Effect of woodchips on water infiltration into soil : evidence of mitigating clogging and hydraulic conductivity reduce

The infiltration of urban water is an increasingly adopted practice downstream of wastewater treatment plants for the disposal of treated effluents. To maintain optimal infiltration conditions, materials such as gravel are commonly added to soils receiving the treated water. However, these materials involve significant economic and environmental costs, and their long-term effectiveness when combined with soil remains limited.

This study investigates the use of woodchips as an alternative material for the infiltration of treated wastewater. This material, primarily composed of organic matter, is less costly and may offer advantageous properties to sustain soil infiltration over time, both in terms of hydraulic performance and treatment capacity.

To this end, two pilot columns composed solely of a low-infiltration-capacity soil layer and two columns composed of the same soil layer overlain by a woodchip layer were hydraulically monitored over a four-year period. The objective was to assess the potential of woodchips to maintain or enhance infiltration in soils over time and to follow water infiltration into the two systems soil (control) and soil with an upper layer of woodchips. Two large columns—one consisting of soil alone and the other of soil overlain by woodchips—were subjected to successive infiltrations of treated wastewater volumes (mimicking a wastewater treatment plant outlet). Water infiltration, storage, and drainage were monitored in both systems. One-dimensional hydraulic modeling of the columns was performed using HYDRUS-1D to solve Richards’ equation and simulate the system behavior. The modeling was based on the van Genuchten–Mualem formulation for the water retention and hydraulic conductivity functions, as commonly adopted. Fitting the experimentally measured quantities enabled the estimation of intrinsic soil hydraulic parameters and the characterization of their temporal evolution over the four years of operation.

In addition to inversion and parameter estimation, a sensitivity analysis of the hydraulic parameters—namely saturated hydraulic conductivity (Kₛ), α, and n—was performed to strengthen the reliability of the modeling results and parameter estimates. This analysis highlights the predominant influence of Kₛ on variations in the soil water retention curves, leading to its selection as a key indicator of the evolution of soil infiltration performance. Furthermore, electrical resistivity tomography (ERT) measurements were used to monitor water distribution within the columns during feeding and resting phases. These data also served to further calibrate the model and gave insights on processes at the interface between the woodchips and the soil below, then improving system representation.

The modeling results demonstrate the significant role of woodchips in sustaining infiltration capacity. The Kₛ values estimated for soils amended with woodchips are consistently higher than those obtained for soils without woodchips. Such benefits is expected to result to the release of organic matter with a benefic effect on the soil structure.