WJEC – Geology of the Human Environment (GL3) – June 2001
1.
Figure 1a shows the rise in groundwater levels beneath Central London since
1905.
Figure 1a
(a) What is meant by the term water table?
(1)
…………………………………………………………………………………………..
(b)
Refer to Figure 1a:
(i) Briefly compare the average rate of rise in the water table before
and after 1950.
(2)
………………………………………………………………………………………….
………………………………………………………………………………………….
(ii) In 1999 groundwater levels were rising at a rate of 3 metre/year.
Calculate the year in which the water table would reach street level
(zero metres) if the 1999 rate were maintained. Show your working.
(2)
Year…………………..
(c) Describe one possible hazard associated with an increase in each of the
following:
saturation of the clay
groundwater (pore) pressure.
(4)
Clay saturation…………………………………………………………………………..
………………………………………………………………………………………….
………………………………………………………………………………………….
Groundwater pressure …………………………………….……………………………
………………………………………………………………………………………….
………………………………………………………………………………………….
A network of 50 extra boreholes is planned to reduce the water table levels in the
London basin. Figure 1b shows a borehole situated near to the Thames estuary.
Figure 1b
(d)
(i) On Figure 1b, show how overpumping from the borehole can
locally exhaust the water supply.
(2)
(ii) Suggest two additional hazards associated with overpumping in
Figure 1b. You may annotate the figure if you wish.
(2)
1.……..…………………………………………………………………………….…………….
……………………………………………………………………………….…………………..
2 ..……………………………………………………………………….……………………….
…………………………………………………………………………………………………..
Total 13 marks
Notes for answers
These represent suggested answers and do not necessarily represent all the possible
acceptable alternatives of a mark scheme.
Question 1
1.
(a) Upper surface(level/boundary) of groundwater/saturated rock
(b)
(c)
[1]
(i) Greater rate after 1950
Twice as fast, Accept numbers -0.4 & 0.8 m/yrs Or 20m/45 yrs etc
(2 max)
[2]
(ii) Working :- 40/3 = ~13 years
Year :- 2012 -2013
[2]
Hazard described
Flooding underground/basements
Shorting of electrical equipment
Effect of acid groundwater on foundations
Building subsidence as clay becomes ductile
Clay expansion
Other sensible (2 max)
[2]
Hazard related to pressure
Pressure on building foundation - “popping”
Tube tunnels become distorted
Pressure on pipes causing damage etc
Expansion of clay causing stress leading to landslip.
Not rising water table - causing flooding but
Flooding of basements/tube etc caused by pressure of groundwater
Instability needs to be qualified for a mark.
Other sensible (2 max)
[2]
(d)
(i)
(ii)
Water table depressed around the borehole
Cone of exhaustion indicated by annotation
(2 max)
Surface subsidence
Groundwater pollution
Saltwater contamination
[2]
[2]
Total 13 marks
ESTA would like to thank the WJEC for allowing this question to be published on GEOTREX. Any
comments should be directed to the Subject Officer for Geology at the WJEC. 15/04/2007