Ground water flow in a fractured carbonate aquifer inferred from combined hydrogeological and geochemical measurements

A conceptual model for ground water flow is presented for a fractured Silur ian dolomite in the Niagara Escarpment area of southern Ontario. Such a mod el is necessary to facilitate remedial efforts of a PCB-contaminated site l ocated in Smithville, Ontario. Both physical and chemical hydrogeological o bservations obtained from field investigations were used to deduce the stru cture of the ground water flow system in the fracture network. The field st udy was conducted using observations obtained from six boreholes drilled in the vicinity of the town of Smithville. The boreholes were diamond cored t hrough the entire thickness of the dolomite formation (approximately 45 m), hydraulically tested using a 2 m packer spacing and then completed using m ultipacker casing strings. Measurements of hydraulic head were obtained on a weekly basis over a period of two gears, and ground water from each boreh ole interval was collected for geochemical analyses for inorganic and isoto pic composition. Transmissivity measurements indicate that the dolomite is divided into two ground water flow systems separated by an extensive unit o f low transmissivity that is pervasive throughout the region. The upper flo w system is characterized by water enriched in Mg2+ and SO42-. Below the lo w transmissivity zone, ground water increases in salinity and is enriched i n Ca2+ and SO42-. Based on the geochemistry, the rate of ground water migra tion in the lower flow system is surmised to be much less than that in the upper system. Measurements of hydraulic head in conjunction with the result s of the analyses of the environmental isotopes (delta(18)O and delta(2)H) suggest that ground water flow is mainly horizontal and likely governed by enlarged bedding plane fractures. The isotope geochemistry; and topographic al features further suggest that ground water recharge is occurring approxi mately 2 km to the north of Smithville.