Hydraulic properties of coarsely and finely ground woodchips

Recent 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 (15.3cm high, 12.1cm wide) to measure saturated hydraulic conductivity (K _s ), cumulative outflow and water content at equilibrium with pressure potentials of -2, -10 and -40cm. Water retention at pressure potentials between -200 and -10,000cm 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) . K _s was independent of PSD with values between 55 and 80cm/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 0.24cm/h at -10cm and 3×10 ^-3 cm/h at -50cm for the DPeM model, suggesting that water would move slowly through stockpiles except during intense rainfalls.