Nitrogen dynamics in the shallow groundwater of a riparian wetland zone of the Garonne, SW France: nitrate inputs, bacterial densities, organic matter supply and denitrification measurements

Abstract

This study highlights the role of interactions between surface and sub-surface water of the riparian zone of a large river (the Garonne, SW France). Information is given about the role of surface water in supplying Dissolved Organic Carbon (DOC ) to the riparian zone for nitrate removal processes. The densities of bacteria (up to 3.310 6 cell m L -1 ) in groundwater are strongly conditioned by the water moving during flood events. Total bacterial densities in groundwater were related to surface water bacterial densities. In sediment, total bacteria are attached mainly to fine particles (90 % in the fraction < 1 mm). Spatial variations in organic carbon and nitrate content in groundwater at the site studied are correlated with exchanges between the groundwater and the river, from the upstream to the downstream part of the meander. Total bacterial densities, nitrate and decressing organic carbon concentrations follow the same pattern. These results suggest that, in this kind of riparian wetland, nitrate from alluvial groundwater influenced by agricultural practices may be denitrified by bacteria in the presence of organic carbon from river surface water.

Spatio-temporal analysis of factors controlling nitrate dynamics and potential denitrification hot spots and hot moments in groundwater of an alluvial floodplain

“…Agricultural land use, in combination with factors such as shallow groundwater, high permeability of alluvial deposits and interconnections with surface water, make alluvial aquifers particularly vulnerable to nitrate diffuse pollution (Arauzo et al, 2011). As a result, nitrate concentrations exceeding the limit of 50 mg-NO 3 − L −1 set for groundwater systems in Europe by the Nitrate Directive (91/676/EEC) and the Groundwater Directive (2006/118/EEC) have been reported for several shallow aquifers in floodplain areas (Baillieux et al, 2014;Sánchez-Pérez et al, 2003).…”

“…In these areas, where surface water rich in organic matter (OM) meets groundwater containing abundant nutrients, denitrification is promoted Sánchez-Pérez et al, 2003). Therefore riparian areas have been shown to be efficient in nitrate pollution mitigation (Vidon and Hill, 2006;Dosskey et al, 2010;Naiman et al, 2010).…”

“…In these systems, the hydrological exchanges at the river/groundwater interface can have a significant impact on denitrification rates in the aquifer (Baker and Vervier, 2004;Lamontagne et al, 2005). Such exchanges recharge aquifer with river water rich in OM, stimulating denitrification in groundwater (Iribar, 2007;Sánchez-Pérez et al, 2003). The location of the denitrification process is driven by advective flux, where flow paths lead organic matter and nitrate to meet (Seitzinger et al, 2006), at points defined as hot spots by McClain et al (2003).…”

Mimicking floodplain reconnection and disconnection using &lt;sup&gt;15&lt;/sup&gt;N mesocosm incubations

“…Our mass balance approach may over-estimate assimilation into biomass by attributing lost N to this pool, but it nevertheless indicates that a large portion of N can be used up by autotrophs and heterotrophs. Although we did not measure bacterial density or microbial community structure in this study, previous studies have shown that the bacterial community is conditioned to respond quickly and efficiently to flooding events (Sánchez-Pérez, 2003). Nitrogen saturation can occur when bacterial communities are overloaded with constantly elevated nitrate concentrations.…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…This has implications for drinking water utilities in the cost and reliability of treatment to meet the regulation quality. Elevated nitrate concentrations in groundwater may also impact on surface water where baseflow is significant and on groundwater dependent ecosystems (Rozemeijer and Broers, 2007;Sánchez-Pérez et al, 2003). The roles of nitrate and phosphate are interlinked in the surface water eutrophication process (Smolders et al, 2009).…”