A Practical Methodology for Determining Trading Ratio ffrom TMDL Allocation Matrix in Support of Watershed-based Permitting

摘要

In recent years, an innovative approach to the National Pollutant Discharge Elimination System (NPDES) program is watershed-based NPDES permitting, which aims at achieving new efficiencies and environmental results at a watershed level. It also provides a process for considering all stressors / pollutant sources within a hydrologically-linked drainage basin. Driven by pollution reduction requirements from Total Maximum Daily Load (TMDL) allocation, watershed-based pollutant trading has emerged as a viable compliance option for TMDL implementation and NPDES permitting strategy for industrial and municipal point sources. However, setting of trading ratios in watershed-based trading can be a contentious issue given the inherent scientific uncertainty of the tradeoffs between point and nonpoint sources. In fact, it was already listed as an obstacle by several pollutant trading programs. This paper presents a practical methodology for estimating “equivalent trading ratio” (ETR) and links uncertainty analysis with trading ratio determination from a TMDL allocation matrix. Determination of ETR can provide a good initial evaluation of “tradeoffs” between various combinations of point and nonpoint source control strategies on ambient water quality improvement. In summary, a greater portion of nonpoint load reduction in overall TMDL load reduction generally correlates with greater uncertainty and thus requires a larger trading ratio. The rigorous quantification of the trading ratio will enhance the scientific basis in water quality trading and allow for more informed decisions for a successful watershed-based permitting program.