Change in Groundwater Nitrate Concentration in a Large River Floodplain: Denitrification, Uptake, or Mixing?

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We examined the buffering capacities of different riparian vegetation (natural riparian forest, 3- and 15-y-old poplar plantations, and a wet meadow) on non-point-source nitrogen pollution along a 7th-order reach of the Garonne River in southwest France. Groundwater nitrate concentration was measured monthly for 1.5 y in 51 wells installed within the aquifer of the river floodplain and in River Garonne water. The mixing of river water and groundwater was estimated using a 2-end-member model based on measured concentrations of chloride. Nitrate concentrations in groundwater decreased significantly along groundwater flow paths crossing the riparian zones. Mixing of nitrate-rich groundwater with nitrate-poor river water accounted for most of the change in nitrate concentration along groundwater flow paths. The fraction of river water in wells increased from the margin of the floodplain with an alluvial terrace (31% river water) to the river; an average of 80% river water occured in the natural riparian forest. However, observed concentrations of nitrate were always less than or equal to the concentration expected from mixing alone, indicating biological effects. Denitrification rates were low or 0 below the depth of the water table in all seasons, suggesting that denitrification was not an important nitrate sink. The remaining change in nitrate concentration not accounted for by mixing was likely the result of vegetation uptake and soil microbial immobilization. Indeed, the largest decrease in nitrate concentration (70 μg N L^-1 m^-1 of groundwater flow) along a groundwater flow path was measured under the natural riparian forest during the growing season. Because mixing is not a retention process, nitrogen retention in large river margins can be highly over-estimated if the importance of mixing between groundwater and river water is not accounted for. Nonetheless, our results confirm that the effort to reduce non-point-source pollution should be concentrated on wetlands and vegetated riparian zones along small streams.