Groundwater chemists and hydrologists are keenly interested in expanding the knowledge of environmental tracers that can be used to determine groundwater age. The age of groundwater is a valuable parameter that serves to inform many types of groundwater availability studies.
Many environmental tracers — such as chlorofluorocarbons (CFCs), SF6, and tritium — are of atmospheric origin. However, there are several classes of atmospheric trace gases whose application as groundwater age tracers have not been fully explored. Hydrofluorocarbons and hydrochlorofluorocarbons (HFCs and HCFCs) are among them.
USGS scientists have recently developed a high-sensitivity technique to measure two of these compounds (HCFC-22 and HFC-134a) in groundwater and the unsaturated zone.
The investigators found that, contrary to many simpler laboratory studies, these compounds can be degraded by bacteria in the environment. Consequently, both classes of compound (HFCs and HCFCs) are not likely to be useful for dating groundwater. Since they are depleted in the unsaturated zone, this reduction implies a weak environmental sink (a few percent or less) that has not been previously discussed.
The study by USGS hydrologists Haase, Busenberg, Plummer, Casile, and Sanford has been published in the journal Chemical Geology.