Measured concentrations of relatively nonreactive, anthropogenic halocarbon tracers (CFC-11, CFC-12, CFC-113) were used to infer the time since recharge, or age, of groundwater collected from the Upper Glacial and Magothy Aquifers underlying Brookhaven National Laboratory on Long Island, NY. On the basis of the reconstructed historical atmospheric concentrations of CCl4, the initial CCl4 concentration for the precipitation that recharged the aquifer was estimated as a function of age. Correlation of measured and estimated initial CCl4 concentrations within the aquifer, over inferred ages of 0-50 yr, suggested that CCl4 was being removed in situ with a half-life of 14 ± 4 yr. Groundwater samples collected at the water table had CCl4 concentrations that were ≤50% of equilibrium with contemporary atmospheric concentrations, suggesting that removal was also significant in the unsaturated zone. Soil gas profiles confirmed that atmospheric CCl4 was being removed from the unsaturated zone, with only ~25% of the initial CCl4 being present in the gas phase at a depth of 30 cm, and with no evidence for removal of CFC-11, CFC-12, or CFC-113. A time-series of soil gas profiles collected before and after a major rainfall event indicated that most removal occurred in the top 15 cm of soil. The flux of CCl4 into the soil was estimated to be ~8600 ± 5100 pmol m-2 d-1, and removal of CCl4 in soils therefore has the potential to significantly affect the global atmospheric lifetime of this compound. The observed degradation in bulk aerobic environments raises questions concerning the conventional wisdom that CCl4 is degraded significantly only within reducing environments.
ASJC Scopus subject areas
- Environmental Chemistry