Numerical investigations of in situ bioremediation in variably saturated aquifers under nonisothermal conditions.

Abstract

<italic>In situ</italic> biodegradation is an increasingly popular choice for remediation of subsurface environments contaminated by volatile organic pollutants. Currently, few biodegradation models incorporate non-isothermal processes. Experimental and field observations suggest, however, that temperature and water content variations can have a significant influence on <italic> in situ</italic> biodegradation. This research effort focuses on the numerical investigation of <italic>in situ</italic> biodegradation under non-isothermal conditions by examining (1) advection-induced phase change during <italic> bioventing</italic> and (2) natural and artificial temperature variation during <italic>intrinsic bioattenuation.</italic> The first component of this research includes the development and application of a one-dimensional finite element water balance and energy transport simulator. Findings indicate that advection-induced phase change can significantly increase or decrease biological activity during condensation and evaporation, respectively. The study also demonstrates the importance of including liquid water flow and appropriately representing effective thermal conductivity in bioventing simulations. The development and application of a two-dimensional subsurface intrinsic bioattenuation simulator constitutes the second component of the dissertation research. The finite element simulator solves a water flow equation and tracks multiple chemical components in the solid, gaseous, aqueous, and NAPL phases by solving a biologically reactive transport equation for each component. Three separate but related field scale phenomena are investigated: (1) seasonal temperature variation, (2) microbial heat generation due to respiration, and (3) bioattenuation enhancement by artificial temperature elevation. The hypothetical site simulations are based on the JP-4 fuel-contaminated KC-135 crash site at Wurtsmith Air Force Base. Seasonal temperature variation is shown to affect plume size and biodegradation mass removal both seasonally and over long simulation time periods. Microbial heat generation due to respiration is thermodynamically possible, but is limited as a bioattenuation indicator in nutrient-poor environments. Artificial warming using resistance heating or using the application of heated water can improve overall contaminant bioremoval rates. However, poor design of these warming technologies can greatly compromise their operational efficiency and may exacerbate contaminant mobilization.