THE SOURCES AND BUDGET FOR DISSOLVED SULFATE IN A CARBONATE AQUIFER IN THE SOUTHERN SACRAMENTO MOUNTAINS, NEW MEXICO

In order to better understand the evolution of water chemistry and the sources of aqueous sulfate in these semi-arid settings, we analyzed chemical and sulfur isotope compositions of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico. Our 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 δ³⁴S of dissolved sulfate in spring and groundwater samples varied from +9.0 to +12.8 ‰, reflecting the mixing of sulfate from the dissolution of Permian gypsum/anhydrite (+12.3 to +13.4 ‰) and oxidation of sulfide minerals (-24.5 to -4.2 ‰). According to sulfur isotope mass balance constraints, the contribution of sulfide-derived sulfate was considerable in the high-mountain recharge areas, accounting for up to ~10 % of the total sulfate load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO₄ input of ~2-4 % was contributed by atmospheric wet deposition. This study demonstrates that the variation in δ³⁴S of sulfate in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here we find that sulfuric acid dissolution due to sulfide oxidation contributes up to 5 % of the total carbonate weathering budget, while most of the sulfate is released from bedrock sources during de-dolomitization.