Monitoring of biophysicochemical changes in a silty clay soil contaminated with LNAPLs

Abstract The interdependence between time domain induced polarization measurements at uncontaminated and artificially contaminated soil samples with diesel and biophysicochemical alterations caused by diesel degradation was investigated during 12 months. The research was performed on a slightly alkaline soil, with high content of organic matter (148 g/kg) and silty clay texture. Soil clay mineralogy was mainly composed of plagioclase, amphibole, biotite, interstratified mica-smectite, goethite and some vestigial kaolinite. A decrease on resistivity and induced polarization (IP) in contaminated soil was observed during the 12 months. This reduction on geoelectrical parameters was related to: 1) the increase on the electrolyte conductivity; 2) the increase on interfacial surface area, as result of biotite transformation and weathering of other ferromagnesian minerals and 3) a slight increase in soil aggregation, caused by diesel-degrading microorganisms (a significant increase of the numbers of specific carbon degraders was observed). After 4 months, the IP response was lowest in the contaminated soil which was explained by inhibition of the cation exchange capacity due to two possible processes: 1) clay particles coating by organic molecules and 2) attachment of microbial cells (biofilms) to clay particles and/or soil aggregates. The results suggest that the content and mineralogy of the clay fraction as well as the aggregation state of the soils contaminated with LNAPLʹs affect the IP response. This response is a diagnostic of the biophysicochemical alterations occurring during diesel degradation as a result of biological activity.