The physical and geochemical controls on heavy metal cycling in mangal sediments, Wynnum, Brisbane

Abstract

Mangroves in Australia are recognised as having considerable environmental and economic value and as such are accorded legislative protection by state governments. However, despite this recognition and protection, mangrove forests and adjacent areas are still being used as dumping sites for sewage effluent and solid refuse. This study investigates a site at Wynnum, Brisbane, where mangroves provide a barrier between a source of metallic pollutants and the sea; abundant solid metallic refuse is also scattered through the mangrove forest. Chemical analyses (for Cu, Cd, Pb, Zn, Hg, Cr, Ni, Sn and Fe) of sediments and waters within the mangrove forest and from adjacent salt marsh reveal elevated metal loads and suggest that metals are entering the mangrove ecosystem from the tip by surface runoff and via groundwater. The mangroves form an effective baffle to tidal current and wave action that promotes the accumulation of fine grained organic matter-rich sediment, within and shoreward of the mangrove forest; this sediment in turn provides provides a suitable habitat for large populations of sulphate reducing bacteria. Direct metal adsorption onto the fine sediment, complexing with organic matter in the sediment, and the formation of insoluble sulphides by reaction with bacterially generated sulphide, all contribute to the trapping of metals in and near the mangrove forest. The fine sulphidic mangal sediments make the major contribution to the biogeochemical metal trap but metal uptake by the mangrove plants is also important. Sediment grainsize is an important control on the vertical distribution of metals in the study area, but the vertical distribution of metals of the sediment column also reflects the position of geochemically distinct horizons which favour either metal trapping (fine grained, neutral to high pH, low Eh horizons) or metal mobilisation (coarse grained, low pH, high Eh horizons). Consequently, the distribution of metals in the sediment column is not fixed because the position of the geochemical horizons can change seasonally in response to changes in rainfall induced ground water influx and depression of the water table during prolonged dry periods. The lateral distribution of metals over the surface of the study area is largely controlled by subtle changes in Eh and pH conditions caused by a combination of biological (e.g., the presence or absence of an algal cover) and physical factors (e.g., the presence or absence of surface depressions, the frequency of tidal inundation and the extent of seasonal desiccation). The lateral distribution of metals in sediment beneath the surface is largely controlled by proximity to the tip face and lateral variations in sediment grainsize and in the hydrological conditions that influence the position of geochemically distinct horizons. The findings of this study show that for environments such as the mangrove community at the Wynnum Tip site, it is important to understand the geochemical conditions in the environment before developing management strategies. Because the reduced sulphidic sediment makes such an important contribution to the metal trapping in the mangrove forest, future land management planning for the area will need to ensure that these sediments are not drained and allowed to oxidise. Oxidation, which would be a likely result of draining the sediment, would not only release many of the presently trapped metals but would probably also create the type of problems typically associated with acid sulphate soil formation.