A methodology for pH TMDLs: Application to Kentucky watersheds

Section 303(d) of the Clean Water Act and EPA's Water Quality Planning and Management Regulations (40 CFR Part 130) require states to develop total maximum daily loads (TMDLs) for their water bodies that are not meeting designated uses under technology-based controls for pollution. The TMDL process establishes the allowable loadings of pollutants or other quantifiable parameters for a water body based on the relationship between pollution sources and in-stream water quality conditions. This method exists so that states can establish water-quality based controls to reduce pollution from both point and non-point sources and restore and maintain the quality of their water resources (USEPA, 1991). This paper presents a methodology for characterizing TMDLs for pH impaired streams that characterizes the pH impact in terms of an associated hydrogen ion load. Because pH and an equivalent ion load can be related as a function of discharge and ionic strength, a functional relationship can be developed between discharge and the associated ion loading for a given pH value. By specifying a minimum pH value (e.g. 6) and a minimum activity correction factor, an envelope of maximum ion loads may be obtained as a function of discharge. By identifying a critical discharge for a given watershed, the maximum hydrogen ion load and associated TMDL can be obtained. Determination of the associated ion load reduction can further be accomplished by developing functional relationships between measured discharges and measured ion loads for a given watershed. The proposed approach has been applied to several watersheds in the state of Kentucky and was recently accepted by EPA Region IV as an acceptable protocol for pH TMDLs. Copyright ASCE 2004.