Modeling biotic and abiotic decay of trichloroethylene (TCE) using the Reactive Multi-Species Transport in 3-Dimensional Groundwater Aquifers (RT3D) code

Removal of dense non aqueous phase liquids, DNAPLS, such as trichloroethylene, TCE, is vital to improving the health of groundwater systems. TCE contamination of groundwater systems is of significant concern and its removal a significant challenge. One of the main causes of delays in cleanup of a TCE contaminated site results from back diffusion. Back diffusion occurs when the TCE mass in the high permeability zones, HPZ, is removed and the TCE trapped in the low permeability zones, LPZ, of the heterogeneous aquifer diffuses out due to concentration gradient reversal and re-contaminates the site. Several studies have indicated that TCE can be transformed into less harmful products of interest via biotic and abiotic processes. These processes are slow but may potentially have an impact since TCE can spend long time periods in LPZs as the mass transport is mainly by diffusion. The biotic process uses an organic solute such as lactate as an electron donor and the halogenated compounds as electron acceptors to biologically transform TCE into dichloroethylene (DCE), vinyl chloride (VC), and ethene. Additionally, the abiotic process transforms TCE into acetylene using reduced iron species as an electron donor. Furthermore, oxidized iron produced from the abiotic process can be converted back into the reduced form by iron reducing bacteria using lactate as a donor. These feedbacks between the biotic and abiotic processes can thus extend the transformation of TCE into acetylene. Reactive transport modeling is a useful tool to study these feedbacks. This thesis successfully develops a clear quantitative model of these decay processes using the Reactive Multi-Species Transport in 3-Dimensional Groundwater Aquifers (RT3D) code. RT3D is part of the MODFLOW family of codes that is commonly used in engineering practice. In addition, this thesis explores the mitigation of the effects of back diffusion by implementing these decay processes in a 2-Dimensional flow cell model. The flow cell is built using Aquaveo Groundwater Modeling System, GMS, while the 2-D flow simulation is performed using USGS MODFLOW, United State Geological Survey Modular Groundwater Flow Model, and the transport simulation is done using RT3D. Lastly, this thesis explains the procedures used in implementing the RT3D user-defined dynamic link library option, which is necessary when user-defined reactions are required