ENHANCED BIOREMEDIATION TREATMENT TREATABILITY STUDY AT THE MEMPHIS DEPOT, MEMPHIS, TENNESSEE: COMPARISON OF TWO ELECTRON DONORS IN A FLUVIAL AQUIFER

The Memphis Depot is a former military supply base located in the southwestern area of Memphis, Tennessee. The Depot received, stored, and warehoused military and civilian goods from inception in 1942 until closure under the Base Realignment and Closure (BRAC) program in 1997. Hazardous materials were released into the relatively highly oxygenated fluvial aquifer that underlies the Depot. The contaminants present today consist mainly of chlorinated volatile organic compounds (CVOCs), principally tetrachloroethene (PCE) and trichloroethene (TCE), but also carbon tetrachloride and chloroform. An element of the selected remedy in the site 2001 Record of Decision (ROD) is enhanced bioremediation of CVOCs in the most contaminated parts of the plume. As part of the Remedial Design for this project, an enhanced bioremediation treatment (EBT) treatability study was conducted using two types of electron donors to spur the natural attenuation process ahead through VOC biodegradation. The two electron donors are 60 percent sodium lactate and soybean oil mixed with the surfactant lecithin (more commonly known as vegetable oil). The results will be compared against each other and the fluid that achieves a favorable reduction will be used in a full-scale implementation. The fluids were injected at two separate sites using similar injection methods and to similar depths. The depth to water in the fluvial aquifer varies from 90 to 100 feet below ground surface. The fluvial aquifer is composed of a silty clay to sand and gravel matrix. Contaminant levels within the plume in the aquifer are as high as 298 micrograms per liter (ug/L) of PCE, 255 ug/L of TCE, 120 ug/L of carbon tetrachloride, and 130 ug/L of chloroform. The principal objective of the study is to determine whether the electron donors can create a more anaerobic environment, thereby spurring microbes to begin the reductive dechlorination process and reduce the CVOC contaminants to levels established for the Depot. Preliminary performance monitoring indicate that reduction of CVOC levels are showing positive results. Performance monitoring events will continue until early 2003.