Hydrogeological and geochemical investigations were conducted on four fractured Champlain Sea clay deposits in Eastern Ontario. The results from water level monitoring, maximum seasonal variations, and hydraulic head profiles revealed a hydraulically active fractured zone existing at all four sites. The depth of this fractured zone varies from site to site, ranging from 3.2 to 6.0 m. Slug test analysis indicated that bulk hydraulic conductivity values in the upper fractured zone range from 1.8 × 10−8 to 2.0 × 10−5 m/s. In contrast, the measured hydraulic conductivity values from the deepest piezometers range from 8.2 × 10−10 to 1.4 × 10−9 m/s. The geochemical analysis indicated the presence of three hydrochemical facies: a shallow "active" facies, a deep "inactive" facies, and an intermediate "transition" facies. The presence of tritiated groundwater well below the groundwater table indicates that the upper fractured zone at all four sites is hydraulically active. Key words : fractures, Champlain Sea clay, in situ testing, hydrogeology, geochemistry, hydraulic conductivity.