Metal contamination of riverine sediments below the Avoca mines, south east Ireland
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Environmental Geochemistry and Health (1997), 19, 7382
Metal contamination of riverine sediments below
the Avoca mines, south east Ireland
Claudia Herr and N.F. Gray
Environmental Sciences Unit, Trinity College, University of Dublin, Dublin 2, Ireland
The River Avoca is severely polluted by discharges of acid mine drainage (AMD) from the abandoned sulphur and
copper mines at Avoca. The riverine sediments were studied during a low flow period to establish the degree of
contamination and to identify the major processes affecting sediment metal concentrations. pH plays a major role in
the regulation of zinc adsorption and desorption in sediments, showing a significant correlation (p8.0), some 7 km below the mine. In contrast Cu and
Fe significantly increased (p
Materials and methods
Collection and analysis of subsurface sediment
Samples were taken from both the sediment layer(030 mm) and from the deposited ochre materialon the river bed. The former is defined here as thesubsurface sample, while the latter is the surfacesample. Approximately 3 kg of the subsurfacesediment was collected with a plastic scoop at eachsite and sub-site from five locations along the riveron 29 June 1994 and 4 August 1994 during lowflow conditions. All samples were pre-sieved in thefield through 1 mm aperture mesh using riverwater. The
- Zinc concentration in the subsurface sediment(Figure 2a) decreased significantly between site 1(662.2 g g1) and sites 4, 5 and 6 with values of533.5 (p
However, significant differences (ANOVA) werenot detected between sites.
Metals on surface samples
To compare metal concentration of the surfacelayer with that of the subsurface layer (
site 1 do not show any great variation and nosignificant differences between the two samplingmethods.
Comparison of surface sampling from two sam-pling intervals at site 4 showed a significantincrease of Zn from 330.0 to 919.1 g g1(p
The study showed that Zn concentrations in thesubsurface sediment (
regimes in the river.
Examination of organic matter content (Table 3)in the sediment showed highest levels at sites 4 and5 (21.6 and 18.2% respectively). An increasedgrowth of periphyton was noticed from visualexamination at sites 4, 5 and 6 on the substrateduring sampling in July and August. Althoughorganic contents in the sediment was lower at site6, almost 100% of the substrate was covered byperiphyton. However, periphyton consists to 9598% of water and therefore an increased organicmatter content due to periphyton is stronglydependent on the amount collected in relation tothe sediment volume.
The low carbonate content of the sediment indi-cates that the acid inputs from mine drainage orthe production of acids from microbial activity areeffectively not neutralised by the sediment. Thesurface water of the Avoca River is soft (hardness
at site 4, while Zn concentrations decreased(Table 4). This decrease in Zn confirms the earlierassumption that Zn is not deposited at site 4.Copper was enriched in the ochre layer by a factorof 26.3 compared with 15.3 in the subsurfacesediment (Table 9). This suggests that Cu ispreferentially co-precipitated with iron hydroxide(Chapman et al., 1983; Kondos et al., 1991), ratherthan adsorbed onto the subsurface sediment. Fugeet al. (1994) reported variable ochreous sedimentsranging in Fe content of 13.340%, and found thatCu was highly concentrated in the ochre andincreased with increasing pH. The Fe content inochreous sediments in the Avoca River rangedfrom 1027%, while Cu concentrations rangedfrom 11632029 g g1. It has been proposed byBigham et al. (1990) that the degree of adsorptionof Zn, Cu and Cd is controlled by the nature andstructure of the ochreous precipitate which hasbeen found to vary with pH.
The current study showed that Fe is largelyconcentrated in the surface sediment showing asediment enrichment factor (SEF) of 4.0 at site 4(Table 9), but less concentrated in the subsurfacesediment (
further sediment sampling.
ANOVA showed significant differences betweenthe mean of the sub-site and the mean of thereplicates for Zn (p
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82 C. Herr and N.F. Gray
Materials and methodsResultsDiscussionConclusions