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

Ambient air, soil air, and groundwater were monitored for volatile halogenated organic compounds in a pristine spruce forest at Klosterhede, Denmark. Although this location is remote from industrial areas and free of any point sources of either soil or groundwater contamination, several volatile chlorinated hydrocarbons were detected. The concentration profiles of chloroform indicated a formation of this compound in the soil. Low concentrations were found in ambient air (0.02 ng L21), higher concentrations in the upper soil layers (9.6 ng L21 at 0.75 m below the surface), and a decrease with increasing depth (down to 1.5 ng L21 just above the groundwater table at 7.5 m below the surface). For the other identified chlorinated compounds the concentration profiles were different from those of chloroform and exhibited less systematic variation between ambient air and soil air and with increasing depth. In the phreatic groundwater, high concentrations (up to 1.6 mg L21) of chloroform and low concentrations (,0.01 mg L21) of the other chlorinated compounds identified in air were found. Of the seven volatile brominated and iodinated compounds analyzed in ambient air, soil air, and groundwater, only bromoform was found and only in the upper soil layer. For chloroform a formation in the spruce forest soil is suggested, leading to concentrations in the groundwater in the micrograms per liter range.