Page 73 - Soil and water contamination, 2nd edition

60 Soil and Water Contamination

Groundwater is replenished by percolation water from the unsaturated zone. In some cases,
surface water may also be a source of groundwater if the surface water level is higher than
the water table , so the surface water infiltrates. The replenishment of groundwater is also
referred to as recharge . As mentioned above, groundwater flow s to areas of low hydraulic
head , which are usually surface water bodies, such as rivers and lakes , and low-lying wetlands.
In these areas, groundwater is discharged and becomes surface water. Groundwater can also
be discharged into the atmosphere via evapotranspiration , leaving dissolved salts behind in
the soil (see Section 3.2).
Groundwater flow velocities are slow and range between stagnant to tens of metres per
day; they are proportional to the gradient in hydraulic head and the hydraulic conductivity .
The hydraulic conductivity refers to the ability of the porous material to transmit water
and is largely determined by the texture of the porous material. Sediments made up of
coarse materials have a larger hydraulic conductivity than those made up of fine particles.
At equal gradient in hydraulic head, the groundwater flow velocity is much slower in clay
than in sand. The hydraulic conductivity also depends on the sorting of the particles.
In poorly sorted materials, small particles block the pore space between larger particles,
thereby reducing the hydraulic conductivity. Well-sorted soil material contains particles of
similar size, giving the soil a relatively large hydraulic conductivity compared to material
with a similar average size but poorly sorted. The apparent groundwater flow velocity is also
inversely related to the effective porosity of the material, i.e. the volume of interconnected
pores relative to the volume of the bulk sediment. Thus, groundwater flows faster in
sediments with a small porosity than in sediments with a large porosity, because the same
volume has to flow through a smaller fraction of the total cross-sectional area. Typical values
of the porosity of sediments range between 0.2 and 0.4.
Groundwater flow in the different aquifer types as mentioned above obeys the same
basic principles, but the different aquifers react differently to changes in hydraulic head .

In the case of a lowering of the hydraulic head in an unconfined (phreatic ) aquifer, pores
are drained and partly filled by air in the zone that is vacated by the groundwater. This

drainage cannot happen in confined aquifers because they are not in direct contact with the
atmosphere. Instead, the pore space that is released due to a lowering of hydraulic head is
compensated by compression of the aquifer or expansion of the water. Consequently, the
amount of water released by a drop in hydraulic pressure is much larger in an unconfined
aquifer than in a confined aquifer.
3.3.2 Sources of groundwater pollution

The major sources of groundwater pollution are principally the same as those of soil
pollution and include landfills (waste dumps), accidental spill s, agriculture, septic tanks ,
and atmospheric deposition . Dissolved pollutants move with the percolating soil water into
groundwater, while organic liquid pollutants may reach the groundwater autonomously. In
addition, in areas where surface water infiltrates to groundwater, surface water pollution is a
potential source of groundwater contamination.
The input of pollutants into groundwater from the unsaturated zone and surface water
is often closely related to fluctuations in groundwater level. A rise in groundwater level is
often accompanied by an increased percolation by which pollutants are transferred from soil
to groundwater. Furthermore, the raised water table may also capture pollutants present in
soil for further transport. Conversely, a falling groundwater level results in changing redox
conditions in soil, which may influence the leaching of pollutants from soil. Whether the
leaching is enhanced or decreased depends on the chemical properties of the pollutant
Another source of pollutants in groundwater is the artificial input of contaminated water
by deep well injection. One application of deep well injection is for storing liquid wastes in

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