Laboratory Measurement of Electrical and Hydraulic Properties of Regolith Over Granitic Bedrock
Authors: Bienvenue Taylor James 
Published: November 2021

Characterizing water flux within the critical zone (CZ) is essential for a multitude of studies and applications related to irrigation,
drainage, water management, and contaminant transport. Trying to measure water flux in the critical zone, specifically in the subsurface, is
difficult due to the associated structural heterogeneity and complex interactions taking place between biological, chemical, and physical
processes. Current methods (i.e., inferred from soil suction and soil moisture measurements) to characterize water flux within the critical
zone can be time consuming and are not directly related to water flux. Recent literature has provided evidence that self-potential (SP) is a
promising tool to characterize water flux within the vadose zone. In hydrological settings, SP is the electric potential signal generated mainly
by water-flux within a porous medium through the electrokinetic phenomenon. By combining SP with more established methods, such as soil suction
measurements, it is possible to efficiently and completely describe water flux within the subsurface.

The use of SP for field CZ hydrological applications has been halted by a lack of understanding of the relationship between the hydraulic and
electrical properties of earth materials in the CZ. Thus, the aim of this study is to improve the measurement technique and understanding of the
electrical properties and hydraulic properties of CZ soils. Samples from the regolith overlying a granitic bedrock at the Treeline study site
within the greater Dry Creek Experimental Watershed (Boise, ID) are used in this study. The central hypothesis is that, in addition to texture,
mineralogical composition has a significant effect on the hydraulic and electrical properties of CZ soils. In order to evaluate this hypothesis,
a novel soil column system has been built to measure the electrical and hydrological properties of soils at both saturated and unsaturated
conditions. For saturated tests, the soil column system mirrors that of a constant-head setup for permeability tests. The soil column system includes
two newly designed hydrogeophysical probes that can make SP and pressure head measurements simultaneously. In this study, the design, fabrication,
and evaluation of the proposed experimental setup is presented. Moreover, the regolith samples, and a sand sample (for reference), are evaluated via
saturated testing (to measure the saturated soil properties) and unsaturated testing (to measure the unsaturated soil properties).

Experimental results demonstrate both the vertical heterogeneity in the regolith and the significant impact of the mineralogy (specifically clay
minerals) on both the electrical and hydrological properties of CZ soils. Within the subsurface of the CZ, clay mineral content tends to increase
with distance from the bedrock due to chemical weathering. The associated increase in clay minerals is matched by a proportional decrease in saturated
hydraulic conductivity, streaming potential coupling coefficient, and increase in surface conduction. Moreover, it was found that in addition to
permeability (textural influence), the zeta potential at the mineral-water interface (i.e., mineralogical influence) also controls the magnitude
of the streaming potential coupling coefficient, and therefore the SP response relative to changes in pore water pressure head.

The quality of experimental data suggests the proposed experimental setup, at its current state, is ideal for evaluating the electrical and
hydrological properties of coarse-grained soils. The measured response of the electrical and hydrological properties of the coarse-grained soils
were closely aligned with the theoretical expectations. The measured response for soils with a greater quantity of clay minerals identified some
limitations of the experimental setup. However, with some minor modifications, the experimental setup may become a preferred method for measuring
both clay-bearing soils and coarse-grained soils. Moreover, the results of the unsaturated tests produce unsaturated soil properties in line with
theoretical trends. The quality of parameters obtained from the unsaturated tests indicate the ability of the test setup to obtain useful information
regarding the unsaturated portion of the critical zone.