Bank storage in karst aquifers: The impact of temporary intrusion of river water on carbonate dissolution and trace metal mobility

Abstract

Storms can trigger changes in river stage that alter the hydraulic gradients between rivers and adjacent aquifers. In eogenetic carbonate karst aquifer systems, storms enable river water to intrude >1km into the adjacent aquifer systems for days to weeks. This process is similar to bank storage of streams in siliciclastic sediments but can have longer temporal and larger spatial scales. River intrusion triggers changes in mineral saturation states and redox conditions in the aquifer due to the input of low pH, low specific conductivity (SpC), and high dissolved organic carbon (DOC) flood water. To assess the effects of river intrusion into karst aquifers, we measured SpC, temperature, pH, redox state, and concentrations of dissolved major and trace elements through an intrusion event at Madison Blue Spring in northern Florida, USA. River water displaced groundwater in the conduit at least 1km into the aquifer and flowed into the pores of the unconfined aquifer matrix. Distinct Cl− concentrations between river water and groundwater provide estimates for mixing fractions. The location and magnitude of oxidation of organic matter in the subsurface controlled trace metal concentration, redox state and saturation state of the water with respect to calcite (SIcal). Organic matter oxidation in the phreatic conduits was limited by the terminal electron acceptors (TEAs) present in the conduit water. Calcite dissolution and trace metal sorption were limited by the lower surface area to porosity ratios in the conduits than the matrix. Organic matter oxidation was enhanced in the matrix by mixing with matrix waters with available DO and NO3−, resulting in greater CO2 production and calcite dissolution than in the conduit. After the intruded river water discharged, conditions remained more reducing in the aquifer than baseflow conditions due to the reduction of DO and NO3− in the matrix water. The organic matter transported into the aquifer during river intrusion drives carbonate dissolution, alters redox state, and impacts trace metal mobility, impacting groundwater and surface water quality.