'Advances in understanding the hydrogeology of the London Basin', poster presented at the London Basin Forum meeting, 14 October 2010.

1. Advances in understanding the hydrogeology of the London Basin
Steve Buss1 and Travis Kelly2
With grateful acknowledgements to the rest of the project team:
Mike Jones, Rob Sage, Peter Isherwood, Vin Robinson, Rory Mortimore, Malcolm Anderson,
Nigel Hoad, Jane Dottridge, Mike Streetly, Paul Daily, Victoria Price and Nancy Proudfoot
Background
The Chalk and Thanet Sands aquifer
provides the main groundwater resource
in the London area and supports
significant abstraction for a variety of uses
including public water supply. Beneath
London the aquifer is confined by clayey
strata of the Lambeth Group and London
Clay. Outcrop geology of the study area is
mapped below.
Stratigraphic control of
aquifer properties
It has been recognised that Chalk aquifer properties can be correlated to the new
stratigraphic classification. The uppermost strata beneath London, the Newhaven and
Seaford Chalk formations, have well-developed intersecting fracture sets and therefore
readily permit groundwater flow. Beneath these the Lewes Nodular Chalk, while also
relatively well-fractured, contains several tabular flint and marl horizons which restrict
vertical groundwater flow. Deeper formations are less well fractured because they are
more clay-rich and plastic (although aquifer properties can be developed at outcrop).
Re-analysis of existing data: hydraulic properties chalk core from a borehole at Faircross,
near Reading (Bloomfield, 1997), and numerous down-hole geophysical logs from the
Environment Agency, show physical evidence of this. Clear demarcations between
regions of similar aquifer properties are seen at formation-level stratigraphic boundaries.
In some of the down-hole logs, properties can be seen to vary at bed-level.
Burial depth control of
aquifer properties
The figure (right) compares the distribution of
transmissivity from Downing et al. (1972) with the
current depth of the top of the Chalk aquifer. There is
excellent correspondence between areas where the
depth exceeds 80 m and the areas of identified low
transmissivity.
Topography associated with river valleys is reflected in
the depth of burial: notably the River Lee and River
Wandle both lie above areas in the aquifer where the
Chalk would be buried deeper than 80 m if it were not
for the overlying river valleys.
Some areas (e.g., beneath the River Roding) have the
top of the Chalk above 80 m below ground level but
apparently low transmissivity. However, since there is
no link from these areas to outcrop, a flux of
groundwater to cause solution enhancement cannot be
provided. Similarly, there is not a high transmissivity
channel beneath the River Hogsmill because there is
nowhere for the fresh groundwater to discharge.
Structural controls on
groundwater flow
Faulting often alters the hydraulic conductivity of Chalk:
both to increase and decrease permeability. Along the
length of faults, permeability can be enhanced by
fracturing (then can be further enhanced by
dissolution). However, after displacement the
permeability can be reduced by mineralisation. Field
evidence and the results from existing calibrated
models demonstrate the importance of both increased
and reduced permeability of faults in the basin.
A new interpretation of the fault pattern in Central
London was provided for the project by the BGS.
Groundwater levels either side of each major fault were
reviewed to assess whether consistently steep hydraulic
gradients developed across the faults. Most of the
major faults (shown in the figure right) demonstrated
discontinuity of groundwater levels. After this, all
groundwater level data for 2000 and 2007 were
contoured based on this interpretation. Groundwater
levels appear to be particularly discontinuous in the
south of the area, where there is most faulting (figure
upper right).
Using high resolution stratigraphic data from the BGS
geological model, and from a version of the existing
London Basin Groundwater Model, we were able to
compare these with the interpreted groundwater level
surface. The figure right shows the formation that the
water table/piezometric surface was in, in 2007. There
are considerable areas beneath the London Clay where
the Chalk and Thanet Sands aquifer is unconfined.
T = 500 m/day Tx:Ty = 5
TF = 5000 m/day TF = 50 000 m/day TF = 50 000 m/day
TF = 50 m/day TF = 0 m/day TF = 0 m/day
High or low permeability faults?
Despite the evidence that there are steep hydraulic gradients perpendicular to some fault
zones, there appears to be no direct evidence that these zones are less permeable than the
surrounding aquifer. There are, however, several lines of field evidence that suggest some fault
zones may be more permeable than the aquifer. Simple numerical modelling of a pumping test
in an otherwise homogeneous aquifer shows how:
1. Narrow zones of high permeability cause a change in the aspect ratio of the cone of
depression: its long axis develops parallel to the fault zones. This is similar to what would
be observed in an anisotropic aquifer.
2. Drawdown does not propagate across the high permeability zones, resulting in steep
hydraulic gradients perpendicular to them.
3. While low permeability zones also lead to steep hydraulic gradients, the long axis of the
cone of depression forms perpendicular to the fault zones.
We learn from this that quantification of these effects in the field, and indeed the
discrimination of narrow high permeability zones from narrow low permeability zones from
regional anisotropy, depends on using several strategically-placed observation boreholes, and
having an open mind when it comes to pumping test analysis.
Sources of groundwater
The Trafalgar Square hydrograph clearly shows that historically groundwater demand exceeded
supply and that the abstraction considerably depleted aquifer storage. Having available the
results of adjacent groundwater models, particularly: the South West Chilterns, Vale of St
Albans, Essex, and Swanscombe models, we have been able to determine inflows to the aquifer
from other outcrop areas.
The plot below shows groundwater inputs vs. outputs for the confined London Basin since 1965.
Abstraction from the confined aquifer is currently approximately balanced by inflows from the
Chilterns and North Downs (which is an indication of the success of the GARDIT programme).
Compare the total inflows below with the maximum historical abstraction of almost 500 Ml/day.
ABOVE (after Bloomfield, 1997): Even deeply
confined, and without solution enhancement,
the Seaford Chalk exhibits typically high
porosity; then there is a gradual decrease
within the Lewes Nodular Chalk to the Chalk
Rock at its base. Below that the porosity is
variable. This transition (at about 155 m
depth) is a transition between diagenesis due
to mechanical compaction above, and
pressure solution compaction below. The
overall decline in porosity is matched by a
corresponding, and more regular, decrease in
the gas permeability of intact chalk.
LEFT: Most boreholes penetrate about 50-60m
of the Chalk because hydrogeologists
understood that the top 60 m was the zone of
groundwater flow and drilling deeper would
be into progressively less productive Chalk.
Several boreholes show a zone of widening
around the Cuckmere Beds, which are a
particularly weak stratum in the middle of the
Seaford Chalk Formation. Others show
development at the sub-Palaeogene
unconformity, possibly due to the effects of
acid groundwater from the Thanet Sands.
This poster presents some of the findings of
an Environment Agency-funded study into
the hydrogeology of the London Basin
aquifer. This conceptual model study
provides a robust, quantitative foundation
for a forthcoming numerical modelling
project. The numerical model will be used
by the Environment Agency to regulate the
diverse abstraction pressures on the aquifer.
Historical abstraction caused the development of a regional cone of depression
beneath Central London, in the centre of which the Chalk aquifer developed an
unsaturated zone. Since 1960 there has been a rise in groundwater levels over
most of the Central London area as a response of the reduction in pumping. This
recovery would potentially cause issues with structural integrity of infrastructure:
this has led to a strategy of increasing abstraction in the area (GARDIT: General
Aquifer Research Development and Investigation Team). Aquifer storage and
recharge (ASR) schemes have been developed in the areas where historical
abstraction caused the development of an unsaturated zone in the Thanet Sands.
-400
-300
-200
-100
0
100
200
300
400
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
Flows(Ml/day)
River Thames Storage Chilterns North Downs Net Abstraction
FlowintoaquiferFlowoutofaquifer
2007 (all elevations in m AOD)
More details on the project and its outcomes can be obtained from the authors:
1 Dr Steve Buss, ESI Ltd., 160 Abbey Foregate, Shrewsbury SY2 5HH (stevebuss@esinternational.com)
2 Travis Kelly, Environment Agency, Red Kite House, Howbery Park, Wallingford OX10 8BD
2007