Natural formation of chloroform and brominated trihalomethanes in soil

We studied the occurrence of halogenated organic compounds in soil air of r ural areas. Chloroform appeared to occur in elevated concentrations compare d to those in atmospheric air, while the concentrations of other chlorinate d solvents were almost equal or lower than those in atmospheric air. We rep ort conclusive evidence that chloroform is naturally produced from in situ Na-37 Cl enrichment field studies in soil top layers. The concentration of chloroform in soil air increased in deeper soil layers, but spiking of thes e soil layers by (NaCl)-Cl-37 did not result in the formation of chloroform enriched with Cl-37. Bromodichloromethane shows similar concentration grad ients in sail air as does chloroform. It seems also to be formed naturally in soil, even although this could not be confirmed by the (NaCl)-Cl-37 enri chment field studies, because of the low concentration levels encountered. No detectable concentrations of chlorodibromomethane and bromoform in soil air were observed. In situ enrichment of a soil top layer by KBr showed tha t soil has the potential to form chlorodibromomethane and bromoform natural ly. The formation mechanisms of the trihalomethanes are discussed, and a hy pothesis is given to explain the natural formation in the soil top layer an d the concentration gradients in soil air.