Modeling and Optimization of GEDIT for In Situ Contaminant Destruction in the Vadose Zone

Abstract : Gaseous electron donor injection technology (GEDIT) involves injection of gaseous electron donors into the soil with the purpose of promoting anaerobic biodegradation of perchlorate. This report describes the application of TMVOC, a vadose zone simulation code developed at Lawrence Berkeley National Laboratory (LBNL), to perform an alternatives and optimization analysis of GEDIT for a demonstration site in California. TMVOC was calibrated to data from a demonstration conducted at the Inactive Rancho Cordova Test Site (IRCTS) in California. A series of optimization runs were conducted to evaluate the effect of horizontal and vertical injection well spacing, gas composition, and gas flow rate on hydrogen, propane, and oxygen transport through the vadose zone. A unifying relationship between a normalized hydrogen concentration and the pneumatic residence time was observed. This empirical relationship was used to explore the effects of design variables on GEDIT cost. For this site, injection hydrogen concentrations of about 10% were found to be more cost effective than lower concentrations and a horizontal well spacing of 24 meters (80 feet) was theoretically sufficient to meet minimum concentration requirements for hydrogen to ensure perchlorate degradation. Unit GEDIT costs were determined to be less than $20/m3 ($15/yd3) under certain conditions. The first-order rate constant for hydrogen consumption strongly affected the fate and transport of hydrogen and thus the economics. The rate constant is anticipated to be site-specific and thus costs for a given site may not be translatable to other sites. The approach outlined in this report provides a practical approach for determining engineering design parameters and economics at any site.