The evolution of London chalk groundwater.
The chemical and isotopic composition of groundwater from 53 sites in the
London area was determined as part of a project aimed at assessing the spatial
variation in age of Chalk groundwater, and determining the relationship between
fracture and matrix groundwater in this dual porosity system.
Systematic changes in groundwater chemistry take place in the downgradient
direction in response to several chemical processes. These processes include
early concentration by evaporation and congruent dissolution of calcite, and
widespread incongruent dissolution and ion exchange in addition to local
oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a
result of the above processes, Chalk groundwater follows an evolutionary path
from calcium bicarbonate type to sodium sulphate bicarbonate or sodium
chloride bicarbonate type groundwaters.
The age of Chalk groundwater was modelled using 4He, 14C and tritium
concentrations. This work indicated that there is a general increase in
groundwater age in a downgradient direction with the oldest water found in the
Hammersmith area. Groundwater in the unconfined zones and in an area south
of the Greenwich fault is almost entirely of unevolved, modern composition.
With the exception of several sites adjacent to the axis of the Basin, Chalk
groundwater in the south Basin is generally less than 10,000 years old.
Groundwater in the north Basin is generally between 10,000 and 25,000 years
old. This implies that while Chalk groundwater in the south of the Basin is
Holocene in age, groundwater in the north is mainly of late Pleistocene age.
The above conclusion is confirmed by the palaeorecharge temperatures which
were calculated from noble gas contents. These calculations indicate that
southern groundwaters yield typical Holocene temperatures of 9-12 °c,
whereas those in the north are characterized by average recharge temperatures
of 5-8 °C.
The results of age modelling imply that average linear groundwater velocities in
the Basin are equivalent to those related to matrix flow. These values are
several orders of magnitude lower than those related to well test analysis and
imply that there is a significant interplay between matrix and fracture
groundwater. This conclusion is confirmed by analysis of stable chlorine
isotopes which indicates that diffusion processes are active in the Chalk
A model of the development of the Chalk recognises that it is a classic dual
porosity aquifer in which groundwater flow occurs predominantly in the fracture
system. The upper 50 m of the aquifer was flushed with fresh water during the
2-3 million years of the Pleistocene and therefore meteoric water largely
replaced the Tertiary and Cretaceous marine water that previously saturated the
system. Most processes which control the chemistry of the groundwater occur
in the matrix where the surface area is exceptionally high. Although fracture
flow dominates the flow regime, diffusion from the matrix into the fracture
porosity controls the chemistry of Chalk groundwater.