Environmental Engineering Reference
In-Depth Information
Climatic conditions are also significant. In areas of high rainfall, the pollution potential
is greater than in less moist areas. In semiarid regions there may be little or no pollution
potential because all infiltrated water is either absorbed by the material holding the con-
taminants, or is held in soil moisture and eventually evaporated.
The character and strength of the contaminant are dependent, in part, on the length of
time that infiltrated water is in contact with the waste and the amount of infiltrated water.
The maximum potential for groundwater pollution occurs in areas of shallow water tables,
where the waste is in constant contact with the groundwater and leaching is a continual
process.
Liquid Waste Disposal
Sewage plant treatment
prior to discharge into the surface or groundwater system is the
most effective means of preventing groundwater pollution, provided that the treatment
level is adequate for the particular sewage.
Lined reservoirs for industrial waste
are constructed with plastic or clay liners to provide a
barrier to infiltration, but storage capacity limitations usually require that the waste even-
tually be treated. Moreover, open reservoirs can affect air quality.
Deep-well injection
into porous sandstones, limestones, and fractured rock masses can be
a risky solution unless it is certain beyond doubt that an existing or potential aquifer can-
not be contaminated.
Septic Tanks and Leaching Fields
Sanitary wastes from homes and other buildings are often disposed into septic tanks with
or without leaching fields. The biological decomposition of solids by anaerobic bacteria
takes place in the septic tank and a part of the solids remains. A large proportion of the
harmful microorganisms, however, is not removed from the waste.
Effluent from the tank may be discharged into the ground by means of a seepage bed,
pit, or trench. The rate at which the soil absorbs the effluent is critical to the system's oper-
ation. If it is not absorbed rapidly enough, it may back up into the drains from the build-
ing and eventually rise to the ground surface over the seepage area. If it drains too rapidly,
it may travel unfiltered into wells or surface water supplies and contaminate them with
various types of disease-bearing organisms. If the flow rate is intermediate, nature will act
as a purification system through microbial action, adsorption, ion exchange, precipitation,
and filtration.
Septic tanks and leaching fields will not work in clayey or silty soils, below the water
table, or in frozen ground, since flow is required for their effectiveness. They are suitable
in lightly populated areas where land is adequate and geologic conditions are favorable,
or in developments where use is intermittent, such as vacation dwellings. When used in
impervious materials without leaching fields, the waste should be pumped periodically
from the septic tank into trucks. Continuous use over a long period of time in densely pop-
ulated areas, even with favorable geological conditions, may eventually result in the pol-
lution of surface and subsurface waters.
Protection against Spills
Spills from chemical plants and other industrial sources often cannot be avoided in many
industrial processes, especially over long time intervals. It is best, therefore, to locate
plants where spills cannot contaminate valuable aquifers.
Control can be achieved, however, with systems of impervious barriers such as a slurry-
wall cutoff and a wellpoint dewatering system. In the example illustrated in
Figure 8.60,
