Following on the U.S. Environmental Protection Agency's 'Arsenic Rule' decision to lower the allowable arsenic content in drinking water from 50 to 10 parts per billion (ppb) or less, public water systems were required to meet the lower limit by January 23, 2006. Private well owners are not subject to the rule because, under Arizona law, it is the sole responsibility of the private well owner to determine the quality (potability) of their private well water. However, private well owners across the state have realized the importance of testing their own water supply for arsenic.