Multidisciplinary perspectives on a natural attenuation zone in a PCE-contaminated aquifer

Abstract: The old dry-cleaner in Hagfors, is one of the most seriously tetrachloroethene (PCE) contaminated sites in Sweden. Ever since the pollution was discovered in the groundwater in the 1990s, the site has undergone remedi-ation two times with limited success. The Geological Survey of Sweden (SGU) has estimated that tens of tonnes of PCE are still left in the subsurface, where dissolved PCE is transported via groundwater and discharged to the adja-cent Örbäcken stream. In this study, I applied high-resolution direct current induced polarization (DCIP) and sam-pled the groundwater and the near-surface sediment for chloroethenes in a study area located adjacent to Örbäcken. The chemical results indicate that natural attenuation is an actively ongoing process in the study area, while the DCIP model result provided information about subsurface structures that correlate well with the aquifer-aquitard boundary indicated from already existing core descriptions. The different DCIP model results were displayed to-gether with borehole data, as well as chemical reference data, in a 3D model. These combined results clearly showed two structures that are continuous throughout the study area. These two structures include one zone of high resistivity, correlating with an area of high amount of the biological degradation product cis-1.2-dichloroethene (cisDCE), and the other is a tilted zone with low resistivity upstream of the high-resistivity zone. The low-resistivity tilted zone crosses the aquifer-aquitard boundary and is interpreted as a fracture zone in the aquitard. The work is part of the research project Monitoring of In situ RemediAtion of CHLorinated hydrocarbon contami-nants, MIRACHL (Funder by Formas 2016-00808 and 942-2016-99, SGU and Tyréns Stiftelse).The work was also partly supported by the European Union, Eurostars Programme, together with Innovation Fund Denmark and The Swedish innovation agency Vinnova under the project “Mapping Geology in Cities” (E10096 MAGIC).