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The maximum chlorine residual in water samples from a potable water system for reliable laboratory results is set at 4 parts per million. This level is crucial because it balances the need for effective disinfection while minimizing interference with laboratory analyses. Higher chlorine levels can lead to issues such as the degradation of certain compounds or the production of chlorinated byproducts, which may skew the results or hinder the detection of specific contaminants.

Maintaining the chlorine residual at or below 4 parts per million helps ensure that tests provide accurate readings of the water's microbiological and chemical quality, supporting both public health standards and regulatory compliance.