Project Summary:

Water-quality data collected from 1,426 wells during 1993–2003 as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) program were evaluated to characterize the water quality in glacial and bedrock aquifers of the northern United States. One of the goals of the NAWQA program is to synthesize data from individual studies across the United States to gain regional- and national-scale information about the behavior of contaminants. This study focused on the regional occurrence and distribution of uranium and ²²²Radon in ground water in the glacial aquifer system of the United States as well as in the Cambrian-Ordovician and the New York and New England crystalline aquifer systems that underlie the glacial aquifer system. A combination of information or data on (1) national-scale ground-water regions, (2) regional-scale glacial depositional models, (3) regional-scale geology, and (4) national-scale terrestrial gamma-ray emissions were used to confirm and(or) refine the regions used in the analysis of the water-chemistry data.

Significant differences in the occurrence of uranium and ²²²Radon, based primarily on geologic information were observed and used in the report. In general, uranium was highest in the Columbia Plateau glacial, West-Central glacial, and the New York and New England crystalline aquifer groups (75th percentile concentrations of 22.3, 7.7, and 2.9 micrograms per liter (µg/L), respectively). In the Columbia Plateau glacial and the West-Central glacial aquifer groups, about 10 percent of wells sampled had concentrations of uranium that exceeded the U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Level of 30 µg/L; in the New York and New England crystalline aquifer group, 4 percent exceeded 30 µg/L. Ground-water samples with high concentrations of uranium were commonly linked to geologic sources rich in uranium. The highest ²²²Radon concentrations were present in samples from wells completed in the New York and New England crystalline aquifer group; the median value (2,122 picocuries per liter (pCi/L)) was about 10 times the median value of the glacial and Cambrian-Ordovician aquifer groups. More than 25 percent of the samples from the New York and New England crystalline aquifer group wells had ²²²Radon concentrations that exceeded the proposed USEPA Alternative Maximum Contaminant Level (AMCL) of 4,000 pCi/L.