AbstractA retrospective analysis of 20 water systems from the U.S. Environmental Protection Agency's Arsenic Demonstration Program demonstrated three patterns of arsenic levels at the tap after arsenic treatment of the source well water. Following an initial destabilization period, Pattern A systems (6/20 with low iron/manganese in source water and plastic piping) had arsenic concentrations that did not change as water traveled to consumer taps (conservative contaminant behavior). Pattern B systems (8/20 with high iron/manganese in source water and iron piping) had consistently higher arsenic concentrations at consumer taps, above the arsenic content of incoming treated water, for months to more than a year after arsenic treatment (nonconservative behavior). Pattern C systems (6/20 with additional occasional arsenic treatment complications) experienced multiple arsenic spikes at consumer taps (nonconservative and unpredictable behavior). These field observations suggest that, in some water distribution systems, arsenic may linger long after it has been removed at its source.