Walt W. McNab

Experimental Study of Cement - Sandstone/Shale - Brine - CO 2 Interactions

BackgroundReactive-transport simulation is a tool that is being used to estimate long-term trapping of CO2, and wellbore and cap rock integrity for geologic CO2 storage. We reacted end member components of a heterolithic sandstone and shale unit that forms the upper section of the In Salah Gas Project carbon storage reservoir in Krechba, Algeria with supercritical CO2, brine, and with/without cement at reservoir conditions to develop experimentally constrained geochemical models for use in reactive transport simulations.ResultsWe observe marked changes in solution composition when CO2 reacted with cement, sandstone, and shale components at reservoir conditions. The geochemical model for the reaction of sandstone and shale with CO2 and brine is a simple one in which albite, chlorite, illite and carbonate minerals partially dissolve and boehmite, smectite, and amorphous silica precipitate. The geochemical model for the wellbore environment is also fairly simple, in which alkaline cements and rock react with CO2-rich brines to form an Fe containing calcite, amorphous silica, smectite and boehmite or amorphous Al(OH)3.ConclusionsOur research shows that relatively simple geochemical models can describe the dominant reactions that are likely to occur when CO2 is stored in deep saline aquifers sealed with overlying shale cap rocks, as well as the dominant reactions for cement carbonation at the wellbore interface.

Palladium-catalyzed reductive dehalogenation of dissolved chlorinated aliphatics using electrolytically-generated hydrogen

Recent studies have shown that reductive dehalogenation of chlorinated hydrocarbons by hydrogen occurs rapidly in the presence of a palladium catalyst. The speed and completeness of these reactions may offer advantages for groundwater remediation. A practical design challenge arises with the need to expeditiously saturate the aqueous phase with hydrogen. To address this, a two-stage treatment column (hydrogen generator and catalytic reactor) has been developed. The first stage consists of an undivided electrolyzer cell which generates hydrogen by electrolyzing the influent water. The second stage contains a catalyst bed of palladium metal supported on alumina beads. This reactor has been tested with groundwater containing various chlorinated aliphatics. Using a flow rate of 300 ml/min and a current of 4 amps under a potential of 8 volts, removal efficiencies greater than 95% were achieved for PCE, TCE, 1,1-DCE, and carbon tetrachloride with residence times on the order of 2 minutes. Chloroform and 1,2-DCA appear less susceptible to dehalogenation by this process. These results imply that dissolved oxygen present in solution does not completely inhibit reduction of the chlorinated hydrocarbons on the catalyst.

Degradation of Chlorinated Hydrocarbons and Groundwater Geochemistry: A Field Study

Published laboratory studies suggest that certain chlorinated hydrocarbons are subject to chemical degradation in groundwater through abiotic and biologically-mediated processes. However, relatively few field investigations have been conducted on the degradation of these compounds. We examine the issue of degradation with regard to several chlorinated aliphatics dissolved in groundwater at the Lawrence Livermore National Laboratory in California. The highly oxidized state of the local groundwater, as indicated by geochemical observations, suggests that reductive degradation reactions of chlorinated ethenes and ethanes are not thermodynamically or microbially favorable. In contrast, it is known that certain chlorinated compounds, such as 1,1,1-TCA, may degrade through redox-independent elimination or hydrolysis reactions. Indeed, statistical analyses and numerical modeling have provided evidence for the nonredox-driven transformation of 1,1,1-TCA to 1,1-DCE and to possibly acetic acid at the site. Similar analyses conducted for PCE and TCE failed to indicate any evidence for reductive dehalogenation. 21 refs., 7 figs.

Simulation of Nitrate Biogeochemistry and Reactive Transport in a California Groundwater Basin

Nitrate is the number one drinking water contaminant in the United States. It is pervasive in surface and groundwater systems, and its principal anthropogenic sources have increased dramatically in the last 50 years. In California alone, one third of the public drinking-water wells has been lost since 1988 and nitrate contamination is the most common reason for abandonment. Effective nitrate management in groundwater is complicated by uncertainties related to multiple point and non-point sources, hydrogeologic complexity, geochemical reactivity, and quantification of denitrification processes. In this paper, we review an integrated experimental and simulation-based framework being developed to study the fate of nitrate in a 25 km-long groundwater subbasin south of San Jose, California, a historically agricultural area now undergoing rapid urbanization with increasing demands for groundwater. The modeling approach is driven by a need to integrate new and archival data that support the hypothesis that nitrate fate and transport at the basin scale is intricately related to hydrostratigraphic complexity, variability of flow paths and groundwater residence times, microbial activity, and multiple geochemical reaction mechanisms. This study synthesizes these disparate and multi-scale data into a three-dimensional and highly resolved reactive transport modeling framework.

A Monte Carlo simulation method for assessing biotransformation effects on groundwater fuel hydrocarbon plume lengths

Abstract Biotransformation of dissolved groundwater hydrocarbon plumes emanating from leaking underground fuel tanks should, in principle, result in plume length stabilization over relatively short distances, thus diminishing the environmental risk. However, because the behavior of hydrocarbon plumes is usually poorly constrained at most leaking underground fuel tank sites in terms of release history, groundwater velocity, dispersion, as well as the biotransformation rate, demonstrating such a limitation in plume length is problematic. Biotransformation signatures in the aquifer geochemistry, most notably elevated bicarbonate, may offer a means of constraining the relationship between plume length and the mean biotransformation rate. In this study, modeled plume lengths and spatial bicarbonate differences among a population of synthetic hydrocarbon plumes, generated through Monte Carlo simulation of an analytical solute transport model, are compared to field observations from six underground storage tank (UST) sites at military bases in California. Simulation results indicate that the relationship between plume length and the distribution of bicarbonate is best explained by biotransformation rates that are consistent with ranges commonly reported in the literature. This finding suggests that bicarbonate can indeed provide an independent means for evaluating limitations in hydrocarbon plume length resulting from biotransformation.

Evaluating Chlorinated Hydrocarbon Plume Behavior Using Historical Case Population Analyses

A nationwide survey of chlorinated volatile organic compound (CVOC) plumes was conducted across a spectrum of sites from diverse hydrogeologic environments and contaminant release scenarios. The goal was to evaluate significant trends in the data that relate plume behavior to site variables (e.g., source strength, mean groundwater velocity, reductive dehalogenation regime) through correlation and population analyses. Data from 65 sites (government facilities, dry cleaners, landfills) were analyzed, yielding 247 individual CVOC plumes by compound. Data analyses revealed several trends, notably correlations between plume length and maximum observed concentration (presumably reflecting the source term) and mean groundwater velocities. Reductive dehalogenation, indicated by daughter products and groundwater geochemistry, appears to exert a relatively subtle effect on plume length, apparent only after the contributions of source strength and groundwater velocity are factored out. CVOC properties (K oc , Henrys Law constant) exert significant effects on variability in maximum observed concentrations between sites but hold little influence on plume length. Probabilistic plume modeling, entailing Monte Carlo simulation of an analytical solution for average plume behavior with parameter distributions derived from site data, was used to produce a synthetic plume set for comparison with field data. Modeling results exhibited good agreement with field data in terms of parameter sensitivity.

Ascertaining the Effect of Reductive Dehalogenation on Chlorinated Hydrocarbon Plume Lengths in Groundwater: Analyses of Multisite Data

Chlorinated hydrocarbon groundwater plume data from a multisite study were evaluated by a variety of statistical techniques (correlation, analysis of covariance, principal components) to quantify the effects of biotransformations (reductive dehalogenation) on plume length. After accounting for the effects of groundwater velocity, source strength, and biases in the data collection process, chlorinated hydrocarbon plume lengths at sites where reductive dehalogenation was occurring were found to be significantly shorter on average, by roughly a factor of two, than those where it was not. Moreover, principal component analyses indicated significant differences in the behavior of chlorinated hydrocarbon plumes between sites with and without evidence of reductive dehalogenation, respectively. The advantage in examining plume behavior from this population-oriented perspective is that overall trends in plume behavior can be evaluated despite site-specific influences such as heterogeneities and unique release histories. Ultimately, it is these average trends that would be of the most interest to policymakers because they represent the ranges of conditions that will be encountered. This is especially important in the case of chlorinated hydrocarbons because they will biotransform at rates significant for appreciable natural attenuation only in certain instances.

Simulation of reactive geochemical transport in groundwater using a semi-analytical screening model

Abstract A reactive geochemical transport model, based on a semi-analytical solution to the advective-dispersive transport equation in two dimensions, is developed as a screening tool for evaluating the impact of reactive contaminants on aquifer hydrogeochemistry. Because the model utilizes an analytical solution to the transport equation, it is less computationally intensive than models based on numerical transport schemes, is faster, and it is not subject to numerical dispersion effects. Although the assumptions used to construct the model preclude consideration of reactions between the aqueous and solid phases, thermodynamic mineral saturation indices are calculated to provide qualitative insight into such reactions. Test problems involving acid mine drainage and hydrocarbon biodegradation signatures illustrate the utility of the model in simulating essential hydrogeochemical phenomena.