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001220 IX.l IX. CHEMICAL, PHYSICAL, AND BIOLOGICAL QUALITr General considerations . In regard to its quality--chemical, physical, or biological--no water body should be assumed to be uniform or unchanging with time. In fact, lack of uniformity and continual change with time are the rule and not the exception. Thus, the analysis of a single water sample can show no more than the chemiCal, physical, or biological character of the water body at one point in space and at one moment in time. It may tell little about the water-quality situation, or even may be misleading, unless it is supple- mented by full information as to nature of the water environment, as to changes taking place in that environment, and as to the manner in which the sample was taken. Such Wormation is quite as meaningful, and may be even more so, as assurance that methods of analysis were competent and accurate. Water quality varies greatly through the several phases of the hydrologic cycle. In nature, water approaches maximum purity in its vapor phase, immediately as it is rejuvenated by evaporation from an ancestral water body such as one of the oceans, a lake, or a stream. Shortly after the vapor phase, water quality begins to change in accord with the physical and chemical characteristics of the environment. In the atmospheric environment, noticeable but generally.minor changes occur in the content of the solutes from both natural and artificial sources; also, in the basic chemical makeup of the water, as in the ratio of hydrogen to tritium. After it encounters the surface of the earth, water undergoes major change in its chemical and physical characteristics. Thus, when water falls into an ocean as rain or snow, its dissolved-solids content soon equals the average salt content found in the oceans. Precipitation on land results in slower but diverse changes in water quality. Erosion of soil particles can cause 'Water to become very turbid as it moves across the land surface. Also, many COl1llllon minerals enter into solution whether the wate;r is on the land surface or in the ground; solubility, chemical reactivity, and temperature control the resulting concentration of the salts. Bacterial growth and other biological activity vary significantly with changes in the physical and chemical characteristics of the environment. In turn, this activity affects the physical and chemical quality of the water through changes which include concentration of dissolved gases and solids, pH, temperature,. color, taste, and odor. Biological growth also may make the water toxic to other life forms.