GEOMICROBIOLOGY AND SOIL MINERALOGY IN ASSESSING THE NATURAL ATTENUATION OF PERCHLOROETHYLENE (PCE) AT A FORMER AIR BASE

Extensive chlorinated solvent use over the past decades, have resulted in soil and groundwater contamination at the former Kelly Air Force Base in San Antonio, TX. Perchloroethylene (PCE), a suspected carcinogen, has contaminated a shallow aquifer and has moved off base into neighboring communities. The Air Force’s final remediation plan for PCE in groundwater is to apply: 1) pump and treat, 2) permeable reaction barriers, and 3) monitored natural attenuation (MNA). Out of the three, MNA is considered the most passive cleanup strategy. The goal of this research was to evaluate natural attenuation of PCE utilizing soils obtained from the saturated zone of the shallow aquifer. To investigate PCE degradation, an understanding of environmental factors such as soil mineralogy, hydrologic conditions, and the geomicrobiology are needed. These environmental factors will provide insight into sorption kinetics, soil characteristics, nutrient availability, PCE degraders, and whether the presence of an electron donor will impact MNA. Both batch experiments and column experiments will be used to evaluate environmental factors while soil characteristics such as texture and clay mineralogy will be determined using sieving and X-Ray Diffraction. Batch experiments were preformed using 20mL vials with 1:1 soil to water ratio spiked with PCE at different concentrations while the degradation was done using 75mg/L PCE. Vials were shaken, centrifuged, and subsampled under headspace utilizing a gas chromatograph with a flame ionization detector. A column was packed with contaminated Kelly soil and saturated conditions were established. A conservative tracer (CaBr2) was run. Currently a column is exposed to a PCE solution that mimics concentration levels at the former base (~40mg/L). Effluent samples are collected and analyzed for PCE concentration as well as the PCE-degrader counts. The initial PCE-degraders are robust and are an order of one magnitude greater than the heterotrophic populations. The sorption study obtained a Kd=6.97 indicating high sorption of PCE to the soil probably caused by soil mineralogy. The conservative breakthrough curve indicated that transport was non-ideal due to heterogeneity of the soil and sorption. The high sorption (~60%) of PCE to the soil will play a significant role in MNA at the former base.