Ocmulgee River basin dissolved oxygen TMDLs

Ocmulgee River Basin Dissolved Oxygen TMDLs
Submitted to:
U.S. Environmental Protection Agency Region 4 Atlanta, Georgia
Submitted by:
Georgia Department of Natural Resources Environmental Protection Division Atlanta, Georgia
June 2001

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Table of Contents

Section
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0

Title

Page

TMDL Executive Summary ..................................................................................3

TMDL Background .............................................................................................. 6

Problem Understanding....................................................................................... 8

Water Quality Standards..................................................................................... 12

Source Assessment ............................................................................................ 13

Technical Approach ............................................................................................ 16

Loading Capacity ................................................................................................ 21

Waste Load and Load Allocations....................................................................... 21

Margin of Safety.................................................................................................. 22

Seasonal Variation.............................................................................................. 23

Monitoring Plan ................................................................................................... 23

Point and Nonpoint Source Approaches ............................................................. 23

Public Participation.............................................................................................. 23

References ......................................................................................................... 23

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1.0 TMDL Executive Summary

Basin Name :

Ocmulgee River

Table 1-1. 2000-303(d) Listed Segments for Dissolved Oxygen

in the Ocmulgee River Basin.

SEGMENT

1999

SEGMENT

LENGTH,

MONITORING

NUMBER

STREAM

(Miles)

STATION 12 DIGIT HUC ID

1

12

030701050302

Alligator Creek

02216028

,

030701050301

2

Big Horse Creek

15

02215400 030701040703

3

Cabin Creek

16

02211380 030701031104

4

Doless Creek

2

Historical 030701030102

5

19

030701050201

Gum Swamp Creek

02215735

,

030701050103

6

House Creek

8

02215276 030701040603

7

Limestone Creek

7

02215040 030701040303

8

Little Ocmulgee River

12

02216000 030701050203

9

Sugar Creek

5

02216190 030701050404

10

24

030701050403

Turnpike Creek

02216187

,

030701050402

Description of Analysis USGS water quality data collected in 1999 identified dissolved oxygen (DO) impairments for Ocmulgee stream segments listed in Table 1-1, and further indicated that these impairments occurred during, and were limited to, summer months, low flow and high temperature conditions. Stream flows during periods of impairment were at, or below, 7Q10 (the minimum 7-day average flow that occurs once in 10 years on the average), which is consistent with the 3-year drought experienced in Georgia from 1998 to 2000. Since the observed DO impairments were clearly driven by persistent low flows and high temperatures, occurring over several summer months, a steady state modeling approach was adopted as appropriate for DO TMDL analysis.

Applicable Water Quality Standards The applicable dissolved oxygen water quality standards for waters in the Ocmulgee River Basin are as follows:
Numeric - GAEPD. A daily average of 5.0 mg/L and no less than 4.0 mg/L at all times for waters supporting warm water species of fish. 391-3-6-.03 (c) (l). (GAEPD, 2000) Natural Water Quality GAEPD. It is recognized that certain natural waters of the State may have a quality that will not be within the general or specific requirements contained herein. This is especially the case for the criteria for dissolved oxygen, temperature, pH and fecal coliform. NPDES permits and best management practices will be the primary mechanisms for ensuring that the discharges will not create a harmful situation. 391-3-6-.03 (7). (GAEPD, 2000) Natural Water Quality EPA. Where natural conditions alone create dissolved oxygen concentrations less than 110 percent of the applicable criteria means or minima or both, the minimum acceptable concentration is 90 percent of the natural concentration. (USEPA, 1986).

Due to naturally occurring low dissolved oxygen in the impaired segments, the EPA natural water quality standard was appropriate to support the proposed allocations. That is, if a model result

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showed a natural dissolved oxygen less than 5.0 mg/L, the natural model result would define the DO standard to be applied. In this case, the standard would become 90 percent of the computed natural DO.

Technical Approach

Model Adopted:

Georgia DoSag steady-state water quality model developed by Georgia Environmental Protection Division.

Calibration Data:

USGS field data from June 1999.

Calibration Conditions:

(1) USGS flows measured in June 1999. (2) USGS Temperatures measured in June 1999. (3) Point source DMR data for June 1999. (4) SOD values for `mixed land uses' based on year-2000 TMDLs for the
South 4 Basins. (5) Depths, velocities, kinetic rates, reaeration, and boundary conditions
based on 1999 USGS field data and/or GAEPD standard modeling practices.

Critical Conditions:

(1) 7Q10 flows recomputed to include data through 1998. (2) Temperatures derived from historic trend monitoring data. (3) Point source discharges at current permit limits. (4) Same SOD for `mixed land uses' as calibration conditions. (5) Same depths, velocities, kinetic rates, reaeration, and boundary
conditions as calibration conditions.

Natural Conditions:

(1) Same flows as critical conditions. (2) Same temperatures as critical conditions. (3) All point sources completely removed. (4) SOD for natural (i.e., fully forested) land use based on year-2000
TMDLs for the South 4 Basins. (5) Same depths, velocities, kinetic rates, reaeration, and boundary
conditions as calibration conditions.

MOS:

Implicit, based on the following conservative assumptions: (1) Drought streamflows persist through the critical summer months at monthly 7Q10 flow values. (2) Hot summer temperatures, based on the historical record, persist for the same critical period. (3) All point sources discharge continuously at their NPDES permit limits for the same critical period. (4) DO saturation, for all flows entering the system, equal those measured during the low DO period in the summer of 1999. (5) Water depths are shallow, generally less than one foot, which aggravates the effect of SOD. (6) Water velocities are sluggish, generally 0.5 fps or less, which intensifies the effect of BOD decay.

Seasonality: Monitoring: Approach:

Dissolved oxygen data showed no impairments outside of the hightemperature, low-flow conditions which occur during the summer months. Follow-up monitoring according to 5-year River Basin Planning cycle (Georgia EPD, 1996) NPDES Permits for point sources; Best management practices for nonpoint sources.

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Date Submitted:

Draft on June 30, 2001.

Table 1-2. Summary of TMDLs for Dissolved Oxygen Listed Segments

in the Ocmulgee River Basin.

SEGMENT

TMDL

NUMBER

STREAM

(lbs/day)

1

Alligator Creek

92

2

Big Horse Creek

139

3

Cabin Creek

767

4

Doless Creek

6

5

Gum Swamp Creek

141

6

House Creek

72

7

Limestone Creek

51

8

Little Ocmulgee River

548

9

Sugar Creek

131

10

Turnpike Creek

204

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2.0 TMDL Background
The State of Georgia is required to develop Total Maximum Daily Loads (TMDLs) for waters not meeting water quality standards, in accordance with Section 303(d) of the Clean Water Act and the U. S. Environmental Protection Agency (EPA) Water Quality Planning and Management Regulations (40 CFR Part 130). Water quality data collected by the United States Geological Survey (USGS) in 1999 indicate that a number of waterbodies in the Ocmulgee River Basin did not achieve water quality standards for dissolved oxygen (DO). These waterbodies were included in the state's 2000-303(d) list, along with the historical list of impaired segments, and proposed by the Georgia Environmental Protection Division (GAEPD).
There are 38 segments listed on the state's 2000-303(d) list as impaired for dissolved oxygen, with 23 in the Altamaha, 10 in the Ocmulgee, and 5 in the Oconee River Basins. The TMDLs are organized in 3 separate reports, 1 for each basin. This report presents the dissolved oxygen TMDLs for the 10 listed segments in the Ocmulgee River Basin identified in Table 1-1. The Ocmulgee River Basin is part of the Middle Three Basins in Georgia as shown in Figure 2-1. The three river basins, the Altamaha, Ocmulgee, and Oconee, are required by the consent decree to have TMDLs developed by June 30, 2001 (Sierra Club v. Hankinson 1997). As shown in the Table 2-1, there are three 8-digit Hydrologic Unit Codes (HUCs) within the Ocmulgee River Basin: the Upper Ocmulgee River Basin, HUC 03070103; the Lower Ocmulgee River Basin, HUC 03070104; and the Little Ocmulgee River Basin, HUC 03070105. The Upper Ocmulgee River Basin flows into the Lower Ocmulgee River Basin near Macon, GA. The Little Ocmulgee River Basin flows into the Lower Ocmulgee River Basin and it flows into the Altamaha River Basin near Hazelhurst, GA.

River Basin
Oconee
Ocmulge e
Altamaha

Table 2-1. Summary of Georgia Middle 3 Dissolved Oxygen TMDLs.

Drainage Area
(square miles)

Impaired Segment
s

1999 WQ Stations

8-digit Hydrologic Unit Codes

5,333

5

66

Upper Oconee (HUC 03070101) Lower Oconee (HUC 03070102)

Upper Ocmulgee (HUC 03070103)

6,086

10

86

Lower Ocmulgee (HUC 03070104)

Little Ocmulgee (HUC 03070105)

2,852

23

35

Altamaha (HUC 03070106) Ohoopee (HUC 03070107)

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GEORGIA
Upper Ocmul gee River Basin

Lower Ocmulgee River Basin

Little Ocmulgee River Basin

8- DIGIT HUC
COUNT Y PO PU LAT ED PLACE

20

0

20

40 Miles

Figure 2-1. Location of the Ocmulgee River Basin.

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3.0 Problem Understanding
USGS collected water quality data in Georgia during 1999. There were a total of 214 stations, including 187 in the Middle Three Basins. There were 86 water quality stations in the Ocmulgee River Basin for 1999. These data showed that dissolved oxygen impairments occurred exclusively during the summer months. Furthermore, all of the impairments were limited to small, headwater streams where the drainage areas are relatively small and dry weather flows are low, or zero. In the downstream reaches of larger watersheds where the flows are higher and not intermittent, and the assimilative capacity is therefore greater, the dissolved oxygen concentrations always met the minimum standard of 4.0 mg/L, and the daily average of 5.0 mg/L.

14 USGS02213700 - OCMULGEE RIVER NEAR WARNER ROBINS, GA

12

USGS02216187 - TURNPIKE CREEK NEAR LUMBER CITY, GA

Free Flowing River in Summer

10

DO (mg/L)

8

6 Intermittent Stream in Summer
4

2

0

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Year 1999

Nov

Dec

Figure 3-1. Comparison of Dissolved Oxygen Data at Two Ocmulgee River Basin Locations.

Figure 3-1 illustrates this important finding by comparing measured DO levels in a non-intermittent, free-flowing stream to DO levels found in a small stream with low or no observable flow. The free flowing river data were collected at USGS 02213700, which is located on the Ocmulgee River near Warner Robbins. The other data in Figure 3-1 were collected on Turnpike Creek, a small headwater stream in the Little Ocmulgee River Basin. These two sets of data are representative of DO conditions observed at other stations in the basin.

GAEPD staff visited the Ohoopee River Basin on April 11, 2001 to observe stream characteristics such as velocities, depths, floodplain widths and vegetation, and adjacent land use. These characteristics have a direct bearing on low dissolved oxygen concentrations. At all sampling stations visited on the Ohoopee River mainstem and its tributaries, the streams flowed through dense, forested swamps with forested stream buffers. All of the visited sites were similar in that the stream would flow out of the shaded forested swamp, which receives a significant amount leaf litterfall, into small clearings for bridges and road access. At each clearing, direct sunlight, small patches of aquatic plants, and heavily vegetated floodplains were observed. Figure 3-2 depicts a

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sampling site on the Little Ohoopee River. Even though the site visit occurred during a period of higher flow, these essential characteristics are still apparent.

Figure 3-2. Picture of Sampling Site on Little Ohoopee River (SR56) Near Covena, GA. Figures 3-3 and 3-4 show the impaired segments for dissolved oxygen and the water quality stations that indicated each impairment. Nine segments shown in Table 1-1 were listed as a direct result of the 1999 dissolved oxygen data, with the exception being Doless Creek as the historical listing. All field data relevant to the Middle Three Basins TMDLs were compiled by GAEPD and included in electronic database files. The data are managed in the Water Resources Data Base (WRDB), a software database that was developed by GAEPD. Project data file(s) contain the following information comprising over a half million records:
1. Historic trend monitoring data through December 31, 1998, 2. 1999 GAEPD/USGS water quality data, and 3. Historic USGS daily average flow data through December 31, 1998.
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ATLANTA

Doless Creek

Cabin Creek

N
8-D I GIT H U C D O IM P AIR E D S EGM EN T RF1 SE GM ENT COUNTY P OPUL ATED PL ACE

MACON

Limestone Creek

GumSwamp Creek Alligator Creek

Turnpike Creek House Creek

Little OcmulgeeRiver
Sugar Creek Big Horse Creek

Figure 3-3. 303(d) Listed Segments for Dissolved Oxygen in the Ocmulgee River Basin. 11

Ocmulgee River Basin Dissolved Oxygen TMDLs

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N

%

%

%%%%%%%%%%%%%

% %%
% %

%%

ATLANTA

%

%%

%%%%%%%%

% %

%

%% %

%%

%

%%

% %%%

02211380 %

%

%%

%

%

%

%

%

% 1999 OBSERVAT ION STATION
8-DIGIT HUC DO IMPAIR ED SEGMENT
RF1 SEGMENT CO UNTY
POPU LAT ED PLACE

%

%

MACON % %
% %%

% %

% %

%

% % 02215040
% %
%

%
% 02215735

%

02216028 %

02216187

02216000

% %

%

%

%

% 02215276

% 022%15400

02216190

20

0

20

40 Miles

Figure 3-4. 1999 USGS Water Quality Stations in the Ocmulgee River Basin. 12

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4.0 Water Quality Standards
All DO impaired waterbodies in the Ocmulgee River Basin have been assigned a water use classification of fishing. Georgia's water quality standards specify the following DO criteria for this use classification:
Numeric. A daily average of 5.0 mg/L and no less than 4.0 mg/L at all times for waters supporting warm water species of fish*. A daily average of 6.0 mg/L and no less than 5.0 mg/L at all times for waters designated as trout streams by the Wildlife Resource Division. (*There are no designated trout streams in the Ocmulgee River Basin). Georgia EPD, 2000
Certain waters of the state may have conditions where dissolved oxygen is naturally lower than the numeric criteria specified above and therefore cannot meet these standards unless naturally occurring loads are reduced or streams are artificially or mechanically aerated.
Natural Water Quality. "It is recognized that certain natural waters of the State may have a quality that will not be within the general or specific requirements contained herein. This is especially the case for the criteria for dissolved oxygen, temperature, pH and fecal coliform. NPDES permits and best management practices will be the primary mechanisms for ensuring that the discharges will not create a harmful situation." 391-3-6-.03(7) Georgia EPD, 2000
EPA Dissolved Oxygen Criteria were used to address these situations. Alternative EPA limits are defined as 90% of the naturally occurring dissolved oxygen concentration at critical conditions.
"Where natural conditions alone create dissolved oxygen concentrations less than 110 percent of the applicable criteria means or minima or both, the minimum acceptable concentration is 90 percent of the natural concentration." Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Freshwater), EPA440/5-86-003, April 1986. US EPA, 1986
Accordingly, if the naturally occurring dissolved oxygen exceeds GAEPD numeric limits at critical conditions then the GAEPD numeric limits apply. If naturally occurring DO is lower than the GAEPD numeric limits then 90% of the natural DO will become the minimum allowable.

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5.0 Source Assessment

Point Sources
GAEPD maintains a database of current NPDES Permits and GIS files that locate each permitted outfall. Monthly Discharge Monitoring Reports (DMRs) for 1999 were downloaded from the Permit Compliance System (PCS). Table 5-1 shows the 8 point sources in the Ocmulgee River Basin that discharge into or upstream of an impaired segment. Table 5-2 contains the June 1999 DMR data that were available for model development. Figure 5-1 shows the location of each facility relative to the impaired segments.

Table 5-1. Contributing Point Sources in the Ocmulgee River Basin.

NPDES Permit

Facility Name

Receiving Water 8-Digit HUC

County

GA0003409 Spring Industries, Inc.

Cabin Creek

Upper Ocmulgee

Spalding

GA0020214

Griffin (Cabin Creek WPCP)

Cabin Creek

Upper Ocmulgee

Spalding

GA0021440

Alamo Pond

Little Ocmulgee River Little Ocmulgee

Wheeler

GA0032344

Scotland Pond

Little Ocmulgee River Little Ocmulgee

Telfair

GA0049280

Middle GA Nursing Home Eastman

Gum Swamp Creek

Little Ocmulgee

Dodge

GA0025887

Cadwell WPCP

Alligator Creek Little Ocmulgee

Laurens

GA0026298 McRae Gum Swamp Creek Little Ocmulgee River Little Ocmulgee

Telfair

GA0026310

Eastman (Roach Branch WPCP)

Little Ocmulgee River Little Ocmulgee

Dodge

Table 5-2. NPDES Permit Limits for Contributing Point Sources.

June 1999 Monthly Average Permit Limits

NPDES Permit

Facility Name

Flow (mgd)

DO (mg/L)

BOD5 (mg/L)

NH3 (mg/L)

GA0003409 Spring Industries, Inc.

1.0

5

20

NA

GA0020214

Griffin (Cabin Creek WPCP)

1.5

5

30

17.4

GA0021440

Alamo Pond

0.07

6

30

NA

GA0032344

Scotland Pond

0.18

6

30

NA

GA0049280

Middle GA Nursing Home Eastman

0.017

6

30

NA

GA0025887

Cadwell WPCP

0.048

6

30

NA

GA0026298 McRae Gum Swamp Creek

0.2

6

30

NA

GA0026310

Eastman (Roach Branch WPCP)

0.9

6

15

10

Notes: NA = not available.

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Figure 5-1. Contributing Point Sources in the Ocmulgee River Basin.
Nonpoint Sources Surface Washoff and Leaf Litter Decay
In 1999, many streams in the basin were dry or had ponded areas and stagnant pools as a result of a 3-year drought in Georgia. Due to the absence of rainfall during the summer months of 1999, the critical time period, stormwater did not contribute any washoff of materials into the streams. Any constituents that may have washed off of disturbed land surfaces in previous months or years have either: (1) already flushed out of the system along with the water column flow; or, (2) a portion may have settled out to become a part of the stream channel bottom. In this manner, the historic washoff of settleable material could accumulate and exert an additional sediment oxygen demand attributable to man's land disturbing activities. The constituents of concern from surface washoff include the fraction of ammonia and BOD5 that become an integral part of channel bottom sediments and thus become a potential source of sediment oxygen demand. Table 5-3 describes the land use distributions associated with each impaired stream. Note the relatively high percentages of forested and wetland land uses combined and the low percentages of built up areas.
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Table 5-3. Land Uses Associated with Impaired Segments in the Ocmulgee River Basin.

Total

Built-Up Built-Up

Contributing Cropland Pasture Forest Wetland Impervious Pervious

Seg. #

Stream

Area (acres) (%)

(%)

(%)

(%)

(%)

(%)

1

Alligator Creek

43,795

30.4

8.5

45.8

5.6

1.0

8.6

2

Big Horse Creek

97,926

27.7

8.4

50.9

4.2

0.9

7.9

3

Cabin Creek

21,502

5.1

17.5

68.7

0.7

2.8

5.2

4

Doless Creek

1,029

0.1

0.0

39.1

0.0

26.7

34.1

5 Gum Swamp Creek 138,102 30.5

7.6

39.1

12.3

1.3

9.1

6

House Creek

52,307

31.4

10.2

44.3

3.4

1.1

9.5

7

Limestone Creek

36,956

43.0

15.0

24.4

11.5

0.9

5.2

8 Little Ocmulgee River 229,350 27.1

6.6

43.9

10.7

1.4

10.2

9

Sugar Creek

55,381

21.1

7.6

52.9

4.3

1.5

12.7

10

Turnpike Creek

51,098

21.3

7.8

52.2

4.2

1.5

13.0

Most of the streams in the Ocmulgee Basin receive significant natural contributions of oxygen demanding organic materials from local wetlands and forested stream corridors, in addition to the aforementioned nonpoint sources of sediment oxygen demand associated with man's land disturbing activities. The following sources of naturally occurring organic materials have been identified:
Adjacent wetlands and swamps with organically rich bottom sediments; and, Direct leaf litterfall onto water surfaces and adjacent floodplains from overhanging trees
and vegetation.
Leaf litterfall is a major contributor to the amount of dissolved organic matter in the stream water column and the amount of sediment oxygen demand being exerted. Many streams in southern Georgia are also referred to as "blackwater" streams because of highly colored humic substances leached from surrounding marshes and swamps. In addition, low dissolved oxygen in blackwater streams is very common in the summer months when the temperatures are high and the flows are low (Meyer, 1992). The oxygen demanding effects of leaf litterfall were reflected here in two ways: (1) by lowering the DO saturation of water entering the channel from adjacent swampy areas caused by decaying vegetation; and, (2) by increasing SOD associated with vegetation decaying on stream channel bottoms.

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6.0 Technical Approach
The technical approach is described by the steps below:
Model Selection and Setup, Calibration Data, SOD Representation, Calibration Conditions, Critical Conditions, Natural Conditions, and Allocations
Model Selection and Setup Initially, an analysis was performed to correlate indicated impairments to basic causes such as point and nonpoint contributions, flow conditions, stream and watershed characteristics, seasonal temperature effects, and others. From this analysis it was obvious that impairments coincided with low or zero flows, slow stream velocities, shallow water depths, and high temperatures. Inflows of very low dissolved oxygen waters from adjacent marshes and forested swamps compounded the situation. Since all of the impairments noted in 1999 occurred during sustained periods of consistently low flows, a steady-state modeling approach was adopted as appropriate to represent the relevant conditions in the impaired streams. The steady-state Georgia DoSag, developed by the Georgia Environmental Protection Division, was selected for the following reasons:
It's simplified without unnecessary complexity. Conforms to GAEPD standard practices for developing wasteload allocations. Works well for low flow and high temperature conditions. Can be developed with a limited dataset, which existed in 1999. Able to handle branching tributaries and both point and nonpoint source inputs.
Georgia DoSag also provides a complete spatial view of a system, upstream to downstream, indispensible for understanding important differences in stream behavior at various locations throughout a basin. The model computes dissolved oxygen using an enhanced form of the Streeter-Phelps equation (Thomann and Mueller, 1987). It is applied to each stream reach over small incremental distance intervals.
There were a total of 6 DOSAG models developed to represent the 10 listed segments in the Ocmulgee River Basin. USGS quadrangle maps along with Arcview and MapInfo spatial graphics files were used to develop drainage areas, stream lengths, bed slopes, and other physical input data for each model.
Calibration Data The model calibration period was determined from an examination of the USGS 1999 water quality data, for each station located on an impaired segment. The data were plotted and evaluated for streamflow, dissolved oxygen, water temperature, BOD5, and ammonia to determine a worst case for dissolved oxygen. The combination of the lowest, steady flow period with the lowest dissolved oxygen, and highest BOD concentrations, defined the critical modeling period. For all 10 of the impaired segments in this report, June 1999 was adopted as the critical period for model calibration. There is one stream gage in the Little Ocmulgee on Turnpike Creek (USGS02216187) that had zero flow during the summer months of 1999. A USGS site with historical stream flow records was used to compute a productivity factor. The 7Q10 published for the Little Ocmulgee River at Towns, GA (USGS 02216000) an average of 5.3 cfs for June through September. A recomputed 7Q10 of 3.85 cfs was calculated to include data through 1998. A productivity factor of 0.012 cfs/mi2 (using a drainage area of 329 square miles). This productivity factor was applied
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consistently to the basin. The June 1999 average of dissolved oxygen, BOD5, and ammonia were also extracted from the dataset for each sampling station. BOD5 was converted to CBODU by multiplying by an f-ratio of 2.5 (standard GAEPD modeling practice) and ammonia was converted to NBODU by multiplying by the stoichometric conversion factor of 4.57. These values, thusly determined, were incorporated into the DoSag model calibration files.
SOD Representation There were no field sediment oxygen demand (SOD) measurements in the Middle Three Basins. For the 2000 TMDLS in the South 4 Basins, there were several SOD measurements that ranged from 0.9 to 1.9 g/m2/day. SOD is an important part of the oxygen budget in these shallow streams. Accordingly, it is necessary to be realistic in the development and application of SOD values in the Middle Three Basins models and to be consistent with last year's findings from the South 4 Basins. For this reason, an examination of SOD results from the year-2000 TMDLs in the South 4 Basins was performed. Results from all calibrated models of existing conditions in June 1998 were compiled and summarized. An average value of existing SOD was determined to be 1.35 g/m2/day. This represented 12 models that had mixed land uses and varying degrees of point source activity. When the same 12 models were re-run under natural conditions (assuming zero point source discharges and completely forested watersheds), SOD averaged 1.25 g/m2/day. These two values were adopted for the Middle Three Basins to represent SOD for: (1) mixed land uses, including agriculture; and, (2) natural or totally forested watersheds, respectively. From this, the anthropogenic nonpoint source contributions, those caused by man's land disturbing activities, are accounted for in the 0.1 g/m2/day difference between the two adopted SOD values.
Calibration Conditions Monthly average values of DO, CBODU, and NBODU for June 1999 were used as in-stream targets to calibrate the models as discussed previously. Calibration flows throughout the basin were set equal to 0.012 cfs/mi2 derived from daily flow records at USGS 02216000 (Little Ocmulgee River at Towns, GA). Water temperature varied across the basin using June 1999 monthly averages from each sampling station. Point source discharges were put into the model at their June 1999 DMR values for DO, BOD5, NH3, and flow. Headwater and tributary water quality boundaries were developed from in-stream field data, expected low DO saturation values (Meyer, 1992), and GAEPD standard modeling practices. SOD was set to 1.35 g/m2/day to reflect mixed land uses. Figure 6-1 depicts a longitudinal dissolved oxygen calibration curve for the mainstem of the Little Ocmulgee River developed using this approach. The Little Ocmulgee River serves as a good illustrative example for this basin because it had more instream sampling stations than any other listed segment and thus can provide the best indication of the success to be expected from this modeling approach throughout a river system and for other river basins. The reader is free to judge the success of this calibration. However, considering the serious scarcity of field data to work with and the fact that major portions of the Ocmulgee River Basin had low or no observable flow, this calibration is viewed as exceptionally good. Accordingly, the DoSag models developed for TMDL analysis can be viewed as dependable and instructive.

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8 Representing June 1999

Little Gum

7

Swamp

Creek

1 Wtf

Roach Branch

1 Wtf

6

1 Wtf

Joiner

Alligator

5

Creek

Creek

1 Wtf

2 Wtfs

4

3

Dissolved Oxygen, mg/L

2

Sugar and

Turnpike

Model Results Creeks

1
Field Data

0

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

River Mile

Figure 6-1. Dissolved Oxygen Calibration for the Little Ocmulgee River.

Critical Conditions Model critical conditions were developed, in accordance with GAEPD standard practices, to assess dissolved oxygen standards, to determine if a problem exists requiring regulatory intervention, and to establish a level of protection if necessary. To do this, each calibrated model was modified in the following manner. For flows, a 7Q10 flow (the minimum 7-day average flow that occurs once in 10 years on the average) was adopted consistent with provisions in Georgia's Water Quality Regulations. Older published 7Q10 flow values in the USGS report (Carter and Fanning, 1982) only considered data through 1979. To account for the droughts in the late 1980s and 1990s, new monthly 7Q10 flow values were calculated; and, an average of the June through September 7Q10 low flows was adopted. Productivity factors were then calculated from the revised 7Q10 flow values and applied uniformly throughout the basin. The adopted productivity factor equaled 0.04 cfs/mi2. Critical water temperatures were developed by examining the long-term trend monitoring data and fitting a harmonic sine function to all of the historical data at a given station. A June through August average temperature from the harmonic fit was used to represent each trend station. Critical temperatures in other locations in the basin were prorated on the basis of June 1999 field data. Point sources were incorporated into the critical conditions models at their current NPDES Permit limits. Water quality boundaries and all other modeling rates and constants were the same as those in the calibrated models, including SOD = 1.35 g/m2/day representing mixed land uses. To determine the effects of point sources alone, at critical conditions, a parallel set of model runs were made with point source flows set equal to zero.
The Little Ocmulgee River calibration was identical to the critical conditions model. The point sources in the Little Ocmulgee River Basin had only permitted data available through the PCS. No DMR data was found, so permitted values were used. It was assumed since the flows were at or below 7Q10 and temperatures were high, that the model would be calibrated at critical conditions. Therefore, 0.012 cfs/mi2 for a productivity factor was used in calibration, critical, and natural

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Ocmulgee River Basin Dissolved Oxygen TMDLs

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conditions.
Natural Conditions For the natural conditions runs, two relevant changes were made to the critical conditions models. First, SOD was changed from 1.35 g/m2/ day to 1.25 g/m2/day to reflect the change from mixed land uses to natural or completely forested land uses. And second, all point source discharges were completely removed. All other model parameters remained the same. The results of the natural conditions runs are plotted in Figure 6-2 along with the June 1999 and critical conditions results for comparison.

8 Representing June 1999
7

6
DO Standard = 5.0 mg/L 5
4

Natural Conditions Critical Conditions without PS

Dissolved Oxygen, mg/L

3

2
Critical Conditions with PS
1

0

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

River Mile

Figure 6-2. June 1999 Critical and Natural Conditions for the Little Ocmulgee River.

Allocations Allocations were based on EPA Dissolved Oxygen Criteria that states if the natural dissolved oxygen is less than the standard, then only a 10% reduction in the natural condition is allowed. Or, the target limits are defined as 90% of the naturally occurring dissolved oxygen concentration at critical conditions. This target and critical dissolved oxygen results for the Little Ocmulgee River are plotted in Figure 6-3.
There are 2-point source reductions that will be implemented with this TMDL. Eastman Roach Branch WPCP (GA0026310) will need to be reduced 100% to meet the target requirements for Gum Swamp Creek. It is currently permitted at a monthly average of 15 mg/L for BOD5, 10 mg/L for NH3, and 0.9 mgd for flow. These values compute to a load for 681 lbs/day into a segment that can only assimilate 141 lbs/day. Its effect is apparent in Figure 6-2 at river mile 45. Griffin - Cabin Creek WPCP (GA0020214) will also need to reduced by 75% so that Cabin Creek will meet its designated use. The 995 lbs/day of ammonia load alone, not including CBOD, contributes to the low dissolved oxygen in the stream. This point source will need to be reduced

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Ocmulgee River Basin Dissolved Oxygen TMDLs

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Figure 6-3 also shows the influence of agricultural nonpoint sources, that is, with point sources removed. The plot includes the target dissolved oxygen concentration (equal to 90% of the natural conditions) and two other sets of model results: (1) with both point and nonpoint sources; and, (2) with nonpoint sources alone. The critical condition without point sources but including nonpoint source (labeled No PS) shows dissolved oxygen above the target line for almost all of the Little Ocmulgee River. From this, the agriculture nonpoint source by itself does not exceed the 10% of natural limit and therefore would not need regulatory intervention.

8 Representing June 1999
7

6 DO Standard = 5.0 mg/L
5 4

Target DO = 90% of Natural
Critical Conditions without PS Critical Conditions without 1 PS

Dissolved Oxygen, mg/L

3

2
Critical Conditions with PS
1

0

90

85

80

75

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

River Mile

Figure 6-3. Critical Condition Scenarios and 90% of Natural Conditions of the Little Ocmulgee River Basin.

Figure 6-4 represents a different type of allocation scenario, a watershed that is impaired for DO but does not contain any point sources. The plots in Figure 6-4 are from the Turnpike Creek where agriculture, which comprises 30% of the watershed, contributes the anthropogenic, nonpoint source load. Moreover, this level of agricultural contribution is typical for all impacted segments in the basin. The results show that the DO at critical conditions, reflecting the full effect of agricultural activities, is consistently above the target DO of 90% of natural conditions. Therefore, no load reductions would be necessary in this watershed because nonpoint contributions do not consume more than 10% of the naturally occurring DO.

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Ocmulgee River Basin Dissolved Oxygen TMDLs

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6 Representing June 1999
5

Dissolved Oxygen, mg/L

4 Critical Conditions
3

2
Target DO = 90% of Natural 1

0

45

40

35

30

25

20

15

10

5

0

River Mile

Figure 6-4. Critical Conditions and 90% of Natural Conditions for a Watershed without Point Sources (Turnpike Creek).

7.0 Loading Capacity
The first step in the process was to determine naturally occurring dissolved oxygen concentrations for the impaired waterbodies. By doing so, the applicable water quality standard used for TMDL development can be identified.
To determine naturally occurring dissolved oxygen concentrations, the steady-state DOSAG models were run, at critical conditions, with zero point source inputs and nonpoint source inputs representing forested or wetland conditions free from man's influences. According to EPA Dissolved Oxygen Criteria, the target limits were identified as 90% of the naturally occurring concentration.
After identifying the dissolved oxygen target limits, the models were run at critical conditions to determine the loading capacity of the waterbody. This was accomplished through a series of simulations aimed at meeting the dissolved oxygen target limit by varying source contributions. The final acceptable scenario represented the TMDL (and loading capacity of the waterbody).
8.0 Waste Load and Load Allocations
Two critical components of the TMDL are the Waste Load Allocations (WLAs) and the Load Allocations (LAs). The WLAs represent the load allocations to point source facilities contributing to impaired waterbodies, while the LAs represent load allocations to the nonpoint source contributions. WLAs and LAs sum to represent the entire TMDL, because MOS is implicitly considered through model assumptions.
The partitioning of allocations between point (WLA) and nonpoint (LA) sources shown in Table 8-1 was based on modeling results and professional judgment to meet the TMDL. The existing WLA is separated into `Direct' and `Upstream' contributions. The `Direct' loads are the point source loads discharging directly into the impaired stream segment. The `Upstream' load is one that discharges in an upstream segment and is transported downstream into the impaired segment. The model

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Ocmulgee River Basin Dissolved Oxygen TMDLs

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was used to account for in-stream, kinetic processes that would occur from the discharge point to the upstream boundary of the impaired segment. The WLAs may be modified by GAEPD during the NPDES permitting process. The TMDLs will be used to assess the permit renewals in the impaired segments. The nonpoint source loads for the Existing LA and TMDL were computed from the model boundary conditions, which include the stream, tributary, and headwater model boundaries.

Table 8-1. Existing and TMDL Loads for Impaired Segments in the Ocmulgee River Basin.

Existing

Existing Upstream

Segment

Direct WLA WLA

Number

Stream

(lbs/day) (lbs/day)

1

Alligator Creek

NA

20

2 Big Horse Creek NA

NA

3

Cabin Creek

1807

NA

4

Doless Creek

NA

NA

5

Gum Swamp

Creek

NA

689

6

House Creek

NA

NA

7

Limestone

Creek

NA

NA

8 Little Ocmulgee

River

190

NA

9

Sugar Creek

NA

NA

10 Turnpike Creek NA

NA

Existing LA
(lbs/day) 72 139 44 6
132 72
51
358 131 204

Total Existing
Load (lbs/day)
92 139 1851
6
141 72
51
548 131 204

TMDL (lbs/day)
92 139 767
6

%

%

Reduction Reduction

WLA

LA

None

None

NA

None

75%

NA

None

141

100%

None

72

NA

None

51

NA

None

548

None

None

131

NA

None

204

NA

None

9.0 Margin of Safety
The margin of safety (MOS) is required in the TMDL development process. There are two basic methods for incorporating the MOS into the TMDL (USEPA 1991):
Implicitly incorporate the MOS using conservative model assumptions. Explicitly specify a portion of the total TMDL as the MOS.
MOS was incorporated implicitly in this dissolved oxygen TMDL development based on the following conservative assumptions:
Drought streamflows that persist through the critical summer months at monthly 7Q10 flow values.
Hot summer temperatures, based on the historical record, that persist for the same critical period.
All point sources discharge continuously at their NPDES Permit limits for the same critical period.
DO saturation, for all flows entering the system, equal to those measured during the low DO period in the summer of 1999.
Shallow water depths, generally less than one foot, which aggravates the effect of SOD. Slow water velocities, generally 0.5 fps or less, which intensifies the effect of BOD decay.

10.0 Seasonal Variation
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Ocmulgee River Basin Dissolved Oxygen TMDLs

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The Statute and regulations require that a TMDL be established with consideration of seasonal variations. Since impairments occurred only during critical summer months, and not during other times of year, a seasonal variation in the TMDL was neither necessary nor appropriate.

11.0 Monitoring Plan
The GAEPD has adopted a basin approach to water quality management; an approach that divides Georgia's major river basins into five groups. Each year, the GAEPD water quality monitoring resources are concentrated in one of the basin groups. One goal is to continue to monitor 303(d) listed waters. This monitoring will occur in the next monitoring cycle for the Ocmulgee in 2004 and will help further characterize water quality conditions resulting from the implementation of best management practices in the watershed.

12.0 Point and Nonpoint Source Approaches
Permitted discharges will be regulated through the NPDES permitting process described in this report. GAEPD is working with local governments, agricultural, and forestry agencies such as the Natural Resources Conservation Service, The Regional Development Councils, the Georgia Soil and Water Conservation Commission, and the Georgia Forestry Commission to foster the implementation of best management practices to address nonpoint sources. In addition, public education efforts will be targeted to individual stakeholders to provide information regarding the use of best management practices to protect water quality.

13.0 Public Participation
A thirty-day public notice will be provided for this TMDL. During this time, the availability of the TMDL will be public noticed, a copy of the TMDL will be provided as requested, and the public will be invited to provide comments on the TMDL.
14.0 References
Carter, R.F. and J.D. Fanning, 1982. Monthly Low-Flow Characteristics of Georgia Streams. United States Geological Survey prepared in cooperation with the Georgia Department on Natural Resources Environmental Protection Division, Open-File Report 82-560.
Dyar, T.R. and S.J. Alhadeff, 1997. Stream-Temperature Characteristics in Georgia. United States Geological Survey prepared in cooperation with the Georgia Department on Natural Resources Environmental Protection Division, Water-Resources Investigations Report 964203.
Edwards, R.T. and J.L. Meyer, 1987. Metabolism of a subtropical low gradient blackwater river." Freshwater Biology, 17:251-263.
Georgia EPD, 1978. Modeling Procedures Manual. Prepared for Georgia Department of Natural Resources, Environmental Protection Division by Larry A. Roesner and Robert P. Shubinski.
Georgia EPD, 1996. Georgia's Watershed Protection Approach: River Basin Management Planning, Draft Program Description. February 1996.
Georgia EPD, 1997. Water Quality Investigation of the Ohoopee River and Tributaries in the Swainsboro and Oak Park, Georgia Area, July-September, 1997.
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Georgia EPD, 2000. Rules and Regulations for Water Quality Control. Chapter 391-3-6. Revised July 2000. Georgia Department of Natural Resources. Environmental Protection Division. Atlanta, GA.
Meyer, J.L., 1992. Seasonal patterns of water quality in blackwater rivers of the Coastal Plain, Southeastern United States. Water Quality in North American River Systems, Battelle Press, Columbus, Ohio, pages 249-276.
Meyer et al., 1997. Organic matter dynamics in the Ogeechee River, a blackwater river in Georgia, USA. Journal of the North American Benthological Society, 16(1):1.
Sierra Club v. Hankinson. Civil Action File No. 1: 94-CV-2501-MHS. Consent decree signed on July 15, 1997.
Thomann, R.V. and J.A. Mueller, 1987. Principles of surface water quality modeling and control. Harper Collins Publishers Inc., New York.
Tsivoglou, C. and L.A. Neal, 1976. Tracer measurement of reaeration: III. Predicting the reaeration capacity of inland streams, Journal of Water Pollution Control Facilities, December, pages 2669-2689.
USEPA, 1991. Guidance for Water Quality Based Decisions: The TMDL Process. EPA 440/4-91001. U.S. Environmental Protection Agency; Assessment and Watershed Protection Division, Washington, DC.
USEPA, 1986. Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Freshwater). Office of Water Regulations and Standards Criteria and Standards Division, EPA440/5-86003.
Wharton, C. H. and M. M. Brinson, 1979. Characteristics of southeastern river systems. Pages 3240 in R. R. Johnson and J. F. McCormick, editors. Strategies for protection and management of floodplain wetlands and other riparian ecosystems. U. S. Forest Service General Technical Report WO-12. National Technical Information Service, Springfield, VA.

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