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In this paper, the details of RT3D, a general purpose, multispecies, reactive transport code, are presented. The code uses MODFLOW to simulate flow and several MT3D sub‐programs to simulate advection and dispersion. A set of reaction modules were developed and incorporated into RT3D to simulate various types of multispecies reactive transport. This new computer model can be used for analyzing different types of subsurface contaminant reactions, microbial metabolisms, and microbial transport kinetics. Details of the model and numerical solution procedure are presented. The numerical formulation of the code is general enough to allow description of any type of reaction with any number of mobile/immobile species. Several example problems are presented to test the performance of the code, and to illustrate its features. The presented numerical model is shown to be a useful tool for analyzing different types of subsurface bioremediation systems. Prediction based on this model can be used for screening remediation alternatives including natural attenuation and/or for forecasting contaminant exposure levels and environmental risks at sensitive, downgradient receptors.

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a b s t r a c tIn this paper we describe potential impacts to groundwater quality due to CO 2 and brine leakage, discuss an approach to calculate thresholds under which "no impact" to groundwater occurs, describe the time scale for impact on groundwater, and discuss the probability of detecting a groundwater plume should leakage occur. To facilitate this, multi-phase flow and reactive transport simulations and reduced-order models were developed for two classes of aquifers, considering uncertainty in leakage source terms and aquifer hydrogeology. We targeted an unconfined fractured carbonate aquifer based on the Edwards Aquifer in Texas and a confined alluvium aquifer based on the High Plains Aquifer in Kansas, which share characteristics typical of many drinking water aquifers in the United States. The hypothetical leakage scenarios centered on the notion that wellbores are the most likely conduits for brine and CO 2 leaks. Leakage uncertainty was based on hypothetical injection of CO 2 for 50 years at a rate of 5 million tons per year into a depleted oil/gas reservoir with high permeability and, one or more wells provided leakage pathways from the storage reservoir to the overlying aquifer. This scenario corresponds to a storage site with historical oil/gas production and some poorly completed legacy wells that went undetected through site evaluation, operations, and post-closure.For the aquifer systems and leakage scenarios studied here, CO 2 and brine leakage are likely to drive pH below and increase total dissolved solids (TDS) above the "no-impact thresholds"; and the subsequent plumes, although small, are likely to persist for long periods of time in the absence of remediation. In these scenarios, however, risk to human health may not be significant for two reasons. First, our simulated plume volumes are much smaller than the average inter-well spacing (1-2.6 wells/km 2 ) for these representative aquifers, so the impacted groundwater would be unlikely to be pumped for drinking water. Second, even within the impacted plume volumes little water exceeds the primary maximum contamination levels. These observations point to: BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 154 S.A. Carroll et al. / International Journal of Greenhouse Gas Control 29 (2014) 153-168• The potential utility of uncertainty quantification methods to evaluate the risk of leakage and inform monitoring and corrective action plans of a potential site for long-term CO 2 storage by capturing storage reservoir, leakage pathway, and aquifer heterogeneity. • The importance of establishing baseline groundwater chemistry that captures the pre-injection variability of underground sources of drinking water (USDW) above the reservoir because the EPA has adopted a "no net degradation" policy toward the protection of groundwater resources. • The need to test and develop spatially diverse monitoring techniques capable of detecting leakage early to employ effective mitigation strategies, and more importantly to add confidence to as...

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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.

A high-fidelity model of kinetic and equilibrium sorption and diffusion is developed and exercised. The gas-diffusion model is coupled with a triple-sorption mechanism: Henry's law absorption, Langmuir adsorption, and pooling or clustering of molecules at higher partial pressures. Sorption experiments are conducted and span a range of relative humidities (0-95 %) and temperatures (30-60 °C). Kinetic and equilibrium sorption properties and effective diffusivity are determined by minimizing the absolute difference between measured and modeled uptakes. Uncertainty quantification and sensitivity analysis methods are described and exercised herein to demonstrate the capability of this modeling approach. Water uptake in silica-filled and unfilled poly(dimethylsiloxane) networks is investigated; however, the model is versatile enough to be used with a wide range of materials and vapors.