Application of ERT and IP for localisation of chlorinated hydrocarbons at a former dry-cleaning facility

English. At Hagforstvätten a dry-cleaning facility operated 1970 – 1993 and leached huge amounts of tetrachloroethene (PCE) to soil and groundwater. This study was conducted at the contaminated site and focused on applying the geophysical methods Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) to investigate if it was possible to locate source zone, degradation zones and groundwater plumes of PCE. A background survey including GPR, DUALEM and magnetic gradiometer was conducted to study geological features and man-made structures that could interfere with the ERT and IP measurements. Drillings and chemical samplings were used as a complement to ERT and IP measurements to validate and strengthen the interpretation of resistivity and IP as caused by geological units, PCE or man-made disturbances. Due to the large heterogeneity in the subsurface of Hagforstvätten it is our opinion that it is a challenge to distinguish anomalies in resistivities and chargabilities as caused by pollutants. A promising outcome from the survey was however that a source of PCE located south of the dry-cleaning facility was distinguished as a degradation zone. In the northern part of the area indications of a groundwater plume were detected, however the sparse reference data in the area makes the interpretation more uncertain. Even though the interpretation of the pollutant situation is difficult at the site, ERT and IP have increased the understanding of the complex subsurface. This information is important to understand how PCE can be stored and spread in the area. Based on our results, we believe that ERT and IP are most suitable for monitoring how the high concentrations of PCE at the secondary source changes with time, for example during a future remediation.

English. In Sweden, as in many countries in the world, a large concern has risen about contaminated soil and water caused by pollution from industries. A widespread group of contaminants with severe environmental and health impacts is chlorinated hydrocarbon. These compounds have mainly been used by industries or dry-cleaning facilities as de-fatting solvents. One site contaminated with chlorinated hydrocarbons is situated at the former dry-cleaning facility Hagforstvätten in Hagfors (Värmland) that operated 1970 - 1993. As a solvent, the industry used tetrachloroethene (PCE) which today is known for being carcinogenic and toxic for humans and ecosystems even at very low doses if the exposure proceeds over a longer time. The spill of PCE to the ground at Hagfors was huge, and today it is estimated that at least 20 to 30 tons of PCE rest underneath the laundry facility (SWECO, 2013). Hagforstvätten is part of a national research project called MIRACHL that improves techniques for monitoring remediation of chlorinated hydrocarbons at contaminated sites. As a part of MIRACHL, the applicability of two methods for mapping the pollutant situation at Hagforstvätten have been investigated. The used methods where Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) that measure the electrical properties of the subsurface. The electrical properties vary with geology, buried man-made objects or presence of chlorinated hydrocarbons. PCE works as an electrical insulator when it displaces water in the porous space in the soil, hence it decreases the resistivity of the ground. The investigation was complicated by the large heterogeneity of the subsurface. Nevertheless, a known source of PCE still resting in the ground could be detected by using ERT and IP. In addition, the measurements showed indications of contaminated groundwater downstream the dry-cleaning facility. The two methods showed that the contaminated site has a very varied geology that probably is highly influenced by man since a former railway station was located at the site. The inhomogeneous geology complicated the work with distinguishing low or high zones of conductivity as caused by pollutants, geology or man-made objects. Therefore, the methods contributed with sparse additional information of the pollutant spreading at the site. Even though the interpretation is difficult at inhomogeneous sites such as Hagforstvätten, the methods covered a large area and thus have given valuable information about the complexity of the site. The increased understanding of the complex mix of geology, man-made structures and pollutants would easily be missed by the previous performed pointwise measurements with geological and chemical sampling. Although it will be a challenge to monitor PCE with ERT and IP at the site, it is important to continue the work. Even if the contamination was caused by the past, it is the responsibility of present generations to preserve a healthy and sustainable environment for humans and ecosystems in the future. References: SWECO. (2013). Hagforstvätten, huvudstudie. Karlstad.