Hydrogeophysical characterization of shallow light non-aqueous phase liquid contamination at a karst aquifer
J.A. Rajab, A. El-Naqa and M. Al-Qinna
Journal name: Near Surface Geophysics
Issue: Vol 16, No 6, December 2018 pp. 643 - 662
Info: Article, PDF ( 2.2Mb )
Price: € 30
Two wells at an industrial facility were recently removed from service due to signs of long-term hydrocarbon contamination. Groundwater monitoring at the polluted wells documented the presence of a massive body of light non-aqueous phase liquid materials that were accumulated at a depth of 50 m in the water table. We have used transient electromagnetic and electrical resistivity tomography to characterize the ground at the light non-aqueous phase liquid contaminated wells. Additionally, analyses of hydrochemical samples and calculations of hydrogeological parameters have been implemented to interpret and describe the geophysical signatures of several anomalous hydrocarbon contaminations. Resistivity models, which are based on the three-dimensional inversion of transient electromagnetic data and the two-dimensional inversion of electrical resistivity tomography lines conducted at the source of leaks and contaminated wells, confirmed the presence of a hyper-conductive zone (i.e. 0.5 m), 15 m deep and 20 m thick, that contained a mixture of light non-aqueous phase liquid, hot water, and chemical acids. In addition, resistivity levels up to hundreds of ohm metre can be attributed to near-surface cavernous structures. Furthermore, the resistivity models can resolve a 30-m-thick conductive layer underlying cavernous structures, yielding resistivity values ranging from 0.78 to 5 m. The ease of light non-aqueous phase liquid migration is promoted due to zones of cavernous structures acting as preferential flow pathways down to the water table, where light non-aqueous phase liquid materials follow the hydraulic gradient of the limestone aquifer, which has a calculated porosity of 20%. Chemical analysis of groundwater samples at contaminated well sites shows the depletion of sulphate and nitrate concentrations and an increase in bicarbonate at locations where the highest hydraulic conductivities are observed. These findings facilitates the construction of a conceptual model for contamination which is used to better understand the evolution of light non-aqueous phase liquid pollution at the study area.