Page 87 - Hydrogeology Principles and Practice

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70 Chapter Two

3 −1
Table 2.4 Groundwater resources of the principal Mesozoic aquifers in England and Wales (in km a ). Abstraction data are for the year
1977. After Downing (1993).
Aquifer Infiltration (I) Abstraction (A) Balance A/I
Cretaceous Chalk 4631 1255 3376 0.27
Cretaceous Lower Greensand 275 86 189 0.31
Jurassic Lincolnshire Limestone 86 43 43 0.50
Middle Jurassic limestones 627 65 562 0.10
Permo-Triassic sandstones 1443 587 856 0.41
Permian Magnesian Limestone 247 41 206 0.17
Total 7309 2077 5232 0.28

the Chalk Formation, the most important source
of groundwater in the south and east of England
(Fig. 2.30) (Downing et al. 1993). The Chalk is a pure,
white, microporous limestone made up of minute
calcareous shells and shell fragments of plankton
together with bands of harder nodular chalk and
flints, marly in the lower part. In total the Chalk is
up to 500 m in thickness. The intrinsic permeability
of the Chalk matrix is low, so that good yields of
−1
3 −1
typically 10 m s depend on the intersection of
fissures and fractures, solutionally developed along
bedding plains and joints. The permeability is best
developed in the upper 80–100 m in the zone of great-
Fig. 2.29 Site of a groundwater source developed in alluvial
est secondary permeability development. Below this
deposits in the Rheidol Valley, west Wales. In general, the well-
sorted fluvial and glaciofluvial sands and gravels reach a thickness level the fissures are infrequent and closed by the
of 30 m and are exploited for locally important groundwater overburden pressure and the groundwater becomes
supplies. At this site, Lovesgrove, transmissivities are 0.05– increasingly saline. In Northern Ireland, a hard micro-
2
−1
2 −1
0.07 m s (4000–6000 m day ), specific yield about 5% and porous and fissured Chalk is found with recrystallized
3 −1
sustainable yield 0.05 m s . The number 1 borehole is positioned
calcite partly infilling pore spaces. The Chalk attains a
below the top of the concrete chamber shown at left and reaches a
depth of 30 m through a sequence of river gravels (Hiscock & Paci maximum thickness of only 150 m and is largely cov-
2000). ered by Tertiary basalt lavas. Recharge via the lavas
supports numerous springs at the base of the outcrop
unit with overlying glacial sands and can yield sup- along the Antrim coast. Borehole yields from the
−3
−2
3 −1
plies of 10 m s . Elsewhere in south-east England, Chalk beneath the lavas are typically less than 10 m 3
−1
Tertiary strata form a variable series of clays, marls s and the number of boreholes is few. Beneath the
and sands ranging in thickness from 30 to 300 m. Chalk, the Lower Cretaceous glauconitic and ferrugin-
The Eocene London Clay, up to 150 m thick, is an ous sands and sandstones of the Upper and Lower
important confining unit in the London Basin. The Greensand Formations and the alternating sequence
underlying Lower London Tertiaries include clays, of sandstones and clays of the Hastings Beds occur
fine sands and pebble beds and where permeable and form locally important aquifers in southern
sands rest on the underlying Chalk, these Basal Sands England.
are generally in hydraulic continuity and can yield The Jurassic Period also resulted in the formation
small supplies. of important limestone aquifer units, namely the
The Cretaceous Period resulted in the transgression Corallian and Lincolnshire Limestone aquifers. The
of shallow, warm tropical seas and the deposition of Corallian is well developed in Yorkshire where well-