Relationship Between Total Dissolved Solids And Bulk Conductivity At A Hydrocarbon-Contaminated Aquifer

A recent conceptual model links high conductivities observed in soils impacted by<br>hydrocarbon to higher total dissolved solids (TDS) resulting from mineral weathering from acids<br>produced during biodegradation. Although not a direct evidence of biodegradation, TDS is a<br>geochemical parameter that closely links groundwater electrical properties to hydrocarbon<br>degradation. In this study, we sampled groundwater from discrete depths from several locations<br>within a hydrocarbon-impacted aquifer in order to investigate the relationship between TDS and<br>electrical conductivity of soils. TDS was measured in the field using a conduc tivity/TDS meter<br>and by the gravimetric method in the laboratory. The results show similar values of TDS for both<br>techniques in uncontaminated groundwater. Slight differences were observed between the two<br>techniques for groundwater from locations contaminated with hydrocarbons. In general, both<br>TDS and electrical conductivity of soils increased with depth at contaminated and<br>uncontaminated locations. However, TDS was 40% higher in contaminated groundwater relative<br>to uncontaminated groundwater. Our results further showed that there was no clear relationship<br>between groundwater specific conductance and electrical conductivity of soils, suggesting that<br>electrical conductivity of soils cannot be simply related to groundwater ionic concentration or<br>TDS at the contaminated locations. We deduce from these observations that other petrophysical<br>properties and lithologic variations in addition to changes in the aquifer matrix induced by<br>biogeochemical reactions may have contributed to the poor correlation between electrical<br>conductivity of soils and groundwater specific conductance. More work is needed to fully<br>understand the impact of biogeochemical changes on the soil matrix and the potential influence<br>of such changes on the electrical properties of soils.