Treatment of drainage from acidic canelands using a constructed wetland

Abstract

This paper describes trials of a constructed freshwater wetland to treat highly acidic drainage from acid Sulfate soil in a sugarcane farm. A constructed freshwater wetland was used to treat acidic discharge from drained acid sulfate soils on a sugar cane farm in the Tweed River flood plain, northern New South Wales (NSW). The bunded 1.44 ha wetland was laser levelled into 6 segmented bays with an overall hydraulic gradient of 0.13%. Water retention time varies between 19 and 82 days dependent on the prevailing evapotranspiration rate. The wetland, which receives about 12% of runoff from a hydraulically isolated 100 ha sugarcane area by pumping, is designed to treat the highly acidic groundwater-dominated recession phase of drainage with large concentrations of dissolved aluminium, iron and manganese. Common couch grass (Cynodon dactylon) and Eleocharis reeds were established in the wetland by natural recruitment. Transects of water quality wetland during filling and while in operation revealed that pH increased while electrical conductivity (EC), dissolved oxygen (DO), redox potential (RP), dissolved sulfate, total potential and total actual acidity (TPA and TAA) decreased dramatically through the wetland. Iron oxyhydroxide flocks are deposited in the inlet bay of the wetland and the redox-pH relation is consistent with ferrolysis. As water moves further into the wetland it is titrated by the organic matter present, resulting in the reduction of protonic acidity, sulfate, dissolved metal concentrations and EC. The advantages and disadvantages of using the wetland as a practical method to treat drainage within a farming system are discussed. The wetland successfully treated acid drainage.