Ground water geochemistry of 228Ra, 226Ra and 222Rn

Abstract

228Ra, 226Ra, and 222Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log 228Ra and log 226Ra ( n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median 228Ra 226Ra activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal 232Th 238U activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log 222Rn and log 226Ra for Piedmont ( n = 68, r = 0.62) and Coastal Plain ( n = 89, r = 0.64) ground water. However, the median 222Rn 226Ra activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess 222Rn activities may be due to greater retention of 226Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log 228Ra and log 226Ra was used to predict total Ra ( 228Ra + 226Ra ) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of 226Ra for 228Ra analysis.