Surani Liyanage Thesis Proposal

Title: A laboratory investigation of the effect of weathering on electrical and hydraulic properties of granitic regolith.

Abstract: The critical zone (CZ) of the Earth is a dynamic region where various geological, hydrologic, and biological processes interact with each other, providing valuable resources to human society. In particular, the bedrock in the CZ is constantly transformed into regolith as a result of chemical and physical weathering, forming a permeable layer to support life on the Earth. The hydrological properties and processes within the CZ are complex and heterogeneous, and we still lack a full understanding of the water dynamics in the CZ. Recently, geoelectrical methods have been frequently used in CZ hydrology to characterize subsurface structures and to monitor water dynamics. The correct interpretation of these field geophysical data relies on a mechanistic understanding of the influences of weathering on the geophysical properties of CZ materials. In this study, the research focuses on how weathering affects the water retention curve, electrical resistivity, and induced polarization of granitic regolith. It is hypothesized that weathering-induced texture and mineralogy changes in the regolith control its water-holding capacity and electrical properties. With an increased degree of weathering, more fines (such as clays) are introduced, and the materials can hold more water due to the increased surface area. Similar, the increased surface area will also increase the electrical conductivity and induced polarization of the sample at a given moisture content. To test this hypothesis, it is proposed to collect granitic regolith samples from the field and measure their water retention and electrical properties in the laboratory. Preliminary work includes the collection of regolith samples from three soil pits in the Dry Creek Experimental Watershed, Idaho, and laboratory determination of the hydraulic and electrical properties of six samples. Analysis of these preliminary results shows that samples from different depths have different textures, water retention curves, and electrical properties, implying a strong influence of weathering. More sample collection and laboratory experiments are planned to generate more data that can be quantitatively analyzed to test the hypothesis. The scientific knowledge generated from this study will help better characterize the CZ subsurface structure and monitor various hydrologic processes with geoelectrical methods.

Advisor: Qifei Niu

Committee Members: Bhaskar Chittoori, Lee Liberty