Methodology for pH total maximum daily loads: Application to Beech Creek watershed

This paper presents a methodology for developing total maximum daily loads (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., 6.0) and an associated ion activity correction factor, an envelope of ion loads may be obtained as a function of discharge. By identifying a critical discharge for a given watershed, the 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. Finally, the required mass loading of an associated neutralizing agent (e.g., CaCO₃) can be determined by constructing a functional relationship between the neutralizing agent and the required hydrogen ion reduction. The proposed approach has been applied to several watersheds in the state of Kentucky and was recently approved by EPA Region IV as an acceptable protocol for pH TMDLs. An application of the methodology is presented for the Beech Creek watershed, which is located in Muhlenberg County, Kentucky.