A Novel Case Study on Achieving Site Closure - EM31 and Pseudo-3D OhmMapper Surveys, Calibrated with Physical Analytical Soil Data, to Create Impacted Soil Volume Estimates before Remediation

Adam Peake, David

Dans les comptes rendus d’articles de la conférence: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Contaminant Hydrogeology 

ABSTRACT: 1.0 INTRODUCTION Volume estimation and three-dimensional mapping of salt-impacted soils is an integral part of both remediation and risk management activities in order to create accurate project budgets, alternative remedial options, and to properly characterize the site for risk management. This extended abstract reviews a case study documenting the process and final results after the combined acquisition and interpretation of novel geophysical survey data using innovative techniques for mapping chloride contaminated soils in the vicinity of a historical pipeline release. The integration of geophysical data from the OhmMapper and EM31 systems with borehole measured concentrations and conductivities enables quantifiable constraints for the geophysical data to be used. For this case study, a start to finish methodology can be recognized, highlighting impacted soil volume estimates compared to actual impacted soil volumes removed from site during a remedial excavation to achieve site closure. 2.0 SITE HISTORY Contamination of the site occurred due to a pipeline break that released approximate volumes of 29 m3 of crude oil and 75 m3 of produced water. Immediate cleanup efforts resulted in the excavation of 1793.95 tons of impacted soil which was disposed of at an off-site facility. The results of a previously conducted Phase 2 Environmental Site Assessment (ESA) showed that the spill area had elevated electrical conductivity (EC) and that the Sodium Absorption Ratio (SAR), Benzene, Toluene, Ethylbenzene, Xylenes (BTEX), and Petroleum Hydrocarbon (PHC) fraction F3 concentrations exceeded the regulatory guidelines. Analyses of soil samples were compared to the Alberta Environment and Parks (AEP) Tier 1 Soil and Groundwater Remediation Guidelines (SGRG). It was determined that 5775 m3 of impacted soil may exist in the former spill area. The final extent of impact was not determined as part of this Phase 2 ESA. The Site was re-assessed by the Owner in order to characterise the background and on-site soil chemistry. The results would be used to develop site-specific Tier 2A guidelines using the Subsoil Salinity Tool (SST). Additional boreholes were drilled and soil samples collected at specific depth intervals for the analysis of salinity parameters. A more traditional frequency domain electromagnetics (EM31) survey was conducted to target the drilling assessment. The volume of salinity impacted soil, as determined by this Phase 2 ESA, SST assessment, and EM31 survey was estimated 13600 m3. DMT Geosciences Ltd. and Pinchin Ltd. were then involved by the Owner to create a better model of the impacted plume. DMT Geosciences used EM31 and advanced pseudo-3D capacitively coupled resistivity (OhmMapper) surveys to create a better model. 3.0 GEOPHYSICAL INVESTIGATION METHODS 3.1 EM31 - Fixed Frequency Electromagnetic Method The Geonics EM31 MKII conductivity meter was used to map variations in ground conductivity, which can be indicative of variations in Total Dissolved Solids (TDS). The system is a self-contained portable instrument that consists of a transmitter dipole that produces a primary field and a receiver dipole that measures secondary fields dependent on subsurface conditions. The ratio of the primary and secondary fields is related to ground conductivity. Apparent conductivity varies due to many factors including mineral composition (clays are of high electrical conductivity for example), variations in volume and concentration of TDS, presence of buried metal, and thermal state. The depth of

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