Dissolve Inorganic Carbon (DIC) and Stable Carbon Isotope Evolution in a Tailings Pile

The Old Lead Belt of SE Missouri (USA) was the nation's major source of lead for close to 100 years. As a result mine spoils estimated at more than 254 billion kg are disposed of in more than 6 major tailings piles that cover more than 12 sq km within the Big and Flat River watersheds. Sediments of mine tailings, groundwater below mine spoils, soil water, surface water and soil gas from tailings were analyzed for physical parameters, major ions and carbon isotope ratios of DIC and carbon dioxide. Surface water was collected from lakes surrounding the tailings, groundwater from wells between 4.6 and 30.5m deep, soil water and soil gas from lysimeters and gas samplers between 0.5 and 3m. Results show that in ground-, soil- and surface water, pH ranged from 6.3 to 8.1, alkalinity from 60 to 513 mg/l and DIC range from 0.6 to 4.9 mM/l for groundwater, 3.4, to 8.5 mM/l for soil water, 1.3 mM/l in lake water and 2.7 and 3.3 mM/l at tailings seeps. Soil carbon dioxide ranged between 0.4 and 21.9 mM C. The isotope ratios of DIC was -9.0 per mil for tailing seeps, -8.4 to -9.3 per mil for lake water, -2.9 to -12.5 for groundwater, -3.5 to -5.8 per mil for soil water and -1.4 for tailings sediments. The stable carbon isotope ratio of carbon dioxide from soil gas was -11.4 to -16.4 per mil and background soil gas (without mine spoils) -22.1 per mil. The results of this study show that mine tailings can generate high DIC in ground and soil water that degas as carbon dioxide to the unsaturated soil zone. Isotope values intermediate between tailings sediment and background soil carbon agree with input of heavier carbon from tailings carbonate into ground, soil and surface water DIC.