Preliminary assessment of biosolids in covers with capillary barrier effects

Abstract

The cost of sourcing material for cover material is often a barrier to implementation of multilayer covers to reclaim mined waste or other disturbed land. This study investigated the use of municipal biosolids blended with stabilizing materials as a low-permeability material in covers with capillary barrier effects (CCBEs) for reactive mine tailings. A blended biosolids referred to as Custom Reclamation Mix or CRM (1:1 volumetric mix of anaerobically digested biosolids and leaf and yard waste) was evaluated as a candidate low-permeability layer in a CCBE using material characterization, physical simulation of the CCBE in columns, and unsaturated flow modelling. The CRM exhibited low saturated hydraulic conductivities (k = 4 × 10 −7 cm/s @ e = 4.0) and an air-entry value of approximately 300 kPa. During laboratory column testing comparing a monolayer biosolids cover to a biosolids CCBEs, biosolids layers within the CCBEs remained highly saturated, acting as a barrier to oxygen diffusion and water flux. Pore-water samples in the tailings of the monolayer cover show some evidence of increased concentrations of nitrate relative to CCBEs columns. No significant change in pH was observed in any of the columns. Numerical models showed reduction in water flux at the tailings surface by 98% and a reduction in oxygen diffusion by up to three orders of magnitude when using biosolids CCBEs relative to uncovered tailings. Cover function was not significantly impacted by the characteristics of the underlying tailings. Whereas substantial cracking occurred in the biosolids monolayer cover, the low-permeability layers in the CCBEs remained sufficiently intact to remain both saturated and apparently retained low hydraulic conductivity. The use of amended biosolids as a low-permeability layer in a CCBE therefore appears promising and should be evaluated at larger scales.