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[1] We compare two models that have been used frequently for describing biodegradation during contaminant transport in groundwater. One is a ''simple'' model based upon macroscopic properties only, and the other is a ''biofilm'' model that accounts for contaminant diffusion and reaction in biofilms. Although the simple model has been used frequently, its mathematical formulation appears inadequate to describe the physics of the biodegradation process. Hence it is unclear when the simple model should be considered valid, and analyses predicated upon the simple model are called into question. We resolve this issue by arguing that the simple model should be considered valid when it is mathematically equivalent to the conceptually superior biofilm model. We demonstrate that the two models are exactly equivalent at the macroscopic scale when steady state conditions prevail. Under these conditions the equivalent macroscopic degradation rate coefficient k can be related to microscopic rate parameters that describe mass transfer across a boundary layer, diffusion within the biofilm, and reaction within the biofilm. Under transient (nonsteady state) conditions the two models are not strictly equivalent. However, the error between the two models is negligible in certain cases. In particular, when the rate-limiting step for biodegradation is either mass transfer across the boundary layer or diffusion within the biofilm, there is no distinguishable difference between the predictions of the two models. Thus this paper can be considered a theoretical foundation for use of the commonly employed simple biodegradation model as well as an elucidation of the conditions for its validity.

This paper presents a new joint inversion approach to shape-based inverse problems. Given two sets of data from distinct physical models, the main objective is to obtain a unified characterization of inclusions within the spatial domain of the physical properties to be reconstructed. Although our proposed method generally applies to many types of inverse problems, the main motivation here is to characterize subsurface contaminant source-zones by processing down gradient hydrological data and cross-gradient electrical resistance tomography (ERT) observations. Inspired by Newton's method for multi-objective optimization, we present an iterative inversion scheme in which descent steps are chosen to simultaneously reduce both data-model misfit terms. Such an approach, however, requires solving a non-smooth convex problem at every iteration, which is computationally expensive for a pixelbased inversion over the whole domain. Instead, we employ a parametric level set (PaLS) technique that substantially reduces the number of underlying parameters, making the inversion computationally tractable. The performance of the technique is examined and discussed through the reconstruction of source zone architectures that are representative of dense non-aqueous phase liquid (DNAPL) contaminant release in a statistically homogenous sandy aquifer. In these examples, the geometric configuration of the DNAPL mass is considered along with additional information about its spatial variability within the contaminated zone, such as the identification of low and high saturation regions. Comparison of the reconstructions with the true DNAPL architectures highlights the superior performance of the model-based technique and joint inversion scheme. *

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Column experiments were conducted to evaluate the effect of pore velocity on the extent of biodegradation of cis-dichloroethene (cis-DCE) during transport in porous media. Columns were filled with homogeneous glass beads and inoculated with a culture capable of complete dechlorination of tetrachloroethene to ethene. A constant concentration of cis-DCE was maintained in the columns' influent. Three different pore velocities were tested in duplicate, subjecting each column to a constant velocity. At high flow velocity, degradation of cis-DCE to ethene was nearly complete within the residence time of the columns. However, at medium and low flow velocities, incomplete dechlorination was observed. After 7 weeks, DNA was harvested from the columns to determine differences in the microbial populations. Results suggest that Dehalococcoides sp. were present in higher quantities in the high-velocity columns, consistent with the observed dechlorination. These results suggest that, at contaminated groundwater sites, heterogeneity of groundwater velocity may be one factor that contributes to heterogeneous distribution of biological activity.