A survey of national groundwater supplies conducted from June 1998 to June 1999 found that arsenic (As) concentrations exceeding 0.010 mg/L are widespread in the United States. In January 2001, the US Environmental Protection Agency published a final rule to lower the As maximum contaminant level from 0.05 to 0.010 mg/L. To meet this new standard, many utilities may need to upgrade current facilities or investigate new treatment options. They also need an accurate appraisal of the ability of existing treatment technologies to consistently reduce As to low levels. The authors conducted a year-long study to evaluate the performance of five full-scale facilities for removing As from raw water. The two coagulation-filtration plants evaluated consistently achieved low levels of As. However, when one facility changed its coagulant from alum to polyaluminum chloride, average removal efficiency plunged from 84 to 43%, a reduction that might be attributable to difference in pH. Of the two iron-removal facilities, one consistently reduced As concentrations; addition of a coagulant, however, should help improve the other plant's As removal. The lime-softening plant was unable to consistently reduce As to low levels, but raising the operating pH through increased lime dosage may improve removal efficiencies. Conventional treatment technologies are capable of removing As from water supplies to levels < 10 ug/L. To ensure optimal As removal, however, treatment processes may need modifications. Utilities intent on optimizing As removal must consider the effect these modifications may have on system performance and costs, as well as other performance issues that may be triggered by changes in water chemistry. Includes 33 references, tables, figures.