Title: A laboratory investigation of the dominant controls on hydrologic and electrical properties of granitic regolith

Prorgam: Geophysics MS

Committee Chair: Qifei Niu

Committee: Qifei Niu, Jennifer Pierce, Lee Liberty, Bhaskar Chittoori

Abstract: The Critical Zone (CZ) of Earth is a dynamic region where various geological,
hydrologic, and biological processes interact with each other. The bedrock in the CZ is
constantly transformed into regolith as a result of chemical and physical weathering. This
creates the porous regolith layer that supports life on Earth. Understanding how
weathering alters regolith properties is essential for interpreting subsurface processes
such as water retention. Recently, geoelectrical methods have been increasingly used in
CZ hydrology to characterize subsurface structure and to monitor water dynamics in CZ.
However, to interpret these field geoelectrical data accurately, one needs to know how
weathering affects the geophysical properties of CZ materials. This research focuses on
how weathering changes basic soil properties in granitic regolith and how those changes
affect its hydraulic and electrical properties.
This study collected regolith samples from different depths along a climatic
gradient within the Dry Creek Experimental Watershed (DCEW), Idaho. Various
laboratory analyses were performed to determine mineralogy, texture, surface area,
surface chemistry, hydraulic properties, and electrical properties. The hydraulic and
electrical data were modeled using the van Genuchten and Waxman-Smits equations
respectively. Model parameters were then correlated with the measured physical and
chemical properties of regolith to determine any relations.
The results demonstrate that weathering transforms primary minerals (such as
feldspar) into secondary clays and alters basic soil properties such as mineralogy, texture,
vi
and surface chemistry. These changes influence hydrological and geophysical properties
of the weathered regolith. Soil hydraulic behavior is best captured by the van Genuchten
model. According to the results, fitting parameter n ¯ (pore-size distribution index/ shape
parameter) is controlled by the uniformity coefficient (Cu), the inverse air-entry pressure
(a) is best predicted by effective grain size (D10), and the residual water content (?r) is
most strongly associated with clay content. The complex electrical behavior of regolith is
captured by the Waxman-Smits model. According to the results, the saturation exponent
(n) is linked to Cu. The real surface conductivity (s's) and imaginary surface conductivity
(s"s ) are strongly controlled by the cation exchange capacity (CEC). Together, these
results show how weathering influences the relationships between basic soil properties
and hydraulic and electrical properties of granitic regolith within the CZ.