BIOLOGICAL INFLUENCES ON FLOW BETWEEN GROUNDWATER AND SURFACE WATER

Hydrogeologists commonly determine flow across the sediment-water interface by measuring vertical hydraulic gradient and estimating vertical hydraulic conductivity. However, direct measurements of exchange between groundwater and surface water indicate that biological processes can create flows of water and gas that are much faster than gradient-driven exchange. Biological influences are broad and range from near-shore transpiration of groundwater, to filtering animals above, on and in the sediment bed that create easily measurable seepage rates, to decomposers that generate gas that produces constant or episodic ebullition. High-temporal-resolution seepage measurements indicate the durations over which these biological influences occur and record their relative rates of induced seepage. Fast upward seepage averaging 21.6 cm/d at the shoreline of a Minnesota lake varied diurnally by 7.0 cm/d as plants growing on an adjacent peat surface intercepted groundwater that would have discharged to the lake. No concomitant changes in vertical hydraulic gradient were measured at these locations. Shrimp present on or near the sediment bed created seepage in the opposite direction of that indicated by adjacent seepage meters in Puget Sound. At an estuary in New York City, a different type of shrimp created highly variable seepage rates that averaged 20 times faster than other measurements made in the same location. A few hundred meters away, where a greater organic fraction was present in the sediments, more than 3 L of gas was released during seepage measurements that spanned low tide. Timing of gas release was determined from erratic seepage pulses created as gas displaced water that exited through the flowmeter. Assuming all gas was released during the 4.25 hours of highly variable seepage surrounding low tide, the inferred seepage rate caused by ebullition was 2.0 cm/d, four times larger than the average seepage measured at that location.