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Porous rock
P
Permeable rock
A rock that water can flow through easily.
There are many types of rock that water can flow through. Some, like sandstone and chalk, have small connecting spaces called pores that water can flow through. Others, like limestone, have no spaces for water inside the rock, but they are broken into large blocks, and the water flows between the blocks. (See also: Aquifer and Porous).
Playa, playa lake
A desert lake, often dry, and marked by white salt crusts. Playas are formed in the center of inland drainage basins. The minerals dissolved in the rock of
 Polluted water—A river polluted with industrial waste, including heavy metals and detergents.
the surrounding land are washed into the lake by rain. When the rainwater evaporates, the minerals are left behind as sparkling white sheets (see: Evaporation).
Polluted water
All water contains some dissolved chemicals. Therefore no naturally occurring water is absolutely pure. Polluted water is defined as water that contains sufficient impurities to make it unfit for its intended use.
There are many causes of water pollution, but it is usually defined as a high level of impurities as a result of human action.
Most water pollution is caused when contaminated water seeps from a concentrated source into rivers, lakes, or groundwater reserves.
The three main sources of pollution are sewage, fertilizers,
and industrial wastes (usually metal compounds).
Poor people living close to a river often deposit all of their wastes in the river because they have no alternative. These human wastes are very rich in nitrogen and phosphorus. In the countryside farmers use high quantities of nitrogen and phosphorus fertilizers. Both nitrogen and phosphorus are soluble and are carried by water through the soils and into rivers. This provides an excess of the nourishment that microscopic water plants (algae) need, and so they grow rapidly. As they grow, they remove oxygen from the water, making it impossible for other forms of life to exist.
In industrialized countries rivers often become polluted by industrial wastes. These wastes often contain metals that are poisonous both to river life and to people. (Compare with: Clean water.)
Porous rock
A rock containing many small spaces, or pores.
The main porous rocks
that conduct water and are
also permeable are chalk and sandstone. Water seeps into and flows through them easily.
A porous rock may hold water but not allow water to flow through it. Clay is an example of this type of rock. Although clay has pores, it is not permeable.
Water flows from homes and industry directly into rivers, and also seeps into groundwater.
Fertilizers flow from farm fields into groundwater that eventually reaches the river.
 Polluted water—The sources of pollution to a river.
Groundwater
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