Microbial mineralisation of benzene and characterisation of microbial biomass in soil above hydrocarbon-contaminated groundwater

Abstract

Rates of microbial mineralisation of benzene were measured in a soil profile above gasoline-contaminated groundwater. The fastest mineralisation rate of benzene occurred in soil with overlapping concentration gradients of hydrocarbon volatile organic carbon and oxygen, in which neither oxygen nor hydrocarbon volatile organic carbon was at a concentration considered to be rate limiting (13.8% oxygen and 1.1 mg l −1 hydrocarbon volatile organic carbon). At our site, the fastest rate of microbial mineralisation of [ 14C]benzene was 83±13 μmol kg −1 day −1 with a half-life ( t 1/2) of 11±1 days in soil from a depth of 0.25 m below the ground surface. Microbial mineralisation rates were slower in soil from a depth of 0.5 m (27±6 μmol kg −1 day −1, t 1/2=72±16 days) as were mineralisation rates in surface soils (52±13 μmol kg −1 day −1, t 1/2=26±7 days). In the anoxic soils just above the groundwater table, microbial mineralisation rates were extremely slow (0.04±0.01 μmol kg −1 day −1, t 1/2=173±31 years). Microbial biomass at 0.25 m below the ground was comparable to the microbial biomass in surface soils (2.0±0.2×10 8 and 1.4±0.4×10 8 stationary phase Escherichia coli equivalent cells g −1 dry wt., respectively). The mono-unsaturated fatty acids in the phospholipids in the microbiota in soils from 0.25 and 0.5 m below the ground surface were greatly enriched in their trans isomers. As the surface soil microbiota was poor in trans isomers of mono-unsaturated fatty acids and showed no better mineralisation rate than microbes from similar environments that were not previously exposed to hydrocarbons, it would seem likely that aromatic hydrocarbons do not pass through the surface of the soil due to microbial mineralisation in the soil profile. This hypothesis was supported by the absence of hydrocarbon volatile organic carbons in air collected from above the soil surface and a comparison of the measured mineralisation rates with the mass flux of aromatic hydrocarbons.