Nappe captive et sous-écoulements : le rôle des alluvions en Basse-Seine

Abstract

The alluvial deposits of the river Seine and its tributaries consist of penglacial bottom gravel in the lower part, and of Holocene peaty clay and loam in the upper part. In this paper, some examples are presented to describe the main influences of these deposits upon groundwater storage and flows. Water reserves which are contained in the chalky substratum are drained by the rivers and by a strong groundwater outflow located in the bottom gravel. The Holocene deposits are semipermeable or impermeable and therefore the underlying aquifers (gravel, chalk) are confined. Artesian spnngs and wells do exist. The fine-grained upper deposits do not constitute wholly impervious beds. Some strong exchanges are found. The groundwater supplies the nver at the places where Holocene deposits are lacking or semipermeable. The reserve is noticed during rises in river level, or high tides, or pumpings. Thanks to the fine-grained Holocene deposits, the groundwater is protected from surface pollutions dunng supplying by the rivers through the gravel. The long transfer duration enserres a protection of the water resource.