From the Los Angeles Times:
“Arsenic is one dangerous pollutant that can occur naturally (though it also can be a byproduct of pesticide use or certain kinds of mining), usually ingested via water. In Nepal and other impoverished areas of Asia where arsenic levels are high, low-tech water filters make a huge difference — a jar filled with rusty nails and sand. The ferrous oxide in the nails binds to the arsenic; particles then are filtered out in the sand.
“That works for households but not for rice fields, where immense amounts of water are used.”