ELEVATED PH, ALKALINITY, CALCIUM, AND SULFATE ASSOCIATED WITH HIGH DISSOLVED URANIUM IN GROUND WATER IN GRANITIC ROCK, SOUTH CAROLINA PIEDMONT

High uranium concentrations in home well water (to 10 mg/L; the drinking water limit is 0.03 mg/L) are found in an area of granitic rock in Greenville County. In 25 investigation wells (high and low U, including a few mafic-rock and shallow regolith wells) the higher U concentrations (say, >0.1 mg/L) are associated with higher pH, alkalinity, and calcium and sulfate in water. Generally, one expects slightly acidic to circumneutral and relatively low alkalinity and low TDS ground water from granitic rock, and from regionally more typical metamorphosed felsic rocks surrounding and perhaps underlying the granitic body. More basic and perhaps more reactive mafic rocks are found nearby and as dikes within the study area and perhaps contribute, but seem insufficient as a general cause for mineralization of water. Geochemical evidence and tests using pumped water help constrain plausible alternatives. Elevated pH, alkalinity, Ca, and especially depleted ¹⁴C in HCO₃ suggest some mineral carbonate involvement, but abundant dissolved silica (avg. ca 28 mg/L) shows considerable weathering of silicate rock also. Elevated sulfate might suggest weathering of sulfide minerals (conceivably induced by better oxygenation via routine pumping) but instead may merely reflect human additions (like the appreciable nitrate, e.g., from fertilizer, septic tanks). Cl is insufficient to explain Na from NaCl (e.g., septic tank effects) and a Na source from rock weathering is indicated. Alkalinity (as HCO₃) vs Ca molar ratios are mostly around the 2:1 ratio that suggests dissolution of mineral CaCO₃ (e.g., speculated marble veins in underlying metamorphic rock, hydrothermal calcite in fracture fill, even much younger precipitate in fractures), but the highest Ca concentrations are closer to a 1:1 relationship that at lower concentration might suggest direct reaction of recharge CO₂ with Ca-bearing silicate rock. Carbon isotopic ratios in HCO₃ show that one of the carbon sources is depleted in ¹⁴C and enriched in ¹³C relative to soil-derived CO₂ dissolved in recharge. This strongly suggests some dissolution of ancient carbonate. The alternative of upwelling of older mineralized ground water from deeper geologic zones is not yet rejected, though not favored topographically.