HYDRAULIC CONDUCTIVITY OF GRAVEL AND SAND AS SUBSTRATES IN ROCK-REED FILTERS

Long-term use of a constructed wetland to treat landfill leachate requ ires that the saturated hydraulic conductivity be maintained and clogg ing avoided to prevent overland now, which bypasses the treatment proc ess. This paper describes the application of an equation developed for prediction of cumulative drainage volume from hillslopes to measure t he saturated hydraulic conductivity (K-s) of substrates used in rock-r eed filters. Outflow was measured at five intervals during the first 2 6 months of operation. The values of K-s obtained by the drainage equa tion compared favorably with values calculated from a more difficult m ethod based on Darcy's law. Results indicate that the finest substrate (a sand-and-gravel mixture) became almost completely clogged, and tha t the presence of reeds (Phragmites australis) did not maintain or inc rease the conductivity. Hydraulic conductivity of pea-gravel (0.5-cm d iameter) and coarse-gravel (3-cm diameter) substrates with reeds did d ecrease in the 26-month period.