Determining sources of nitrate in the semi-arid Rio Grande using nitrogen and oxygen isotopes

The Rio Grande, a semi-arid river in the American Southwest, is a major source of surface water for agriculture and drinking supplies in New Mexico and Texas. In addition to increasing salinity, considerable increases of NO3- [nitrate] have been observed in the semi-arid portion of the Rio Grande. It is possible that elevated water salinity inhibits denitrification on irrigated fields and, thus, fails to mediate excess nutrient load from anthropogenic activities. Therefore, two major goals of this project were to 1) characterize and quantify major NO3- sources, and 2) assess whether elevated water salinity affects microbial denitrification. In fall 2014 and summer 2015, surface water, irrigation drains, urban runoff, and municipal waste effluents were sampled between Elephant Butte, New Mexico and Tornillo, Texas for stable isotope analysis. Highest NO3- concentrations were observed in waste effluents and nearby agricultural drains irrigated with reclaimed water. Conversely, NO3- concentrations in river and agricultural drains were significantly lower in areas farther away from urban centers. Two major NO3- sources were identified using chemical and isotope tracers: fertilizers, with low δ15N [delta fifteen nitrogen] and high δ18O [delta eighteen oxygen] (average 0.6 and 18.3‰ [permille], respectively), and waste water effluents from cities, with high δ15N and low δ18O (average 10.5 and -5.1‰, respectively). According to nitrogen and oxygen isotope mass balance constraints, waste effluent-derived NO3- contribution was the smallest in upstream locations and accounted for up to 24-47% near Las Cruces compared to fertilizer-derived NO3-. Further downstream, effluent contributions increased and accounted for up to 41-77% between Las Cruces and El Paso. The highest fertilizer-derived NO3- contributions of 90-100% were measured in the agricultural district located below El Paso where reclaimed city water is commonly used for irrigation. Elevated salinity did not appear to control microbial denitrification. In fact, the strongest isotopic evidence of microbial denitrification was observed in water samples showing elevated salinity. Results suggest urban centers are important NO3- contributors into aquatic system of the watershed and microbial processes do not appear to significantly reduce NO3- loads from anthropogenic sources