Fresh water is any naturally occurring water existing in liquid or frozen form that contains low concentrations of dissolved salts and other dissolved solids. It is usually defined as water with a salinity of less than one percent of that of the oceans, which have an average salinity of 3.5 percent. It also includes non-salty, mineral-rich waters such as from mineral springs.
Only about 2.5 percent of the roughly 1.4 billion cubic kilometers the earth's total water is fresh water. And of this, only about 0.3 percent exists in liquid form on the surface, with the remainder mostly locked up in the polar ice caps and other glacial ice. The Antarctic ice sheet holds about 90 percent of the earth's surface fresh water, and the total for this plus the Greenland ice sheet exceeds 99 percent. Nearly all of the remaining fresh water lies underground as groundwater. Of the surface fresh water that is in liquid form, the Great Lakes in the U.S. account for 21 percent, and Lake Baikal in Russia, the world's deepest and oldest fresh water lake, holds another 20 percent.
The vast majority of higher plants as well as most insects, amphibians, reptiles, birds and mammals require fresh water for survival. However, fresh water can easily become polluted by naturally occurring substances or processes and, increasingly, by human activity, and much of the earth's easily accessible fresh water (i.e., on the surface and groundwater) is now unsuitable for human consumption without treatment.