Nitrate concentration in wetlands: assessing the contribution of deeper groundwater from anions

Denitrification in wetlands is useful for removing nitrate from the surface water, although it can be difficult to assess wetland functioning particularly where it overlies a hard-rock aquifer whose fractures and joints form pathways that mix waters with different chemical composition. The variability of NO3 concentrations in such waters, which partly transit through wetlands, can obscure the effect of denitrification. To address this question, we monitored groundwater chemistry at different depths on three pilot sites overlying (mica)schist aquifers with almost no NO3 contamination at depth, probably due to denitrification. The spatial variability of NO3 concentrations, both along the flowpath and with depth, is related in each site to at least one of the following factors: (1) upward flux of deeper NO3-free groundwater; (2) in situ heterotrophic denitrification; (3) application of different types of fertilizer and other amendments. These factors are efficiently discriminated by the monitoring of just three, easily affordable, parameters: NO3, SO4 and Cl.