The sources and budget for dissolved sulfate in a fractured carbonate aquifer, southern Sacramento Mountains, New Mexico, USA

摘要

Climate change in the SW USA is likely to involve drier conditions and higher surface temperatures. In order to better understand the evolution of water chemistry and the sources of aqueous SO _4 in these semi-arid settings, chemical and S isotope compositions were determined of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico, USA. The results suggest that the evolution of water chemistry in the semi-arid carbonate aquifer is mainly controlled by dedolomitization of bedrock, which was magnified by increasing temperature and increasing dissolution of gypsum/anhydrite along the groundwater flow path. The δ ~(34)S of dissolved SO _4 in spring and groundwater samples varied from +9.0‰ to +12.8‰, reflecting the mixing of SO _4 from the dissolution of Permian gypsum/anhydrite (+12.3‰ to +13.4‰) and oxidation of sulfide minerals (-24.5‰ to -4.2‰). According to S isotope mass balance constraints, the contribution of sulfide-derived SO _4 was considerable in the High Mountain recharge areas, accounting for up to ~10% of the total SO _4 load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO _4 input of ~2-4% was contributed by atmospheric wet deposition. This study implies that the δ ~(34)S variation of SO _4 in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here it was found that H _2SO _4 dissolution due to sulfide oxidation contributes up to 5% of the total carbonate weathering budget, while most of the SO _4 is released from bedrock sources during dedolomitization.