ELEVATED RADIUM-226 IN GROUNDWATER DUE TO OXIDATION OF SEDIMENTARY IRON SULFIDE; UPPER COASTAL PLAIN, SOUTH CAROLINA

Groundwater in the Upper Coastal Plain is known to be occasionally enriched in radium, which is derived from radioactive mineral grains with a Piedmont provenance. However, causes of local scale variations in groundwater radium concentrations within the Coastal Plain are not well understood.

Monitoring wells associated with a landfill at the Savannah River Site in Aiken County, South Carolina are screened at various levels within the Steed Pond Aquifer, which is composed of yellow and tan quartz sands of the Eocene Congaree Formation. Background wells (upgradient of the landfill) screened in the upper part of the aquifer have time-averaged Ra-226 concentrations of 0.5 to 2 pCi/L; these values are typical of the region. On the other hand, wells screened near the base of the aquifer are enriched in Ra-226, with time-averaged concentrations of 2 to 5 pCi/L. Most of the wells with elevated Ra-226 concentrations have total radium (Ra-226 + Ra-228) concentrations which exceed the EPA MCL of 5 pCi/L.

Groundwater samples with enriched radium-226 also have elevated sulfate concentrations, and are more acidic than rainwater. Flow direction considerations and mass balance calculations rule out landfill leachate as the origin of this groundwater. A reduced, lignitic, sulfidic clay layer, the Paleocene Lang Syne / Sawdust Landing Formation, underlies the Steed Pond Aquifer, with the top of the clay only 5 to 10 feet below the screens of several radium-rich wells. Elevated radium-226 concentrations in the lower part of the aquifer can be accounted for by a model involving infiltration of oxygenated groundwater into the reduced sulfidic sediments, with consequent oxidative dissolution of iron sulfide, acidification of groundwater, and mobilization of naturally occurring radium.