Pre- and postrestoration assessment of stream water-groundwater interactions: effects on hydrological and chemical heterogeneity in the hyporheic zone

Reach-scale stream restoration with natural channel design is often used to improve stream ecosystem structure and function. Some investigators have studied the effects of restoration on the hyporheic zone, but most used space-for-time substitution instead of comparing the same reach before and after restoration. We examined spatial patterns of hyporheic exchange rates and geochemistry during base flow in a 30-m pool-riffle-pool sequence before and 1 y after the stream was restored by installation of a cross-vane and engineered rock-riffle. Prerestoration vertical hyporheic exchange rates were relatively uniform across the riffle bedform. Average downwelling was 30 cm/d at or upstream of the riffle, and average upwelling was 30 cm/d downstream of the riffle. Downwelling hyporheic exchange rates increased up to an order of magnitude adjacent to the cross-vane and in the engineered rock-riffle. Prerestoration porewater [NO3-] was distributed along a wide and continuous gradient (0.1-3.8 mg/L), with higher concentrations in areas of downwelling and lower concentrations in areas of upwelling. Postrestoration [NO3-] was distributed bimodally with relatively high concentrations (>1.6 mg/L) adjacent to the cross-vane and low concentrations (<1 mg/L) elsewhere. These results suggest that rapid flushing of surface water through the subsurface creates residence times too short to yield net changes in [NO3-] along flow paths around the cross-vane. Distinct zones of groundwater upwelling were present before and after restoration, but the size of the zone of upwelling increased downstream of the cross-vane after restoration, possibly because higher permeability material was placed over the original, lower permeability sediment during restoration, creating an effective anisotropy that favored horizontal flow and produced larger apparent zones of groundwater discharge.