Calibration of a new device to measure water content of rocks.

Large-scale problems of water quality and water availability require knowledge of water travel times through the unsaturated zone and substances carried with it. A number of methods and devices are available for measuring the water content in soils but usually they are not usable with the rocks too and very few are the attempts to verify their applicability. In applying the devices for water content estimation in rocks, the main difficulty is related to the installation of the probes. The installation procedure have to ensure a good contact between the rock and the sensor in order to reduce the uncertainty due to the gap effects. In this study a non-traditional method, known as electrical impedance spectrometry (EIS), was used. Using the new apparatus Z-meter, the electrical impedance in complex form and its changes were monitored in order to verify the suitability of the device with the rock. One kind of stainless steel probes have been installed into samples of calcarenite, a sedimentary rock coming from a quarry located in Canosa (South of Italy). The relationship between water content and resistance and between the reactance and the rock characteristics, such as texture, structure, etc., were investigated within a frequency range from 100Hz to 10000Hz, by using water with different electrical conductivity (EC) values. The experimentation was performed in environment with controlled temperature, due to the influence of temperature variation on all indirect moisture measurement methods. The experimental tests have been performed three times using exactly the same experimental set up, in order to check the repeatability of the data. The results allowed to define the calibration curves at different EC values for the calcarenite. This represent an important result because defines the possibility of using a new device, the Z-meter, to measure the rock water content by overcoming the issue of the gap effect that causes uncertainty of the measured values. This opens new scenarios linked to the possibility to monitor in situ flow and transport processes, that occur in the vadose zone constituted from rocks, measures rarely performed until now.