STABLE AND RADIOGENIC ISOTOPIC CHARACTERIZATION OF ACID MINE DRAINAGE IN A FRACTURED ROCK SETTING, CHALK CREEK, COLORADO

Acid mine drainage (AMD) affects many streams throughout the western United States.  Understanding flow dynamics and sources within a fractured rock setting is necessary in outlining a potential remediation strategy for AMD. Radiogenic and stable isotopes of water were used in the Mary Murphy Mine, Chalk Creek, Colorado, in order to characterize flowpaths and sourcewaters.  Moreover, results from this research provide insights into groundwater flow systems in mountain environments of the Colorado Rockies. The radiogenic isotope sulphur-35 (³⁵S) with a half-life of 87 days is useful for identifying this year’s snowmelt runoff, while tritium (3H) with a half-life 12.43y is useful for studying hydrologic processes at the decadal time scale. Hydrometric information showed that discharge from the mine adit exhibited a hydrograph characteristic of snowmelt runoff. However, mixing models using stable water isotopes (D and ¹⁸O) found less than 7% of the mine’s peak discharge was from snowmelt, suggesting a regional groundwater dominated system. Sulfur-35 and tritium were then sampled after snowmelt in July using a synoptic strategy that included interior mine sources, groundwater wells, and nearby Chalk Creek. Measured tritium from the Golf Portal and 1400’ Portal was 14 TU and 14.5 TU, respectively; suggesting an older water source at least in part. The average tritium value within Mary Murphy Mine was 13.55 TU. The highest value occurred in seep XC-3 within the 1400’ portal, at 18 TU. Mine samples showed little ³⁵S, measuring –2+/-20 and –1+/-5 mBq/L for MVN-1 and MVC-1, respectively. Chalk Creek also showed little ³⁵S, -1.2+/-1.3 mBq/L.  Groundwater sample results show ³⁵S however; MM-6 measured 17.5+/-2 mBq/L, while MM-11 measured 1.4+/-.4 mBq/L. The degree of uncertainty for the ³⁵S was high due to a large range in recovery, from 0.8% to 80%, especially within the mine. The combination of radiogenic and stable isotopes within and near the Mary Murphy Mine may provide a useful tool for studying interactions between groundwaters and surfacewaters in a fractured rock setting. Remediation techniques can be directed more appropriately, and cost effectively, by the characterization of flowpaths within the mine as well.