High resistivities associated with a newly formed LNAPL plume imaged by geoelectric techniques ‐ a case study

Abstract

In April 10, 1997, a rice field in Nankan county, northwestern Taiwan was flooded by sixty to seventy kilolitres of fuel oil as a result of an accidental underground pipeline leakage. Immediate action to excavate contaminated soil and remove it to a landfill were done. However, it was believed that a significant quantity of light non‐aqueous phase liquid (LNAPL) contaminant might have remained in the soil and infiltrated into the groundwater. This paper emphasizes the use of multiple geoelectric techniques to detect and map the LNAPL plume in this uncontrolled real‐world site, and help monitor the effectiveness of the clean‐up operation. A significant change in resistivity values was detected between polluted (> 140 ohm‐m) and non‐polluted areas (< 140 ohm‐m). Repeated measurements were conducted at 1, 4 and 10 month intervals after the first measurements. These data were used to monitor variation and a possible spreading of the LNAPL plume over time. The total LNAPL masses were concentrated or diluted in the soil matching the variations of the water table. Two additional resistivity profiles were conducted to investigate the spatial distribution of the LNAPL contaminant plume within the study area. Electromagnetic induction and ground penetrating radar were also used to outline the resistivity zone defined by the plumes. The results of the survey serve to provide insight into the sensitivity of geoelectrical methods for detecting a shallow subsurface LNAPL plume, and help to provide valuable information related to monitoring the movement of an LNAPL plume over time in a study area.