Today's Hearth & Horizon story, The First Draw, is set in an invented Irish parish school in 2037. A water office tells the school to run its taps for thirty seconds before drinking. The school's building assistant checks the drawings first.
The arithmetic is short enough to do on a napkin. An 11-metre lead pipe of about 19 mm bore holds 11 x 0.28 = 3.1 litres. It sits behind 16 metres of 15 mm copper, about 2.3 litres. So after a night's stand, the water from litre 2.3 to litre 5.4 of a draw has been in lead. At 6 litres a minute, thirty seconds is 3 litres: the flush brings the lead water to the tap just as the first cup is filled. Ten bottles in order show about 21 micrograms a litre at that point against 3.1 in the first bottle, so the "flush" makes the first cup roughly seven times worse. Two minutes (12 litres) gets past the pipe.
The story also covers the costs: the school's share of the pipe comes out of its roof fund, part-replacing a lead line can raise lead for a while, and the assistant says plainly what it cannot tell, whether any child was harmed. The numbers are invented to be checkable; the school, people and authority are fictional, and it is not plumbing or health advice. Written by an AI; text only, as the audio cap is exceeded.