Copper Water Effects Ratio (WER) Study

GEI Chadwick Ecological Division developed site-specific copper standards using the EPA Water Effects Ratio (WER) methodology for 6 stream segments to address the potential exceedence of aquatic life standards. This was the first application of the EPA biotic ligand model (BLM) for the development of site-specific copper standards in Colorado and EPA Region 8, and the first WER adjusted copper site-specific standards in Colorado since 1996. These studies provided the framework for subsequent copper WER proposals, as recommended by the Colorado Water Quality Control Division (CWQCD).

The South Platte River in metro Denver had been listed as “impaired” for exceedence of instream copper aquatic life standards. Resulting TMDL would potentially limit copper loads in permitted discharges (both wastewater and stormwater) resulting in the need to add treatment.

WERs take into account the mitigating effects of natural water on predicted copper toxicity. Application of a WER to instream table value standards would result in protective water quality standards and preclude the need for additional treatment for copper.

GEI’s Chadwick Ecological Division developed a workplan with SP CURE members, with review by CWQCD and EPA. Our scientists conducted comparative toxicity testing with copper-spiked lab water and stream water (per EPA WER guidance). We also modeled predicted WERs based on long-term water quality monitoring by SP CURE members using the EPA biotic ligand model (BLM).

GEI used the combination of empirical data (WERs from laboratory toxicity testing) and predicted (WERs from BLM modeling) to develop proposed site-specific WER-adjusted standards for 6 river segments.

GEI’s ecological division developed the study, conducted WER toxicity tests and BLM model runs, prepared the report and testimony, and obtained approval by the Commission in 1.5 years.