Capturing carbon dioxide (CO(2)) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO(2) concentrations in the atmosphere. Sequestering CO(2) underground has its own set of environmental risks, including the potential migration of CO(2) out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO(2) was initiated to investigate potential groundwater impacts. Dissolution of CO(2) in the groundwater resulted in a sustained and easily detected decrease of ~3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.

TitleMonitoring CO2 intrusion and associated geochemical transformations in a shallow groundwater system using complex electrical methods Specifically, resistivity initially decreases due to increase of bicarbonate and dissolved species.As pH continues to decrease, the resistivity rebounds toward initial conditions due to the transition of bicarbonate into non-dissociated carbonic acid, which reduces the total concentration of dissociated species and thus the water conductivity. An electrical phase decrease is also observed, which is interpreted to be driven by the decrease of surface charge density as well as potential mineral dissolution and ion exchange. Both laboratory and field experiments demonstrate the potential of field complex resistivity method for remotely monitoring changes in groundwater quality due to CO 2 leakage.3

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13The dissolution of CO 2 in water leads to a pH decrease and carbonate content increase in degassing by de-pressurization is also explored using simple geochemical models, and shows that 30 2 the sequestration of metals by resorption and re-precipitation upon CO 2 exsolution is quite 31 plausible and may warrant further attention.

14One of the risks that CO 2 geological sequestration imposes on the environment is the 15 impact of potential CO 2 /brine leakage on shallow groundwater. The reliability of reactive 16 transport models predicting the response of groundwater to CO 2 leakage depends on a 17 thorough understanding of the relevant chemical processes and key parameters affecting 18 dissolved CO 2 transport and reaction. Such understanding can be provided by targeted 19 field tests integrated with reactive transport modeling. A controlled-release field 20 experiment was conducted in Mississippi to study the CO 2 -induced geochemical changes 21 in a shallow sandy aquifer at about 50m depth. The field test involved a dipole system in 22 which the groundwater was pumped from one well, saturated with CO 2 at the pressure 23 corresponding to the hydraulic pressure of the aquifer, and then re-injected into the same 24 aquifer using a second well. Groundwater samples were collected for chemical analyses 25 from four monitoring wells before, during and after the dissolved CO 2 was injected. In 26 2 this paper, we present reactive transport models used to interpret the observed changes in 27 metal concentrations in these groundwater samples. A reasonable agreement between 28 simulated and measured concentrations indicates that the chemical response in the aquifer 29 can be interpreted using a conceptual model that encompasses two main features: (a) a 30 fast-reacting but limited pool of reactive minerals that responds quickly to changes in pH 31and causes a pulse-like concentration change, and (b) a slow-reacting but essentially 32 unlimited mineral pool that yields rising metal concentrations upon decreased 33 groundwater velocities after pumping and injection stopped. During the injection, calcite 34 dissolution and Ca-driven cation exchange reactions contribute to a sharp pulse in 35 concentrations of Ca, Ba, Mg, Mn, K, Li, Na and Sr, whereas desorption reactions control 36 a similar increase in Fe concentrations. After the injection and pumping stops and the 37 groundwater flow rate decreases, the dissolution of relatively slow reacting minerals such 38 as plagioclase drives the rising concentrations of alkali and alkaline earth metals observed 39 at later stages of the test, whereas the dissolution of amorphous iron sulfide causes slowly 40 increasing Fe concentrations. 41 42

During a carbon sequestration field study to simulate the impact of CO 2 migration on shallow groundwater chemistry, the stable isotope composition of dissolved inorganic carbon ( 13 C DIC) and dissolved strontium (87 Sr/ 86 Sr) were evaluated as tracers. Dissolved CO 2 in groundwater was introduced using a closed-loop dipole-style well field situated in a shallow sand-dominated aquifer. Baseline  13 C DIC values, oxygen and hydrogen isotope ratios, and 87 Sr/ 86 Sr values of groundwater were established in four monitoring wells (MW-1 to 4) and one up-gradient background well (BG-1) prior to the introduction of dissolved CO 2. Baseline groundwater  13 C DIC-PDB , oxygen ( 18 O SMOW) and hydrogen (D SMOW) stable isotope values averaged-17,-4.1 and-19.5 ‰, respectively. Groundwater 87 Sr/ 86 Sr baseline values averaged 0.70840 at MW-3 and 0.70818 at MW-2. Arrival of the dissolved CO 2 plume at the monitoring wells is modeled using a 1-D analytical solution, and yields breakthrough curves with flow velocities that are consistent with prior numerical modeling estimates. The  13 C DIC-PDB rose to an average steady-state value of 0.16  0.3 ‰ during the test;  18 O and D of water did not change from their baseline values. 87 Sr/ 86 Sr dropped sharply by 0.00022 at MW-3 and 0.00005 at MW-2 in the first two weeks after plume arrival at the wells, and then slowly increased toward baseline values, correlating with the behavior of dissolved Na, K, Ca, Sr and Si. Carbonate dissolution and desorption from organic matter and Fe-bearing phases at the low-pH plume front is the likely mechanism producing this behavior. The  13 C DIC and the 87 Sr/ 86 Sr of dissolved strontium served as excellent tracers of plume movement during this experiment.

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