A large borehole cavity was constructed to study the transport of agricultural chemicals through the vadose zone and to the shallow groundwater system. A metal culvert (3.05 m dia and 3.65 m long) was installed vertically in the excavated cavity to provide a permanent structure. Stainless steel suction lysimeters were installed radially from the cavity wall to collect water samples for chemical analysis from the vadose zone and the shallow groundwater system at five depths in the soil profile. A monitoring system consisting of electronic tensiometers and a data logger were also installed to measure soil water pressure head at three depths in the vadose zone.

A rainfall simulation experiment was conducted to test the performance of this large field monitoring system. Field data on the water and chemical transport through the vadose zone were collected during and after the rain simulation period. A rainfall of 127 mm was applied during a 7.5 h period over the area where suction lysimeters were installed at various depths. Chloride was applied with the rain water while bromide was applied to the soil surface (prior to rainfall) at a rate of 175 kg/ha. Metolachlor and metribuzin were applied at the experimental site three weeks prior to conducting this rainfall simulation experiment. Increased chloride concentrations were detected at 180 and 240 cm depths within three hours of initiating rainfall. Bromide was detected at 240 cm depth within three hours of initiating rainfall and increased from 0 to nearly 20 mg L–1 in about seven hours. Herbicide was detected at 120 cm depth in about an hour. Breakthrough curves for chloride and bromide indicated that preferential flow can cause rapid transport of agricultural chemicals to the shallow groundwater system. The results of this rainfall simulation study indicated that this field monitoring facility works well and has the potential for future field scale studies on chemical transport to shallow groundwater systems.