Environmental Engineering Reference
In-Depth Information
groundwater Supply
Unbeknownst to many of us, our Earth possesses an unseen ocean. This
ocean, unlike the surface oceans that cover most of the globe, is freshwater:
the groundwater that lies contained in aquifers beneath Earth's crust. This
gigantic water source forms a reservoir that feeds all the natural fountains
and springs of this planet, but how does water travel into the aquifers that lie
under the surface of the Earth?
Groundwater sources are replenished from a percentage of the average
approximately 3 feet of water that falls to Earth each year on every square
foot of land. Water falling to Earth as precipitation follows three courses.
Some runs off directly to rivers and streams (roughly 6 inches of that 3 feet),
eventually working its way back to the sea. Evaporation and transpiration
through vegetation takes up about 2 feet. The remaining 6 inches seeps into
the ground, entering and filling every interstice, each hollow and cavity.
Although groundwater accounts for only 1/6 of the total 1,680,000 miles of
water, if we could spread this water out over the land, it would blanket it to
a depth of 1000 feet.
Almost all groundwater is in constant motion through the pores and crev-
ices of the aquifer in which it occurs. The water table is rarely level; it gen-
erally follows the shape of the ground surface. Groundwater flows in the
downhill direction of the sloping water table. The water table sometimes
intersects low points of the ground, where it seeps out into springs, lakes, or
streams.
Usual groundwater sources include wells and springs that are not influ-
enced by surface water or local hydrologic events. As a potable water source,
groundwater has several advantages over surface water. Unlike surface water,
groundwater is not easily contaminated. Groundwater sources are usually
lower in bacteriological contamination than surface waters. Groundwater
quality and quantity usually remain stable throughout the year. In the
United States, groundwater is available in most locations.
As a potable water source, groundwater does present some disadvantages
compared to surface water sources. Operating costs are usually higher,
because groundwater supplies must be pumped to the surface. Any con-
tamination is often hidden from view, and removing contaminants can be
very difficult. Groundwater often possesses high mineral levels and thus
an increased level of hardness, because it is in contact longer with miner-
als. Near coastal areas, groundwater sources may be subject to saltwater
intrusion.
Note:
Groundwater quality is influenced by the quality of its source.
Changes in source waters or degraded quality of source supplies may
seriously impair the quality of the groundwater supply.
