ISOTOPIC AND GEOCHEMICAL APPROACHES TO CHARACTERIZING WATER MOVEMENT THROUGH ABANDONED MINE WORKINGS, NELSON-WOOSTER-HUMPHREY TUNNEL CREEDE, COLORADO

Long term acid mine drainage (AMD) discharging from the portal of the Nelson Tunnel near Creede, CO is currently impacting water quality in West Willow Creek and the Rio Grande River. We are using established isotope and geochemical tracer techniques to quantitatively determine the sources, ages and pathways of waters in the mine.

Preliminary results indicate that waters draining the mine are well mixed and composed to some degree of old groundwater not just meteoric inputs. The stable isotope (¹⁸O) of the mine water is steady at -15 ‰ throughout the year, suggesting a well-mixed groundwater system composed of equal parts winter snow (-20 ‰) and summer monsoon rain (-10 ‰). Tritium (³H) values within the tunnel are primarily “tritium-dead” indicating water that is at least older than the “bomb-spike” waters of nuclear weapons testing in the 1960s. Additionally, dissolved inorganic carbon (DIC) δ¹⁴C testing indicates mine water on the order several thousands of years of age. Results therefore suggest that mine waters are largely not directly connected to surface waters, or to the shallow groundwater (sampled from springs and domestic wells), but rather are likely entering the tunnel at intersections with a system of watershed-wide faults.

To provide age verification for the DIC δ¹⁴C results the mine water samples were also analyzed for the δ¹⁴C of dissolved organic carbon (DOC). To isolate the aquatic humic components of the DOC, the XAD Chromatographic technique was employed. This method involves first passing an HCl-acidified sample through a column packed with XAD-8 resin to selectively sorbs hydrophobic (fulvic) weak acids. Followed by an elution with sodium hydroxide and a rinse with DI water to remove chloride. Salts are then removed in the final step using a cation exchange resin-filled column, leaving only these isolated organic carbon constituents. Additionally, fluorometer analysis of the final isolates resulted in fluorescence index values indicative of terrestrial carbon sourcing and minimal microbial influence.

The results from this study have been used to develop a hydrogeologic conceptual model of the mine complex, which will aid in the development and feasibility analysis of targeted remediation strategies.