FREQUENCY, MAGNITUDE, AND DURATION: CURRENT POLICY AND PRACTICE

USEPA's Technical Support Document for Toxics (1991) provides detailed recommendations on evaluating frequency, magnitude, and duration issues within the context of the existing USEPA criteria approach. These recommendations include use of statistical permit limit derivations and techniques for dynamic wasteload analysis. We surveyed state permitting agencies and found that few of these recommendations have been put into practice. Instead, most jurisdictions continue to rely on steady-state wasteload analyses at design conditions. Reasons for lack of adoption of the recommended procedures include lack of familiarity and perceived technical difficulty.Use of a steady-state, design condition approach to wasteload allocations can result in permit limits that over-protect or under-protect the biological uses of a waterbody in terms of meeting a target frequency of biologically-based excursions of criteria. A dynamic modeling analysis can potentially result in a more accurate wasteload allocation, but at a considerable expenditure of effort. The decision to undertake a dynamic analysis requires an evaluation of the potential savings associated with a more accurate allocation, the costs of undertaking the study, and the regulatory response to the study. As part of an ongoing WERF research project (98-HHE-3) we are developing an Implementation Guidance document and accompanying software to aid permittees in deciding whether to undertake dynamic analyses of frequency, magnitude, and duration issues; and to aid in implementing the analyses under existing guidance. The research includes extensive analyses and examples of the benefits and potential drawbacks of implementing the approach recommended by USEPA. This work lays the foundation for a more general evaluation of the use of criteria to protect designated uses in real-world situations in which exposure concentrations fluctuate in frequency, magnitude, and duration.