Change in groundwater chemistry as a consequence of suppression of floods: the case of the Rhine floodplain

Spatio-temporal variations of nitrogen, phosphorus and base cation concentrations in groundwater were related to the drastic change in hydrological conditions of the Rhine alluvial floodplain (Eastern France), which has been disconnected from the river by canalisation. The Groundwater chemistry was studied in two alluvial forests with contrasting hydrological conditions: one in a sector unflooded for 30 years, the second one in a sector still subject to flooding. Nutrient concentrations were measured at two levels, in the root zone (1.5 m depth) and in the gravel below the root layer (4.5 m depth). In the unflooded sector, the average nitrate concentration was significantly lower in the shallow groundwater (2.06 mg l−1 NO3−) than in the deeper layer (5.84 mg l−1NO3−). In contrast, in the flooded sector the nitrate concentrations in the shallow groundwater (5.02 mg l−1 NO3−) were not significantly different from those in the deep groundwater (3.98 mg l−1 NO3−). The concentration of phosphate was similar in shallow and deep groundwater in the unflooded sector (46 and 35 μg l−1, respectively) but significantly lower in the deep groundwater of the flooded sector (47 μg l−1), than in the shallow groundwater (58 μg l−1). The major elements (cations: Ca2+, Mg2+, Na+ and associated anions: HCO3−, SO4−2, Cl−) concentrations were significantly higher in the groundwaters than in the surface water. The results are discussed in terms of changes that accompanied suppression of floods, and processes that take place during the transfer of nutrients through the groundwater–soil–plant compartments. The reduction of groundwater fluctuations in the unflooded sector modified the transfer of nitrate by reducing the resolubilisation of locally produced nitrate, and/or denitrification. The transfer of phosphate was affected to a lesser extent, because of precipitation and adsorption. Base cation concentrations reflect exchange between groundwater and the calcareous gravel