Analytical equations are developed to model changes in porosity, specific surface area, and permeability caused by biomass accumulation in porous media. The proposed equations do not assume any specific pattern for microbial growth but instead are based on macroscopic estimates of average biomass concentrations. For porous media with a pore‐size distribution index value (λ) equal to 3, the macroscopic model predictions of porosity, specific surface area, and permeability changes are in exact agreement with biofilm‐model predictions. At other values of λ between 2 and 5, simulated porosity profiles are identical and relative specific surface area and permeability profiles show minor deviations. In comparison to biofilm‐based models, the macroscopic models are relatively simple to implement and are computationally more efficient. Simulations of biologically reactive flow in a one‐dimensional column show that the macroscopic and biofilm approach based transport codes predict almost identical porosity and permeability profiles. The macroscopic models are simple and useful tools for estimating changes in various porous media properties during bioremediation of contaminated aquifers.

Abstract. A direct method for transforming multiple solute transport equations, coupled by linear, series, and/or parallel first-order, irreversible reactions, into a series of simple transport equations having known solutions is developed. Using this method, previously published analytical solutions to single-species transport problems, in which the transported species reacts with first-order kinetics, can be used to derive analytical solutions to multispecies transport systems with parallel, serial, and combined reaction networks. This new method overcomes many of the limitations that were implicit in previously published methods. In particular, the number of species that can be described is unlimited, and the reaction stoichiometry does not have to be unimolar. To illustrate the method, an analytical solution is derived for a five-species serial-parallel reactive transport system. The analytical solution obtained for this problem is compared with a numerical solution obtained with a previously developed code. This analytical method is applicable to the verification of new numerical codes. IntroductionTo test and verify numerical descriptions of reactive transport in porous media, a variety of analytical solutions must be developed. However, the number of available analytical solutions, and the particular problems described by these solutions, is limited because of difficulties in solving such problems. All the above solutions are based on the unimolar assumption; that is, the stoichiometry of the reaction is such that 1 mol of product is produced from the reaction of 1 mol of reactant. However, this assumption is limiting if the species react according to serial-parallel reaction networks. In such instances a parent species may react to produce more than one daughter. Further, for such reactions the stoichiometric yields may be less than or more than unity.

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and Technology. This report was prepared to document the results of the modeling effort and facilitate discussion of characterization and remediation options for the carbon tetrachloride (CT) plume among the ITRD participants.As a first step toward implementation of innovative technologies for remediation of the CT plume underlying the 200 West Area, modeling was performed to provide an indication of the potential impact of the CT source on the compliance boundary approximately 5000 m away. The primary objective of the modeling was to bracket the amount of CT source that will most likely result in compliance/non-compliance at the boundary and the relative influence of the various model input parameters.The modeling was based on the assumption that about 750,000 kg of CT were discharged to the soil in the 200 West Area. Previous work has shown that, of this 750,000 kg, about 65% cannot be accounted for (after totaling atmospheric losses [21%]; unsaturated zone inventory in soil gas, soil moisture, and adsorbed phases [12%]; and the dissolved phase in the aquifer [2%]). Therefore, model simulations were performed using 65%, 30%, 10%, and 1% of the 750,000 kg as possible source amounts of CT that could reach the groundwater (approximately 1 to 2% of the original CT inventory now exists in the distal plume based on averaged CT groundwater measurements). The modeling simulations conducted for this study examine the migration of CT from the source area to the compliance boundary. The simulations did not examine migration of the existing distal plume and did not attempt to match the historical disposal and migration of CT (i.e., did not attempt to reproduce the current CT plume distribution).Other model input parameters including the groundwater Darcy velocity; inlet concentration (the concentration of CT leaving the source area); porosity; soil/water equilibrium partition coefficient (K d ); abiotic degradation rate (K a ); dispersivity; and stream tube cross-sectional area were also varied to assess sensitivity of the results to each of the parameters.The CT transport simulation was conservatively modeled as a stream tube 1000 m wide by 5000 m long in one-dimensional flow (i.e., no vertical or transverse dispersivity or convection). Regional flow-modeling results using the Hanford Site-Wide Groundwater Model (SGM) flow grid provided groundwater velocity estimates for the simulation. The basic assumptions used in developing the model were as follows:• The major source of contamination is within a 500m x 500m box.• The contaminant plume was in equilibrium with the source immediately before pumpand-treat efforts.• Processes considered in the model are one-dimensional convective-dispersive transport of reactive solutes subject to adsorption and first-order abiotic degradation (hydrolysis).• Volatilization and first-order natural biodegradation are negligible.ivThe one-dimensional van Genuchten model simulated convective-dispersive transport of CT through a homogenous medium along the centerline of the contaminant plume f...

Kinetics were determined for methanogenic activity and chlorinated ethylene dehalogenation by a methanol-enriched, anaerobic sediment consortium. The culture reductively dechlorinated perchloroethylene (PCE) to trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), vinylchloride (VC), and ethylene and ethane. The absence : of methanol or the addition of 2-bromoethanesulfonic. acid in the presence of methanol suppressed both methanogenic activity and dechlorination. In contrast, acetate production continued in the presence of 2-bromoethanesulfonic acid. These results suggest that dechlorination was strongly linked to methane formation and not to acetate production. A kinetic model, developed to describe both methanogenesis and dechlorination, successfully predicted experimentally measured concentrations of biomass, methane, substrate, and chlorinated ethylenes. The average maximum specific dehalogenation rates for PCE, TCE, 1,1-DCE, and VC were 0.9 +/- 0.6, 0.4 +/- 0.1, 12 +/- 0.1, and 2.5 +/- 1.7 mumol contaminant/ g. DW/day, respectively. This pattern for dechlorination rates is distinctly different than that reported for transition metal cofactors, where rates drop by approximately one order of magnitude as each successive chlorine is removed. The experimental results and kinetic analysis suggest that it will be impractical to targeting methanol consuming methanogenic organisms for in situ ground-water restoration. (c) 1995 John Wiley & Sons, Inc.