This report summarizes research completed from May 2001 to November 2004 to evaluate processes controlling the natural attenuation (NA) of petroleum hydrocarbons and methyl tertiary butyl ether (MTBE) in the Upper Chalk aquifer at the site of an unleaded fuel spill in St Albans, southeast England. The unsaturated zone and saturated zone of the aquifer are contaminated with benzene, toluene, ethylbenzene and xylenes (BTEX), MTBE, tertiary amyl methyl ether (TAME) and tertiary-butyl alcohol (TBA). A mixed BTEX/MTBE/TAME/TBA plume extends 125 m from the site and <10 m below the water table, whereas a MTBE/TAME/TBA plume extends 220 m from the site and >15 m below the water table.
MTBE can be biodegraded in the aquifer under aerobic conditions at concentrations up to 6 mg/L, with the production of TBA. Anaerobic degradation of MTBE is not significant, although the aquifer microorganisms have the potential for this. Measured first-order aerobic MTBE degradation rates obtained from uncontaminated aquifer microcosm experiments were 0.0074/day compared with 0.0025/day for plume microcosms. The presence of TBA as an impurity in MTBE and temporal variation in the release of TBA and MTBE from the source area, prevent the deduction of MTBE degradation at field scale from the distribution of TBA. Stable isotope compositions of MTBE in groundwater indicate that there has been no significant MTBE degradation in the plume, although the sensitivity of the technique will be low if isotope enrichment factors are small in this setting. Aerobic MTBE degradation may be inhibited by preferential aerobic degradation of BTEX or other more biodegradable hydrocarbon compounds in the groundwater. Aerobic MTBE degradation is unlikely to be an important process for MTBE attenuation within the plume, particularly in the presence of other hydrocarbons, but could be significant at the advancing plume fringe where O2 is more available. Aerobic MTBE degradation should be considered in risk assessments for unleaded fuel releases at sites.