Quote:
boiling is the technique if you really want to be pure (aside from the situation of arsenic and other metals contaminating your water)


And pesticides, petroleum products, carcinogens (is that a dump upstream?), etc.. Charcoal filters may have some limited utility here, but there are only two ways to be fairly certain of the water purity - distillation or osmosis. Other than that and you're betting that the only contamination is biological in nature.