Hydraulic properties of coarsely and finely ground woodchips

- Hydraulic properties of packed woodchips of various sizes measured in the laboratory.
- Water retention and hydraulic conductivity functions estimated by inverse modeling.
- Cumulative outflow best predicted by a model with two interacting pore domains.
- Results suggest that water moves very slowly through unsaturated woodchip columns.

SummaryRecent evidence suggests that leachate from woodchips stockpiled at recycling facilities could negatively impact water quality. Models that can be used to simulate water movement/leachate production require information on water retention and hydraulic conductivity functions of the stockpiled material. The objectives of this study were to (1) determine water retention and hydraulic conductivity functions of woodchips with particle size distributions (PSDs) representative of field stockpiled material by modeling multistep outflow and (2) assess the performance of three pore structure models for their ability to simulate outflow. Six samples with contrasting PSDs were assessed in duplicate. Samples were packed in cylindrical columns (15.3 cm high, 12.1 cm wide) to measure saturated hydraulic conductivity (Ks), cumulative outflow and water content at equilibrium with pressure potentials of −2, −10 and −40 cm. Water retention at pressure potentials between −200 and −10,000 cm were obtained using pressure plate extractors and used to supplement data from the outflow experiment. Hydraulic parameters of the pore models were derived from these measurements using HYDRUS-1D run by DREAM(ZS). Ks was independent of PSD with values between 55 and 80 cm/h. Cumulative outflow at each pressure potential was correlated with the PSD geometric mean diameters, and was best predicted by a model having two interacting pore domains, each with separate hydraulic conductivity and water retention functions (DPeM). Unsaturated conductivities were predicted to drop on an average to 0.24 cm/h at −10 cm and 3 × 10−3 cm/h at −50 cm for the DPeM model, suggesting that water would move slowly through stockpiles except during intense rainfalls.