Interpretation of geophysical and geochemical data in a 3D geological model construction of a contaminated site

Abstract: The Alingsås dry-cleaning facility is one of the heavily halogenated hydrocarbon contaminated sites in Sweden. The source of contamination dates back to the 1970s when a large scale tetrachloroethylene leakage outbroke inside the dry-cleaning building. After years of propagation and dissemination, the tetrachloroethylene together with its degradation products was widely found in soil and groundwater at the site. Although no drinking water source is located in the contaminated zone, the potential risk due to the spreading plume cannot be neglected in a long run. Albeit, the natural self-cleaning potential was demonstrated by the previous investigations, the whole process can be enhanced and accelerated by the engineered biological remediation. Funded by Formas, SGU, and Tyréns, the “MIRACHL” project is set up to realize the monitoring of in-situ bio-geochemical remediation process with electrical imaging tomography. The first pilot injection of remediation products was conducted in November 2017. This master’s thesis work serves as a part of the “MIRACHL” project. In this thesis work, a refined three-dimensional conceptual model was built up via GeoScene3D, a 3D geological modelling software. The three-dimensional model consists of terrain surface, interpolated bedrock surface, interpolated groundwater surface, soil and groundwater chemistry data, borehole logs, as well as geophysical data. Of them, the geophysical data came from the 2D inversion results of electrical resistivity and induced polarization (DCIP) measurements along four buried horizontal arrays. The DCIP measurements data were collected before the first pilot injection. Through the visualization in GeoScene3D, it can be clearly seen that most halogenated hydrocarbons concentrate on the lower part of the unconsolidated layer, closely situated to the bedrock surface. In the latest set of measurement, the highest tetrachloroethylene concentration body was found nearby the source of leakage. The highest concentration bodies of trichloroethylene and cis-1,2-dichloroethene are situated at the same location with a slight difference in elevation. Vinyl chloride generally has a relatively low concentration level at the site, but a zone with extremely high concentration level was found at the northern edge of the measurement zone. Also, the concentration of halogenated hydrocarbons in groundwater samples is much lower than that measured via Membrane Interface Probe soundings (MIP-Soundings). Three possible reasons were assumed for such phenomenon: diffusional limitation, temperature factor, and contaminants multi-phase pattern. The DCIP inversion results were also correlated with the superposed geological and geochemistry reference data. However, no strong correlations were found between halogenated hydrocarbons and the DCIP responses due to the varieties of factors such as the phase pattern of the contaminants. In spite of this, several changes are expected to be seen in the inversion results after a couple of time steps, due to the boosted biological degradation process after the first pilot injection as well as the injection itself. However, due to the limited information provided, it is not possible to predict these changes at this stage.