Town Creek Nutrients TMDL
June 2007
DRAFT
Total Maximum Daily Load (TMDL)
for *
The Town Creek Watershed
In the Tombigbee River Basin of Mississippi
To Address Impairment due to Nutrients
Prepared by:
US EPA Region 4
61 Forsyth Street SW
Atlanta, Georgia 30303
June 2007
Region
southeast
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Town Creek Nutrients TMDL
June 2007
FOREWORD
This report has been prepared in accordance with the schedule contained within the federal
consent decree dated December 22, 1998. The report contains one or more Total Maximum
Daily Loads (TMDLs) for waterbody segments found on Mississippi's 1996 Section 303(d) List
of Impaired Waterbodies. Because of the accelerated schedule required by the consent decree,
many of these TMDLs have been prepared out of sequence with the State's rotating basin
approach. The implementation of the TMDLs contained herein will be prioritized within
Mississippi's rotating basin approach. The amount and quality of the data on which this report is
based are limited. As additional information becomes available, the TMDLs may be updated.
Such additional information may include water quality and quantity data, changes in pollutant
loadings, or changes in landuse within the watershed.
Conversion Factors
To convert from
mile2
km2
m3
ft3
ft3
cfs
cfs
m3
m3
To
acre
acre
ft3
gallons
liters
gal/min
MOD
gallons
liters
Multiply by To convert from
640 acre
247.1 days
35.3 | meters
7.48 ft3
28.3 hectares
448.8 miles
0.646 | tonnes
264.2 ug/l*cfs
1000 ug/l*MGD
To
ft2
seconds
feet
gallons
acres
meters
tons
gm/day
gm/day
Multiply by
43560
86400
3.28
7.48
2.47
1609.3
1.1
2.45
3.79
Fraction
io:l
10"2
io-3
10"6
io-9
io-12
io-15
io-18
Prefix
Deci
Centi
Milli
Micro
Nano
Pico
Femto
Atto
Symbol
d
c
m
H
n
P
f
a
Multiple
"T6~~
io2
io3
io6
io9
IO12
IO15
IO18
Prefix
deka
hecto
kilo
mega
giga
tera
peta
exa
Symbol
"da^
h
k
M
G
T
P
E
11
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Town Creek Nutrients TMDL June 2007
TABLE OF CONTENTS
FOREWORD ii
TABLE OF CONTENTS iii
Index of Tables iii
Index of Figures iv
TMDL INFORMATION PAGE v
EXECUTIVE SUMMARY vii
1. INTRODUCTION 1
1.1 Background 1
1.2 Applicable Waterbody Segment Use 2
1.3 Applicable Waterbody Segment Standard 3
1.4 Nutrient Target Development 5
1.5 Pollutants of Concern: Total Nitrogen and Total Phosphorus 6
2.0 WATERBODY ASSESSMENT 8
2.1 Town Creek Water Quality Data 8
2.2 Assessment of Point Sources 8
2.3 Assessment of Non-Point Sources 13
3.0 ANALYTICAL APPROACH 16
3.1 Introduction 16
3.2 Simple Method for Estimating Non-point Source Loads 16
3.3 WASP Model 17
3.3.1 WASP Segmentation 18
3.3.2 WASP Parameterization and Loadings 18
3.4 Model Results for Existing Conditions 19
4.0 DEVELOPMENT OF THE TMDL 21
4.1 Wasteload Allocation 22
4.2 Load Allocation 23
4.3 Determination of Watershed Load Reductions 23
4.4 Margin of Safety 23
4.5 Critical Conditions and Seasonal Variation 24
5.0 CONCLUSION 25
5.1 Public Participation 25
REFERENCES 26
Index of Tables
Table 1. Town Creek Instream Nutrient Data 8
Table 2. NPDES Permitted Facilities in the Town Creek watershed 8
Table 3. NPDES facilities used in the Town Creek watershed water quality model 12
Table 4. BODs and Ammonia Permit Levels and nutrient levels for Significant NPDES
Facilities 13
Table 5. Nutrient Loadings for Various Land Uses 14
Table 6. Landuse in Town Creek Watershed 14
Table?. EMC s for Storm Events 17
Table 8. Parameterization of Stream Segments Used in the WASP model 18
Table 9. Average Annual Non-point Source Loads 19
iii
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Town Creek Nutrients TMDL June 2007
Table 10. Average Annual Loads for Existing Conditions 20
Table 11. TMDL Components 21
Table 12. Instream Nutrient and Chi a concentrations for Existing and TMDL Conditions 22
Index of Figures
Figure 1. Map of the Listed Segment, Town Creek 4
Figure 2. NPDES dischargers in the Town Creek watershed included in the water quality model
12
Figure 3. Pie chart representing land use types of Town Creek watershed by percentages 15
Figure 4. Predicted DO Concentrations For Existing Conditions 20
Figure 5. Predicted DO and Chi a Concentrations for TMDL Conditions 22
IV
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Town Creek Nutrients TMDL
June 2007
TMDL INFORMATION PAGE
Table i. Listing Information
ID
MS013TE
Name
Town
Creek
County
Union, Lee,
Pontotoc
Impairment
Nutrients
HUC
03160102
Mon/Eval
Evaluated
Location: Near Nettleton from headwaters to watershed 016 boundary
Table ii. Applicable Mississippi Water Quality Standards
Parameter
Nutrients
Dissolved Oxygen
Beneficial use
Aquatic Life Support
Aquatic Life Support
Narrative Water Quality Criteria
Mississippi's current standards contain a narrative criteria that can
be applied to nutrients which states "Waters shall be free from
materials attributable to municipal, industrial, agricultural, or other
discharges producing color, odor, taste, total suspended or
dissolved solids, sediment, turbidity, or other conditions in such
degree as to create a nuisance, render the waters injurious to public
health, recreation, or to aquatic life and wildlife, or adversely affect
the palatability of fish, aesthetic quality, or impair the waters for
any designated use (MDEQ, 2002)."
DO concentrations shall be maintained at a daily average of not less
than 5.0 mg/1 with an instantaneous minimum of not less than 4.0
mg/1.
Table iii. NPDES Facilities in the Watershed
NPDES ID
MS0020940
MS0021733
MS0022845
MS0023302
MS0023655
MS0033235
MS0033464
MS0034444
MS0036111
MS0039501
MS0042048
MS0048046
MS0052639
Facility Name
Plantersville POTW
Saltillo POTW
Webb Utility Systems— Indian Hills
Subdivision
Natchez Trace Tupelo, Headquarters
Guntown POTW
East Union Attendance Center
Mississippi Department of Wildlife —
Fisheries and Parks, Tombigbee State
Park
Elvis Presley Park
Tupelo POTW
Super 8 Motel — Belden Inn
Verona POTW
City of Tupelo, Deer Park Estates
Facility
Garden Park Estates
Permitted
Discharge
0.275
0.98
0.134
.025
0.2
0.03
0.007
0.015
10.5
0.004
1.05
0.027
0.072
Receiving
Water
Town Creek
Sand Creek
Little Sand
Creek
Tributary of
Mud Creek
Sand Creek
Bridge
Creek
Town Creek
Middle
Tulip Creek
Town Creek
Town Creek
Coonewah
Creek
West Tulip
Creek
Yonaba
Creek
City
Plantersville
Saltillo
Tupelo
Tupelo
Guntown
Ellistown
Tupelo
Tupelo
Tupelo
Tupelo
Verona
Tupelo
Saltillo
County
Lee
Lee
Lee
Lee
Lee
Union
Lee
Lee
Lee
Lee
Lee
Lee
Lee
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Town Creek Nutrients TMDL
June 2007
NPDES ID
MS0055972
MS0060011
Facility Name
Heardtown Estates Subdivision
Sherman POTW*
Permitted
Discharge
0.085
0.2
Receiving
Water
Bustaloba
Creek
Town Creek
City
Tupelo
Sherman
County
Lee
Pontotoc
Table iv. Total Maximum Daily Loads
Parameter
TN
TP
BOD
WLA (Ibs/day)
209
22
1045
LA (Ibs/day)
946
180.4
2574
MOS
implicit
implicit
implicit
TMDL (Ibs/day)
1155
202.4
3619
VI
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Town Creek Nutrients TMDL June 2007
EXECUTIVE SUMMARY
The State of Mississippi originally placed Town Creek on the Mississippi 1996 Section 303(d)
List of Impaired Waterbodies due to evaluated causes of pesticides, siltation, nutrients, and
organic enrichment (OE)/low dissolved oxygen (Low DO). The Mississippi Department of
Environmental Quality (MDEQ) was not able to complete biological monitoring on Town Creek
as it is a non-wadeable stream. MDEQ submitted, and EPA approved, a delisting package for
OE/Low DO based on recent monitoring data that showed that the water quality standards for
DO were being attained in Town Creek. This TMDL addresses impairment due to nutrients in
Town Creek by providing an estimate of the total nitrogen (TN) and total phosphorus (TP) loads
allowable in the stream for point sources and non-point sources in the Town Creek watershed.
Mississippi does not have numeric water quality standards for allowable nutrient concentrations;
however, MDEQ is currently working on the development of numeric nutrient criteria to be
adopted in State water quality standards. A Nutrient Task Force (NTF) has been established to
assist the State in this effort, and MDEQ is progressing according to the State's Nutrient Criteria
Development Plan, which has been mutually agreed upon with EPA.
In the 1999 Protocol for Developing Nutrient TMDLs, EPA suggests several methods for the
development of TMDL targets for nutrients (USEPA, 1999). According to this document, "The
target value for the chosen indicator can be based on: comparison to similar but unimpaired
waters; user surveys; empirical data summarized in classification systems; literature values; or
professional judgment." Mississippi's method is based on a comparison between similar but
unimpaired waters within the same region. This method is dependent on adequate data and
utilizes data collected in support of nutrient criteria development, as per the plan mutually agreed
upon with EPA. Based on MDEQ's methodology and studies, an annual concentration range of
0.6 to 0.7 mg/1 is an applicable target for Total Nitrogen and 0.06 to 0.10 mg/1 for Total
Phosphorus in Ecoregion 65.
The limited nutrient data and estimated ecoregion concentrations indicate reductions of nutrients
are needed. The allocations proposed for Town Creek are the results of a water quality model
EPA developed for the impaired streams. The EPA Water Quality Analysis Simulation Program,
Version 7 (WASP7.2) was applied as the in-stream water quality model. The purpose of the
modeling exercise was to determine reductions in nutrient loads that would have to occur in
order to protect the designated use of the streams, and achieve water quality standards. Point and
non-point sources were reduced in the WASP model so that simulated DO concentrations
continued to achieve water quality standards and simulated in-stream TN and TP concentrations
reflected Ecoregion 65 target levels. Chlorophyll-a concentration in the water column was
simulated in the model as a surrogate for the total productivity of the system. The target
chlorophyll-a concentration was in the range expected in a "natural" or least-disturbed condition.
In the absence of site specific chlorophyll-a data and in consideration of the modeling work and
technical analysis completed for the TMDL development process, water column chlorophyll-a
concentrations less than 20 micrograms per liter are indicative of conditions that should be
protective of aquatic life in these Mississippi waters. In recognition of the lack of data available
for Town Creek as well as the absence of numeric nutrient criteria for these waters, EPA is
asking for comments on the approach used in this report to determine the TMDL.
vii
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Town Creek Nutrients TMDL June 2007
1. INTRODUCTION
1.1 Background
The identification of waterbodies not meeting their designated use and the development of total
maximum daily loads (TMDLs) for those waterbodies are required by Section 303(d) of the
Clean Water Act and the Environmental Protection Agency's (EPA) Water Quality Planning and
Management Regulations (40 CFR part 130). The TMDL process is designed to restore and
maintain the quality of those impaired waterbodies through the establishment of pollutant
specific allowable loads. This TMDL has been developed for the 2006 §303(d) listed segment
shown in Figure 2.
Town Creek is listed as an evaluated waterbody impaired due to nutrients, organic enrichment (OE)/low
dissolved oxygen (low DO). EPA approved a delisting package submitted by MDEQ for OE/Low DO as
recent monitoring data showed that DO standards were being attained in Town Creek. Town Creek is in the
Tombigbee River Basin, which comprises east-central Mississippi. The drainage area for the Town Creek
watershed is approximately 375.8 square miles. The watershed is predominantly comprised of forest and
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Town Creek Nutrients TMDL
June 2007
agricultural lands. A map of the watershed is provided in
Figure 1.
1.2 Applicable Waterbody Segment Use
The designated beneficial use for the listed segment is fish and wildlife. The water use
classifications are established by the State of Mississippi in the document State of Mississippi
Water Quality Criteria for Intrastate, Interstate, and Coastal Waters (MDEQ, 2003).
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Town Creek Nutrients TMDL June 2007
1.3 Applicable Waterbody Segment Standard
The water quality standard applicable to the use of the waterbody and the pollutant of concern is
defined in the State of Mississippi Water Quality Criteria for Intrastate, Interstate, and Coastal
Waters (MDEQ, 2003). Mississippi's current standards contain a narrative criteria that can be
applied to nutrients which states, "Waters shall be free from materials attributable to municipal,
industrial, agricultural, or other discharges producing color, odor, taste, total suspended or
dissolved solids, sediment, turbidity, or other conditions in such degree as to create a nuisance,
render the waters injurious to public health, recreation, or to aquatic life and wildlife, or
adversely affect the palatability offish, aesthetic quality, or impair the waters for any designated
use (MDEQ, 2002)."
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Town Creek Nutrients TMDL
June 2007
Legend
Figure 1. Map of the Listed Segment, Town Creek
Cities of Tupelo and Verona
• Natchez Trace Parkway
• MS Waterbodies-Unimpaired
• MS013TE-Town Creek
HUG 03160102
Counties
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Town Creek Nutrients TMDL June 2007
1.4 Nutrient Target Development
Since there are no specific numeric criteria for nutrients in Mississippi, the State's narrative
water quality standard must be translated to quantify a level of nutrients that are protective of
aquatic life. Numeric criteria for Total Nitrogen (TN) and Total Phosphorus (TP) are currently
being developed by the Mississippi Nutrient Task Force (NTF) in coordination with EPA Region
4. MDEQ has a State nutrient criteria development plan that has been mutually agreed upon
with EPA and MDEQ and is on schedule according to that plan in development of nutrient
criteria (MDEQ, 2004). Data were collected for wadeable streams to calculate the nutrient
criteria.
In the 1999 Protocol for Developing Nutrient TMDLs, EPA suggests several methods for the
development TMDL targets for nutrients (USEPA, 1999). According to this document, "The
target value for the chosen indicator can be based on: comparison to similar but unimpaired
waters; user surveys; empirical data summarized in classification systems; literature values; or
professional judgment." Mississippi's method is based on a comparison between similar but
unimpaired waters within the same region. This method is dependent on adequate data and
utilizes data collected in support of nutrient criteria development as per the plan mutually agreed
upon with EPA. The initial phase of the data collection process for wadeable streams in
Mississippi is complete.
Nutrient data were collected quarterly at 99 discrete sampling stations state wide where
biological data already existed. These stations were identified and used to represent a range of
stream reaches according to biological health status, geographic location (selected to account for
ecoregion, bioregion, basin and geologic variability) and streams that potentially receive
nonpoint source pollution from urban, agricultural, and silviculture lands as well as point source
pollution from NPDES permitted facilities.
Nutrient concentration data were not normally distributed; therefore, data were log transformed
for statistical analyses. Data were evaluated for distinct patterns of various data groupings
(stratification) according to natural variability. Only stations that were characterized as "least
disturbed" through a defined process in the Mississippi Benthic Index of Stream Quality (M-
BISQ) process or stations that resulted in a biological impairment rating of "fully attaining" were
used to evaluate natural variability of the data set (MDEQ, 2003).
The M-BISQ, a regionally calibrated benthic index of biotic integrity, was developed through a
partnership between MDEQ and Tetra Tech, Inc. in 2001 from 434 wadeable (perennial, lst-4th
order streams) in the State excluding the Yazoo Delta. This index defined five bioregions for the
State, and established the 25th percentile of the least disturbed condition for each bioregion as
the threshold of impairment of the State of Mississippi's wadeable streams.
Each of the two groups—"least disturbed sites" and "fully attaining sites"—was evaluated
separately. Some stations were used in both sets; in other words, they were considered "least
disturbed" and "fully attaining." The number of stations considered "least disturbed" was 30 of
99, and the number of stations considered "fully attaining" was 53 of 99.
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Town Creek Nutrients TMDL June 2007
Several analysis techniques were used to evaluate nutrient data. Graphical analyses were used as
the primary evaluation tool. Specific analyses used included: scatter plots, box plots, Pearson's
correlation, and general descriptive statistics. In general, natural nutrient variability was not
apparent based on box plot analyses according to the four stratification scenarios. Bioregions
were selected as the stratification scheme to use for TMDLs in the Pascagoula Basin. However,
this was not appropriate for some water bodies in smaller bioregions. Therefore, MDEQ now
uses ecoregions as a stratification scheme for the water bodies in the remainder of the state. In
order to use the data set to determine possible nutrient thresholds, nutrient concentrations were
evaluated as to their correlation with biological metrics.
For the preliminary target concentration range, the means of the data at each of the nutrient sites
were taken. The 75th and 90th percentiles of the means of the nutrient sites in that ecoregion
fully supporting aquatic life were selected according to the M-BISQ scores. For the estimate of
the existing concentrations, the median was taken of the data from the sites that were not
attaining and had nutrient concentrations greater than the target. In Ecoregion 65, an annual
concentration range of 0.6 to 0.7 mg/1 is an applicable target for TN and 0.06 to 0.10 mg/1 is an
applicable target for TP.
1.5 Pollutants of Concern: Total Nitrogen and Total Phosphorus
The following is an adaptation of the State of Washington Department of Ecology's Citizen's
Guide to Understanding and Monitoring in Streams and Lakes and provides a brief description
and basic understanding of the pollutants of concern for this TMDL report:
The two primary nutrients of concern are nitrogen and phosphorus. Both elements commonly
are measured in several forms. Phosphorus can be reported as total phosphorus (TP), which
includes a particulate form and a dissolved form. The dissolved form is measured and reported
as soluble reactive phosphate (SRP), phosphate (PO/f3), or orthophosphate (ortho-P); all different
terms used to describe the fraction of TP that is soluble, and therefore more immediately
available to organisms for growth.
Nitrogen can be reported as total nitrogen (TN), either measured directly or calculated from its
constituents, which are organic-N, ammonia-N (unionized or ionized), nitrite-N, and nitrate-N.
Of these, organic-N and ammonia-N are measured as total Kjeldahl nitrogen (TKN), while
nitrite+nitrogen (N(V) and nitrate-nitrogen (N(V), are usually measured as nitrate+nitrite-
nitrogen (N(V + NCV)- As is the case with TP, there are fractions of TN that are more
bioavailable. TKN includes the organic form of TN, which is less immediately bioavailable for
growth versus the more readily available component of TKN, which is NHa or NH4+. Together,
the fractions of NH3 or NH4+ and N(V + N(V represent forms of nitrogen that are most
immediately available for growth.
Organically bound TP and TN, while not immediately available, can be converted to bioavailable
forms at predictable rates; and may be significant drivers of primary productivity. One chemical
form of an element can be converted into another, and the conditions under which the conversion
occurs are influenced by many factors; such as pH, temperature, oxygen concentration, and
biological activity. The original form of the nutrient and the prevailing physical conditions will
determine if an increase in total nutrient concentrations will result in higher available nutrient
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Town Creek Nutrients TMDL June 2007
concentrations and therefore, a corresponding immediate increase in growth or productivity. If
nutrients enter the stream as organic matter, that has to be decomposed before the nutrients can
be utilized for additional growth. This process consumes oxygen and is important due to its
effect on the rate of decomposition. During warmer month, nutrients enter the system as intact
organic matter and decompose relatively quickly as compared to cold, wet-weather months when
decomposition is slow. These dynamics are further complicated by the fact that increased
growth leads to greater numbers of organisms that need even more nutrients. So, as nutrients
become available they are often immediately utilized.
Increases in anthropogenic nutrient concentrations and their impacts are considered nutrient
pollution. Municipal and industrial discharges usually contain nutrients, and overland flow from
developed watersheds contains nutrients from lawn and garden fertilizers as well as the
additional organic debris so easily washed from urban surfaces. Agricultural areas also
significantly contribute to nutrient increases through poor manure management, fertilizing
practices, and increased nutrient-bearing soil erosion from plowed surfaces.
Increased nutrient loading typically results in increased algal growth, where sufficient conditions
of light, temperature, substrate and flow (residence time) are met. And, the resulting enhanced
growth in algal biomass will occur locally and/or downstream. In flowing stream segments
where conditions are right, attached forms of algae tend to dominate, i.e., periphyton attached to
rocks, logs, aquatic macrophytes, and other substrate. In slower flowing streams, algae
suspended in the water column, i.e., phytoplankton, may tend to dominate. Excessive growths of
algae, both periphyton and phytoplankton, can adversely affect other aquatic life through
habitat/life cycle disruption and exaggerated fluctuations of normal dissolved oxygen cycles;
eventually resulting in a DO crash. In addition, unsightly conditions, odors and poor habitat
conditions for aquatic organisms can also be attributed to excessive algae (WDOE, 1994).
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Town Creek Nutrients TMDL
June 2007
2.0 WATERBODY ASSESSMENT
2.1 Town Creek Water Quality Data
Nutrient data for the Town Creek Watershed were collected in September 1999. The data are
presented in Table 1.
Table 1. Town Creek Instream Nutrient Data
Station
TUP 1
TUP1
TUP 1
TUP1
TUP 2
TUP 2
TUP 2
TUP 2
TUPS
TUPS
TUPS
TUP 4
TUP 4
TUP 4
AVERAGE
TN (mg/1)
2.9
2.7
2.6
2.7
7.8
9.4
6.3
8.4
5.2
7.2
6.1
4.8
4.7
5.6
5.03
TP (mg/1)
0.1
0.2
0.1
0.1
1.8
2.1
1.1
1.51
1
1.1
1
0.9
0.8
0.8
0.83
2.2 Assessment of Point Sources
An important step in assessing pollutant sources in the Town Creek watershed is locating the
NPDES permitted sources. There are nineteen facilities permitted to discharge into the Town of
the nineteen facilities (labeled "terminated" in Table 2) are no longer discharging in the Town
Creek watershed. Out of the active permitted dischargers, three wastewater treatment facilities
(WWTFs) represent the greatest effluent contributors (by flow) to Town Creek.
Table 2. NPDES Permitted Facilities in the Town Creek watershed
NPDES ID
MS0002178
MS0020940
MS0021733
MS0022845
MS0023302
MS0023655
MS0033235
Facility Name
Krueger International Inc,
Tupelo
Plantersville POTW
Saltillo POTW
Webb Utility Systems— Indian
Hills Subdivision
Natchez Trace Tupelo,
Headquarters
GuntownPOTW
East Union Attendance Center
Permitted
Discharge
Terminated
0.275
0.98
0.134
0.025
0.2
0.03
Receiving Water
NA
Town Creek
Sand Creek
Little Sand Creek
Tributary of Mud
Creek
Sand Creek
Bridge Creek
City
Tupelo
Plantersville
Saltillo
Tupelo
Tupelo
Guntown
Ellistown
County
Lee
Lee
Lee
Lee
Lee
Lee
Union
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Town Creek Nutrients TMDL
June 2007
NPDES ID
MS0033464
MS0034444
MS0035246
MS0036111
MS0039501
MS0042048
MS0042471
MS0046621
MS0048046
MS0052639
MS0055972
MS0060011
Facility Name
Mississippi Department of
Wildlife — Fisheries and Parks,
Tombigbee State Park
Elvis Presley Park
Oak Grove Mobile Home Park
Tupelo POTW
Super 8 Motel — Belden Inn
Verona POTW
Tri State Mack Truckwash
Crafton Warehouse
City of Tupelo, Deer Park
Estates Facility
Garden Park Estates
Heardtown Estates
Subdivision
Sherman POTW
Permitted
Discharge
0.007
0.015
Terminated
10.5
0.004
1.05
Terminated
Terminated
0.027
0.072
0.085
0.2
Receiving Water
Town Creek
Middle Tulip
Creek
NA
Town Creek
Town Creek
Coonewah Creek
NA
NA
West Tulip Creek
Yonaba Creek
Bustaloba Creek
Town Creek
City
Tupelo
Tupelo
Plantersville
Tupelo
Tupelo
Verona
Tupelo
Tupelo
Tupelo
Saltillo
Tupelo
Sherman
County
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Pontotoc
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Town Creek Nutrients TMDL
June 2007
Table 3 and
NPOES Dischargers
Cities of Tupelo and Verona
Natchez Trace Parkway
Chickasaw
MS Waterbodies
MS013TE-Town Creek
HUC 03160102
Counties
12
16
• '•1
Figure 2 present all the point sources permitted to discharge to the Town Creek watershed. For
the water quality model for Town Creek, EPA included only the active NPDES dischargers as
the inactive facilities would have to recommence discharging in order to have an impact on water
10
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Town Creek Nutrients TMDL
June 2007
quality in Town Creek. The location of Sherman POTW is not represented on
NPOES Dischargers
Cities of Tupelo and Verona
Natchez Trace Parkway
Chickasaw
MS Waterbodies
MS013TE-Town Creek
HUC 03160102
Counties
024
12
16
• '•1
Figure 2, as it is not geo-referenced. Table 4 presents BODs and ammonia permit limits for the
significant NPDES facilities discharging in the Town Creek watershed.
11
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Town Creek Nutrients TMDL
June 2007
NPOES Dischargers
Cities of Tupelo and Verona
Natchez Trace Parkway
'///, Cities
- MS Waterbodies
MSQ13TE-Town Creek
HUC 03160102
Counties
Chickasaw
Figure 2. NPDES dischargers in the Town Creek watershed included in the water quality model
Table 3. NPDES facilities used in the Town Creek watershed water quality model
Map
Number
0
1
NPDES ID
MS0020940
MS0021733
Facility Name
Plantersville POTW
Saltillo POTW
Permitted
Discharge
0.275
0.98
Receiving
Water
Town Creek
Sand Creek
City
Plantersville
Saltillo
County
Lee
Lee
12
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Town Creek Nutrients TMDL
June 2007
Map
Number
2
3
4
5
6
7
8
9
10
11
12
13
Not on
map
NPDES ID
MS0022845
MS0023302
MS0023655
MS0033235
MS0033464
MS0034444
MS0036111
MS0039501
MS0042048
MS0048046
MS0052639
MS0055972
MS0060011
Facility Name
Webb Utility Systems-
Indian Hills Subdivision
Natchez Trace Tupelo,
Headquarters
Guntown POTW
East Union Attendance
Center
Mississippi Department
of Wildlife — Fisheries
and Parks, Tombigbee
State Park
Elvis Presley Park
Tupelo POTW
Super 8 Motel — Belden
Inn
Verona POTW
City of Tupelo, Deer Park
Estates Facility
Garden Park Estates
Heardtown Estates
Subdivision
Sherman POTW*
Permitted
Discharge
0.134
.025
0.2
0.03
0.007
0.015
10.5
0.004
1.05
0.027
0.072
0.085
0.2
Receiving
Water
Little Sand
Creek
Tributary of
Mud Creek
Sand Creek
Bridge
Creek
Town Creek
Middle
Tulip Creek
Town Creek
Town Creek
Coonewah
Creek
West Tulip
Creek
Yonaba
Creek
Bustaloba
Creek
Town Creek
City
Tupelo
Tupelo
Guntown
Ellistown
Tupelo
Tupelo
Tupelo
Tupelo
Verona
Tupelo
Saltillo
Tupelo
Sherman
County
Lee
Lee
Lee
Union
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Lee
Pontotoc
* Sherman POTW is not geo-referenced and is not presented on the map, but it was used in the model.
Table 4. BOD5 and Ammonia Permit Levels and nutrient levels for Significant NPDES Facilities
Facility
Tupelo POTW
Saltillo POTW
Verona POTW
Flow
(MGD)
10.5
1.0
1.0
BOD5
(mg/L)
10.0
10.0
10.0
NH3-N
(mg/L)
2.0
2.0
2.0
TN* (mg/L)
10.0
10.0
10.0
TP* (mg/L)
5.0
5.0
5.0
*Estimated nutrient levels based on literature values
2.3 Assessment of Non-Point Sources
The two primary nutrients of concern are nitrogen and phosphorus. TN is a combination of
many forms of nitrogen found in the environment. Inorganic nitrogen can be transported in
particulate and dissolved phases in surface runoff. Dissolved inorganic nitrogen can be
transported in groundwater and may enter a stream from groundwater infiltration. In agricultural
areas, studies have indicated that human activities have increased nitrate concentrations in
ground water. In urban areas, it is likely that nitrogen sources are relatively localized when
compared with the generally more intensive and widespread use of fertilizers on cropland. The
greater the percentage of impervious surfaces in a watershed (usually more predominant in urban
areas) generally results in surface runoff of nutrient-laden water, rather than seepage to ground
water (USGS, 1999). Finally, atmospheric gaseous nitrogen may enter a stream from
atmospheric deposition.
13
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Town Creek Nutrients TMDL
June 2007
Unlike nitrogen, phosphorus is primarily transported in surface runoff when it has been sorbed
by eroding sediment. Phosphorus may also be associated with fine-grained particulate matter in
the atmosphere and can enter streams as a result of dry fallout and rainfall (USEPA, 1999).
However, phosphorus is typically not readily available from the atmosphere or the natural water
supply (Davis and Cornwell, 1988). As a result, phosphorus may be a limiting nutrient in non-
point source dominated rivers and streams, with the exception of watersheds which are
dominated by agriculture and have high concentrations of phosphorus contained in the surface
runoff due to fertilizers and animal excrement or watersheds with naturally occurring soils which
are rich in phosphorus (Thomann and Mueller, 1987). Table 5 presents an estimate of typical
nutrient loading ranges associated with various land uses. Figure 3 presents a diagrammatic
representation of the types of land uses in the Town Creek watershed by percentage.
Table 5. Nutrient Loadings for Various Land Uses
Landuse
Roadway
Commercial
Single Family-Low Density
Single Family -High Density
Multifamily Residential
Forest
Grass
Pasture
Total Phosphorus (Ib/acre-yr)
Minimum
0.53
0.61
0.41
0.48
0.53
0.09
0.01
0.01
Maximum
1.34
0.81
0.57
0.68
0.72
0.12
0.22
0.22
Median
0.98
0.71
0.49
0.58
0.62
0.10
0.12
0.12
Total Nitrogen (Ib/acre-yr)
Minimum
1.2
1.4
2.9
3.6
4.2
1.0
1.1
1.1
Maximum
3.1
7.8
4.2
5.0
5.9
2.5
6.3
6.3
Median
2.1
4.6
3.6
5.2
5.0
1.8
3.7
3.7
Source: Horner et.al. in 1994 in Protocol for Developing Nutrient TMDLs (USEPA 1999)
The drainage area of the Town Creek watershed is approximately 240,514 acres. The watersheds
contain many different landuse types, including urban, forest, cropland, pasture, water, and
wetlands. The land use information for the watershed is based on the 1999 National Land Cover
Dataset (NLCD). The land use categories were grouped into the land uses of urban, forest,
cropland, pasture, disturbed, wetlands, and water. In both watersheds, the area directly
surrounding the creeks is predominantly rural and dominated by pastureland. Landuse
distribution in the watersheds is summarized in Table 6.
Table 6. Landuse in Town Creek Watershed
Landuse
Agriculture -
Cropland
Agriculture - Pasture
Barren or Mining
Forest
Transitional
Urban
Water/Wetlands
TOTAL
Town Creek
Percentage
17%
11%
0%
58%
3%
1%
11%
100%
Area (acres)
119744
75701
127
411384
18310
4806
75914
240,514
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Town Creek Nutrients TMDL
June 2007
Town Creek, MS
Landuse to Segment MS013TE (areas in acres)
• 1 % —
D 39% -i
D0%
D Agriculture - Cropland
• Agriculture - Pasture
D Barren or Mining
D Forest
• Transitional
D Urban
• Water/Wetlands
—D31%
Figure 3. Pie chart representing land use types of Town Creek watershed by percentages
15
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Town Creek Nutrients TMDL June 2007
3.0 ANALYTICAL APPROACH
3.1 Introduction
Town Creek was listed as an evaluated stream that was impaired due to nutrients. Even though
the stream is not impaired for Low DO, the TMDL allocations were determined by analyzing the
effects of TN and TP loads on in-stream response variables of algal biomass and achieving the
DO standard. Excess chlorophyll-a (Chi a) concentration was used as a surrogate for total
productivity of the system resulting from nutrient enrichment. The EPA Water Quality Analysis
Simulation Program, Version 7 (WASP7.2) was applied as the in-stream water quality model
(Wool et. al., 2001). WASP7.2 contains an eutrophication component that simulates complex
nutrient transport and cycling in the streams, as well as models any dissolved oxygen sag
resulting from point source discharges. The purpose of the modeling exercise was to determine
the level of reductions in nutrient loads that would have to occur in order to protect the
designated use and achieve water quality standards in Town Creek. For existing conditions, the
NPDES facilities were assumed to have existing effluent BODs concentrations of 10.0 mg/L and
nutrient concentrations of 10.0 and 5.0 mg/L for TN and TP, respectively; however, to achieve
in-stream nutrient concentrations in the range of those expected in Ecoregion 65, the effluent
concentration from the facilities was reduced to 2.0 mg/L of TN and 0.2 mg/L of TP. Non-point
source loads of TN and TP were calculated using EPA's Simple Method for estimating
watershed loads. Pollutant loads were reduced until model simulations indicated that the DO
criteria and nutrient targets were attained in stream.
3.2 Simple Method for Estimating Non-point Source Loads
Average annual non-point source loads of BOD5 and nutrients were estimated using a
spreadsheet based on EPA's Simple Method formula from the Better Assessment Science
Integrating Point and Non-point Sources (BASINS) Pollutant Loading (PLOAD) model
(USEPA, 2001):
LP = Eu (P * PJ* RVu * Cu* Au * 2.72 / 12)
Where: LP = Pollutant load, Ibs
P = Precipitation, inches/year
PJ = Ratio of storms producing runoff (default = 0.9)
RVu= Runoff Coefficient for land use type u, inches of runoff/inches of rain
RVu=0.05 + (0.009 * lu); lu = percent imperviousness
Cu = Event Mean Concentration for land use type u, milligrams/liter
Au = Area of land use type u, acres
The PLOAD model was used to estimate nutrient loadings to Town Creek based on existing land
use conditions. The 1999 National Land Cover Dataset (NLCD) is the most recent data available
to reflect land use conditions in the Town Creek watershed.
The PLOAD model uses average annual rainfall and event mean concentrations (EMCs) to
estimate pollutant loading transported off a particular land use. The EMC is the measure of
pollutant levels during a runoff event, and is expressed as the loading for a specific constituent
16
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Town Creek Nutrients TMDL
June 2007
divided by the stormwater volume. The model assumes all lands are connected to the stream,
resulting in a conservative estimate of average annual loads. An annual average rainfall of
approximately 55 inches was assumed to fall on the impaired watershed, based on mean annual
rainfall information available in the NHDplus dataset. The default ratio of 0.9 for storms
producing runoff was assumed in the model. Land use data entered into the spreadsheet were
based on the 1999 land use/cover features categorized according to the NLCD (see Table 6).
EMC values assumed for the various land uses are shown in Table 7 and represent default values
contained in the BASINS PLOAD model (USEPA, 2001).
Table 7. EMCs for Storm Events
Land Use
Forest/rural open
Urban open
Agriculture
Low-density residential
Medium-density residential
High-density residential
Communication and transportation
Rangeland
Water
Wetlands
BOD5
(mg/1)
1.00
5.0
4.0
4.0
7.0
3.0
2.00
2.00
1.00
5.0
Total N
(mg/1)
0.30
1.10
2.30
1.60
2.40
2.30
1.00
1.00
0.30
1.10
Total P
(mg/1)
0.060
0.20
0.350
0.200
0.50
0.350
0.10
0.10
0.060
0.20
The PLOAD model was used to estimate nutrient concentrations expected to runoff the
watershed in the absence of development. In this scenario, disturbed landuses in the watershed
were changed to least disturbed conditions (i.e., wetland and forest). Results of this scenario
indicate annual average TN and TP concentrations of 0.45 and 0.07 mg/L, respectively. The
predicted TN and TP concentrations are within the range of concentrations identified by MDEQ
as preliminary nutrient criteria for the ecoregion; therefore, the upper limits of the ecoregion
values (i.e., TN of 0.7 mg/L and TP of 0.1 mg/L) were selected as in-stream water quality targets
in the TMDL analysis.
3.3 WASP Model
The WASP model helps users interpret and predict water quality responses to natural phenomena
and man-made pollution for various pollution management decisions. WASP is a dynamic
compartment-modeling program for aquatic systems, including both the water column and the
underlying benthos. The time-varying processes of advection, dispersion, point and diffuse mass
loading and boundary exchange are represented in the basic program. This analysis used the
conventional pollutant (eutrophication) module. The conventional pollutant module represents
the reaction kinetics for nutrients (nitrogen, phosphorus), carbon sources (detritus, algae, and
three CBOD groups), algal growth and dissolved oxygen. The WASP model was used to
provide an estimate for the existing growing season in-stream Chi a concentrations based on
least disturbed land use conditions. The WASP model estimated growing season Chi a
concentrations to be less than 20 |ig/L, which should be protective of aquatic health.
17
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Town Creek Nutrients TMDL
June 2007
3.3.1 WASP Segmentation
Segmentation of the WASP model is based on data included in the National Hydrography
Dataset Plus (NHDPlus) GIS coverage, and is provided in Table 8. Segments of Town Creek
were assumed to have rectangular shape. Cross-sectional area was calculated by multiplying the
depth by the width. The volume of each segment was calculated by multiplying the cross-
sectional area by the segment length.
Table 8. Parameterization of Stream Segments Used in the WASP model
Segment
IFlat
2Flat
3Flat
ITown
2Town
3 Town
4Town
5Town
6Town
7Town
STown
9Town
lOTown
Length
(m)
4478
5467
3291
4655
5217
3417
4513
4536
3752
4041
4014
10000
10000
Width
(m)
6
11
11
122
143
149
141
194
215
220
205
210
220
Depth
(m)
1
1
1
2
2
3
3
3
3
3
3
3
3
Volume
(m3)
16121
36082
21721
1362984
1790474
1323746
1654466
2639952
2581376
2844864
2633184
6720000
7040000
3.3.2 WASP Parameterization and Loadings
The WASP model was parameterized using standard rates, constants and environmental
parameters. Daily flows used in the model were estimated using flows recorded at a nearby U.S.
Geological Survey (USGS) Gage # 72435020 on Town Creek. A weighted drainage area
approach is used to estimate streamflow in Town Creek. The USGS hydro separation program
(HYSEP) was used to separate estimated flows into storm and base flows. Base flows in the
model were assigned BODs, TN and TP concentrations based on the 25th percentile of measured
data. Storm flows were assigned values based on PLOAD results and default EMC values.
EMC values in the WASP model were adjusted until simulated annual average storm load were
comparable to the PLOAD annual load results. Pollutant watershed loadings were entered in the
model on a daily time scale based on results from the PLOAD and USGS HYSEP spreadsheets.
For existing conditions, point source dischargers were represented in the model by using constant
loadings based upon facility design flow and estimates of effluent concentrations, which were set
at TN and TP limits of 10.0 and 5.0 mg/L, respectively. In the TMDL scenario it was necessary
to reduce these limits to achieve the nutrient targets.
Average annual non-point source loads of TN, TP, and BOD5 generated from the PLOAD
spreadsheet (in Ib/yr) were converted to units of kilograms per day (kg/day) for input to the water
quality model. The PLOAD results for TN were partitioned into ammonia, nitrate, and organic
nitrogen; and TP was partitioned into orthophosphate and organic phosphorus. TN was
partitioned as follows: 60% nitrate-nitrite; 15% ammonia-nitrogen; and 25% organic nitrogen.
18
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Town Creek Nutrients TMDL
June 2007
TP was partitioned as follows: 80% orthophosphorus; and 20% organic phosphorus. In the water
quality model BOD5 is simulated as CBOD, and represents the complete biochemical oxidation
of organic matter. Table 9 contains the average annual non-point source loads assumed in the
WASP model for existing conditions.
Table 9. Average Annual Non-point Source Loads
Parameter
BODS
TN
TP
NFS Load (kg/day)
5620
3260
530
NFS Load (Ibs/day)
12364
7172
1166
3.4 Model Results for Existing Conditions
The WASP model for Town Creek was set up with time varying model stresses such as flows
and pollutant loads. This allows the model to simulate both low flow and wet weather
conditions. A ten-year simulation period was used to model two scenarios: (1) represent existing
loads of BOD5 and nutrients from point and non-point sources; and (2) reduce all sources so that
the water quality standard for DO continues to be attained and nutrient targets are attained. In
these TMDLs, the nutrient targets are an in-stream TN concentration of 0.7 mg/L, a TP
concentration of 0.1 mg/L, and Chi a concentrations comparable to values calculated from the
WASP analysis using a least disturbed landuse condition. The model was run with and without
the NPDES facilities to evaluate their impact on in-stream water quality.
A "critical zone" was identified in the model where DO was the lowest and Chi a concentrations
were the highest. Achieving water quality standards at this segment in the model ensures
compliance with standards at all other modeled segments. Model results presented in
Town Creek Critical Zone DO (mg/l) and Cilia (ug/l)
Exisitng Conditions
14
DO mg/L
- Phyto Chi a ug/L
10
1993 1994 1995 1996 1997 1998 1999
1993-2003
2000
2001
2002
2003
Figure 4 depict the predicted DO and Chi a water quality in Town Creek at the critical zone
under existing loads for BOD5 and nutrients. As previously noted, Town Creek was recently
delisted for Organic Enrichment/Low DO based on recent monitoring data. The model for
existing conditions does indicate DO values that do not meet the water quality standard over the
19
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Town Creek Nutrients TMDL
June 2007
ten year period. However, the model was run over a ten year period from 1993-2003 and
therefore, does not include the most recent monitoring data that indicate that a DO impairment
no longer exists in Town Creek.
Town Creek Critical Zone DO (ing/I) anil Chla (ug/
Exisitng Conditions
14
DOmg/L
Phyto Chi a ug/L
1993 1994 1995 1996 1997 1998 1999
1993-2003
2000
2001
2002
2003
Figure 4. Predicted DO Concentrations For Existing Conditions
Table 10 summarizes average annual loads simulated in the model for TN, TP, and BOD5. These
loads include both natural and anthropogenic sources. Point sources loads are calculated by
multiplying the facility design flow by the assumed effluent concentration.
Table 10. Average Annual Loads for Existing Conditions
Parameter
Existing Point
Source Load
(kg/day)
Existing Point
Source Load
(Ibs/day)
Existing Non-point
Source Load
(kg/day)
Existing Non-
point Source Load
(Ibs/day)
Town Creek Watershed
TN
TP
BOD5
95
10
475
209
22
1045
3260
530
5620
7172
1166
12364
20
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Town Creek Nutrients TMDL
June 2007
4.0 DEVELOPMENT OF THE TMDL
The TMDL process quantifies the amount of a pollutant that can be assimilated in a
waterbody, identifies the sources of the pollutant, and recommends regulatory or other
actions to be taken to achieve compliance with applicable water quality standards based
on the relationship between pollution sources and in-stream water quality conditions. A
TMDL can be expressed as the sum of all point source loads (WLA), non-point source
loads (LA), and an appropriate margin of safety (MOS), which takes into account any
uncertainty concerning the relationship between effluent limitations and water quality:
TMDL = Z WLAs + Z LAs + MOS
The objective of a TMDL is to allocate loads among all of the known pollutant sources
throughout a watershed so that appropriate control measures can be implemented and
WQS achieved. 40 CFR §130.2 (i) states that TMDLs are expressed in terms of mass per
time (e.g. pounds per day), toxicity, or other appropriate measures.
The TMDLs for Town Creek Watershed are expressed as the allowable loadings of TN
TP, and BODs that are still expected to achieve the DO standard and an algal biomass
protective of aquatic health (Table 11). The in-stream target concentrations for TN and
TP are 0.7 and 0.1 mg/L, respectively, and are expressed as average annual
concentrations. The target Chi a concentration is 20 |ig/L, expressed as a growing season
average.
Table 11. TMDL Components
Parameter
TMDL
(kg/day)
TMDL
(Ibs/day)
WLA
(kg/day)
WLA
(Ibs/day)
LA (kg/day)
LA
(Ibs/day)
NFS
Percent
Reduction
Town Creek Watershed
BOD
TN
TP
1645
525
92
3619
1155
202.4
475
95
10
1045
209
22
1170
430
82
2574
946
180.4
50%
50%
50%
The TMDL scenario was achieved by reducing effluent BOD, TN, and TP concentrations
to 10-2-0.2 for the significant NPDES facilities and a 50 percent reduction from non-
point sources. Simulated DO and Chi a concentrations for this scenario are shown in
Figure 5. As previously noted, the nutrient impairment in Town Creek is addressed by
reducing BOD and nutrient loads to attain the DO standard and average annual in-stream
nutrient targets (i.e., 0.7 mg/1 TN and 0.1 mg/1 TP). This reduction scenario results in a
growing season average Chi a concentration of 20 jig/1. EPA believes these in-stream
nutrient and Chi a concentrations should be protective of aquatic health for this system.
As previously noted, Town Creek was recently delisted for Organic Enrichment/Low DO
based on recent monitoring data. Model results for existing conditions indicate simulated
DO values that do not meet the water quality standard over a portion of the ten year
period. However, the modeling period was from 1993-2003 and does not include the
21
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Town Creek Nutrients TMDL
June 2007
time period of the most recent monitoring data. The TMDL provides WLAs for BOD
based on modeling results, not on in-stream DO data that indicated that DO water quality
standards are now being attained in Town Creek.
Town Creek Critical Zone DO img.li anil Chla (ug/l)
TMDL Conditions
13
DO mg/L
- Phyto Chi a ug/L
10
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
1993-2003
Figure 5. Predicted DO and Chi a Concentrations for TMDL Conditions
Figure 5 shows that the DO improved to meet the 5.0 mg/1 water quality standard and the
resultant Chi a values with a 50 percent reduction in all nutrient loads from non-point
sources and significant NPDES facilities set at effluent concentrations of 2 and 0.2 for
TN and TP, respectively. Table 12 presents TN, TP, and Chi a concentrations for
existing and TMDL conditions at the critical zone in the Town Creek model.
Table 12. Instream Nutrient and Chi a concentrations for Existing and TMDL Conditions
Existing Conditions
at Critical Zone
TMDL Conditions
at Critical Zone
Annual Average
TN (mg/1)
3.8
0.7
Annual Average
TP(mg/l)
1.5
0.1
Growing Season
Chlfl(ng/l)
60
20
4.1 Wasteload Allocation
Facilities regulated by the NPDES program (including MS4 areas, if any) are assigned a
WLA. The WLAs are expressed separately for continuous discharge facilities (e.g.,
waste water treatment plants) and MS4 areas, as the former discharges during all weather
conditions, and the latter discharges in response to storm events. There are no MS4 areas
in the Town Creek watershed.
The WLAs for Town Creek require reductions of TN, including ammonia, TP, and BODs
to achieve the water quality standard for DO, and in-stream nutrient Chi a concentrations
assumed to be protective of aquatic health for this system. The WLA for the facilities is
22
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Town Creek Nutrients TMDL June 2007
based upon the facilities continuing to meet the daily minimum DO limitation currently
required by their permit. In the absence of facility specific nutrient data and a calibrated
water quality model, the WLAs call for reductions of effluent concentrations from the
significant dischargers presented in Table 4 to 10-2-0.2 in order to achieve water quality
and protect aquatic life in Town Creek. EPA recommends that the State conduct effluent
discharge studies in order to verify the targets proposed in the TMDL. As previously
noted, the TMDL provides WLAs for BOD based on modeling results, not on in-stream
DO data that indicated that DO water quality standards are now being attained in Town
Creek.
4.2 Load Allocation
The primary mode for transport of nutrients to streams is during a storm event.
Modification of the land surface from a pervious land cover to an impervious surface
results in higher peak flow rates that wash nutrient enriched water into the stream. The
load allocation calls for reductions in average annual nutrient loadings from non-point
sources throughout the watershed equal to the percent reductions provided in Table 11.
These reductions are expected to allow DO concentrations to continue to meet standards
and nutrient loads to attain the targets.
4.3 Determination of Watershed Load Reductions
The TMDL scenario was achieved by reducing effluent concentrations from the NPDES
facilities to 10-2-0.2 and a 50 percent reduction from nonpoint sources. Best
management practices (BMPs) should be encouraged in the watershed to reduce potential
TN and TP loads from non-point sources. The watershed should be considered a priority
for riparian buffer zone restoration and other nutrient reduction BMPs. For land
disturbing activities related to silviculture, construction, and agriculture, it is
recommended that practices, as outlined in Mississippi's BMPs: Best Management
Practices for Forestry in Mississippi (MFC, 2000), Planning and Design Manual for the
Control of Erosion, Sediment, and Stormwater (MDEQ, et. al, 1994), and Field Office
Technical Guide (NRCS, 2000), be followed, respectively.
4.4 Margin of Safety
TMDLs shall include a margin of safety (MOS) that takes into account any lack of
knowledge about the pollutant loading and in-stream water quality. In this case the
measured water quality was used directly to determine the reduction to meet the water
quality standard. In this case the lack of knowledge concerns the data, and how well it
represents the true water quality. There are two methods for incorporating a MOS in the
analysis: 1) implicitly incorporate the MOS using conservative model assumptions to
develop allocations; or 2) explicitly specify a portion of the TMDL as the MOS and use
the remainder for allocations. An implicit MOS was incorporated in the analyses through
the use of conservative modeling assumptions.
23
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Town Creek Nutrients TMDL June 2007
4.5 Critical Conditions and Seasonal Variation
The critical conditions can be defined as the environmental conditions requiring the
largest reduction to meet standards. By achieving the reduction for critical conditions,
water quality standards should be achieved during all other times. Critical conditions are
accounted for in the water quality model by selecting the segment requiring the greatest
reductions in pollutant loads. By targeting this segment for reductions all other segments
in the model meet water quality standards. Seasonal variation must also be considered to
ensure that water quality standards will be met during all seasons of the year. Seasonal
variation was considered by targeting the growing season as this is when the greatest
nutrient loadings enter the creek. In addition, the model scenario used a time period of
ten years and accounts for numerous seasons.
24
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Town Creek Nutrients TMDL June 2007
5.0 CONCLUSION
The TMDL allocations proposed for Town Creek are the result of a water quality model
scenario developed by EPA for the impaired streams. Point and non-point source
contributions were reduced in the WASP model so that simulated DO concentrations
continued to achieve water quality standards and simulated in-stream TN and TP
concentrations reflected Ecoregion 65 target levels. In recognition of the lack of data
available for Town Creek as well as the absence of numeric nutrient criteria for these
waters, EPA is asking for comments on the approach used in this report to determine the
TMDL.
In the absence of facility specific nutrient data and a calibrated water quality model, the
WLAs call for reductions of effluent concentrations from the significant dischargers
presented in Table 4 to 10-2-0.2 in order to achieve water quality and protect aquatic life
in Town Creek. EPA recommends that the State conduct effluent discharge studies in
order to verify the targets proposed in the TMDL. As previously noted, the TMDL
provides WLAs for BOD based on modeling results, not on in-stream DO data that
indicated that DO water quality standards are now being attained in Town Creek.
The TMDL calls for a 50% reduction in nonpoint source loads of BOD, TN and TP. It is
recommended that Town Creek be considered as a priority for riparian buffer zone
restoration and other nutrient reduction BMPs. Implementation of these BMPs should
reduce the nutrient load entering the creek. This will provide improved water quality for
the support of aquatic life in the waterbodies and will result in the attainment of the
applicable water quality standards.
5.1 Public Participation
This draft TMDL is being proposed for public review and comment for a 30-day period.
The EPA is notifying the public by publishing a notice of the TMDL through a legal ad in
the statewide newspaper, the Clarion-Ledger. EPA is also providing an e-mail to
members of the public who have requested that MDEQ include them on a TMDL mailing
list. The TMDL is also available for review and comment on EPA Region 4's website:
(http://www.epa.gov/Region4/water/tmdl/mississippi/).
The public may request a copy of the TMDL report by mail from:
Attention: Ms Sibyl Cole,
U.S. EPA Region 4, Water Management Division
61 Forsyth Street, S.W.
Atlanta, Georgia 30303
The public may also submit comments by mail at the address listed above or by email at
cole.sibyl@epa.gov or by phone at 404-562-9437. All comments received during the
public notice period will become a part of the public record for this TMDL.
25
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Town Creek Nutrients TMDL June 2007
REFERENCES
Davis and Cornwell. 1998. Introduction to Environmental Engineering. McGraw-Hill.
MDEQ. 2004. Mississippi's Plan for Nutrient Criteria Development. Office of Pollution
Control.
MDEQ. 2003. Development and Application of the Mississippi Benthic Index of Stream
Quality (MBISQ). June 30, 2003. Prepared by Tetra Tech, Inc., Owings Mills, MD, for
the Mississippi Department of Environmental Quality, Office of Pollution Control,
Jackson, MS. (For further information on this document, contact Randy Reed [601-961-
5158).
MDEQ. 2003. State of Mississippi Water Quality Criteria for Intrastate, Interstate, and
Coastal Waters. Office of Pollution Control.
U.S. EPA. 2001. BASINS PLOAD Version 3.0 Users Manual. Office of Water,
Washington, D.C.
U.S. EPA. 2000. Nutrient Criteria Technical Guidance Manual: Rivers and Streams.
EPA-822-B-00-002. Office of Water, Washington, D.C.
U.S. EPA. 2000. Stressor Identification Guidance Document. EPA/822/B-00/025.
Office of Water, Washington, D.C.
U.S. EPA. 1999. Protocol for Developing Nutrient TMDLs. EPA 841 -B-99-007. Office
of Water (4503F), Washington D.C. 135 pp.
U.S. Department of the Interior, U.S. Geological Survey. 1999. The Quality of Our
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