49
Exercise G - Water abstraction from a well in a confined
aquifer
Theory
An aquifer is a water-bearing layer in the ground in which horizontal flow is possible
due to its inter-connecting void structure. These pores through which the flow takes
place may be very small indeed and, generally, are between the limits 2mm -
0.02mm. The movement is slow compared with surface run-off and the flow is usually
laminar. The Reynold's Number in flows of this kind is very low.
A confined aquifer is one which is capped by an impermeable stratum and, assuming
horizontal plane boundaries, all subsequent water movement must be in horizontal
paths. Recharge on the aquifer can therefore occur only where there is a break in the
permeable cover.
If the aquifer is fully saturated then the water will rise in a borehole, which penetrates
the cover until an equilibrium pressure is obtained at the bottom of the borehole.
Figure G1
The imaginary surface which contains the water surfaces in any such boreholes is
called the piezometric surface and can be thought of as extending in all directions,
see Figure G1. If the piezometric surface lies above the ground surface, water will
flow from a borehole penetrating the aquifer without the assistance of a pump. This
now constitutes an artesian well.
When there is no ground water movement the piezometric surface must be horizontal
and plane. Under these circumstances, the water will rise to the same level in any
boreholes present. If water is removed from a well by pumping, the piezometric
surface is depressed locally as water flows towards the well through the aquifer. This
is the situation shown in Figure G2, and Darcy's Equation relates the local water
velocity in the aquifer to the slope of the piezometric surface above.
Figure G2
Summary of Contents for S12-MKII
Page 1: ...Advanced Environmental Hydrology System Instruction Manual S12 MKII ISSUE 21 February 2014...
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Page 8: ...4 Equipment Diagrams Figure 1 Front View of S12 MKII Hydrology System...
Page 9: ...Equipment Diagrams 5 Figure 2 Plan View of S12 MKII Hydrology System...
Page 10: ...Armfield Instruction Manual 6 Figure 3 End View of S12 MKII Hydrology System...