Integrated monitoring technologies for the estimation of water content in unsaturated sandy soils

The evaluation of water content of fine-grained unsaturated soils is important in the hydrogeological investigation because it play a key role in the recharge of aquifers, in the characterization and remediation of contaminated lands, etc. Depending on the observation scale and on the aim of investigation, different techniques may be used to evaluate the soil moisture in the laboratory and in the field. These include gravimetric direct measurement (θg) and volumetric indirect measurement (θv). It is generally known that there is a growing interest in the estimation of θv, so that much equipment for assessing this quantity in an indirect way have been developed. Although the indirect techniques are less expensive, and less time consuming than laboratory gravimetric direct measurements, results are deeply affected by the calibration of equipment. The present work has taken as reference the Frequency Domain Reflectometry (FDR) using a probe, PR2 Delta-T Device, that has been acquired thanks to the “Ricerca di base 2014 Project – DIMBASE14”. The probe is constituted by six pairs of sensors, which allow the estimation of water content at different depths: 10, 20, 30, 40, 60 and 100 cm. In order to achieve reliable measures, the probe was calibrated at laboratory scale on two sandy soils widely outcropping in Central Italy: a limestone soil from “Conca di Terni” and a flyschoid sand from the Tiber River alluvial plane (Perugia). Several tests have been carried out at different dry density values belonging to the dry side of the Proctor curves. By using gravimetric water content and dry density obtained on laboratory samples, the volumetric water content was calculated and plotted vs. the permittivity measured by FDR probe. Results - coupled with calibration obtained by other Authors in the literature - allowed to obtain a general equation valid to estimate the water content on sandy soils of different composition and grain size distribution. After the calibration, the infiltration process was investigated through a five-layered soil column with 50 cm diameter and 110 cm height. We also show results discussed by checking changes of degree of saturation and the influence of entrapped air along the column.