Hydrogeologic controls on radon-222 in a buried valley-fractured bedrock aquifer system

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A field study was conducted to evaluate the distribution of radon (222Rn) and the hydrogeologic controls on its occurrence in the glacial and fractured rock aquifers of the Pawcatuck River Basin, Rhode Island. A total of 91 ground water samples were collected and analyzed for major chemical constituents as well as uranium and 222Rn. The three bedrock types underlying the study area include the Proterozoic Esmond and Sterling gneisses, and the Devonian Scituate Granite, each having average uranium contents of 1.9 ppm, 3.3 ppm, and 4.1 ppm, respectively (Nevins 1991). All wells sampled in this study yielded radon levels above the proposed U.S. EPA limit of 300 pCi/L. Wells in areas underlain by the Esmond Suite had the lowest radon content (range 500 to 30,400 pCi/L, median 1400 pCi/L), areas underlain by the Sterling Suite were not significantly different but showed slightly higher concentrations (range 700 to 27,300 pCi/L, median 1600 pCi/L), and the areas underlain by the Scituate Suite had significantly higher levels (range 1600 to 83,500 pCi/L, median 5900 pCi/L). High fluoride and alkalinity concentrations in ground water were found to be hydrogeochemical markers for elevated radon concentrations. These markers are believed to represent chemical reactions that produce remobilization of uranium in the host crystalline rock to more favorable siting along grain and fracture boundaries. Physical characteristics such as porosity and the location of uranium are controlling factors in the distribution of radon between surficial and bedrock wells, with bedrock aquifers exhibiting higher radon concentrations than surficial-materials aquifers in this basin.

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Ground Water