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

Abstract

Nitrate (NO3−) contamination of freshwater systems is a global concern. In alluvial floodplains, riparianareas have been proven to be efficient in nitrate removal. In this study, a large spatio-temporal datasetcollected during one year at monthly time steps within a meander area of the Garonne floodplain (France)was analysed in order to improve the understanding of nitrate dynamic and denitrification process infloodplain areas. The results showed that groundwater NO3−concentrations (mean 50 mg NO3−L−1) wereprimarily controlled by groundwater dilution with river water (explaining 54% of NO3−variance), butalso by nitrate removal process identified as denitrification (explaining 14% of NO3−variance). Dilutionwas controlled by hydrological flow paths and residence time linked to river-aquifer exchanges and floodoccurrence, while potential denitrification (DEA) was controlled by oxygen, high dissolved organic car-bon (DOC) and organic matter content in the sediment (31% of DEA variance). DOC can originate bothfrom the river input and the degradation of organic matter (OM) located in topsoil and sediments of thealluvial plain. In addition, river bank geomorphology appeared to be a key element explaining potentialdenitrification hot spot locations. Low bankfull height (LBH) areas corresponding to wetlands exhibitedhigher denitrification rates than high bankfull height (HBH) areas less often flooded. Hydrology deter-mined the timing of denitrification hot moments occurring after flood events. These findings underlinethe importance of integrating dynamic water interactions between river and aquifer, geomorphology, anddual carbon source (river and sediment) when assessing nitrate dynamics and denitrification patterns infloodplain environments.