Vertical heterogeneity of chromium as a proxy for reactive metallic contaminants in the pore water of a municipal solid waste landfill subject to leachate recirculation

As part of the experiment introduction sustainable landfill management (iDS), three pilot projects were instigated in which landfills are being stabilized through leachate recirculation and/or aeration. The ultimate goal of these projects is to improve the leachate quality until it complies with previously derived environmental protection criteria. The hydrology and leachate transport of one of the three landfills – a municipal solid waste landfill being subjected to leachate recirculation - has been difficult to characterize. To further elucidate transport processes in this landfill, two vadose zone monitoring systems (VMS) have been installed. The VMS are installed at a 45 degree incline, and allow for pore water sampling at 0.4 metre depth intervals between 3 – 6.5 metres and 9.6 – 13 metres below the landfill surface. These systems provide a unique opportunity to collect and analyse in-situ pore water samples for a multitude of parameters of importance for contaminant speciation and transport, thereby providing valuable information on the vertical heterogeneity within the landfill. Starting from July 2022, the VMS are sampled every 3 months and analysed for parameters relevant for understanding contaminant availability and transport throughout the landfill, i.e. contaminant concentrations, concentrations of reactive colloidal particles, pH, and electrical conductivity. A separate paper focuses on the amount and composition of dissolved organic matter in the pore water as a function of depth in the landfill. This paper builds further on that study and describes how these parameters ultimately govern the speciation and vertical distribution of contaminants and its change with time, using Chromium as a proxy for contaminant behaviour