IMPLEMENTATION OF THE BIOTIC LIGAND MODEL IN SITE-SPECIFIC STANDARDS SETTING

In November 2013, the Upper Thompson Sanitation District (District) initiated a stream study that employed the United States Environmental Protection Agency’s (EPA) copper Biotic Ligand Model (BLM) and Fixed Monitoring Benchmark (FMB) statistical method. The purpose of the study was to develop site-specific stream standards for copper that would subsequently be used to modify the District’s wastewater treatment facility permit. Stream standards for copper have been historically derived from hardness-based table value standards (TVS). The copper BLM considers a comprehensive list of water chemistry parameters to predict copper speciation, bioavailability, and toxicity. As such, it serves as a more sophisticated tool for setting stream standards than the hardness-based TVS. The District formally proposed site-specific copper standards that were adopted by the Colorado Water Quality Control Commission in January 2015. In June 2015, the District’s permitted copper effluent limitations were formally modified based on the revised stream standards.