Geochemical Processes Controlling In Situ Contaminant Solubility in Landfills

Sustainable management of landfills through actively reducing contaminant emissions requires knowledge of underlying geochemical processes, which is currently limited. To this end, pore water was sampled across a vertical transect from a landfill subject to leachate recirculation with an in situ collection system. The samples were analyzed for pH and the concentration of inorganic contaminants and dissolved organic matter fractions; all showed similar distributions with depth. The underlying mechanisms of this analogous behavior were identified using geochemical modeling, which captured measured concentrations adequately for most contaminants. While nonreactive contaminant concentrations were best predicted considering limited interaction with the solid matrix and reactive anions were best predicted by metal (hydr)oxide binding, adequate predictions for reactive cations with a high organic matter binding affinity required a description of humic substance fractions. Predictions of other reactive cations were steered by the formation of carbonate or sulfide minerals. The importance of mineral precipitates deviates from previous lab experiments on sampled solid waste, highlighting the importance of modeling in situ samples to explore processes in complex systems such as landfills. The obtained understanding can be used to test landfill simulation reactor results, aid in understanding impacts of proactive waste management strategies, and ultimately facilitate their implementation.