IN SITU RADIATION MEASUREMENTS IN SUBSURFACE INVESTIGATIONS
A commercial small-diameter NaI detector and winch system was used to determine the width, depth and downgradient extents of groundwater plumes containing 90Sr, a nuclide that emits only moderate-energy beta radiation. Bremsstrahlung radiation generated by the more energetic beta decay of its 90Y daughter can, however, be quantified with the NaI gamma detector when subsurface 90Sr concentrations exceed 10 Bq/g. Vertical PVC access tubes are emplaced in transects perpendicular to the axis of 90Sr groundwater plumes and at various distances from source areas. Repeated scans over time have tracked 90Sr movement. The NaI gamma detector was also used to locate and quantify the 90Sr that has accumulated within an underground permeable reactive barrier that has been capturing one of these aquifer plumes over the past 12 years.
A 0.02 L Geiger detector was lowered into vertical access tubes to monitor the mobility and decay of 60Co at locations of pipeline leakage. In situ measurements minimized our radiation doses, showed that 60Co was relatively immobile and provided assurance that the contamination remained several metres above the water table during radioactive decay.
A 1.8 L Geiger detector was configured for towing in contact with river sediments behind a boat moving at a rate of less than 1m/s. Three field days of field work provided more than 45,000 data points. A semi-quantitative map was generated and it revealed that a 200 m X 400 m area of riverbed, in water 10 to 30 m deep, contained above-background radiation. Eight months of analysis of 62 cores provided quantitative identities and concentrations of the radionuclides present and confirmed the areal extent of contamination provided by the towed detector. Spatial resolution provided by the in situ bottom-tow approach, however, was much better.