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 14C 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 13C/12C 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 18O 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.

SAMPLES of groundwater have been collected from the Chalk of the London Basin and the Lincolnshire Limestone in eastern England to determine the radiocarbon and the 13C/12C ratio in the dissolved carbonate species, and the 18O/16O and D/H ratios of the water. The results will be published more fully elsewhere, but here we report preliminary findings of the 13C/12C and 18O/16O ratios and their implications.