Preliminary Scoping and Screening Exercises
If no information about contamination on the site exists in the scoping stage then it may be necessary to collect samples in order to determine if the site requires further investigation. At this stage it is only necessary to collect as many samples as would be necessary to determine if the site exceeds guidelines or not.
Investigating Site Contamination
If the site has been proven to be contaminated and an investigation is authorized then larger sample numbers provide more precise and reliable estimates of mean concentrations and variance. For blood and milk samples it is recommended that 50 samples are taken. For environmental variables and surveys more than 30 samples should be collected to ensure the results are statistically rigorous. The number of samples will largely depend on the area being sampled. A larger area will require more samples than a small area. Equally as important as the number of samples is the location of where samples were take (will they accurately reflect the extent of contamination), the sampling methodologies and the QA/QC.
Before deciding on the number of samples needed read chapter 3 (Statistical Considerations) from the UNEP document: Guidance on the Global Monitoring Plan for Persistent Organic Pollutants (PDF file - 2007).