لخّصلي

Ninety seven per cent of water on the Earth is marine (salt- water), while only 3% is freshwater (Figure 3.1).With re- gard to the freshwater, 79% is stored in polar ice caps and mountain glaciers, 20% is stored in aquifers or soil moisture, and 1% is surface water (primarily lakes and rivers).Possible effects of global warming, combined with continued increases in human population and economic development are resulting in critical concern for the future sustainability of freshwater resources.The limited supplies of surface waters and groundwa- ter receive significant amounts of the pollutants generated by humans.An es- timated 110,000 km3 of rain, snow, and ice falls annually on land surfaces, and this is what replenishes fresh water re- sources.Lakes across the planet have an average reten- tion time of 100 years, meaning it takes 100 years to re- place that volume of water.

Ninety seven per cent of water on the Earth is marine (salt- water), while only 3% is freshwater (Figure 3.1). With re- gard to the freshwater, 79% is stored in polar ice caps and mountain glaciers, 20% is stored in aquifers or soil moisture, and 1% is surface water (primarily lakes and rivers). An es- timated 110,000 km3 of rain, snow, and ice falls annually on land surfaces, and this is what replenishes fresh water re- sources. Possible effects of global warming, combined with continued increases in human population and economic development are resulting in critical concern for the future sustainability of freshwater resources.
The limited supplies of surface waters and groundwa- ter receive significant amounts of the pollutants generated by humans. Lakes across the planet have an average reten- tion time of 100 years, meaning it takes 100 years to re- place that volume of water. Rivers, on the other hand, have a much shorter retention time. The relatively long retention time in lakes highlights the danger of introducing pollutants that will be present for a long time (i.e., they are “environ- mentally persistent”). The short retention time in rivers means that pollutants are transferred rapidly to other areas such as groundwater or oceans. The retention time of groundwater is measured in hundreds if not thousands of years. In the groundwater environment, persistent pollu- tants may remain intact for extremely long periods because of constraints to transformation. The characteristics of groundwater are described in Section 3.10. Pollution of groundwater and surface water is discussed in Chapters 17 and 18, respectively.
Pollutants in the ocean may be introduced into the food chain by filter-feeding organisms or possibly may be se- questered in cold, deep basins where they are resistant to degradation by natural processes. Much of the world’s pop- ulation inhabits coastal areas, making oceans especially