A NATURAL SOLUTION TO AN UNNATURAL PROBLEM? UNDERSTANDING REMEDIATION OF ACID MINE DRAINAGE IN LITTLE PEAK CREEK, ORE KNOB, NORTH CAROLINA

Little Peak Creek (LPC), a tributary of the South Fork of the New River, drains the abandoned workings of the former Ore Knob copper mine in Ashe County, North Carolina. Weathering of pyrite and pyrrhotite-rich mine tailings produces acid mine drainage and lowers the pH of Little Peak Creek to 4.5-4.8. Approximately 2 km downstream from headwaters, an unknown natural remediation process abruptly raises pH to 5.7 over 250 meters of channel length. Two small surface waters entering LPC near the point of initial pH rise clearly do not supply the 694-1,484% volume increase necessary to raise the pH by simple dilution. Titration of LPC water samples with these surface waters (24.4 mg/L and 14.6 mg/L HCO₃^-) indicates the combined streams would have to deliver the equivalent of 94% LPC volume to accomplish remediation, a discharge well beyond the size of their collective basins. Groundwater in equilibrium with unusual carbonate-bearing (~1%) metasedimentary rock identified in the area may be responsible for the neutralization process. Reaction of the rock with a sealed pH 4.8 H₂CO₃ solution for 96 hrs yielded a solution containing 102.48 mg/L HCO₃^-. Titration of LPC samples with this solution indicated the addition of a volume equivalent to 12.5% of the LPC sample volume is sufficient to produce the pH change observed in the natural setting. The potential for longer interaction of natural waters with the rock at the site would likely yield higher HCO₃^- concentration. Analysis of the streambed in the vicinity of the pH change suggests a topographic explanation for a sudden influx of groundwater. LPC abruptly incises 10 m into bedrock, potentially breaching a saturated zone and introducing bicarbonate-buffered groundwater into the stream. Further investigation will attempt to locate and chemically characterize groundwater seeps within the incised channel to better understand this natural remediation process.