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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Water Part II: Water, Water Everywhere, Nor Any Drop to Drink

By Jeff Danner Posted March 26, 2012 at 1:17 am

After last week’s hiatus, I am picking back up with my series on water.   If you want to start at the beginning check out “Water Part I: Is God a Mathematician?”   Part II was going to be about the desalination of water.  As I started to write it, I began explaining some of the background on why the ocean is salty and why most plants and animals cannot tolerate salt water and, low and behold, it became this stand alone column.   I’ll move on to desalination next week.
 
Ocean water is quite salty, with an average salinity of 3.5%, which is 220 times higher than the average lake or stream.  The oceans contain just about every salt known on earth.  But far and away, the most common salt is sodium chloride (NaCl) which represents 85% of the ocean’s salinity.  Sodium chloride should sound familiar as there is likely a small container of it sitting on your dinner table.
 
The oceans became salty over the last several hundred million years via several mechanisms.  Although the fresh water which flows to the ocean contains low concentrations of salt, the salts they do contain accumulate in the ocean.  When water evaporates from the ocean it leaves the salt behind.  Then some of this evaporated water  falls as rain on the land, dissolves more salts, and carries them to the ocean.  As this cycle repeated itself over the eons, the levels of salt in the ocean rose.
 
While the process described in the paragraph above is the one most commonly presented to explain how the oceans became salty, it cannot explain the preponderance of NaCl.  The concentration of NaCl in land rocks is quite low, so river water does not carry much of it to the oceans.  The secret to understanding the high concentration of NaCl in the ocean emmanates from submerged volcanic vents on the ocean floor.  The gases coming from the vents have high concentrations of hydrogen chloride (HCl).  When the HCl dissolves in water it forms hydrochloric acid which then leaches sodium out of the rock on the sea floor.  The sodium from the rock and the chlorine from the hydrochloric acid combine to make the Na Cl. 
 
Given that water is essential for all life and, thus, all of our food production; it’s rather ironic that most of the water on earth is of little use to us because it is too salty.  The title of this column comes from Samuel Taylor Coleridge’s famous poem, The Rime of the Ancient Mariner, the world’s most famous articulation of this irony, in which thirsty sailor gaze longingly at the ocean while adrift at sea due to the curse the mariner brought upon them by killing an albatross.
 
If you drink salt water, the sodium concentration in your blood begins to rise above its equilibrium level.  When this happens your body takes two actions to try to bring things back to normal.  Your kidneys pull sodium out of your blood and divert it to your urine.  In addition, your body extracts water from other cells in to dilute the sodium in your blood that your kidneys have not been able to remove.  However, given the high concentration of sodium in ocean water, these two systems cannot manage to overcome any substantial ingestion of salt water.   As noted in Mr. Coleridge’s poem, those adrift at sea who resort in desperation to drinking salt water only hasten their own deaths.
 
Land plants cannot tolerate salt water either.  Plants extract water from the soil through osmosis.  In osmosis, first fresh water in the soil encounters the roots of the plant.  Across the root’s membrane, the water within the plant contains dissolved salts and nutrients which the plant needs to live.  Therefore, the water within the plant is saltier than the water in the surrounding soil.  Nature is always trying to even things out (that’s a layman’s explanation of entropy) so the fresh water from the soil passes through the membrane into the plant to dilute the salty water within the plant.  Then plant loses water to the air through transpiration which concentrates the salts in the root again, restarting the cycle.
 
If you put salt water in the ground, now the water outside the root membrane is saltier than the water inside the root.  When this happens the water flows in the opposite direction, out of the plant and into the soil.  This dehydrates the plant, killing it.
 
All of these problems with the salt water could be solved, if we just removed the salt.  This is not difficult, but, as I’ll explain next week, it’s expensive both in terms of financial and energy inputs.
 
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