CONSTRUCTION AND TESTING OF A FACILITY FOR MEASURING WATER AND AGROCHEMICAL TRANSPORT THROUGH THE VADOSE ZONE

A large borehole cavity was constructed to study the transport of agri cultural chemicals through the vadose zone and to the shallow groundwa ter 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 ca vity wall to collect water samples for chemical analysis from the vado se 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 rest the performance of this large field monitoring s ystem. Field data on the water and chemical transport through the vado se 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 whe re suction lysimeters were installed at various depths. Chloride was a pplied with the rain water while bromide was applied to the soil surfa ce (prior to rainfall) at a rate of 175 kg/ha. Metolachlor and metribu zin were applied at the experimental site three weeks prior to conduct ing this rainfall simulation experiment. Increased chloride concentrat ions were detected at 180 and 240 cm depths within three hours of init iating rainfall. Bromide was detected at 240 cm depth within three hou rs 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 prefe rential flow can cause rapid transport of agricultural chemicals to th e 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.