NATURAL AND ANTHROPOGENIC FACTORS CONTROLLING GROUNDWATER SALINITY IN THE MESSILA VALLEY OF THE SEMI-ARID RIO GRANDE WATERSHED

Aquifer groundwater is an important water source for flood irrigation and municipal users in the semi-arid Rio Grande watershed. However, the increasing concentrations of sulfate, chloride, and sodium significantly decrease groundwater quality in the Rio Grande aquifers. One of the biggest uncertainties related to elevated salt is their poorly-defined sources and how agricultural and municipal practices contribute to increasing salinity. In order to address this problem, in 2015-2016 we sampled ~50 wells of the Messila Valley in southern New Mexico for sulfur and oxygen isotope composition of dissolved sulfate to characterize major salinity sources of groundwater. Additionally, a limited number of groundwater samples were analyzed for (²³⁴U/²³⁸U) ratios. The measured δ³⁴S and δ¹⁸O of sulfate varied in a wide range, from -4 to +10‰ and +2 to +12‰, respectively, and were in a good agreement with the sulfate isotope composition of the Rio Grande surface water (-2 to +8‰ and 0 to +10‰) and municipal waste effluents (0 to +9‰ and -1 to +5‰). In contrast, smaller sulfate inputs were from direct application of fertilizers (-2 to +5‰ and +9 to +17‰) and sedimentary bedrock dissolution/brine inflow (+8 to +12‰ and +10 to +12‰). In the shallow groundwater samples, the (²³⁴U/²³⁸U) ratios were indicative of fertilizer-derived uranium. Overall, our isotope results suggest that the groundwater of this region have been already impacted by human activities and recharge of water affected by surface processes such as i) the return flows from the Rio Grande surface water used for irrigation, ii) municipal discharges, and iii) irrigation with the reclaimed city water.