ISOTOPIC AND NOBLE-GAS STUDY OF CHALK GROUNDWATER IN THE LONDON BASIN, ENGLAND

The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at as sessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater i n this dual porosity system. Systematic changes in groundwater chemist ry take place in the downgradient direction in response to several che mical processes. These processes include early concentration by evapor ation and congruent dissolution of calcite followed by widespread inco ngruent dissolution and ion exchange in addition to local oxidation-re duction reactions, gypsum dissolution and saline intrusion. As a resul t of the above processes, Chalk groundwater follows an evolutionary pa th from Ca bicarbonate type to Na bicarbonate type. The age of Chalk g roundwater was modelled using C-14, delta(13)C, H-3, delta(2)H and del ta(18)O. There is a general increase in the groundwater age in a downg radient direction with the oldest water found in N central areas of th e basin. Groundwater in the unconfined zones and in areas S of the Gre enwich fault is almost entirely of unevolved, modern composition. Carb on-14 modelling suggests that Chalk groundwater in the S basin is gene rally less than 10 000 a old while that in the north is generally betw een 10 000 and 25 000 a old. The presence of H-3 in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however , indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temper ature determinations using delta(2)H, delta(18)O and noble gas analyti cal results suggest that significant Devensian recharge did indeed occ ur in the aquifer. A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow o ccurs predominantly in the fracture system. The upper 50 m of the aqui fer was flushed with fresh water during the 2-3 x 10(6) a of the Quate rnary and therefore meteoric water largely replaced the Tertiary and C retaceous marine water that previously saturated the system. Most proc esses which control the chemistry of the groundwater occur in the matr ix where the surface area is exceptionally high. Although fracture flo w dominates the flow regime, diffusion from the matrix into the fractu re porosity controls the chemistry of Chalk groundwater. (C) 1997 Else vier Science Ltd. All rights reserved.