The age of groundwater in the chalk of the London Basin

Abstract

Isotope techniques have been applied to a study of groundwater movement in the Chalk of the London Basin. The 'age' of the water as determined by •4C measurements increases toward the central confined part of the basin where it exceeds 25,000 years and therefore originated during the Pleistocene. The age distribution supports previous interpretations of the permeability pattern, partly based on hydrochemistry. Measurements of tritium and the Stable isotope ratios of carbon, oxygen, and deuterium were also made. The •aC?2C ratio varied from -13%o at outcrop to values less negative than .-19oo in the central part of the basin, which is approaching the value of +2.35%o for the aquifer matrix. The •80 and deuterium ratios indicate that the Pleistocene waters were recharged at a mean air temperature less than 1 øC cooler than that of present day recharge, probably because recharge in the Pleistocene was limited to summer periods because of frozen ground in the winter. INTRODUCTION The London Basin is a synclinal structure in Cretaceous and Eocene strata in southeast England (Figure 1). The principal geological formations and their general thicknesses are [Water R'esources Board, 1972]' for the Eocene, London clay, 60-160 m, and Lower London Tertiaries, 12-40 m; and for the Cretaceøus, Chalk, 180-250 m, Upper Greensand, 6 m, and Gault, 50 m. The London clay is a relatively impermeable, overConsolidated clay, but the Lower London Tertiaries are represented by a series of sands, loams, and clays. In the central and eastern parts of the basin, the lower part of the Lower London Tertiaries is arenaceous. This facies, which has been referred to as the Basal Sands [Water Resources Board, 1972], attains a thickness of more than 30 m east of London and is commonly in hydraulic continuity with the Chalk. The Chalk is a soft, fine-grained limestone. Groundwater flow in the aquifer is predominantly through fissures, which are mainly in the upper 60 m and tend to be well developed along valleys, particularly in the unconfined areas but sometimes also in the confined area. The Chalk is underlain by the Upper Greensand and the Gault; the latter, being an argillaceous deposit, forms the base of the aquifer system. Much of the London Basin is drained by the River Thames. Groundwater flows from the outcrops on the northern and southern flanks of the basin through the confined part of the aquifer to natural discharge areas in the Thames valley. Since the eighteenth century, groundwater in the Chalk and the Lower London Tertiary sands has been developed for water supply. Groundwater levels have fallen in the center of the basin by as much as 70 m, and an increasing proportion of the natural discharge has been intercepted by pumped wells which are now the main outlet from the confined area, natural discharge no longer taking place from the central and eastern parts of the basin. The groundwater in the Chalk and Lower London Tertiary sands at outcrop is of the calcium bicarbonate type. Generally, this changes into a sodium bicarbonate type...

Fluctuation of NO3-N in groundwater of the reservoir of the Sunagawa Subsurface Dam, Miyako Island, Japan

“…Foster and Smith-Carington (1980) identified by-pass flow through macropores (both microfissures and large pores) in chalk from a balance calculation of tritium in the unsaturated zone and suggested that up to 15% of the tritium was transported by the preferential flow of water through macropores. The fraction of water moving in this way in chalk in England has been estimated as 6-15% of total water movement (Smith et al 1976). This type of flow is currently thought to occur principally during heavy rainstorms.…”

The O and H stable isotope composition of freshwaters in the British Isles. 2. Surface waters and groundwater

“…fingerprinting sources of water leakage (Butler and Verhagen, 1997;Maréchal and Etcheverry, 2003), to the regional, e.g. groundwater resource and quality studies (Smith et al, 1976;Rauert et al, 1993), the archaeological (Iacumin et al, 1997;White et al, 1998), the environmental (Andrews et al, 1997;Iacumin and Longinelli, 2002), the quasi-forensic (Keenlyside et al, 1997) and ultimately as part of the global mosaic which aids the understanding of climatic processes (Rozanski et al, 1993;Hoffmann et al, 2000). In contrast to some areas of Europe, such as Germany and Italy (Förstel and Hützen, 1983;Longinelli and Selmo, 2003), the stable isotopic variations of waters in the British Isles have hitherto not been summarised in any systematic way.…”

Recharge velocity and geochemical evolution for the Permo-Triassic Sherwood Sandstone, Northern Ireland

“…The other end member is the d 13 C value of soil CO 2 . Smith et al (1976) quoted an average UK value of K26.1G 0.3 ‰. Other factors affecting the d 13 C TDIC values of the groundwater are the degree of openness of the system and the extent of bacterial respiration in the aquifer.…”