RECENT COMPARISON STUDIES TO ASSIST IN SELECTION OF ADVANCED MODELING TOOLS FOR TMDL DEVELOPMENT

The degree of analysis, and complexity of tools, required to develop TMDL components ranges from simple, screening level approaches based on limited data to detailed investigations driven by extensive data. A variety of interrelated factors determine the analysis approach and tools that are appropriate for developing each TMDL. The factors include the type of impairment, the physical, biological and chemical processes occurring in the waterbody and its watershed; the size of the watershed; the number and type of sources; the amount of time, data and resources available for TMDL development; and the types and costs of actions needed to implement the TMDL. Generally speaking, more detailed and effort-intensive TMDL development techniques are warranted where there is significant uncertainty concerning whether pollutant sources are attributable to human or natural sources and the anticipated cost of controls is especially high. In such situations professionals responsible for TMDL development are likely to be faced with the need to select and use one or more complex, or advanced, environmental simulation models to support their efforts.This paper focuses on four recent studies performed to compare the capabilities of advanced simulation models. All studies were performed in a research and development context, but all have yielded results that are useful in guiding model selection for TMDL development efforts. Within the TMDL context, models are commonly used to establish the linkage of a variety of water quality targets and sources, and to provide a means for comparing various allocation strategies. The models that were compared in the studies include hydrodynamic, sediment, toxic chemical, eutrophication, bioaccumulation and groundwater models. This paper deals collectively with the materials, conclusions and recommendations that have resulted from the efforts expended in performing recent model comparison studies for the Water Environment Research Foundation (WERF) and the U.S. Environmental Protection Agency Office of Research and Development.