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Groundwater
 Groin—The Rhine in Germany. The groins are the low banks that stick out into the water on the far shore.
are especially common close to towns where a beach is of major tourist importance.
Groundwater
The water held within water- bearing rocks, or aquifers, in the ground.
Water may flow naturally to the surface and come out as a spring or bubble upward as a flowing pool, or it may be taken out of the rocks by using a well or a bore hole.
Limestone, chalk, and sandstone are the main rocks
that are permeable enough for groundwater to flow through them readily. Water that is trapped in aquifers is called artesian water. It is a particularly important source of water in dry areas, but is exploited in all parts of the world. (See also: Water cycle.)
H
Hardness
A measure of the amount of mineral matter, and in particular calcium bicarbonate, that is dissolved in water. Water is said to be hard if it is difficult to make soap lather. Water
is said to be soft if soap
lathers easily. The more bicarbonate dissolved in
the water, the harder it is.
Because minerals are less soluble in hot water than cold water, heating water causes
 Hardness—In hard water areas, heating elements in hot water tanks quickly develop thick lime scale.
some of the minerals to settle out. This produces the scaling seen
on kettles and the inside of hot water pipes.
Hard water is produced when
rainwater percolates through limestone or chalk rocks. While it is percolating, the water dissolves some of the limestone (calcium carbonate), turning it into soluble calcium bicarbonate.
Water hardness can be beneficial to health, reducing the amount of heart disease.
People who want to lower
the hardness of water can use chemical filters. They work by causing a chemical reaction between the filtering chemical (often calcium hydroxide, slaked lime, or sodium carbonate, soda ash) and the hard water.
Another method of removing hardness is to use a column of resin. This process (called ion exchange) uses sodium chloride (common salt).
Head
The height of the water, for example, in a tank or reservoir above the tap or drain where the water flows out. The higher the head of water, the greater the pressure on the water as it flows out of the tap, and the faster it flows.
Headwater
One of the small streams that is at the start, or head, of a river system. Headwater streams may be known as a branch or a fork.
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