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3.1 THE WATERY PLANET
3.1.1 Distribution
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
Figure 3.2 The hydrologic cycle. (http://www.ec.gc.ca/water/en/ nature/prop/e_cycle.htm)
vulnerable to pollutants introduced directly or from surface water and groundwater drainage.
3.1.2 The Hydrologic Cycle
Water covers much more of earth’s surface than does land. The continual movement of water across the earth due to evap- oration, condensation, or precipitation is called the hydrologic cycle (Figure 3.2). The consistency of this cycle has taken mil- lennia to establish, but can be greatly altered by human activ- ities including global warming, desertification, or excessive groundwater pumping. Water, in its constantly changing and various forms, has been and continues to be an important factor driving evolutionary processes in all living things.
Evaporating water moderates temperature; clouds and water vapor protect us from various forms of radiation; and precipitation spreads water to all regions of the globe, allow- ing life to flourish from the highest peaks to the deepest caves. Solar energy drives evaporation from open water sur- faces as well as soil and plants. Air currents distribute this vaporized water around the globe. Cloud formation, conden- sation, and precipitation are functions of cooling. When va- porized, water cools to a certain temperature, condensation occurs, and often results in precipitation to the earth’s sur- face. Once back on the surface of the earth, whether on land or water, solar energy then continues the cycle. The latent heat of water (the energy that is required or released as wa- ter changes states) serves to moderate global temperatures, maintaining them in a range suitable for humans and other living organisms.
Some processes involved with the hydrologic cycle aid in purifying water of the various contaminants accumulated during its cycling. For instance, precipitation reaching the soil will allow weak acids absorbed from air to react with various minerals and neutralize the acids. Suspended sedi- ments entrained through erosion and runoff will settle out as the water loses velocity in ponds or lakes. Other solids will be filtered out as water percolates through soil and vadose zones and ultimately to an aquifer. Many organic com- pounds will be degraded by bacteria in soil or sediments.