Chloroform in a pristine aquifer system: Toward an evidence of biogenic origin

Ambient air, soil air, and groundwater were monitored for volatile halogena ted organic compounds in a pristine spruce forest at Klosterhede, Denmark. Although this location is remote from industrial areas and free of any poin t sources of either soil or groundwater contamination, several volatile chl orinated hydrocarbons were detected. The concentration profiles of chlorofo rm indicated a formation of this compound in the soil. Low concentrations w ere found in ambient air (0.02 ng L-1), higher concentrations in the upper soil layers (9.6 ng L-1 at 0.75 m below the surface), and a decrease with i ncreasing depth (down to 1.5 ng L-1 just above the groundwater table at 7.5 m below the surface) For the other identified chlorinated compounds the co ncentration profiles were different from those of chloroform and exhibited less systematic variation between ambient air and soil air and with increas ing depth. In the phreatic groundwater, high concentrations (up to 1.6 mu g L-1) of chloroform and low concentrations (<0.01 mu g L-1) of the other ch lorinated compounds identified in air were found. Of the seven volatile bro minated and iodinated compounds analyzed in ambient air, soil air, and grou ndwater, only bromoform was found and only in the upper soil layer. For chl oroform a formation in the spruce forest soil is suggested, leading to conc entrations in the groundwater in the micrograms per liter range.