Page 72 - Soil and water contamination, 2nd edition

Environmental compartments 59

3.3 GROUNDWATER
3.3.1 Definition of groundwater

Groundwater is defined as the water present in the saturated zone beneath the water table .
As noted above, the pore spaces between the particles of the porous medium are completely
or nearly completely filled with water, so that all water is in contact and the water pressure
increases proportionally with depth. The groundwater slowly flows from places with high
hydraulic head , i.e. a measure of potential energy consisting of an elevation term and
a pressure term, to places with low hydraulic head. The hydraulic head is measured by
determining the position of the water table in an unpumped well or piezometer (i.e. a pipe
open at the top and bottom and placed vertically in the soil) relative to a reference surface.
A water-yielding rock formation that contains and is able to transmit sufficient
groundwater to be a source of water supply is called an aquifer . Although aquifers are often
comprised of a stratum or combination of strata of coarse unconsolidated rock, such as
gravel or coarse sand, they can also consist of porous rock, such as sandstone , or non-porous
but fractured rock, such as limestone or granite. Aquifers are bounded by a layer of low
permeability (aquitard ) or an impermeable body of rock (aquiclude ). If there is no aquitard
or aquiclude on the top of an aquifer, the aquifer is said to be unconfined or phreatic . A

phreatic aquifer is bounded from above by a phreatic surface, which is defined as the surface
at every point of which the pressure in the water equals the atmospheric pressure. A special
case of a phreatic aquifer is the perched aquifer , which is a phreatic aquifer formed on a semi-
permeable, or impermeable, layer below which unsaturated material is present. Confined
aquifers are aquifers that are bounded by aquitard or aquiclude at both the top and bottom.

The hydraulic pressure at the top of a confined aquifer may well be higher than atmospheric
pressure. Artesian aquifers are a special case of confined aquifers and are characterised by a
hydraulic head that is higher than the local ground surface level, so that water can flow out of
wells without any need for pumps. Figure 3.4 depicts the different types of aquifers.

Groundwater Groundwater Groundwater
recharge discharge recharge
area Hydraulic head area area
unconfined aquifer Hydraulic head
(water table) confined aquifer
Perched Piezometer
aquifer
Artesian well
adose zone
V V Vadose zone
adose zone
Stream
Groundwater
Groundwater
Groundwater f f fl l lo o ow w w l l li i in n ne e e
Unconfined aquifer
Unconfined aquifer
Aquitard
Aquitard
Aquitard Unconfined aquifer
Confined aquifer
Confined aquifer
Confined aquifer
Artesian aquifer
Artesian aquifer
Artesian aquifer
Bedrock
Bedrock
Bedrock
6642 6642 6642
Figure 3.4 Different types of aquifers.

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