We studied the occurrence of halogenated organic compounds in soil air of rural areas. Chloroform appeared to occur in elevated concentrations compared to those in atmospheric air, while the concentrations of other chlorinated solvents were almost equal or lower than those in atmospheric air. We report conclusive evidence that chloroform is naturally produced from in situ Na{sup}37Cl enrichment field studies in soil top layers. The concentration of chloroform in soil air increased in deeper soillayers, but spiking of these soil layers by Na{sup}37Cl did not result in the formation of chloroform enriched with 37Cl. Bromodichloromethane shows similar concentration gradients in soil air as does chloroform. It seems also to be formed naturally insoil, even although this could not be confirmed by the Na{sup}37Cl enrichment field studies, because of the low concentration levels encountered. No detectable concentrations of chlorodibromomethane and bromoform in soil air were observed. In situenrichment of a soil top layer by KBr showed that soil has the potential to form chlorodibromomethane and bromoform naturally. The formation mechanisms of the trihalomethanes are discussed, and a hypothesis is given to explain the natural formation in the soil top layer and the concentration gradients in soil air.
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