The NECB NAWQA study evaluated arsenic, iron, and manganese concentrations in selected bedrock public water-supply wells throughout the study area (Ayotte and others, 1999). Variability in concentrations, associated with bedrock lithochemical groups and land use were explored. Concentrations of arsenic at or above 0.005 mg/L (milligrams per liter) were detected in more samples of water from wells completed in bedrock than in water from wells completed in stratified drift. Arsenic concentrations are higher in water from wells in metamorphosed variably calcareous marine sediments than in water from wells in igneous and other metamorphic rocks. Iron and manganese were detected at approximately the same frequency in water from wells in both types of aquifers.

Private domestic bedrock wells also were sampled for arsenic and selected radionuclides (radon and radium). Analyses of water samples from 58 wells show that concentrations of arsenic are highest in waters with high pH and low dissolved oxygen. These results indicate that solubility and mobility in ground water are affected by factors other than just the presence of arsenic concentrations in rock.

Radon, a colorless and odorless gas, that decays to other elements and releases radiation in the process, was detected in water from bedrock wells in New England at concentrations greater than 2,000 picocuries per liter (pCi/L) in most samples, but exceeded 100,000 pCi/L in some samples. The USGS analyzed samples from bedrock wells around eastern New England and found that the median radon level for wells in metamorphosed marine calcareous sediments is about 2,185 pCi/L, whereas the median for wells in other metamorphosed marine sediments is about 3,750 pCi/L.

Radium isotopes were also detected in water samples but generally at low concentrations (less than 1 pCi/L). At least one radium isotope, however, was detected in concentrations greater than 1 pCi/L in 33 percent of the bedrock wells.

--Joseph Ayotte (603) 226-7810 or jayotte@usgs.gov

For more information on arsenic, radon, and radium, visit the web site at URL
http://co.water.usgs.gov/trace/

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