Water and the Hydrologic
is one of the central materials that determines the conditions of life
on Earth. Water makes up eighty to ninety percent of all living organisms.
Most organisms can survive only in a limited range of temperatures. Water
provides one of the main buffers between the living entity and the environment,
preventing sudden changes in temperature.
Greek philosophers considered water to be one of the four elements along
with earth, wind, and fire. At the end of the eighteenth century, chemists
were able to decompose water and show that it is made of oxygen and hydrogen.
As it is a vital compound in life—our bodies are 70% water—and
as the compound that keeps the Earth's environment fit for life, it is
indeed an "element" of what makes up the conditions on Earth.
to have been on the Earth throughout its 4.5 billion year history. The
Earth is the only planet of the triplet of "identical" planets—Mars,
Earth and Venus—with surface oceans. Mars and Venus have dry surfaces.
The clouds on Venus are not made up of H2O like the Earth's
clouds, rather they are made up of CO2. Most of the Earth's
water resides in the oceans.
that water was part of the original primordial soup in which complex molecules,
including proteins, were first formed—eventually leading to life.
has several properties that make it a unique compound in its ability to
support life. The properties are: its latent heat, its density, and its
ability to dissolve so many substances. All of these properties come from
the peculiar molecular composition and the geometry of the water molecule.
Because of the importance of the structure in determining the properties
of water as liquid water and as ice, we describe the molecular structure
and the liquid state in some detail.
peculiar geometry of water with an angle of approximately 105° (104.5°
to be precise) between the hydrogen bonds turns out to be the basis of
the miracle of our planet. First, let us look at how the molecule is formed
when two hydrogen atoms and an oxygen atom combine. Figure W1 shows the
valence electrons of H and O atoms, and the covalent bonding of H20.
W1. Scheme of formation of covalent bonds between H and O to form
water. The two "open" pairs of electrons in oxygen are called
the non-bonding pairs.
that with the sharing shown, H and O atoms complete their shells for part
of the time. However, as oxygen has a higher positive charge in the nucleus
the shared electron in each bond spend a larger fraction of time in the
oxygen orbital. This makes the oxygen side of the molecule more negative
than the hydrogen sides. The actual reason that the molecule is bent rather
than linear has to do with the mutual repulsion of the unshared pair of
electrons in oxygen and is beyond our scope here. The molecule has a shape
shown schematically and in terms of the actual spatial configuration in
Figure W2 a and b.
shape of the water molecule is an isosceles triangle in which the H—O—H
bond angle is approximately 105°. The illustration on the right
shows the scheme of covalent bonds in water. Altogether, there are eight
valence electrons in the water molecule: six originally belonging to the
oxygen atom and one each to hydrogen. Four are involved in the O—H
bonds, two in each. The remaining four belong to oxygen, and are in nonbonding
orbitals. This gives water its peculiar geometry and charge character.
The molecule is electrically polar, that is, it has a net positive charge
at the hydrogen end of the triangle and a net negative charge at the oxygen
atom. In a group of water molecules clustered together, a positively charged
region in one molecule tends to be attracted to the negatively charged
region in another. There are two positive regions in each water molecule
(the two hydrogen atoms). There are two negative regions projected by
the nonbonding electrons. Each of the nonbonding pairs of electrons attract
a positive hydrogen atom on a neighboring water molecule, and each of
the hydrogen ends attracts the oxygen end of a neighboring water molecule.
This results in each water molecule having four nearest neighbors. This
type of bonding between molecules, due to hydrogen atoms forming a "bridge,"
is called a hydrogen bond.
W3 a and b show the scheme of a hydrogen bond, and the spatial configuration
of water. Figure W3c shows how the continuously moving liquid water molecules
make transient hydrogen bonds with one another, forming a fluid network.
W3 (a). The
hydrogen bond is the weak covalent bond between the hydrogen in
one water molecule with the two electrons in the non-bonding orbital
of oxygen in another water molecule. The hydrogen bond is very weak
(about 5 kcal/mole) compared to the H-O bond in its original molecule
(about 110 kcal/mole).
configuration results from the hydrogen bonding in liquid water, magnified
about a million times the original size. The shadows suggest the constant
motion of the molecules.
W3 (c). Models of the three-dimensional network formed by the
liquid water molecules, creating a fluid network.
temperatures, this structure is highly geometrical and coordinated. This
ordered structure is the structure of ice that in effect gives the hexagonal
shape to snowflakes. At a temperature close to the freezing point, the
kinetic energy of the water molecules is small and the hydrogen bonds
keep the molecules in place but still not very closely packed. This is
sketched in Figure W4.
Figure W4. The structure
of ice, magnified a million times. The atoms come together in a hexagonal
hydrogen bonding means that ice has a lot of empty space. When ice melts,
the "frozen" geometry is removed, but not all the hydrogen bonds
are broken. The molecules begin to pack more closely together so as to
fill some of the empty space. Thus liquid water is denser than ice. Water
has its greatest density at 4°C. This is why the top of a lake freezes
first. The cooler part freezes and the more dense water at slightly higher
temperature sinks to the bottom. The bottom freezing last helps protect
fresh water organisms that live in the bottom. The empty space also means
that ice does not conduct heat very well. So, the frozen top of the lake
keeps the heat from the water below it from escaping too readily, maintaining
it as liquid. This characteristic is central to maintaining aquatic life
Even when ice converts to water at 0°C, only about 15% of the bonds
are broken. So, cold water molecules are still relatively bound together.
Although molecules are constantly in motion, local order is still mostly
maintained, as molecules remain bound to one another even as they move
fast. Thus, it is able to absorb a lot of heat without significant change
in temperature. That is, the heat capacity of water is higher than most
substances including air. Oceans do not suffer from sudden changes in
temperature, which is important for living systems in the oceans. The
ocean also buffers the climate on its shore. The large quantity of water
on Earth serves to prevent sudden rises of temperature, between night
and day for example .
a lot of heat and thermal motion of the molecules to finally break all
the bonds, and to vaporize the water. All of this means that it takes
high amounts of energy to change the states of water from solid to liquid
and liquid to gas - water has a high latent heat of fusion (energy required
for melting) and a high latent heat of vaporization (energy to vaporize).
Land plants and animals are able to dissipate a lot of heat simply by
evaporation of water through transpiration or by sweating, in the case
of animals. Under extreme conditions such as in a desert a human body
may evaporate as much as one liter of water per hour by sweating to rid
itself of heat.
nature and empty spaces in water also make it a good solvent. The polar
nature gives rise to the high surface tension of water. This high surface
tension makes water capable of rising in capillary structures of roots
and stems. It also gives firmness to the surface of lakes so that light
insects can actually sit on the surface.
is a greenhouse gas. Both the capability to keep heat in, and to transfer
heat from the tropics serve to buffer temperatures on Earth. For example,
as one half of the Earth rotates away from the sun, the fall in temperature
is much more gradual than it would have been if there were no water vapor
in the atmosphere. Thus water has a combination of properties that accounts
for its central role in preserving life on Earth: liquid denser than solid;
high surface tension; high heat capacity; and high solubility.
Cycle and Water Balance
a fundamental necessity for all ecological systems, as it is a cornerstone
of life. Estuaries (where river and sea meet) are an aquatic environment
that is important to the life cycle of many species. Water is cycled through
evaporation and transpiration from plants into the atmosphere, and precipitation
back to Earth.
Four-fifths of the water in the global water cycle comes from the oceans.
Of a total of about 1.36 billion cubic kilometers of water, about 97%
is in oceans. An additional 2% is locked up in glaciers and icecaps, and
0.31% is stored in deep groundwater reserves.
only about 4.2 million cubic kilometers of relatively accessible fresh
water. Water evaporates from the large surfaces of the Earth's oceans.
It is estimated that 41,000 cubic kilometers of water returns to the sea
from the land per year, balancing the transport of water from sea to land
through the atmosphere as precipitation. About 32,000 cubic kilometers
return to the sea as runoff that cannot be captured. The remaining 9,000
km3, is potentially available as water supply for people. This
could theoretically supply 20 billion people. During the past 300 years,
human water use has increased 35-fold. But the availability is far from
uniform on the Earth's continents. There are places in the Middle East
and Africa which have no access to natural fresh water. In addition, some
people have lifestyles that consume much more water than others. The average
U.S. resident consumes 70 times as much water per year as an average resident
States Geological Survey website has a section completely devoted to the
Water Resources of the
United States. The water withdrawn for public supply during 1995 was
an estimated 40,200 Mgal/d. Public suppliers served about 225 million
people during 1995. Total public supply withdrawals in 1995 averaged 1979
gal/d for each person served. The pie charts below show the amount of
surface water used and ground water used for public consumption. Public
supply refers to water withdrawn by public and private water suppliers
and delivered to multiple users for domestic, commercial, industrial,
and thermoelectric power uses.
cycle also cleans the environment. Clouds and run off transport and deposit
pollutants into lakes and oceans. The transport and deposition of SO2
and NOx in pollution is the environmental problem known as
acid rain. SO2 and NOx come from sources such as
fossil fuel burning as well as from some natural sources such as volcanoes.
The amounts of water cycled per day is enormous and highly variable. One
thousand gigatonnes of water evaporate from the oceans each day. In some
places like the southern coastal regions of Peru, decades may pass with
no rain at all. The water cycle is one cycle that has been manipulated
by technology through building dams and hydroelectric energy plants.
W5: Hydrologic Cycle in Quantities
and Pollution of Water
water is so abundant on Earth that we call the Earth a "water planet,"
97% of this water is salt water. Only about 3% of water is fresh water,
and less than 0.01% is readily available from rivers and lakes.
of availability of fresh water arise from agriculture--both the vast amounts
used in irrigation and the pollution arising from pesticides and fertilizers--,
industrial pollution, and pollution from sewage. Some types of industrial
pollution of water (like heavy metal pollution) have been realized only
relatively recently. Water had historically been thought of a "bottomless"
sink for pollutants. A statement often used by industry as recently as
a couple of decades ago was "dilution is the solution to pollution."
These attitudes and the fact that water is relatively plenty in highly
industrialized nations delayed our recognition and addressing of water
hydrologic cycle is continuously at work globally, local conditions of
rain and fresh water supply vary tremendously. This uneven distribution
of water determines the nature of many of the problems related to fresh
water management and use. The average residence time of a molecule of
water in the atmosphere is about eight days. The residence time of water
in deep ground water aquifers, or large glaciers, may be hundreds, thousands,
or hundreds of thousands of years.
shows the major salt and freshwater stocks on Earth, and the small amount
of freshwater available. Less than 1% of this is actually usable.
of total water
3: Water stocks on Earth.
W6 shows the annual fresh water availability for selected countries.
W6. Average Annual Water Availability for Selected Countries,
1921-1985. The average amount of fresh water available for various
countries is shown here, in cubic kilometers per year, as measured
over the period 1921 to 1985. This figure shows the vast differences
in the natural distribution of fresh water among different regions.
Source: The World's Water 2000-2001: The Biennial Report on Freshwater
speak for themselves. While these amounts themselves are low, we also
add to the problem by polluting our groundwater. Groundwater lies mainly
deep underground in aquifers which are geological niches of porous materials
or space between rocks. Farms, cities, and factories all have run-off
that can get deep into the ground and pollute even aquifers. Industrialized
agriculture has also been responsible for digging deep into the water
of Ground Water
and pesticides applied to cropland, organic wastes from farmlands, and
sewage from cities all pollute ground water. Nitrate pollution of groundwater
due to these sources have become very severe. In California's central
valley, nitrate level in ground water almost tripled between 1895 and
1980. One of the effects of nitrates in groundwater is the so-called blue-baby
syndrome or methenoglobinemia in which the oxygen-carrying capacity of
the baby's blood decreases.
of various kinds have entered many aquifers and are one pathway for organochlorine
compounds--also known as endocrine disrupters--to enter our systems. The
effects of organochlorines are discussed in the unit on Risk
& Human Health.
the problem of water pollution was first realized, the remedies sought
were end-of-pipe--cleaning up polluted water--rather than conservation
of fresh water or prevention of pollution. Water purification technologies
were developed. Creative redesign of industrial processes and water conservation
technologies like low-flush toilets have begun to get serious attention
only recently. Industry is just beginning to design and implement methods
that reduce water pollution. The use of "grey water" (water
that has only been partially re-cleaned) for high water use applications
such as agriculture has not received much attention in the U.S.
resources in the US
Drinking Water Act (1974) - national interim primary drinking water
regulations - established by EPA in 1977.
1986, with the passage of the amendments to the Safe Drinking Water Act,
(EPA was mandated specifically to regulate microbiological constituents,
inorganic and organic compounds, and radioactivity) 83 contaminants were
to be regulated in the initial stage with an additional 25 contaminants
to be added every 3 years. In 1988, regulatory efforts focused on lead.
The latest amendments to the Safe Drinking Water Act occurred in 1996.
The criteria for the selection and regulation of contaminants was a key
amended item. EPA no longer has to set 25 new standards every three years.
Regulated contaminants either need to have adverse health effects or are
present at levels sufficiently high to warrant public concern. Development
of regulatory levels are to be based on risk assessment, cost-benefit
analysis, and minimizing overall risk.
demand *make link with oxygen cycle see last paragraph*
note = Hydrologic Cycle = "natural model" for scientific model