Some consumers choose to purchase bottled drinking water, rather than relying on city tap water supplies. Bottled water has typically been extracted from underground sources. If water exists underground, but has no natural exit points, bottling companies may construct a water table well by drilling down to extract water from an unconfined aquifer. This is done when the Earth’s natural water level – known as a water table – is much lower than the Earth’s surface. In some cases, as with a valley or gully on a mountain, the level of the water table may be higher than the Earth’s surface, and a natural spring can emerge. Bottling companies are permitted to extract this water from a hole drilled into the underground spring, but the composition of the water must be identical to that of the naturally surfacing variety nearby.
Artesian water is drawn from a confined aquifer, a deep underground cavity of porous rock that holds water and bears pressure from a confining layer above it. This water can be accessed if companies drill a vertical channel down into the confined aquifer. Due to the pressurised nature of this aquifer, water will often rise up from within it and form a flowing artesian well, which appears as an explosive fountain at the earth’s surface. However, this only occurs when the surface is lower than the natural water table. If the surface is not lower than the natural water table, it is still possible to draw artesian water by using an extraction pump.
Some bottled water is advertised as ‘purified’, which means it has been subjected to a variety of different cleansing processes. A common filtering procedure, known as reverse osmosis, involves the water being pressed through microscopic membranes that prevent larger contaminants from passing through. The microscopic size of these holes is such that they can even obstruct germs, but they are most effective against undesirable materials such as salt, nitrates and lime scale. One disadvantage of reverse osmosis is that a lot of unusable water is generated as a by-product of the procedure; this must be thrown away.
For treating pathogens, an impressive newer option is ultraviolet (UV) light. Powerful UV light has natural antibacterial qualities, so this process simply requires water to be subjected to a sufficient strength of UV light as it passes through a treatment chamber. The light neutralises many harmful germs by removing their DNA, thereby impeding their ability to replicate. A particularly impressive quality of UV light is its ability to neutralise highly resistant viral agents such as hepatitis.
The overall effects of UV light treatment are variable, however, which leaves many municipal water treatment processes relying on chlorination. Its powerful and comprehensive antimicrobial effect notwithstanding, chlorination is also extremely inexpensive and remains the only antimicrobial treatment capable of ensuring water remains contaminant-free all the way through the pipes and to the taps of domestic homes. Many members of the public remain suspicious of water that has been treated with such a harsh chemical. Its ease of use and affordability has meant that chlorine often plays an important role in making tainted water supplies safe for consumption immediately after natural disasters have occurred.
On the basis of your understanding of the passage, answer any five questions from the six given below:
- Under what conditions does a natural spring emerge ?
- What is a confined aquifer?
- What forms a flowing artesian well ?
- Explain the term ‘purified’ water.
- How are pathogens treated ?
- Why is municipal water treated by chlorination?