Since 1940, the amount of fresh water used has roughly
quadrupled as world population has doubled. Some water
experts estimate the practical upper limit of usable
renewable fresh water lies between 9,000 and 14,000 cubic kilometers
yearly. That suggests a second quadrupling of world water
use is unlikely.
The planet’s renewable fresh water is finite—10,000 cubic miles’
worth is available each year on average—and constraints on its availability
and use are increasingly evident. The growth of the human population inevitably
limits the average availability fresh water per person, and the growing
human thirst for fresh water comes at the expense of natural ecosystems
and threatens the survival of animal and plant species.
In the past, water scarcity was at most a local or temporary problem,
but it is now becoming pervasive and persistent in some regions of the
world.
Desalination
is too labor- and energy-intensive to add much to the world’s supply of
fresh water or to contribute to the availability of fresh water for agriculture.
Pricing water appropriately can encourage more efficient use, but much
of the world’s use of water is not even metered and will be difficult to
price.
Even if technologically feasible, any solution to water shortages that
involves moving massive amounts of water over long distances would have
major impacts on the environment, altering or destroying wetlands and riparian
habitats essential to the survival of other species.
In 1995, about 386 million people in 31 countries lived in conditions of
water stress or water scarcity, based on hydrological benchmarks of the
minimum annual per capita availability of renewable fresh water needed
for economic development. By 2020 the number of people living in such conditions
could be as high as 2.9 billion or as low as 1.2 billion, depending on
the rate of population growth over the next 24 years. By one recent estimate,
more than half of all the world’s accessible renewable fresh water is already
being used, indicating the problems the world may face if population doubles.
Lack of water is already a problem in densely populated urban environments
such as Mexico City and Beijing. This is not just a developing world problem.
Rapidly growing cities in Texas, California, Florida, Arizona and Nevada
are finding that the availability of renewable fresh water is constraining
their prospects for continued growth.
Fresh water is essential for farming, for industry, for human health
and life itself. Every living being on land and in lakes and rivers requires
it, and the more water humans use the less remains for these nonhuman species,
many of them already threatened by habitat loss.
Water Use in the United States:
The bars that stand out most are the blue ones -- water
for electricity production. Electricity water use increased almost
500% from 1950 to 1990. Irrigation
water use increases by about 50% -- it takes more water to grow
food for our increasing population.
Water pollution
occurs when some substance degrades a body of water to such a degree that
water cannot be used for a specific purpose
Major pollutants:
disease-causing agents, such as bacteria
oxygen-demanding waste - organic wastes decomposed by aerobic bacteria
Nutrients such as phosphorus & nitrogen in the form of fertilizers,
manure, sludge, irrigation water, legumes, and crop residues are applied
to enhance agricultural production. When they are applied in excess of
plant needs, nutrients can wash into aquatic ecosystems where they can
cause excessive plant growth, which reduces swimming and boating opportunities,
creates a foul taste and odor in drinking water, and kills fish.
Farmers can implement nutrient management plans which help maintain high
yields and save money on the use of fertilizers while reducing NPS pollution.
Silt in the water can damage some fish’s gills and make breathing difficult.
Cloudy water also absorbs more sunlight than clear water. This may raise
the water temperature. A temperature that’s too high can stress or kill
aquatic organisms.
Silt that settles to the stream bottom is known as sediment. Fish find
some of their food (like aquatic insects) on stream bottoms. An increase
in sediment can kill aquatic insects by suffocating them.
Sediment can also smother fish eggs and alter natural repopulation patterns.
It can also fill in the living spaces and destroy habitat.
Mountaintop mining
Water pollutants vary in degree to which they're degradable
by natural processes:
non-persistent pollutants
Sewage, fertilizers, and some household cleaning products are examples
of non-persistent or degradable pollutants. They are generally less harmful
because they can eventually be broken down and their damaging effects can
be reversed.
persistent pollutants
degrade slowly and remain in the water for years. They may bioaccumulate
up the aquatic food chain, exposing animals, birds, and people who eat
fish to unacceptably high concentrations of chemicals. This can result
in animal and human health risks and in serious environmental damage.
include some pesticides, oil, or specific compounds of oil, and metals
such as cadmium, lead, and mercury.
Sources of water pollution:
Point sources vs. non-point sources
Point sources include sewage-treatment plants, industrial plants, and animal
feedlots
Non-point sources
Nonpoint source (NPS) pollution, unlike pollution from industrial and sewage
treatment plants, comes from many diffuse sources. NPS pollution is caused
by rainfall or snowmelt moving over and through the ground. As the runoff
moves, it picks up and carries away natural and human-made pollutants,
finally depositing them into lakes, rivers, wetlands, coastal waters, and
even our underground sources of drinking water.
agriculture is the leading contributor to water quality impairments, degrading
60% of the impaired river miles and half of the impaired lake acreage surveyed
by states, territories, and tribes. Runoff from urban areas is the largest
source of water quality impairments to surveyed estuaries.
The most common NPS pollutants are sediment and nutrients. These wash into
water bodies from agricultural land, small and medium-sized animal feeding
operations, construction sites, and other areas of disturbance. Other common
NPS pollutants include pesticides, pathogens (bacteria and viruses), salts,
oil, grease, toxic chemicals, and heavy metals.
Beach closures, destroyed habitat, unsafe drinking water, fish kills, and
many other severe environmental and human health problems result from NPS
pollutants. The pollutants also ruin the beauty of healthy, clean water
habitats.
1. Screening.
Wastewater entering the treatment plant includes items like wood, rocks,
and even dead animals. Unless they are removed, they could cause problems
later in the treatment process. Most of these materials are sent to a landfill.
2. Pumping. The wastewater system relies on the force of gravity
to move sewage from your home to the treatment plant. So wastewater-treatment
plants are located on low ground, often near a river into which treated
water can be released. If the plant is built above the ground level, the
wastewater has to be pumped up to the aeration tanks (item 3). From here
on, gravity takes over to move the wastewater through the treatment process.
3. Aerating.
One of the first steps that a water treatment facility can do is to just
shake up the sewage and expose it to air. This causes some of the dissolved
gases (such as hydrogen sulfide, which smells like rotten eggs) that taste
and smell bad to be released from the water. Wastewater enters a series
of long, parallel concrete tanks. Each tank is divided into two sections.
In the first section, air is pumped through the water. As organic matter
decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen
through the water also keeps the organic material suspended while it forces
'grit' (coffeegrounds, sand and other small, dense particles) to settle
out. Grit is pumped out of the tanks and taken to landfills.
4. Removing
sludge. Wastewater then enters the second
section or sedimentation tanks. Here, the sludge (the organic portion
of the sewage) settles out of the wastewater and is pumped out of the tanks.
Some of the water is removed in a step called thickening and then the sludge
is processed in large tanks called digesters.
5. Removing scum. As sludge is settling to the bottom of the
sedimentation tanks, lighter materials are floating to the surface. This
'scum' includes grease, oils, plastics, and soap. Slow-moving rakes skim
the scum off the surface of the wastewater. Scum is thickened and pumped
to the digesters along with the sludge. Many cities also use filtration
in sewage treatment. After the solids are removed, the liquid sewage is
filtered through a substance, usually sand, by the action of gravity. This
method gets rid of almost all bacteria, reduces turbidity and color,
removes odors, reduces the amount of iron, and removes most other solid
particles that remained in the water. Water is sometimes filtered through
carbon particles, which removes organic particles. This method is used
in some homes, too.
6. Killing bacteria. Finally, the wastewater flows into a 'chlorine
contact' tank, where the chemical chlorine is added to kill bacteria, which
could pose a health risk, just as is done in swimming pools. The chlorine
is mostly eliminated as the bacteria are destroyed, but sometimes it must
be neutralized by adding other chemicals. This protects fish and other
marine organisms, which can be harmed by the smallest amounts of chlorine.
The treated water (called effluent) is then discharged to a local river
or the ocean.
R. Wastewater Residuals. Aother part of treating wastewater is
dealing with the solid-waste material. These solids are kept for 20 to
30 days in large, heated and enclosed tanks called 'digesters.' Here, bacteria
break down (digest) the material, reducing its volume, odors, and getting
rid of organisms that can cause disease. The finished product is mainly
sent to landfills, but sometimes can be used as fertilizer.
Freshwater Ecosystems
Wetlands:
once considered useless, disease-ridden places (e.g., malaria and yellow
fever)
provide many benefits to society:
fish and wildlife habitats
natural water quality improvement
flood storage
shoreline erosion protection
opportunities for recreation and aesthetic appreciation
among the most productive ecosystems in the world, comparable to rain forests
and coral reefs. They also are a source of substantial biodiversity in
supporting numerous species from all of the major groups of organisms –
from microbes to mammals.
Littoral zone - light penetrates to the bottom, allowing aquatic
plants to grow
Limnetic zone - the open water area where light does not generally
penetrate all the way to the bottom
Euphotic zone - the layer from the surface down to the depth
where light levels become too low for photosynthesis
Benthic zone - the bottom sediment
Major threats to our lakes:
An overabundance of nutrients. This leads to algal blooms and excessive
plant growth which ultimately deplete oxygen supplies for fish and some
other aquatic life.
An overabundance of sediment. This "runoff" soil can fill lakes and destroy
habitat for plants and animals, as well as clog fish gills and smother
fish eggs.
Metals and other organic chemicals such as polychlorinated biphenyls (PCBs),
contaminating fish and shellfish.
Sources of lake pollution:
Agricultural management practices can lead to pollutants like nitrogen,
phosphorous, sediment, & pesticides and entering a lake.
Runoff from pavement and lawns in urban areas picks up oil, metals, bacteria
(including E. coli), nutrients, and transports them through the
storm sewer system.
Septic systems also contribute to lake pollution when they leak into the
shallow groundwater. This can also increase the load of nutrients, bacteria
(including E. coli) and other organic wastes.
Oceans
70% of the earth’s surface
Zones:
The ocean bottom is the benthic zone and the water itself (or
the water column) is the pelagic zone. The neritic zone is
that part of the pelagic zone that extends from the high tide line to an
ocean bottom less than 600 feet deep. Water deeper than 600 feet is called
the oceanic zone, which itself is divided on the basis of water
depth into the epipelagic, mesopelagic, and bathypelagic zones. These zones
roughly correspond to the three other zones divided on the basis of the
amount of sunlight they receive. In the sunlit zone, enough light
penetrates to support photosynthesis. Below that lies the twilight zone,
where very small amounts of light penetrate. Ninety percent of the space
in the ocean lies in the midnight zone, which is entirely devoid
of light.
Two important communities found in the neritic province are:
a partially enclosed body of water formed where freshwater from rivers
and streams flows into the ocean, mixing with the salty sea watere
among the most productive ecosystems on earth, creating more organic matter
each year than comparably-sized areas of forest, grassland, or agricultural
land
provide habitat for more than 75% of America's commercial fish catch, and
for 80-90% of the recreational fish catch
the unintended catch of animals associated with commercial fishing operations,
the vast majority of which is discarded back into the ocean already dead
or dying.
Bycatch is pervasive the world's fisheries. It includes undersized or juvenile
fish of targeted species as well as
non-target species of fish, turtles, marine mammals, birds, and
other wildlife.
