CAREER: Advancing the Mechanistic Understanding of Field-Scale Preferential Flow and Transport Processes in Soils using Geophysics

Clemson University, Clemson SC

Investigators

Abstract

CAREER: ADVANCING THE MECHANISTIC UNDERSTANDING OF FIELD-SCALE PREFERENTIAL FLOW AND TRANSPORT PROCESSES IN SOILS USING GEOPHYSICS Stephen Moysey Clemson University Preferential flow and transport processes cause water and contaminants to be isolated within distinct flow paths that can by-pass a large portion of a soil. Despite the universal occurrence of this phenomenon and its fundamental control on water and solute movement in the subsurface, there are few tools available to investigate the mechanisms leading to preferential flow at the field scale. As a result, there is a gap in knowledge regarding how interactions between different mechanisms causing preferential flow can alter observed flow behaviors, particularly as soil and rainfall conditions undergo seasonal changes throughout the year. This project will address the knowledge gap by testing the hypotheses that: (1) mechanisms controlling preferential flow and transport shift throughout the year due to seasonal dependence on antecedent water content and hydrologic forcing, and (2) ground penetrating radar (GPR) and electrical resistivity data can be used to detect preferential flow behaviors and discriminate the mechanistic causes of these observations. This research will be carried out by performing transient infiltration and steady-state tracer tests in two large (>32m3 volume) hydrogeophysical grid lysimeters under different water content and rainfall characteristics. The bottom face of each lysimeter is designed to collect distributed discharge of water and solutes from a grid of 1m x 1m outflow cells. One of the lysimeters will be constructed in the lab and filled with synthetic soil heterogeneities representative of conditions causing preferential flow. The design of this lysimeter will allow for evaluation of active preferential flow mechanisms based on patterns of the outflows. The second lysimeter will be located on a hillslope in the Clemson Experimental Forest. This lysimeter will represent natural soil conditions and capture seasonal effects of changing antecedent water content and variable rainfall characteristics. In each experiment, the lysimeters will be continuously monitored using time-lapse 3D GPR surveys and electrical resistivity measurements. The hydrologic and geophysical data from the lysimeter experiments will be analyzed using data reduction techniques that allow for quantitative comparison of the results for each set of control variables. As a result, the project will provide unique, quantitative insights into how the details of transient preferential flow and transport processes evolve in response to changing environmental conditions. Understanding preferential flow processes is of fundamental importance to society as it affects many critical issues in watersheds, including flooding, contaminant fate and transport, agriculture, infrastructure stability, and ecosystem health. For example, the contribution of preferential flow processes to ecosystem services to be worth is estimated to be over US$304 billion per year. The integration of novel geophysical imaging techniques with infiltration and transport studies will provide unique insights into the dynamics of preferential flow at the field scale ? including an improved understanding of seasonal controls on these processes. There is an urgent need for understanding these basic processes to enable prediction and informed adaptation to shifts in watershed behavior as climate change causes perturbations in traditional soil behaviors. The high-resolution data obtained in this study will allow for testing and refinement of existing conceptual models for representing preferential flow in soils under varying hydrologic conditions. The educational aspects of this project will help to introduce and prepare a new generation of hydrologists ? spanning middle school girls to working professionals ? to extract information from an increasingly overwhelming mass of available data that ranges from hydrologic repositories to high-resolution geophysical images. Specific educational activities include the involvement of undergraduate students in the project through the Creative Inquiry pedagogy at Clemson, a hands-on educational module on water resources to be delivered to eighth grade girls through Clemson?s Project WISE summer camp, and an annual continuing education workshop for professionals to increase the awareness and understanding of geophysical applications in hydrology. In all three of these activities, there will be an emphasis on using a new generation of methods for sensing the environment and mining of large data sets to advance our fundamental understanding of hydrologic processes at real field sites.