Abstract
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.
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Acknowledgments
We thank Chris Browne, Dennis Colautti, Sonia Campagnolo, Graham Carlyle, Dan Fitzgerald, Daryl Leblance, Camelia Rusmir, Jason Shabaga, Greg Taylor, and Cherie Westbrook for field assistance and Shan Sanmugadas and Jackson Langat for laboratory assistance. Thanks are also due to Carolyn King in the York Geography department cartographic office for the figures. We are grateful to four anonymous reviewers for their helpful comments on the manuscript. Funding was provided by the National Sciences and Engineering Research Council of Canada operating grants to A.R. Hill and K.J. Devito.
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Hill, A.R., Devito, K.J. & Vidon, P.G. Long-term nitrate removal in a stream riparian zone. Biogeochemistry 121, 425–439 (2014). https://doi.org/10.1007/s10533-014-0010-2
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DOI: https://doi.org/10.1007/s10533-014-0010-2