AGSO 10urnal of Australian Geology & Geophys ics, 17(5/6), 193- 201 © Commonwealth of Australia 1999 Behaviour of aluminium and iron in acid runoff from acid sulphate soils in the lower Richmond River catchment Angus Ferguson l & Bradley Eyre l Aluminium and iron concentrations and partitioning between par• ticulate, colloidal and dissolved forms were examined in acid run• off from known acid sulphate soil environments in the lower Richmond River catchment during dry season conditions. Chroni• cally acid drains in the Tuckean Swamp and Rocky Mouth Creek exhibited extremely high concentrations of dissolved metals (8- 10 mg/L AI , 5mg/L Fe). Dissolved aluminium and iron were quickly transformed to solid hydroxide species, which were rapidly Introduction Potential acid sulphate soils occur in Holocene backswamp de• posits along much of Australia's eastern and northern coast• lines. They remain 'potential' and, therefore, harmless as long as the local water table is kept high enough to maintain reducing conditions in the soil. If soils are disturbed or the water table is lowered either during an extended drought or by artificial drain• age, pyrite in the soil can oxidise, producing large amounts of sulphuric acid. The soil then becomes an actual acid sulphate soil. Subsequent rainfalls can flush accumulated acid from the soil into adjacent drains and waterways, reducing pH to below 3.0, mobilising dissolved metals such as aluminium and iron, and reducing dissolved oxygen. High levels of dissolved metals in acid runoff may poten• tially have a range of acute and chronic effects on estuarine biota. Large fish kills after rainfall in acid sulphate soil environ• ments are believed to be linked to toxic levels of aluminium in acid runoff combined with low dissolved oxygen levels (Brown et al. 1983, Sammut et al. 1993). Flocs of iron and aluminium oxides in acid runoff may smother aquatic plants and benthos (Dent 1986), and cause shifts in the makeup and diversity of aquatic plant communities towards a simpler suite of acid- and aluminium-tolerant species (Klepper et al. 1992, Sammut et al. 1994). Acid runoff and associated aluminium toxicity may also impact directly on human populations if allowed to contami• nate domestic drinking water, as in coastal areas of Vietnam and Indonesia (Dent 1995). Increasing concern over environmental impacts on estuarine ecosystems has prompted numerous studies looking at acid run• off from acid sulphate soils (e.g. Callinan et al. 1992, Lin & Melville 1992, Virgona 1992, Willet et al. 1992, White et al. 1993, Sammut & Melville 1995). However, most of the hydro• logical and physicochemical studies have focused on isolated events (e.g. Sammut et al. 1993), and small areas (e.g. White & Melville 1993), with no long term regional studies undertaken to assess the off-site seasonal impacts of acid runoff in the wider estuarine environment. Estuaries function as important sinks, sources and transformers of trace metals, thus modifying the quantity and quality of trace metals transported from the land to the ocean (Eyre & McConchie 1993). These estuarine processes also control the partitioning and, ultimately, the ef• fect that high metal concentrations in acid runoff will have on the estuarine ecosystem (Benoit et al. 1994). This study looks at aluminium and iron concentrations and behaviour in acid waterways of the lower Richmond River catch• ment to the Richmond River estuary. Filtration through pro• gressively fine filters was used to separate particulate, colloidal and dissolved fractions, while thermodynamic computer mod• elling was used to estimate likely metal speciation. I Centre for Coastal Management, Southern Cross University, PO Box 5125 East Lismore, NSW 2480, Australia removed from the water column by the aggregation and precipita• tion of diaspore and hematite in distinct flocculation zones, as water was subjected to steep pH and salinity gradients. This sug• gests that high metal concentrations may be found in benthic sediments and biota. Dissolved metals in acid runoff represent a major source of environmental pollution and, combined with the effects of acidity and low dissolved oxygen levels, pose a signifi• cant threat to estuarine ecosystems. Study area Geomorphology The Richmond River catchment on the north coast of New South Wales is part of the Clarence- Moreton Basin and has an area of about 6900 km 2 (Fig. I). The underlying geology of the catch• ment comprises various metasediments , dating back to the Palaeozoic, partially overlain by basaltic rocks of the Tertiary Lamington Volcanics-lava flows from the Mt Warning/ Wollumbin shield volcano Neranleigh- Fernvale Group (McTaggart 1961). The present lower Richmond River catchment consists of an extensive floodplain of Quaternary alluvium, extending in• land past Lismore and Casino. The Richmond River estuary is believed to be a mature infilled barrier type (Roy 1984), with barrier systems and mixed sediments of Pleistocene origin largely overlain by Holocene alluvium and estuarine sediments. The Post Marine Transgression, occurring since the end of the last ice age inundated the Pleistocene landscape of the lower Rich• mond, and infilling with Holocene sediments and reworking of Pleistocene sands have created highly variable floodplain envi• ronments (Morand 1994). Acid sulphate soils in the Richmond River catchment Significant areas of potential acid sulphate soils have accumu• lated in protected estuarine embayments and behind barrier sys• tems during the formation of Holocene floodplains in the lower Richmond River catchment. Large acid sulphate soil deposits have been identified in the Tuckean Swamp (R. Smith pers. comm. 1995) and in the Rocky Mouth Creek area (Naylor 1992), with smaller more localised deposits occurring around North Creek, Maguires Creek, and Bungawalbyn/Sandy Creeks. Climate and hydrology The Richmond River region enjoys the highest annual rainfall of New South Wales, with summer maxima and fairly high interannual variation. The climate is controlled by two major influences: the subtropical high-pressure belt during winter/ spring, bringing clear, mainly dry conditions; and easterly mon• soonal tradewinds during summer/autumn, bringing warm hu• mid conditions. Tropical cyclones may affect the region from about January to April, bringing heavy rainfall and flooding. Baseflow conditions Baseflow conditions exist in the Richmond River catchment from approximately July to December. Surface runoff during this period ceases and streams in the upper Richmond and Wil• son River catchments are fed by groundwater. In the floodplains of the lower catchment, including acid sulphate soil environ• ments, watertables drop, owing to evapotranspiration, and fresh• water flows all but cease. Tidal influence becomes stronger in floodplain waterways as the estuary migrates upstream. The freshwater/saltwater interface may reach as far inland as Coraki. In areas where tidal inundation has been excluded by one• way flood gates (e.g. Tuckean Swamp ; Rocky Mouth Creek)