FRACTURE CONTROLLED VARIATIONS IN REGIONAL TRANSMISSIVITY AND GEOCHEMICAL EVOLUTION OF GROUNDWATER, CARBONATE AQUIFER, SOUTHERN MANITOBA, CANADA

A variety of data indicate that the Carbonate Aquifer in southern Manitoba, a major regional water supply, is a heterogeneous and anisotropic medium, wherein groundwater flow follows preferred-flow path networks. Specific capacity tests show that aquifer transmissivity can vary up to four orders of magnitude within one km. Geostatistical analysis reveals a strong anisotropy in the transmissivity field, with better spatial continuity in NE-SW and NW-SE directions, coincident with the dominant orientations of fractures observed in bedrock exposures, as well as lineaments observed from satellite images. Inflowing fresh waters from major recharge zones show a geochemical evolution along the flow paths characteristic of dedolomitization reactions. Stable isotope and geochemical data show evidence of fingering along high transmissivity networks that we interpret to be related to preferential flow paths along regional fracture systems, suggesting enhanced water/rock interaction as compared to low transmissivity zones.