Automatic sampling of perched water from vadose zone shallow wells

Since the late 1970s there has been increased interest in understanding groundwater movement and solute transport in the vadose zone. While it is widely recognized that the potential for significant lateral flow exists as water perches above impeding layers in the vadose zone, conducting meaningful research on this phenomenon requires control of land management over large areas. A 7500-ha holding available to the Tennessee Agricultural Experiment Station at the Ames Plantation in West Tennessee provides such an opportunity for extensive vadose zone monitoring over large areas. In 1991 an interdisciplinary team of researchers at The University of Tennessee began developing a research facility in a 994-ha agricultural watershed to monitor off-site movement of agricultural chemicals. A 10-ha no-tillage test site was equipped with a centrally located 0.4-ha test plot and a monitoring system to characterize surface and subsurface hydrology. Shallow wells to collect perched water with screens at the loess-paleosol interface (1.8 to 2.8 m beneath the soil surface), were installed at 23 locations on concentric circles with radii of 45, 60, 90, and 150 m from the center of the 0.4-ha test plot. Potassium bromide was surface applied on 9 March 1993 to the 0.4-ha test plot at the rate of 300 kg/ha of Br-. Data from shallow wells at the Ames Plantation Water Quality Project indicated extensive lateral movement of water in the vadose zone. Based on the spatial distribution of the shallow wells that yielded bromide, 17 additional shallow wells were installed in March 1995 to more accurately characterize water movement at the loess-paleosol interface. An automatic sampling system was installed to collect samples from the 17 new wells as well as from four of the original wells. The sampling system uses a liquid level switch and a datalogger to open a solenoid valve allowing vacuum from supply tanks to extract the sample from the well into sample collection jugs and to record the time the samples are extracted. The sampling system is working well and is easily maintained by the field technician.