Abstract
Elevated concentrations of naturally occurring radionuclides are found in ground-water samples collected from the Newark basin in central New Jersey and are associated with uranium enrichment in the bedrock formations. Elevated radionuclide concentrations are present primarily in the lower part of the Passaic Formation but also are common in the underlying Lockatong Formation and, locally, in the Stockton Formation. These formations are exposed along the southern and eastern margins of the Newark basin and in the Flemington and Hopewell fault blocks, where the formations are stratigraphically repeated. Other formations in the Newark basin contain significantly lower concentrations of radionuclides. Borehole geophysical testing, analysis of lithologic cores, and autoradiographs were used to determine that the source of the natural radioactivity in the lower part of the Passaic Formation and the upper part of the Lockatong Formation is thin, laterally extensive, uranium-enriched black mudstones that locally contain carbonate-rich laminae. Radioactive zones are found dispersed along horizontal bedding planes throughout the matrix and in arcuate bands that crosscut bedding. Autoradiographs from the Stockton Formation show that the radioactive sources are dispersed but are not conclusive as to whether the source of the radioactivity is in the sandstone matrix or in the fracture-fill materials. Results of radiochemical analyses of ground-water samples show that gross alpha-particle activities range from less than 1 to 124 picocuries per liter, uranium concentrations range from less than 0.4 to 60 micrograms per liter, radium-226 concentrations range from less than 0.6 to 22.5 picocuries per liter, radon-222 concentrations range from 71 to 15,900 picocuries per liter, and gross beta-particle activities range from less than 1 to 49.7 picocuries per liter. The local ground-water chemistry determines the distribution of dissolved uranium and radium. Elevated uranium concentrations are associated with high gross alpha-particle activity in oxidizing ground water that is low in iron and manganese and commonly high in sulfate and bicarbonate. In contrast, elevated concentrations of radium-226 are associated with high gross alpha-particle activity in reducing ground water that is high in iron, manganese, and sometimes barium and low in sulfate. High concentrations of uranium and radium typically are not found concurrently in the same sample. Radon-222 concentrations are weakly correlated with radium-226 concentrations but not with uranium concentrations or the concentrations of any other dissolved chemical constituent. Chemical processes that may govern the distribution of the radionuclides in the ground water of the Newark basin are postulated on the basis of the chemical composition of the ground water and published laboratory experiments. Uranium concentrations are controlled by the solubility of the U + 6 ion and by carbonate complexing. Radium-226 concentrations are restricted by adsorption onto iron and manganese hydroxides, adsorption onto or ion-exchange with secondary clay minerals, and coprecipitation with sulfate minerals. These reactions, in turn, are limited by the Eh (oxidation-reduction potential) and the pH of the ground water. Radon-222 concentrations are governed by the abundance and distribution of radium-226 in the rock matrix, the emanation efficiency of radon-222 from the rock matrix, and the physical characteristics that determine ground-water flow in the rock.
Published Version
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