United States
Environmental Protection
Agency
Office Of Water
(WH-553)
EPA841-F-33-007
August 1993
Number 9
4xEPA TMDL Case Study
Appoquinimink River,
Delaware
Key Feature:
Project Name:
Location:
Scope/Size:
Land Type:
Type of Activity:
Pollutants:
TMDL Development:
Data Sources:
Data Mechanisms:
Monitoring Plan:
Control Measures:
A phased TMDL for phosphorus on
a tidal freshwater river reach
Appoquinimink River
EPA Region Hi/New Castle
County, Delaware
River, watershed 30,200 acres
Flat plains
Agriculture, urban
Phosphorus (algae)
Phased, PS/NPS
Local, STORET
WASP4 model
Yes
NPDES permit, BMPs
* TMDL Site
FIGURE 1. Location of the Appoquinimink River
watershed
Summary: The Appoquinimink River watershed is located in
eastern Delaware (Figure 1). Delaware's Department of
Natural Resources and Environmental Control (DNREC)
chose the tidal freshwater segment of the Appoquinimink
River as the site of its first total maximum daily load
(TMDL) because intensive monitoring indicated the criteria
for dissolved oxygen (DO) were being violated; an NPDES
permit decision was pending for the only point source
discharger to the river, the Middletown-Odessa-Townsend wastewater treatment plant, making the TMDL relatively
simple; and information on nonpoint source loadings was available. DNREC used available ambient water quality data
and existing point and nonpoint source loading data to conduct the initial assessment and characterization of the
Appoquinimink's water quality problems. In addition, the EUTRO4 version of EPA's WASP4 water quality model was
used to analyze the DO and nutrient economy of the river. Phosphorus overenrichmenl was determined to be the ultimate
cause of excursions of applicable DO criteria. To avoid exacerbating the problem, DNREC developed a total maximum
daily load (TMDL) for phosphorus whose first phase (1) limits point source loads at existing levels to prevent increasing
the frequency DO standard violations; (2) further characterizes nonpoint source nutrient loads and their impact on river
water quality; (3) describes the water quality monitoring and modeling studies necessary to determine the second-phase
TMDL; and (4) plans and schedules activities that will lead to the adoption and implementation of the Phase n TMDL
(DNREC, 1992). The Phase I TMDL of 18,947 Ib/yr was calculated as the sum of the point source allocation (6,862
Ib/yr) and the background/nonpoint source allocation (12,085 Ib/yr). These allocations reflect a reasonable margin of
safety to prevent further water quality degradation until the TMDL can be refined in Phase n to meet water quality
standards.
Contact: Rick <3reene, Delaware Department of Natural Resources and Environmental Control, Division of Water
- Jtesources, 89 Kings Highway, P.O. Box 1401, Dover, Delaware '19903,, phone (302)739-4590
Recycled/Flecyclable
Printed on paper that contains
at leas!: 50% recycled fiber
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BACKGROUND
The Appoquinimink River watershed is located in the
flat coastal plain of eastern Delaware (Figure 1). The
river's headwaters and major tributaries drain
agricultural lands and feed four major impoundments:
Shallcross Lake, Silver Lake, Noxontown Fond, and
Wiggins Mill Pond.
In its natural state, the 30,200-acre watershed is
dominated by oak, hickory, pine, southern floodplain
forest, and southern mixed forest (Omernik, 1987).
Foresflands make up 15 percent of its area. The
wetlands that cover another 9 percent represent the only
large marsh system in Delaware that is essentially
undisturbed by human activity. These wetlands are
highly valued as waterfowl, shorebird, and wildlife
habitat and as a spawning and nursery area for fish and
aquatic life. About 61 percent of the watershed is
actively cultivated to produce corn, soybeans, small
grains, and specialty crops such as potatoes and
tomatoes. There are 130 farms in the watershed, each
averaging 150 acres of cropland.
The tidal freshwater segment of the AppCKminimink is
bounded by the head of tide at Noxontown Pond and
Silver Lake (river mile 10.2) at the upstream end and by
Drawyer Creek's confluence with the Appoquinimink
River (river mile 5.0) at the downstream end. At the
upstream end, the river flows at approximately 30 cubic
feet per second. Salinity within this 5-mile reach
generally remains below 5 parts per thousand. Under
the definitions provided in the State of Delaware Surface
Water Quality Standards (DNREC, 1990), this salinity
level classifies the reach as freshwater.
Aquults make up the majority of soils in the watershed,
with Matapeake-Sassafras Association constituting
approximately 83 percent of the area. These soils are
mainly limited by the risk of erosion unless close-
growing plant cover is maintained. They are deep, well-
drained, and medium- to coarse-textured. Except for the
degree of slope and the hazard of erosion in some areas,
the major soils have few limitations for farm and
nonfarm use. Slopes range from nearly level in the
uplands to steep near the stream channels.
The remainder of the basin consists of Tidal Marsh
Association soils that exist within the marshlands along
the Delaware River and protrude inland along the
Appoquinimink and its tidal tributaries. Marsh
vegetation covers most of these soils. Tidal marsh
cannot be used for crops or pasture, but it is used as
wildlife habitat and for some recreational purposes.
Middletown, Odessa, and Townsend cover
approximately 11 percent of the Appoquinimink
watershed, and most of the watershed's population
(4,500 people) is located in these towns. The population
is expected to expand within the near future. Although
the watershed's economy is essentially agrarian, some
light industry does exist in Middletown. The only point
source discharger to the Appoquinimink River is the
Middletown-Odessa-Townsend wastewater treatment
plant (MOT WWTP).
The designated uses of the tidal freshwater portion of the
Appoquinimink are: primary contact recreation;
secondary contact recreation; fish, aquatic life, and
wildlife; industrial water supply; and agricultural water
supply. The 1986 report of the Rural Clean Water
Program's Appoquinimink River Basin Project stated
that recreational uses such as swimming have been
sharply curtailed because of water quality constraints,
especially the excessive algal growth and DO deficit that
have resulted from phosphorus loadings (Water
Resources Agency, 1986).
The reach is impaired by low DO levels. For freshwater
systems, section 11.1 of the Standards requires a
representative daily (24-hour) average DO concentration
of 5.5 mg/L from June through September and an
instantaneous minimum DO concentration of 4.0 mg/L.
Although there are no numerical standards for nutrient
concentrations, section 7 of the Water Quality Standards
does recognize that nutrient overenrichment is a
significant problem in some of Delaware's surface
waters. For this reason, it is DNREC's policy to
minimize nutrient input to surface waters from any
controllable source, establishing the types of, and need
for, nutrient controls on a site-specific basis. Nutrient
controls may include, but are not limited to, effluent
limits on point sources or the institution of best
management practices (BMPs) for nonpoint sources.
ASSESSING AND CHARACTERIZING
THE PROBLEM
Targeting and Prioritizing
DNREC chose the Appoquinimink River as the site of its
first TMDL because it was identified as being water
quality limited and requiring a TMDL; a wastewater
management decision was pending at the MOT WWTP;
a single point source made the TMDL relatively simple;
and information on nonpoint source loadings was
available.
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+ Water Quality Station
• O;jtfall
FIGURE 2. The Appoquinimink River watershed
Monitoring and Data Bases
DNREC used available ambient water quality data and
existing point and nonpoint source loading data to
conduct an initial assessment and characterization of
water quality problems in the Appoquinimink River.
Most of the ambient data came from intensive water
quality surveys that were conducted for New Castle
County from September through October 1990 to assess
the human health and environmental impacts that might
be caused by increasing the MOT WWTP discharge.
DNREC's ambient water quality monitoring program
data from 1985 through 1990 were retrieved from EPA's
STORET data base to supplement the intensive survey
data. STORET data on nitrate nitrogen (NO3-N) and
soluble orthophosphorus (SOP) concentrations were
particularly important because the intensive survey failed
to quantify them. The value for NO3-N is provided in
Table 1, which summarizes the available water quality
data. Only one sample of SOP (0.04 mg/L) was
available from March of 1990 at river mile 6.0, while
levels measured at the upstream boundary of the reach
were below the detection limit of 0.02 mg/L. No data
on chlorophyll-^ concentrations were available.
As part of the intensive surveys, water quality samples
were collected during high and low slack tide conditions
at stations located at the upstream boundaries, at river
mile 7.95 (Route 13 bridge), at river mile 6.4 (Route
299 bridge at Odessa) just upstream of the existing
discharge, and at river mile 3.2 below the downstream
boundary of this reach (Figure 2). A contractor
collected data on DO, temperature, 5-day biological
oxygen demand (BODS), ammonia nitrogen (NH3-N),
total Kjeldahl nitrogen (TKN), total phosphorus (TP),
SOP, salinity, and pH. A diel DO profile was also
developed at the river mile 6.4 station, just above the
treatment plant discharge.
The diel DO data collected during October 1990
indicated violations of the daily average criterion of
5.5 mg/L. Periodic grab samples collected from 1985 to
1990 also indicated several violations of the minimum
criterion of 4.0 mg/L. To more completely characterize
the factors contributing to these violations of the DO
standard, DNREC prepared a plan to intensively monitor
the Appoquinimink and its major tributaries. The plan
includes synoptic water quality surveys of the tidal river
and major tributaries; measurement of tributary flows
and nutrient concentrations to estimate nutrient loads;
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TABLE 1. Applicable water quality standards (DNREC, 1990) and the results of intensive water quality surveys
conducted for New Castle County on the Appoquinimink River, Delaware, from river mile 5.0 to river mile 10.2 during
September and October 1990 (DNREC, 1992)
Parameter
Dissolved oxygen \
Total phosphorus
Soluble
orthophosphorus
Total KjckUhl nitrogen
Ammonia-nitrogen
Nitrate-nitrogen
Applicable Water Qualify
Standard
5.5 mg/L daily average
4.0 mg/L instantaneous minimum
(both apply to freshwater systems)
None
None
None
None
None 'v
Intensive Survey
Diel Station
Diel Station
Upstream Boundary
Downstream Boundary
All stations
Diel station
Diel station
Diel station
for New Castle County, mg/L
4.2 - 6.1 (5.2 mg/L 24-hour avg at
an avg water temperature of 21.5°C
0.18 - 0.25
< 0.10
< 0.10
Less than detection (0.05 mg/L)
0.35 - 1.14
0.18 - 0.33
1.37 (std dev 0.9)'
" Data from EPA's STORET database for the State of Delaware (1985-1990); not available from the intensive surveys.
analyses of sediment nutrient content; and diel
monitoring of DO, temperature, and salinity at selected
stations in the tidal river for periods of several
consecutive days. The plan also provides for a data base
to calibrate a water quality model of the Appoquinimink.
This plan was submitted to EPA Region HI for review
and approval as part of DNREC's Ambient Surface
Water Quality Monitoring program.
Monitoring began in November 1991 and is still under
way. The additional data .will allow DNREC to calibrate
the WASP4 model to a higher order of complexity,
making the model more predictive. Previous modeling
efforts that were conducted at lower levels of complexity
only mimic river responses. With the more sophisticated
model, the effects of combinations of various BMPs and
point source reductions can be predicted. When
modeling is complete, DNREC will identify appropriate
pollution reduction controls and their impacts on water
quality.
TMDL DEVELOPMENT
Determining the Pollutants of Concern
The EUTRO4 version of EPA's WASP4 water quality
model was used to analyze the DO and nutrient economy
of the Appoquinimink River so that the cause of the DO
criteria violations could be determined. The WASP
model runs were steady-state, tidally averaged
simulations of a one-dimensional channel to represent the
tidal freshwater portion of the Appoquinimink River.
Model simulations were run using the Full Linear DO
Balance (Level 3 order of complexity), as defined in the
WASP4 user's manual. Key processes modeled included
carbonaceous biological oxygen demand (CBOD),
oxidation, nitrification, reaeration, and sediment oxygen
demand (SOD). Although they were considered
important, algal photosynthesis and respiration rates
were not modeled as part of this initial effort because
there were no chlorophyll-a data. Instead, algal
photosynthesis and respiration rates were estimated using
screening-level analyses (discussed below) that involved
evaluating available STORET data.
The diel variation of DO concentrations (1.8 mg/L) that
was noted during the intensive water quality surveys
suggested that pnytoplankton productivity and respiration
were occurring at significant rates. It was therefore
important to determine whether nitrogen or phosphorus
was limiting algal growth. Analysis of STORET data
yielded a nitrogen/phosphorus ratio of 40 with a standard
deviation of 23, indicating that phosphorus is more likely
to be limiting pnytoplankton growth (Thomann and
Mueller, 1987).
DNREC postulated that most of the phosphorus available
for biological uptake is being used for phytoplankton
growth and that additional loadings of phosphorus would
contribute to increased phytoplankton productivity. In
streams with heavy algal growth, differences in algal
catabolism during light and dark periods can result in
wide diurnal variations in DO.
Excessive algal biomass production and subsequent die-
off and sedimentation of organic matter can contribute to
higher-than-normal SOD that causes DO levels to fall
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below criteria. To prevent more frequent violations of
the DO standard, DNKEC decided to establish an initial
TMDL that capped existing phosphorus loads to the
reach until a more refined TMDL that ensures
compliance with the standard can be established.
Point Source Wasteload Allocation
Because most of the phosphorus from the treatment plant
would be bioavailable as SOP, it is likely that eutrophic
conditions would result throughout the reach and
possibly in downstream waters if limits were not set. At
the time of the Phase I TMDL analysis, the MOT
WWTP was permitted and operating at 0.5 mgd with
effluent BOD5 at 15 mg/L. The permit conditions were
assumed in the modeling analysis of BOD, with values
for effluent nitrogen concentrations assumed to be
10.0 mg/L of ammonia-nitrogen and 5.0 mg/L of
organic-nitrogen, as reported for similar treatment
facilities by Thomann and Mueller (1987).
Using 24-hour composite samples, DNREC analyzed
effluent phosphorus concentrations (TP and SOP) on a
weekly basis from February 6, 1991, through April 3,
1991. Concentrations of TP in the effluent ranged from
2.61 to 4.88 mg/L, and concentrations of SOP ranged
from 2.18 to 4.88 mg/L. These concentrations were
multiplied by the measured daily discharge to estimate
actual phosphorus wasteloads from the treatment plant,
as presented in Table 2.
The point source load limit was established by statistical
analysis of the effluent phosphorus load measurements.
The data were statistically analyzed to define a monthly
average load limit at a 95 percent confidence level. The
monthly average phosphorus load was determined to be
14.57 Ib/day, with a standard deviation of 2.57 Ib/day.
The 95th percentile value of a normal distribution with a
mean of 14.57 and standard deviation of 2.57 is 18.8
Ib/day. This translates into equivalent loads of 572
Ib/month and 6862 Ib/yr. This allocation was
incorporated into the NPDES permit as final effluent
limits for MOT WWTP. These caps go into effect May
9, 1994. No interim limits have been set for phosphorus
while the plant works to meet compliance.
Nonpoint Source and Background Load
Allocation
Ambient water quality measurements showed that
background concentrations of TP in the tidal portion of
the Appoquinimink are below 0.1 mg/L.
Rural Clean Water Program studies that were conducted
from 1980 through 1986 measured nonpoint source
loading rates of phosphorus and nitrogen in the
Appoquinimink's Wiggins Mill subwatershed. These
studies provided data on loads based on the following
agricultural seasons: fallow season, 151 days from
November through March; planting season, 61 days
from April through May; and growing season, 153 days
from June through October. There were a total of seven
data points for each season.
Using a log-transformed distribution of the seasonal
data, a Monte Carlo simulation program entitled PC-MC
was run to generate iinnual loads by repeated random
sampling of the seasonal distributions. A total of 2,000
annual load simulations were run to develop an entire
distribution of annual loads based on random sampling
of the seasonal load (distribution. The median annual
phosphorus load for the Wiggins Mill sub-basin was
determined to be 1,760 Ib/yr. This value was
extrapolated to the entire watershed tributary to the tidal
freshwater segment of the Appoquinimink by multiplying
by the ratio of watershed area (14,900 acres/2,170 acres)
to yield an annual nonpoint source phosphorus load limit
of 12,085 Ib/yr.
DNKEC decided that although these readily available
estimates were adequate to use for the first phase of the
Appoquinimink TMDL, they were not appropriate to use
for developing the final TMDL. Land use patterns and
the widespread implementation of BMPs since the last
studies were conducted in 1986 have certainly altered
nonpoint source loading rates. The validity of
extrapolating the Wiggins Mill loading rates to the rest
of the Appoqumirnink watershed is also questionable,
because of differences in land use patterns, soils, and
geologic-hydrologic conditions among the subwatersheds.
Additional studies to characterize nonpoint source
nutrient loads to the reach and to assess the effect of
Noxontown Pond, Silver Lake, and Shallcross Lake on
the nonpoint source loads actually delivered to the reach
were therefore proposed as part of the Phase I TMDL.
The Margin of Safety
TMDLs should reflect a margin of safety based on the
uncertainty or variability in the data, the point and
nonpoint source load, estimates, and/or the modeling
analyses.
The point source phosphorus loads were well defined
based on the recent effluent monitoring. The nonpoint
source phosphorus load measurements were more
variable, and therefore had a greater level of uncertainty
associated with their estimates. Selecting die 95*
percentile value of the estimated load distribution, as
was selected for the point source loads, allowed nonpoint
source phosphorus loads equivalent to those that
occurred prior to implementation of BMPs during the
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TABLE 2. Summary of measured phosphorus loads in Middletown-Odessa-Townsend effluent
Date
6 WEB 91
13 FEE 91
20FEB91
27 FEE 91
6 MAR 91
14 MAR 91
20 MAR 91
27 MAR 91
3 APR 91
Flow
(mgd)
0.550
0.530
0.517,
0.509
0.418
0.505
0.529
0.551
0.523
Effluent Concentration (mg/L)
Total Phosphorus Soluble OrthoPhosphorus
3.89
2.90
3.05
3.08
2.74
2.73
2.61
4.43
4.88
2.81
2.24
2.49
2.18
2.61
2.47
2.40
2.90
2.67
Mass Loads (Ibs/day)
Total Phosphorus Soluble OrthoPhosphorus
17.84
12.82
13.15
13.07
9.55
11.50
11.51
20.36
21.29
12.89
9.90
10.74
9.25
9.10
10.40
10.59
13.33
11.65
Rural Clean Water Program project. Limiting nonpoint
source loads to the median value (equivalent to the 50*
perceatile value) of the estimated distribution yields a
phosphorus load that is approximately 30 percent less
than the pre-BMP loads, and is representative of the
nonpoint source loads measured after implementation of
BMPs during the Rural Clean Water Program. DJNREC
therefore believes that selection of the median value of
the estimated nonpoint source load distribution provides
a reasonable margin of safety from a water quality
perspective.
BVIPLEMENTAHON OF POLLUTION
CONTROLS
Point Sources
New Castle County identified and explored several
potential pollution control options for the MOT WWTP.
The principal options were land application (i.e., spray
irrigation) of the treated effluent and relocation of the
discharge to the Delaware River. Land treatment proved
to be both more environmentally responsible and less
costly.
Relocating the discharge to the Delaware River was
rejected because, even if DNREC could obtain a permit
to cross miles of wetlands, costs would be exorbitant.
Spray irrigation at a site other than one adjacent to the
plant was also rejected because it would require
additional infrastructure change. However, spray-
irrigating to adjacent land would take advantage of the
existing infrastructure (i.e., pumps).
DNREC has implemented the point source phosphorus
wasteload allocation by incorporating it into MOT
WWTP's NPDES permit effluent limits. The NPDES
permit, which was issued in November 1992, capped
phosphorus loads at 6862 Ib/yr. Effluent discharge was
limited to 0.5 mgd, except for the first 18 months when
the plant can discharge up to 0.65 mgd to accommodate
current growth while a method of compliance is
evaluated and installed. There are no current plans to
expand the stream discharge, although spray irrigation
may expand up to 1.2 mgd for a total discharge of 1.7
mgd.
Nonpoint Sources
The Rural Clean Water Program report for the
. Appoquinimink River identified 14 BMPs potentially
applicable to the Appoquinimink watershed. The BMPs
included permanent vegetative cover; animal waste
control systems; stripcropping systems; terrace systems;
diversions; grazing land protection; waterways; cropland
protective cover; conservation tillage systems; stream
protection; permanent vegetative cover on critical areas;
sediment retention, erosion control structures, or water
control structures; fertilizer management; and pesticide
management. Most of these BMPs were implemented to
some degree during the Rural Clean Water Program
study prior to the TMDL analyses. Continuation,
expansion, and refinement of these practices throughout
the Appoquinimink watershed are potential control
measures for agricultural areas. Other BMPs, such as
erosion and sediment controls, may become necessary to
control runoff and nutrient loads from developing areas.
DNREC will coordinate with New Castle County, New
Castle County Water Resources Agency, and New Castle
County Conservation District in establishing additional
BMPs for nonpoint sources, if necessary.
FOLLOW-UP FOR THE NEXT PHASE
Table 3 presents the proposed schedule of activities that
will support completion of the TMDL.
Public Hearing
Before sending the preliminary TMDL to EPA Region
HI for approval, DNREC published a Hearing Notice in
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the News Journal and the Delaware State News to obtain
written and oral comments on the first-phase TMDL
from interested parties. A report on the development of
the preliminary TMDL (DNREC, 1992) was made
available to the public in Dover, New Castle, and
Georgetown, Delaware. The TMDL hearing was held in
conjunction with the hearing to consider comments on
the New Castle County Department of Public Works'
application for reissuance of the NPDES permit for the
MOT WWTP (NPDES Permit No. DE 0050547). As
stated previously, the purpose of the permit was to
establish effluent limitations, monitoring requirements,
and other terms and conditions needed to protect the
designated uses of the Appoqninimink River.
Nonpoint Sources
The studies to estimate existing nonpoint source loads
for the entire Appoquinimink watershed were completed
by the end of 1992. DNREC monitored the overflows
of Silver Lake and Noxontown Lake in order to
determine actual nonpoint source loads to the upper
boundary of the tidal river. DNREC also funded a
cooperative study between the University of Delaware,
the Water Resources Agency for New Castle County,
and the New Castle County Soil and Water Conservation
District.
These studies are documented in Nutrient Budgets for the
Appoquinimink Watershed (Ritter and Levin, 1992),
which outlines the nonpoint source nitrogen and
phosphorus budgets that were developed using the unit
loading rate method, and also details land uses that were
determined from 1989 aerial photographs, national
wetlands inventory maps, and parcel base maps. The
nutrient budget study concluded that Noxontown Pond
may remove from 60 to 70 percent of the nitrogen and
30 to 50 percent of the phosphorus from nonpoint
sources; that Silver Lake may remove from 30 to 50
percent of the nitrogen and 50 to 70 percent of the
phosphorus; and that Shallcross Lake is probably
removing some nitrogen and phosphorus in the Drawyer
Creek watershed.
The researchers also found that (1) cropland is the
largest land use in die Appoquinimink watershed and
contributes over 75 percent of the nitrogen and
phosphorus loads from nonpoint sources; (2) die nitrogen
load from nonpoint isources is much greater than the
nitrogen load being discharged by the MOT WWTP; (3)
if land use changes from cropland to urban-high density
development with central sewer in the future, nitrogen
loads from nonpoint sources would decrease and
phosphorus loads would remain at present day levels; (4)
the nitrogen contribution from septic tanks is greater
than the nitrogen load from the WWTP; (5) the MOT
WWTP phosphorus load constitutes approximately 32
percent of the phosphorus load in the Appoquinimink
watershed; (6) nitrogen loads may be able to be reduced
by lowering nitrogea fertilizer application rates, but crop
yields would also be reduced; and (7) phosphorus loads
may be able to be r<5duced by constructing ponds and
filter strips in criticd areas. The loading rates
determined by this stud)' are being applied in the water
quality modeling study of the Appoquinimink River as
described below to better define the impact of nonpoint
source nutrient loads on the water quality of the
Appoquinimink and to provide a basis for refining the
established TMDL.
Water Quality Model Calibration and
Application
EPA's WASP4 model was applied and tentatively
calibrated to simulate the observed nutrient and DO
concentrations in the Appoquinimink River as part of the
first-phase TMDL. The initial modeling study helped
identify the major sinks of DO and indicated impacts of
point source loads on ambient nutrient concentrations.
Because of limitations in the existing data base, the full
eutrophication version of the WASP4 model was not
TABLE 3. The proposed schedule of activities to support development of the final phase of the Appoquinimink
phosphorus TMDL
Scheduled Activity Start Date Completion Date
Public Hearing: Phase I TMDL and NPDES Permit
Nonpoint Source Nutrient Budget Study
Intensive Water Quality Monitoring
Water Quality Modeling Study and TMDL Determination
Identification of Feasible Point and Nonpoint Source Wasteload Allocation Options
Preparation of Phase n TMDL Document
August 11, 1992
July 15, 1992
October 1, 1991
October 1, 1992
March 1, 1993
August 1, 1993
-
September 30, 1992
ongoing
August 1, 1993
August 1, 1993
December 31, 1993
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implemented. As a consequence, a predictive
relationship between nutrient loads, algal productivity,
and DO concentrations could not be precisely
determined. The latest modeling study used the
information obtained from the nonpoint source
nutrientload study and the intensive water quality
monitoring study to calibrate the EUTRO4 version of
WASP for the Appoojwiimink. The calibrated model
was applied to project DO levels under a variety of point
and nonpoint source nutrient loading scenarios.
The original plan was to develop cost and confidence
curves for different pollution control scenarios.
However, the modeling study found that even the most
aggressive pollution control scenario—which consisted of
total removal of point source loads; 50 percent removal
of nonpoint source phosphorus and nitrogen loads; and
50 percent removal of SOD, ammonia, and phosphorus
flux of sediments—provided only a marginal difference
in DO levels, indicating that the system is driven by
SOD.
Given this information, the Phase n TMDL will:
• define the phosphorus load reductions necessary
to meet DO criteria;
• require additional characterization of nonpoint
source nutrient loads;
• require continued monitoring and modeling,
address the SOD issue; and
• specify how the TMDL will be implemented.
There are essentially two methods to address SOD. The
short-term solution is to dredge and fill the mucky
bottom, alleviating the oxygen sink. The long-term
solution is to limit phosphorus loads, preventing the
proliferation of algae whose death and sedimentation
cause the SOD and allowing the current SOD to
gradually decrease over time.
A Jack of SOD data will make the long-term solution
difficult to quantify within the context of a TMDL.
However, over the next several years DNREC will begin
planning and conducting SOD measurements so this
information can be available for future watershed
studies. These results have demonstrated that sediment
can significantly impact the quality of water systems
along Delaware's coast, and DNREC plans to conduct
future TMDL studies accordingly.
REFERENCES
DNREC. 1988. Clean water strategy. State of
Delaware, Department of Natural Resources and
Environmental Control, Division of Water Resources
March 30, 1988.
DNREC. 1990. State of Delaware Surface Water
Quality Standards, as amended February 2, 1990. State
of Delaware, Department of Natural Resources and
Environmental Control, Division of Water Resources.
DNREC. 1992. Development of a phase I total
maximum daily load (TMDL) for the Appoquinimink
watershed. State of Delaware, Department of Natural
Resources and Environmental Control.
Mills, W.B., D.B. PorceUa, M.J. Ungs, S.A. Gherim,
K.V. Summers, Lingfung Mok, G.L. Rupp, G.L.
Bowie, and D.A. Haith. Water quality assessment: A
screening procedure for toxic and conventional pollutants
in surface and ground waters, parts I and II. U.S.
Environmental Protection Agency, Washington, DC
EPA/600/6-85/002a.
Omernik, J.M. 1987. Ecoregions of the conterminous
United States. Annals of the Association of American
Geographers 77(1): 118-125.
Ritter, W.F., and M.A. Levin. 1992. Nutrient budgets
for the Appoquinimink river watershed - Draft. State of
Delaware, Department of Natural Resources and
Environmental Control.
Thomann, R.V., and J.A. Mueller. 1987. Principles of
surface water quality modeling and control. Harper and
Row, Publishers, New York.
Water Resources Agency for New Castle County. 1986.
Appoquinimink river basin project, Rural Clean Water
Program, final report.
This case study ws prepared by T«n» Tech, &&,„ Fairfax,
Virginia, ui conjunction with, EPA's Office of Wetlands,
Oceans and Watersheds, Watershed Management Section.
To obtain copies, contact your EPA Regional 3Q3$)fTMIXL
Coordinator.
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