GEOCHEMISTRY AND STABLE CARBON ISOTOPE CHARACTERIZATION OF THE IMPACT OF MINE SPOILS ON SURFACE, SOIL AND GROUNDWATER AT THE ST JOE STATE PARK, SE MISSOURI

This study investigated the production and fate of carbon dioxide and DIC in ground-, soil- and surface-water in a mine tailings pile. Samples were collected from the Federal Mine tailings pile at the St Joe State Park, SE Missouri, and analyzed for field parameters major ions, alkalinity, DIC, and stable carbon isotope ratio of DIC and soil carbon dioxide gas. Surface water, groundwater and soil water was sampled. Soil gas was sampled from perforated piezometers installed at discrete depths.

Results show that sulfate concentrations for all samples are much higher (up to 1617.3 mg/l) than concentrations of metals from oxidized sulfide ores of Pb, Zn, and Fe with maximum concentrations of 0.09, 9.6, and 5.1 mg/l respectively. In shallow ground and soil water, Ca concentrations are much higher (113.8 to 531.5 mg/l) than Mg (46.2 to 143.2 mg/l), while in deeper groundwater Mg concentrations (47.8 to 349.2mg/l) are higher than Ca (46.2 to 143.2 mg/l). In ground-, soil- and surface-water, alkalinity range from 60 to 513 mg/l and DIC range from 12.9 to 132.5 mg C/l. The stable carbon isotope ratio of DIC ranged from 12.9 to 3.2 per mil and that of carbon dioxide from soil gas at the shallowest depth of 0.5 m is much heavier ( 13 to 16 per mil) than background soil gas (-22 per mil) without mine spoils. The relationship between sulfate and pH suggest buffering by dolomite in the tailings. High sulfate and low metals indicate removal of metals by precipitation. Alkalinity correlates negatively with pH indicating DIC generation by acid produced during metal oxidation. Carbon isotope ratios are consistent with the weathering of dolomite in ground- and soil water, and carbon dioxide loss in surface waters. Observed decreases in isotope ratio with increase in DIC may be due to oxidation of organic matter that supplies isotopically light carbon to the DIC pool. Heavier isotopic ratios of carbon dioxide within the mine spoils compared to background soil gas suggest generation of carbon dioxide in mine spoils from dolomite weathering that can potentially be lost to the atmosphere. Results show that oxidation of metal sulfides in mine-tailings generate high carbon dioxide and DIC in ground and soil water. This may contribute carbon dioxide to the atmosphere through the unsaturated zone and from groundwater discharged to the surface as mine drainage.