Denitrification and patterns of electron donors and acceptors in eight riparian zones with contrasting hydrogeology

Abstract

A better understanding of nitrate removal mechanisms is important for managing the water quality function of stream riparian zones. We examined the linkages between hydrologic flow paths, patterns of electron donors and acceptors and the importance of denitrification as a nitrate removal mechanism in eight riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10–30 mg L−1 near the field-riparian edge to < 1 mg L−1 in the riparian zones throughout the year. Chloride data suggest that dilution cannot account for most of this nitrate decline. Despite contrasting hydrogeologic settings, these riparian zones displayed a well-organized pattern of electron donors and acceptors that resulted from the transport of oxic nitrate-rich groundwater to portions of the riparian zones where low DO concentrations and an increase in DOC concentrations were encountered. The natural abundances of d15N and in situ acetylene injection to piezometers indicate that denitrification is the primary mechanism of nitrate removal in all of the riparian zones. Our data indicate that effective nitrate removal by denitrification occurs in riparian zones with hydric soils as well as in non-hydric riparian zones and that a shallow water table is not always necessary for efficient nitrate removal by denitrification. The location of ‘hot spots’ of denitrification within riparian areas can be explained by the influence of key landscape variables such as slope, sediment texture and depth of confining layers on hydrologic pathways that link supplies of electron donors and acceptors.