Hydrological controls on copper, cadmium, lead and zinc concentrations in an anthropogenically polluted mangrove ecosystem, Wynnum, Brisbane, Australia

Biogenic sulphides, particularly pyrite and hydrated iron- monosulphides, are common in sediments in mangrove ecosystems where they may provide a geochemical trap for heavy metals. However, oxidation of the sulphides can lead to remobilisation of metallic species and the development of acid sulphate soil conditions. In this paper the effects of prolonged dry conditions on the metal binding capacity of sediments in a mangrove forest are investigated. Data from 1989 show that the mangrove forest is acting as a buffer between a domestic garbage tip and the open waters of Moreton Bay. However, prolonged dry conditions during 1991 removed much of this buffering capacity and some metals were mobilised to deeper layers in the sediment or transported down the hydraulic gradient until chemical conditions more favourable for metal trapping were encountered. This study shows that understanding the transient nature of geochemical conditions in natural environments is important in environmental management because: (a) consideration needs to be given to how environmental data collected at one time may be affected by natural changes (commonly cyclic) in geochemical conditions; (b) consideration needs to be given to how human activities may have affected natural changes in geochemical conditions; and (c) planned development or environmental remediation work will need to anticipate the consequences of natural changes in geochemical conditions.