GEOLOGIC CONTROLS ON WATER QUALITY IN A MINERALIZED REGION OF COLORADO

The Snake River Basin, located within Summit County, Colorado supplies water to the Dillon Reservior, a source of potable water for the metropolitan Denver area. Natural acid drainage resulting from the oxidation of disseminated pyrite has created low pH (3.5 to 5) waters near the headwaters of the Snake River. These acidic waters have dissolved elements such as manganese, aluminum, iron, and zinc from the metamorphic gneisses and schists within the area. These elements have either remained in solution to mix with low pH, metal-impacted water originating from downstream Peru Creek, or precipitated onto the riverbed.

The geologic, hydrologic, and geochemical controls on the contributions of subsurface and surface water to stream quality in an area of natural acid drainage are investigated. Multiple conceptual models of the flow system are defined using the MODFLOW code, including rapid flow through the shallow regolith and slow flow through the deep bedrock. The mass fluxes from the flow model are used to constrain PHREEQC reaction and transport models. The chemical models simulate stream water chemistry resulting from the alternative conceptual flow models. Ultimately, this project will assist with the environmental assessment of mineralized areas by developing an approach for determining natural background concentrations of trace metals in naturally-impacted drainages.