CAREER: Dynamic water table controls on reactive solute transport near the groundwater-surface water interface

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Water table fluctuations influence the transport of nutrients and contaminants between soils, groundwater, and surface water. The water table moves up and down over short and long timespans, particularly near rivers. Water table fluctuations introduce oxygen to shallow groundwater, which can react with ecologically important dissolved substances like organic carbon, nitrate, iron and manganese and have cascading effects on contaminants like arsenic. This project aims to understand how groundwater chemistry evolves in response to water table fluctuations over a range of timescales. Field observations, controlled laboratory column experiments, and computer models will quantify the movement of reactive solutes such as oxygen, nitrate, iron, and manganese beneath a fluctuating water table. Results will improve our understanding of dynamic changes in both groundwater and river water quality and the management of these vital water resources. This project will also introduce a broad audience of citizens and future scientists to dynamic surface water-groundwater interactions through the creation of a hands-on outdoor laboratory facility and museum exhibit. Water table fluctuations play an important role in reactive solute transport and water quality near rivers. When the water table rises, entrapped soil air exchanges oxygen with surrounding pore water, which travels downward when the water table falls. The enhanced supply of oxygen stimulates aerobic respiration, nitrification-denitrification, and oxidation of dissolved iron and manganese in groundwater. Water table fluctuations can propagate tens to hundreds of meters into an aquifer in response to changes in river stage. Yet the implications for reactive transport and water quality near rivers remain poorly understood. The overarching goal of this project is to quantify relationships between water table fluctuations, redox transformations, and solute export from aquifers to rivers. This project leverages novel sensor technology to observe the effects of water table fluctuations on groundwater chemistry in controlled column experiments and natural riparian aquifers. The four field sites in this study experience a range of water table fluctuations in response to tides, storms, and seasons. Each site has unique properties that influence reactive transport near the water table. However, pairs of sites also have commonalities in climate, lithology, catchment scale, land use, and geomorphology. Numerical fluid flow and reactive transport models will quantify dynamics in biogeochemical transformation rates and solute fluxes in these riparian aquifers. Observations and model results will be synthesized to generate a quantitative understanding of the effects of water table fluctuations on reactive solute transport in dynamic riparian aquifers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.