The Cedar River Basin Pathogen Total Maximum Daily Load — A Case Study for Watershed-Scale Total Maximum Daily Loads

A recent analysis of the Total Maximum Daily Load (TMDL) program by the Environmental Protection Agency (EPA) concluded that states are confronted with a significant increase in the number of TMDLs that need to be developed in the next decade. The majority of these TMDLS will be more complex than those developed to date and the level of resources and funding are not anticipated to increase. To exacerbate the problem, many states are behind the curve and others have to comply with Consent Decrees that establish an accelerated pace of TMDL completion. EPA concluded that one strategy to address the maximum number of impairments in the most expeditious and cost-effective manner was developing TMDLs using a holistic approach on hydrologically connected impaired segments; i.e., a watershed-scale approach to TMDL development and implementation. This paper presents a case study of the watershed approach applied to Cedar River Basin located in Iowa. The watershed is approximately 8,000 square miles in size, has nine waterbodies impaired for bacteria, and is under a Consent Decree that requires an aggressive schedule for TMDL development. The source allocation was complicated by the fact that there are 11 Municipal Separate Storm Sewer Systems (MS4s) within the watershed, over 100 National Pollutant Discharge Elimination System (NPDES) facilities, and over 80 percent of the watershed is used for raising crops and a large population of livestock. This paper will present a framework that was developed to perform the allocation analysis and estimate the impact reductions to loads from these stressors had on in-stream concentrations throughout the watershed. The framework uses the Bacterial Indicator Tool (BIT), Hydrologic Simulation Program-Fortran (HSPF), the Load Duration Curve (LDC) model, and MATLAB to analyze the data and perform postprocessing on model results. The framework ensures that water-quality impacts from source reductions upstream to downstream adequately account for the impacts of pollutant fate and transport mechanisms. This ensures the TMDLs and implementation plans within the watershed are linked.