Always test after a flood. Or if you notice a weird smell. Or if your water suddenly tastes like the bottom of a pond.
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Also test if you’re pregnant or have a baby. Babies shouldn’t drink nitrate-soup. And definitely test if you’re buying a house with a well. Never trust the seller’s old test. They might have collected it from a different well. Or from a hose. You don’t know their life.
Rule of thumb: Test every single year. Yes, every year. It’s cheaper than a monthly Netflix subscription. And it won’t cancel your water supply after three seasons.