MadSci Network: Earth Sciences

Re: Why do we call groundwater in most areas a “nonrenewable source”?

Date: Wed Aug 23 16:13:15 2000
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Earth Sciences
ID: 966393418.Es

	Water sinks back into the ground if we let it.  Some areas of our 
country have several different groundwater levels, called "aquifers," 
stacked on each other, with deep, dry layers in between them.  This depends 
on the "geological history" of the area (whether glaciers deposited gravel, 
etc.).  The low-tech approach is to use the water that is closest to the 
surface, sometimes called the "water table"; but water in deeper layers may 
be purer or available in larger quantities.   	
        Before there was running water, country people got their water from 
shallow wells; and when they had used it, they threw it out on the ground — 
some soaked in, and some evaporated.  Because people didn't have running 
water, they didn't have flush toilets.  Now, people in the countryside 
usually get their water from deeper wells and treat their wastewater in 
septic tanks.  Most of this wastewater goes back into the ground, but it 
may not go all the way back down to the aquifer from which it was drawn.
	Many cities use surface water (lakes and rivers) and discharge 
their treated wastewater to rivers that carry it away.  Other cities, 
downstream, may wind up using this same water later; but they have to 
devote extra effort to purifying it before they do.  Some cities get all of 
their water from wells; but they usually discharge their wastewater to the 
surface (e.g., a river), so it doesn't get back into the ground.  Most 
rivers eventually flow to the sea, where the fresh water gets mixed with 
salt water and becomes undrinkable.  
	In some areas of our country, treated wastewater (we aren't 
supposed to call it "sewage") is applied to the surface of land for rapid 
infiltration (soaking in) or is pumped back down where it came from.  This 
is fine, if the water is properly purified before it is sent back down.  
Water that soaks in from the surface may get considerable purification as 
it filters down through the soil (sometimes hundreds of feet); but if water 
is pumped down directly into the aquifer or applied to soil that is 
gravelly and doesn't do much filtering, contaminated water may wind up in 
someone else's well (or the city's) and cause illness.  Even the septic 
tank-well system in the countryside sometimes causes contamination and 
	Israel is a leading site for recharge of groundwater aquifers.  
They collect water during rains (there aren't many) and use it to fill up 
their aquifers.  Underground storage is preferred to surface reservoirs 
because Israel is always short of water, and this makes their stored water 
especially subject to terrorist attacks.  So every year the groundwater is 
recharged, mostly by a company called Mekorot, and then used very carefully 
until the next "rainy season."  If groundwater were as precious in the 
U.S., we would probably be doing almost the same thing.  At least, some 
American communities are trying.  

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