MODELLING CHEMICAL DISCHARGE DURING FLOOD EVENTS FROM SUGARCANE-DOMINATED, COASTAL LOWLAND, ACID SULFATE SOILS

FLOOD events in coastal lowland acid sulfate soils (CLASS) result in the increased export of metals and acidity that will have a greater deleterious impact on the surrounding environment compared to a non-flooding system. The role in acidity production and behaviour of iron oxide/sulfate materials (especially jarosite) under these flood/flow systems are contentious issues for land management. A flood event was monitored at two sugarcane-dominated field sites on the Tweed valley floodplain in northern NSW. The water quality results show that goethite and hematite phases control dissolved iron within the discharge waters rather than jarosite. The oxides are likely present as clay coatings and deposited as either direct soil inputs or simply disturbed bottom sediments that are solubilised. Modelling of these data, along with similar reported data values in the literature, indicated that the same conclusions can be drawn at similar CLASS sites. It is concluded that in CLASS sites where water table management allows the upper horizon in acid sulfate environments to remain permanently oxidised, such as under sugarcane cropping, the mineral jarosite will not readily contribute to acidic discharge events, even under flood conditions. Therefore, the likely source of acidity and metals in canefield discharge waters is likely to be from immediate field drain areas and drainbottom sediments. This information can help develop improved management techniques to reduce discharge in sugarcane-dominated CLASS areas.