Effect of the initial soil water saturation on the behaviour of a mixed LNAPL and heavy metal contaminated glaciofluvial deposit

Abstract

SummarySoil contamination by mixtures of petroleum hydrocarbons and heavy metals is common in urban and industrial localities. Interactions between these contaminants have an impact on the mobility and the management of contamination. We have characterized the modifications to the transport of heavy metals (Cd, Cu, Pb, Zn) in soil induced by residual light non‐aqueous phase liquid (LNAPL) for two conditions of trapping. Experiments on the elution of tracers and heavy metals in columns of soil were performed with a glaciofluvial material as the soil. Tracer experiments were modelled with the mobile–immobile (MIM) system of partial differential equations. The experiments were designed to compare water flow and metal transport in LNAPL‐contaminated soil with a control set. Residual LNAPL was trapped in water‐saturated and dry soil to ensure preferential wettability of soil surfaces, namely either water‐wet or LNAPL‐wet.In water‐wet soil columns, LNAPL decreased water flow by two orders of magnitude and increased the fraction of immobile water. Solute residence times (SRTs) suggested that heavy metals resided mainly in mobile water where the reaction time was sufficient to reach steady‐state retention. The SRTs also indicated that a fraction of the heavy metal flux diffused to the immobile water where its retention was limited by diffusion. Retention of heavy metals was significantly greater than in the control columns. In LNAPL‐wet soil columns, the obstruction of small pores and surface coating by residual LNAPL significantly decreased the attenuation capacity of the soil by decreasing the diffusion of heavy metals to immobile water and surface sites. Evidently, the individual behaviour of heavy metals can be significantly modified by non‐miscible organic contaminants. These modifications can have important implications for risk evaluation, contamination management and in situ remediation of soil that is contaminated by mixtures of petroleum hydrocarbons and heavy metals.