He, C, and Sr isotopes are used to infer spring sources in a water-stressed area. Spring-water origins and pathways in the Cuatrociénegas Basin are revealed by linking structure and geochemistry via regionally extensive fault networks. This study presents the fi rst dissolved noble gas and He isotopic data from northeastern Mexico. Basementinvolved faults with complex reactivation histories are important in northeastern Mexico tectonics and affect hydrogeologic systems. The importance of faults as conduits for northeastern Mexico volcanism is recognized, but connections between faulting and the hydrogeologic system have not been extensively investigated. This research tests the hypothesis that Cuatrociénegas Basin springs are divided into two general classes based upon discharge properties: (1) regional carbonate aquifer discharge (mesogenic) mixed with contributions from deeply sourced (endogenic) fl uids containing 3 He and CO 2 from the mantle that ascend along basement-involved faults; and (2) carbonate aquifer discharge mixed with locally recharged (epigenic) mountain precipi tation. Carbonate and/or evaporite dissolution is indicated by Ca-SO 4 hydrochemical facies. He isotopes range from 0.89 to 1.85 R A (R A is the 3 He/ 4 He of air, 1.4 × 10 -6 ) and have minimal 3 H, from which it is inferred that basement-involved faults permit degassing of mantle-derived He (to 23% of the total He) and CO 2 ( pCO 2 ≤ 10 -1 atm). Mantle degassing is compatible with the thinned North American lithosphere, as shown in tomographic images. Sr isotopes in both Cuatrociénegas Basin springs and spring-deposited travertine ( 87 Sr/ 86 Sr = 0.707428-0.707468) indicate that carbonate rocks of the regional Cupido aquifer ( 87 Sr/ 86 Sr = 0.7072-0.7076) are the main source of Sr. Rock-water interactions with mafi c volcanic rocks ( 87 Sr/ 86 Sr = 0.70333-0.70359) are not inferred to be an important process. Groundwater-dissolved inorganic C origins are modeled using major elements and C isotopes. C isotope data show that ~30% ± 22% of CO 2 in spring water is derived from dissolution of aquifer carbonates (C carb = 30%), 24% ± 16% is from soil gas and other organic sources (C org = 24%), and 46% ± 33% is from deep sources [C endo (endogenic crust and mantle) = 46%]. This study demonstrates the presence of mantlederived 3 He and deeply sourced CO 2 that ascend along basement-penetrating faults and mix with Cupido aquifer groundwater before discharging in Cuatrociénegas Basin springs.
This research integrates data procedures for the delineation of regional ground water flow systems in arid karstic basins with sparse hydrogeologic data using surface topography data, geologic mapping, permeability data, chloride concentrations of ground water and precipitation, and measured discharge data. This integrative data analysis framework can be applied to evaluate arid karstic aquifer systems globally. The accurate delineation of ground water recharge areas in developing aquifer systems with sparse hydrogeologic data is essential for their effective long-term development and management. We illustrate the use of this approach in the Cuatrociénegas Basin (CCB) of Mexico. Aquifers are characterized using geographic information systems for ground water catchment delineation, an analytical model for interbasin flow evaluation, a chloride balance approach for recharge estimation, and a water budget for mapping contributing catchments over a large region. The test study area includes the CCB of Coahuila, Mexico, a UNESCO World Biosphere Reserve containing more than 500 springs that support ground water-dependent ecosystems with more than 70 endemic organisms and irrigated agriculture. We define recharge areas that contribute local and regional ground water discharge to springs and the regional flow system. Results show that the regional aquifer system follows a topographic gradient that during past pluvial periods may have linked the Río Nazas and the Río Aguanaval of the Sierra Madre Occidental to the Río Grande via the CCB and other large, currently dry, upgradient lakes.
15Claims of CO 2 leakage at the Kerr farm near the IEAGHG Weyburn-Midale CO 2 Monitoring and Storage 16 project in Saskatchewan, Canada in 2011 presented an opportunity to test years of research in near-17 surface leakage detection in an operational setting. The claim was based on a study conducted by a private 18 consultant that drew criticism from geologic CO 2 storage experts. To assess the origin of the claimed 19 leakage, a soil gas assessment was conducted at the site using a relatively new process-based soil gas 20 approach (Romanak et al., 2012). In order to assess leakage without years of background data or complex 21 statistical analyses, this simple approach uses three fixed soil gas ratios (augmented by isotopic and 22 hydrocarbon data when needed). Using this approach at the Kerr site, the relative contributions of various 23 processes creating soil gas geochemical signatures were determined including: 1) biologic respiration, 2) 24 CH 4 oxidation, 3) CO 2 dissolution/reaction with carbonate, 4) mixing with atmosphere, and 5) CO 2 25 leakage from the storage reservoir. For additional verification, the results of the Kerr assessment were 26 compared to new data collected at the ZERT controlled release site in Bozeman, Montana and published 27 data from a leakage proxy at the Mt. Etna volcano, Italy. The results from the Kerr site show no evidence 28 for CO 2 leakage from the storage reservoir and also verify that a process-based approach can be used to 29 quickly and economically assess leakage above geologic carbon storage sites. In addition, data from the 30 ZERT release suggest quantification of a leakage signal apart from natural processes may be most easily 31 accomplished using a process-based approach.
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