Based on reference methods and other literature studies, decanting of water samples for organic analyses is not an acceptable approach.
This is because organic contaminants like PAHs, PCBs, petroleum hydrocarbons and dioxins and hydrophobic - meaning they adsorb well to the walls of the container. This is why the reference methods, as well as the regulatory guidelines, require sample extraction and analysis on a sample as submitted, in the original container.
Decanting can potentially lead to a biased low result.
Decanting for water samples may be permissible in some situations, such as when a significant amount of fine sediment is present. However, this approach implies that the initial analysis completed follows standard guidelines and the subsequent analysis meant to demonstrate that quantifiable concentrations of target parameters are largely associated with particulate matter and not a true representation of dissolved phase impact.
In situations where decanting is used, the data is qualified as such, with appropriate commentary. Based on data reviewed from numerous projects, this is not a common occurrence, as the use of low-flow sampling often addresses the sediment issue.