WETLANDS (Australia) 21(2) Pulsative discharge of dissolved metals PULSATIVE DISCHARGE OF DISSOLVED METALS FROM COPPER SLAG EMPLACEMENTS INTO THE WINDANG UNCONFINED SANDY AQUIFER ADJACENT TO LAKE ILLAWARRA

Granulated copper slag is a waste residue of copper smelting activities. Copper slag is mainly composed of a ferro-silica matrix (Fayalite) embedded with numerous, submicron sulfide particles of various metals originating from the parental copper ore. In Wollongong, some 120,000 to 150,000 tonnes of slag produced annually by a local smelter has been disposed of in landfills. A large quantity of granulated copper slag has been emplaced within the Windang Peninsula’s unconfined sandy aquifer since 1945. A portion of the emplacement, situated within the fluctuating water table zone (unsaturated zone of the aquifer), exhibits intensive biological and/or chemical weathering, resulting in the release of soluble metals such as zinc, copper and cadmium into the aquifer. Groundwater quality data recorded from 1993 to 2003 suggests that in the aquifer beneath the slag emplacement, zinc occurs totally in the soluble form. Concentrations of zinc in groundwater beneath the emplacement rise after each rainfall event and gradually decrease in dry weather due to diffusion and dispersion processes. The pulsative nature of metal discharge into the aquifer is partially due to continuous biological and chemical reactions in the unsaturated zone in dry weather and the lack of advection forces in the Windang Peninsula’s sandy aquifer. Continuous oxidation of sulfides in slag produces iron and silica oxy-hydroxides which coat slag granules or precipitate in the interparticle pore space. Ferrous oxyhydroxides act as a temporary trap to adsorb metal ions, solubilized in the sulfide oxidation process in dry weather. In the next wet period, the infiltrating rainwater removes these adsorbed metals and drives them through the unoxidised mass of slag below the water table, into the deeper part of the aquifer.


INTRODUCTION
The Windang Peninsula is located some 85 km south of Sydney and a further 15 km south of the Wollongong CBD (Figure 1).The peninsula is a narrow strip of estuarine sand barrier of Holocene age and runs in a northsouth direction between the Tasman Sea on the east and Lake Illawarra to the west.

Copper Emplacements
The Windang Peninsula has been the subject of significant sand extraction activities, where a large volume of material has been removed to depth of >2 m below the surface.The voids which were created during sand mining activities have had a secondary use for emplacement of granulated copper slag.Figure 2 shows the location of the major copper slag emplacements on the Windang Peninsula and the boreholes used in this study.Only anecdotal information is available on the timing of when copper slag emplacement activities began on the Windang Peninsula, but examination of early aerial photography suggests that the emplacement activities may have begun as early as 1945.Windang Garden Estate and the Windang Driving Golf Range emplacements are the subject of this study.

Copper Slag Mineralogy and Chemical Composition
Copper slag consists mainly of a ferrosilica matrix known as fayalite (Biswas and Davenport, 1980), with minor amounts of aluminium, magnesium, zinc, copper, lead and cadmium.Table 1 shows the elemental composition of unweathered (S1) and weathered slag (W1-W10), using electron probe and X-ray analysis techniques.

Sulfides within Copper Slag
Sulfur in copper slag occurs in various forms of sulfide, mainly at submicron size, embedded in a ferro-silica matrix.The composition of these sulfide particles plays a critical role in the chemical and biological processes and reactions occurring within the copper slag emplacement.
Examination of polished copper slag granules using a reflected light microscope indicates that approximately 95 -95% of the sulfide particles are present as iron sulfide (FeS2, FeS), with the remainder being composed of 1 to 1.5% copper-iron sulfide (CuFeS2), 0.5-1% copper sulfide (Cu2S, CuS) and the remainder being zinc sulfide and lead sulfide (ZnS and PbS).
Segregation and packing of sulfide particles takes place during the cooling and solidification stage of the viscous slag lava.As can be seen in the reflected light micrographs of the polished slag granules, submicron sulfide particles are segregated mainly in concentric or pseudo-concentric layers (Figures 3 and 4), which follow the viscous fayalite matrix.Some crystalline sulfide spherules up to 50 µm in diameter are observed in the amorphous fayalite matrix.

Sulfide Oxidation Processes
Fluctuation in groundwater levels within the Windang copper slag emplacement creates a wet-and -dry, unsaturated vadose zone (Figure 5).In this zone, relatively rich in oxygen, the presence of soluble organic carbon resulting from direct infiltration of rainwater or remnant humic substances favours two major biochemical reactions in the copper slag emplacement (Crecelius, 1986, Gay, 1995, McGregor et al., 1998) • In the vadose zone, oxidative reactions caused by aerobic or facultative aerobic iron and sulfur oxidising bacteria such as Thiobacillus ferroxidans, Thiobacillus thiooxidans and Gallionella spp bacteria takes place as follows (Ribet et al., 1995): • Ferric sulfate produced from the oxidation of iron sulfide (Equation 2) is an even more effective oxidant, and which is generally considered to be one of the main agents of mineral leaching: 2Fe 2 (SO 4 ) 3 + Cu 2 S +H 2 O+3/2O 2 4FeSO 4 + 2CuSO 4 + H 2 SO 4 (Equation 3) Parallel to these reactions, another group of iron bacteria, Ferrobacillus ferrooxidans oxidise the ferro compounds to ferric hydroxide (Gay, 1995).With the excess of oxygen in this zone, silica and iron oxyhydroxides precipitate, causing the formation of an agathoid silica and iron oxy-hydroxide coating (hematite or limonite) on the slag particles (Prominco, 1996, Forbes Rigby, 1996).An idealised chemical process and reactions of copper slag within the vadose zone of the Windang Peninsula's unconfined sandy aquifer is shown in Figure 5.

Mechanisms of Copper Slag Breakdown and Chemical Weathering
Oxidation of sulfide in the slag takes place mainly along the matte prill (submicron sulfide segregation zone) (Yassini, 1993;Gee et al, 1997).Microfractures and voids enhance penetration of the oxygenated groundwater and bacteria into the mass of the slag particles.Oxidationhydration reactions increase the hydrostatic pressure along the matte prill and create compression tension and expansion forces.These forces result in cracks and ultimately exfoliation of slag granules (Yassini, 1993).

Leaching Processes in Weathered Copper Slag
Examination of the polished surface of weathered and exfoliated copper slag M metals across the leached zone present a typical layered structure, where the spatial pattern of accumulation of various elements is consistent.This can be seen in the X-ray mapping micrographs and X-ray spectrum (Figures 10-17): • Silica is usually en immediate surrounds of the faya matrix.
homogenous across the l zone.Three to four bands of high accumulation of iron were detected close to the outer edge.Aluminium, similar to silica, shows a higher accumulation in zone (x W1).A second, relatively enriched zone is also found at the periphery of the leached region.
Calcium is highly enriched in the silica-aluminium zone with several bands of accumulation across the leachate zone.
Copper is enric parallel bands across the leach zone with higher accumulation along the zone of titanium enrichment.
• Zinc is the most leached metal with homogenous accumulation across the leached zone.• Sulfur oxidation products are homogenously distributed across the leached zone.Interestingly, a localised band (x W7) appears parallel to highest copper accumulation zone.
The zoning pattern of metal accumulation across the leached aureole suggests a pulsative activity in metal transport and accumulation.Xray mapping micrographs show fluctuation in metal concentration across the fayalite matrix and the leached zone (Figure 15).

Physical and Chemical Characteristics of Windang Peninsula's Unconfined Sandy Aquifer
The Windang Peninsula's estuarine sand barrier consists of 30 -40 m of quartzose sand with a few local intercalations of silt and clayey sand, underlain by Permian Budgeon lithic sandstone (Longworth &McKenzie, 1982, Roy andPeat, 1973) Figure 18 shows an idealised East-West cross section of the Windang aquifer and the various sub-surface lithological units.
Figure 19 shows the results of the grain size analysis of marine sand and oxidised and unoxidised slag from the Windang Peninsula.The Windang unconfined aquifer is mainly composed of medium size well sorted marine sand.In the unsaturated vadose zone of the copper slag emplacement, where copper slag oxidises due to precipitation of iron oxyhydroxide around the slag surface, grain size gradually increases in size, and oxidised slag generally becomes coarser.Increase in grain size is also accompanied by a reduction in porosity and permeability (Table 2) Groundwater in the Windang Peninsula is, as for any other unconfined coastal aquifer, composed of a freshwater aquifer set on top of a saline aquifer.The aquifer forms a wedge which increases in thickness to the east.The thickness of the freshwater aquifer in the immediate foreshore of Lake Illawarra (BH 7, BH6, and BH10) is 3 to 3.5 m, with an electrical conductivity (EC) ranging from 400 to 800 µS/cm.At a distance of 200 m from the foreshore of Lake, in the Port Kembla Golf Course, at BH2A, the freshwater wedge attains 4.5 to 5 m thickness.Some 400 m distance from the foreshore of the Lake in BH1, BH8, E1, and E2 it occurs a depth of 8m with the electrical conductivity ranging from 100 to 400 µS/cm.At Port Kembla Golf Course, in Borehole BH2 at the depth of 6 m, electrical conductivity in dry weather fluctuates between 4,800 to 5,500 µS/cm.Here this depth is considered as a mixing zone between theoverlying freshwater aquifer on top and the underlying saline aquifer.

Windang Freshwater Aquifer Redox Potential Behaviour
The freshwater aquifer in the Windang Peninsula is generally in a reducing condition.Negative redox potential values fluctuate between -137 mV to -217mV.However, beneath the slag emplacement where active oxidation of copper slag occurs, positive redox potentials range from + 30mV to + 200 mV.
Figure 20 shows the Eh (mV)-pH values in the boreholes listed above, within and down gradient of the copper emplacement sites.Background bores are clustered below the -75 mV sulfate redox line.Locally observed positive redox values in boreholes WCC3,         BH12 and BH 9 relate to the sulfate induced into the aquifer from the copper slag oxidation process.Organic carbon in the Windang freshwater aquifer ranges from 3 to 28 mg/L (Coffey Partners International, 1995,Gay, 1995 and is a remnant of residual humic substances and is an indication of the swampy nature of the land in historical times (Yassini, 1993) Tables 3, 4, and 5 show mean analytical values of groundwater chemistry in background bores (above the copper slag emplacement), beneath the copper slag emplacement and down gradient of the site along the foreshore of the Lake Illawarra

Pulsative Release of Metals in the Aquifer beneath the Copper Slag Emplacement
Groundwater quality data recorded from 1993 to 2003 suggests that: In the aquifer beneath the slag emplacement, all the zinc is present in soluble forms (i.e., there is no particulate form of zinc in the aquifer).
The concentration of zinc in groundwater beneath the emplacement rises after each rainfall event (Table 6) and gradually decreases during dry weather, due to diffusion and dispersion processes.In dry weather, biological and chemical reactions continue in the unsaturated zone (in the water table fluctuation zone), and sulfide oxidation releases various metals which are temporarily adsorbed to oxy-hydroxide compounds on slag particle coatings or onto inter-particle hydroxide precipitates.In the next wet period, the infiltrating rain water will mobilise these adsorbed metals and drive them through the unoxidised mass of slag below the water table into the deeper part of the aquifer.As a result of oxidation of copper slag, soluble metals such as zinc, copper lead and arsenic are released into the aquifer (Hollings et al, 1999).This release takes place in pulses and follows the rain fall regime.Table 6 shows aquifer zinc concentration in dry and wet weather conditions Despite the availability of high concentrations of zinc, negative redox potential and the presence of H2S in the aquifer, no neogenic zinc sulfide was found in the aquifer beneath the copper slag emplacement.Scanning electron imaging and X-ray mapping of the 0.45 µm filter residue, did not show any autogenic sulfide except iron sulphide (framboidal pyrite).It seems that the hydrogen sulfide pump selectively reacts with available soluble metals.The preference is given first to iron, and until the iron is totally removed, no other sulfide precipitate can be formed).During wet weather, infiltration of rainwater or rising of the water table, removes the adsorbed metals from the oxyhydroxides and drives them through the mass of unoxidised slag into the aquifer beneath the copper slag emplacement.Elevated concentration of totally soluble zinc recorded in the aquifer immediately after rainfall events, suggests a pulsative nature in solubilized metal discharge.
While the Windang Peninsula's unconfined aquifer is generally in a reduced condition below the sulphate reduction limit and organic carbon is available in the reservoir, no neogenic zinc sulfide particles were found.The abundance of autogenic iron sulfide in 0.45µm filtration residues suggests that the hydrogen sulfide pump selectively reacts with available soluble metals.
The preference is given first to iron and no other sulfide precipitate can be formed until the iron is totally removed.

Figure 1
Figure 1 Regional map showing the location of the Windang Peninsula and Lake Illawarra.

Figure 2
Figure 2 Aerial photograph of the major copper slag emplacements in Windang Peninsula and the location of the boreholes used in this study.

Figure 3 Figure 4
Figure 3 Micrograph showing polished surface of unweathered slag using a reflected light microscope showing the pseudo-concentric layers of submicron sulphide particles Figures(6)(7)(8)(9) are scanning electron micrographs of a weathered slag

Figure 5
Figure 5 Subsurface profile of unsaturated vadose zone in Windang Golf Driving Range emplacement.

Figure 6
Figure 6 SEM micrograph of the surface of a weathered copper slag granule showing the fracture pattern of an exfoliated scale.

Figure 7
Figure 7 SEM micrograph of an oblique view of an exfoliated weathered slag granule showing exfoliated scales and early stage of compression and tensional crack formation on the unweathered surface.

Figure 8 Figure 9
Figure 8 SEM micrograph of the beginning of exfoliation process in a weathered slag granule

Fig. 10
Fig. 10 SEM micrograph and EDXR spectra of a weathered slag granule showing the coating of ferrous hydroxide and amorphous silica on the surface of a slag granule.

Figure 11
Figure 11 SEM micrograph and EDXR spectra of an iron sulfide spherule embedded in the fayalite matrix in an unweathered slag granule.

Figure 12
Figure 12 SEM micrograph and EDXR spectra of an unweathered slag granule showing the major component of fayalite matrix.

Figure 13
Figure 13 SEM micrograph and EDXR spectra of a iron sulfide spherule denuded from it's fayalite matrix.

Figure 14
Figure 14Micrograph of an oxidised slag scale using a reflected light microscope showing a leached zone surrounding the unweathered matrix and the accumulation of ferrous hydroxide at the periphery.

Figure 15
Figure 15 SEM micrograph of a microprobe analysis transects and X-ray mapping of the leached zone and the unweathered fayalite matrix.

Figure 16 X
Figure 16 X-ray spectrums of Al, Ca, and Zn in the fayalite matrix and the leached zone.

Figure 17 XFigure 18 Figure 19
Figure 17 X-ray spectrum of Fe and Si in the fayalite matrix and the leached zone

Figure 20
Figure 20 pH and redox relation diagram in the Windang Peninsula's unconfined aquifer.

Figure 21
Figure21An idealised model of the slag oxidation process and the release of metals into the groundwater aquifer.

Table 2
Physical characteristics of Windang Peninsula's marine sand, un-oxidized and oxidised slag

Table 3
Up-gradient groundwater chemistry

Table 6 .
Aquifer zinc concentration beneath the copper slag emplacement in dry weather and wet weather sampling