The presence and spatial distribution of groundwater arsenic was evaluated in an area of approximately 6000 square kilometers that extends along the southeast border of the subhumid pampa plains of southeast Buenos Aires Province, Argentina. Groundwater constitutes the only easily exploitable source of water for the large rural and suburban population in the area, both for human consumption and for use in agricultural and livestock production. In the study area, 56.3 percent of water samples had seriously elevated arsenic levels (0.06 to 0.5 milligrams per liter [mg/L]), constituting a risk for human consumption and agricultural and livestock use. In 27.2 percent of the samples, arsenic content ranged from just below the World Health Organization-recommended maximum level of 0.01 mg/L to 0.05 mg/L. Only in 16.5 percent of the samples did the water contain no or very low concentrations. The contamination of domestic water supplies in the study area constitutes a serious water quality problem since this element has been identified as the cause of a number of health problems, including cancer.