U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Sampling Throughout The Hydrologic Cycle To Characterize Sources Of Volatile Organic Compounds In Ground Water

By Arthur L. Baehr, Leon J. Kauffman, Emmanuel G. Charles, Ronald J. Baker, Paul E. Stackelberg, Mark A. Ayers, and Otto S. Zapecza U.S. Geological Survey, West Trenton, New Jersey

ABSTRACT

Results of three studies in New Jersey demonstrate that analysis of samples collected throughout the hydrologic cycle can improve understanding of the sources of volatile organic compounds (VOCs) in ambient ground water. Results of the first study indicate that atmospheric concentrations of methyl-tert butyl ether (MTBE) are sufficiently high to cause detection in ground water, whereas atmospheric concentrations of the chlorinated VOCs trichloromethane (chloroform), 1,1,1-trichloroethane (TCA), and tetrachloroethene (PCE) are not. Results of analysis of unsaturated-zone gas and ground-water samples indicate that point sources of MTBE, such as spills or infiltration of urban runoff, are also prevalent and that these are the only possible sources of the chlorinated compounds. In the second study, VOC concentrations in three ground-water monitoring networks in a surficial aquifer were compared. Although it was anticipated that deeper ground water from the public supply wells would be associated with less frequent detections and lower concentrations of VOCs than water from the shallower observation wells, the opposite was found for many chlorinated VOCs. The integration of chlorinated VOCs from multiple point sources within the larger contributing areas of the public supply wells may explain this result. Chloroform, however, is detected frequently throughout the ground-water- monitoring network, indicating that its sources are widespread. MTBE is detected less frequently in observation wells that tap 10- to 15-year-old ground water than in shallow observation wells that tap recently recharged ground water and public supply wells. This result is not contradictory because some of the ground water in a given sample from a public supply well would have been recharged during the past 10 years, a period of increased MTBE use due to oxygenated-fuel programs. In the third study, the occurrence of MTBE in a lake, caused by the use of gasoline-powered watercraft, may explain the frequent occurrence of MTBE in samples from lakeside wells in the fractured-rock aquifer. The probable lake/well interaction complicates investigations of ground-water contamination with MTBE due to fuel spillage because of the difficulty of determining point-source plume dimensions with a nonpoint-source signature superimposed on the ground water.