Natural chloroform in soil gas below four coniferous forest sites was studied. High concentrations were found within narrow areas-Hot Spots-varying from similar to 25 to > 400 m(2) in size, with chloroform concentrations being typically 20-100 times those in corresponding Low Spots.Attempts to localize Hot Spots by visual inspection with regard to type and density of vegetation failed. Possible differences between Hot and Low Spots could be emission, leaching or degradation of chloroform. However, emissions of chloroform from Hot Spots were similar to 10 times higher than from Low Spots and similarly the chloroform concentration in groundwater below a Hot Spot was similar to 10 times higher than below the corresponding Low Spot. No differences in chloroform mineralization rates were observed between sites and incubation of soil cores confirmed a larger net formation of chloroform in the Hot Spots. Various soil parameters were measured in order to compare the soil sampled from Hot and Low Spots. The halogenation degree of organic soil samples was in the same range, although slightly higher in the H-horizon of the Hot Spot. The chloroform formation potential of the soil organic matter showed differences between soil horizons but not between sites. The high levels of chloroform in the Hot Spots are probably best explained by differences in chloroform forming activity caused by an uneven distribution of yet unidentified microorganisms, since differences in soil organic matter quality and in emission, leaching and degradation of chloroform as well as a number of additional soil parameters could be completely ruled out.10341699317334Danish Agency for Science, Technology and Innovation [09-061119/FTP]Danish Agency for Science, Technology and Innovation [09-061119/FTP]