Removal of Atmospheric CCl₄under Bulk Aerobic Conditions in Groundwater and Soils

Abstract

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 col lected 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 environ ments.

Microbial Removal of Atmospheric Carbon Tetrachloride in Bulk Aerobic Soils

“…4 , indicating that the process is widespread. This conclusion is consistent with the observations of other researchers (24,36,65). Removal occurred under bulk aerobic conditions in both field ( Fig.…”

“…Removal occurred under bulk aerobic conditions in both field ( Fig. 1; Table 1) and laboratory incubations, consistent with previous reports by Happell and Wallace (24) and Happell and Roche (23).…”

“…The estimated total atmospheric lifetime of CCl 4 is 26 years (68) without inclusion of a soil sink. CCl 4 is generally considered to be recalcitrant to aerobic biodegradation (11,41); however, removal of atmospheric concentrations of CCl 4 has been reported for aerobic groundwater and soils (24,36,65), with the global soil removal rate estimated at 27.2 kilotons year Ϫ1 (23). The estimated total atmospheric lifetime of CCl 4 is reduced to 20 years if the presence of a soil sink is considered, suggesting that CCl 4 emission estimates may be underestimated by approximately 60% (23).…”

“…Separate triplicate or duplicate syringes were used for each treatment. Glass syringes were kept gas tight by the standard method of storing them immersed in water in order to maintain a film of water between the plunger and barrel to prevent air/gas leakage (21,23,24).…”

Biodegradability of chlorinated solvents and related chlorinated aliphatic compounds

“…Removal of atmospheric CT was demonstrated in the unsaturated zone of soil profiles, with only 25% of the initial atmospheric CT present in the gas phase at 30 cm depth (Happell & Wallace 1998). A CT flux of 8300 pmol m )2 soil surface d )1 was estimated in the latter study.…”