Distribution and seasonal variation of methanotrophs in trichloroethylene contaminated groundwater

Seasonal variations in distribution of methanotrophs in trichloroethylene (TCE)-contaminated groundwater in Chikura, Chiba, Japan were investigated. The hydrostratigraphic units of the area consisted of three aquifers which extended over 30-m depth. High concentrations of dissolved methane and high electric conductivity were observed in the bottom aquifer influenced by fossil water confined below, because of its location on the edge of a naturally dissolved methane field. Methanotrophs were distributed throughout the three aquifers. Their abundance was low in the zone of low-dissolved methane and high-dissolved oxygen, while high numbers were observed in the zone of high-dissolved methane and low-dissolved oxygen. Groundwater level decreased in the season of low precipitation. In the first aquifer, seasonal variation of dissolved methane showed the opposite pattern to that of groundwater level. In this aquifer, a seasonal variations in the number of methanotrophs and dissolved methane were positively correlated. This was explained by calculating that dissolved methane regulated the abundance of methanotrophs in the first aquifer. Although in every aquifer, the pH was within the suitable range for TCE degradation by methanotrophs and indigenous methanotrophs had TCE degradation ability, variations in the numbers of methanotrophs did not show significant correlation with TCE concentrations in groundwater in any aquifer.