Impact of microforms on nitrate transport at the groundwater–surface water interface in gaining streams

• Microforms could load more NO3- to a river compared with a macroform-only condition.
• Microforms affect NO3- flux by introducing more mass flux and reducing residence time.
• Microforms affect the spatial distribution of solutes in the shallow subsurface.
• Microforms induced pathlines could remain oxic or switch from NO3- producing to consuming.

Small streambed structures (or microforms, 0.01–1 m in length) exist ubiquitously in riverbed systems. Small-scale topography is potentially important in controlling hyporheic exchange flow and transport of conservative and reactive solutes at the groundwater–surface water interface. The role of microforms on NO3- transfer in a riffle-scale (macroforms of >1 m length) hyporheic zone within a gaining river setting is investigated using a 2-D flow and transport model which accounts for both nitrification and denitrification. Results show that the short pathlines caused by microforms lead to more NO3- discharge to the river compared with a macroform-only condition due to shortened residence times of both surface water and groundwater in mixing zones. Short hyporheic exchange flow pathways caused by microforms could remain oxic along their entire length or switch from nitrate producing to nitrate consuming as oxygen concentrations decline. Microforms affect net NO3- flux by the combined effect of introducing more stream mass flux and reducing their residence time in mixing zones under different hydrological and biogeochemical conditions. Our findings underscore that ignoring microforms in river beds may underestimate NO3- load to the river and have practical implications for pore water sampling strategies in groundwater–surface water studies.