An automatic sampling system to monitor perched water in the vadose zone

Increasing concern about ground water movement and solute transport in the vadose zone requires further development of monitoring systems to characterize subsurface hydrology. An interdisciplinary team of researchers at The University of Tennessee has developed a research facility on a 994-ha agricultural watershed in West Tennessee. Since the fall of 1990 a 10-ha test site of the watershed has been in conservation tillage. A square 0.4-ha test plot has been established within the 10-ha site. A monitoring system is installed in, and surrounding, the field-test plot to characterize surface and subsurface hydrology. Shallow wells are installed with a screen at the loess-paleosol interface to collect perched water caused by differences in the physical characteristics of the soil layers. Shallow wells were installed, in 1992, 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 (Yoder et al., 1994).

On March 9,1993, a bromide tracer was surface applied to the plot at the rate of 300 kg/ha. Monitoring of the bromide movement began immediately after the application and is continuing (Yoder et al., 1994). Based on the spatial distribution of the shallow wells that have yielded bromide, 17 additional shallow wells were installed in April 1995 to more accurately characterize the loess-paleosol interface and subsurface hydrology. These additional shallow wells were installed on the west side of the 0.4-ha test plot. An automatic sampling system was installed to collect samples from 21 shallow wells, including the 17 additional shallow wells. The system is divided into three separate sampling systems, each monitoring seven shallow wells.

The primary components of each sampling system include a microcontroller card with 32K of memory and 24 input/output lines, liquid level switches, flow sensors, isonic valves, two vacuum tanks, a 12-V battery, and a solar panel. The automatic sampling system records the time each sample is taken.

Several operational problems during the first six months of operation and a generally dry season resulted in a limited amount of data being acquired. The data suggest the new shallow wells on the 45-m radius are approaching the spatial resolution necessary to define possible flow paths of lateral subsurface flow at the interface.