Environ. Sci. Technol. 1998, 32, 1244-1252
Removal of Atmospheric CCl4 under Bulk Aerobic Conditions in Groundwater and Soils JAMES D. HAPPELL* AND DOUGLAS W. R. WALLACE Department of Applied Science, Oceanographic and Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, New York 11973-5000
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 e50% 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.
and field (20-23) experiments, there are many studies that conclude that microbially mediated aerobic degradation of perchlorinated compounds, such as CCl4, does not occur (18, 20, 22, 23). On the other hand, Castro (24) reports that two types of bacteria (a Pseudomonas and a CH4 oxidizer), normally considered aerobic oxidizers, are able to reductively dehalogenate CCl4 at a rate faster than an anaerobic methanogen. Additionally, CCl4 distributions in the warm (>10 °C) ocean surface waters (25-27) and some colder waters (28) show very clear evidence for the degradation of background levels of CCl4 at rates far greater than expected for hydrolysis, even in well-oxygenated waters. In this paper, we examine the distribution of background levels of CCl4 in groundwater and soil gas and compare these distributions with those of closely related anthropogenic compounds that are much less reactive and that can be used to provide information concerning the kinetics of CCl4 degradation. Notably we use the measured concentrations of CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CCl2FCClF2) as nonreactive analogues of CCl4 in soil gas and as a tool to estimate the time since recharge of groundwater (see below). TCA (CH3CCl3) concentrations were also measured as a part of this study. By background levels, we mean the concentrations found within the atmosphere as a result of global anthropogenic releases. CCl4 has been released to the atmosphere as a result of its widespread use as a solvent, particularly during the first half of the 20th century. Studies of the deep ocean, which to some extent records past atmospheric concentrations, suggest that, prior to the anthropogenic releases of this century, the atmospheric mole fraction of CCl4 was
