Thesis
Geochemical cycling of heavy metals in Brisbane river estuary dredge sediments during sub-aerial disposal, and consolidation at the Port of Brisbane Authority's Fisherman Islands reclamation paddocks
Southern Cross University, School of Resource Science and Management
Doctor of Philosophy (PhD), Southern Cross University
2000
Metrics
2 Record Views
Abstract
The disposal of dredge spoil from many of the world's ports is becoming an increasing environmental problem. Many of the dredged sediments are contaminated with heavy metals or organic wastes, or contain sufficiently high concentrations of sulphide minerals that disposal can lead to sulphide oxidation, which may deplete dissolve oxygen concentrations in the water column, or create acute acidity problems if disposed of in a subaerial setting. In addition, the disposal of dredge spoil at sea may result in the smothering of benthic communities, siltation problems outside of the disposal area, and visual pollution through suspended sediment. Consequently innovative dredge spoil disposal techniques are required. One such technique is to use the dredge spoil as reclamation land fill, but the same environmental issues of sediment contamination by heavy metals or organic pollutants and potential acid generation during disposal and drying still remain. This Thesis investigates the dredging and disposal of sediments from the Brisbane River estuary, carried out by the Brisbane Port Corporation. It pays particular attention to the changes in heavy metal geochemistry (speciation, concentration, and mobility), the development of an acid sulphate soil character, and the production of acidity during dredging, disposal, drying and consolidation of sediments within the reclamation paddocks at Fisherman Islands. The Thesis also addresses the management implications of the reclamation process and also looks at the possible use of an industrial waste (seawater neutralised bauxite refinery residues; SWNRM) to control heavy metal mobilisation and acidification problems that may develop. The use of an industrial residue for environmental remediation has added benefits because it not only solves the environmental problem treated, but utilises an otherwise wasted resource.
Brisbane River estuary sediments dredged by the Brisbane Port Corporation are, by world standards, uncontaminated. Although metal concentrations are low, except for Mn where it appears that the guideline is set too low for areas where there is a strong natural supply of Mn, metal speciation in the estuarine sediment is dominated by sulphides. Despite the domination of metal speciation by sulphides, degree of pyritisation (DOP) data suggest that pyrite concentrations are typical for well oxygenated marine waters. Degree of sulphidisation data (DOS) and sediment textural data suggest that pyrite production in the estuarine sediments is limited by the supply of sulphate to the sediment and not by the concentration of ferric iron available for reduction. The estuarine sediments of the Brisbane River do, however, contain sufficiently high concentrations of pyrite to be of concern as a potential source of sulphide acidity during sediment drying, but the estuarine sediments are well buffered by carbonate (mostly shell material), even when size separation is performed on the sediments to remove the >63 µm shell fragments.
Heavy metal concentrations in the reclamation paddocks at the Fisherman Islands site remain well within the recommended guidelines, excepting for localised hot spots of Hg and Cd; Mn is affected by the same problem as in the estuarine sediments. Apart from the upper few centimetres of the sediments, metal speciation is dominated by sulphides, and these metals are strongly bound to the sediment and are not lost during disposal, or de watering. Elutriation data for the estuarine and the youngest disposed sediments (Paddock SB 1) agree well with geochemical speciation data. Adsorption experiments using the <63 µm fraction of estuarine sediments suggest that the sediments have a strong capacity to trap even greater amounts of heavy metals from the water column than they already contain.
During disposal into the reclamation paddocks, there is some size separation of the fine (<63 µm) pyrite-rich silts and clays from the coarser (>63 µm) pyrite-poor sands and gravels; the sand and gravel contains most of the shell material. In the most recently reclaimed paddock this size separation is of little consequence because the sediments remain well buffered and any acid produced by sulphide oxidation is readily neutralised. However, as more sediment is added to the paddock, sediments become anoxic and sulphate reduction within the sedimentary pile occurs. This reduction rapidly produces pyrite and black iron monosulphides and causes DOP and DOS readings to rise. Pyrite production in the sediments is sulphate limited because the fine grained texture of the sediment restricts permeability. Consequently, there is a clear relationship between DOP and DOS readings and the increasing pyrite content of the paddocks as they age; the carbonate buffering capacity is readily exceeded in these sediments and they become potentially acid sulphate. To compensate for this effect, the management strategies used by the Brisbane Port Corporation, to aid de-watering of the sediments, act to protect the sediments from sulphide oxidation because elevated water tables are maintained and oxygen diffusion is restricted by of a 3 m thick sand covering. However, any disturbance and exposure of the sediments after deposition is likely to generate large amounts of highly acidic metal-rich leachates.
Geochemical characterisation of seawater neutralised bauxite refinery residue (SWNRM) indicates that it has very good acid neutralising capabilities ( 3.5 mol kg-1 ),an excellent metal binding capacity, and a physical character that lends itself for blending with the dredge spoil to combat acidity and metal mobilisation during disposal. The grainsize of SWNRM is similar to that of the fine grained fraction of the disposed dredge spoil and, consequently, will be deposited with the fine fraction where the greatest acid generating capacity, and metal releasing potential is held. Although blending the SWNRM with the <63 µm fraction of the estuarine sediments increases heavy metal removal, there are changes in the metal binding preference in the blended sediment. The SWNRM is shown to be effective at controlling the acid sulphate soil nature of the reclaimed dredge spoil, but the effects of the addition of an iron-rich additive to sediments that are actively undergoing reduction needs to be investigated further. Although sulphide production within the paddocks is controlled by sulphide availability, the tidally influenced ground waters in the paddocks may eventually supply sufficient sulphate to convert all the iron in the SWNRM to pyrite by reaction with biogenic sulphide. It is therefore recommended that long term stability trials of the SWNRM be conducted to evaluate the benefits of using SWNRM to treat acute acidity and metal mobilisation in the paddocks during disposal of dredge spoil, against the long term risks associated with potentially increasing the pyrite content of the sediments.
Details
- Title
- Geochemical cycling of heavy metals in Brisbane river estuary dredge sediments during sub-aerial disposal, and consolidation at the Port of Brisbane Authority's Fisherman Islands reclamation paddocks
- Creators
- Malcolm Clark
- Contributors
- David McConchie (Supervisor) - Southern Cross University
- Awarding Institution
- Southern Cross University; Doctor of Philosophy (PhD)
- Theses
- Doctor of Philosophy (PhD), Southern Cross University
- Publisher
- Southern Cross University, School of Resource Science and Management
- Number of pages
- xiii, 339
- Identifiers
- 991013312528802368
- Copyright
- © Malcolm W. Clark 2000
- Academic Unit
- Faculty of Science and Engineering
- Resource Type
- Thesis