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

Hydrogeology WorkshopPresent 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)

Hydrogeology WorkshopPresent Geology (2)

Note: Relative thickness of the Glacial Clay,

Sands and Gravel and Alluvium is greatly

exaggerated

Hydrogeology Workshop

Part 2

Introduction to Hydrogeology

Hydrogeology WorkshopIntroduction 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

Hydrogeology WorkshopIntroduction 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.

Hydrogeology WorkshopIntroduction 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

Hydrogeology WorkshopIntroduction 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).

Hydrogeology WorkshopIntroduction to Hydrogeology (5)

Groundwater Flow

Bath Tubs and Black BoxesHow 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...........

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.