Vertical heterogeneity of organic matter in the pore water of a municipal solid waste landfill subject to leachate recirculation

As part of the sustainable landfill management experiment (iDS), three pilot project were instigated in which landfills are being stabilized through leachate recirculation and/or aeration. The 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, such as contaminant concentrations and reactive colloidal particles that these contaminants can be associated with. Thus, the VMS provide valuable information on the vertical heterogeneity within the landfill. Every three months, since July 2022, landfill pore water is analysed for a) a wide range of inorganic contaminants that will be presented in a separate paper, and b) dissolved organic matter composition in terms of humic substances, which will be presented in this paper. Organic matter in landfill waste contributes both to the mobilization of contaminants (when bound to dissolved organic matter) and immobilization (when bound to solid organic matter). Understanding the dynamics of organic matter in a landfill affected by full scale leachate recirculation is essential to assess the effectiveness of leachate recirculation as an active landfill treatment. Humic substances, in the form of humic and fulvic acids, have in soil science extensively been shown to be valid proxies to predict the binding of metals to organic matter. Similar behaviour is expected in waste bodies. Results of humic substances dynamics during the first year of frequent pore water monitoring in the waste body will be shown and related to solid waste analyses and bulk leachate data.