Short-term variations in groundwater arsenic (As) concentrations were characterized at four water treatment plants in Illinois and one in Indiana. At each plant, water samples were collected at 45-minute intervals over a six-hour period. Concentrations of As remained steady during the entire test at three facilities. At one facility, the As concentration increased from 58 mg/L in the first sample to 82 mg/L after 2.5 hours. At another facility the As concentration increased from 48 mg/L in the first sample to 68-70 mg/L in all subsequent samples. Variations in As over several years four of these facilities were characterized using data from the Water Surveys groundwater-quality database. The long-term As variations were unique at each facility. For example, at one facility, the minimum and maximum As concentrations were 20 mg/L and 120 mg/L, a factor of 6. At another facility, the range in As was relatively narrow, with minimum and maximum As concentrations of 34 mg/L and 46 mg/L.