EVALUATING THE ECOLOGICAL AND BIOGEOCHEMICAL IMPLICATIONS OF PREFERENTIAL FLOW PROCESSES AT THE GROUNDWATER/SURFACE WATER INTERFACE

In his commentary piece: “On the Effectiveness of Remediating Groundwater Contamination: Waiting for the Black Swan,” Don Siegel explicitly identifies the characterization of preferential flow process as a fundamental challenge of hydrogeological characterization and contaminant remediation. Preferential flow is a scale-invariant concept, impacting pore- to landscape-scale processes of water movement and exchange. We are working to advance geophysical and heat tracing methodology to identify and characterize preferential flow processes that are particularly influential to streambed redox zonation and surface-water ecology. At the centimeter-scale in bed sediments, we use bulk electrical measurements paired with co-located fluid sampling during salt tracer injections to quantify differential tracer loading. When bulk and fluid electrical conductivity are plotted together, the resulting hysteretic relation in the presence of less-mobile porosity can be used to quantify preferential-flow parameters. We have recently identified the geoelectrical signature of less-mobile porosity exchange in sandy bed sediments, even though all pores are apparently well-connected, due to effect of cobble inclusions on the advective flow field. In these glacial kettle pond sediments, less-mobile porosity exchange parameters are sensitive to flow rate and direction. At the stream reach to network scale, we are using infrared and fiber-optic distributed temperature sensing to locate preferential groundwater discharge points that can be incorporated into stream restoration design. The understanding of preferential brook trout spawning has also benefitted from heat tracing-based identification of discrete groundwater discharge. Although the complete cleanup of some contaminated aquifers may always remain a “Black Swan” event, we are making progress at the surface water/groundwater interface in evaluating ecologically important preferential flow processes.