TMDL: Development of an Optimized WLA Strategy

Almost 25 years after the passing of the Clean Water Act (CWA), the Environmental Protection Agency (EPA) has introduced the concept of Total Maximum Daily Load (TMDL). This concept encompasses the determination of the Load Capacity (LC) of a stream and the associated Waste Load Allocation (WLA) for point and Load Allocation (LA) for non-point source pollutants. A fundamental pillar of TMDL is the recognition that the conflicting interests and demands of potential users of an aquatic system need to be balanced. It is therefore imperative to include a framework that allows optimization as the omission of the latter causes excess costs that otherwise could potentially be reduced. This paper presents a methodology to develop a very efficient WLA procedure. Specifically, it incorporates the imposed environmental constraints and at the same time optimizes the discharge rates into the aquatic system. It is shown (using the Potomac River Estuary as an example domain) that using a 2-dimensional dynamic modeling approach with a high temporal discretization allows identifying optimum loadings for a large number of discharges along a stream. Various aspects like concentration of dischargers in a small stretch of stream, number and placement of monitoring nodes and varying water quality standards along the stream are investigated.