Most principal aquifers in upstate New York are unconsolidated glacial and alluvial deposits within bedrock valleys. Groundwater in these aquifers can be under either water table (unconfined) or artesian (confined) conditions. Farms, industries, or towns and cities have been built upon many of these aquifers because they form level areas suitable for development and generally provide an ample groundwater supply. This development, coupled with the generally high permeability of these deposits and the typically shallow depth to the water table, makes groundwater in these aquifers susceptible to contamination from point sources such as landfills, road salt stockpiles, hydrocarbon fuel storage, and industrial facilities with a potential for contaminant leakage, in addition to urban and agricultural runoff, septic tank leachate, and other nonpoint sources. The report summarizes the geohydrology of the aquifer system in the Olean area. The large amount of geohydrologic data collected in the Olean area during previous and current investigations enabled the construction of nine maps: location and wells and test holes (plate 1), surficial geology (plate 2), geologic sections (plate 3), generalized bedrock topography (plate 4), potentiometric surface (plate 5), saturated thickness of the outwash aquifer (plate 6), generalized soil permeability (plate 7), land use (plate 8), and estimated well yields (plate 9). (Lantz-PTT)