Contribution of stratigraphy to groundwater motion understanding in chalk: examples of karstogenic horizons of the Pointe de Caux, France

Abstract

Abstract Chalk groundwater is the main renewable drinking water resource for many cities of the Paris–London Basin. Understanding karst groundwater motion enhancement appears to be a major issue in order to better protect drinking water, to define hydrogeological surveys and to explore the aquifer. In Normandy, the stratigraphy of chalk was investigated in the 1970s and 1980s but this newly developed stratigraphy was not introduced to hydrogeology where chalk aquifers are studied without considering the sequence boundaries and key event surfaces. Upper Normandy is a unique hydrogeological region where both stratigraphy and hydrogeology can be studied together. In this article we focus on field observations and their direct application to scientific theory. Eight hydrogeological surfaces, linked to sequence boundaries or key event surfaces, are identified. They increase porosity and permeability sufficiently to develop karstic features, hereafter called karstogenic horizons. These field observations lead us to propose a new stratified chalk groundwater model. Palaeokarsts and perched springs not aligned to the current base level can be explained from a geodynamic perspective. Global eustatism and regional uplift during the Quaternary Period have to be taken into account with the hydrogeological stratified model, as the controlling factors of the groundwater motion and the karstogenic horizon development. This theory will help hydrogeologists to determine the probability of encountering palaeokarsts above the piezometric level and thereby define well locations with a greater degree of confidence according to the karstogenic horizon drilled. Chemical studies may also be applied to show if this stratified model can enhance water quality by a new well design.