Hydrochemical trends, palaeorecharge and groundwater ages in the fissured Chalk aquifer of the London and Berkshire Basins, UK

The hydrochemical, radiochemical, stable isotope, C-14 and dissolved noble gas composition of groundwaters has been determined along two profiles acro ss the confined, fissured Chalk aquifer of the London Basin of southern Eng land, and for selected sites in the adjacent Berkshire Basin. During downgr adient flow in the London Basin aquifer, the groundwater chemistry is modif ied by water-rock interactions: congruent and incongruent reaction of the c arbonate lithology resulting in enhanced Mg/Ca and Sr/Ca ratios and C-13 co ntents with increased residence times; redox and ion exchange reactions; an d towards the centre of the Basin, mixing with a residual saline connate wa ter stored in the Chalk matrix. There is evidence from anomalous water chem istries for a component of vertical leakage from overlying Tertiary beds in to the confined aquifer as a result of historical dewatering of the aquifer . Dissolved noble gas contents indicate the climate was up to 4.5 degrees C cooler than at present during recharge of the waters now found in the cent res of both Basins; stable isotope (H-2 and O-18) depletions correspond to this recharge temperature change, For evolved waters having delta(13)C > -8 parts per thousand PDB a negative linear correlation is demonstrated betwe en derived recharge temperatures and delta(13)C values, which is interprete d as mixing between relatively warm, light isotopic, Fracture-borne waters and cooler stored waters of the matrix having a C-13 signature more or less equilibrated with the Chalk. From geochemical (C-14, He-4) age estimates, the abstracted water is interpreted as being either of wholly Holocene/post -Devensian glacial origin, or an admixture of Holocene and Late Pleistocene pre glacial (cold stage interstadial) recharge. Devensian pheniglacial sta ge waters of the Last Glacial Maximum are not represented. (C) 1999 Elsevie r Science Ltd. All rights reserved.