1. The water cycle begins with water vapor-moisture in the air from evaporation of water from the surfaces of oceans, lakes, etc.; transpiration from ground water through plants into the air from their leaves; and water vapor belched up from volcanoes. The vapor condenses under the influence of reduced temperature to form clouds, and then condenses further onto the surface of particles to form precipitation. Rain washes much of the air pollution out of the air, making rain a significant cause of water pollution. Acidic gases from combustion sources cause acid precipitation (“acid rain”), but even without any man-made acids in the air, normal levels of carbon dioxide (about 0.04%) dissolve in the rain to produce carbonic acid, H2CO3.
2. This slightly acidic rainwater lands on the ground and percolates through the soil where it begins to dissolve grains of limestone (CaCO3) and dolomite (Ca/MgCO3). The resulting solution then contains calcium and magnesium ions (Ca+2 and Mg+2), called hardness; plus carbonate (CO3+2) and bicarbonate (HCO3) ions, together called alkalinity: These are the dominant ions in most waters, and they form the basis of water chemistry. The water also begins to dissolve many other minerals and organic matter in the soil, and excess acidity can cause leaching of valuable nutrients, which then pollute streams. As the rainwater soaks into the soil and moves downward through many layers of Earth, it is filtered by particles of dirt, which also adsorb many dissolved contaminants becoming relatively pure ground water. If it penetrates as deep as 10 meters, this “natural purification” may be sufficient to render the water potable without any further treatment, but wells from shallower depths cannot be considered free of “influence from the surface” (penetration of pathogens from surface contamination, especially protozoan cysts). The water collects above impervious layers of clay or stone, often extending over vast regions, and moves very slowly through pores, cracks, and other channels in the ground as an aquifer. Most are very ancient, and it may have been 50,000 – 250,000 years or more since the water there fell as rain. Thus, well water from deep aquifers is considered to be a non-renewable resource, like petroleum. Pumping out water faster than it can be renewed can lead to a significant lowering of surface terrain, called subsidence (pronounced “sub-SI-dence”). Areas with varied geologic histories may have several aquifers stacked on top of one another, each receiving water from its own regional source and having its own unique chemical composition. The upper level of an aquifer is called water table, and in most places the water table of the uppermost aquifer coincides with the water level of the lakes and rivers in the region. That water flows directly to the surface, but water from the deeper aquifers may be pumped out by wells, spewed out of volcanoes, or drawn into the roots of plants by osmotic pressure and transpired out of the leaves to become water vapor again, completing the cycle.