PERSISTENT SCIENCE CHALLENGES IN LEGACY MINE LAND SITES: RESULTS FROM A MULTI-SCALE GEOCHEMICAL STUDY IN TOMICHI CREEK, COLORADO, USA

Despite considerable progress in remediating legacy mine land (LML) sites in the western U.S., many continue to negatively impact water quality. Financial and regulatory constraints, and scientific challenges remain, making it difficult to identify where remediation efforts could be most beneficial. We collaborate with land managers to identify and evaluate persistent LML impacts using multi-scale geochemical techniques. Tomichi Creek is highly impacted by the Akron Mine and Mill near Whitepine, CO. It was mined from 1880s-1950s with primary commodities Pb, Ag, Zn, Cu, and Au. In 2015-16, the USEPA, USFS, and Trout Unlimited coordinated reclamation efforts along Tomichi Creek including excavation of the mill properties and wetlands, moving waste rock away from the stream, regrading slopes, and encapsulating waste. Post-remediation, Pb and Cd concentrations decreased, but remain close to aquatic life standards with evidence of impacted fish populations.

In 2021, we performed an in-stream synoptic tracer study along a ~4-km stretch of Tomichi Creek starting just above the Akron Mine, collecting stream water, groundwater (piezometers installed adjacent to the stream), and stream bank sediment. Stream bank sediments collected adjacent to piezometers show elevated Pb, Zn, and Cd, with highest concentrations near the mine/mill sites (8,600 mg/kg Pb; 6,250 mg/kg Zn; 50 mg/kg Cd), decreasing downstream (3,900 mg/kg Pb; 3,800 mg/kg Zn; 24 Cd mg/kg). Dissolved groundwater Pb and Zn concentrations are highest (128 µg/L Pb and 7,800 µg/L Zn) near the Tomichi Mill site. Stream water Pb concentrations increase from <1 to 3.4 µg/L near this site and persist downstream. Zinc increases from 5 to 16 µg/L and increases downstream (up to 72 µg/L). Cadmium is greatest in groundwater at the same site (28 µg/L) and stream water Cd concentrations steadily increase downstream (0.39 µg/L). Differences in Pb and Zn concentrations between total (unfiltered) and dissolved (filtered <0.45 µm) in stream and groundwater suggests a large fraction of trace elements are transported downstream via suspended sediment or colloids. Work is ongoing to improve understanding of the residual sources and geochemical processes contributing to trace element mobilization to assess long-term impacts to ecosystem health.