25 Hydrogeology and Geochemistry

Tuesday, 13 March 2001
3:30 PM - 5:00 PM

ARSENIC AND RADIONUCLIDES IN BEDROCK GROUND WATER, EASTERN NEW ENGLAND

Paper number: 25-3
Program: Technical Sessions
Time: 4:10 PM – 4:30 PM

Authors

Abstract

Water quality in bedrock aquifers is studied as part of the New England Coastal Basins study unit of the USGS National Water Quality Assessment Program. Analyses of water samples from more than 100 domestic bedrock wells show that arsenic concentrations are higher in water from wells in metamorphosed marine sediments derived from variably calcareous protoliths, than in water from igneous and other metamorphic rocks. Arsenic concentrations are highest in waters with high pH and low dissolved oxygen concentrations. Few samples with dissolved oxygen concentrations greater than 1 milligram per liter had arsenic greater than 1 microgram per liter. This result suggests that factors other than the bedrock arsenic concentration control its solubility and mobility in ground water. Further, arsenic in bedrock well water appears to be primarily dissolved and not sorbed to particulate or colloidal material. Unfiltered samples and samples collected through a 0.45, 0.0046, and 0.0029 micrometer filter show no difference in arsenic concentration, indicating that all of the arsenic is dissolved.

Radon concentrations in water from the bedrock wells are commonly above 4,000 picocuries per liter (pCi/L) and can exceed 100,000 pCi/L. The median for metamorphosed sediments, derived from variably calcareous protoliths, is about 2,185 pCi/L while the median for other metamorphosed sediments is about 3,750 pCi/L. Concentrations of radium isotopes were generally low but at least one radium isotope was above 1 pCi/L in 33 percent of the bedrock wells. Radium 224 was above 1 pCi/L in only 5 percent of the wells while radium 226 was above 1 pCi/L in 13 percent, and radium 228 in 19 percent. Radium 226 and 228 were detected only in metamorphosed marine sediments (16 and 20 percent of 31 samples, respectively). Radium 226 was associated with high gross alpha activity in water with low dissolved oxygen and high dissolved iron. Uranium was associated with high gross alpha activity in water with high dissolved oxygen and low dissolved iron.