USE OF STABLE AND RADIOISOTOPES IN AN INTEGRATIVE APPROACH FOR CHARACTERIZING GROUNDWATER FLOW IN A MODERATELY FRACTURED BEDROCK AQUIFER

Assessing risk of mining-related activities to bedrock aquifers and downgradient environmental receptors requires an understanding of the influence of fractures on groundwater flow. The hydrogeology of fractured bedrock systems is complex and requires an integrated assessment approach. Stable and radioisotopes provide a relatively quick and affordable means in further evaluating the influence of fractures on groundwater flow. Isotopes were used as part of a hydrogeological investigation at a proposed underground mining operation located in the mountainous terrain of the Swannell Ranges of north central BC. Sampling for – stable (d²H, d¹⁸O, d¹³C) and radiogenic (³H, ¹⁴C) isotopes was conducted during routine water quality sampling events to augment the physical and chemical hydrogeological data to help assess estimates of groundwater recharge conditions and history. Groundwater level hydrographs, core logs, hydraulic packer testing and radioisotope distributions indicate that the granitoid bedrock has experienced a complex pattern of groundwater flow in faulted and fractured intrusions and altered bedrock. Hydrogeologic evidence of these variations was observed to a depth of approximately 250 m below ground surface (bgs).

Different isotopic models were used to interpret the d¹³C and ¹⁴C data to calculate mean groundwater residence times. Mean groundwater ages across the study area range from modern to about 5,000 years before present, however the mean age of most groundwater samples results range from about 2,000 to 4,000 years before present. Tritium levels indicate a component of modern (post 1950) recharge in all groundwater samples. Overall, isotopic data indicate that groundwater in the relatively shallow fracture network (i.e., < approximately 250 m bgs has a component of ancient water (i.e., 2,000 to 4,400 years), and that under natural conditions, the travel time for the older groundwater to reach surface receptors is thousands of years. The steepness of the terrain, along with the absence of downgradient trends in groundwater age (¹⁴C data) indicates that groundwater moves along numerous, non-interconnected groundwater flow paths. This, in turn, suggests that a large, laterally continuous, shallow (less than 150 m below ground surface) fracture network does not exist across the study area.