Integration of geophysical, geochemical and microbiological data for a comprehensive small-scale characterization of an aged LNAPL-contaminated site

Characterization of aged hydrocarbon-contaminated sites is often a challenge due to the heterogeneity of subsurface conditions. Geoelectrical methods can aid in the characterization of such sites due to their non-invasive nature, but need to be supported by geochemical and microbiological data. In this study, a combination of respective methods was used to characterize an aged light non-aqueous phase liquid-contaminated site, which was the scene of a crude oil blow-out in 1994. As a consequence, a significant amount of crude oil was released into the subsurface. Complex resistivity has been acquired, both along single boreholes and in cross-hole configuration, in a two-borehole test site addressed with electrodes, to observe the electrical behaviour at the site over a two-year period (2010-2011). Geoelectrical response has been compared to results of the analysis of hydrocarbon contamination in soil and groundwater samples. Geochemical parameters of groundwater have been observed by collecting samples in a continuous multi-channel tubing (CMT) piezometer system. We have also performed a biological characterization on soil samples by drilling new boreholes close to the monitoring wells. Particular attention has been given to the characterization of the smear zone that is the sub-soil zone affected by the seasonal groundwater fluctuations. In the smear zone, trapped hydrocarbons were present, serving as organic substrate for chemical and biological degradation, as was indicated by an increase of microbial biomass and activity as well as ferrogenic-sulfidogenic conditions in the smear zone. The results show a good agreement between the intense electrical anomaly and the peaks of total organic matter and degradation by-products, particularly enhanced in the smear zone.