Can We Detect Regional to Local Groundwater Flow systems using Electromagnetic Methods?

Abstract:

Understanding regional groundwater flow systems (RGFS) and their characteristic temporal and spatial scales is fundamental for sustainable management of water and energy resources. We designed a series of numerical experiments to address three questions. (1) What are the roles of changes in topography, geology, and weathering rates on the dominant features of RGFS? (2) How deep is the active circulation zone were significant amounts of water, solutes, and energy are moving? And (3) can geophysical techniques detect these circulation patterns? Our modeling domain is two dimensional (10 km long by 5 km deep) with heterogeneous and anisotropic permeability and porosity variations that are consistent with empirical observations in regional systems. Flow is driven by topography (similar to the classic conceptual model introduced by Tóth), and the flow of water and transport of solutes and heat is modeled with a fully-coupled multiphysics approach and state-of-the-art equations of state. Our simulations show the accumulation of solutes and heat near stagnation points, which location is highly sensitive to topography and geology but relatively insensitive to weathering rates. This accumulation creates contrasting electrical resistivity patterns that separate local, intermediate, and regional flow paths, and therefore provide a tool to characterize the effective circulation depth within the RGFS. Our results highlight that regional flow paths have the potential to convey significant amounts of water, energy, and solutes through relatively impervious depths, informing our definition of circulation depth and offering a new perspective for a holistic definition of the critical zone. Furthermore, the high-contrast patterns of resistivity open the door to effectively using electromagnetic and inverse techniques to image and understand circulation in deep, crystalline basements.