THERMAL NEUTRON LOGGING FOR CHLORINATED SOLVENT CONTAMINATION IN AQUIFERS
Anthony L. Endres and John P. Greenhouse
Waterloo Centre for Groundwater Research, Department of Earth Sciences,
University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Displacement of groundwater by relatively hydrogen-poor solvents affects the response of thermal neutron devices by changing the scattering and thermal neutron capture properties of a contaminated aquifer. Since chlorine has a large capture cross-section compared to other elements commonly found in aquifers, the capture phenomenon significantly influences thermal neutron count rates in the presence of chlorinated solvent contamination.
Thermal neutron logging was performed during a controlled release of perchloroethylene (PCE) with tools having short (8 cm) and long (41 cm) source-detector spacings. These results demonstrated that thermal neutron logging successfully detected and monitored immiscible phase chlorinated solvents in the subsurface at residual concentrations. Comparison of the data from the two different tools indicates that the predominate mechanism affecting tool response is the change in macroscopic capture cross-section associated with the presence of solvent-based chlorine.
Laboratory tests were conducted to determine the response of the short spacing neutron moisture probe used in the field experiments (i) to changes in chlorine concentration in a homogeneous environment and (ii) to the spatial distribution of chlorine-rich layers. These tests demonstrated that this device is sensitive to the presence of chlorine in a homogeneous medium to concentrations below 0.1 moles Cl/l and that chlorine-rich layers as thin as 0.2 cm (1/16 in) caused an appreciable tool response. These laboratory results were used to convert the field data into apparent PCE saturation measurements that compare favorably with PCE saturations obtained from core samples.
The results of this combined field and laboratory study conclusively show that this simple, readily available technology can be effectively used to evaluate and monitor chlorinated solvent contamination at residual concentrations under suitable geological conditions.
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