Salinization of aquifers at the regional scale by marine transgression: Time scales and processes
Résumé
Saline fluids with moderate concentrations have been sampled and reported in the Armorican basement at the regional scale (northwestern France). The horizontal and vertical distributions of high chloride concentrations (60-1400mg/L) at the regional scale support the marine origin and provide constraints on the age of these saline fluids. The current distribution of fresh and “saline” groundwater at depth is the result mostly of processes occurring at geological timescales - seawater intrusion processes followed by fresh groundwater flushing -, and only slightly of recent anthropogenic activities. In this study, we focus on seawater intrusion mechanisms in continental aquifers. We argue that one of the most efficient processes in macrotidal environments is the gravity-driven downconing instability below coastal salinized rivers. 2‑D numerical experiments have been used to quantify this process according to four main parameter types: (1) the groundwater system permeability, (2) the salinity degree of the river, (3) the river width and slope, and (4) the tidal amplitude. A general expression of the salinity inflow rates have been derived, which has been used to estimate groundwater salinization rates in Brittany, given the geomorphological and environmental characteristics (drainage basin area, river widths and slopes, tidal range, aquifer permeability). We found that downconing below coastal rivers entail very high saline rates, indicating that this process play a major role in the salinization of regional aquifers. This is also likely to be an issue in the context of climate change, where sea-level rise is expected.