Root Penetration of Sealing Layers Made of Fly Ash and Sewage Sludge

Fly ash and sewage sludge are suggested materials for constructing sealing layers covering mine tailings impoundments. Little is known, however, of their effect on vegetation or resistance to root penetration. We investigate: (i) the ability of different plant species to grow in sealing layers comprising fly ash and sewage sludge, (ii) the impact on plant growth of freshly hardened fly ash compared to aged and leached ash, and (iii) the plant stress response to fly ashes of different properties. A 6‐mo greenhouse study using birch ( Betula pendula Roth.), Scots pine ( Pinus sylvestris L.), Kentucky bluegrass ( Poa pratensis L.), and willow ( Salix viminalis L.) demonstrated that no roots could grow into a compacted layer consisting only of ash, while a 6:4, ash–sludge mixture admitted roots into the upper part and a 1:9, ash–sludge mixture was totally penetrated (to 15 cm in depth) by roots of willow and Scots pine. Freshly hardened ash prevented root growth more effectively than aged ash did, as was observed in tests using reed canarygrass ( Phalaris arundinacea L.) and pea ( Pisum sativum L.). Furthermore, extracts of highly alkaline ash were more toxic to pea in a 48‐h toxicity test than less alkaline ash was. However, stress responses to diluted ash extracts of lower pH, measured as enzyme capacities in dwarf bean ( Phaseolus vulgaris L.), were more related to the metal and ion contents. Root penetration of sealing layers is most effectively prevented if little sewage sludge is added, and if ash of high alkalinity is chosen.