EPA questions the current “safe level” of arsenic in water.

Current level is 10ppb for arsenic in water.  This is deemed “safe”. Arsenic occurs naturally in the ground and can also be the result of contamination of hazardous waste. Water softeners (ion-exchange), activated carbon and reverse osmosis filters can reduce and remove arsenic from drinking water.

http://www.newscientist.com/article/dn21870-new-concerns-over-safety-of-arsenic-in-drinking-water.html