Page 60 - Hydrogeology Principles and Practice

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Physical hydrogeology 43

BO X
Continued
2.5

water levels in the borehole that at first supplied the fountains in
Trafalgar Square under an artesian condition (Kirkaldy 1954) had
declined to 60 m below sea level (Fig. 3), a drop of 40 m since the
borehole was drilled in 1844, causing the groundwater condition in
the Chalk aquifer to change from confined to unconfined. The
decrease in hydraulic head caused a loss of yield from 2600 m 3
3
−1
−1
day to 36 m day by 1900.
By 1950, and over an area extending from the centre of London
south-west to Richmond (Fig. 2b), groundwater levels were more
2
than 60 m below sea level over an area of 200 km . In the 100
years up to 1950, groundwater abstractions are estimated to have
6
3 −1
6
3 −1
increased from 9.0 × 10 m a to 73.0 × 10 m a (Marsh &
Davies 1983).
The general decline in groundwater levels had a number of
impacts on groundwater resources in the London Basin, including a
loss of yield and eventual failure of supply boreholes, and saline
water intrusion into the Chalk aquifer. Saline water was recorded as
far west as Lambeth in a zone 5–8 km wide centred on the River
Thames. As discussed in Box 2.8, falling groundwater levels in a
confined aquifer system can cause land subsidence, although the
well-consolidated nature of the marine London Clay formation pre-
vented substantial compaction in this case. Nevertheless, the over-
all settlement approached 0.2 m in Central London over the period
1865–1931 (Wilson & Grace 1942) as a consequence of the com-
bined effects of dewatering the aquifer system, heavy building
development and the removal of fine sediment during pumping
from sand and gravel horizons.
An increasing awareness of the problems of over-abstraction
from the Chalk aquifer together with a reduction in industrial
abstractions since 1945 have led to a virtual end to further aquifer
exploitation in London. Water supplies switched to piped supplies
drawn predominantly from reservoirs in the Thames and Lea valleys,
or from major abstraction boreholes located outside of Greater Fig. 3 Variation in groundwater level at Trafalgar Square
London. Since 1965, the reduction in groundwater abstraction has (borehole TQ28/119) showing the changes in groundwater
resulted in commencement of a recovery of groundwater levels at a conditions in the Chalk aquifer below London. In the period
−1
rate of about 1 m a in the centre of the cone of depression (Lucas before groundwater exploitation, the natural potentiometric
& Robinson 1995; see Fig. 5.3b). The re-saturation of the Tertiary surface was near to ground surface at about 7.5 m above sea level.
strata has implications for building structures because of increased At this time, the Chalk aquifer was confined by the Eocene
hydrostatic pressure on deep foundations and the flooding of tun- London Clay. By 1965, the groundwater level had declined to
nels constructed during the main period of depressed groundwater 83 m below sea level as a result of over-exploitation of the
levels between 1870 and 1970. To combat this problem it is recog- groundwater resource. The lower section of the Tertiary strata
nized that pumping will be required to control groundwater levels. includes sands of the Lower London Tertiaries which are in
Regional groundwater modelling of the London Basin indicated hydraulic contact with the Chalk and these extend the aquifer
3
3
that an additional 70 × 10 m day −1 of pumping would be neces- vertically. By 1900, the Chalk potentiometric surface was within
sary to control the rise in groundwater to an acceptable level assum- these strata and the groundwater condition had become
ing that existing abstractions continued at their current level (Lucas semiconfined. For most of the twentieth century until the
& Robinson 1995). To be effective, much of this abstraction would mid-1960s when groundwater levels began to rise, the Chalk
need to be concentrated in an area incorporating Central London potentiometric surface was below the base of the Tertiary strata
together with an extension eastwards along the River Thames and the groundwater condition was unconfined. After Marsh and
where structural engineering problems due to groundwater level Davies (1983).
rise are predicted (Simpson et al. 1989).