The Reston Groundwater Dating Laboratory

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Vulnerability

U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 03-4278

Aquifer Susceptibility in Virginia, 1998-2000
Prepared in cooperation with
Virginia Department of Health
Office of Drinking Water

By David L. Nelms, George E. Harlow, Jr., L. Niel Plummer, and Eurybiades Busenberg

ABSTRACT

The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Health, sampled water from 171 wells and springs across the Commonwealth of Virginia between 1998 and 2000 as part of the Virginia Aquifer Susceptibility study. Most of the sites sampled are public water supplies that are part of the comprehensive Source Water Assessment Program for the Commonwealth. The fundamental premise of the study was that the identification of young waters (less than 50 years) by multiple environmental tracers could be used as a guide for classifying aquifers in terms of susceptibility to contamination from near-surface sources. Environmental tracers, including chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), tritium (3H), and tritium/helium-3 (3h/3he), and carbon isotopes (14C and d13C) were used to determine the age of water discharging from wells and springs. Concentrations of CFCs greater than 5 picograms per kilogram and 3H concentrations greater than 0.6 tritium unit were used as thresholds to indicate that parts of the aquifer sampled have a component of young water and are, therefore, susceptible to near-surface contamination.

Concentrations of CFCs exceeded the susceptibility threshold in 22 percent of the wells and in one spring sampled in the Coastal Plain regional aquifer systems. About 74 percent of the samples from wells with the top of the first water zone less than 100 feet below land surface exceeded the threshold values, and water supplies developed in the upper 100 feet of the Coastal Plain are considered to be susceptible to contamination from near-surface sources. The maximum depth to the top of the screened interval for wells that contained CFCs was less than 150 feet. Wells completed in the deep confined aquifers in the Coastal Plain generally contain water older than 1,000 years, as indicated by carbon-14 dating, and are not considered to be susceptible to contamination under natural conditions. All of the water samples from wells and springs in the fractured-rock terrains (the Appalachian Plateaus, Valley and Ridge, Blue Ridge, and Piedmont regional aquifer systems) contained concentrations of CFCs and 3H greater than one or both of the thresholds. Because all of the water samples exceeded at least one of the threshold values, young water is present throughout most of these regional aquifer systems; therefore, water supplies developed in these systems are susceptible to contamination from near-surface sources. No relation between well depth and presence of CFCs is evident from samples in the fractured-rock terrains.

More than 95 percent of the samples for which the dating methods were applicable contained waters with apparent ages less than 35 years. About 5 percent of these samples, most of which were from the Blue Ridge and Piedmont regional aquifer systems, contained young waters with apparent ages of less than 5 years. Most of the samples from the Valley and Ridge Carbonate, Blue Ridge, and Piedmont regional aquifer systems had young water fractions of more than 50 percent, whereas samples from the Coastal Plain Shallow and Appalachian Plateaus regional aquifer systems contained less than 40 percent young waters.

Concentrations of CFCs in excess of air-water equilibrium, which can indicate that nonatmospheric sources (such as sewage effluent) have introduced CFCs into the ground-water system, were measured in 6 and 48 percent of the water samples from the Coastal Plain and fractured-rock regional aquifer systems, respectively. The nitrate (NO3) concentrations greater than the USGS detection level of 0.05 milligrams per liter generally increase as the apparent age of the young water fraction decreases, with the highest NO3 concentrations for samples in which one or more of the CFCs are above modern atmospheric mixing ratios (commonly referred to as "contaminated" for ground-water dating purposes). Most of the samples in which NO3 was detected were from the fractured-rock regional aquifer systems, especially the Valley and Ridge Carbonate regional aquifer system, where 90 percent of the samples had concentrations greater than the detection level. Numerous halogenated volatile organic compounds were detected at low concentrations (parts per quadrillion) in the samples from the regional aquifer systems in the fractured-rock terrains and the Coastal Plain Shallow regional aquifer system.

The ratio of the percentage and apparent age of the young fraction in binary mixtures of young and old (greater than 50 years) waters provides an indication of the relative degree of susceptibility. Large ratios are associated with samples in which the fraction of the young water is large (greater than 75 percent) and apparent age is extremely young (less than 5 years). Samples from the fractured-rock regional aquifer systems generally have the largest ratios. Analysis-of-variance tests indicate that the ratios in the samples from the Blue Ridge regional aquifer system are significantly higher (p<0.05) than those in the other regional aquifer systems. Results from the multiple tracer approach indicate that shallow wells (less than 100 feet deep) and springs in the Coastal Plain and wells and springs in the fractured-rock terrains contain a component of young ground water and are, therefore, susceptible to contamination from near-surface sources.

References

Shapiro, Stephanie Dunkle, Plummer, L. Niel, Focazio, Michael J., Busenberg, Eurybiades, Kirkland, Wandee, and Fernandez, Mario, Jr., 2003, Occurrence of volatile organic compounds in drinking water from the United States: Results from archived chromatograms and water samples, 1989-2000. U.S, Geological Survey Water-Resources Investigations Report, WRIR 02-4173, 20 p.

Nelms, D.L., Harlow, G.E., Jr., Plummer, L.N., and Busenberg, Eurybiades, 2003, Aquifer susceptibility in Virginia, 1998-2000: U. S. Geological Survey Water-Resources Investigations Report 03-4278, 58p.

Shapiro, S.D., Busenberg, E., Plummer, L.N., and Focazio, M.J., 2003, Data from archived chromatograms on halogenated volatile organic compounds in untreated ground water used for drinking water in the United States, 1997-2000. U.S. Geological Survey, Open-File Report 03-352, 31pp.

Shapiro, S.D., Busenberg, E., Focazio, M.J., and Plummer, L.N., 2004, Historical trends in occurrence and atmospheric inputs of halogenated volatile organic compounds in untreated ground water used as a source of drinking water. Science of the Total Environment, v. 321, 201-217.