Arsenic levels exceeding 10 μg/L are present in hundreds of private supply wells distributed over ten counties in eastern and southeastern Michigan. Most of these wells are completed in the Mississippian Marshall Sandstone, the principal bedrock aquifer in the region, or in Pleistocene glacial or Pennsylvanian bedrock aquifers. About 70% of ground water samples taken from more than 100 wells, have arsenic contents ≥10 μg/L with a maximum value of 220 μg/L. Water samples and continuous cores were taken from two test wells. Arsenic content of core samples ranges from <5 to more than 300 ppm, with the highest values found for pyritic black shales. Authigenic cements in the Marshall Sandstone include patchy authigenic pyrite that locally contains arsenic-rich (up to 8.5 wt. % As) domains. Bulk arsenic contents of pyrite-bearing intervals, sampled in well cuttings, are a high as 1020 ppm. Arsenic-rich pyrite is likely the ultimate source of arsenic in eastern and southeastern Michigan ground water, but evidence for pyrite oxidation at depth in bedrock aquifers is generally lacking. Pyrite oxidation may occur or have occurred in tills derived from the Marshall Sandstone and Coldwater Shale, which were found to contain arsenic-rich (up to at least 0.7 wt. % As) iron oxyhydroxides. Plausible mechanisms for widespread arsenic mobilization in eastern and southeastern Michigan ground water include weathering of pyrite in tills, reductive dissolution of iron oxyhydroxides in tills, and potentially, pyrite oxidation in bedrock aquifers, due to drawdown in wells or lowering of water-table levels in response to Pleistocene glaciation.