12). There are at least
two possible explanations for these data. It may be that the effluent plume
found in the immediate vicinity of the beds was dissipated somewhat, probably
by dilution and dispersion within the groundwater. Another explanation is
that samples withdrawn from well 12 may not have been from within the effluent
plume but on its fringe.
The mean concentration of ammonia nitrogen measured at well 12 was only
0.72 mg-N/L, but still significantly higher than the mean level at well 1-3 of
0,21 mg-N/L (Table A13). The mean concentration of phosphorus was not signif-
icantly different from the background levels at well 1-3. The chemical oxygen
demand (COD) measured in two samples from well 12 were 19 and 38 mg/1 (Table
A14). The COD concentrations were higher than the 5 mg/L or less determined
in two samples from well 1-3, a background well. Fecal coliform bacteria were
not detected in the groundwater samples extracted from well 12 (Table A14).
The groundwater quality data from well 12 revealed reductions in the
elevated levels of potential pollutants measured in the groundwater near the
-------
-85-
SILVER Cf
BE
BED 3
390
380
330
300
o
Figure _A14. Computer Generated Plot of Total Dissolved Solids
Levels on August 27, 1981 (Scale: 1 unit = 3.75 ft).
-------
-86-
- u
tr
LU
. w
— N
I 1 I 1 1 I I I I I I 1 I I I I 1 I I I ' I I I I 1
Figure A15. Computer Generated Plot of Ammonia Nitrogen
Levels on August 27, 1981 (Scale: 1 unit =
3.75 ft).
-------
beds (Table 8). These data suggest that pollutant attenuation through di-
lution and dispersion or removal and degradation may have been achieved within
the saturated zone. The groundwater extracted from well 11, approximately
82.3 m.downgradient and adjacent to Silver Creek, exhibited water quality
essentially equal to that of the upgradient groundwater (Tables 8 and
A12-A14).
Nitrogen Transformations --
The groundwater quality data collected in this study regarding ammonia
nitrogen levels clearly demonstrated that anaerobic conditions were present
under the wastewater absorption beds. As shown in Table 8, the mean ammonia
concentrations in the groundwater increased from near zero at well 1-3 to over
40 mgN/L at wells 18 and 13-3. These data were consistent with the soil
atmospheric data which revealed high levels of methane gas in the soil atmo-
sphere under Bed 2 (Table 6).
Early post-startup monitoring of the absorption system (Fall, 1977)
demonstrated higher concentrations of nitrate nitrogen at well 13-3. Figure
A16 depicts the concentrations of nitrate and ammonia nitrogen measured over
time since system startup in this well. Since wells 17-22 were not installed
until 1979, well 13-3 was the only shallow well in the immediate vicinity of
the beds. The figure shows NCL levels in the groundwater from well 13-3
increasing after system startup, however, nitrate levels started decreasing
while ammonia nitrogen levels increased. This trend continued until nitrate
levels stabilized at levels consistently below 5 mgN/L and ammonia nitrogen
levels at levels above 30 mgN/L. In June of 1979, continuous ponding was
noted in all three beds at Westboro (see following section on operation and
maintenance). This corresponded to the time when ammonia levels were most
rapidly rising in well 13-3 (Figure A16). Once the infiltrative surfaces of
the absorption beds became continuously ponded, the area available for oxygen
diffusion into the soil was greatly reduced. It was theorized that continuous
ponding caused anaerobic conditions to develop under the beds at Westboro,
resulting in the high ammonia nitrogen and low nitrate nitrogen levels
measured in the underlying groundwater.
Surface Water Quality
Water quality monitoring in Silver Creek was conducted upstream and
downstream of the wastewater absorption beds (Figure 3). Though various
concentrations of potential pollutants were measured, levels upstream and
downstream of the system were not significantly different (Table A16). This
is not surprising since the minimum stream flow of Silver Creek is approxi-
mately 750 times greater than the wastewater flow (Carl C. Crane, 1976).
Assuming complete mixing in the stream prior to the downstream sampling point,
to raise the concentration of a pollutant 1 mg/L in the stream would require
an input concentration of 750 mg/L.
Surface water samples were also collected from the intermittent flow in
the drainage ditch located approximately 18 m (60 ft) south of Bed 3 (Table
-------
Continuous Pond
ng
12/ 12/ 12/ 12/ 12/ 12/
77 W8 79 80 81 x82
Date , mo./yr.
Figure 1116. Ammonia and Nitrate Nitrogen Concentrations at Weil 13-3 Versus Time.
-------
Tabic A10. Surface Water Quality in the Vicinity of the Wastewater Absorption Ucds.
Location
Stati stic
TDS
(ng/L)
Cl-
(nig/L)
Cond.
(ijPlhOS/C.Tl)
TKN
(mg-N/L)
NH
(mg-N/L)
NO. P
(rrg-fvL) (mg-P/L)
COD
(mg/L)
T. Col i.
(#7100 mL)
Si 1vcr
Mean
1 54
3.0
108
1 .55
0.16
1 .36
0.06
66.5
Creek -
S.D.
57.8
1 .2
23
1 .49
0.05
0.65
0.0b
-
-
Ups tream
Min.
55
2.0
85
0.7
<0.1
<0.1
0.02
33
88
Max.
222
5.0
138
4.2
3.4
2.0
0.13
100
720
n
6
8
6
5
7
8
'J
2
5
Si 1vcr
Mean
169
2.3
108
0.59
0.50
1 .50
0.05
43.5
-
Creek -
S.D.
43.0
0.7
25
0.29
0.80
O./O
0.03
-
-
0 own st. re am
M i n.
100
2
90
0.3
0.1
0.9
0.02
20
M
Max.
215
4
144
3.8
2.0
2.0
0.08
67
6,100
n
7
9
5
6
9
9
5
2
6
Urainage
Moan
350
28
440
5.6
5.3
7.6
0.03
23.5
-
Di tch
S.D.
55.6
9
39
1.9
1.9
1.2
0.03
-
-
Min.
292
13
396
3.6
3.3
6.0
0.01
15
<1
Max.
n
42§
rs5
484
5
8.1
5
8.1
5
9.3
5
0.0/
4
32
2
1 ,780
5
-------
-90-
A16). These data indicated higher concentrations of most parameters compared
to those measured in Silver Creek. The ditch bottom was at a lower elevation
than the bottom of the absorption beds and adjacent groundwater levels and
groundwater flow patterns indicated flow toward the ditch. Percolating
effluent form the absorption beds could have seeped into the ditch after soil
treatment or dilution via the groundwater. Since this ditch collected runoff
from nearby fields and the discharge from an interceptor drain, it was impos-
sible to ascertain the impact, if any, due specifically to the absorption
beds.
Operation and Maintenance
Some uncertainty surrounded the operation and maintenance characteristics
of the wastewater absorption beds between start up in September, 1977 and the
initiation of this study in June, 1981. The design concept was to have two
beds in service at any time, annually rotating a third resting bed into
service. In practice, this has not been the case. Due to a malfunction of
one siphon, Bed 1 received all of the wastewater during the first year of
operation (estimated to be 2.8 cm/d). After correcting this problem in Fall,
1978, Bed 1 was taken out of service and Beds 2 and 3 were put on line. Beds
2 and 3 have been used almost continuously since that time. Attempts to
utilize Bed 1 have been unsuccessful as it becomes continuously ponded soon
after putting it on line. This is probably due in large part to the soil
conditions in the area of bed 1 which were identified as poorly suited to long
term wastewater absorption (refer to discussions of site and soil characteris-
tics).
Periodic malfunctions of the siphon dosing system have occurred since
startup. Constant "dribbling" of the siphon feeding Bed 1 was noted within
the first year of operation. Investigation revealed that a faulty siphon
assembly was responsible for the problem. This was corrected during Spring,
1978. While Bed 1 was out of service, the two siphons discharging to Beds 2
and 3 should have automatically alternated discharges to the two beds.
However, on several occasions during this study, the siphon dosing system was
found to be malfunctioning. Wastewater would dribble out one or the other
siphons, often the siphon supplying Bed 3. This condition would persist until
air was pumped under the siphon bell. During Fall, 1983, the problem was
corrected by cleaning and tightening all joints in the piping.
After discovering that the absorption beds were continually ponded, it
was suspected that the beds were being adversely impacted by large amounts of
subsurface water moving laterally downslope into the systems. To correct this
an interceptor drain was designed and installed during Fall, 1980. A per-
forated 10-cm (4-in) diameter pipe was installed in a gravel filled trench.
The trench was dug along the west edge of the beds to the drainage ditch
running along the south side of Bed 3 (Figure 3). The drain pipe was laid
with its invert at the north terminus set at an elevation of approximately
1500.5 ft (457 m). The pipe was laid with approximately 0.2% slope to the
outlet point. Thus, the invert of the drain was roughly 3.2 ft (1.0 m) below
the bottom of the absorption beds.
-------
During the spring and fall seasons, the interceptor drain discharged a
small amount of water. This was believed to be shallow, laterally-flowing
subsurface water which occurred intermittently in response to snow melts and
rainfall. Three grab samples of the discharge from the interceptor drain
exhibited characteristics similar to that measured at well 1-3.
-------
TECHNICAL REPORT DATA
(Please read tnstmcHor*- on the nvcrsi befun curtplcttng/
1. REPORT NO.
EPA/600/2-86/02 3
4 TITLE AND SUBTITLE
Large Soil Absorption Systems for Wastewaters from
Multiple-Home Developments
3. RECIPIENT'S ACCESSLONnNO
M-*?
5 REPORT DATE
Fe bruar.y 19.8 6_
^ 7M>
6 PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Siegrist, Robert L., Anderson, Damann L.5 and
Hargett, David L.
8 PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
RSE Group
Sci enti sts/Engi neers
2445 Darwin Road
Madison, WI 53704
10. PROGRAM ELEMENT NC.
AZB1B
1 1 CONTRACT/GRANT NO
68-03-3057
i
12. SPONSORING AGENCY NAME AND ADDRESS
Water Engineering Research Laboratory—Cincinnati, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final (1980-1983)
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
JamesyF. Kreissl FTS 684-7611 (513) 569-7611
performance of large soil absorption systems for treatment and disposal
of wastewaters from multiple-home developments. The objectives were to
investigate absorption system performance and identify potential deficiencies
in presently u^d design criteria. Where possible, recommendations regarding
more appropriate design and operation practices were to be made.,.
The study was conducted in three parts. _,A survey of state regulatory
agencies was conducted to enable characterization of the distribution,
regulatory structures, design restrictions and state attitudes associated.""
with multiple-home systems. An in-depth field investigation of the community
wastewater absorption system at Westboro, Wisconsin was conducted between
June 1981 and May 1983.r"CThis work included delineation of the soil and site
characteristics and monitoring of the applied wastewater, abosrptlon system
soil infiltrability, subsystem soil .moisture and pore gas, and groundwater
impacts. The final part of this study included a more casual field investi-
gation of six multiple home systems located in the State of Washington. The
basic characteristics of each system were determined including the facility
design, soil conditions, applied wastewater character and the absorption
system performance.
17.
KEY WORDS ANO DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED Tfc.RMS
19 SECURITY CLASS (This Report)
Unclassified
c. cosati Held/Group
18. DISTRIBUTION STATEMENT
Release to Public
20. SECURITY CLASS (This pagei
Unclassified
21. NO. OF FAGES
104
22. PRICE
EPA Foim 2220-1 (Rev. 4-77) previous eOiTiON is OBSOLETE
i
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