New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Grande is the most important source of water for the irrigation of agricultural fields in the Rio Grande Valley of West Texas. Return flows from this irrigation likely add to the salinity of the Rio Grande and may include the addition of excess nutrient chemicals like nitrate (NO₃). To date, however, the seasonal concentrations of NO₃ within the Rio Grande and the sources that contribute this nutrient chemical have not been well-defined. To address this problem, we measured NO₃ concentrations of the Rio Grande surface water in five locations of the Mesilla Valley, stretching approximately 100 kilometers between Radium Springs, New Mexico and El Paso, Texas. Samples were collected monthly from these five sites in 2010. In general, NO₃ concentrations were significantly lower in the upper part of the Mesilla Valley, ranging from 0.1 to 1.3 mg/L, as compared to higher values of 3.9 to 5.0mg/L in downstream locations near El Paso. However, the highest NO₃ fluxes (as large as 27 mg/L) were found in the central part of the Mesilla Valley, adjacent to the town of Vado, New Mexico. This NO₃ flux is most likely associated with a surface water and/or groundwater discharge from dairy farms common in this area. The flux is diluted as the river moves downstream. The seasonal variation of NO₃ shows that during non-irrigation season we see slightly higher concentrations compared to lower concentrations during irrigation season. The significance of these findings leads us to believe that there are various contamination issues related to return irrigation flows and/or discharges of municipal/dairy effluents. This initial work demonstrates that NO₃ concentrations in the Rio Grande increase in downstream locations. The contamination of nitrate in the Rio Grande is additional factor causing the decrease of water quality in the region.