| The hydrological functioning and nitrate removal effectiveness of eight southern Ontario riparian zones developed on glacial till and outwash landscapes with contrasting hydrogeology was studied from March 2000 to September 2002. Riparian sites with more than 2 m of permeable sediments in the upland had a permanent hydrologic connection to the upland, whereas sites with less than 2 m of permeable sediments in the upland only received significant groundwater flow during the spring and autumn. As the upland aquifer size and slope gradient increased, the magnitude of flow entering the sites increased and the seasonality of inputs decreased. Seeps were often observed at sites with a concave topography and connected to large aquifers. Subsurface flow data indicated a consistent subsurface flowpath in areas of the riparian zones where the slope gradient was >5%, whereas significant changes in flow directions were observed in areas with flatter topography. Upland aquifer size, topography and riparian sediment stratigraphy also affected the nitrate removal capacity of the sites. With the exception of a site where a steep slope was connected to the stream, nitrate removal efficiency at the sites was >90%. Riparian zones more than 25 m wide were necessary to achieve a 90% nitrate removal at sites with gravel textured soils, whereas less than 20 m was enough at sites with sandy loam to loamy sand textured soils. Sites with a steep topography at the perimeter and linked to a thick upland aquifer (>6 m) with less than 6 m of permeable sediments in the riparian zone were generally large nitrate sinks within the landscape, whereas sites with gentle topography and a thin aquifer were only minor nitrate sinks. Spatial patterns of groundwater nitrate suggested that plant uptake was not a major nitrate removal mechanism at the sites, and acetylene block experiments and 15N-NO3- measurements indicated that denitrification occurred actively at all sites in spite of contrasting hydrogeology. However, slope gradient, soil texture and soil depth, by influencing hydrologic pathways that link supplies of electron donors and acceptors, influenced the location of hotspots of denitrification within the riparian areas.; A conceptual model linking topography, upland permeable sediment depth and riparian sediment stratigraphy to riparian hydrologic types and riparian nitrate removal effectiveness has been developed. Topographical, surficial geology and soil maps can help in determining the broad classes of riparian zone hydrological and biogeochemical functioning identified by the model. The conceptual model has been validated using independently published data and results suggest that this conceptual framework could be used for landscape management purposes in most temperate landscapes and that it could be adapted for contaminants other than nitrate. |
