Climate change is thought to have major effects on groundwater resources. There is however a limited knowledge of the impacts of past climate changes such as warm or glacial periods on groundwater although marine or glacial fluids may have circulated in basements during these periods. Geochemical investigations of groundwater at shallow depth (80–400 m) in the Armorican basement (western France) revealed three major phases of evolution: (1) Mio-Pliocene transgressions led to marine water introduction in the whole rock porosity through density and then diffusion processes, (2) intensive and rapid recharge after the glacial maximum down to several hundred meters depths, (3) a present-day regime of groundwater circulation limited to shallow depth. This work identifies important constraints regarding the mechanisms responsible for both marine and glacial fluid migrations and their preservation within a basement. It defines the first clear time scales of these processes and thus provides a unique case for understanding the effects of climate changes on hydrogeology in basements. It reveals that glacial water is supplied in significant amounts to deep aquifers even in permafrosted zones. It also emphasizes the vulnerability of modern groundwater hydrosystems to climate change as groundwater active aquifers is restricted to shallow depths.

processes. Peat soils have a significant impact on water quality, ecosystem productivity and 3 greenhouse gas emissions. However, the extent of peatlands is decreasing across the world, 4 mainly because of anthropogenic activities such as drainage for agriculture or groundwater 5 abstractions in underlying aquifers. Potential changes in precipitation and temperature in the 6 future are likely to apply additional pressure to wetland. In this context, a methodology for 7 assessing and comparing the respective impacts of groundwater abstraction and climate 8 change on a groundwater-fed wetland (135 km²) located in Northwest France, is presented. A 9 groundwater model was developed, using flexible boundary conditions to represent surface-10 subsurface interactions which allowed examination of the extent of the wetland areas. This 11 variable parameter is highly important for land management and is usually not considered in 12 impact studies. The model was coupled with recharge estimation, groundwater abstraction 13 scenarios, and climate change scenarios downscaled from 14 GCMs corresponding to the 14 A1B greenhouse gas (GHG) scenario over the periods 1961-2000 and 2081 -2100 show that climate change is expected to have an important impact and reduce the surface of 16 wetlands by 5.3 % to 13.6 %. In comparison, the impact of groundwater abstraction (100 % 17 increase in the expected scenarios) would lead to a maximum decrease of 3.7 %. Results also 18show that the impacts of climate change and groundwater abstraction could be partially 19 mitigated by decreasing or stopping land drainage in specific parts of the area. Water 20 management will require an appropriate compromise which encompasses ecosystem 21 preservation, economic and public domain activities. 22

International audienceIn recent decades, saline fluids have been sampled worldwide at great depths in continental basements. Although some of them have been attributed to marine transgressions the mechanisms allowing their circulation is not understood. In this paper, we describe the horizontal and vertical distribution of moderately saline fluids (60 to 1400 mg L−1) sampled at depths ranging from 41 to 200 m in aquifers at the regional scale of the Armorican Massif (northwestern France). The horizontal and vertical distributions of high chloride concentrations are in good agreement with both the altitudinal and vertical limits and succession of the three major transgressions between the Mio-Pliocene and Pleistocene ages. The mean chloride concentration for each transgression area is exponentially related to the time spanned until present. It defines the potential laws of leaching of marine waters by fresh meteoric waters. The results of the Armorican aquifers provide the first observed constraints for the time scales of seawater circulation in the continental basement and the subsequent leaching by fresh meteoric waters. The general trend of increasing chloride concentration with depth and the time frame for the flushing process provide useful information to develop conceptual models of the paleo-functionning of Armorican aquifers