The Host

It's considered a bad idea to dump hazardous chemicals down your drains.  Those chemicals you dump down the drain can end up back in your drinking water, or they can get into the local water table.  When a tidy mortician orders his assistant to dump his concoction of chemicals into the sink because they have too much dust on them, and are obviously "dirty", bad things start going on in Seoul's Han River in the 2006 Korean movie The Host (aka Gwoemul).