Environmental Engineering Reference
In-Depth Information
9 Water and Wastewater
THE WATER RESOURCE
Freshwater sources
Of all the water existing in the planet, 97.5 percent is saline water contained in oceans, seas,
bays, underground deposits, and saline lakes. The rest (2.5 percent) is freshwater that is not
completely available for use. About 68.7 percent of freshwater is locked in icecaps, glaciers,
and permanent snow; 30.1 percent is groundwater, 0.3 percent is surface water, and 0.94 per-
cent is freshwater trapped in biological tissues, the atmosphere, in soil moisture, and in ground
ice and permafrost. Of that small 0.3 percent that corresponds to surface water, 2 percent is in
rivers, 11 percent in swamps, and 87 percent in lakes. So, of the total estimated volume of
water on the planet that is 1.386 × 10 9 km 3 only 1,063 × 10 7 km 3 is available freshwater that is
distributed as follows: 91,000 km 3 in lakes, 11,470 km 3 in swamps, 2,120 km 3 in rivers, and
10,530,000 km 3 as groundwater (Gleick, 1996).
These numbers are a snapshot at certain point in time, but that does not mean the system
is static. Water is moving above and below the ground at any time as a result of the never-
ending hydrologic cycle. Water evaporates from the surface (mostly from oceans) and
then condensates and falls as precipitation that recharges subterranean deposits (aquifers)
that then seep out and recharge rivers (or vice versa) that end up carrying water back to
oceans.
Water aquifers
Water aquifers are underground water reservoirs capable of providing water through water
wells. Water is held in aquifers by porous consolidated rock or by unconsolidated materials
such as sand or gravel. Also, water can be held in rock fractures.
Aquifers can be unconfined or confined (Fig. 9.1). An unconfined aquifer is one open at the
top where water is free to fluctuate up and down depending on rate of use and rate of recharg-
ing. The upper boundary of an unconfined aquifer is commonly known as “the water table,”
and the water pressure at this level is equivalent to the atmospheric pressure. Below the water
table, all spaces are filled with water (the saturated zone) in contrast with the unsaturated zone
above the water table. Unconfined aquifers are more easily recharged because they are opened
to receive rainwater that infiltrates from the surface all across the aquifer; on the downside,
they are more susceptible to contamination.
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