HIGH-FREQUENCY MEASUREMENTS INDICATE EXCHANGES BETWEEN GROUNDWATER AND SURFACE WATER THAT ARE UNRELATED TO TRADITIONALLY MEASURED HYDRAULIC GRADIENTS (Invited Presentation)

Hydrogeologists typically assume exchange between groundwater and surface water is driven by hydraulic gradients and governed by hydraulic conductivity. Automated seepage meters with temporal resolution of seconds to minutes indicate flow across the sediment-water interface that is substantially more complex. Concurrently measured hydraulic gradients are often poorly related to automated seepage data, either because of insufficient frequency of gradient measurements or because other processes are controlling the exchange. Seiches and waves are an example of short-term seepage variability commonly detected by automated meters and commonly missed when hydraulic-head sensors are programmed to output data at 30-minute intervals or longer. A seepage bias unrelated to gradients can exist when upward flow buoys fine-grained sediment in the interstices of a coarser matrix, enhancing exchange rates, but downward flow pushes fines into intergranular traps, reducing flow. The largest non-gradient-based influences often are biological. Numerous species that live on or beneath the sediment-water interface alter the sediment structure, creating conduits for preferential flow. Some actively filter water and create substantial exchanges across the sediment-water interface. Biogeochemical processes can generate gas that accumulates and occludes pore spaces, reducing transmission of water; gas that continues to accumulate until it is released to the surface, either gradually or episodically, further changing hydraulic conductivity, sometimes abruptly. Devices that can measure fluxes at these time scales have thus far been rare. They reveal processes that have been little measured and are poorly understood, processes that could be very important when quantifying transmission of water, nutrients, and anthropogenic compounds between groundwater and surface water.