Long-term nitrate removal in a stream riparian zone

The long-term capacity of riparian zones in regulating groundwater nitrate fluxes is not well understood. This study analyses patterns of nitrate removal for the period 1994-2012 at two sites in a river floodplain that have received high groundwater nitrate loading from a large upland aquifer for over 32 years. During the study, mean NO3 (-)-N concentrations entering the riparian zone varied between 20-30 and 30-42 mg/L at the upstream and downstream sites respectively, but did not show any clear inter-annual trend. A permeable sand layer in the riparian zone is underlain by a regional aquitard at a depth of 5-6 m and 4 m at the upstream site and downstream site respectively. Denitrification resulted in a decline in nitrate concentrations as lateral groundwater flow in the sand layer interacted with buried peat and channel bar deposits that range up to 3 m in depth at both riparian sites. This interaction was greater at the downstream site where the organic deposits extend down to < 1 m from the aquitard in some locations. At the upstream site nitrate removal efficiency in the sand layer, at depths of 3-4 m 20 m from the river bank, declined from 68 % in 1996-1998 to 42 % in 2009-2012. A smaller decline from 92 to 82 % occurred in the sand layer 10 m from the river bank during the study. In contrast, no clear pattern of change was evident at the downstream site where a nitrate removal efficiency of 98-100 % occurred at the river bank in most years between 1994 and 2012. These data suggest that the long-term nitrate removal performance of some riparian zones may decline if carbon availability for denitrification becomes limited as a result of variations in the quantity, quality and location of subsurface organic deposits that interact with deeper groundwater flowpaths.