The goal of sampling is to obtain for analysis a portion of the main body of water that is representative. The most critical factors necessary to achieve this are points of sampling, and materials selection, system design, time of sampling, frequency of sampling, and proper procedures to maintain the integrity of the sample prior to analysis.

Homogeneity of the process to be sampled is frequently lacking, necessitating multiple-point sampling. If it is impractical to utilize a most-representative sampling point, it may be practical to determine and understand interrelationships so that results obtained at a minimum number of points may be used to characterize the system.

Samples collected from a single point in a system are always recognized as being non-representative to some degree. For this reason, total representativeness of samples cannot be a prerequisite to the selection of a sampling point. The degree of representativeness of the sample shall be assessed and the assessment made a part of the permanent record. This will prevent an artificial degree of accuracy from being assigned to the data derived from tests on the sample.

The samples shall be of sufficient volume and shall be taken frequently enough to permit reproducibility of testing requisite for the desired objective, as required by the method of analysis to be used.

Laboratories or facilities conducting water sampling should be in compliance with Guide D3856.