A Procedure for Setting Environmentally Safe Total Maximum Daily Loads (TMDLs) for Selenium

This article presents a seven-step procedure for developing environmentally safe total maximum daily loads (TMDLs) for selenium. The need for this information stems from recent actions taken by the U.S. Environmental Protection Agency (EPA) that may require TMDLs for selenium and other contaminants that are impairing water bodies. However, there is no technical guidance from EPA or elsewhere that deals exclusively with selenium. This leaves biologists and environmental contaminant specialists without the tools needed to effectively address the TMDL issue for selenium. This article provides guidance by laying out an assessment method that links the basic components of EPAʹs TMDL process to the contaminant-specific information required for selenium. The underlying principle in this process is that selenium concentrations be kept below levels that threaten reproduction of fish and aquatic birds. The steps are: (1) Delineate and characterize the hydrological unit (HU, i.e., water body) of interest. (2) Determine selenium concentrations and assess biological hazard. (3) Determine sources, concentrations, and volumes of selenium discharges; calculate existing selenium load. (4) Estimate retention capacity of HU for selenium. (5) Calculate the total allowable selenium load and specify reductions needed to meet the target loading. (6) Allocate selenium load among discharge sources. (7) Monitor to determine effectiveness of selenium load reduction in meeting environmental quality goals. Proper application of this procedure will ensure compliance with EPA regulatory requirements and also protect fish and wildlife resources.