Radium and radon in ground water in the Chickies Quartzite, southeastern Pennsylvania

Abstract

The Chickies Quartzite, a Lower Cambrian-age formation comprised of quartzite and slate overlying a basal conglomerate, forms narrow ridges and crops out discontinuously over 112 square miles in the Piedmont physiographic province of southeastern Pennsylvania. The formation is a low-yielding, fracturedrock, water-table aquifer recharged primarily by local precipitation. It is the sole source of water suppty for thousands of domestic users. Ground water in the Chickies Quartzite generally is soft and acidic. During 1986-88, the U.S. Geological Survey sampled water from 160 wells that penetrate the Chickies Quartzite to determine the magnitude and distribution of radium-226 (Ra-226), radium-228 (Ra-228), and radon-222 (Rn-222) activities in ground water in the formation and to characterize the geochemical environment associated with elevated activities of radium (Ra). In addition, 28 wells penetrating adjacent geologic units and 1 well in the Hardyston Quartzite were sampled to determine relative background Ra and Rn-222 activities in ground water. Analyses included determination of activities of dissolved Ra-226, Ra-228, and Rn-222, and concentrations of dissolved uranium (U), dissol^«d organic carbon (DOC), and major and minor dissolved inorganic ions. Rock samples were analyzed for U and thorium (Th) and geophysical logs were run to determine sources of Ra and Rn-222 in the Chickies Quartzite. Activities of up to 41 pCi/L (picocuries per liter) for Ra-226, 160 pCi/L for Ra-228, and 32,300 pCi/L for Rn-222 were measured in ground water in the Chickies Quartzite. Forty-seven percen t of the samples contained Ra-226 and Ra-228 activities greater than 5 pCi/L. Median activities measured v^re 1.2 pCi/L for Ra-226, 2.6 pCi/L for Ra-228, 4.2 pCi/L for combined Ra-226 and Ra-228, and 2,400 pCi/L for Rn-222. Ra-228 activity exceeded Ra-226 activity in about 92 percent of 100 water samples; the median Ra-228/Ra-226 activity ratio was 2.4. Ra-228/Ra-226 activity ratios commonly were greater in ground water than calculated Th/U ratios in rock samples, suggesting preferential leaching of Ra-228 from aquifer solids. Of ground water in the adjacent geologic units, the highest activities (up to 2.9 pCi/L for Ra-226, 12 pCi/L for Ra-228, and 25,300 pCi/L for Rn-222) were measured in ground water in the Harpers Phyllite and Antietam Quartzite. Nonparametric (Spearman rho test) statistical correlations show that the activity of dissolved Rr is inversely related to pH and directly related to concentrations of total dissolved solids, DOC, barium, and sulfate. Low pH decreases adsorption of Ra onto the aquifer matrix. The other factors may favor Ra mobility by enhancing complexation or increasing solubility. Rn-222 activity does not correlate with and is not supported by the activity of its parent, Ra-226, in solution. Ra-226 activity correlates positively, bu4; weakly, with U concentrations. Ra-226 does not appear to be supported by its parent, U-238, in solution. Observed distributions of Ra-228, Ra-226, and Rn-222 activities in ground water in different lithologies of the Chickies Quartzite reflect different geochemical controls on adsorption and distribution of parent thorium-232 (Th-232) and uranium-238 (U-238) in the formation. Radium activities were greatest in acidic ground water in the conglomerate and quartzite (median pH of 5.0 and 5.2, respectively) and least in the more neutral water in the slate (median pH of 6.4). For ground water in the conglomerate, quartzite, and slate, respectively, median activities measured were: 1.3, 1.5, and 0.2 pCi/L for Ra-226; and 3.7, 2.5, and 1.0 pCi/L for Ra-228. Natural-gamma-ray geophysical logs and results of rock analyses indicate that the conglomerate may contain more Th and U than the quartzite and that the conglomerate may be more enriched in Th with respect to U than the quartzite; Th and U distribution in both lithogies is variable. Median Rn-222 activities in ground-water samples generally are greater progressively from the slate (1,400 pCi/L) to the quartzite (2,000 pCi/L) to the conglomerate (3,400 pCi/L) and may reflect difference? in U content of the lithologies. Ra-226, Ra-228, and Rn-222 activities in ground water vary locally and temporally. Lithology controls the geochemical environment, topography, and ground-water flow paths, and, therefore, the spatial distribution of Ra and Rn-222 activities. Temporal variations are seasonal; maximum Ra and Rn-222 activities in ground water occur with water-table lows in autumn and minimum Ra and Rn-222 activities occur with water-table highs in the spring.