We present results from field studies at two central California dairies that demonstrate the prevalence of saturated-zone denitrification in shallow groundwater with 3H/ 3He apparent ages of < 35 years. Concentrated animal feeding operations are suspected to be major contributors of nitrate to groundwater, but saturated zone denitrification could mitigate their impact to groundwater quality. Denitrification is identified and quantified using N and O stable isotope compositions of nitrate coupled with measurements of excess N2 and residual NO3(-) concentrations. Nitrate in dairy groundwater from this study has delta15N values (4.3-61 per thousand), and delta18O values (-4.5-24.5 per thousand) that plot with delta18O/delta15N slopes of 0.47-0.66, consistent with denitrification. Noble gas mass spectrometry is used to quantify recharge temperature and excess air content. Dissolved N2 is found at concentrations well above those expected for equilibrium with air or incorporation of excess air, consistent with reduction of nitrate to N2. Fractionation factors for nitrogen and oxygen isotopes in nitrate appear to be highly variable at a dairy site where denitrification is found in a laterally extensive anoxic zone 5 m below the water table, and at a second dairy site where denitrification occurs near the water table and is strongly influenced by localized lagoon seepage.

A groundwater treatment technology based on catalytic reductive dehalogenation has been developed to efficiently destroy chlorinated hydrocarbons in situ using a reactive well approach. The treatment process utilizes dissolved H2 as an electron donor, in the presence of a commercial palladium-on-alumina catalyst, to rapidly reduce common chlorinated aliphatics such as trichloroethylene and tetrachloroethylene into nonchlorinated hydrocarbons such as ethane. Rapid reaction rates permit the deployment of a treatment unit within a dual-screened well bore, allowing contaminated groundwater to be drawn from one water-bearing zone, treated within the well bore, and discharged to an adjacent zone with only one pass through the system. A demonstration groundwater treatment system based on this concept was evaluated in a chlorinated hydrocarbon contaminated aquifer at a major Superfund site. The system rapidly destroyed a variety of common contaminants such as TCE and PCE and maintained its performance for a test period of 1 year. Operation of the treatment system was optimized to maintain catalyst activity and to prevent formation of intermediate compounds.

The Mount Simon sandstone and Eau Claire shale formations are target storage and cap rock formations for the Illinois Basin-Decatur Geologic Carbon Sequestration Project. We reacted rock samples with brine and supercritical CO(2) at 51 °C and 19.5 MPa to access the reactivity of these formations at storage conditions and to address the applicability of using published kinetic and thermodynamic constants to predict geochemical alteration that may occur during storage by quantifying parameter uncertainty against experimental data. Incongruent dissolution of iron-rich clays and formation of secondary clays and amorphous silica will dominate geochemical alterations at this CO(2) storage site in CO(2)-rich brines. The surrogate iron-rich clay in the model required significant adjustments to its thermodynamic constants and inclusion of incongruent reaction terms to capture the change in solution composition under acid CO(2) conditions. This result emphasizes the need for experiments that constrain the conceptual geochemical model, calibrate mean parameter values, and quantify parameter uncertainty in reactive-transport simulations that will be used to estimate long-term CO(2) trapping mechanisms and changes in porosity and permeability.

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Almost 4 million metric tons of CO 2 were injected at the In Salah CO 2 storage site between 2004 and 2011. Storage integrity at the site is provided by a 950-m-thick caprock that sits above the injection interval. This caprock consists of a number of low-permeability units that work together to limit vertical fluid migration. These are grouped into main caprock units, providing the primary seal, and lower caprock units, providing an additional buffer and some secondary storage capacity. Monitoring observations at the site indirectly suggest that pressure, and probably CO 2 , have migrated upward into the lower portion of the caprock. Although there are no indications that the overall storage integrity has been compromised, these observations raise interesting questions about the geomechanical behavior of the system. Several hypotheses have been put forward to explain the measured pressure, seismic, and surface deformation behavior. These include fault leakage, flow through preexisting fractures, and the possibility that injection pressures induced hydraulic fractures. This work evaluates these hypotheses in light of the available data. We suggest that the simplest and most likely explanation for the observations is that a portion of the lower caprock was hydrofractured, although interaction with preexisting fractures may have played a significant role. There are no indications, however, that the overall storage complex has been compromised, and several independent data sets demonstrate that CO 2 is contained in the confinement zone.carbon sequestration | geomechanics