Tracer and nitrate movement to groundwater in the Northern Great Plains

An experiment was conducted to study the movement of surface-applied tracers and fertilizer to groundwater on an agricultural field under dryland farming practices. Movement of surface-applied bromide, chloride, and fluoride tracers, and fertilizer nitrate (NO₃) through a coarse-loamy glacial till soil and vadose zone to saturated glacial till at about 3 to 4 m, and to a confined aquifer at 6.8 m, were monitored for 5-yr. Crop treatments were a dryland rotation of wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], and sunflower (Helianthus annuus L.). Applied bromide, chloride, and No₃ could be detected in trace amounts at all levels, including the aquifer, within a few days after applications and following large summer storms. Within the root zone, NO₃ and chloride concentrations varied with root extraction and application placement. The largest stable concentration of solute occurred within the shadow vadose zone beneath the root zone. Below the root zone, tracer and NO₃ concentrations decreased exponentially with depth. Following a large rainfall the shallow vadose layer served as a 'feeder zone' for solute movement to deeper layers. Elevated concentrations following recharge events were spatially sporadic, occurring at some locations and not at others. Spatially variable and temporary increases in solute concentrations were caused by spatially variable recharge, governed by microtopographic controls. Water concentrated in microtopographical 'low' areas and moved in concentrated plugs to the water table carrying solute. NO₃-N did not increase in the saturated till or the aquifer over the 5-yr period.