3-DIMENSIONAL MODEL FOR SUBSURFACE TRANSPORT AND BIODEGRADATION

This paper describes and demonstrates a numerical model for subsurface solute transport with aerobic and sequential anaerobic biodegradation . The model can depict multiple constituents in a three-dimensional (3 D), anisotropic, heterogeneous domain. Hydrocarbon contaminants are si mulated as electron donors for microbial growth, and available electro n accepters (EAs) may be utilized simultaneously or in the following s equence: O-2, NO3-, Mn(IV), Fe(III), SO42-, and CO2. The model can acc ount for Mn(II), Fe(II), H2S, CH4, and a user-defined nitrogenous comp ound as products of biodegradation. Biodegradation of each hydrocarbon substrate follows Monod kinetics, modified to include the effects of EA and nutrient availability. Inhibition functions allow any EA to inh ibit the utilization of all other EAs that provide less energy to the microbes. Microbial biomass is conceptualized as scattered microcoloni es attached to the porous medium. The model assumes that interphase di ffusional limitations to microbial growth are negligible and no geomet rical parameters are assigned to the colonies. The behavior of the mod el was demonstrated using simple, hypothetical test cases. Transport o f a biodegradable hydrocarbon was compared to a nonbiodegradable trace r in a 3D, hypothetical domain. Anaerobic biodecay significantly reduc ed predicted contaminant concentrations and travel distance. Biodegrad ation of the total contaminant mass depended on EA availability and di d not follow first-order kinetics.