Surface Water-Groundwater Exchange Dynamics in Buried-Valley Aquifer Systems

Groundwater is a primary source of drinking water worldwide, but excess nutrients and emerging contaminants could compromise groundwater quality and limit its usage as a drinking water source. As such contaminants become increasingly prevalent in the biosphere, a fundamental understanding of their fate and transport in groundwater systems is necessarily to implement successful remediation strategies. The dynamics of surface water-groundwater (hyporheic) exchange within a glacial, buried-valley aquifer systems are examined in the context of their implications for subsurface transport of nutrients and contaminants. High permeability facies act as preferential flow pathways which enhance nutrient and contaminant delivery, especially during storm events, but transport throughout the aquifer also depends on subsurface sedimentary architecture (e.g. interbedded high and low permeability facies). Sediment analyses reveal high stratigraphic heterogeneity, with cross-stratified open-framework gravel facies throughout the aquifer. Temperature and specific conductivity measurements indicate extensive hyporheic mixing near the river, but surface water influence was also observed far from the stream-aquifer interface. Measurements of river stage and hydraulic head indicate that significant flows during storms alter groundwater flow patterns, even between consecutive storm events, as riverbed conductivity and hydraulic connectivity between the river and aquifer change. Given the similar mass transport characteristics of buried-valley aquifers, these findings are likely representative of glacial aquifer systems worldwide. Our results suggest that water resources management decisions based on average (base) flow conditions may inaccurately represent the system being evaluated, and could reduce the effectiveness of remediation strategies for nutrients and emerging contaminants.