New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande is the fourth longest river system in United States and is the primary source of irrigation water for the Rio Grande Valley. Evaporation, groundwater recharge associated with salt-rich sedimentary rocks, geothermal heating, and flood irrigation are believed to increase the solute content of the river. To assess the salt loads from various geologic and anthropogenic sources in the Rio Grande Valley, we have been working to determine the chemistry and S isotope composition (³⁴S of SO₄ ^2-) for the Rio Grande and its shallow aquifers in New Mexico and West Texas. Our results suggest that SO₄^2- fluxes in the Rio Grande are mainly controlled by surface sulfide weathering in upstream locations (δ34S < -2 ‰) and application of fertilizers during irrigation in downstream locations (-2 to +4 ‰). The sedimentary SO₄^2- flux associated with thermal and non-thermal flows (+8 to 10 ‰) appears to be minor, but does account for some of the increase of δ³⁴S (up to 5 ‰) in the distal end of Mesilla Basin at the end of the non-irrigation season. The geothermal flows in the Mesilla Basin are characterized by high Cl- (up to 578 mg/L) and lower SO₄^2- concentrations (up to 236 mg/L), thus, they cannot account for significant SO₄^2- increase (546 mg/L) observed in this basin. However, agricultural drains in the Mesilla show considerably high Cl^- and SO₄ ^2- concentrations (up to 1203 mg/L and 1531 mg/L, respectively) suggesting the important salt loads to the Rio Grande from the near-surface environment. In addition to geological sources, the elevated Cl^- fluxes can also be partly linked to anthropogenic sources such as fertilizers and municipal waste discharges.