DETERMINING ENVIRONMENTAL FACTORS CONTROLLING NITROGEN CYCLING IN THE SEMI-ARID RIO GRANDE USING NITROGEN AND OXYGEN ISOTOPES

Abstract:

The Rio Grande is a semi-arid river in the American Southwest supporting agriculture and large populous centers in New Mexico and west Texas. In addition to increasing salinity, considerable increases of nitrate (NO₃), up to ~50 mg/L, have been previously observed in the Rio Grande between Las Cruces, New Mexico and El Paso, Texas. This is particularly a problem during non-irrigation season when little surface water is released from upstream reservoirs, substantially reducing stream flows in the Rio Grande. While both irrigation runoff and municipal waste effluents are likely important NO₃ contributors, there are no quantitative studies assessing NO₃ fluxes to the Rio Grande from these two sources. Therefore, in this study we used 𝛿15N and 𝛿¹⁸O values of NO₃ as environmental tracers to characterize major NO₃ sources in the Rio Grande and its agricultural drains between Las Cruces and El Paso. Surface water of the semi-arid Rio Grande, drains and major wastewater treatment plants were collected in October 2014 (non-irrigation season) and August 2015 (irrigation season). The water samples from the 2014 sampling campaign showed that the 𝛿¹⁵N and 𝛿¹⁸O values of NO₃ in the Rio Grande and two agricultural drains located south of El Paso varied in relatively narrow range from +9.8 to +15.7‰ and -5.9 to -0.2‰, respectively. These ranges were similar to 𝛿¹⁵N and 𝛿¹⁸O values of local wastewater treatment plants in Las Cruces and El Paso, from +8.2 to +10.2‰ and -9.7 to -2.5‰ respectively. Municipal wastewater effluents are important tributaries to the semi-arid Rio Grande in the studied area, particularly during non-irrigation season. Furthermore, irrigation of agricultural fields south of El Paso is to a large extent supported by reclaimed municipal wastewater. Consequently, these explain the observed higher contributions of NO₃ from urban sources in the investigated area.