The Savannah River DO TMDL and Recent Advancements in Developing Point Source Permit Loads from Time Variable Models

A group of municipal and industrial wastewater dischargers requested an ongoing review of the Savannah River and Harbor Dissolved Oxygen (DO) modeling and TMDL performed by the EPA Region 4. EPA developed and calibrated a coupled time-variable hydrodynamic (EFDC)/water quality (WASP) model of the tidal Savannah River for an eleven year time period (1997–2007). The purpose of the time variable model is to develop allowable point source effluent BOD5 and ammonia loads. The State regulatory agencies from Georgia and South Carolina have limited the decrease in river DO from point source discharges to 0.10 mg/L during the critical period of March-October, which is applied as a daily average and must be met 90% of the time. The regulatory agencies original proposed approach to computing allowable point source BOD5 and ammonia loads was to apply the conventional approach of assuming all point sources are simultaneously discharging at their monthly BOD5 and ammonia permit loads every day during a critical low flow condition. This approach is highly conservative and can result in overly stringent point source effluent permit limits. It was recommended that this conventional approach, which was developed when steady-state models were common, be abandoned and replaced with a more scientific-based approach. This new approach evaluates the effect of point source dischargers on river DO with consideration of the day to day variability of effluent nd ammonia and also the long term variability of river flows. Both State regulatory BOD5 agencies and EPA Region 4 accepted this approach for developing point source BOD5 and ammonia loads as part of the TMDL. In addition to accepting the representation of point source loads as time variable inputs to the model, the agencies also accepted a recommendation for developing monthly and daily effluent permit limits from time-variable loads. The point source permit BOD5 and ammonia loads derived from this more rigorous time-variable approach were approximately twice the values developed from the conventional steady-state approach while still complying with the maximum decrease in river DO as established by the State regulatory agencies. This new approach allowed a more scientific and cost effective derivation of point source permit limits eliminating the costs associated with the conservatism of conventional approaches. Additionally, a practical computational tool was implemented to facilitate the evaluation of multiple scenarios and the application of the new approach. This paper summarizes the development and application of this new methodology implemented to consider the point source effluent loading and long term river flows variability during the modeling stage as well as during the actual DO TMDL implementation.