From Mountains to Bedforms: Multiscale Groundwater Dynamics and its Influence on Solute and Energy Fate and Transport

Abstract

The emergence of multiscale, nested flow systems is intrinsic to hydrologic systems across landscapes. This complex nesting and its implications for the flow of water and the transport of solutes and energy are the common threads of this dissertation.

First, we explore the role of deep Regional Groundwater Flow (RGF) in mountainous terrains and their importance for the lowlands’ overall water, energy, and solute budgets. To this end, we implemented flow and transport models for a series of synthetic mountain-to-valley transition systems and the Tularosa Basin, NM. These models underscore the critical role of topography and geology in the RGF and the spatial patterns of solutes and energy, resulting in unique patterns of subsurface electrical resistivity and constraining our ability to image the subsurface with electromagnetic geophysical methods. Our analyses assess the potential of magnetotelluric surveys to map the nested nature of mountain groundwater flow and provide vital information to characterize unconventional groundwater resources.

The second part of the dissertation explores the role of meanders as natural biogeochemical reactors along river corridors. We used groundwater flow and transport models to assess the role of the meander’s geometry and topology and the RGF in the hydrodynamics and denitrification potential of the sinuosity-driven hyporheic zone. Our results show that a narrow meander neck shields the hyporheic zone from the modulating effects of RGF. Moreover, this model allows us to explore when a meander acts as a net nitrogen source or sink by using a handful of dimensionless physical and biogeochemical parameters. Finally, to upscale these analyses from individual reaches to watersheds or continents, we developed a novel Python package that enables the supervised and unsupervised identification of meandering features along river networks using a spectral decomposition approach. These efforts pave the way for more accurate quantification of sinuosity-driven hyporheic exchange and provide critical information for river restoration strategies.