Final cover performance in the Australian Environment - the A-ACAP field trials

Containment of waste or other contaminated materials has been based on entombing the material and preventing infiltrating rainfall from draining into the waste. The A-ACAP field trials were conducted in 5 Australian states with climatic regimes from tropical to almost semi-arid and investigated the water balance in compacted clay barriers approved for each site and phytocaps using large non-weighing lysimeters. These trials showed that the compacted clay barriers still allow drainage (0 - 233 mm/yr) and within a short period of time show evidence of desiccation cracking. Although the clay may swell upon wetting, the reduced permeability remains. Drainage is more difficult to predict from these caps using readily available water balance models, due to the presence of these preferential pathways, as the majority of models assume moisture moves through the soil mass. The A-ACAP also showed that phytocaps can perform equivalently (0 - 255 mm/yr) and potentially outperform compacted clay barrier caps, particularly when predictability, sustainability and recovery from damage are taken into account. The keys to designing these systems are understanding the performance of the natural system, matching the seasonality of rainfall with plant growth and rapidly establishing vegetation on the capping system.