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SIMULATION OF ONE-DIMENSIONAL NITRATE TRANSPORT THROUGH SOIL AND CONCOMITANT NITRATE DIMINUTION.

SADEGHI, ALI M.; KUNISHI, HARRY M.

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Abstract

A small core of intact soil provided information about the lateral transport and diminution of nitrate in the presence and absence of a carbon source capable of supporting microbial growth. The core (6.2 cm in diameter by 15 cm in length) was obtained by pushing a plastic cylinder horizontally into the wall of a trench excavated in a riparian zone where water moves laterally. Under conditions favorable for nitrate diminution, pulses of nitrate solution containing carbon were passed through the core and leached with water. Effluents were collected at fixed intervals and analyzed. Nitrate losses ranged from 4% to 72%, presumably via denitrification. Effluent nitrate concentrations and pore water velocities were entered into a model developed by Parker and van Genuchten for one-dimensional convective-dispersive solute transport and for solute decay and production. The model accounted for the experimentally determined transport and loss of nitrate (r2 = 0.92-0.98). The model also provided values for dispersion coefficient, nitrate exclusion factor, and nitrate decay rate constant. Nitrate loss was reasonably accounted for by a decay rate constant (avg. = 2.75/day) that increased as observed nitrate loss increased from 53% to 72%. A sizable factor for anion exclusion and/or the presence of immobile-water regions was necessary to account for the rapid passage of nitrate through the soil core. Peclet numbers, which are inversely related to dispersion coefficient, were low. These low numbers and a retardation factor of less than 1 suggested that the flow through the soil core may have been heterogeneous.

(C) Williams & Wilkins 1991. All Rights Reserved.

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