| Soils are thought to be sinks for atmospheric CCl4. To provide evidence of CCl4 degradation by soils, a two-year soil air samples monitoring program was initiated in New York and New Jersey. Greenhouse gases were also monitored along with CCl4, i.e. CFC-11, CFC-12, CFC-113, and CHCl3.; Soil air depth profiles showed that degradation of atmospheric CCl 4 occurred in all tested soils, and was affected by soil properties such as TOM and pH. Degradation rate constants at Sparkill, NY, were estimated as 0.6 d-1, 0.2 d-1, and 0.05 d-1 for soil depth segments of 0-1 m, 1-2m, and 2-3m, respectively. A time series of CCl4 concentrations at Sparkill, NY, showed a seasonal cycle with enhanced degradation in spring.; Microcosm studies were performed with unsaturated soil samples from Sparkill, NY, under controlled aerobic and anaerobic environments with methanol as the primary substrate, to determine whether atmospheric CCl4 degradation is biological in nature. Results suggested that indigenous soil microorganisms were responsible for the degradation. The pseudo-first order CCl4 degradation rate constants ranged from 0.04 +/- 0.01 d-1 to 0.56 +/- 0.02 d-1 at 20 °C in the aerobic microcosms, and were about 0.14 +/- 0.02 d-1 in the anaerobic microcosms at 20 °C. This is the first time that such rate constants have been reported for ppt levels of background atmospheric CCl4 in soils.; Field soil air profiles of CHCl3 indicated that it was formed rather than degraded in soils. It is speculated that abiotic mechanisms contributed to the formation of CHCl3 along with biotic processes.; Field soil air profiles of CFCs indicated that CFCs could be considered stable enough with soil depth. In contrast, the microcosm study showed that when native microorganisms were stimulated with appropriate substrates they could transform CFC-11 by both aerobic and anaerobic attack, and CFC-12 under anaerobic conditions.; To conclude, a number of processes investigated in this study contribute to the transformation of compounds of CCl4, CHCl3, CFC-11 and CFC-12 in soils. |
