Abstract
The effect of preferential flow in soil pipes on nitrate retention in riparian zones is poorly understood. The characteristics of soil pipes and their influence on patterns of groundwater transport and nitrate dynamics were studied along four transects in a 1β to >3βm deep layer of peat and marl overlying an oxic sand aquifer in a riparian zone in southern Ontario, Canada. The peatβmarl deposit, which consisted of several horizontal layers with large differences in bulk density, contained soil pipes that were generally 0.1 to 0.2βm in diameter and often extended vertically for 1 to >2βm. Springs that produced overland flow across the riparian area occurred at some sites where pipes extended to the peat surface. Concentrations of NO~3~^β^βN (20β30βmgβL^β1^) and dissolved oxygen (DO) (4β6βmgβL^β1^) observed in peat pipe systems and surface springs were similar to values in the underlying sand aquifer, indicating that preferential flow transported groundwater with limited nitrate depletion. Low NO~3~^β^βN concentrations of <5βmgβL^β1^ and enriched Ξ΄^15^N values indicated that denitrification was restricted to small areas of the peat where pipes were absent. Groundwater DO concentrations declined rapidly to <2βmgβL^β1^ in the peat matrix adjacent to pipes, whereas high NO~3~^β^βN concentrations of >15βmgβL^β1^ extended over a larger zone. Low dissolved organic carbon values at these locations suggest that supplies of organic carbon were not sufficient to support high rates of denitrification, despite low DO conditions. These data indicate that it is important to develop a greater understanding of pipes in peat deposits, which function as sites where the transport of large fluxes of water with low biogeochemical reaction rates can limit the nitrate removal capacity of riparian zones. Copyright Β© 2011 John Wiley & Sons, Ltd.