Wetlands are key environments in watersheds with several essential hydrological functions such as flood mitigation, support for low water flow or improvement of water quality. Some of them, in particular, the peat bogs, are also known for their carbon storage capacity. Nevertheless, peat bogs are poorly studied in the Mediterranean while in such context carbon storage capacity could be strongly threatened by their drying up due to global warming.
A hydrogeological study is currently conducted on one of the southernmost low altitude Mediterranean peat bog located in Moltifao, Corsica Island, France. The Moltifao peat bog (40.000 m² and from 240 to 270 masl) is connected to an alluvial aquifer and situated between the Ascu torrential River and surrounding granitic massifs which makes its major ecological specificity and its high international importance (RAMSAR). The objective of this study is to evaluate the potential of a combined isotope and geochemical approach to quantify seasonal variations of the water components that may condition water fluxes and carbon cycle processes in such a Mediterranean context.
To reach this objective, 5 quarterly field campaigns in contrasted hydrological conditions were carried out from May 2018 to June 2019 through a network of 26 sampling points. The investigations involved a hydrodynamic and multi-tracer approach, including stream flow and groundwater level monitoring, physico-chemical parameters, major ions, stable isotopes of the water molecule (18O, 2H) and tritium (3H). In addition, rainwater and stream water were collected monthly for the characterization of the local stable isotopes framework.
Groundwater level measurements showed the relative contribution of the alluvial aquifer, rainfall regime and direct surface runoff from the surrounding granites on the recharge of the Moltifao peat bog. Geochemical and isotope tracers also highlighted seasonal variations in the alluvial groundwater recharge from the granites and from the Ascu River. The Ascu River water is characterized by low electric-conductivity (EC) and neutral water type, whereas groundwater from the granites controls the mineralization with higher EC and a Na-SO4 water type. Water from the alluvial aquifer shows an intermediate EC, a Ca-Na-Cl water type and a higher Si content. These strong variations in geochemical and isotope signatures allows quantifying the relative input from all water bodies involved in the peat bog recharge over the seasons.
From these first data, a water budget of such Mediterranean peat bog is proposed. This is an initial stage before a further study on carbon cycle processes from organic (DOC, TOC) and inorganic (DIC) carbon contents and associated isotopes (13C-DIC and 14C-DIC) intended to inform on the peat bog carbon storage capacity facing drying up related to seasonality and climate projections.