Hydro geology on the Sandstone Ridge. Workshop slides presented by Gordon Coombs of AMEC

1. 1. Prior to the start of the final
retreat of the ice sheet ~
18,000 years ago.
Whole area covered by ice up
to >400m thick.
2. Ice retreats to north of
Norley/Cuddington, leaving
deposits of Boulder Clay
(blue), then stops.
Unable to retreat west of
Ridge because of pressure
exerted by ice-flows from
Welsh highlands.
Retreat of ice-
sheet halted here.
3. Outwash from the static
ice-front to north of
Norley/Cuddington and
through gaps in the Ridge
(blue arrows).
Leave deposits of glacial
sands and gravel.
4. Final total retreat of the
ice-sheet leaves further
extensive accumulations of
Boulder Clay.
Sandstone Ridge
high ground

2. Hydrogeology Workshop
Present Geology (1)
• And it has all resulted in this:
Go to British Geological Survey (BGS) ‘Geology of Britain Viewer’ @
http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/viewer.html
(Follow instructions at rear of handout)

3. Hydrogeology Workshop
Present Geology (2)
Note: Relative thickness of the Glacial Clay,
Sands and Gravel and Alluvium is greatly
exaggerated

4. Hydrogeology Workshop
Part 2
Introduction to Hydrogeology

5. Hydrogeology Workshop
Introduction to Hydrogeology (1)
What is groundwater and its place in the hydrological cycle?
Ocean
Evaporation
Heat/Radiation
Clouds form
Circulation in the
atmosphere
Precipitation
Runoff
Transpiration and
evaporation
Recharge
Water table
Groundwater flow
Discharges
Groundwater discharges:
•River/lakes
springs/wetlands;
•Sea/estuaries
•Abstractions
(wells/boreholes)
Saline
water

6. Hydrogeology Workshop
Introduction to Hydrogeology (2)
Some basic terminology
Aquifer:
• Rock which allows water to flow in
‘significant’ quantities (Ridge area examples:
Terrace/Glacial Sands and Gravel, Sherwood
Sandstones)
Aquiclude:
• Rock through which virtually no water
flows. (Ridge area examples rare strictly speaking
but could include Mercia Mudstones, Halite
formations, Glacial Boulder Clay)
• Can be a layer above a ‘confined’ aquifer.
Aquitard:
• Rock which allows ‘small’ amounts of
water to flow through it. (Ridge area examples
Mercia Mudstones, Halite formations, Glacial
Boulder Clay)
• Can be a layer above a ‘semi-confined’
aquifer.
Porosity; Void space, expressed as
% of rock volume.
• Primary porosity – intergranular;
• Secondary porosity –
fractures/fissures;
• Effective porosity – space in which
groundwater flow is ‘active’ (also
related to ‘storage’).
Permeability (Hydraulic
Conductivity)
• Property which allows fluid (water)
to move through rock/soil.

7. Hydrogeology Workshop
Introduction to Hydrogeology (3)
Some basic terminology
• Water table:
– Surface of groundwater, interface between the saturated aquifer and
the unsaturated zone.
• Piezometric Level:
– ‘Pressure’ (head) of water in a confined or semi-confined aquifer.
Effectively where the water level will be in a borehole drilled into the
aquifer.
• Aquifer conditions:
– Unconfined: Aquifer which contains the water table. Water
can enter it from recharge.
– Confined: Fully saturated aquifer with piezometric surface
(e.g. level of water in a borehole) above its top. Sometimes known
as ‘artesian’. Strictly speaking, no water enters as recharge from
above.
– Semi-confined: Fully saturated aquifer with piezometric
surface (e.g. level ,of water in a borehole) above its top. BUT water
can leak into it from above.
– Perched aquifer: An upper unconfined aquifer (i.e. It
contains a water table) which is separated from a lower one , with an
intervening unsaturated zone.
Based on Brassington, R. (1988) Field Hydrogeology, Geological Society Handbook, Open
University Press, Milton Keynes/Halstead Press, New York – Toronto, P.4, fig. 1.2
Confined or semi-confined Unconfined

8. Hydrogeology Workshop
Introduction to Hydrogeology (4)
Groundwater Flow
Bath Tubs and Black Boxes
Where does it want to go?
• In at the top (recharge/leakage) and out at the bottom (discharge to
rivers, lakes, sea etc).
• From high groundwater level/pressure (higher ground) to low
level/pressure (lower ground/lower pressure).

9. Hydrogeology Workshop
Introduction to Hydrogeology (5)
Groundwater Flow
Bath Tubs and Black Boxes
How does it get there?
• Driven by hydraulic gradients
– Lateral hydraulic gradients
– Vertical hydraulic gradients
• Downwards
• Upwards
– How do they work (e.g from recharge to discharge at rivers, springs etc).
• ‘Controlled’ by Permeability
• Primary – intergranular – flow tends to be fairly slow
• Secondary - fractures/fissures/bedding planes and geological faults – flow can be very fast.
Sherwood Sandstone aquifers benefit from Primary AND Secondary
porosity/permeability:
– Lots of water ‘stored’ in intergranular primary pores;
– Rapid flow of groundwater through fractures/fissures;
– (very good for water supplies)
And...........

10. Photographs from CCTV down a
public water supply borehole in
the Sherwood Sandstone.
• Primary porosity in the rock-mass;
•Clear fissures showing secondary porosity;
•Nice clear water under these ‘static’
conditions.