AASHTO R 24:99(2018) Standard Practice for Collection and Preservation of Water Samples.
1.1. The objective of sampling is to collect a portion of material small enough in volume to be conveniently transported to and handled in the laboratory while still accurately representing the material being sampled. This implies, first, that the relative portions or concentrations of all pertinent components must be the same in the sample as in the material being sampled, and second, that the sample must be handled in such a way that no significant changes in composition occur before the tests are perfornied. Complete and unequivocal preservation of samples, whether domestic sewage, industrial waste, or natural waters, is a practical impossibility. Regardless of the nature of the sample, complete stability for every constituent can never be achieved. At best, preservation techniques can only retard the chemical and biological changes that inevitably continue after the sample is removed from the parent source.
1.2. The values stated in SI units are to be regarded as the standard.
2.1. Grab or catch Samples—Strictly speaking, a sample collected at a particular time and place can represent only the composition of the source at that time and place. However, when a source is known to be fairly constant in composition over a considerable period of time or over substantial distances in all directions, then the sample may be said to represent a longer time period or a larger volume, or both, than the specific point at which it was collected. In such circumstances, some sources may be quite well represented by single grab samples. Examples are some water supplies, some surface waters, and, rarely, some wastewater streams.
2.1.1. When a source is known to vary with time, grab samples collected at suitable intervals and analyzed separately can be of great value in documenting the extent, frequency, and duration of these variations. Choose sampling intervals on the basis of the frequency with which changes may be expected. which may vary from as little as 5 mm to as long as 1 h or more.
2.1.2. When the composition of a source varies in space rather than time, a set of samples collected from appropriate locations with less emphasis on timing may provide the most useful information.
2.1.3. Use great care in sampling wastewater sludges, sludge banks, and muds. No definite procedure can be given, but every possible precaution should be taken to obtain a representative sample.AASHTO R 24 pdf download.