THE USE OF HYDROCHEMISTRY AND ENVIRONMENTAL ISOTOPES TO IDENTIFY GROUNDWATER FLOW SYSTEMS IN COAHUILA STATE, MEXICO

The Eagle Ford Formation in southern Texas is nowadays one of the fast growing oil and gas fields not only on the United States of America, but also on the planet. The Eagle Ford shale Formation from south Texas continues to Coahuila state in northern Mexico, some shale gas/oil exploration wells drilled by PEMEX close to the USA border showed moderate results; however, prospective regions for oil and gas development are extensive, covering more than 1/3 of the Coahuila area. Coahuila state has a semi-arid condition (precipitation 300-400 mm/year), and groundwater is the main source. This investigation contributes with the geochemical characterization of groundwater flow systems in the southern region of the Coahuila state, including a karstic regional aquifer unit in the Coahuila Platform. Local flow systems, circulation depth is restricted to less than 150 m, HCO₃-Ca nature and 21.5 °C, high dispersion of δ¹⁸O and δ²H values reflecting different recharge elevation, measureable tritium concentration (1.6-1.9 TU) and high C-14 (67 pmC). The intermediate flow systems, have a circulation depth up to 200 m, the water types are HCO₃-Ca and HCO₃-Mg, with about 23 °C, have a wide deviation of δ¹⁸O and δ²H values, are tritium free (<0.5 TU), and a C-14 corrected age between 5,000-6,000 years BP. A second group of groundwater flow systems shows a longer travel path as indicated by a Mixed and Mg-SO₄ water type, temperature of 26 °C, and a narrower range of δ¹⁸O and δ²H values, they are also tritium free; and a C-14 corrected age between 6,000-12,500 years BP. Deep wells (>200 m) in karstic conditions tapping regional groundwater flow systems, with water 28 °C, long and deep travel paths are confirmed by δ¹⁸O and δ²H values with a very small deviation and C-14 corrected age between 12,500-25,000 years BP. The major, minor, isotopic (C-13 and S-34) composition is consistent with a series of water-rock interactions including dissolution of limestone and/or dolomite and gypsum, oxidation of sulfide minerals in the carbonate rocks was identified as an additional source for dissolved sulphate. Some dissolution of dolomite and precipitation of calcite occur as water dissolves gypsum. This baseline characterization is useful information to evaluate future potential effects of unconventional oil and gas development in the region.