Nitrate Removal in Stream Riparian Zones

Abstract This review considers the role of stream riparian zones in regulating the transport of nitrate (NO − 3 ) in groundwater flow from uplands to streams. The current consensus is that most riparian zones effectively remove NO − 3 from subsurface water. However, research has not focused on the relationship between hydrology and chemistry within the context of the riparian zone hydrogeologic setting. Most riparian zones that remove NO − 3 occur in landscapes with impermeable layers near the ground surface. In this setting, small amounts of groundwater follow shallow horizontal flow paths that increase water residence time and contact with vegetation roots and organic‐rich riparian soils. Limited research suggests that riparian zones have less effect on NO − 3 transport in hydrogeologic settings where groundwater has little interaction with vegetation and sediments because flow occurs mainly across the surface, or at depth beneath the riparian zone before discharging to the stream. Considerable uncertainty surrounds the relative importance of vegetation uptake and microbial denitrification in NO − 3 removal from subsurface water in riparian zones. Plant NO − 3 uptake requires the presence of the root zone below the water table. Information is lacking on the vertical distribution and seasonal dynamics of fine root biomass in relation to water table fluctuations. High denitrification rates have been reported in 0 to 10 cm surface soils of riparian zones in the USA, France, and New Zealand. However, rapid NO − 3 removal from groundwater also occurs in riparian locations where the water table is always >0.5 m below the surface. Denitrification at depth within the saturated zone has been studied to a limited extent and has been found not to occur at some sites. An interdisciplinary approach in which patterns of NO − 3 depletion and the role of NO − 3 removal processes are related to groundwater flow paths is needed to provide a better understanding of NO − 3 regulation in riparian zones.