Toxic Substances Hydrology Program
MTBE Biodegrades Naturally in Stream Sediments
USGS scientists have discovered the potential for the widespread geographic capability of microorganisms indigenous to stream-bottom sediments to break down methyl tert-butyl ether (MTBE) to harmless carbon dioxide. This is the first significant microbial process that has been shown to degrade MTBE in the environment. This capability to degrade MTBE is related to the interface between oxygenated (oxic) and oxygen-depleted (anoxic) ground water, characteristic of ground-water/surface-water interactions. At the Toxics Program Research Site, Laurel Bay, South Carolina, a plume of MTBE-contaminated ground water from a leaking underground storage tank has moved through the subsurface to a drainage ditch 200 meters away. USGS scientists have shown that at the oxic-anoxic interface within the stream-bottom sediments underlying the ditch where MTBE-contaminated ground water is discharging, highly efficient biodegradation is occurring. This biodegradation results in highly efficient removal of MTBE in the last 2 meters of ground-water flow prior to discharge to the ditch. This shows that highly efficient MTBE biodegradation in ground-water systems is possible, and depends on microorganisms that live at the interface between anoxic and oxic conditions. The biodegradation at the interface is a dramatically more efficient when compared to the biodegradation in the aquifer that permitted the MTBE to travel more than 200 meters away from the source area.
Given that MTBE biodegradation is stimulated at oxic-anoxic interfaces in ground-water systems, it is possible that artificially placing oxic-anoxic interfaces in MTBE plumes may stimulate biodegradation. This hypothesis was tested at the Laurel Bay research site. USGS scientists placed an oxygen-releasing compound into the anoxic MTBE plume and instrumented the site with closely spaced observation wells. MTBE concentrations decreased from more than 20 milligrams per liter (mg/L) to less than 2 mg/L across this artificial interface. This, in turn, indicates that engineered bioremediation of MTBE-contaminated ground water is feasible.
Additional USGS Information on MTBE