The U.S. Environmental Protection Agency (EPA) has encouraged states to use the "watershed approach" to water quality permitting. Using this approach, National Pollutant Discharge Elimination System (NPDES) permits are issued on a watershed-by-watershed basis, rather than one permit at a time. Currently, few states conduct water quality permitting on a watershed basis. Instead, evaluations for renewal and issuance of individual permits are conducted independently. This paper will present a case study for a discharger in Ohio, a state that uses watershed permitting, discussing some of the pitfalls to avoid when establishing watershed-based permits. There are a number of organizational and regulatory benefits to the watershed permitting approach. A watershed approach can facilitate development of Total Maximum Daily Loads (TMDLs), which are required by the Clean Water Act. Also, the watershed approach can ease implementation of pollution controls across the watershed and facilitate public comment on permits. However, there are a number of technical pitfalls associated with applying traditional point source permitting approaches to a watershed. Traditional approaches use a single set of "critical" environmental and discharge conditions to calculate effluent limitations. The "safety factors" inherent in the calculations multiply when more than one discharge is evaluated. The resulting effluent limitations are usually much more stringent than needed to protect water quality, placing an unnecessary (and often expensive) burden on the dischargers. This paper describes the problems encountered by a discharger in eastern Ohio when traditional methods were used for watershed-based permitting. A probabilistic modeling evaluation demonstrated that the "critical" steady-state limits derived by the state regulatory agency were more stringent than necessary to protect water quality. Alternative permit limitations were developed that were fully protective of water quality, but were less stringent and more practical to achieve.
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