Composite sampling is a strategy in which multiple individual or "grab" samples (from different locations or times) are physically combined and mixed into a single sample so that a physical (rather than mathematical) averaging takes place. Thus, for a well-formed composite, a single measured value should be similar to the mean of measurements of the individual components of the composite. The analysis results from multiple composite samples can be used to perform the statistical calculations described in Chapter Nine (PDF, 79 pp, 684K). Multiple composite samples can provide improved sampling precision and reduce the total number of analyses required compared to noncomposite sampling. This strategy is sometimes employed to reduce analysis costs when analysis costs are large relative to sampling costs.
If the objective of the waste characterization effort is to classify all of the drums collectively as either hazardous or nonhazardous, then collection of multiple composite samples could be employed to reduce analysis costs. This approach, however, is not recommended if there is substantial drum-to-drum variability such that one or more drums should clearly be classified as hazardous and others as nonhazardous. If some of the drums might be classified as hazardous when characterized individually, then we recommend you make a waste classification decision on each drum to avoid the possibility of mixing hazardous waste with nonhazardous waste.
Composite sampling should not be used if the integrity of the individual sample values changes because of the physical mixing of samples. The integrity of individual sample values could be affected by chemical precipitation, exsolvation, or volatilization during the pooling and mixing of samples. For example, volatile constituents can be lost upon mixing of samples or interactions can occur among sample constituents. In the case of volatile constituents, compositing of individual sample extracts within a laboratory environment may be a reasonable alternative to mixing individual samples as they are collected.
Note that compliance with the land disposal regulation (LDR) numeric treatment standards is to be determined using "grab" samples rather than composite samples. Grab samples processed, analyzed, and evaluated individually normally reflect maximum process variability, and thus reasonably characterize the range of treatment system performance. In those cases in which only composite data were available to develop a treatment standard, the EPA used these data. For wastes for which the standards are based on composite data, enforcement of the standard is to be based on composite data.
A detailed discussion of the advantages and limitations of composite sampling is presented in the Standard Guide for Composite Sampling and Field Subsampling for Environmental Waste Management Activities (ASTM D 6051-96).
See also RCRA Waste Sampling Draft Technical Guidance
We have to sample a very large number of drums. Is it practical to composite samples from different drums?
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