科技英语（地质英语）.doc

READING MATERIAL The quality of a body of water refers to its temperature and the amount and character of its content of mineral particles, solutes and organic matter (chiefly bacteria), in relation to its intended use. The most common source of pollution of water from wells and springs is sewage, and the infection most commonly communicated by polluted water is typhoid. Drainage from septic tanks, broken sewers, privies, and barnyards contaminates ground water. If the water contaminated with sewage bacteria passes through material with large openings such as very coarse gravel or the cavernous limestone, it can travel for miles without much change. If, on the other hand, it percolates through sand or permeable sandstone, it can become purified within short distances, in some cases less than 30m. The difference lies in the aggregate internal surface area of the material through which it percolates. The large aggregate force of molecular attraction holds the water and promotes its purification by (1) mechanical filtering-out of bacteria (water gets through but most of the bacteria do not), (2) destruction of bacteria by chemical oxidation, and (3) destruction of bacteria by other organisms, which consume and oxidize them. Purification goes on both in the zone of aeration and in the zone of saturation. Because clay particles are much smaller than sand particles, we might suppose that clay, with its much larger internal surface area, would be the ideal medium for purification. But it is not, because as we have seen, it is almost impermeable. Particles of sand are large enough to permit rapid percolation, yet small enough to permit purification within short distances. For this reason treatment plants for purification of municipal water supplies and processing of sewage percolate these fluids through sand. A substantial proportion of domestic sewage passes through septic tanks and then mingles with ground water. It percolates into streams, which it generally reaches in a