In 2007, EPA proposed a new approach for developing numeric nutrient water quality criteria. This approach abandoned the long-held position that water quality impairments attributed to nutrients are mediated through excessive plant growth. This new approach relies, instead, on “weight-of-evidence”,
an amorphous consideration of distributional, literature, and stressorresponse nutrient concentrations. The “weight-of-evidence” approach largely abandons cause and effect type relationships with the exception of the stressor-response evaluations. These stressor-response evaluations
were based on conditional probability analysis to identify total phosphorus endpoints that allegedly represent thresholds of impairment. A cursory review of the data used in the conditional probability analysis suggests that the variability around the endpoint is too great to be of use in
TMDL application. Further review indicates that the endpoints identified through conditional probability analysis are not associated with significant changes in the response metrics for macroinvertebrates or periphytic diatoms. Consequently, this method should not be used to derive numeric
nutrient water quality criteria.
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