HR: 0800h
AN: H41C-0650    [Abstracts]
TI: Unsaturated Zone Nitrate beneath Natural and Agricultural Ecosystems in a Semiarid Region, Southern High Plains, USA
AU: * Reedy, R C
EM: bob.reedy@beg.utexas.edu
AF: Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Rd., Bldg. 130, Austin, TX 78758, United States
AU: Scanlon, B R
EM: bridget.scanlon@beg.utexas.edu
AF: Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Rd., Bldg. 130, Austin, TX 78758, United States
AU: Strassberg, G
EM: gil.strassberg@beg.utexas.edu
AF: Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Rd., Bldg. 130, Austin, TX 78758, United States
AB: Quantifying nitrate reservoirs in the unsaturated zone is important for linking land surface processes and groundwater contamination. The purpose of this study was to characterize nitrate reservoirs beneath natural ecosystems and rainfed (nonirrigated) agricultural ecosystems using data from an area of high groundwater nitrate contamination (20% of wells > 10 mg/L nitrate-N) in the southern High Plains, Texas, USA as an example. Profiles were drilled beneath natural ecosystems (5), and rainfed (19) agricultural ecosystems in these regions. Previous studies have shown that conversion of land from natural grasslands to cropland increases recharge, displacing chloride bulges that previously accumulated under natural conditions, downward in the profile. Median nitrate-N for profiles beneath natural grassland and shrubland ecosystems is low (0.4 mg/kg dry sediment, range 0.1 – 0.6 mg/kg). Lack of nitrate accumulation under natural ecosystems in the southern High Plains contrasts with large nitrate inventories in the central High Plains and Nevada found in previous studies. Median nitrate-N in the shallow, chloride flushed portion of profiles beneath rainfed agriculture is moderately high (2.6 mg/kg) as a result of fertilizer application. Large nitrate-N inventories (median 10.6 mg/kg) in the transition from low to high chloride concentrations at depth is attributed to nitrate mineralization related to initiation of cultivation. Although nitrogen and oxygen isotopes of nitrate could not distinguish natural from fertilizer nitrate, the chloride profiles and associated age dating of soil pore water can be used to distinguish pre-anthropogenic natural nitrate from fertilizers. Nitrate associated with initial cultivation of soil should pass into the aquifer system as a pulse with a lag time in decades; however, over-application of fertilizers and leaching below the root zone will continue to provide a source of nitrate to the underlying aquifer. Unsaturated zone data are extremely useful in linking surface loading with groundwater nitrate levels and developing a comprehensive understanding of controls and timing of groundwater nitrate contamination.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1813 Eco-hydrology
DE: 1831 Groundwater quality
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting