Geoelectrical signatures of redox processes

Common redox potential sensors provide point measurements and are error prone. Destructive sampling can cause contact with air which will influence the redox state. Hence, assessing redox conditions in soil is challenging. Redox sensitive reactions like the degradation of organic contaminants will change water chemistry and geophysical properties. We explored geophysical techniques ERT and SIP to observe the degradation processes of Propylene Glycol (PG), a de-icing chemical used at airports. Two laboratory experiments were performed to monitor changes caused by PG degradation: 1) 3D ERT was measured, every week, in four sand tanks. One rich in oxides. 2) SIP was measured, every three days, in twenty-six oxides rich sand columns. In both experiments PG was applied just above the water table. ERT - When iron and manganese oxides were available, degradation reduced resistivity. When oxides were unavailable, the electrical resistivity increased, most likely due to methane production which reduced water saturation. SIP - An increase of the real conductivity was associated with the metal ions release but independent of the frequency. Also an increase of phase angle and imaginary conductivity at frequencies below 1Hz that precede the ion release was observed in the water.