Assessment of a constructed wetland to reduce acidity discharge in caneland drains

ACIDIC drainage from pyrite oxidation in acid sulfate soils (ASS) is recognised by Australian growers as an important environmental issue needing best management. Constructed wetlands, where the dissolved metals responsible for most of the acidity load are precipitated for later treatment, have been suggested as an acidity management tool for sugar cane. The Sugar Research and Development Corporation (SRDC) therefore supported a project for the construction and assessment of a 2 ha wetland on Robert Quirk's 100 ha Tweed River cane farm. A major concern expressed by some growers was that if the expensive technique was successful it might be foisted on the industry. Also, that the precipitation of metals and other components of the acidic discharge might permanently damage and remove from any future production, the land under the wetland. Robert Quirk's wetland has been monitored and assessed for over 3 years: it has now been decommissioned and is being returned to cane production. This paper shows that while the wetland very successfully treated about 10% of the acidity load from the 100 ha farm, it would require up to 20% of the cane farm to treat all of the acidity load. Such a requirement is completely impracticable! However, the successful reduction of acidity discharge with the wetland suggests that encouraging and managing suitable vegetation in drains (rather than having drains always clean) may be a new way of reducing acidity discharge from canelands. In terms of the permanent removal of the land from any future production, it has been shown that the precipitation of acidic metals is small and comparable in magnitude and management necessary for the disposal of sediment on caneland during drain cleaning. The NSW industry already has formulated and adopted best management practices for drain cleaning in ASS.