Study found that certain atmospheric trace gas compounds are not good indicators of groundwater age
U.S. Geological Survey (USGS) scientists recently developed a high-sensitivity technique to measure two compounds (HCFC-22 and HFC-134a) in groundwater and the unsaturated zone.
Groundwater chemists and hydrologists are 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. There are several classes of atmospheric trace gases, however, with applications as groundwater age tracers that have not been fully explored. Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are among them.
The investigators found that, contrary to many simpler laboratory studies, HCFC-22 and HFC-134a can be degraded by bacteria in the environment. Consequently, both classes of compound are not likely to be useful for dating groundwater. Because 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 was published in the journal Chemical Geology.