In this topic you will learn about Earth's water supply.

The water in Earth's oceans, salt-water lakes, and inland seas contains seven times more salt than a person can drink. Although these waters supply us with resources like seafood and minerals, a person cannot survive drinking it. Getting fresh water from salt water takes a process called desalination. Seawater contains dissolved rock salt and other materials. As water evaporates it leaves the dissolved materials behind. The liquid water that collects at the end of the process is free of dissolved materials.

Only a tiny fraction of Earth's water is usable fresh water. Fresh water is constantly renewed by the water cycle. When water falls to Earth, some water seeps into the ground, becoming groundwater. Groundwater seeps into the ground into spaces between bits of rock and soil. It seeps downward until it is blocked by rock that has few spaces. Then the water starts to back up and fill the spaces in the soil and rocks above. The top of the water-filled spaces is called the water table. If the water table reaches above the surface, a pond, a lake, or a stream forms..

An underground layer of rock or soil that is filled with water is called an aquifer. Water can move through an aquifer for great distances.

Some groundwater seeps out of the ground in what is called a spring. Springs occur where the water table meets the surface. They can feed water into streams and lakes long after it stops raining.

Long ago people learned to tap into groundwater by digging wells. A well is a hole dug below the water table that water seeps into. People get the water out of some wells with pumps. Wells can also be dug deep into aquifers sandwiched between tightly packed layers. Water spouts up in these wells because the rock layers are squeezing it..

Most supplies of fresh water for large towns and cities come from reservoirs. Reservoirs are storage areas for fresh water supplies. They may be human made or natural lakes or ponds. Pipelines transport water from reservoirs.

Freshwater supplies for large areas can be cleaned up on a large scale. The water is often treated with chemicals that make particles clump together. These chemicals sink to the bottom. The water then flows through filters to remove the smaller particles. Chlorine is added to kill bacteria. Many cities add fluoride, which helps prevent cavities in your teeth.