TALE OF TWO FLOWPATHS THROUGH A MINE-WASTE DUMP—CLEAR CREEK COUNTY, COLORADO

Infiltration of adit water through a waste-rock dump along two separate flowpaths,one shallow,the other deep,resulted in distinct and sometimes opposite metal loading and transport trends. The two flowpaths were delineated using electromagnetic (EM) conductivity and direct current (DC) resistivity surveys in conjunction with a NaCl tracer. The tracer was used to tag adit water that infiltrated the dump and emerged as seeps at the base. A relatively shallow flowpath through the unsaturated waste dump contained chloride tracer with an estimated flow rate of 92 m/day and a hydraulic conductivity of 1.6 x 10⁴ cm³/s. A second, deeper, flowpath was identified by the geophysical methods as moving below the adit along the base of the waste dump to seeps near the toe. After 24 hours, this flowpath did not contain chloride tracer. The chemistry of the adit water before infiltration, derived from hydrothermally-altered fractured granite, is circum neutral, characterized by a pH of 7.5, alkalinity of 72 mg/L HCO3, 960 µg/L Sr, 8,830 µg/L Mn, and 5,330 µg/L Zn; this compared with water from a nearby fen with a pH of 5.5, alkalinity of 2.1 mg/L HCO3, 20 µg/L Sr, 26 µg/L Mn, and 100 µg/L Zn. Along the shallow unsaturated flowpath over a distance of tens of meters, the pH decreased to 6.6, Mn decreased 10-fold to 950 µg/L, and Zn by half to 2,430 µg/L. Inverse modeling using PHREEQCI indicates precipitation of Mn-oxide minerals is causing sorption and reduction of Zn and other base metals including Cd, Co, Cu, and Ni. In contrast, the deeper flowpath had a pH of 4.6, alkalinity of 2.9 mg/L HCO3, 680 µg/L Sr, 5,370 µg/L Mn, and 3,260 µg/L Zn. About 30 percent of water along this flowpath is probably from the fen as indicated by the decrease in Sr, which behaves conservatively in this system. The copper concentration along the deeper flowpath increased to 350 µg/L, compared to 122 µg/L for the adit water and 111 µg/L for the shallow flowpath. More highly reduced conditions along the deeper flowpath result in less precipitation of Mn-oxide, lower pH, and leaching of Cu. In-situ reduction of metals by sorption along the shallow flowpath has important implications for passive remediation.