Groundwater and surface water are significant resources for rural water supplies, and certain agricultural practices may have substantial effects on these resources. An 11-yr study was started in 1989 near Oakes, ND that continuously monitored NO3–N concentrations in subsurface water of a field that was converted from dryland to center-pivot irrigation in 1989. The vadose zone was monitored with four disturbed and 16 undisturbed-profile lysimeters, and the groundwater of the surficial aquifer was monitored with 18 sets of nested wells, which sampled shallow, intermediate, and deep depths. The depth to water table of the surficial aquifer was approximately 3 m and the saturated thickness extended to a depth of 7 m. Also, NO3–N levels from two subsurface drains were monitored. The time series NO3–N concentration data from each of the monitoring locations exhibited the similar three-phase trend where NO3–N concentrations first increased, then decreased, and finally reached a steady-state level that was maintained. The first and second phases of this trend were shorter (∼3 yr total) for the lysimeters and increased as the depth of observation increased (5 and 8 yr total for shallow and intermediate wells, respectively). Also, the peak NO3–N concentration decreased as the observation went deeper into the profile (ranging from 150 mg L−1 in lysimeters, to 50 mg L−1 in shallow wells, and to 40 mg L−1 in intermediate wells). The NO3–N levels in the deep wells averaged 0.48 mg L−1, had a maximum of 1.59 mg L−1, and exhibited a slight increase through time. The subsurface drainage NO3–N levels were an average of 77% lower than the groundwater concentrations, which may have been caused by biotic and abiotic reduction. The increase in NO3–N concentrations in subsurface waters as a result of the initiation of irrigation can be partially explained by the residual N in the soil from dryland agriculture. As soil moisture increased, the availability and mobility of nitrogen increased, which attributed to the flush of NO3–N through the soil profile.

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