The influence of phosphate, silicate, calcium, magnesium and humic acid on iron flocculation and arsenic removal has been shown in synthetic waters, but these findings may not necessarily be representative for actual groundwaters. In our study, three different groundwaters were selected which varied in composition, and with jar test, the removal of As and changes in iron floc size were monitored. The groundwaters were used as is, or after modification. The main findings are that the iron floc size can be related to the molar (Ca+Mg)/P ratio, and that As removal is closely related to Fe removal, which would indicate that adsorption competition is of lesser importance than hindered flocculation