|MadSci Network: Earth Sciences|
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 illness. 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.
Try the links in the MadSci Library for more information on Earth Sciences.