In a natural analog study of risks associated with carbon sequestration, impacts of CO 2 on shallow groundwater quality have been measured in a sandstone aquifer in New Mexico, USA. Despite relatively high levels of dissolved CO 2 , originating from depth and producing geysering at one well, pH depression and consequent trace element mobility are relatively minor effects due to the buffering capacity of the aquifer. However, local contamination due to influx of brackish waters in a subset of wells is significant. Geochemical modeling of major ion concentrations suggests that high alkalinity and carbonate mineral dissolution buffers pH changes due to CO 2 influx. Analysis of trends in dissolved trace elements, chloride, and CO 2 reveal no evidence of in situ trace element mobilization. There is clear evidence, however, that As, U, and Pb are locally co-transported into the aquifer with CO 2 -rich brackish water. This study illustrates the role that local geochemical conditions will play in determining the effectiveness of monitoring strategies for CO 2 leakage. For example, if buffering is significant, pH monitoring may not effectively detect CO 2 leakage. This study also highlights potential complications that CO 2 carrier fluids, such as brackish waters, pose in monitoring impacts of geologic sequestration.

[1] It is well known that terrestrial photosynthesis and 13 C discrimination vary in response to a number of environmental and biological factors such as atmospheric humidity and genotypic differences in stomatal regulation. Small changes in the global balance between diffusive conductances to CO 2 and photosynthesis in C3 vegetation have the potential to influence the 13 C budget of the atmosphere because these changes scale with the relatively large one-way gross primary production (GPP) flux. Over a period of days to years, this atmospheric isotopic forcing is damped by the return flux consisting mostly of respiration, Fire, and volatile organic carbon losses. Here we explore the magnitude of this class of isotopic disequilibria with an ecophysiological model (SiB2) and a double deconvolution inversion framework that includes timevarying discrimination for the period of [1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994]. If the net land carbon sink and plant 13 C discrimination covary on interannual timescales at the global scale, consistent with El Niño-induced drought stress causing a decline in global GPP and C3 discrimination, then less interannual variability in ocean and land net carbon exchange is required to explain atmospheric trends in d 13 C and CO 2 as compared with previous studies that assumed discrimination was invariant.

We measured seasonal and interannual variations in delta(13)C values within the carbon reservoirs (leaves and soil) and CO(2) fluxes (soil and ecosystem respired CO(2)) of an old growth coniferous forest in the Pacific Northwest USA with relation to local meteorological conditions. There were significant intra-annual and interannual differences in the carbon isotope ratios of CO(2) respired at both the ecosystem (delta(13)C(R)) and the soil levels (delta(13)C(R-soil)), but only limited variations in the carbon isotope ratios of carbon stocks. The delta(13)C(R) values varied by as much as 4.4 per thousand over a growing season, while delta(13)C(R-soil )values changed as much as 6.2 per thousand. The delta(13)C of soil organic carbon (delta(13)C(SOC)) and needle organic carbon (delta(13)C(P)) exhibited little or no significant changes over the course of this study. Carbon isotope discrimination within leaves (Delta(p)) showed systematic decreases with increased canopy height, but remained fairly constant throughout the year (Delta(p)=17.9 per thousand -19.2 per thousand at the top of the canopy, Delta(p)=19.6 per thousand -20.9 per thousand at mid-canopy, Delta(p)=23.3 per thousand -25.1 per thousand at the canopy base). The temporal variation in the delta(13)C of soil and ecosystem respired CO(2) was correlated ( r=0.93, P<0.001) with soil moisture levels, with dry summer months having the most (13)C-enriched values. The dynamic seasonal changes in delta(13)C of respired CO(2) are hypothesized to be the result of fast cycling of recently fixed carbon back to the atmosphere. One scaling consequence of the seasonal and interannual variations in delta(13)C(R) is that inversion-based carbon-cycle models dependent on observed atmospheric CO(2) concentration and isotope values may be improved by incorporating dynamic delta(13)C(R) values to interpret regional carbon sink strength.