If you have ever had your water tested for lead, you may have been asked not to use the tap overnight. That is because the timing of a sample can change the result as much as the water itself.
First-draw samples. A first-draw sample is the first water out of a tap after it has been sitting still, usually for at least six hours. That water has been in contact with the faucet and the pipes just behind it, so it shows the highest exposure someone is likely to get, for example when filling a kettle first thing in the morning.
Flushed samples. A flushed sample is taken after the water has run for a set time. It represents water that has come from further back in the system, closer to the main.
Sequential samples. Some investigations collect several samples in a row from the same tap. Comparing them can help show where lead is coming from: the faucet, the pipes inside the walls or the service line.
Why it matters. A single low result from a flushed sample does not prove that the first water of the day is low as well. A good sampling plan matches the question being asked, which is why professional testers usually discuss the purpose of the test before choosing how to sample.
