PATHOGEN TMDL DEVELOPMENT FOR SHELLFISH HARVESTING

The United States Environmental Protection Agency (EPA) Region 2 sponsored a technical study to assist New York State Department of Environmental Conservation (DEC) in developing and implementing pathogen TMDLs to achieve the beneficial use of shellfish harvesting. HydroQual and Battelle worked with EPA Region 2 and DEC to develop a flexible yet technically sound TMDL development methodology that can be applied to water bodies in New York State (NYS) that are currently closed for shellfish harvesting. Application of this methodology to two water bodies, namely, Oyster Bay Harbor (OBH) and Mill Neck Creek (MNC) and tidal tributaries is presented here.The water quality data relevant for development of TMDLs in the study area were compiled and compared with NYS standards for total coliforms. Consistent with National Shellfish Sanitation Program's (NSSP) guidelines, these standards include a geometric mean of less than 70 MPN/100mL and an estimated 90th percentile of less than 330/100mL which are also used to determine target load reductions. Appropriate modeling approaches were selected for linking pollutant loads to receiving water responses. A Storm Water Management Model (SWMM) developed by EPA was used to characterize MNC's urban watershed and to assess the hydrologic/hydraulic model parameters using the United States Geological Survey (USGS) flow data in MNC. In OBH, a Watershed Treatment Model (WTM) developed by the Center for Watershed Protection was used to characterize the relative contributions of point and non-point source loadings.A statistical roll-back method was used to develop target load reductions for the individual sources. A margin of safety was incorporated both implicitly through conservative assumptions in the models, and explicitly through inclusion of a percentage of load (10%) in the allocation process. TMDLs were then calculated using the sum of waste load allocations and load allocations. Based on the target load reductions, several management scenarios and a reasonable assurance plan to eventually achieve the desired water quality goals were developed. Some of the recommendations include additional monitoring to confirm pollutant loadings, source load reductions and bacterial source tracking. Discussions in this paper will primarily focus on model development in support of the TMDL development process. Additional information can be found in the draft TMDL document available at NYS website.