IMPORTANCE OF THE HYPORHEIC ZONE IN MODIFYING GROUNDWATER CHEMICAL REACTIONS DURING DISCHARGE TO SURFACE WATERS

Groundwater discharging to or recharging from surface waters is rarely isolated from hyporheic flow. Instead groundwater and hyporheic flow typically co-occur just beneath the sediment interface, with the hyporheic flux component of the total streambed flux often exceeding the groundwater component. Although groundwater discharge or recharge may compress the size of the hyporheic zone, typically the groundwater flux must far exceed the hyporheic flux before the magnitude of the hyporhec flux is affected. Groundwater usually has a distinct physicochemistry characterized by constant temperature, no light, and relatively low oxygen which differs greatly from relatively well oxygenated hyporheic flow with variable temperature derived from surface water entering hyporheic flow paths. Mixing between groundwater and surface water in the hyporheic zone can enhance carbon, nutrient, and contaminant reactions before groundwater discharges to surface water. Chemical reactions in streambed sediment also may continue after groundwater discharges as a result of repeated exchanges between surface water and the hyporheic zone. An example illustrates how high metal concentrations in groundwater affected by acidic mine drainage may be only modestly attenuated as groundwater approaches discharge zones but is more substantially attenuated by chemical reactions in hyporheic flow paths. A contrasting example illustrates nitrate attenuation in an agricultural setting. I illustrate the use of dimensionless factors that combine physical and biogeochemical factors for characterizing the relative roles of groundwater and hyporheic-zone influences on chemical transformations in watersheds.