Total Maximum Daily Load Toogoodoo Creek (Hydrologic Unit Code 030502060401) Impaired Stations 12B-34, 12B-35, 12B-45 Fecal Coliform Bacteria
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Total Maximum Daily Load Toogoodoo Creek (Hydrologic Unit Code 030502060401) Impaired Stations 12B-34, 12B-35, 12B-45 Fecal Coliform Bacteria September 2010 SCDHEC Technical Document Number: 012D-21 Prepared for: Bureau of Water Prepared by: Ann-Marie Altman i Abstract The delineated watershed surrounding impaired stations 12B-35, 12B-34, and 12B-45 (Toogoodoo Creek and Lower Toogoodoo Creek) located within Shellfish Management Area 12B in Charleston County, South Carolina consists of approximately 23.08 square miles of shellfish growing area habitat. Water quality monitoring stations 12B-34, 12B-35, and 12B-45 are listed on the 2008 303(d) list as impaired for shellfish use support due to exceeding the fecal coliform standard. All of these stations have previously been listed in 2006 while 12B-35 was also listed in 2004. Stations are listed as impaired on the basis of at least 30 monthly samples taken over a period of 3 years as required by the National Shellfish Sanitation Program. Meeting the percentage reduction or the water quality standard (WQS) at each station will effectively protect the shellfish harvesting beds in the referenced watershed for human consumption. Station 12B-45 requires a reduction of 66 % while station 12B-35 requires a reduction of 32 % and 12B- 34 requires a reduction of 30% (Table Ab-1). This TMDL document is based on 77-157 data points per each monitored station from 1994-2007 to ensure greater temporal variability. The primary land use of the watershed is forested area (35.45%) followed by wetlands/open water (34.73%). Probable sources of fecal coliform (FC) bacteria are large populations of water birds and wildlife, agricultural runoff and failing septic systems. Existing conditions and percent reductions were calculated using cumulative probability distributions. Compliance with terms and conditions of existing and future NPDES sanitary and stormwater permits (including all construction, industrial and MS4) may effectively implement the wasteload allocation (WLA) and demonstrate consistency with the assumptions and requirements of the TMDL. For SCDOT, existing and future NPDES MS4 permittees, compliance with terms and conditions of its NPDES permit is effective implementation of the WLS to the Maximum Extent Practicable. For existing and future NPDES construction and Industrial stormwater permitees, compliance with terms and conditions of its permit is effective implementation of the WLA. Required load reductions in the load allocation (LA) portion of this TMDL can be implemented through voluntary measures and are eligible for CWA 319 grants. The Department recognizes that adaptive management/implementation (i.e. WLA and LA) of this TMDL might be needed to achieve the water quality standard and we are committed towards targeting the load reductions to improve water quality in the watershed used in the development of this TMDL document. As additional data and/or information becomes available, it may become necessary to revise and/or modify the TMDL target accordingly. ii Table Ab-1. Total Maximum Daily Load for the Toogoodoo Creek TMDL Watershed. Loads are expressed as colony forming units (cfu) per day. WLA LA 90th %tile of 1,2 Margin of Non- Station Existing TMDL WQ Target Non- % Reduction Safety (MOS) Continuous Continuous ID Load (cfu/100ml) (cfu/100ml) Continuous to Meet (cfu/100ml) Sources3 SCDOT7 (cfu/100ml) Sources4,7 Load (cfu/100ml) (% (% Reduction) Allocation7 Reduction) See Note 5 12B-34 58.3 43 40.9 2.1 30% 0% 30% Below See Note 12B-35 60.3 43 40.9 2.1 32% 32%6 32% Below 12B-45 122 43 40.9 2.1 See Note 66% 0%5 66% Below Table Notes: 1. TMDL is expressed as a concentration. If daily average tidal exchange estimates were available, this number could be converted to load in cfu/day by multiplying flow by concentration and a conversion factor. 2. Shellfish WQS = No more than 10% of the samples shall exceed 43cfu/100 ml 3. WLA is expressed as a daily maximum; N/A = not applicable, no point sources. Existing and future continuous discharges are required to meet the prescribed loading for the pollutant of concern. Loadings are developed based upon permitted flow and an allowable permitted maximum concentration of 43/100ml. 4. Percent reduction applies to all NPDES-permitted stormwater discharges, including current and future MS4, construction and industrial discharges covered under permits numbered SCS & SCR. Stormwater discharges are expressed as a percentage reduction due to the uncertain nature of stormwater discharge volumes and recurrence intervals. Stormwater discharges are required to meet percentage reduction or the existing instream standard for pollutant of concern. 5. As long as the conditions within the SCDOT MS4 area remain the same the Department deems the current contributions from SCDOT negligible and no reduction of FC bacteria is necessary. SCDOT must continue to comply with the provisions of its approved NPDES stormwater permit. 6. By implementing the best management practices that are prescribed in either the SCDOT annual SWMP or the SCDOT MS4 permit to address fecal coliform, the SCDOT will comply with this TMDL and its applicable WLA To the maximum extent practicable (MEP) as required by its MS4 permit. 7. Percent reduction applies to existing concentration. iii TABLE OF CONTENTS Abstract.......................................................................................................................................... ii TABLE OF CONTENTS ............................................................................................................ iv 1.0 Introduction....................................................................................................................... 7 1.1 Background..................................................................................................................... 7 1.2 Watershed Description.................................................................................................... 7 1.2.2 Precipitation ................................................................................................................ 9 1.2.3 Winds .......................................................................................................................... 9 1.2.5 Land Use and Soils ..................................................................................................... 9 1.3 Water Quality Standard................................................................................................. 13 2.0 Water Quality Assessment ............................................................................................. 13 3.0 Source Assessment and Load Allocation ...................................................................... 15 3.1 Point Sources................................................................................................................ 17 3.1.1 Continuous Point Sources......................................................................................... 17 3.1.2 Non-Continuous Point Sources................................................................................. 18 3.2 Nonpoint Sources.......................................................................................................... 20 3.2.1 Urban and Suburban Stormwater Runoff ................................................................. 20 3.2.2 Agricultural Runoff................................................................................................... 20 3.2.3 Failing Septic Systems.............................................................................................. 21 3.2.4 Wildlife and Domestic Animals................................................................................ 21 3.2.5 Boat Traffic............................................................................................................... 21 3.2.6 Hydrographic Modification...................................................................................... 22 4.0 Methods............................................................................................................................ 21 5.0 Development of TMDL................................................................................................... 23 5.1 Critical Conditions........................................................................................................ 23 5.2 Wasteload Allocation.................................................................................................... 22 5.2.1 Continuous Point Sources......................................................................................... 23 5.2.2 Non-Continuous Point Sources................................................................................. 23 iv 5.3 Load Allocation............................................................................................................ 23 5.4 Existing Load................................................................................................................ 24 5.5 Margin of Safety ........................................................................................................... 25 5.6 Calculation of the TMDL.............................................................................................. 25 6.0 IMPLEMENTATION .................................................................................................... 28 6.1 Implementation Strategies............................................................................................ 29 6.1.1 Continuous Point Sources........................................................................................