It may in fact be an essential nutrient!
"Some studies have suggested that inorganic arsenic is an essential nutrient in goats, chicks, mini pigs, and rats. However, no comparable data are available for humans."
This is from an [link|http://www.epa.gov/ttn/uatw/hlthef/arsenic.html|EPA report.]
So, maybe before too long, there will be a recommended minimum arsenic content in drinking water.
Incidentally, do you have a deck made of treated lumber? What do you suppose they treat the lumber with? Yep, arsenic.