Geographic-Specific Water Quality Criteria Development: A Conditional Probability Analysis Approach

John F. Paul and Michael E. McDonald

U.S. Environmental Protection Agency, NHEERL (B205-01), Research Triangle Park, NC 27711

A conditional probability analysis (CPA) approach using monitoring data to develop geographic-specific water quality criteria for protection of aquatic life is presented. This approach expresses a biological threshold of impact as the likelihood of exceeding a given value of a pollution metric. Uncertainty and natural variability are inherently incorporated into the analysis. Application with survey data (probability-based sample design) alleviates two issues typical with existing methods: (1) extrapolation to an entire geographic area and (2) possible bias in site selection. Data from wadable streams in the mid-Atlantic area of the U.S. are used to illustrate the approach. Benthic community index values (EPT taxa richness) were used to identify impacted stream communities. Thresholds of impact were identified using four changepoint techniques for water column turbidity, percent fines in the sediment bed, and geometric mean diameter of sediment bed particles. Finally, a demonstration is provided of how these thresholds could be used to develop criteria for protection of aquatic life in streams. Although final development of water quality standards is a management decision, scientifically-defensible approaches for establishing criteria based on significant deviations from expected community condition across a wide range of geographic conditions could be a useful first step.

Keywords: water quality criteria, sedimentation, wadable streams, mid-Atlantic region, benthic community condition, conditional probability, statistical analysis, aquatic ecosystems