Review: “The Hydrogeology of the Chalk of North-West Europe”

Abstract

The chalk deposits of Late Cretaceous and Early Paleocene age in and around the present North Sea Basin form an important regional aquifer in the shallow subsurface on the basin perimeter, and a major hydrocarbon reservoir in the deeper parts of the basin. Currently, about 8.5 million cubic meters of water are pumped daily from the chalk. About 80 percent of the withdrawal is in southern England and northern France. This book provides an overview of the regional hydrogeology of the chalk, with primary emphasis on the fresh-water reservoir. The first six chapters of the book discuss topics applicable to the aquifer as a whole. Chapter 1, "The Making of an Aquifer," briefly describes the regional structure and stratigraphy and the diagenesis and modification of the chalk into a regionally extensive aquifer. Chapter 2, "The Formation and Diagenesis of the Chalk," poses the question, "Why so much chalk?" It explains how the combination of a rise in sea level, lack of erosion in nearby land areas, and higher temperatures and higher C02 in the atmosphere during Late Cretaceous time led to the extensive accumulation of chalk ooze. The resulting rock consists almost entirely of skeletal fragments of algae composed of lowmagnesium calcite. Chapter 3, "The Chalk as an Aquifer," describes the dual-porosity nature of the chalk, which generally consists of a highly porous matrix of low permeability and a fracture network that contains both tight tectonic fractures and highly permeable karstic fissures. Flow in the unsaturated and saturated zones is described in detail, and downgradient changes in the groundwater chemistry are discussed. The other three introductory chapters discuss simulation of flow and transport, engineering geology, and groundwater pollution in the chalk. The middle section of the book provides descriptions of the chalk aquifer in France, Belgium, the Netherlands, Denmark, Sweden, and the United Kingdom. These six chapters generally describe the aquifer extent, stratigraphy and structure, aquifer properties, groundwater levels and potentiometric surfaces, recharge and discharge characteristics, natural water chemistry and pollution, and current groundwater development in each country. The section on France provides a detailed discussion (15 pages) that relates groundwater levels and flow in the chalk of the Paris Basin to geologic structures, topography, and karstic networks. Ground pollution by nitrates and pesticides also receives a lot of attention. The chapter on Belgium consists largely of descriptions of the three major chalk aquifers: the Hesbaye aquifer, the Mons Basin, and the confined aquifers of northern Belgium, with emphasis on modeling approaches. The chapter describes a complex, 5-layer finite-element model of the Hesbaye aquifer, incorporating pumping from wells and galleries, faults, and various geologic unit boundaries. The stratigraphy and lithology of the chalk in the Netherlands are described for the two separate areas of occurrence in the northern and southern parts of the country. Detailed hydrogeologic information is presented only for the southern part of the Netherlands, where the aquifer is near surface and characterized by a matrix of high porosity and very low permeability; fractures and dissolution openings provide high secondary permeability. The chapter on Denmark contains brief summaries of the structure, stratigraphy, karst features, and aquifer properties; the principal focus of the chapter is on hydrochemistry, including natural chemical constituents and pollution problems. In Sweden, the occurrence ofthe chalk is limited to four areas in the southernmost part of the country. The structural setting, stratigraphy, aquifer properties, karstic features, water chemistry, and groundwater development are described for these areas. The chalk