Lateral Hyporheic Zone Chemistry in an Artificially Constructed Gravel Bar and a Re‐Meandered Stream Channel,Southern Ontario,Canada1

Abstract: The effect of stream restoration on hyporheic functions has been neglected, although channel rehabilitation projects have a potential to alter stream‐ground‐water interactions. The present study examined the effect of an artificially constructed gravel bar and re‐meandered stream channel on lateral hyporheic exchange flow and chemistry in two lowland N‐rich streams in southern Ontario, Canada. Nitrate concentrations were relatively high, ranging from 0.5 to 1.3 mg N/l in both streams during spring through fall months. However, nitrate concentrations showed a steep decline as stream water entered the gravel bar and the meander bends. Differences between observed and predicted nitrate concentrations based on conservative ion concentration patterns indicated that 40‐100 and 68‐98% of the nitrate entering the hyporheic zone was removed in the gravel bar and meanders, respectively. Rapid depletion of dissolved oxygen concentrations along lateral hyporheic flow paths and denitrifying potentials assayed by the acetylene block technique in hyporheic sediments suggests that denitrification was an important mechanism of nitrate depletion. Despite the high rate of nitrate removal, the flux of stream water laterally entering the constructed gravel bar and meander bends was very small, and hyporheic nitrate removal was <0.015% of the daily stream load during base‐flow periods in summer and fall. The effects of restoration projects on hyporheic zone dynamics are often limited in lowland streams by low channel gradients and fine floodplain sediments with low interstitial flows that restrict the magnitude of the stream‐hyporheic connection.