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001227 IX.2 Man's activities are a principal cause of change in the quality of both surface and ground waters. Municipal and industrial sewage dis- charged into streams often contribute extreme concentrations of certain chemical materials that are completely foreign to natural waters. Com- pounds of nitrogen are a common example. Irrigation-return waters carry increased concentrations of salts, which swell the dissolved load in some streams and some ground-water bodies. Even so simple an act of man as providing artificial water storage leads to changes in physical, chemical and biological quality. Man's methods of measuring water quality have been subject to con- tinuous change and improvement as new techniques and equipment are developed. Both the precision of measurement and the method of sllmpling, however, usually depend upon the purpose to be served. In many cases partial analyses are satisfactory; in other cases, approximate analyses are adequate as indicators. Thus, in interpreting water-quality data both the purpose of, and the conditions surrounding, the original collection of samples and the laboratory determinations "must be considered. An investi- gator of water quality should evaluate what each particular analysis or sample represents in relation to the body of water under study. Further, the choice of field or laboratory procedures must be weighed in relation to the economic value of the study. Some of the constituents dissolved in water may change after a sample is collected; in this situation, fixing or field determination of these constituents must be considered. Even in the face of all these potential complexities, considerations of cost are such that most investigations of water quality have been and are limited to measurement of a few, pre-selected key components. Thus, a particular deleterious substance may escape detection. For example, in connection with a major construction project, a long series of stand- ardized chemical tests of the stream failed to identify certain organic compounds that were adverse to mixing sound concrete. Commonly, theI'efore, it is highly advisable that any comprehensive water-quality investigation open with a critical reconnaissance including at least one exhaustive analysis. It should be noted here that I'equirements of biolOgical sampling are quite unlike those of physical and chemical sampling in several respects, For example, because there is no single universal indicator species, analysis of biological samples must depend upon interspecific I'elationships. Thus a rather long series of samples is required. In addition, the analyst must detect and interpret either extreme or widely variable conditions, because water quality must be keyed to critical or threshold levels when aquatic life is concerned. This rules out composite samples,