The distribution and geochemistry of arsenic in two large aquifers in central Illinois was characterized. The areal distribution was complex; wells with high arsenic concentrations were often located less than 1km from wells with low or undetectable arsenic. High arsenic concentrations were associated with high concentrations of iron, organic carbon, bicarbonate, and ammonia, which is consistent with iron oxide reduction as the arsenic source. There was no clear pattern of arsenic concentration vs. depth and no correlation with chloride, so saline groundwater from the bedrock was not a significant source. High arsenic concentrations were also associated with low sulfate concentrations which may indicate sulfate reduction and sorption of arsenic to sulfide minerals. In most samples As(III) made up over 90% of the total arsenic, which is consistent with the redox conditions.