D2 Soil and Subsoil Characterization

Topic

Hydraulic characterization of the soil and subsoil aims to determine the properties governing how water is stored and moves within the ground: soil moisture content; the matric potential that retains water within soil pores; hydraulic conductivity, which dictates the rate at which water moves through the soil; and the water table, which marks the beginning of the saturated zone. It is precisely this characterization process that renders these four properties—previously invisible underground—measurable in the field.

Soil moisture content is measured using a technique based on time-domain reflectometry, which involves measuring the soil's apparent dielectric constant. This constant—which depends heavily on the amount of water in the soil—allows the propagation speed of an electromagnetic signal through the soil to be directly converted into the desired moisture content value.

Matric potential, meanwhile, is measured directly using tensiometers: a porous ceramic tube is placed in contact with the soil, and the suction the soil exerts on the water inside the tube generates a negative pressure reading within the tensiometer itself, equivalent to the matric potential of the surrounding soil.

Saturated hydraulic conductivity is determined in the field using permeameters—either constant-head or falling-head types—by applying Darcy's Law; this allows the measured water flow through a soil sample to be directly related to the pressure gradient driving that flow.

Pumping tests conducted in wells allow for the determination of two aquifer properties: transmissivity and the storage coefficient. By fitting the water-table drawdown curve—recorded during pumping—to established analytical solutions (such as the Theis solution), it becomes possible to derive both properties from a single pumping test. Finally, electrical resistivity tomography maps the spatial distribution of apparent resistivity throughout the subsurface—a resistivity that varies with both water content and salinity; it is precisely this variation in resistivity that enables the generation of two-dimensional subsurface cross-sections without the need for drilling.

Thus, ranging from the moisture content and matric potential with which this document begins to the two-dimensional resistivity cross-sections with which it concludes—and encompassing saturated hydraulic conductivity as well as aquifer transmissivity and storage coefficient—the hydraulic characterization of soil and subsurface allows the invisible properties governing groundwater to finally be measured, quantified, and compared across different locations.

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