Influence of plant root aging on water percolation in three earthen landfill cover systems: A numerical study

Vegetated earthen cover is a common practice after the landfilling of municipal solid waste. However, the impact of vegetation growth, particularly plant ageing and root depth, on the performance of landfill cover systems remains unclear. This study aims to numerically investigate the influence of vegetation growth-induced changes in soil hydraulic properties on percolation under heavy rainfall conditions in three types of landfill cover systems: one-layer (Case I), two-layer capillary barrier (Case Ⅱ), and two-layer capillary barrier over a low permeability layer (Case Ⅲ). The study considers vegetation growth-dependent soil water retention and water permeability, as well as root depth. The results show that under heavy rainfall with a 100-year return period in Southern China (36. 8 mm for 12 h), when vegetation is absent, Case III outperforms Case I and Case Ⅱ. In cover systems susceptible to water breakthrough, fresh root systems (e.g., 3 months old) at a depth of 0.4 m extends the water breakthrough time (WBT) of the cover system by an average of 43 %, while the ageing of root system decreases this duration by approximately 52 %. In cover systems with 1.0 m long roots, the trend of this effect becomes more pronounced. When young and fresh roots dominate in the cover system, water infiltration in all cover systems are reduced, resulting in an average 58 % decrease in bottom percolation, compared with the case without vegetation. However, with elder roots (19 months old) and deeper plant roots (1.0 m deep), the effectiveness of all types of landfill cover systems reduces, leading to an average 202 % increase in bottom percolation. Among all the scenarios considered, only Case Ⅲ continues to perform satisfactorily and meet the design standards even with severe root decay due to plant ageing. This study implies that in practice Case Ⅲ with short root systems should be selected to minimize the negative impact of vegetation on the hydraulic performance of landfill cover systems.