Chemical and Isotopic Evaluation of Sulfur Sources and Cycling in the Pecos River, New Mexico, USA

The use of stable isotopes in studies of watershed biogeochemical processes has increased greatly throughout the last several decades. Much of the sulfur cycling research has addressed the influence of changes in atmospheric acid deposition on sulfur dynamics in temperate ecosystems. Little is known about sulfur cycling in dryland ecosystems such as those in the American Southwest. To identify the sources and assess the cycling of sulfur in dryland ecosystems, chemical and isotopic compositions of water were measured on samples collected from the Pecos River (New Mexico, USA) during a reconnaissance survey in spring 2010. Based on the chemical and isotopic results, the Pecos River is readily divided into an upper basin of 6,000 km2 above Santa Rosa Lake and a lower basin of 44,000 km2 above Red Bluff Reservoir in western Texas. The upper basin contains river water with low concentrations of chloride (3 mg/L) and sulfate (13 mg/L), low values of δD (− 87‰) and δ18O (− 12.3‰), and low δ34S (− 4.3‰) and δ18O values (2.6‰) of dissolved sulfate (δ34SSO4 and δ18OSO4). Three different sources contributing to the pool of dissolved sulfate are identified, namely the oxidation of sulfide minerals, the soil processing of atmospheric sulfate, and the dissolution of ancient evaporites. The relative contributions of the three different sulfate sources change from reach to reach. Sulfate from evaporite dissolution primarily of Permian age dominates in the lower reaches while sulfate from sulfide oxidation dominates in the upper part of the Pecos River. Despite significant lithologic variations across the lower basin, δ34SSO4 values of river water are quite constant, with an average value of 11.8‰. In contrast, a 5‰ decrease was observed in δ18OSO4 values of river water between upstream and downstream reaches of the lower Pecos River, indicating that 63% of the dissolved sulfate had been recycled. Surprisingly, most of the sulfur cycling observed occurs in two small irrigation districts (the Fort Sumner Irrigation District and the Carlsbad Irrigation District), whereas there is only a minimal decrease (0.7‰) in δ18OSO4 in the largest irrigation district (the Pecos Valley Artesian Conservancy District). This study implies that the influence of land use activities on sulfur cycling may be more profound than previously thought