Water flow dynamics in bottom ash landfills.

To obtain a mechanistic understanding of material reactivity and transport properties of bottom ash landfill leachate, this study investigates hydrological processes across seven Swiss bottom ash landfills of varying ages, focussing on competitive solution supply from the matrix flow and fast preferential transport paths during and after heavy precipitation events. Monitoring data of a selected landfill were compared with conceptual fluid flow models described by Richards Equation. The modelling domain of a small landfill section comprises infiltration layer, transition layer, vertical preferential flow path and matrix zone. Several simulations were conducted to examine the effects of material properties and rainfall intensity on the hydrogeochemical behaviour of water transport systems. The results show that preferential flow paths dominate leachate discharge during precipitation events, while matrix flow controls flux during dryer periods. A third, high flow-through drainage system is needed to explain measured peak arrival times linked to the heavy precipitation events. Based on data analysis and the modelling, water transport in landfill is governed into i) matrix flow through the landfill body, ii) preferential flow within the landfill body, and iii) preferential flow along edge zones and the drainage layer at the landfill boarder. Surface ponding and low permeability subsurface layers lead to local water accumulation and funnelled flow, which enhances water transport towards the edge zones and the drainage layer. This allows significant water fractions of the precipitation to bypass the bottom ash material, while backflow processes occurring after peak flux rates mobilise highly interacted porewater from the matrix zone.