United States
Environmental Protection
Agency
Office Of Water
(WH-553)
EPA841-F-93-001
April 1992
Number 1
&EPA TMDL Case Study
Denver Metro — The South
Platte River Segment 15
Key Features:
Project Name:
Location:
Scope/Size:
Land Type:
Type of Activity:
Pollutant(s):
TMDL Development:
Data Sources:
Data Mechanisms:
Monitoring Plan:
Control Measures:
Revision of TMDLs to meet Water
Quality Standards
Denver Metro — The South Platte
River Segment 15
EPA Region Vffl/Denver, Colorado
River, drainage area 380 mi2
Smooth to irregular plains
Urban
Toxic ammonia (NH3), BOD/DO,
toxics, metals
PS, NPS, Toxics
Site-specific data from NPDES
permittee and localities
STREAMDO and Colorado Ammonia
Model
Yes
NPDES permit
Summary: In 1986, low dissolved oxygen (DO) and the
presence of toxic ammonia, other toxics, and metals convinced
the Colorado Water Quality Control Commission to identify
Segment 15 of the South Platte River as water quality unpaired
and a high priority for TMDL development. EPA Region VIII
developed TMDLs for the segment after assuming authority to
issue the NPDES permit for the Denver Metro Wastewater
Reclamation District's Central Facility. Although numerous
point sources discharge to the river upstream from Segment 15
and nonpoint source pollution was also known to contribute to
its pollutant load, these were considered insignificant when
compared with the discharge from the Central Facility at low
flow. Almost the entire flow of the South Platte is diverted
immediately above Metro's discharge. Using the extensive data
that were key to identifying the water quality problems in this
segment of the South Platte, TMDL development proceeded for
metals, nitrates, nitrites, and ammonia. Ammonia was of
primary concern because it contributed to both un-ionized ammonia toxicity and DO problems. Wastelpad allocations
based on the TMDLs were incorporated into the Central Facility's NPDES permit when it came up for renewal in 1986.
Unfortunately, water quality data gathered since then indicate that DO continues to be below required concentrations in
sections of Segment 15. As a result, the Region is now working to revise the TMDLs for those parameters that affect DO
so that the DO standard will be achieved throughout the reach. The revised TMDLs may include "non-chemical"
components, such as habitat restoration.
FtUptcn
BwrUk*
Ammorit Toneity PtBtH«n»
FIGURE 1. South Platte River/Segment 15
Contacti Bruce Zander, U.S. EPA Region Vffl, Water Division, 999 18th St., Ste. 500, Denver, CO 80202-2466,
- phone {303)293-1580 - ' " > ' '
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BACKGROUND
Programmatic Issues
The Metro Wastewater Reclamation District (Metro
District) provides wastewater treatment for a major portion
of metropolitan Denver and portions of surrounding
Adams, Arapahoe, and Jefferson Counties. It consists of
20 municipalities and service districts that are "Member
Municipalities," 23 "special connectors," and 17
"connectors to connectors," discharging approximately 210
million gallons per day (mgd) into the South Platte River
near Sand Creek (Figure 1). The river below this
discharge is considered to be effluent dominated because
estimated background flow is only about 20 mgd.
The Colorado Department of Health is delegated to issue
permits in the State of Colorado. EPA Region VIE
assumed authority to issue an NPDES permit for the
Denver Metro municipal facility in 1986, however, because
of the facility's consistent violations under the Clean Water
Act (CWA). When excessive nutrients, low dissolved
oxygen (DO), and the presence of toxic ammonia, other
toxics, and metals convinced the Colorado Water Quality
Control Commission (WQCC) to target Segment 15 of the
South Platte River as a high priority for TMDL
development, the Region established TMDLs to address
these water quality problems. Wasteload allocations
(WLAs) were incorporated into Denver Metro's NPDES
permit Metro District was the major cause of water
quality impairment
TMDL development begins with a thorough evaluation of
available data and information. The Region evaluated the
WLA that had already been established for Denver Metro
by the Department of Health. It was deemed insufficient.
The Region also considered the quality and abundance of
available data to determine whether the data were adequate
to calculate a TMDL and allocate pollutant loads for each
parameter with a reasonable assurance that water quality
standards would be met The data appeared sufficient, and
TMDLs were promulgated. A new NPDES permit, with
limits based on the TMDLs calculated for each pollutant of
concern, was issued to the Denver Metro facility in
December 1986 (USEPA, 1986).
NOTE: Had there been substantial uncertainty that any
TMDL would result in the attainment of water quality
standards, a phased approach to TMDL development could
have been chosen. Under this approach, a formal
monitoring plan is adopted as part of the TMDL to assess
attainment of standards and to support revision of the
TMDL if standards are not attained,
Denver Metro—the NPDES permittee—installed pollution
controls'and collected data over the next 4 years so that the
effectiveness of these controls could be evaluated. A
review of the data in 1990 revealed that past problems with
chlorine toxicity and ammonia toxicity appeared to have
been resolved by upgrades of the Central Facility. The
data also indicated that low DO concentrations continued to
be a problem. Water quality standards had not yet been
attained. (The DO profile along this segment of the river
is shown in Figure 2.) This information forced a
reassessment of the TMDLs for those parameters that affect
DO in the stream, followed by a decision by the Region to
update those TMDLs. The possibility of developing new,
phased TMDLs that incorporate non-chemical parameters is
currently being discussed because, while water quality
problems are largely the result of various point source
discharges, habitat degradation is a significant exacerbating
factor.
To facilitate development of the new TMDLs, the State and
EPA Region VIII are working closely with Metro District
to collect and interpret additional data, as well as to model
water quality. EPA's Office of Water is also continuing to
review new approaches, technology, and tools to better
assist in implementing the TMDL process (USEPA,
1991b). Metro District is funding the monitoring,
modeling, and site-specific criteria development for the
TMDLs. .
CfHTftAL PLAMT
' 70 MOO NTTNffO fWtUWT
US MGO UCONOAJIY tmjJCMT
A
I PutMn
own
A A A
tm urn natty
A
il«n
Miles below Burlington Ditch Headgate
FIGURE 2. Dissolved Oxygen Profile for South Platte
River Segment 15 (Camp, Dresser & McKee, 1992)
The Resource
The South Platte River originates in the center of Colorado
and flows generally northeast for 270 miles to Nebraska.
Approximately 65 percent of the population of Colorado is
concentrated in a 30-mile-wide strip along the South Platte
River, beginning 18 miles south of Denver and extending
80 miles northward. The land type in the watershed is
mainly smooth western high plains. Natural vegetation on
the plains tends toward gana, or buffalo grass. The soil
types typical of this area are dry Mollisols (Omernik,
1987). The Denver area receives approximately 12 to 16
inches of annual rainfall, and annual natural runoff in the
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area ranges from 0.1 to 1 inch (USGS, 1985). Runoff
from urbanized areas, however, is greater. Low-flow
conditions are most common from July to October, and
high flows generally occur during the spring (May to June).
Segment 15 of the South Platte River, shown in Figure 1,
flows north from the Denver metropolitan area to Fort
Lupton, Colorado. This segment of the river extends 26
miles and its drainage area is approximately 380 square
miles.
Upstream of Clear Creek, Segment 15 is characterized by
heavy commercial and industrial land uses. Along
Segment 15 itself there are active gravel mines, flooded
gravel mines, pasture lands, and agricultural lands. Erosion
control, which has extensively modified the upstream
channel of Segment 15, has negatively affected the riparian
zone, river hydrology, and assimilative capacity of the
river. Dewatering flows from gravel-mining operations
along the river contribute sediments and also affect the
river hydrology. DO problems tend to occur in large
ponded areas, which are a result of in-stream gravel mining
and small dams built for irrigation withdrawal and utility
line protection. Low species diversity throughout segments
of the South Platte indicates that poor water quality and
habitat degradation are impairing the health of aquatic
communities.
Figure 1 illustrates the major pollution sources and quality-
impaired areas along Segment 15. The location of point
source inflows, nonpoint source loadings, irrigation return
flows, tributary flows, and water supply withdrawals along
the segment are indicated in Figure 3.
The WQCC has classified Segment 15 for the following
uses: (1) Class 2 warm water aquatic life, (2) Class 2
recreation, (3) water supply, and (4) agricultural use. Table
1 presents the water quality standards that were in effect at
the time of TMDL development so that the river reach
would support these uses. It is possible that in the future
the WQCC will adopt more stringent standards for un-
ionized ammonia and a tiered standard for DO. Effluent
limits for phosphorus are not currently in effect for Metro
District's Central Facility. In addition, the State and EPA
are investigating the development of site-specific standards
for DO in the South Platte River. Changes in water quality
standards could require TMDL revision.
ASSESSING AND CHARACTERIZING THE
PROBLEM
Targeting and Prioritizing
Although each State decides how to prioritize impaired
waters for TMDL development, the CWA provides that the
severity of pollution and the intended uses of a waterbody
be considered. EPA policy guidelines (USEPA, 1991a)
suggest additional criteria. TMDL development for
Segment 15 of the South Platte River was targeted and
assigned a high priority because (1) available data indicated
water quality was impaired, and (2) there was an urgent
need to reissue the Metro Facility's NPDES permit
Monitoring and Data Bases
Denver Metro conducted voluntary ambient monitoring at
various points along Segment 15 for many years prior to
1986 in order to assess the water quality impacts of its
discharges to the river. This information provided the first
indication that there was a DO problem in sections of the
reach, prompting EPA Region VJII to initiate more
intensive water quality monitoring. EPA and the State
conducted a joint data collection effort for approximately 6
months in 1985. The monitoring revealed that, in addition
to low DO, the problems included chlorine and ammonia
toxicity, as well as high concentrations of various metals.
The data from Denver Metro's ambient monitoring, the
EPA/State intensive monitoring efforts, the water quality
records from the South Adams County Water and
Sanitation District and the City of Brighton, and effluent
chemistry data from wastewater treatment facilities on
Segment 15 were combined to form a fairly complete data
base, with a period of record beginning in 1980, for
numerous water quality parameters. This data base was
very useful in characterizing water quality trends in
Segment 15 over time and in completing TMDL analyses
for the pollutants of concern.
After 1986, Denver Metro's ambient monitoring program
was formally modified to provide more complete water
quality data and information. Weekly water quality data
are now collected at designated sampling locations along
the South Platte River, including Segment 15, and in
certain tributaries. In addition, Denver Metro collects a
series of 24-hour (diel) water quality samples two times
each year. Diel data can greatly increase the accuracy of a
water quality model when used to calibrate for diel
variations in DO and other parameters. This monitoring
program was initially outlined in the 1986 EPA-issued
NPDES permit for the Central Facility and has been
sustained through annual CWA section 308 letters to
Denver Metro.
The current data base is particularly valuable because of its
long period of record, broad spatial coverage, and
consistency in sampling and analytical methods. It is quite
useful for (1) determining whether water quality standards
are being met, (2) indicating water quality trends, and (3)
providing better information to revise established TMDLs,
if necessary. In addition, the expanded data base better
supports water quality modeling efforts.
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FOKT UUPTON WWTP
SEGMENT
BOUNDARY-
BASE LIKE ROAD (16BTH AVE) ,
SEGMENT BOUNDARY
AT BURLINGTON
DITCH aa.-
LEGEND
© GROUNDWATER DISCHARGE SITES
• IRRIGATION RETURN FLOWS & NON-POINT
SOURCE LOADING POINTS
• WWTP DISCHARGE POINTS
•DiarHcra CENTRAL
PLANT
FIGURE 3. Segment 15 Pollution Loading Sources (after Camp Dresser & McKee, 1992)
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TMDL, DEVELOPMENT — 1986
Determining the Load/Waste Load Allocation
Scheme
The objective of a TMDL is to allocate allowable loads
among all of the pollutant sources throughout a watershed
so that appropriate control measures can be implemented
and water quality standards achieved. To do this, EPA
Region Vin followed five distinct steps.
The first step taken to develop the TMDLs for Denver
Metro was selecting the pollutants to consider. Water
quality data for Segment 15 of the South Platte were
reviewed to identify existing water quality problems and
their probable causes. In 1986, Denver Metro concentrated
on ammonia and chlorine toxicity, DO problems, and
metals. Ammonia was of particular concern because it
contributed to the un-ionized ammonia toxicity and DO
problems in Segment 15 of the river (B. Zander,
correspondence, May 23, 1991).
The second step taken was to estimate the maximum
allowable loading of the pollutant(s) of concern that would
not violate water quality standards. The critical flow
condition that resulted in the lowest dilution of pollutants
was identified as the 7Q10 (the 7-day consecutive low
flow, reoccurring every 10 years).
Next, all point and nonpoint sources to Segment 15 were
identified and their contributions estimated. Point source
discharges along the South Platte and its tributaries include
publicly owned treatment works and industrial dischargers
(see Figure 4). The Denver Metro District's Central
Facility, at the head of Segment 15, is the largest
discharger in the area with a design capacity of 185 mgd
(287 cfs). The South Adams County Waste and Sanitation
District wastewater facility and the wastewater facility for
the City of Brighton have design capacities of only 4.3
mgd (6.7 cfs) and 2.6 mgd (4.1 cfs), respectively. Various
industrial discharges to the river are also relatively small.
Nonpoint source pollution contributions, including ground
water inflow, were also estimated. When compared with
the discharge from Metro's Central Facility at low flow,
however, these other loadings were considered much less
significant For this reason, TMDL development was
centered around this facility and the other pollutant sources
were considered to be background.
Predictive analysis of pollutants in Segment 15 of the
South Platte and determination of total allowable loads
were performed using the model and mass balance
equations developed for the second step. WLAs for point
sources and load allocations for nonpoint sources were
developed and are shown hi Table 1. The margin of safety
(MOS), which is required when calculating a TMDL, was
incorporated through the conservative assumptions used
during TMDL development. If these conservative
assumptions had been deemed insufficient, an additional
MOS would have been added as a separate component of
the TMDL.
The final step was to determine the limits to be placed on
individual pollution sources so that the total loading for
each pollutant would be within the specified TMDL.
Because the Central Facility was identified as the most
significant source of pollutants to Segment 15, the facility
was required to upgrade to advanced wastewater treatment.
The other municipal facilities in Segment 15 are required to
treat only to secondary levels. Best management practices
to control nonpoint sources were not recommended.
The WLAs for the Central Facility were incorporated into
its NPDES permit. The TMDLs required the Denver
Metro Facility to be very near water quality standards at
the end of the pipe for many pollutants (e.g., metals and
other inorganics) because of the low dilution during critical
conditions.
Modeling
To predict stream response to various pollutant loading
scenarios, the staff of EPA Region Vni developed
STREAMDO, a steady state, one-dimensional water quality
model. STREAMDO was used for Section 15 of the South
Platte to model dissolved oxygen and un-ionized ammonia.
There were numerous advantages to using this model. It
was accepted by the regulatory authorities, was not overly
complex, and was easily modified and understood. Also, it
ran on Lotus 1-2-3, a common spreadsheet software
package.
Features of the STREAMDO model included a mass
balance approach; subdivision of stream segments; and
representative equations for physical, biological, and
chemical processes. To determine allowable concentrations
for effluent parameters other than biological oxygen
demand and ammonia, modelers used a simple mass
balance calculation.
STREAMDO was calibrated and verified in 1986 using
available historical water quality data. The model
coefficients and inputs used to calculate TMDLs for
Segment 15 were also based on these data. The TMDLs
are presented in Table 1.
FOLLOW-UP
Monitoring
Reporting requirements in the NPDES permit and letters
from EPA issued under CWA section 308 required the
Central Facility to collect ambient water quality,
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TABLE 1. Total Maximum Patty Loads for the South Platte River at the Points of Discharge for Denver Metro
PARAMETER
Arsenic (Total)
WASTE LOAD
ALLOCATION'
(e.g., Permit Limit)
Cadmium
Chromium (Hex)
—
Chromium (Tri)
Manganese (Diss)
«_^———
Mercury
m~^—~—~~^~*
Nickel
Selenium (total)
Ammonia
Dissolved Oxygen
May 1 - July 14
July 15 - Apr. 30
PH
Total Residual Chlorine
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
_.
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
Ib/day (ug/L)
) Ib/day (mg/L)b
1 Ib/day (mg/L)b
0 Ib/day (mg/L)
mg/L
mg/L
s.u.
e mg/L
91.0 (52.0) 1
1.8 (1.0)
47.0 (27.0)
95.0 (54.0)
46.0 (26.0)
46.0 (26.0) I
280.0 (160.0)
0.09 (0.05)
189.0 (108.0)
18.0 (10.0)
0.2 (0.10)
252.0 (144.0)
1.8 (1.0)
18.0 (10.0)
June-Sept. 10.5 (6.0)
Oct 15.8 (9.0)
Nov.-Dec. 22.8 (13.0)
Jan. 35.0 (20.0)
Feb.-Mar. 22.8 (13.0)
Apr.-May 15.8 (9.0)
LOAD
ALLOCATION'
(Background)
(30.0)
(1.0)
(8.0)
0.5 (3.0)
0.5 (3.0)
0.5 (3.0)
0.5 (3.0)
0.5 (3.0)
0.5 (3.0)
TMDL
(Ib/day)
WATER QUALITY
STANDARD0
(ug/L)
P«— — ^—
1.0
2.0
^^KH_H^~«
2.0
27.0
0.01
3.0
1.0
0.02
17.0
^ — ^— ™
(12.0)
__^_^^_«_
(18.0)
(14.0)
(160.0)
(0.05)
(11.0)
(10.0)
(0.1)
(99.0)
96.0
48.0
48.0
307.0
0.1
192.0
19.0
0.2
269.0
96.0
1.9
48.0
96.0
50.0 |
1.0 |
25.0 |
50.0
23.U
25.0
160.0
0.05
100.0
10.0
I 0.1
1.9
19.0
11.0
16.3
23.3
36.0
23.3
16.3
140.0
1.0 (mg/L)
10.0 (mg/L)
0.1 (mg/L)
5.0 (mg/L)
4.5 (mg/L)
6.5 - 9.0 (s.u.)
0.003 (mg/L)
Based on 210-mgd effluent flow and 20-mgd background flow in the South Platte River. A+MOS-TMDL1
Reload allocation includes loadings from all PSs and NPSs upstream from the permittee's discharge (SWLA+ZLA+MOS-TMDL).
"Nitrite and nitrate limits are based on meeting State water quality standards at the end of the pipe.
'Site-specific water quality standards in place at the time this TMDL was set
hydrologic, and biological data to ensure that the facility
was complying with its permit, to monitor water quality
trends, and to evaluate whether the TMDLs adequately
protect water quality and the aquatic community. This
monitoring has shown that, while the ammonia toxicity
problem appears to have been resolved, low DO
concentrations persist in specific sections of Segment 15.
This finding has forced EPA to re-evaluate the TMDLs
established in 1986 for pollutants that exert an oxygen
demand.
Modeling
STREAMDO was used for this re-evaluation using the
additional data from the follow-up monitoring. The
Colorado Ammonia Model was also incorporated to
produce the Segment 15 Water Quality Model (Camp
Dresser & McKee, Inc., 1992). This latest round of DO
modeling for the South Platte shows that benthic oxygen
demand plays a key role hi causing excursions below the
standard. As a result, more field work is planned to further
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Segment 14
Segment 15
So Plolte River
Segment 6
N
Point Load
Junction
Storm Drainage Inflow
Wastewater Facility
Water Withdrawal
Cherry Cr Res
FIGURE 4. Pollutant Sources Upstream from Segment 15 (after DRCOG, 1990)
characterize the link between the quality of Denver Metro's
effluent and the benthic oxygen demand. The water quality
model, as well as certain TMDLs, will be updated based on
information collected this year on the benthic processes.
Although it is anticipated that the TMDLs for several
pollutants will change, Metro District has been required to
conduct engineering studies on alternative solutions to the
DO problem. One solution may involve additional
nitrification/denitrification of the effluent of the South
Complex of the Central Facility; although the Segment 15
Model indicated this will not necessarily alleviate the DO
problem in Segment 15. Modeling also indicated that
ponding above the 88th Avenue bridge and at the Fulton
Ditch diversion is a major cause of oxygen depletion in the
river. As a result, several nontraditional solutions are being
examined. Channel restoration to improve natural stream
aeration is one possibility. By restoring the river's natural
cross-section—alternating riffles and pools to improve
reaeration and increase velocities—it is thought that the
benthic layer will have much less influence on DO.
According to the model, a variety of combinations of
improvements at the Denver Metro treatment facility,
physical habitat improvements, and artificial reaeration may
achieve the specified in-stream DO targets. The artificial
reaeration involves an off-channel facility in which water is
pumped over a cascade structure in a park setting.
Alternative Pollution Controls
The alternative methods to increase DO along Segment 15
of the South Plane River are currently being ranked and
costed by the discharger. The alternatives and their
associated costs are presented hi Table 2.
Ranking of the alternatives is based on four criteria:- (1) '
implementability and relative magnitude of activity; (2)
operatability and reliability; (3) environmental
comparability; and (4) public support. Implementability
and relative magnitude rates each alternative on the
likelihood that the alternative will actually be implemented.
At this stage of the screening process, capital and operating
costs are not listed as separate criteria, but are considered
qualitatively in formulating a rating for this criterion.
Operatability and reliability rates each alternative on how
easy or difficult it is to operate from the District's
standpoint The evaluation considers the risk that the
Metro District would assume in the operation if water
quality standards are not met. By necessity, multiple-
jurisdiction involvement would be rated at the low end of
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TABLE 2. Alternative Methods to Increase DO Concentrations and Facilitate Meeting Water Quality Standards for
DESCRIPTION OF ALTERNATIVE
• Nitrification/denitrification faculties for Central Plant's 115-mgd South
Complex
• Stream modification above and below 88th Ave.
• Effluent dispersal to 3 ditches via ditches
• Stream modification above and below 88th Ave.
• Effluent dispersal to 3 ditches via pipeline
• Stream modification above and below 88th Ave.
• Artificial reaeration at 168th Ave.
• Stream modification above and below 88th Ave.
• Artificial reaeration at 168th Ave.
• Effluent dispersal to 2 ditches via ditches
• Stream modification above and below 88th Ave.
• Artificial reaeration at 168th Ave.
• Effluent dispersal to 2 ditches via pipeline
• Stream and drop structure modification at 88th Ave.
• Artificial reaeration below 88th Ave.
• Artificial reaeration at 168th Ave.
• Filter all Metro effluent
« Chlorination/dechlorination
• Stream modification above and below 88th Ave.
• Nitrify South Complex effluent to 5 mg/L (NH3-N)
• Chlorination/dechlorination
CAPITAL COST
($ Million)
72-112'
32
44
32
52
59
56
102
114
ANNUAL O&M
COST
($K/Yr)
2,000-4,500"
630
480
670
1970
840
880
3,950
4,636
* 1989 cost
the scale. Environmental comparability ratings are based
on an alternative's ability to enhance land use, surface
water quality, and fish and wildlife habitat, as well as air
quality considerations. Public support is the most
subjective of all me criteria. Under this criterion each
alternative is rated on its ability to provide additional
community benefits and to garner support from a high
percentage of the surrounding community.
REFERENCES
Camp Dresser & McKee Inc. 1992. Nitrification
alternatives study. Metro Wastewater Reclamation District,
Denver, CO.
DRCOG. 1990. South Platte River Segments 6 and 14
wasteload allocation study. Denver Regional Council of
Governments, Denver, CO.
Omemik, J. M. 1987. Ecoregions of the conterminous
United States. Annals of The Association of American
Geographers 77(1): 118-125.
USEPA. 1986. Statement of basis, Metropolitan Denver
Sewage Disposal District No. 1, CO-0026638, major
municipal renewal permit issued by EPA. United States
Environmental Protection Agency, Washington, DC.
USEPA. 199 la. Guidance for water quality-based
decisions: The TMDL process. United States
Environmental Protection Agency, Office of Water,
Washington, DC.
USEPA. 199 Ib. Workshop on the water quality-based
approach for point and nonpoint source controls, meeting
summary. United States Environmental Protection Agency,
Office of Water, Washington, DC.
USGS. 1985. National water summary 1985. U.S.
Geological Survey Water-Supply Paper 2300.
This case study prepared by Tetta Tech, Inc.", Fairfax, VA1
in conjunction wife USEPA» Watershed Management
Section, Office of Wetlands, Oceans, and Watersheds, and
Region VIH. ' '
8
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