Nitrogen Sources and Sinks Within the Middle Rio Grande, New Mexico¹

Abstract

Relationships between discharge, land use, and nitrogen sources and sinks were developed using 5 years of synoptic sampling along a 300 km reach of the Rio Grande in central New Mexico. Average river discharge was higher during 2001 and 2005 “wet years” (15 m3/s) than during the drought years of 2002‐04 “dry years” (8.9 m3/s), but there were no differences in nitrogen loading from wastewater treatment plants (WWTPs) which were the largest and most consistent source of nitrogen to the river (1,330 kg/day). Average total dissolved nitrogen (TDN) concentrations remained elevated for 180 km downstream of the Albuquerque WWTP averaging 1.2 mg/l in wet years and 0.52 mg/l in dry years. Possible explanations for the constant elevated TDN concentrations downstream of the major point source include reduced nitrogen retention capacity, minimal contact with riparian or channel vegetation, large suspended sediment loads, and low algal biomass. Somewhat surprisingly, agricultural return flows had lower average nitrogen concentrations than river water originally diverted to agriculture in both wet (0.81 mg/l) and dry years (0.19 mg/l), indicating that the agricultural system is a sink for nitrogen. Lower average nitrogen concentrations in the river during the dry years can be explained by the input of agricultural returns which comprise the majority of river flow in dry years.

Water quantity and quality challenges from Elephant Butte to Amistad

“…The study of Oelsner et al (2007) focused on the Rio Grande above and below Albuquerque but the conclusions apply to the Rio Grande below Elephant Butte as well. This study found that the managed hydrology of the Rio Grande is the dominant control on nitrogen concentrations within the river.…”

Use of a dynamic simulation model to understand nitrogen cycling in the middle Rio Grande, NM.

“…Our previous work shows that water in the agricultural conveyance channels has lower nitrate concentrations than water diverted from the river into the conveyance canals [Oelsner et al, 2007]. Both immobilization (uptake) and denitrification are known to be important processes for removing nitrogen and reducing nitrate concentrations in the Middle Rio Grande [Molles et al, 1998;Valett et al, 2005].…”

“…Several studies have looked at how nutrient load patterns change downstream and over time [Moore and Anderholm, 2002;Oelsner et al, 2007;Passell et al, 2004;2005]. Results from these studies suggest that wastewater treatment plants are important point sources of nutrients to the river [Oelsner et al, 2007;Passell et al, 2004] and agricultural diversions can remove nitrogen and improve water quality downstream of wastewater treatment plant point sources [Oelsner et al, 2007].…”

“…Results from these studies suggest that wastewater treatment plants are important point sources of nutrients to the river [Oelsner et al, 2007;Passell et al, 2004] and agricultural diversions can remove nitrogen and improve water quality downstream of wastewater treatment plant point sources [Oelsner et al, 2007]. In addition, Oelsner et al, 2007 showed that there is limited biological modification of dissolved nitrogen and organic carbon in the river within the Middle Rio Grande. However, due to the complex relationships between river water, agricultural conveyance water and groundwater, it is not clear from concentration patterns where and how nitrogen is removed from the system.…”

The potential to improve water quality in the middle Rio Grande through effective wetland restoration

“…Some have suggested that the absence of wetland and riparian vegetation may be partly responsible for elevated nutrient levels throughout the middle Rio Grande (Oelsner et al 2007). Wetlands and riparian areas can provide many ecosystem services such as nutrient retention, sediment attenuation, increased biodiversity, and flood abatement (Zedler 2003;Fisher & Acreman 2004).…”