Geophysical Monitoring Probes and Their Applications to Some Hydrogeological Problems
Geophysical monitoring probes were constructed to monitor the vertical changes of water content and ion concentration in the subsurface. These devices contain electrodes for resistivity measurements and thermistors for temperature measurements. They are designed to monitor the resistivity and temperature of the saturated zone, the transition zone, and the vadose zone near a groundwater/surface water interface. Additional installations at the site include four monitoring wells and four Time Domain Reflectometry (TDR) access tubes, each paired with the geophysical probe installations. Pole-Pole resistivity measurements with an a-spacing of 5 cm are made using a CR 1000 Campbell Scientific, Inc. datalogger. Automated data collection occurs every four hours during normal (non-precipitating) conditions and in three-minute cycles during and through four hours following precipitation events. Manual data collection of water levels from the monitoring wells and TDR measurements from the TDR access tubes were made at irregular intervals.
The temperature measurements, water table measurements, and TDR measurements were to be used to correct, ground-truth, and calibrate the resistivity measurements. These objectives were made difficult by a heterogeneous vadose zone, combined with a marked drop in the top of the water-saturated zone, and difficulties with TDR installation. Numerous interesting transient and long-term changes of water content were observed despite these obstacles. Rapid data collection during and four hours following rain events gives a very intricate depiction of the infiltration of rain water through the vadose zone as well as the rise of the top of the water saturated zone. Precipitation events of greater than 10 mm significantly affect the position of the top of the water saturated zone and have been considered in detail to identify a rapid rise of water near the groundwater/surface water interface and a less rapid rise further from this interface. In addition to transient incidents, this system is ideal for long term monitoring effects. There is an indication of distinctly different infiltration patterns of precipitation and ion content of pore water over the autumn and winter months compared to the spring and summer months. This work has also identified improvements to be made to the monitoring system in the future.