The clean, fresh-water resources of the Earth are not boundless. Nevertheless, they might be sufficient for human needs but for their natural (unaccommodating) distribution and for competing uses.
Fresh water is partitioned into four planetary areas: surface waters, snow and ice, underground aquifers, and the atmosphere. The quantity of water in each of these partitions is vast; each contains thousands of cubic miles of fresh water.* And, with significant limitations of rate and energy requirements, water moves back and forth among them--the "hydrologic cycle." For example, surface waters evaporate into the atmosphere. From there, rain (and sleet and snow) return purified water to the surface of the Earth. Rain runs off into rivers and lakes, or seeps into the ground to replenish aquifers (although some major aquifers are deep and run very slowly, so they are largely "fossil water"; and some are being drained by drilled wells faster than their supplies are replenished). Finally, snow and ice may be melted in the spring, or trapped for years--even millions of years--in glaciers and polar ice caps.
Vast as these fresh water resources are, they are only about 1/40 of the volume of the oceans. However, the salt water of the oceans cannot be used for our metabolic purposes. The salt renders it poisonous to drink or to use for land irrigation. Granted, sea life, both animal and plant, has energy-consuming processes that are strong enough and sharply dedicated enough so that briny water can provide for their metabolic needs. But humans and almost all other land creatures (both animals and plants) cannot perform this chemical magic. We would die in a few days if we did not have access to the renewal of fresh water.
The problem for many human beings is not having adequate access to clean fresh water, both for drinking (and cooking), and for hygiene and sanitation. For children under the age of five, waterborne diseases are the leading cause of death worldwide; at any given time, half of the hospital beds in the world are occupied by patients with waterborne diseases; in fact, 88% of all cases of disease are caused by unsafe drinking water, inadequate sanitation, or poor hygiene. Roughly 1.1 billion people (of the world's population of 6 billion) do not have access to safe drinking water; some 2.6 billion lack adequate clean water for sanitation.
In the past hundred years humanity has become so numerous that natural hydrologic-cycle supplies are now insufficient for our needs. Even with massive purification and reclammation projects (so that, for example, upstream city sewers or farm runoff do not contaminate downstream drinking water), humanity is frankly living waterwise beyond its means. Prehistoric aquifers are being drained--where once a well 10 or 15 feet deep could reach fresh, pure water, now one may need to drill 100 feet or more to reach the aquifer water table. The vast Ogallala Aquifer underlying much of eight U.S. states and serving some 27% of land irrigation in the U.S. is being depleted at the rate of 12 cubic miles of water per year; experts estimate that it will run dry in as little as 25 years.
There are some "fixes" that work. The vast quantities of water that are sprayed on agricultural crops could be used much more efficiently; spraying water onto a field loses as much as 90% of the water to evaporation and runoff whereas drip or seep irrigation can capture 75% of the water for the plants' use. As to ocean water supplies, with enormous energy expenditure (perhaps available from nuclear power sources, ocean thermal energy conversion, or even wind, tide, solar, or geothermal sources) significant quantities of ocean water could be desalinated for human use.
On the other hand, there are some "fixes" that do not work well. For example, dams distort ecological patterns (such as fish migrations) and expose surface water areas to increased evaporation. For another example, an enormous project of the World Bank to introduce tube wells into parched and water-contaminated areas of Bangladesh tapped, inadvertantly, into arsenic-laden aquifers; millions of people in Bangladesh now suffer from acute and chronic arsenic poisoning (and have very limited, grossly inadequate, health resources available to them).**
Humanity has overrun the Earth's fresh water resources, and the problems are getting worse year by year. If civilized human life is to continue on this planet, dramatic actions are urgently needed, both scientific and engineering progress, and political and social changes tantamount to sociocultural revolutions affecting billions.
* Note on partition volumes
About 3,000 cubic miles of water (fresh, of course) is in the atmosphere (as vapor) at any one time
There is about 30,000 cubic miles of surface freshwater on the Earth in lakes and rivers
There is about 3,000,000 cubic miles of freshwater in the world's underground aquifers
And ice caps and glaciers add about 7,000,000 cubic miles more.
The total volume of the oceans is about 32,500,000 cubic miles (this is salt water, of course).
** Update on arsenic in Bangladesh water (from Scientific American, November, 2009)
In 2002 M.I.T. researchers determined that microbes digesting organic carbon were freeing that trapped arsenic. But where did the carbon come from and how was the arsenic getting into the water supply? The M.I.T. team now thinks they have the answers, which they report in the journal Nature Geoscience.
Using a six-square-mile test plot, they found that the organic carbon comes from shallow ponds that were dug to provide soil for flood protection. The carbon compounds sink in the pondwater and seep underground where bacteria digest them, setting up the perfect chemical conditions to free up the soil’s arsenic. Groundwater flow then brings the arsenic-rich water to the wells.
Future wells dug deep enough, to the low-arsenic part of the aquifer, could help. Rice fields filter arsenic from the water, so wells under those fields could also be part of an answer to a problem affecting millions of Bangladeshis.
Bun Gladieux, president of the Presssure Positive Company, has a blog with an interesting series of topics.
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