The earth,an aquatic planet,has fresh and salt water in the hydrosphere (see Table 1.2).The hydrosphere includes land (a part of the earth crust),ocean,and a part of atmosphere.Land water is distributed in soil,rivers,lakes,and polar ice.Groundwater is a part of land water,and it consists of water held in voids in the earth crust.Table 1.2 shows that more than 99 percent of available source of liquid freshwater lies underground.