AMENDING NUTRIENT TMDLS USING UPDATED INFORMATION

The U.S. Environmental Protection Agency, Region III, recently developed Total Maximum Daily Loads (TMDLs) for the Christina River Basin and Wissahickon Creek to ensure compliance with applicable water quality standards for dissolved oxygen. In both cases, the TMDLs were premised on integrated hydrodynamic, water quality, and sediment contaminant models that evaluated the stimulating effect of nutrients on periphyton growth. The periphyton state variable served as the basis for determining the diurnal range of dissolved oxygen concentrations about the mean daily concentration. A review of the modeling report for the Christina River Basin indicated that the periphyton half-saturation constant for phosphorus was unusually high. Through subsequent review, we determined that the unusual value drove the significant load reductions contained in the TMDL. Furthermore, diurnal dissolved oxygen data collected by the USGS under drought flow conditions demonstrated that the model predictions were inaccurate. The USGS data were presented to EPA and the Agency revised the model, using a more-typical half-saturation constant, to better approximate the observed dissolved oxygen range. A review of the Wissahickon Creek TMDL modeling report indicated that it used a typical phosphorus half-saturation constant for periphyton growth, but most of the other kinetics were not supported by data. Rather, values were selected from typical ranges to match the dissolved oxygen profile. In addition, the model used fixed reaeration rates rather than rates calculated based on hydrodynamic considerations. EPA revised its model to address some of the objections we raised. As a result, one discharger avoided additional regulation on phosphorus.