Influence of vegetation-induced water table seasonality on groundwater chloride concentration dynamics in a riparian fen peatland

Peatlands are environments that rely mainly on high water levels to accumulate organic matter. Depending on the chemical species observed, the lowering of the water table can change biogeochemical equilibriums, with various impacts. This paper aims to understand the effect of shallow groundwater seasonality on chloride concentrations in a French riparian peatland by combining water table monitoring, geochemical and stable water isotopes analysis. Water table levels and groundwater samples were recorded and collected for 3 years, every 2 months, in nine observation wells and the nearby river. Chloride concentrations were highly variable in space and time, ranging from 10 to 100 mg L-1. They are shown to be related to the water table dynamics, which are closely linked to the life cycle of the local vegetation. These dynamics were characterized by a significant drawdown between June and October due to plant transpiration and a fast recovering period just after its senescence. Results show that the chloride accumulates within the unsaturated zone during the drying phase and is solubilized back into the groundwater during the rewetting phase, increasing its concentration. Moreover, the water table rises in autumn with various dynamics according to the location in the peatland, which induces some special differences in hydraulic gradients. Such gradients allow lateral transfers from zones of fast recovery to zones of slow recovery, where year-to-year chloride accumulation was observed. These complex 3D processes preclude the use of chloride to constrain how the peatland hydrogeological system functions. Conversely, the use of stable water isotopes in this work emphasizes the importance of the river's role during the summer as a water supplier to counterbalance vegetation transpiration. The relationships between chloride concentration, vegetation, seasonal meteorological parameters, and shallow groundwater in a typical riparian peatland were investigated at a site scale. Under large water table fluctuations due to plant transpiration, chloride is not only accumulated within the unsaturated zone by root exclusion, but it is also subject to lateral transfer leading to spatial heterogeneities in concentrations. It highlights the complex 3D processes that may make chloride alone unsuitable to understand peatland hydrogeological systems.image