Formation of chloroform in soil. A year-round study at a Danish spruce forest site

Soil air from top soil of a Danish spruce forest was investigated monthly from December 1997 to December 1998 for the occurrence of chloroform, 1,1,1-trichloroethane, tetrachloromethane, trichloroethene and tetrachloroethene. Within the monitoring period, three different patterns of soil air concentrations were identified. For chloroform, concentrations peaked in spring and autumn while 1,1,1-trichloroethane and tetrachloromethane peaked during mid winter. Trichloroethene and tetrachloroethene, concentrations remained constant throughout the year. The relative ratios of soil air concentrations to ambient air concentrations indicated a natural production of chloroform, while the other chlorinated compounds investigated probably originated from non-point source pollution. The seasonal variation of the chloroform concentration suggested a production by microorganisms, as high chloroform concentrations were found in the soil in warm and humid periods of the year (spring and autumn) with high microbial activity.