The age of groundwater in the chalk of the London Basin

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 14 C 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 hydro‐chemistry. Measurements of tritium and the stable isotope ratios of carbon, oxygen, and deuterium were also made. The 13 C/ 12 C ratio varied from −13‰ at outcrop to values less negative than −1‰ in the central part of the basin, which is approaching the value of +2.35‰ for the aquifer matrix. The 18 O and deuterium ratios indicate that the Pleistocene waters were recharged at a mean air temperature less than 1°C 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.