Water quality was measured in the vadose zone and groundwater on a newly irrigated field in Southeastern North Dakota from 1990-1995. The study site was 64 ha of which 53 ha were irrigated starting in 1989. Vadose zone water quality was monitored with 20 large undisturbed soil core lysimeters and four reconstructed profile lysimeters. Groundwater quality was measured at shallow oxidized, shallow unoxidized, and deep (unoxidized) levels below the water table with 18 sets of nested wells. Subsurface drainage quality was also measured at two access manholes. Analysis for nitrate-N, nitrite-N, ammonium-N, phosphate-P, sulfate, chloride, calcium carbonate, calcium, magnesium, sodium, and potassium was performed on water samples from the lysimeters, wells, and subsurface drains. Nitrate-N varied the most throughout the study compared to the other ions. Average vadose zone nitrate-N concentrations peaked in 1990. The peak nitrate-N concentration was observed later and with lower concentrations at deeper points in the groundwater. The change in nitrate-N through depth and time was believed to be caused by advective-dispersion and denitrification, as evidenced by other ion observations. Denitrification rates were calculated by comparing nitrate-N mass loss through depth. The resulting rates (mean rates = 0.012-0.116 mg/L/d) were consistent with values reported for aquifers in the upper Midwest.