U and Sr Isotope Tracers of Agricultural Salinity Sources to the Lower Rio Grande River

Abstract:

Elevated salinity of the lower Rio Grande River deteriorates water quality and limits domestic and agricultural water use. Both natural and anthropogenic processes contribute salts in the Rio Grande. Previous studies have focused on natural salt contributions with less emphasis on anthropogenic sources of salinity in the Rio Grande. Using (²³⁴U/²³⁸U) activity ratios (UAR), ⁸⁷Sr/⁸⁶Sr isotope ratios, and major element concentrations, we aim to trace and quantify the salt loads in the Lower Rio Grande watershed which is greatly impacted by agricultural activities. Between 2009 and 2010, we sampled the Rio Grande stretch and irrigation return flows between the Elephant Butte Reservoir, New Mexico and El Paso, Texas. Furthermore, we monitored in monthly intervals the temporal changes of chemical and isotopic compositions of the Rio Grande at Canutillo, Tx.

Our results show higher U and Sr fluxes in the Rio Grande during the irrigation season as compared to the non-irrigation season. The UAR (1.62 to 2.13) and ⁸⁷Sr/⁸⁶Sr ratios (0.7099 to 0.7138) were higher in the non-irrigation season compared to the irrigation season (UAR: 1.69 to 1.77; ⁸⁷Sr/⁸⁶Sr: 0.7100 to 0.7106). These variations of UAR and ⁸⁷Sr/⁸⁶Sr ratios imply multiple sources of U and Sr in the Rio Grande. In contrast, the agricultural return flows show a narrow range of UAR (1.31 to 1.37) and ⁸⁷Sr/⁸⁶Sr ratios (0.7091 to 0.7099) in the studied seasons. This is consistent with salinity contributions from agricultural sources. Rio Grande at Canutillo shows low UAR (1.62 to 1.77) and ⁸⁷Sr/⁸⁶Sr ratios (0.7104 to 0.7105) during the irrigation season as compared to the non-irrigation season (UAR: 2.04 to 2.24; ⁸⁷Sr/⁸⁶Sr: 0.7105 to 0.7109). The low U and Sr signature at Canutillo during the irrigation season is close to that of the agricultural return flows, indicative of agricultural salinity sources. These results provide useful elemental and isotopic constraints for future mass balance calculations of salinity inputs in the Rio Grande River.