4 pp. / Following on the U.S. Environmental Protection Agency's "Arsenic Rule" decision to require public water systems to lower the allowable arsenic content in drinking water from 50 parts per billion (ppb) to 10 ppb by January 23, 2006, private well owners across the state have realized the importance of testing their own water supply for arsenic. Under Arizona law, it is the sole responsibility of the private well owner to determine the quality (potability) of their private well water. This article discusses the geologic prevelance of arsenic across the state, and options available to the well owner to address this water quality concern. Expected to be the first in a 3-part series on ground water quality issues common in Arizona.