THE IMPORTANCE OF TEMPORAL VARIABILITY OF GROUNDWATER-SURFACE-WATER EXCHANGE ON CONTAMINANT MASS LOADING: FIELD EXAMPLES OF PHOSPHORUS, SELENIUM AND COPPER DISCHARGE TO SURFACE WATER

Mass loading of constituents passing through the sediment-water interface commonly is determined based on pore-water concentration multiplied by a volumetric flux rate. Although both concentration and water flux rate can vary over time, water fluxes usually are assumed to be constant or to change slowly or by small amounts. Advances in seepage-meter technology provide the ability to measure flow between groundwater and surface water over periods of a minute or less for durations of days or weeks or more. Three case studies using automated seepage meters demonstrate substantial temporal variability in response to precipitation, wind, changing streamflow, and evapotranspiration. At Ashumet Pond, MA, where a phosphorus plume is discharging to a lake, large rates of groundwater discharge became even larger following a small rainfall event. At Great Salt Lake, UT, where selenium from nearby wetlands may be discharging to the lake, a typical thunderstorm caused a lake seiche that resulted in seepage rates increasing by an order of magnitude. At the Columbia River, WA, copper-rich riverbed sediments were suspected of posing a health risk to organisms living at or near the sediment-water interface. Seepage measured during the day was downward, from the river to the aquifer, thereby potentially minimizing exposure to copper. Nighttime measurements from automated sensors indicated hydraulic gradient and seepage both reversed in response to reduced discharge from an up-river power-production dam. The potential for nightly exposure to copper at the sediment-water interface would have gone undetected based on daytime manual measurements of seepage. For all three of these examples, times of maximum mass loading of contaminants to surface water may be missed, or total mass fluxes underestimated, if temporal variability is not considered or determined.