Sulfur isotopic study of sulfate in the aquifer of Costa de Hermosillo (Sonora, Mexico) in relation to upward intrusion of saline groundwater, irrigation pumping and land cultivation

Groundwater from the Costa de Hermosillo aquifer has been used extensively for irrigation over the past 60 a in the Sonora region of northwestern Mexico resulting in salinization of fresh groundwater resources. Salinization of groundwater is most pronounced on the western/coastal side of the aquifer, with an aerial extent of 26.7 km2, where maximum values are reported for conductivity (31 mS/cm) and Cl− concentrations (16,271 mg/L). Salinization is likely to increase if groundwater pumping continues at levels comparable to the present time. Upward incursion of marine water into the aquifer is inferred from δ2H (−7.2 ‰) and δ18O (+1.6‰) compositions of groundwater samples with the highest conductivity. Compared to modern seawater in the Gulf of California, ratios of SO4/Cl and Cl/Br are small (0.01 and 33, respectively) and the S isotopic composition of SO42- is high (+32.7%) in the most saline portions of the Costa de Hermosillo. This saline groundwater is inferred to result from an earlier phase of dissimilatory bacterial SO42- reduction coupled to decomposition of organic matter in marine blue clays deposited during the Miocene/Pliocene transgression. The isotopic composition of present-day surface discharge from agricultural fields is substantially enriched in 32S due to widespread application of (NH4)2SO4 fertilizers and potential mobilization of S from mineral resources. Surface water discharging from irrigated fields has δ34S values ranging from −2.1 to 3.3 ‰ which are distinctly different from groundwater and surface water in adjacent non-agricultural areas with δ34S values ranging from 5.2 to 13.5 ‰. Prolonged irrigation pumping that promotes the incursion of air to the subsurface could enhance the weathering of S-bearing minerals such as magmatic sulfides, producing 32S-enriched SO42-.