Hydrogeologic Controls on the Occurrence and Distribution of Radon-222 in Glacial Aquifers of Southern Rhode Island

Author

Nicole C. Ruderman, University of Rhode Island

Date of Award

1996

Degree Type

Thesis

Degree Name

Master of Science in Geology

Department

Geology

First Advisor

Anne Veeger

Abstract

A field study was conducted to evaluate the distribution of Radon (²²²Rn) in the glacial aquifers of the Pawcatuck River Basin, Rhode Island. A total of 95 ground-water samples were collected from private wells in stratified-sediment and bedrock aquifers of the Upper Wood, Queen-Usquepaugh, and Chipuxet river basins. Gamma-ray (uranium, thorium, and potassium) emissions from the regolith material throughout the study area were measured. The ground-water samples were analyzed for basic chemical constituents as well as uranium and ²²²Rn to help evaluate the factors controlling ²²²Rn distribution. The granite of the Scituate Igneous Suite underlies the Upper Wood River and Queen-Usquepaugh aquifers. The granite gneiss of the Esmond Plutonic Suite underlies the Chipuxet and the Queen-Usquepaugh aquifers. The granite gneiss of the Sterling Plutonic Suite is found underlying the Upper Wood River aquifer. The uranium-bearing minerals (source of radon) found in the bedrock are zircon, allanite, sphene, and monazite. The average uranium content of the Esmond Gneiss is 1.9 ppm, Sterling Gneiss is 3.3 ppm, and Scituate Granite is 4.1 ppm (Nevins, 199 I). All wells sampled in this study yielded radon levels above the proposed EPA limit of 300 pCi/L, with many being an order of magnitude or more greater. Wells in areas underlain by the Esmond Suite had the lowest radon content (range 500 to 30,400 pCi/L, median 1,400 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 1,600 pCi/L), however, the areas underlain by the Scituate Suite had significantly higher levels (range 1,600 to 83,500 pCi/L, median 5,900 pCi/L). Water chemistry factors play little if any role in influencing radon concentrations. High fluoride concentrations in ground water, however, indicate that the mineral fluorite is present in the underlying bedrock. Fluorite is commonly found with uranium-bearing minerals in A-type granites. The physical processes such as well yield and the siting of uranium are the controlling factors in the distribution of radon between surficial and bedrock wells. Bedrock aquifers exhibited higher radon concentrations than surficial-materials aquifers because surficial-materials aquifers have greater water-transmitting capacity, thus a greater volume of water to dilute the radon. Radon concentrations showed no correlation with the uranium content in the surficial material. However, higher radon levels in ground water correlate with higher uranium contents in the underlying bedrock, therefore underlying bedrock uranium content is perhaps the most important factor in radon distribution. Although the EPA will most likely adopt a standard less rigorous than 300 pCi/L, this study reveals that much of the ground water in southern Rhode Island has elevated radon levels that may be cause for concern.

Recommended Citation

Ruderman, Nicole C., "Hydrogeologic Controls on the Occurrence and Distribution of Radon-222 in Glacial Aquifers of Southern Rhode Island" (1996). Open Access Master's Theses. Paper 2027.
https://digitalcommons.uri.edu/theses/2027