How TMDL allocations are structured can have a large impact on the total cost for water pollution control and on who pays what share of this cost. Development of more cost-effective TMDLs on a watershed basis creates opportunities to reduce the overall cost, maximize the effectiveness
of pollution control, and shift pollution control responsibilities from high cost controls over point source discharges to comparatively low cost controls over non-point sources. However, ad hoc allocation procedures, such as equalization of effluent concentrations, do not address cost, effectiveness
or implementation schedule, and the complexity of real watersheds also rules out the use of most ad hoc allocation procedures. Cost, effectiveness, and implementation metrics must be formalized in a model capable of predicting and optimizing the performance of the whole system. An added benefit
of this systematic approach is that it also allows the “fairness” of the allocation procedure to be addressed. The model described here is being developed for a watershed in Los Angeles with cooperation from the US Environmental Protection Agency Region 9 and the Los Angeles
Regional Water Quality Control Board. This test case will examine TMDL allocation scenarios for heavy metal discharges into a highly urban and industrialized watershed subject to periodic and severe storm events. The model is based on a material balance around the entire watershed and includes
both spatial and temporal effects. Spatial effects are handled using a tanks-in-series mixing model of the interconnected sub-watersheds. Individual sources and sinks are considered within each sub-watershed. Temporal effects are handled by considering various time-average pollutant concentrations
and loadings, such as annual, seasonal, and rain event specific. The cost, effectiveness, and implementation schedule for control technologies and strategies for each source are input data to the model. The optimization procedure is flexible and allows the objective function and constraints
to be easily modified; for example, the overall cost can be minimized subject to implementation schedule constraints. Various allocation procedures, such as equalization of effluent concentrations or waste loads, are built into the model for comparison.
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