In subsurface environment, small-scale heterogeneities usually cause the reduction of the applicability of in situ remedial techniques. Biogeochemical heterogeneities and preferential groundwater flow paths create complex hydrogeologic conditions at most contaminated sites. A thorough understanding of the resulting three-dimensional distribution of contaminants is a necessity prior to determining a need for remediation. In this study, a gasoline spill site was selected to examine the effects of aquifer heterogeneities and geochemical variations on petroleum hydrocarbon biodegradation via different oxidation-reduction process. At this site, two multilevel sampling wells were installed to delineate the lateral (5 m) and vertical (0.5 m) distribution of contaminant concentrations and different biogeochemical parameters. Two 5-cm (I.D.) continuous soil cores [from 4 to 8 m below land surface (bls)] were collected within the gasoline plume to evaluate the distribution of the microbial population in soils. Results show that high microbial activities were observed in soil samples based on the following evidences: (1) high petroleum hydrocarbon degradation rate, and (2) high microbial biomass. Each soil section was used for chemical extraction, microbial enumeration, and grain size distribution. Results show that the soil sections with more permeable sediment materials corresponded with higher biomass (total anaerobes > 2 x 106cells/g) and significant contaminant degradation. However, those sections with less permeable sediments contained lower microbial population. Results indicate that the subsurface microorganisms were distributed unevenly in the aquifer, and some regions were devoid of microorganisms and biodegradation activities. Spatial distribution of microorganisms, soil materials, and biogeochemical characteristics in the subsurface soils control the extent and kinetics of contaminant biodegradation. Thus, using blended aquifer materials for measurement of in situ biodegradation rates may not achieve representative results.

The industrial solvent, trichloroethene (TCE), is among the most ubiquitous chlorinated compounds found in subsurface contamination. Operation of an avionics repair shop at a military base has resulted in past release of solvent chemicals including TCE and other chlorinated aliphatic hydrocarbons. The objectives of this study were to investigate the occurrence of natural remediation process and the feasibility of using natural remediation as the remedial option at this site. The following tasks have been performed: (1) site characterization to delineate the lateral and vertical extent of contaminants in the subsurface; (2) field investigation of natural remediation; and (3) efficiency of TCE removal through natural remediation in the field. Results indicate that TCE biodegradation occurred at this site, and natural remediation is a possible remedial alternative for TCE plume containment. Evidences for the TCE natural remediation included: (1) decreased TCE and other chlorinated compounds concentrations along the transport path; (2) production of the TCE degradation byproducts (including ethane); (3) decreased total organic carbon along the transport path, (4) deceased pH in the spill source area; (5) production of chloride ion and carbon dioxide. Experiences obtained from this study would be helpful in developing a site remedial protocol for other DNAPL sites.