Geophysical Detection Of Perchloroethylene In A Sandy Aquifer Using Resistivity And Nuclear Logging Techniques

As part of a larger programme to understand the behaviour of dense non-aqueous phase<br>liquids (DNAPLs) in the subsurface (Redman, 1992), a controlled release of 770 litres of<br>perchloroethylene (PCE) into a 9 by 9 metre cell within a sandy aquifer was monitored<br>geophysically during the summer of 1991. Surface and in situ resistivity, terrain conductivity<br>and neutron, induction, and density logging data were collected over a period of several weeks<br>to document the movement of the PCE away from the point of injection towards its final resting<br>place in pools on confining layers within the cell.<br>The very high resistivity of the PCE in its liquid state makes it a target for resistivity,<br>induction logging and terrain conductivity methods. Its high density, low hydrogen and high<br>chlorine content make it a good target, in principle, for density and neutron logs. For resistivity<br>measurements, two specifically designed in situ probes were installed one metre on either side<br>of the injection point. Two surface arrays were used to monitor apparent resistivity across the<br>diagonals of the cell. A resistivity scanning system was developed to ensure rapid acquisition<br>of surface and probe resistivity data. The density, neutron and induction logs were performed<br>on a regular basis in nine PVC-cased boreholes arranged on a 3 metre radius circle around the<br>injection point. In all cases, extensive background measurements were made.