Use of phytocaps in remediation of closed landfills - correct selection of soil materials

Abstract

Phytocapping is an emerging technology gaining interest as an alternate capping system in the management of closed landfills, particularly where leachate production occurs primarily through groundwater intrusion rather than surface infiltration. A field trial was undertaken at a former putrescible landfill to compare the performance of a phytocap and an engineered cap. The engineered cap comprised compacted subsoil, overlain by a gravel drainage layer, overlain by sandy clay loam topsoil. The phytocap comprised a layer of topsoil only. A range of native trees, shrubs and grasses were planted into each cap, and irrigated daily with landfill leachate of varying strength (0, 40 or 100%). Laboratory and field-testing evaluated the effect of leachate on soil physical and chemical properties. Deterioration in the topsoil's hydraulic properties impaired leachate infiltration and encouraged surface erosion for both systems. Laboratory studies found that aggregate instability due to slumping was the major cause for the decline in soil hydraulic properties. The topsoil sorbed appreciable amounts of added ammonium, organic nitrogen and organic carbon, and transformed much of the added ammonium to nitrate within a 14-day period. Thus, the capping system has the potential to remove leachate contaminants if the hydraulic properties of the topsoil can be improved. The trial has shown the importance of correct testing prior to selecting soil materials for use in capping systems where infiltration of leachate and rainwater is encouraged rather than discouraged.