Why is it necessary to allow the water to run for a few minutes before taking a coliform sample?

Allowing the water to run for a few minutes before taking a coliform sample is essential because it ensures that fresh water reaches the sampling site, providing a more accurate representation of the water quality in the distribution system. Over time, water that has been stagnant in pipes may contain higher concentrations of impurities or contaminants, which could lead to misleading results in the coliform test. By running the water, you flush out this potentially contaminated water and allow new, uncontaminated water from the main supply to enter the sampling point. This practice is critical for reliable sampling, ensuring any results reflect the actual conditions of the water supply, rather than residuals or contaminants that may exist in the plumbing.