High mean As concentrations of up to 26.6 ??mol/L (1990 ??g/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. Sulfides in the bedrock are the primary source of the As in the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with As concentrations in the ground water barely above the detection limit of 2.0 ??mol/L, shows that there are complicating factors. Chemical analyses of water from 35 bedrock wells throughout a small watershed reveal spatial clustering of wells with high As concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium-bicarbonate-dominated water with low As concentrations (CaHCO 3 type), sodium-bicarbonate-dominated water with moderately high As concentrations (NaHCO3 type), and calcium-bicarbonate-dominated water with very high As concentrations (High-As type). It is proposed that differences in recharge area and ground-water evolution, and possible bedrock composition difference are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption of As is an insignificant control on As concentration. Correlations of As concentrations with Fe and redox parameters indicates that reductive dissolution of Fe(III) oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO3 and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO4 concentrations, probably due to precipitation of Fe(III) oxyhydroxides and adsorption of As under oxidizing conditions. Crown Copyright ?? 2006 Published by Elsevier Ltd. All rights reserved.