Development of a Numerical Simulator for DNAPL Weathering

Abstract

To investigate the effects of bioremediation on DNAPL source zones, we carried out a series of experiments in a two-dimensional tank filled with sand. A microbial assemblage originating from a field site contaminated with chlorinated ethene was used for inoculation without enrichment. Injection of 250 ml tetrachloroethene (PCE) into the tank yielded a residual zone of PCE with a pool at the bottom. After this injection, the tank was continuously flushed with anaerobic water containing sufficient electron donor and various nutrients. Chlorinated ethenes analysis, microbial groups counting, and the visual observation of the dyed PCE showed that PCE was dechlorinated in the source zone. Bio-enhanced dissolution is believed to have occurred as the total chlorinated ethene concentration was about four times higher than the solubility limit of PCE. Moreover, an increased PCE solubility limit in the source zone and mobilization of residual PCE were observed. Dechlorination of cDCE to vinyl chloride (VC) occurred only when PCE concentrations were low (<0.1 mM). Similarly, transformation of VC to ethene occurred only after cDCE concentrations became low. After one year of experiment, approximately 135 ml of PCE had left the tank mainly as cDCE (73%). PCE left in the tank was 90 ml and was only present in the pool. This amounts to a total PCE of 225 ml, which is 10% less than the volume injected into the tank. Spatial moment analysis were performed to investigate the general plume behavior. This was compared to a model simulation with a modified version of RT3D that accounted for DNAPL dissolution and biodegradation.