Water is often referred to as the "universal solvent" and this is appropriate
as its primary use is as a solvent. Approximately three quarters of the earth’s surface is
covered with water and it would be impossible to discuss the science of water without first
looking at its molecular structure.
THE WATER MOLECULE
The formula for water (H²0) by itself, tells us only its composition and molecular
weight. It does nothing to explain the remarkable properties that result from its unique
molecular arrangement.
As can be seen in the diagram two hydrogen atoms are bonded to a single oxygen atom
separated by an angle of 105º. Because of the tendency of the oxygen atom to withdraw
electrons from the hydrogen the molecule is said to be dipolar. This property causes the
individual water molecules to attach to agglomerate with the hydrogen of one molecule
attracting the oxygen of an adjacent molecule and vice versa.
A result of this hydrogen bonding is that the molecules of H²0 cannot leave the surface of a
body of water as readily as they could if there was no inter-molecular attraction. In fact, large
quantities of energy are required to break the hydrogen bond and thus liberate a H²0
molecule to the vapour phase, and as a result water vapour or steam has a high energy
content and is most useful as a medium for transferring energy in industrial and domestic
applications.
Hydrogen bonding is also responsible for the unique crystal arrangement which water forms
upon freezing causing the ice to expand and occupy a greater volume than it would as a
liquid with a consequent change in density. Thus the solid phase (ice) will float on the liquid
phase (water).
Another consequence of this hydrogen bonding is the property of high surface tension. This
property causes water to rise in a capillary tube, a property essential for the survival of
plants.
A meniscus forms (left above) when hydrogen atoms
reach upwards to wet oxide surfaces at the water line
in a glass tube. The drawing at the right shows how
"hydrogen bonding" of water to a thin glass tube
causes the water in the tube to rise above the level
of the surrounding water.
The electric properties of water are also responsible
for its unique solvency and hydration effects which
allow it to dissolve other ionic materials readily and
prevents ions from recombining and
precipitating from solution. For example:
Another important property of water solutions is the
phenomenon of osmotic pressure. This occurs if two
aqueous solutions are separated by a semi-permeable
membrane. Then the solvent (H²0) will move through
the membrane from the more dilute solutions to the
more concentrated one. This process controls
the performance of all living cells.
This process can be overcome by applying a sufficiently
high pressure to the more concentrated solution, and
this is the osmotic pressure. If a higher pressure can be
applied to a concentrated salt solution the solvent can
be made to move through the membrane in the reverse
direction. This is the process of "reverse osmosis" and
can be used in the desalination of water.Percolation
The term "percolation" is employed to describe the passage of water into, through, and out of
the ground. The diagram below shows the conditions in which water occurs below the
surface of the ground. Only water in the saturated zone can be withdrawn from sub-surface
sources, the development of ground water supplies depending upon the yields actually
obtainable and their cost. Unwanted entrance of ground water into manholes and pipes is an
important matter in sewage design.
Ground water is derived directly or indirectly from precipitation: (1) directly as rain water and
snow melt that filter into the ground, seep through cracks or solution passages in rock
formations, and penetrate deep enough to reach the ground water table; (2) indirectly as
surface water from streams, swamps, ponds, lakes, and reservoirs that filters into the ground
through permeable soils when the ground water table is lower than free water surfaces.
Streams that re-charge the ground are known as "influent" streams; streams that draw water
from the ground as "effluent" streams.
The water table tops out the zone of saturation; the capillary fringe overrides it. The fringe
varies in thickness from a foot or so in sand to as much as 10 ft in clay.
Soil water is near enough to the
surface to be reached by the roots
of common plants. Some soil water
remains after plants begin to wilt.
Stored or pellicular* water adheres to
soil particles and is not moved by
gravity.
Gravity or vadose** water moves
down by gravity throughout zone.
Capillary water occurs only in the
capillary fringe at bottom of the zone
of aeration.
Free water occurs below the water
table. Movement controlled by the
slope of the water table.
Confined or artesian water occurs beneath
a confining stratum. Moves laterally as
water in a pressure
conduit.
Fixed ground water occurs in subcapillary
openings of clays, silts, etc.
Not moved by gravity.
Connateî water entrapped in rocks
at the time of their deposition.
It rises and falls with the water table, lagging behind to become thicker above a falling table
and thinner above a rising table. Evaporation is increased when capillarity lifts water to, or
close to, the ground surface. Pollution spreads out along the water table and is lifted into the
fringe. There it is trapped and destroyed in the course of time. Hydraulically, an aquifer
dipping beneath an impervious geological stratum has a piezometric surface, not a ground
water table.
How much rain filters far enough into the ground to become ground water is quite uncertain.
Among governing factors can be listed:
1. Hydraulic permeability.
Permeability, not merely pore space, determines the rate of infiltration of rainfall and its
passage to the ground water table. Only rarely does freezing not reduce permeability.
2. Turbidity.
Suspended matter picked up by erosion of tight soils clogs the pores of open soils.
3. Rainfall patterns and soil wetness.
Light rainfalls have time to filter into the ground, heavy rainfalls do not. Wet soils are soon
saturated; dry soils store water in surface depressions and their own pores. Some stored
water may reach the groundwater table eventually. Heavy rains compact soil, and prolonged
rains cause it to swell. Both reduce surface openings. Air displacement from soils opposes
filtration; sun cracks and biological channels speed it up.
4. Ground cover.
Vegetation retards runoff and increases surface evaporation as well as retention and
transpiration of soil water. Effects such as these are most marked during the growing
season.
5. Geology.
Geological structure has much to do with infiltration. Examples are (a) lenses of
impervious materials which intercept incoming water and keep it from reaching the
groundwater table; and (b) confining layers of tight materials which direct water into closedchannel
flow. Independent zones of saturation above lenses of impervious materials store
perched water; continuous zones of saturation (aquifers) lying between impervious materials
hold artesian water.
6. Surface slope.
Steep slopes hasten surface runoff and reduce infiltration.
The earth’s crust is porous to depths of 3 to 12 kms. Beyond that, pressures are so great
that plastic flow closes all interstices.Groundwater Discharge
In nature, subsurface waters are discharged from the ground: (1) to the surface through
springs and seepage outcrops (hydraulic discharge); and (2) to the atmosphere from the soil
or through vegetation (evaporative discharge).
Hydraulic discharge takes place wherever the groundwater table intersects the land surface.
Geologic and hydraulic conditions that combine to force the return of groundwater to the
earth’s surface as springs include:
(1) outcroppings of impervious strata covered by pervious soils or other water-bearing
formations;
(2) overflows of subterranean basins in limestone or lava;
(3) leakage from artesian systems through faults that obstruct flow; and
(4) steep surface slopes that cut into the water table. In humid regions, groundwater may
seep into streams throughout their length.
The Water Cycle
Precipitation, percolation, runoff, and evaporation are stages in the cycle of water, which is
without beginning or end. Of the water driven to earth, some falls directly upon water
surfaces; some flows over-land and makes its way into brooks and rivers, ponds, lakes, and
reservoirs, or seas and oceans; some is returned at once to the atmosphere by evaporation
from water and land surfaces, and by evaporation and transpiration from vegetation; and
some sinks into the soil. Transpiration is evaporation or exhalation of water or water vapour
from plant cells, leaf cells, for example — and corresponds to perspiration in animals.
Of the water entering the earth’s skin, part is held near the surface whence some of it
evaporates directly and some is taken up by vegetation to be returned to the atmosphere by
transpiration. The remainder of the infiltering water settles downward by gravity until it
reaches the groundwater table to join the subterranean reservoir within the earth’s crust.
Most of the groundwater eventually discharges at the surface of the earth through springs
and seepage outcrops, or it passes, at or below the water line, into streams and standing
bodies of water, including the oceans.
The water flowing in brooks and rivers is derived, only in small part, from direct precipitation,
in largest volume from rain running off the surface of the earth, and in steadiest amounts as
dry-weather flow from the lowering of lakes, ponds, and reservoirs and from groundwater
seepage. Evaporation and precipitation are the principal driving forces in the water cycle.
Solar radiation is the source of needed energy. Runoff and percolation shift the scene of its
evaporation laterally along the earth’s surface; atmospheric circulation does so for its
condensation and precipitation.
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