J. Villinski, J. Hogan, P. Brooks, M. Conklin (UA-HWR)

Nitrogen is generally regarded as the limiting nutrient in semi-arid streams and rivers. Likewise, nitrogen is the element that has been most heavily impacted by anthropogenic means, mainly through the use of man-made fertilizers. Current understanding of nutrient dynamics, and anthropogenic impacts on these dynamics, are not well understood at the scale of the river in semi-arid and arid regions. In mesic, temperate regions, riparian corridors act as buffers, removing and transforming pollutants (i.e., nitrate). In semi-arid regions, compared to mesic systems, the movement of water is away from the stream during most of the year. We cannot, therefore, rely on mesic models to understand nutrient cycling in semi-arid riparian systems. To move from process-level studies of nutrient speciation and retention, we need to gather base-line data from semi-arid rivers under different management schemes.

Science questions

• What are the sources and sinks, and the mass loads of nitrogen and carbon species in the semi-arid rivers, both temporally and spatially?
• How do anthropogenic factors (agriculture, urban centers, groundwater extraction, reservoirs) affect the nitrogen and carbon loads and speciation?

Research approach

Our scientific approach is to perform synoptic sampling of two end-member rivers (the Rio Grande and the San Pedro River), collecting filtered samples from the length of the river at different times of the year (twice yearly on the Rio Grande and seasonally on the San Pedro River), and analyzing the samples for nitrogen species (nitrate, ammonium and dissolved organic nitrogen) and dissolved organic carbon in addition to the measurements being performed by James Hogan (TA2).

Results

This is the first year of this project. Synoptic sampling was performed twice along the Rio Grande, in August 2001 and January 2002, in conjunction with the salinity sampling performed by James Hogan, TA2. Samples were taken from ninety locations along the river from the near the headwaters in Colorado to south of Fort Quitman, Texas, as well as from tributaries, agricultural drains and the conveyance channel. Sampling on the San Pedro River will commence in fall 2002 in collaboration with research being done by the Upper San Pedro Partnership (James Leenhouts, USGS and Juliet Stromberg, ASU/SAHRA).

Results of the two synoptic sampling runs indicate that two major sources of nitrogen to the Rio Grande system are 1) municipal waste and 2) agricultural return flows (e.g., nitrate concentration, see Figure 3-7). Of interest is the fact that after a large increase in nitrate concentrations below the Albuquerque wastewater treatment plant, the nitrate levels do not decrease until the river reaches the Elephant Butte reservoir. This behavior may be due to a lack of primary production occurring in this region, and may be related to high sediment load decreasing light penetration. In contrast, as water moves through the sediments between the river and the conveyance channel, nitrate is removed, suggesting that microorganisms are utilizing nitrate as an electron donor.

Nitrogen levels, both nitrate and ammonium, were generally higher in the winter than in the summer, most likely due to the lack of agricultural production at that time. Mineralization of organic nitrogen results in ammonium, the lack of growing crops decreases the utilization of this nutrient, and the ammonium is then transported to the river. The increased nitrate levels maybe due to flushing of fertilizers. Concomitantly, the carbon to nitrogen levels in the dissolved organic matter in the river were generally lower in the winter than the summer, indicating that the river is not nitrogen-limited during this time.

The results of this project to date are providing a background level understanding of the overall levels, sources, and sinks of nitrogen species in the Rio Grande during two seasons. Most importantly, nitrogen increases occur in well-defined river segments, coinciding with agricultural return drains and the major urban centers of Albuquerque and El Paso/Juarez. Even though the nitrate-nitrogen levels are below the EPA-mandated MCL of 10 mg L-1, recent research has linked bladder cancer to nitrate-nitrogen levels above 2.5 mg L-1. This level of nitrate occurs in the winter months in the Mesilla Valley and El Paso/Juarez regions of the Rio Grande. In addition, the effects of these higher than background levels of nitrogen on the riparian ecosystem are not known at this time. Finally, during times of sustained droughts, with lower flows in the river, the nitrate levels in the river may increase to unhealthy levels (with a greater percent of the flow coming from agricultural drains and wastewater treatment plants).

Plans

• Sampling will commence along the San Pedro River this fall after the monsoon season. This will allow us to assess the differences in nutrient levels and speciation due to different anthropogenic controls.
• Sampling will continue on the Rio Grande this summer and winter, with additional isotope measurements made to confirm nitrogen sources, distinguishing between animal-derived wastes (i.e., waste water treatment) and plant sources.
• Collaboration will commence with Julie Stromberg to study the linkages between riparian structure and diversity and nutrient levels.

This research is important to the goals of SAHRA because this is the first step to up-scale the nutrient work currently being performed under TA3 from the process/reach level to the length of the river. In addition, research will begin to address issues of transferability of results across basins. This work, in conjunction with Julie Stromberg's work, should prove valuable in the creation of an integrated riparian ecosystem model.