A composite water sample is ONE sample which is made by combining water samples from two or more sources/uses/locations. For example, an operation uses two wells, one well is used for handwashing and the other well for filling their dump tank. Instead of taking two separate samples for each well, only ONE sample is taken which is a mixture of water from both wells.
Composite water testing is allowed. The way in which operations choose to sample the water is up to them. As long as operations are taking the appropriate number of samples, from the appropriate place, at the appropriate time, etc. the sampling method is up to them. However, if they receive a positive result (i.e., more than 0 total coliforms and 0 E. coli) they will not know which source (or multiple sources) has the problem, and will have to do more investigation and testing to determine the source of contamination.
To ensure a true representation of the water being tested, treated water should not be combined with untreated water to form a composite sample, doing so may create a sample that does not truly indicate the potability of the water (i.e. the chlorine in the treated water could react with the untreated water and the composite sample results may show the water to be potable even if it was not).
More information about composite water testing can be found in Appendix G.