Evaluation of EPA's Weight-of-Evidence Approach to Nutrient Criteria Development

EPA has developed a series of nutrient TMDLs for southeastern Pennsylvania watersheds that rely on a new method to establish nutrient criteria for phosphorus. This method, the weight-of evidence approach, is not based on cause and effect relationships. Rather, it depends upon a collection of evaluations with dubious ties to nutrient-related use impairments. These “lines of evidence” were then used to establish a target endpoint for the TMDL based on a less than transparent weighting process. A critical review of the evidence used in this approach indicates that the selected endpoints are unlikely to result in designated use restoration. A review of site-specific data for several of the affected watersheds confirms that the selected endpoints are inappropriate as other factors are responsible for the impairment.