Removal of Atmospheric CCl4under Bulk Aerobic Conditions in Groundwater and Soils

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 CCl 4, the initial CCl 4 concentration for the precipitation that recharged the aquifer was estimated as a function of age. Correlation of measured and estimated initial CCl 4 concentrations within the aquifer, over inferred ages of 0−50 yr, suggested that CCl 4 was being removed in situ with a half-life of 14 ± 4 yr. Groundwater samples collected at the water table had CCl 4 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 CCl 4 was being removed from the unsaturated zone, with only ∼25% of the initial CCl 4 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 col lected before and after a major rainfall event indicated that most removal occurred in the top 15 cm of soil. The flux of CCl 4 into the soil was estimated to be ∼8600 ± 5100 pmol m - 2 d - 1, and removal of CCl 4 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 CCl 4 is degraded significantly only within reducing environ ments.