WHAT CAN ELECTROMAGNETIC METHODS TELL US ABOUT THE NESTED NATURE OF GROUNDWATER FLOW IN MOUNTAINOUS WATERSHEDS?

Characterizing groundwater flow in mountainous watersheds and its multiscale nature is critical for the sustainable management of water and energy resources. Traditionally, the interpretation of geophysical observations has focused on the characterization of subsurface architecture or the identification of solute migration patterns. However, it is reasonable to ask how much information about the nested nature of the flow field is captured in such observations and whether this information can be extracted and used to learn more about the flow system and its characteristic time scales. We designed a series of numerical experiments to address the following questions. (1) What is the effect of topography, geology, and weathering rates on the watersheds' groundwater flow and transport patterns? (2) Can electromagnetic geophysical observations detect these patterns? To explore these questions, we use a two-dimensional conceptualization of the mountain to valley transition and perform a qualitative comparison with observations in natural systems. The domain's permeability and porosity are assumed heterogeneous and anisotropic, and fault characterizes the transition from mountain to valley at the piedmont of the mountain range. The flow of water and transport of solutes and heat are fully coupled within a multiphysics framework. As expected, our simulations show a strong correlation between the magnitude and patterns of mountain block recharge with topography and geology. We observe a significant increase in groundwater ages and salinity as we move away from the mountain range, consistent with field observations. The bulk resistivity patterns allow us to separate fresh-to-slightly brackish groundwater transition, showing the deep groundwater flow system entering the lowland aquifer. Furthermore, using MARE2DEM, we find that the electromagnetic signals from a synthetic magnetotelluric observation network are sensitive to the changes in the flow field. This sensitivity serves as evidence that rigorous inversion techniques could isolate the effects of flow processes in these systems. Our results highlight the complex multiscale nature of mountain hydrology and the potential of electromagnetic geophysical tools to provide insights into the spatial variations of flow and its characteristic time scales.