Modeling contaminant transport and biodegradation in groundwater

The field of groundwater transport modeling has grown tremendously over the past ten years. This is mostly due to the need for quantitative estimates of contaminant mass transport in the subsurface. Transport processes in groundwater include advection, dispersion, adsorption, decay, chemical and biological reactions. This chapter deals with modeling biodegradation, the breakdown of organic contaminants in the subsurface by microorganisms. The main focus of the chapter is on how to incorporate microbially mediated reactions into groundwater contaminant transport models. At the field scale, the transport characteristics of an aquifer limit the delivery of the required nutrients and dissolved oxygen to the microbial population, and therefore it is necessary to incorporate these limitations into a model for biodegradation in the subsurface. The emphasis in this chapter is on BIOPLUME II, a biodegradation model developed recently. The BIOPLUME II model utilizes a stoichiometric instantaneous reaction between oxygen and the organics to simulate biodegradation. It is assumed that microbial kinetics occur at a much faster time scale than groundwater flow, and that the biodegradation of the organic contaminants is limited by the mass flux of dissolved oxygen into the contaminated plume. The biodegradation input data requirements for the model are simplified and include a quantitative estimate of dissolved oxygen sources in an aquifer. The BIOPLUME II model was developed by modifying an existing two-dimensional transport model from the USGS. The main modifications included simulating the transport of two substrates in the groundwater: oxygen and the organics. This chapter covers briefly the numerical methods that are utilized in the BIOPLUME II model as well as the input parameters required for the model. Several case studies of modeling biodegradation at field sites are included in addition to a discussion of other models that have been developed to simulate the biodegradation process. The chapter also includes a summary of some of the basic principles that govern the biodegradation of organics in the subsurface and the application of bioremediation as a technology for groundwater clean-up efforts .