Acid mine drainage is an important environmental risk linked to the surface storage of reactive mine tailings. To manage this problem, a cover with a capillary barrier effect (CCBE) can be used. This oxygen barrier cover relies on maintaining a fine-grained material layer (moisture-retaining layer, MRL) with a high degree of saturation. CCBEs can be colonized by surrounding plants. Plant roots pump water and could impact CCBE’s performance. This performance is predicted with unsaturated water flow numerical models in which vegetation parameters can be included. Vegetation parameters may be specific in a CCBE environment. Therefore, analyzing and quantifying the vegetation that colonizes this type of cover is necessary. Plant colonization was investigated through cover and density surveys on 12 transects on a 17-year-old CCBE in the mixed forest of Quebec, Canada. Then, aboveground vegetation and root colonization intensity at three depths in the MRL were characterized on 25 plots of five dominant vegetation types (Salix, Populus, Alnus, Picea sp., and herbaceous species). The mean root length density under plots dominated by Salix sp. was higher than in the other plots. Root colonization of the MRL was concentrated in the first 10 cm and occurred under all woody and herbaceous species as well. This work quantitatively describes, for the first time, the vegetation colonizing a CCBE both at the above- and belowground levels. These data will be useful to better predict the long-term performance of this engineered reclamation cover.

Mine tailings storage poses significant environmental risks such as the formation of acid mine drainage. Engineered covers offer a solution by controlling water ingress. Their performance is based on specific soil hydrogeological properties (SHPs). They must support vegetation which can impact saturated hydraulic conductivity ( k_sat) and water retention curve (WRC). This study assesses the impact of 4-year-old willow root colonization on silty soil covers using flexible wall parameters and water retention tests. The obtained SHPs were compared with root traits. The results suggested that the hydrogeological properties of the studied in situ samples were not significantly affected by roots. The variation of measured and predicted k_sat values spanned an order of magnitude, regardless of the root colonization intensity, up to a root length density (RLD) of 2.98 cm/cm³. RLD showed a significant and positive linear relationship with measured k_sat values ( R² = 0.54). However, when root colonization was low (RLD < 1 cm/cm³), RLD was negatively correlated with the n_vG ( R² = 0.44) parameter of the van Genuchten WRC model, while the opposite relationship was observed for samples with RLD > 1 cm/cm³ ( R² = 0.61). Additionally, RLD and the coarse root (diameter > 1 mm) volume to fine root volume (C/F ratio) influenced WRCs. Over time, coarser roots may have a more pronounced impact on SHPs; further research is needed.