Development of acid sulfate soil in sub-aerially disposed dredge spoil at Fisherman Islands, Brisbane, Australia

Abstract

During disposal of Brisbane River estuary dredge sediments into the reclamation paddocks at Fisherman Islands, a size separation of the fine (<63 µm) pyrite-rich silts and clays from the coarser (>63 µm) pyrite-poor sands and gravels occurs. The sand and gravel fractions contain most of the shell material and, therefore, most of the acid-neutralising capacity. In the most recently reclaimed paddock this size separation is of little consequence because sediments remain well buffered and acid produced from sulfides contained in the oxidising estuarine sediments is readily neutralised by the large volume of seawater and fine-grained carbonate minerals. However, as more sediment is added to the paddock, sediments become anoxic and sulfate reduction within the sedimentary pile occurs. This reduction rapidly produces pyrite and black monosulfides, consequently the pyrite content and degree of pyritisation and sulfidisation (DOP, DOS) of the sediment increase. Sulfide production rapidly removes sulfate from sediment pore waters and the process quickly becomes sulfate-limited because the fine-grained sediment texture restricts sediment permeability and therefore sulfate replenishment. Consequently, a clear relationship between DOP and DOS is observed for the reclamation paddock sediments. However, pre-existing mud-flat, and older unbunded dredge spoil sediments from the 1960s and 1970s, have no clear relationship between DOP and DOS values because they contain fewer fines and the production of pyrite was not limited by sulfate availability, but was constrained by other geochemical factors (e.g. redox potential, organic matter, or iron availability). Because of the increasing pyrite content of the paddock sediments as they age, the carbonate-buffering capacity provided by the sediment is readily exceeded, and the sediments become potentially acid sulfate. Should these sediments subsequently be disturbed and allowed to oxidise, the surrounding environment could be subject to large quantities of metal-rich acidic waters.