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By Alfred R.
Conklin
There are two connections between
storm water and drinking water. One is water, which runs over
the earth’s surface and winds up in streams, ponds, lakes and
rivers from which it is withdrawn to use as drinking water.
The other is through plants into and through the soil to
ground water. From there it is withdrawn from wells to be used
as drinking water.
In the first instance water picks
up contaminants, both organic and inorganic, and deposits them
in bodies of water. Hydrocarbons, in the form of fuel and
grease from automobiles, busses and trucks, constitute common
organic contaminants. In reality they also include any organic
compound used in the home or in industry. They also include
organic matter both living and dead. This could be the bodies
of dead animals as well as their manure and the microorganisms
that go along with both. Inorganic contaminants can range from
acids used in cleaning brick to bases used in cleaning houses.
In the second case water takes a
tortuous path to the surface and then through the pores of
soil. In this process it is acted upon by many physical
processes, chemical reactions and biological processes. Larger
particles of either inorganic or organic material are trapped
and filtered out. Organic and inorganic molecules are ad- and
absorbed by soil clay and organic matter. Some is also trapped
in small soil pores. The form of inorganic chemicals is
changed by chemical reactions and by living organisms. Organic
matter is decomposed by soil microorganisms leaving carbon
dioxide, water and humus.
Microorganisms added to soil in
storm water are introduced into a hostile environment. They
must compete with microorganisms adapted to the varying water
and air content, nutrient availability and pH of soil.
Pathogenic organisms find the constant variation in
temperature and pH particularly difficult to survive.
Organisms are attracted to the charges on soil particles and
are also physically filtered out. These organisms can only
survive in soil by entering a resting or hibernation phase,
which protects them from this hostile environment.
Not all rainwater enters the soil.
No matter how well soil is vegetated some water runs off and
some erosion, called geologic erosion, occurs. This amounts to
1000 kg/ha or less of soil per year (1000 lbs. per acre per
year). It is important to keep in mind that soil erodes from
the surface down. The top most layers are removed first and
anything on the soil surface is moved along with them. This
eroded soil moves into streams, rivers, ponds and lakes. It
may be deposited in streams and rivers or it may be carried by
them to a pond, lake or to the ocean and deposited there. It
may move directly into ponds and lakes where it is immediately
deposited as the water velocity decreases upon entering the
body of water.
Accelerated erosion occurs in
agriculture. When the best soil conservation practices are
used this amounts to 2 000 to 3 000 Kg per hectare per year.
The majority of agricultural chemicals, lime, fertilizer
herbicides and insecticides are applied to the soil surface.
Even in cases where the chemicals are incorporated into the
soil they are most often applied to the surface first and
incorporated some time later. Because of the method of
application, soil chemicals are subject to erosion along with
soil and thus can endup being deposited in bodies of water,
which subsequently become sources of drinking water.
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Agricultural materials which endup
in bodies of water are not limited to agricultural chemicals.
Manure and microorganisms associated with animals and animal
production are also eroded along with soil and wind up in
bodies of water. Both of these components make water unfit for
use as drinking water unless it undergoes extensive
purification. In addition these components result in a high
biological oxygen demand in water. This will also have
detrimental effects on the water quality.
One should not believe that
agriculture is the only source of soil and pollutants in
drinking water. At all construction sites, housing
developments, road construction and others, erosion and
pollution of water occurs. This water also finds its way into
streams, rivers, ponds and lakes. Whenever the soil surface is
bare and left that way for a period of time the soil is
subject to erosion. In addition any chemicals spilled on the
soil in the construction process will be eroded with soil and
wind up in water.
To keep storm water as clean as
possible we need to control water running off houses, and
roads in the city and prevent soil erosion in all other
situations. Roofs and roads are impervious to water and it
runs off these surfaces. In the process it washes off organic
and inorganic debris,. The water then runs into storm water
drains and finally into bodies or water.
If storm water is passed through
sand before entering a body of water it will be significantly
cleaner. Proper construction of sand filters can increase
their percolation rate while maintaining their filtering
capacity.
Storm water from open or
agricultural land can be completely eliminated very simply.
Soil erosion starts by raindrops hitting the bare soil surface
and dislodging primary soil particles, sand silt and clay.
This is called splash erosion (see diagram). It can be
controlled by preventing rain from hitting the soil surface.
The simplest way to do this is to have plants growing on it.
Leaves adsorb the energy in the raindrops and gently move the
water to the soil surface. Plant residues covering the soil
surface, straw for instance, will do the same thing over the
short term but are less effective.
The second step in soil erosion is
the movement of water over the soil surface. Water movement
over the soil surface, called sheet erosion, is again
controlled by having plants growing on it. The more plants the
better. Not only do plants prevent or slow water movement over
the soil surface they also encourage infiltration. Water
infiltration into vegetated soil is much faster than into
unvegetated soil. Water that moves into the soil cannot erode
it. In addition movement through the soil, as mentioned above,
results in it being cleaned.
Other more costly practices can
also be used to control water movement over the soil surface.
Low embankments, sometimes called bunds, can be used to
prevent water movement across a slope. Terracing has been in
common use for preventing or slowing the movement of water
over fields.
Implementation of simple and
inexpensive methods of water control and filtration can
decrease the amount and improve the quality of storm water.
Sand filters and soil erosion control are two such methods.
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