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

Technical Report · Fri Aug 01 04:00:00 EDT 2008

DOI:https://doi.org/10.2172/947333· OSTI ID:947333

Meixner, Tom ^[1]; Tidwell, Vincent Carroll; Oelsner, Gretchen ^[1]; Brooks, Paul ^[1]; Roach, Jesse D

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1. University of Arizona, Tucson, AZ

Water quality often limits the potential uses of scarce water resources in semiarid and arid regions. To best manage water quality one must understand the sources and sinks of both solutes and water to the river system. Nutrient concentration patterns can identify source and sink locations, but cannot always determine biotic processes that affect nutrient concentrations. Modeling tools can provide insight into these large-scale processes. To address questions about large-scale nitrogen removal in the Middle Rio Grande, NM, we created a system dynamics nitrate model using an existing integrated surface water--groundwater model of the region to evaluate our conceptual models of uptake and denitrification as potential nitrate removal mechanisms. We modeled denitrification in groundwater as a first-order process dependent only on concentration and used a 5% denitrification rate. Uptake was assumed to be proportional to transpiration and was modeled as a percentage of the evapotranspiration calculated within the model multiplied by the nitrate concentration in the water being transpired. We modeled riparian uptake as 90% and agricultural uptake as 50% of the respective evapotranspiration rates. Using these removal rates, our model results suggest that riparian uptake, agricultural uptake and denitrification in groundwater are all needed to produce the observed nitrate concentrations in the groundwater, conveyance channels, and river as well as the seasonal concentration patterns. The model results indicate that a total of 497 metric tons of nitrate-N are removed from the Middle Rio Grande annually. Where river nitrate concentrations are low and there are no large nitrate sources, nitrate behaves nearly conservatively and riparian and agricultural uptake are the most important removal mechanisms. Downstream of a large wastewater nitrate source, denitrification and agricultural uptake were responsible for approximately 90% of the nitrogen removal.

• Research Organization: Sandia National Laboratories
• Sponsoring Organization: USDOE
• DOE Contract Number: AC04-94AL85000
• OSTI ID: 947333
• Report Number(s): SAND2008-5431
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
DENITRIFICATION
EVAPORATION
GROUND WATER
MATHEMATICAL MODELS
NEW MEXICO
NITRATES
NITROGEN
NUTRIENTS
New Mexico.
Nitrogen.
REMOVAL
RIO GRANDE RIVER
SOLUTES
TRANSPIRATION
UPTAKE
WATER QUALITY
Water resources development