Evaluation of Four Vadose Zone Probes Used for Leak Detection and Monitoring

Gypsum and fiberglass resistance devices, a heat dissipation sensor, and a miniature resistivity probe were evaluated for vadose-zone monitoring. Tests were performed to determine how well the probes would detect changes in the moisture content of soil or in the chemistry of soil water. The probes were imbedded in a clayey soil. The electrical resistivity of gypsum blocks changed with water content over a broad range of water content in calibration cells. Gypsum blocks successfully indicated changes in water content in columns of soil that were wetted. Wet blocks tended to disintegrate when they were repeatedly removed from and inserted into the soil, but no disintegration of blocks left in unsaturated soil for up to 3 months was observed. The electrical resistance of fiberglass devices decreased with increasing water content and salinity of the soil water in calibration cells. The devices did not work well in tests in which the soil was wetted to near saturation — the readings of the fiberglass devices sometimes changed in the opposite sense to changes in the moisture content of the soil. Heat dissipation sensors provided useful information over a relatively wide range in moisture content, yielded stable readings over periods of several months, and did not appear to be particularly vulnerable to degradation in soil. Miniature electrical resistivity probes yielded stable readings over periods of many months. The probes were found to undergo particularly large changes in readings when the soil was wetted with a liquid having a high electrolyte concentration.