The U.S. Environmental Protection Agency (EPA) was conducting an investigation of the Gold King Mine near Silverton, Colorado in August 2015 to assess the on-going water releases and treatment options for the inactive mine. During excavation conducted as part of the investigation on August 5, 2015, pressurized water began leaking above the mine tunnel. Approximately three million gallons of heavy metal-laden wastewater spilled into Cement Creek, a tributary to the Animas River. Colorado and New Mexico state officials declared a state of emergency and upheld restrictions on residential well water use and recreational river use. Farmers refrained from using irrigation ditches for livestock and agricultural. In August 2016 John Cassels mobilized to the disaster area as a part-time employee of Avatar Environmental,a subcontractor to WESTON under their Region 6 START (Superfund Technical Assessment and Response Team) contract. Once onsite, Cassels joined teams of technicians which collected surface water and sediment samples, assessed river water quality.
River monitoring efforts included the daily collection and analysis of surface water and sediment samples at nine locations along the Animas and San Juan Rivers. These nine locations were strategically located upstream of water intakes which support the towns of Aztec, Farmington, the Lower Valley Water Users Association, Morning Star Water Supply System and the North Star Water Users Association. Surface water samples collected were analyzed for total dissolved and suspended solids, total metals, pH, alkalinity, and anions. In addition, in-situ measurements were recorded and include pH, ORB, conductivity, turbidity, and dissolved oxygen. Sample site photographs looking upstream and downstream were taken daily to document river conditions for each sample.