EFFLUENT LIMITS FOR FLUCTUATING DISCHARGES: THE WERF DYNAMIC LOAD ANALYSIS TOOL

To protect aquatic life in waterbodies that receive effluent discharges, regulators set water quality criteria to define the allowable ambient concentrations of pollutants. The water quality criteria, in turn, are used to allocate pollutant loads to dischargers through the National Pollutant Discharge Elimination System (NPDES). NPDES permit limits on effluent are often determined by a steady-state analysis of critical design conditions that combine maximum loading from the discharger and a lower percentile of instream dilution capacity. These steady state assumptions may not be appropriate for effluent loads that fluctuate in time, particularly when point source loading is correlated with instream dilution capacity. In such cases, permit limits calculated by steady state methods can be either unnecessarily stringent or not sufficiently protective.WERF funded research (98-HHE-3) to improve the understanding of the link between fluctuating exposures and effects. The overall goal was to develop more appropriate, sitespecific permit limits for dischargers. The project was divided into two phases. Phase 1 developed guidance and software to improve the implementation of existing U.S. EPA recommendations, and Phase 2 developed a methodology to allow water quality criteria to better reflect site-specific exposures and biological conditions at to evaluate alternative approaches to calculating water quality criteria for fluctuating exposures. This paper describes the Phase 1 development of a decision process and software tool to implement existing recommendations. The full report is available from WERF (Butcher and Diamond, 2003).