Hydraulic conductivity of gravel and sand as substrates in rock-reed filters

Abstract

Long-term use of a constructed wetland to treat landfill leachate requires that the saturated hydraulic conductivity be maintained and clogging avoided to prevent overland flow, which bypasses the treatment process. This paper describes the application of an equation developed for prediction of cumulative drainage volume from hillslopes to measure the saturated hydraulic conductivity ( K s) of substrates used in rock-reed filters. Outflow was measured at five intervals during the first 26 months of operation. The values of K s) obtained by the drainage equation compared favorably with values calculated from a more dificult method based on Darcy's law. Results indicate that the finest substrate (a sand-and-gravel mixture) became almost completely clogged, and that the presence of reeds ( Phragmites australis) did not maintain or increase the conductivity. Hydraulic conductivity of pea-gravel (0.5-cm diameter) and coarse-gravel (3-cm diameter) substrates with reeds did decrease in the 26-month period.