Spatial and temporal variability in nutrient dynamics in the Middle Rio Grande, New Mexico: The importance of an agricultural irrigation network for nutrient retention

Background/Question/Methods

Natural and human populations in the Middle Rio Grande (MRG) corridor depend on the MRG as one of the few perennial water sources in this water-poor region. To determine anthropogenic impacts on nutrient loading and retention in this arid land river ecosystem, we collected samples once a month for 28 months at 30 sites along the 340 km MRG reach. During 15 months we also collected samples from all of the major tributaries to the MRG. Samples were analyzed for inorganic nutrients (PO₄, NO₃, and NH₄), and discharge data from 27 river and irrigation return flow gauges and four wastewater treatment plants were used to calculate the nutrient loads associated with each sample collected and the water loss due to agricultural irrigation.  

Results/Conclusions

Water entering the MRG from upstream had low concentrations and loads of nitrate (0.02 ± 0.03 mg l^-1, 62 ± 105 kg NO₃ day^-1), soluble reactive phosphorus (SRP) (0.01 ± 0.01 mg l^-1, 33 ± 31 kg PO₄ day^-1), and ammonium (0.003 ± 0.004 mg l^-1, 7 ± 15 kg NH₄ day^-1). During all months the Albuquerque wastewater treatment plant was the major contributor of nitrate, SRP, and ammonium to the MRG, resulting in instream concentration and load increases of ~2000% for NO₃ and SRP. During months when little water was diverted from the MRG for irrigation, nutrient levels remained elevated for 260 km below the wastewater inputs. During months when significant portions of the river flow were diverted for irrigation, nitrate and SRP concentrations and loads declined dramatically in the downstream direction. Total retention of wastewater inputs within the MRG corridor ranged from 18 to 99% and 34 to 99% for nitrate and SRP, respectively, with a strong and significant relationship found between the percentage of water diverted from the MRG for irrigation, and the percentage of nitrate (r² = 0.80) and SRP (r² = 0.73) removed within the reach. Concentrations of NO₃ and SRP in agricultural return drains downstream of urban inputs were on average 28% and 54% lower, respectively, than concentrations in water diverted from the river for irrigation, indicating the MRG agricultural network acts as a sink with respect to nutrient loads and concentrations. Irrigation associated water losses averaged 62% of incoming water, highlighting that the important ecosystem service of nutrient retention provided by the MRG irrigation network comes with the expense of significant water loss.