A stable isotope study of recharge processes in the English Chalk

Abstract

The δ 2H and δ 18O values of water contained in unsaturated Chalk have been measured in drillcore profiles taken over a period of 28 months on arable land in Cambridgeshire, eastern England. The isotopic composition of rainfall and lysimeter drainage on the same site has also been monitored over a period of four years. The distribution of isotope values in the upper few metres of soil and unsaturated Chalk shows that the mechanism of infiltration is not simple piston-like downward displacement since the scale of vertical fluctuations is incompatible with estimated annual infiltration and the apparent rates of movement of matrix water derived from thermonuclear tritium studies. Lysimeter drainage is not isotopically identical to matrix water at the same depth suggesting that different routes to water table are associated with consistently different isotopic content. Although there is an apparent difference between matrix water beneath arable and permanent grassland, these differences appear to cancel out in the composition of bulk recharge beneath these land types. The regional aquifer has a uniform isotopic composition similar in value to weighted average rainfall and lysimeter drainage, suggesting that the recharge-abstraction system has been in isotopic equilibrium for many years despite changes in pumping rates and land use. Two other Chalk sites in southern England have been investigated in less detail but indicate clear difference in infiltration behaviour from that seen in Cambridgeshire; this seems to be related to a higher matrix conductivity and annual rainfall amount.