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


Estuaries are important sinks for sediments derived from both fluvial and marine environments because they offer protection from strong waves and currents, and because water and sediment transport mechanisms within them inhibit the escape of sediments (Salomons and Forstner 1984; Harris 1988). The estuarine circulation patterns that promote the accumulation of sediments (e.g. landward transport by waves and tides) are also important in understanding processes affecting trace metal distribution in estuaries (Meade 1972; Postma 1980). Furthermore, the well-developed chemical, physical, and biological gradients in estuaries provide a wide range of conditions that make particular parts of estuaries very effective metal traps. Because many cities are sited on estuaries, the estuarine environment is often subjected to high metal loads, and an understanding of processes involved in trace metal distribution within these estuaries is essential for sound environmental management (Etcheber et al. 1981; Campbell et al. 1988; Yudan et al. 1988).

Many estuaries in urban areas are dredged to maintain water depths in ports that have developed along their banks, and the dredge spoil has to be disposed of either at sea or on land. The sub-aerial disposal of these dredge spoils may have additional environmental concerns because many estuarine sediments contain reduced forms of inorganic sulfur (mostly as pyrite), and the subsequent oxidation of these pyrites during spoil disposal releases associated heavy metals and considerable amounts of acid, which may further release any heavy metals bound to other sedimentary particles. Where sediments contain high concentrations of pyrite, or contain highly reactive iron monosulfides (e.g. greigite), they may require innovative disposal techniques that minimise sulfide oxidation.

Where sediments are disposed of subaerially they may be contained in 1 of 3 recognised types of disposal paddock: (i) up-land, where dredge spoil is situated above the saturated zone and leachates move from the spoil into the foundation soils to the ground water; (ii) near-shore, where dredge spoil is partially sited in the saturated zone and the leachate tends to flow from the landward to the seaward side of the site; and (iii) in-water, where dredge spoil is sited in the saturated zone and leachates may move in and out of the paddock through bund walls, or the saturated zone.

It would therefore follow that the potential oxidation and trace-heavy metal release will be affected by the type of disposal paddock used (PINAC 1990). An up-land disposal site may develop a well-leached and oxidised surface that progressively moves down through the sedimentary pile releasing acidity and associated heavy metals to underlying ground waters through the action of infiltrating rain water. All in-water disposal site in marine waters may neutralise any acidity produced, especially for sediments with low sulfide contents, because of the buffeting capacity of infiltrating seawater and fine grained carbonate minerals; sea water has an alkalinity (acid-neutralising capacity) of 150 mg/L of CaC[O.sub.3] equivalent. Similarly, the type and intensity of diagenesis that disposed sediments will undergo during dewatering and consolidation will be greatly affected by the type of storage paddock used.

This paper reports on the changes to the distribution of sulfur species, degree of pyritisation (DOP), and degree of sulfidisation (DOS) in the mixed near-shore, and in-water dredge spoil paddocks at the Port of Brisbane Authority's reclamation paddocks at Fisherman Islands (Fig. 1). Dredge spoil in these paddocks is pumped in at the western edge of the paddock and excess water discharged through weir boxes on the eastern edge (Fig. 2). This disposal method imparts to the dredge spoil a strong grain-size separation such that the majority of fines accumulate at the weir boxes, whereas most to the coarser sands and gravels are deposited close to the pumpout point. These sediments then undergo diagenesis under an anoxic and reducing conditions with subsequent sulfide production (similar to the process of illuviation described by Fanning et al. 1988; Fanning and Fanning 1989).



Forty-five cores and 11 grab samples were taken from the Fisherman Islands reclamation paddocks 1, 2, 3, 5, and S2 during 1993 (Figs 2, 3, and 4), representing different stages of sediment deposition and ageing. Paddocks 5 and S2 are material recently deposited and are < 6 months old, paddocks 1 and 2 are from spoils that are 1-2 years old, and paddock 3 is spoil about 3-4 years old at the time of sampling. Eighteen grab samples from the estuary, representing the initial dredge material, were also collected (Fig. 1; dredging now only occurs up to Breakfast Creek, sample location 15, but in the early 1980s continued to sample location 18 City Reach). Cores were frozen at -12[degrees]C for up to 4 weeks before they were thawed, split, and subsampled every 5 cm for the top 50 cm of the core, then every 25 cm after that. Logging the cores showed that there were distinct changes in the sediment character at depth, and it was determined that this was the interface between the bunded disposal of dredge spoil (1990s) and the unbunded disposal of dredge spoil (1960s and 70s) or pre-existing mud-flat sediments; this break was characterised by the presence of a mangrove root mat.


All samples were oven dried at 65[degrees]C for 24 h to give a constant weight, homogenised by grinding, and stored in air-tight containers ready for analysis; 470 samples were analysed for this study. Although this technique induces some oxidation of sulfides and minor pH changes (recorded to be < 0.2 pH units for samples close to neutrality, but about 0.5 pH units for samples that already had a field pH < 5.5), it ensures that all samples are processed uniformly (Loring and Rantala 1992). Sediment samples were analysed for total sulfur, sulfate-sulfur, acid-volatile sulfur, residual sulfur, total carbon, organic carbon and carbonate-carbon, reactive pH, total iron, and reactive iron using the method of Clark et al. (2000). Carbon speciation was determined by LECO for total carbon, an acid titration for carbonates, and organic carbon by difference with cross analysis by the Walkley and Black method (Walkley and Black 1934). Sulfur species were determined by using LECO for total sulfur, HPLC (high performance liquid chromatography) determination of a 5 : 1 water extract for sulfate, back titration of trapped volatilised [H.sub.2]S from HCI for acid volatile sulfur (AVS), back titration of trapped volatilised [H.sub.2]S from chromous chloride for sulfidic sulfur, and sulfidic sulfur--AVS for pyritic sulfur (Clark et al. 2000). From these data, DOP (Berner 1970; Raiswell et al. 1988; Allen et al. 1991; Morse et al. 1993) and DOS (Clark et al. 2000) could be calculated. DOS values calculated here differ from those of Gagon et al. (1995) in that reduced sulfur is determined not just for sulfur bound in pyrites but considers all reduced species. Consequently, DOS values here will tend to be greater than those of Gagon et al. (1995). Reactive iron for DOP calculations was obtained using an overnight dithionite-citrate extraction (Holmgren 1967) and analysis by atomic adsorption spectroscopy. Reactive soil pH in a 5 : 1 water extract was determined on air-dried samples with 0.1 mL 1 M Ca[Cl.sub.2], using a standard pH probe, and the carbonate sulfide ratio (C : S) were determined by calculation.


Textural variations in the reclamation paddocks

Dredge spoil slurries are introduced to the paddocks through a 60-cm-diameter pipe on the western edge of the paddocks and flowed over the paddock surfaces to a pool at the eastern edge of the paddocks. Suspended sediment settles from the pooled water as current velocity falls, and the excess water is discharged to Moreton Bay via a weir box. The dredge spoil discharge procedure has several physical and geochemical consequences for the dredge soil retained in the paddocks.

Vertical distribution

The pattern of sediment texture variations down cores taken from reclamation paddocks suggests that parts of some cores arc missing. The competence of the sediment generally decreases down the core as the water content increases, but in several cores there is a repetition of this sequence and the boundary between incompetent and more competent sediment is marked by a thin layer of highly reduced black mud. It is likely that a substantial proportion of this thixotropic black mud was displaced when the core tubes are driven into the sediment. The position of the black mud in the cores marks the presence of thixotropic mud layers in the sediment that were deposited in topographic depressions formed on the sediment surface in the parts of the paddock most distant from the pump-out point during the pump-out process.

The vegetation that establishes on the dredge spoils during the consolidation process, between pump-out events, is dominated by Phragmites species rushes and remains mangrove-free. Several root mats are observed in some cores, but near the base of most cores there is a well-developed root mat thought to represent original mangrove forest and salt marsh flats buried by dredge spoil as the reclamation paddocks are filled. Irrespective of the origin of the root mat, it marks a boundary across which there are marked geochemical changes in the sediment in the reclamation paddocks.

Lateral distribution

Logs for cores taken from the 3 oldest paddocks (paddocks 1, 2, 3) indicate that there is a strong grain-size sorting during sediment deposition, and this grain-size sorting is also seen in the grab samples taken from paddocks 5 and S2 (Figs 4, 5), which show that as distance from the pumpout point increases, the proportion of fine sediment in the samples increases and the degree of sorting decreases. For example, sample paddock P5S1 is closest to the sediment pumping point and has the greatest sorting and the coarsest grain size, but the distal end of a transect, e.g. sample paddock P5S3, has very poor sorting and a much greater proportion of fines (which means that fine sediments are transported and accumulate close to the weir box). Some cores showed the presence of sedimentary structure, which included cross bedding, cross lamination, and parallel laminations. The position of the cores containing sedimentary structures showed that cores with cross bedding are closer to the pump-out whereas, cores containing parallel laminations are closer to the weir box (Clark et al. 2000).


Chemical analysis

Data for carbon and sulfur speciation, and derived species for the 5 reclamation paddocks and the estuary, are summarised in Table 1; there is an increase in mean DOR DOS, acid-volatile sulfur, total residual sulfur, and total sulfur concentration between the estuary sediments, newly deposited dredge spoils (paddocks 5 and S2), and the older paddocks (paddocks 1, 2, 3) as sediments age. There is a reverse trend in the reactive iron concentration and the reactive pH, where these parameters show decreases with sediment age.

Vertical Distribution

There are distinct distributions of sulfide and carbon species and derived parameters in the paddock sediments (Figs 6, 7, 8). There is a strong association between increasing organic carbon concentrations and increasing and sulfide concentrations (Figs 6, 7). The increases in sulfide concentration and organic carbon content are often coincidental with sharp decreases in sediment reactive pH and reactive iron content, and increases in DOP and DOS (Fig. 8); a fall in the carbonate: sulfide ratio (Fig. 6) is common. The increase in organic carbon content of the sediment is associated with the root mats that are observed in the cores. The distributions of the organic root mats are not, however, consistent throughout all the cores, and may range in depth from 10-20 cm to 75 cm down core, or may be absent in some cores (Fig. 3).



Sediment distribution in the paddock and the sedimentary structures can be described as a deltaic model (Reineck and Singh 1975; Reading 1978). This model consists of 3 components: a bottom set of gently inclined to flat-lying, fine-grained laminated muds; a fore set of beds of sand, silt, and gravel material dipping at 10[degrees]-25[degrees]; and a top set of essentially flat-lying sands and gravels (Reineck and Singh 1975; Reading 1978; Allen and Mercier 1988). The gradients on the reclamation paddocks are low, as is the depth of the receiving waters, and hence there is not the real vertical separation of the sedimentary facies that would be expected (Reineck and Singh 1975; Reading 1978; Allen and Mercier 1988). These observations allow the construction of the mixing model (Fig. 9) for sediment deposition within the reclamation paddocks.


The presence of several organic root mats in the core profiles suggests that a number of revegetation events have occurred. This revegetation is dominated by Phragmites and other rushes; no mangrove succession is evident. Most commonly, the areas that revegetate first are close to the weir box, where excess surface water is discharged, but not right at the weir box, because high concentrations of fine sediments are present. These sediments are relatively high in nutrients and trace elements and are leachable to an extent; salt is removed from the interstitial pore spaces via infiltrating rain waters (Iversen and Jorgensen 1993). Areas right at the weir box receive the salts washed from further up the hydraulic slope and salt crusting may be seen; hence, these areas are slower to revegetate, because of the low salt tolerance of many plants (Jennings 1968; Cooper 1981). Some salt crusting may occur from the oxidation of pyrite and efflorescence of ferrous sulfates to the sediment surface (Fanning et al. 1988; Fanning and Fanning 1989). Sediments closer to the pump-out point are sandier and more readily drained and leached, and do not recolonise readily because of a lack of fine sediment, which is required for bacterial populations to grow and fix nitrogen (Armstrong 1978; Hutchings and Saenger 1987; Mackey et al. 1992). Consequently, some sediment cores show multiple root-zones (e.g. core 10), whereas other cores have only one root-zone towards the lower sections of the core (e.g. core 24; Fig. 6). In addition, the regrowth of vegetation will depend on the periodicity of dredge spoil pumping to the paddock (i.e. several months need to elapse for conditions to become sufficiently hospitable to allow plant growth). Rapid filling of the paddock, which has occurred previously, will not allow sufficient time for regrowth to occur. Consequently, buried surfaces that have had sufficient time to leach salts and have sufficient titles are recolonised by plants (Phragmites) and produce the high organic root-zones.

Clearly, the high carbon content of the developed root-zones will provide an ideal habitat for sulfate-reducing bacteria to flourish (Lion et al. 1982; McKee et al. 1988; Clark et al. 1998), where an ample supply of sulfate from tidal, ground, and dredge waters allows for substantial pyrite formation. The increase in acid-volatile sulfur and decrease in extractable sulfate from these horizons gives a strong indication of the biogenic nature of the sulfide formation (Figs 6, 7, 8; Lion et al. 1982; McKee et al. 1988). Sulfides (pyrites) are, for the oxidative sulfide addition theory, produced by the formation of a monosulfide such as greigite by microbial activity, which then undergoes the transformation to pyrite through oxidation and sulfur addition (Berner 1982; Rickard 1997; Rickard and Luther 1997).

A plot of total organic carbon against total sulfur (Fig. 10) for the estuary sediments shows that the estuary sediments sit within the normal marine conditions. However, the sample from the Swing Basin (sample 6, Fig. 1b), which receives poor water circulation because of a flow separation from a sharp change in water depth (Allen 1994), shows some enrichment in sulfur and plots more towards the euxinic field (highly reducing, oxygen-depleted systems; Fig. 10). Figure 11 shows that the paddocks are diagenetically enriched in sulfur, with many samples plotting above the normal marine field and into the euxinic field and beyond (Leventhal 1995). In contrast, the original dredge sediments from the Brisbane River estuary tend to plot firmly within the normal marine field (Fig. 10); samples from the paddocks that plot well above the euxinic field (Fig. 11) are almost certainly enriched in biogenic sulfur. However, the addition of more recent root material, through grass and rush growth, to the dredge spoil results in an increase in organic carbon and, consequently, gives some sediments a far more terrestrial (non-marine) character (Leventhal 1995; Fig. 11).


In addition, the segregation of the sediment types may also be made on the reactive pH of the sediments, where sediment samples taken below the major root-zone have low reactive pH and higher DOP and DOS increases. Plotting reactive pH against the DOP (Fig. 12) shows that the more recent dredge spoils are well buffered, probably because of the pore water composition not being depleted in bicarbonate (Dyer 1973; Berner 1982; Chester 1990), whereas, the older material have probably undergone several cycles of drought, desiccation, oxidation, acid production, carbonate removal, and sulfide reformation since the 1960s (Canfield et al. 1993; Thamdrup et al. 1994). Hence, the older sediments have little or no acid-buffering capacity left and, once dried, even with minor sulfide oxidation, exhibit significant acid production, and thus lower reactive pH (Fig. 12).


The relatively low DOP of the more recent dredge spoil compared with those recognised as being from the 1960s is similar to the DOP values obtained from the estuary; however, there has been some increase in the DOP values such that they may be considered euxinic (Fig. 11; Raiswell et al. 1988; Canfield et al. 1993). For the more recent dredge spoil the increase in DOP is not accompanied by any substantial decrease in reactive pH, but for the older dredge spoil there is a substantial loss of sediment pH buffering. Because geochemical conditions are never constant and there may be large fluctuations in Eh and pH conditions on an hourly basis, sulfide minerals may be undergoing oxidation even as they are forming. Consequently, iron oxides may undergo up to 300 reduction/oxidation cycles before final sedimentation as a pyrite particle and exclusion from the cycle (Canfield et al. 1993; Thamdrup et al. 1994). Hence, there is small, but significant acid production, which may ultimately diminish the pH buffering capacity of the sediment.

Given that the older sediments have had some 20 years for buffer reduction, it is not surprising that reactive pH is lowered (Canfield et al. 1993; Thamdrup et al. 1994). However, the more recent dredge spoils have not had sufficient opportunity for buffer reduction and consequently maintain high reactive pH, despite significant sulfide concentrations and degrees of pyritisation (Fig. 12).

A plot of the DOS against DOP (Fig. 13) shows that there is a very strong correlation between DOP and DOS of the recent dredge spoil, but that this relationship does not exist for the older dredge spoil. This relationship would tend to suggest that the older sediments are becoming more iron-limited (i.e. they may have a high DOP but the DOS may vary significantly (Raiswell et al. 1988; Leventhal 1995), whereas the more modern dredge spoils are sulfur-limited (i.e. the loss of sulfate is linked closely to the gain in pyrite (Raiswell et al. 1988; Canfield and Des Marais 1993; Leventhal 1995).


Because the older dredge spoils were deposited without bunds, much of the finer sediment was lost to Moreton Bay by tidal processes (Allen 1994; Lewis and McConchie 1994; McAlister 1996). Consequently, at the major root-zone observed in the cores (Figs 6-8) there is also a large increase in mean sediment grain size (Clark et al. 2000). This increased grain size means that sediment permeability is increased and that tidally pumped ground waters are likely to flow in the older sediments (Chanton et al. 1989; Appelo and Postma 1994; Weaver et al. 1996; Fig. 14), whereas the more recent sediments, which are bunded and hence retain all the finer materials, do not experience the flow of ground waters as much, because of pore filling (Risk and Moffat 1977; Molenar 1986). However, small irregularities in the distribution of sediments, thin sand layers, roots, shrinkage cracks, and any permeability contrast may act to enhance fluid flow in the sediment (Weaver et al. 1996). Coupled with the tidal pumping experienced during tidal cycles (Chanton et al. 1989), significant quantities of dissolved species may be transported from the paddocks to Moreton Bay, and from Moreton Bay into the paddocks (Fig. 14).


There is evidence of post-oxic groundwater transport in cores 24 and 28 (Fig. 3), where the metal speciation of the lower section of the core is far more oxidised and carbonate-rich (Clark et al. 2000). Both these cores are very close to the edge of paddock 1, and before paddock 5 and super bund were constructed, were close to the open tidal waters of Moreton Bay. Similar distributions of trace element speciation in the lower sections of generally anoxic-reducing cores have been attributed to post-oxic groundwater flow through the sediment (Clark et al. 1997, 1998; Clark 1998); post-oxic fluids effectively dilute sulfide production because of carbonate and oxide formation rather than sulfate reduction (Aller and Rude 1988; Huerta-Diaz and Morse 1992). However, these fluids, because they are sulfate-rich, often provide an upwardly migrating source of sulfate, which will become available for sulfide reduction as it moves up through the sedimentary pile (Chanton et al. 1989; Clark 1998; Clark et al. 2000).


Disposal of Brisbane River dredge spoils into the Fisherman Islands reclamation paddocks removes the sediments from the estuarine environment. However, the size sorting of the sediments during the disposal process into reclamation paddocks separates fine-grained, sulfide-rich sediments from coarse-grained, sulfide-poor, carbonate-rich sediments. The trapped porewaters within the disposed sediment are readily depleted of sulfate as anoxia within the sediment is established and acid sulfate soil develops in the mid and distal part of the hydraulic slope. This anoxia is fuelled by trapped organic matter disposed with the dredge spoils and by the root mat that developed on the paddocks between successive disposal episodes. The rapid removal of sulfate from the pore waters rapidly increases sediment DOS values but does not readily increase sediment DOP values and limits the production of pyrite (Fig. 14). Because of the fine-grained nature of most of the sediment, porosity and permeability of the sediments are low and sulfate replenishment is severely restricted, in addition, the byproducts of sulfate reduction (e.g. bicarbonate) are not readily removed via the groundwater system, and the more recent (1990s) reclamation sediment remains well buffered against the modest amounts of sulfuric acid generated during oven drying of the sediment, and consequently, their pH remains high. Older dredge spoils have little or no buffering capacity and sediment pH on drying is much lower. However, should the sediments of the reclamation paddocks be disturbed and exposed on a large scale, sulfide oxidation will occur on a large scale and what minor buffering is provided by trapped bicarbonate will be rapidly exceeded, and the surrounding environment will be subject to large amounts of acidic, and potentially metal-rich waters.
Table 1. Maximum, minimum, mean, and standard deviation values (%) of
carbon, sulfur, reactive iron, and derived species in sediments from
the estuarine sediments and youngest (paddocks 5 and S2) and older
(paddocks 1, 2, 3) reclamation paddocks

TC, Total carbon; C[O.sub.3.sup.-2], carbonate as C[O.sub.3.sup.2-],
equivalent; OC, organic carbon; TS, total sulfur; AVS, acid-volatile
sulfur; TRS, total residual sulfur (pyritic sulfur); C : S, carbonate/
sulfide ratio; RFe, reactive iron; DOP, degree of pyritisation; DOS,
degree of sulfidisation; RpH, reaction pH

 TC C[O.sub.3 OC TS S[O.sub.4.
 .sup.2-] sup.2-]-S

Brisbane River

Max. 35.6 26.4 30.3 0.96 0.19
Min. 0.4 0.1 0.2 0.10 0.04
Mean 2.8 3.2 2.2 0.50 0.10
s.d. 5.7 4.2 4.9 0.19 0.04

Paddock 5

Max. 2.2 3.5 2.0 0.98 0.17
Min. 0.2 [less than or 0.1 0.09 0.04
 equal to] 0.1
Mean 1.5 2.2 1.1 0.67 0.09
s.d. 0.6 1.1 0.6 0.30 0.04

Super Bund (paddock S2)

Max. 1.5 3.5 1.1 1.12 0.13
Min. 0.8 0.4 0.5 0.31 0.05
Mean 1.1 1.7 0.8 0.84 0.09
s.d. 0.3 1.0 0.2 0.28 0.03

Paddock 1

Max. 16.1 27.4 15.1 5.30 0.65
Min. 0.7 [less than or 0.1 0.15 0.02
 equal to] 0.0
Mean 3.6 2.8 3.0 1.10 0.17
s.d. 3.1 3.2 3.0 1.08 0.14

Paddock 2

Max. 12.0 10.8 10.5 3.68 0.50
Min. 0.4 [less than or 0.1 0.08 [less than or
 equal to] 0.1 equal to] 0.01
Mean 2.4 2.43 1.9 0.85 0.16
s.d. 1.8 2.0 1.7 0.85 0.11

Paddock 3

Max. 11.5 14.1 11.2 5.46 0.62
Min. 0.1 [less than or [less than or 0.04 0.02
 equal to] 0.1 equal to] 0.1
Mean 1.6 1.7 1.3 1.20 0.14
s.d. 1.6 2.0 1.6 1.17 0.12

 [S.sup AVS TRS C : S

Brisbane River

Max. 0.86 0.17 0.76 55.6
Min. 0.05 0.01 0.03 0.5
Mean 0.40 0.06 0.33 9.9
s.d. 0.18 0.05 0.17 11.4

Paddock 5

Max. 0.91 0.48 0.71 25.4
Min. 0.05 0.04 0.01 [less than or
 equal to] 0.1
Mean 0.58 0.20 0.38 6.2
s.d. 0.29 0.14 0.25 6.7

Super Bund (paddock S2)

Max. 1.02 0.34 0.79 14.6
Min. 0.24 0.04 0.18 0.5
Mean 0.75 0.17 0.58 3.3
s.d. 0.26 0.10 0.22 4.6

Paddock 1

Max. 4.93 1.97 4.04 86.0
Min. 0.09 [less than or 0.05 [less than or
 equal to] 0.01 equal to] 0.1
Mean 0.93 0.20 0.73 6.9
s.d. 0.99 0.28 0.80 12.2

Paddock 2

Max. 3.36 1.70 3.24 174.9
Min. 0.02 [less than or [less than or [less than or
 equal to] 0.01 equal to] 0.01 equal to] 0.1
Mean 0.69 0.20 0.49 11.7
s.d. 0.76 0.32 0.59 22.6

Paddock 3

Max. 5.11 3.96 4.72 173.8
Min. 0.01 [less than or [less than or [less than or
 equal to] 0.01 equal to] 0.01 equal to] 0.1
Mean 1.06 0.30 0.77 7.8
s.d. 1.09 0.46 0.88 24.5


Brisbane River

Max. 3.0 0.20 0.83 10.5
Min. 0.5 0.03 0.25 7.5
Mean 1.5 0.10 0.64 8.0
s.d. 0.4 0.05 0.16 0.5

Paddock 5

Max. 2.0 0.17 0.73 8.4
Min. [less than or 0.05 0.11 7.3
 equal to] 0.1
Mean 1.4 0.12 0.50 7.9
s.d. 0.7 0.04 0.22 0.3

Super Bund (paddock S2)

Max. 1.9 0.23 0.82 8.3
Min. 1.3 0.07 0.58 7.7
Mean 1.5 0.16 0.68 8.0
s.d. 0.2 0.05 0.08 0.2

Paddock 1

Max. 1.5 0.93 0.98 8.5
Min. 0.2 0.03 0.44 3.1
Mean 0.8 0.33 0.80 7.3
s.d. 0.3 0.26 0.10 1.3

Paddock 2

Max. 2.0 0.76 0.99 11.9
Min. 0.2 0.01 0.20 3.0
Mean 0.9 0.25 0.75 7.0
s.d. 0.3 0.20 0.15 2.0

Paddock 3

Max. 1.5 0.85 0.97 9.2
Min. 0.2 0.01 0.24 3.4
Mean 0.6 0.38 0.83 7.2
s.d. 0.2 0.23 0.15 2.0


We would like to thank the Brisbane Port Corporation for assistance in the collection of materials from the Brisbane River estuary and for initially suggesting the project; in particular, these thanks are extended to Dr Mark Pillsworth. We would also like to acknowledge Fiona Davies-McConchie for her most constructive comments on the manuscript. In addition, we would like to thank the reviewers for their highly constructive comments, especially Professor D. Fanning for placing this work in context with his own research and the supply of additional relevant literature. Finally, we would like to thank Mrs. P. Aiammala-Clark for retyping the original manuscript after its mysterious disappearance into the electronic ether.


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M. W. Clark (A,B) and D. M. McConchie (A)

(A) Centre for Coastal Management, Southern Cross University, NSW 2480, Australia.

(B) Corresponding author; email:

Manuscript received 16 May 2003, accepted 30 April 2004
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Author:Clark, M.W.; McConchie, D.M.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Sep 1, 2004
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