Techniques used in isotope hydrology studies at Chalk River

Full text: Techniques in use at Chalk River for studying the hydrology of existing and potential waste management sites include: (a) devices to scan radioactivity in situ and to obtain samples or to inject tracers into groundwater systems; and (b) isotope measurements to follow dispersion, to differentiate between surface water and groundwater, and to obtain information on the age of the ground water. Groundwater discharge into a lake bed is measured by a 'seepage meter' consisting of the end of a steel drum which is embedded in a lake bed and vented to a plastic bag which inflates with the incoming water. Groundwater flow near the shoreline of a lake has been studied using several arrangements for injecting tracers in a uniform plane and collecting samples in the flow path. For sampling in a shallow borehole in rock, a string of simple inflatable packers with sampling intervals is constructed from readily available materials. Dispersion in a sand aquifer has been studied using 131I tracer; this has been monitored over 20 m of travel by gamma-scanning an array of dry PVC pipes in the flow system. This enabled detailed observation of tracer flow without the labour of discrete sample analysis and with no disturbance due to withdrawal of water from the flow system. Results from grain-size analysis and hydraulic conductivity measurements indicated the aquifer to be homogeneous, but the in situ scanning technique revealed three layers of different velocity. Within discrete layers, observed and calculated breakthrough curves yielded surprisingly low dispersivities (∼1 cm), in agreement with laboratory measurements. Measurements of environmental 3H, 14C, uranium isotopes and deuterium are being used to characterize groundwater discharging from bedrock and to estimate its age. Detectable tritium is taken as an indication of the presence of recent surface-water input, either residual drill fluid or leakage from the surface. Carbon-14 is measured by CO2 absorption in an amine-type liquid scintillation cocktail when sufficient sample is available. The accelerator mass-spectrometry technique has been used for measuring samples of a few milligrams, e.g. from groundwater carbonate of 0.5 L samples taken in down-hole samplers or from fracture-coating calcites. (author)