Environmental Engineering Reference
In-Depth Information
consider that subsurface sewage disposal systems (septic tanks) are used by
almost one-third of the U.S. population. Briefly, a septic tank and leaching
field system traps and stores solids while the liquid effluent flows from the
tank into a leaching or absorption field, where it slowly seeps into the soil
and degrades naturally. The problem with subsurface sewage disposal sys-
tems such as septic tanks is that most of the billions of gallons of sewage
that enter the ground each year are not properly treated. Because of faulty
construction or lack of maintenance, not all of these systems work properly.
Experience has shown that septic disposal systems are frequently sources of
fecal bacteria and virus contamination of water supplies taken from private
wells. Many septic tank owners dispose of detergents, nitrates, chlorides,
and solvents in their septic systems or use solvents to treat their sewage
waste. A septic tank cleaning fluid that is commonly used contains organic
solvents (trichloroethylene, or TCE) that are potential human carcinogens
that pollute the groundwater in areas served by septic systems.
landfills
Humans have been disposing of waste by burying it in the ground since
time immemorial. In the past, this practice was largely uncontrolled, and
the disposal sites (i.e., garbage dumps) were places where municipal solid
wastes were simply dumped on and into the ground without much thought
or concern. Even in this modern age, landfills have been used to dispose of
trash and waste products at controlled locations that are then sealed and
buried under the ground. Now such practices are increasingly seen as a less
than satisfactory disposal method, because of the long-term environmental
impact of waste materials in the ground and groundwater. Unfortunately,
many of the older (and even some of the newer) sites have been located in
low-lying areas with high groundwater tables.
Leachate
(seepage of liquid
through the waste) high in biochemical oxygen demand (BOD), chloride,
organics, heavy metals, nitrate, and other contaminants has little difficulty
reaching the groundwater in such disposal sites. In the United States, liter-
ally thousands of inactive or abandoned dumps like this exist.
Agriculture
Fertilizers and pesticides are the two most significant groundwater contami-
nants that result from agricultural activities. The impact of agricultural prac-
tices wherein fertilizers and pesticides are normally used is dependent on
local soil conditions. If, for example, the soil is sandy, nitrates from fertilizers
are easily carried through the porous soil into the groundwater, contami-
nating private wells. Pesticide contamination of groundwater is a subject
of national importance because groundwater is used for drinking water by
about 50% of the nation's population. This especially concerns people living
