Effect of hydrocarbon contamination on streaming potential

Abstract

In the last decade, spontaneous potentials have been used to map reduced groundwater contaminant plumes. The measured signal includes contributions of both the streaming potential and the geo‐chemical electrical potential and for these applications the geochemical electrical potential is of interest. Therefore, the streaming potential must be modelled and subtracted from the measured spontaneous potential. Streaming potential is caused by the displacement of ions in the electrical double layer on sediments due to hydraulic head gradients in the fluid within a porous medium. Commonly, the streaming current coupling coefficient is assumed to be constant when modelling the streaming potential both inside and outside a contaminant plume. We postulated that organic contaminants might change the sediment surface properties, thereby affecting the streaming potential signature.To test this hypothesis, we used sediment and groundwater samples from hydrocarbon impacted field sites. Samples were equilibrated with water from the site for several weeks before the tests. Each sample was split into two and tested with clean water and hydrocarbon polluted water in an apparatus constructed following the design of Sheffer (). The streaming current coupling coefficient of the polluted sub‐samples was lower than that of the unpolluted sub‐samples for five of the six samples tested. Other parameters measured were hydraulic conductivity, cation exchange capacity and for the fluid: electrical conductivity, pH, major ions and hydrocarbons. Based on the values of the laboratory experiments, numerical modelling was used to demonstrate that the impact of these changes on field measurements is negligible.