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.

[1] We examined how landscape hydrogeologic characteristics influence groundwater nitrate removal by eight stream riparian sites on glacial till and outwash landscapes in southern Ontario, Canada. During high water table periods in 2000 -N input concentrations from adjacent cropland to the riparian sites ranged from 0.15 to 44.7 mg L À1 . Seven of the eight sites had a mean nitrate removal efficiency of >90%. This removal occurred within the first 15 m of the riparian zone at three sites with loamy sand and sandy loam soils overlying a shallow confining layer at 1-2 m. However, at four of five sites with more conductive sand and cobble sediments the width required for 90% nitrate removal varied from >25 m to a maximum of 176 m at a site with a confining layer at 6 m. Sites linked to an extensive thick (>6 m) upland aquifer with a slope gradient of >15% at the riparian perimeter had high nitrate inputs throughout the year and were large nitrate sinks. Sites with gentle topography (<4-5%) and <2 m of permeable sediments were minor nitrate sinks because of small nitrate inputs that were limited to the late autumn-spring period. A conceptual model linking landscape hydrogeologic characteristics to riparian zone nitrate removal capacity is developed to understand and predict the effectiveness of riparian buffers at the landscape scale.

No abstract

The efficiency with which riparian zones remove nitrate (NO−3) from contaminated ground water can vary with landscape setting. This study was conducted to determine the influence of flood plain geometry, lithology, hydrologic flow path, and nitrate transport on mechanisms of nitrate depletion of contaminated ground water. Patterns of NO−3−N, chloride, and dissolved organic carbon (DOC) concentrations and δ15N‐NO−3 and δ18O‐NO−3 values in combination with detailed piezometric head measurements were investigated in a river floodplain connected to a large upland sand aquifer in an agricultural region near Alliston, Ontario, Canada. Ground water discharging to the forested floodplain from the sand aquifer exhibited large spatial variability in NO−3−N concentrations (10–50 mg/L). The transport and depletion of NO−3 was strongly influenced by floodplain geometry and lithology. Little ground water flow occurred through the low‐conductivity matrix of peat in the floodplain. Plumes of NO−3‐rich ground water passed beneath the riparian wetland peat and flowed laterally in a 2‐ to 4‐m‐thick zone of permeable sands across the floodplain to the river. Analyses of the distribution of the NO−3−N concentrations, isotopes, and DOC within the floodplain indicate that denitrification occurred within the sand aquifer near the river where nitrate‐rich ground water interacted with buried channel sediments and surface water recharged from peat to the deeper sands. This study shows that the depth of permeable riparian sediments, ground water flow path, and the location of organic‐rich subsurface deposits may be more important than the width of vegetated strips in influencing the ability of riparian zones to remove nitrate.

No abstract