Three surveys of arsenic occurrence in US drinking water supplies were synthesized to estimate how possible arsenic standards would affect compliance. Detectable levels of arsenic (0.5 ug/L) were found in 73 percent of surface water sources and 58 percent of groundwater sources. For finished water, detectable arsenic occurred in 45 percent of surface water systems and 53 percent of groundwater systems. The authors' compliance projections for arsenic standards ranging from 2 to 20 ug/L agreed fairly well with initial estimates by the US Environmental Protection Agency. It was estimated that approximately 25 percent of all community water suppliers (11,550-11,890 systems) would violate an arsenic standard of 2 ug/L. Between 6 and 17 percent of systems (2,775-7,870) were projected to violate an arsenic standard of 5 ug/L, and 1-3 percent of systems (510-1,360) were estimated to violate an arsenic standard of 20 ug/L. Includes 31 references, tables, figures.