Natural Attenuation of a Trichloroethylene Plume in a Basalt Aquifer

This simulation depicts the natural attenuation of a contaminant plume in a basalt aquifer. The broad, volcanic Snake River Plain in the western U.S. possesses one of the largest most productive aquifers in the country drawing its water from surrounding mountain ranges. The aquifer, consisting mainly of fractured basalts, is at least 60 meters below land surface. The Idaho National Laboratory, built on the Snake River Plain, is remediating portions of the aquifer that are contaminated from past waste disposal practices. One such location, Test Area North, has an aquifer plume containing trichloroethylene or TCE. The rapid movement of groundwater in the fractured basalts of the aquifer caused TCE to spread into a two-mile long plume.

As in other locations of the Earth's subsurface, this aquifer is home to a diverse community of microbes adapted to survive in this austere environment. These cells exist where the aquifer chemistry permits their energy needs to be met. Among these cells are methanogens that survive by using hydrogen to reduce dissolved inorganic carbon where oxygen is absent. In the process they produce methane that dissolves into the aquifer. Elsewhere in the aquifer, where oxygen is abundant, cells called methanotrophs derive their energy from the methane produced by methanogens reducing oxygen in the process. To derive energy from methane, these methanotrophs use a unique enzyme called methane monooxygenase. As it turns out, methane monooxygenase can co-metabolize TCE. In other words, using this enzyme these methanotrophs inadvertently remove TCE from the aquifer through their metabolic activity.

With the constant supply of methane methanotroph communities can heal the damage caused by the TCE. Naturally occurring microbes in the aquifer other than methanotrophs may be removing TCE by their own enzymes.

Research over the past 10 years indicates that the TCE is gradually disappearing from the aquifer near Test Area North by this biologically driven natural attenuation. TCE is estimated to have a 13-year half-life in the aquifer so that by late this century it will have practically disappeared. Research continues to investigate the rates at which these cells degrade TCE, the effects that TCE co-metabolism has on the cells, and how our knowledge of natural attenuation at Test Area North can be transferred to other contaminated sites.

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