Evaluation of the rehabilitation procedure of a pyritic mine tailings pond in Avoca, Southeast Ireland

Land Degrad. Develop. 9, 67±79 (1998)

EVALUATION OF THE REHABILITATION PROCEDURE OFA PYRITIC MINE TAILINGS POND IN AVOCA,

SOUTHEAST IRELAND

C. O'NEILL,1 N. F. GRAY2� AND M. WILLIAMS3

1Department of Mechanical and Manufacturing Engineering, University of Glamorgan, Pontypridd, Wales2Department of Civil, Structural and Environmental Engineering, Trinity College, University of Dublin, Dublin 2, Ireland

3Department of Botany, Trinity College, University of Dublin, Dublin 2, Ireland

Received 25 June 1996; Accepted 27 February 1997

ABSTRACT

A 32 ha tailings pond used for the disposal of pyritic mine waste was examined after a period of eight years to determinethe success of the rehabilitation plan used to revegetate the site. This was achieved by examining both the vegetationcover and the quality of the topsoil in order to determine the e�ect of the tailings. A number of ¯oristic habitats wereidenti®ed within the site indicating that succession had occurred since revegetation of the area with metal-tolerant grassspecies. Four main habitats were investigated: leguminous, grass, gorse and low canopy. The soil layer in Shelton Abbeywas 25±30 cm deep and contained levels of nutrients and metals comparable to those found in unpolluted soils. It wasfollowed by a 20±25 cm layer of mixed soil and tailings, followed by the tailings only. The tailings retained elevatedconcentrations of metals indicating their unsuitability for growth of unadapted plant species. Vegetation from allhabitats, analysed both in the summer and winter, contained higher levels of iron only compared with vegetation grownon unpolluted soils. Metals do not appear to be signi®cantly leached from the tailings either into the soil or into surfaceand ground waters, and have not been accumulated to above normal levels by plant uptake. The rehabilitation protocolused at the site appears to have been successful. However, the site needs to be managed on an on-going basis to ensure theintegrity of the bund and revegetated area. # 1998 John Wiley & Sons, Ltd.

KEY WORDS: metals; mine management; mine waste; soil contamination; tailings

INTRODUCTION

The Avoca mines in County Wicklow, Ireland, are located 1.5 km south of the Meeting of the Waters. Theyincline northeast to southwest, and stretch for approximately 1.7 km on either side of the north±southAvoca River valley (Figure 1). They were originally exploited for the abundant copper ores and iron pyritesthey contained, but are now abandoned. The most important base metal sulphides found in Avoca arechalcopyrite (CuFeS2), pyrite (FeS2), sphalerite (ZnS) and galena (PbS) although the last has no commercialvalue (Platt, 1973, 1974; McArdle, 1994). The Avoca mines were exploited mainly for copper, althoughpyrite and ochre were also important products. In recent years the pyrite was sold to a local fertilizer plantfor H2SO4 production and eventually became the main product of the mines.

In 1954 the option of expanding the Avoca mines into a large-scale operation was considered. However,the disposal of the tailings was seen as the most di�cult problem to be surmounted. A containing dam waseventually selected as the best available option for disposal of the tailings produced. The 32 ha meadow atShelton Abbey which is adjacent to the Avoca River was considered to be the most suitable site available,although it is 5 km downstream from the Avoca mines (Figure 1). The area was converted into a tailingspond for the Avoca mines in 1955. The dam walls were constructed from rock and clay removed from thesurrounding area as it was hoped this material would eventually support vegetation similar to that in the

LAND DEGRADATION & DEVELOPMENT

�Correspondence to: Professor Nick Gray, Department of Civil, Structural and Environmental Engineering, Trinity College, Universityof Dublin, Dublin 2, Ireland

CCC 1085±3278/98/010067±13$17.50

# 1998 John Wiley & Sons, Ltd.

locality (Figure 2). The tailings pond was separated into two distinct lagoons divided by an earth bund, butoperated in parallel with a single in¯uent but two e�uent points. By the early 1970s the walls had beensuccessfully colonized by Betula pubescens, Cirsium spp. Dactylis glomerata, Festuca rubra, Agrostis tenuis,and Cerastium spp. (Platt, 1974). The tailings were pumped from the mill at the mines via pipes laid besidethe railway line to Shelton Abbey; the slurry was discharged through a series of outfalls and left to settle ineither of the tailings ponds. Residual water was discharged from the ponds into the adjacent river through aseries of decants. Shortly after the mining operation went into receivership in 1982, the surface of the tailingspond dried out leaving the site open to wind and water erosion. Wind erosion caused tailings to be blownaround the surrounding area causing severe contamination. To resolve both the dust and water pollution

Figure 1. Map showing the location of the Avoca Mining Strip

68 C. O'NEILL, N. F. GRAY AND M. WILLIAMS

# 1998 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 9, 67±79 (1998)

Figure 2. Plan of the tailings pond showing the di�erent habitats outside the site

REHABILITATION OF MINE TAILINGS POND IN IRELAND 69


problems, and to comply with planning conditions for the disposal of tailings set out by the local authority,the receivers arranged for reclamation of the site to take place. Liverpool University was commissioned todevise a method for upgrading the impoundment in 1983.Work on the site commenced in summer 1984 and was completed a year later. Due to the problems with

erosion it was decided not to seed directly on to the tailings as the cover produced by this method wouldinitially be too sparse to prevent further erosion. A shallow cover approach was used to encapsulate thetailings. This involved 20±30 cm of shale being placed on the surface of the tailings followed by 7±10 cm oftopsoil and subsoil removed from the surrounding area (Johnson, 1991). The north side of the impoundmentwas covered with material removed from the surrounding hillside resulting in the removal of part of the largestands of natural deciduous woodland, now designated an Area of Scienti®c Interest. The south end of thesite was covered by subsoil removed from near the fertilizer plant. The hillside soil would have been higher inorganic matter than the subsoil due to the presence of lignin. It is also likely to have contained morenutrients. However, there are other potential sources of di�erence between the north and south of the siteincluding drainage and seed dispersal. Therefore habitat di�erences within the site cannot simply be linkedto di�erences in the cover soil used. The site was graded and levelled to ensure that depressions which mightaccumulate water were removed. Lime with a grade of 2 mm was applied at a concentration of 500 kg haÿ1

and fertilizer was applied at 250 kg haÿ1 consisting of 15 per cent nitrogen, 15 per cent phosphorus, and15 per cent potassium. An agricultural seed mix was sown consisting of 25 per cent Lolium perenne, 20 percent Poa compressa, 15 per cent Festuca rubra, 15 per cent Trifolium repens and 25 per cent Agrostis tenuis cv`Parys'. The `Parys' cultivar was developed speci®cally for use on reclaimed copper mine sites. The seedmixture was applied to a 2 ha trial plot in September 1984 and monitored. As growth was successful theremainder of the site was seeded in April 1985.

The present study was carried out to evaluate the success of the rehabilitation procedure used, and toexamine the current status of the reclaimed tailings pond eight years after rehabilitation.

METHODS

Vegetation surveys were carried out on three occasions, winter 1992, spring 1993 and summer 1993, toidentify the presence of di�erent habitats and to determine the success of site revegetation. Ten 1 m2 quadratswere taken in each of four habitats in each season. Detailed quantitative analysis of the vegetation was carriedout using a system of strati®ed random sampling in order to examine the species present and their abundance.

Soil sampling was carried out in the winter and the summer only. The winter cores were dug manually to adepth of 60 cm in order to estimate soil parameters directly a�ecting vegetation. Summer cores were taken toa depth of 2 m using a mechanical digger in order to determine whether leaching was occurring through thesite. Generally samples taken to 25 cm were found to contain the majority of roots and therefore this depthwas taken to be the rooting zone.

Percentage water content was the principal physical soil parameter measured. This was determined bymeasuring the loss in weight of samples after heating at 105 8C for eight hours (Hesse, 1971). Chemical para-meters measured were organic matter content, pH, metal content and nutrient content. The organic mattercontent was measured by determining the loss on ignition of samples burnt at 500 8C for eight hours. ThepH of samples was found by means of a glass electrode in a soil solution. Hesse's method was employed usinga more dilute soil solution to avoid damage to the electrode (Hesse, 1971). X-ray ¯uorescence spectrometrywas carried out on tailings and soil samples to determine which elements were present (Jenkins, 1988). Thosefound to be present in high quantities were subsequently analysed using atomic absorption spectro-photometry following digestion in nitric acid at 170 8C for 2 hours (Allen, 1989; Perkin Elmer, 1990). Thekey elements analysed were copper, zinc, iron and manganese. The lead concentration of soil samples wasalso determined due to the fact that this element is commonly associated with zinc in ore. Total oxidizednitrogen present in the samples was determined by means of extraction with potassium chloride followedby ¯ow injection analysis (Allen, 1989; Tecator, 1984). Phosphorus content was determined by means of



spectrophotometry following digestion in sulphuric acid at 370 8C for 135 minutes. The concentration ofcopper, zinc, iron and manganese in vegetation samples taken in summer was also analysed. The vegetationwas washed to remove surface contamination, then oven-dried, desiccated and analysed by means of atomicabsorption spectrophotometry following digestion in nitric acid (Allen, 1989; Perkin Elmer, 1990). Corevegetation and roots were analysed separately.

RESULTS AND DISCUSSION

Vegetation Analysis

The dominant vegetation type in Shelton Abbey is grass, with a large and increasing area covered in gorse.An area of the site had a very low canopy. There were a number of bare patches throughout the site and someareas of poor drainage were also observed. Some of the drainage problems appeared to be associated withthe presence of vehicles on the site resulting in signi®cant compaction problems leading to surface-waterretention. Additionally, the drainage channels at the perimeter of the site were ¯ooded at times throughoutthe year. This appeared to be indicative of poor drainage over the area of the tailings pond.Three main habitats were found within the site: leguminous, grass and gorse. These were repeated within

the impoundment and were examined thoroughly. A fourth habitat, which covered less area than the otherhabitats, was also examined as it had a very low canopy and was possibly indicative of a problem withinthe site. Analysis of quadrat data by DECORANA veri®ed such divisions and a representative plot ofDECORANA scores for the summer vegetation is illustrated in Figure 3. The grass habitat was the mostabundant in terms of percentage coverage of the site (45 per cent coverage),while the gorse habitat was thesecond most abundant habitat (35 per cent coverage) and was rapidly expanding, colonizing more of the site.A number of bare patches were found within the site which had rocky soil on the surface. Many of theseareas became ¯ooded after heavy rainfall.

The leguminous habitat (14 per cent coverage) was bright green in appearance due to the presence oflarge quantities of legumes. The predominant legume was Trifolium repens, but Vicia sepium and Lotuspendunculatus were also common. Grasses in this habitat included Festuca rubra, Agrostis capillaris tenuis,Holcus lanatus, Lolium perenne and rarely Poa pratensis. The habitat was patchy in occurrence althoughextensive in parts, and merged with both the grass and gorse habitats. The ground was ®rm although someareas became waterlogged occasionally.

The grass habitat was generally straw-coloured in appearance. The predominant species in this habitatwere grasses, dominated by the rhizomatous Festuca rubra which formed a tussock community. Other grassspecies present included Holcus lanatus, Agrostis tenuis, Lolium perenne, Dactylis glomerata and Festucapratensis. Trifolium repens, V. sepium and occasionally Lotus pendunculatus were also recorded. The groundbecame boggier in many parts of this habitat.

The predominant features of the third habitat were the large quantity of gorse bushes and the relativelylarge number of plant species recorded. The Ulex europaeus grew both in clusters and singly, in many placesforming an entire or semiclosed ring around a central, sheltered, grassy area. Within these areas and beneaththe gorse bushes a number of plant species were found including the grasses F. rubra, A. tenuis, H. lanatusand occasionally Phleum pratense. Legumes found in addition to U. europaeus were L. pendunculatus,T. repens and V. sepium. In the spring months a number of juvenile bushes of U. europaeus were foundbeneath the mature bushes and in other habitats across the site.

The low canopy of the fourth habitat (6 per cent coverage) appeared to be indicative of a problem withinthe site. Polythrinchium trifolii was found growing on the leaves of Trifolium sp. indicating that thevegetation was of a poorer quality than that found throughout the remainder of the site, Grasses observedwere F. rubra, A. tenuis and occasionally Holcus lanatus. Senecio jacobaea individuals were seen to standsingly, providing a stark contrast to the low canopy. Legumes found on this site were Vicia sepium andTrifolium repens. In winter the dominant vegetation of this habitat appeared to be mosses, particularlyEurynchium praelongum and Brachythecium rutabulum. These became overgrown as the spring vegetation



developed. It appeared that the low canopy was caused by extensive grazing of the habitat by rabbits, evidentby the presence of large quantities of rabbit droppings.

Figure 4 gives dominance±diversity curves (rank abundance plots) for the summer vegetation taken fromeach habitat type. Species sequence is plotted against abundance (as log10 percentage frequency). Speciesnumber and composition can easily be determined and clearly illustrate the simpli®ed nature of the sward. Ingeneral less than 10 species were found in each quadrat taken. Although the gorse habitat contained thegreatest number of plant species counted (14) this is in dramatic contrast to the rich and varied ¯orasurrounding the site. For example, native grassland adjacent to the site had on average over 30 speciesrecorded in each quadrat. The dominance±diversity curve for the low-canopy habitat had the steepest slopefollowing an initial plateau, and indicated an increased dominance of only three of the original sown species.

The gorse habitat was found to be increasing in area during the research period as evidenced by theobservation of seedlings across the site in spring. This indicates that the site has not as yet reached a climaxcommunity. A large variety of plants was detected in the area immediately surrounding Shelton Abbey,indicating the presence of a varied gene pool and the potential for further succession on the site. Theoccurrence of dominant species found within the site can be seen in Table I.

Soil Analysis

The pH, organic matter content, water content and concentrations of metals and nutrients found in therooting zone in each habitat are shown in Table II.

Figure 3. Representative plot of DECORANA scores for summer vegetation



Figure 4. Dominance±diversity curve for summer vegetation

#1998JohnWiley

&Sons,Ltd.

LAND

DEGRADATIO

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DEVELOPMENT,9,67±79(1998)

It was found that the water and organic matter contents di�ered signi®cantly between habitats in therooting zone as did the manganese concentrations ( p < 0�05 in all cases). These parameters may thereforehave in¯uenced the formation of di�erent plant communities within the site. As the concentrations of theelements analysed were not found to be toxic, tolerance is unlikely to have evolved within the site.The leguminous habitat was found to contain the highest percentage water content of the habitats exam-

ined. However, the habitat did not appear to be more saturated than the remainder of the site. The grasshabitat became easily waterlogged, despite the fact that the water content in this habitat was lower than thatfound in samples from the other habitats. Signi®cantly higher concentrations ( p < 0�05) of zinc were foundhere and may have contributed to community formation. These factors may also contribute to the absence ofRumex acetosella from this habitat. Holcus lanatus is more abundant in the grass habitat than in any otherhabitat indicating that conditions are particularly suitable for the growth of this species. The gorse habitathad the lowest iron and manganese content, which may have played a role in formation of the community.This habitat contained lessVicia sepium and Trifolium repens than the other habitats. The low canopy habitatwas found to have the lowest organic matter content and the highest concentrations of manganese resulting inless binding of metals at the soil surface than is seen in other habitats and therefore greater availability ofelements to vegetation. The habitat contained fewer, less healthy legumes, than the other habitats indicatingthat nitrogen cycling may be poor. H. lanatus was rare within the habitat indicating that conditions may notbe suitable for its growth. The impact of grazing on this habitat is also likely to have been signi®cant.

Table I. Relative occurrence of dominant species within quadrats examined at each habitatduring the summer sampling period. Ten 1 mÿ2 quadrats were used to assess species domi-nance with presence shown as the percentage of quadrats in which particular species wererecorded

Species Leguminous Grass Gorse Low canopy(%) (%) (%) (%)

Trifolium repens 100 90 50 100Vicia sepium 40 70 20 80Rumex acetosella 80 0 30 30Agrostis capillaris 100 90 100 100Festuca rubra 100 100 100 100Holcus lanatus 40 100 60 10Lolium perenne 100 10 0 0Ulex europaeus 0 0 100 0

Table II. Mean and standard deviation (SD) of the main soil parameters measured at Shelton Abbey for the rootingzone (0±25 cm). Where N is total oxidized nitrogen (mg gÿ1) and OM is organic matter (%). An asterisk indicatesparameter measured from 0±10 cm only

Habitat Leguminous Grass Gorse Low canopycharacter

mean SD mean SD mean SD mean SD

Water (%) 14.3 2.7 7.0 0.9 10.2 0.9 12.9 2.1OM (%)� 5.1 1.4 5.3 4.3 6.0 1.0 4.6 0.7pH 5.6 0.2 5.4 0.1 5.7 0.1 6.0 0.1P (mg gÿ1) 593 107 664 101 445 116 611 88K (mg gÿ1) 673 255 851 181 465 48 802 340N (mg gÿ1�� 8.1 0.9 8.7 4.5 9.0 5.5 10.2 4.0Cu (mg gÿ1) 38 15 33 6.6 41 8.5 43 14Fe (mg gÿ1) 36 434 20 587 42 084 4599 29 747 22 017 41 833 16 943Zn (mg gÿ1) 99 18 146 67 114 35 116 27Mn (mg gÿ1) 2543 1385 2189 157 1162 1444 4031 2135



A wide range of metal concentrations was found within each habitat. Concentrations of both iron andmanganese exceed the normal for agricultural crops which may cause problems if the site is to be usedagriculturally. In the presence of high concentrations of manganese the tops of plants become yellow andstunted (Bradshaw and Chadwick, 1980), however, this was not observed in any of the vegetation fromShelton Abbey, indicating toxicity was not experienced within the site.

Succession

It was observed that the current vegetative composition of the site di�ers from the seed mixture sown in 1985indicating that succession has taken place. Poa sp. was rarely seen within the site during the study, despite thefact that it comprised 20 per cent of the original seed sown. This indicates that either this species was notsuited to the site or was out-competed by other species. Festuca rubra, and Agrostis sp. occur in almost allquadrats, while Trifolium sp. is found in over 80 per cent of the quadrats taken (Table III) Lolium perenneoccurs in over a quarter of the quadrats taken during the sampling period indicating that it has maintainedits position at the site.

Table III. The relative occurrence of the plant species originally sown on the Shelton Abbey site in the summer of 1995compared to the original composition (as a percentage) of the species in the mixture used to reseed the site in 1985. Ten1 mÿ2 quadrats were used to assess species dominance with their presence shown as the percentage of quadrats in whichparticular species were recorded

Species Leguminous Grass Gorse Low canopy Original(%) (%) (%) (%) seed mix (%)

Poa sp. 20 0 0 0 20Festuca rubra 100 100 100 100 15Agrostis sp. 100 90 100 100 25Lolium perenne 100 10 0 0 25Trifolium repens 100 90 50 100 15

Metal Uptake by Plants

Potential metal uptake by the vegetation was determined by examining the roots and vegetative parts of theplants which were separated and analysed independently (Table IV).

Higher concentrations of elements analysed were contained in the roots of vegetation than in the shoots;due to some plant species preferentially storing metals in their roots (Williamson, et al., 1982). No signi®cantdi�erence in the mean concentrations of copper, iron or manganese was observed between vegetationgathered from di�erent habitats ( p > 0�05), although some di�erences were observed in the case of zinc( p < 0�01) (Figure 5). Signi®cant di�erences were found between shoot and root concentrations for thegrass, gorse and leguminous habitats ( p < 0�05). The vegetation in the low canopy habitat had very shortroots which may have resulted in reduced ability to uptake and store zinc from the soil. Concentrations ofcopper in plant shoots and roots were within the normal range for vegetation growth in unpolluted soil.Vegetation examined contained higher concentrations of iron than vegetation growing in normal soils,although the shoot concentrations were within the range for agricultural crops. The root concentrations werehigher than the normal range for crops. This suggests that trials would be required to determine whether ornot root crops in particular could be grown on the site. Most of the lead content of soils is not soluble, withonly 1±2 mg gÿ1 available to plants in uncontaminated soils. The levels found in vegetation were negligible,so it was concluded that lead was not a source of toxicity at the site. The concentrations of zinc found in thesoil were at the top end of the range for normal soils, indicating that some tolerant individuals might befound in the site.

As grasses, the dominant vegetation present, do not normally use exclusion as a method of achievingtolerance, higher concentrations of zinc than normal would have been found in the species if tolerance was



present (Thurman, 1981). This was not observed indicating that it is unlikely that the species are su�eringfrom toxicity. Under certain conditions aluminium can be highly toxic to plants. High concentrations in soilsa�ect root growth (Bradshaw and Chadwick, 1980). Some samples of vegetation were analysed to provide anindication of the toxicity of aluminium at Shelton Abbey. The mean concentration of aluminium found

Table IV. Comparison of the metal concentration in vegetation samples taken from Shelton Abbey during the summerof 1993. Expected ranges in vegetation from unpolluted soils and in crops are also given. Analysis of metalconcentration in vegetation samples taken in summer. Each reading represents the mean of nine separate results

Copper mg gÿ1 Iron mg gÿ1 Manganese mg gÿ1 Zinc mg gÿ1 n

Mean SD Mean SD Mean SD Mean SD

Core vegetationLeguminous 8.5 3.7 1161 547 4962 160 34.4 12.5 9Grass 8.1 2.5 753 160 564 197 27.0 4.1 9Gorse 9.8 2.2 2500 1474 1025 326 38.1 4.3 9Low canopy 9.1 1.5 1469 305 507 138 29.1 8.6 9

Core rootsLeguminous 20.9 12.9 3068 1955 703 245 72.1 35.5 9Grass 21.0 4.9 1792 1024 732 614 65.0 20.2 9Gorse 18.9 5.9 4568 2021 1763 1371 59.3 13.4 9Low canopy 17.7 2.0 2006 960 701 188 38.0 10.7 9

Unpolluted soil and vegetationUnpolluted soil 2.5±25 40±500 50±1000 15±1000(range)�

Agricultural crops 4±15 15±100(range)��

�Allen, 1989; ��Freedman and Hutchinson, 1981

Figure 5. Mean concentration and standard deviation of elements in vegetation gathered from di�erent habitats



within shoots analysed was 882 mm gÿ1, well within the range found in unpolluted soils. Therefore it can beconcluded that aluminium is not a problem in the site.

The concentration of elements in the original tailings as estimated in the 1970s was on average: copper470 mg gÿ1, zinc 755 mg gÿ1 and lead 230 mg gÿ1. It can be seen by comparison with the mean metalconcentrations of tailings samples taken at a depth of 200 cm, that while the current concentration ofcopper and lead are similar, the zinc concentration has fallen signi®cantly (p < 0�001) (Table V). As zinc ismobilized more rapidly than other elements upon acidi®cation, it was concluded that the element has beenleached over the years.

Table V. Mean and standard deviation of metal concentrations found in samples taken from 200 cm depth at SheltonAbbey during the summer sampling period as compared to concentration data from original tailings

Copper Iron Zinc Lead

Mean SD Mean SD Mean SD Mean SD

Concentration at 564 83 51 600 25 934 280 169 288 189200 cm in mg gÿ1

Concentration in 470 ± ± ± 755 ± 230 ±original tailings

The concentrations of zinc currently found in the river below Shelton Abbey are not signi®cantly elevated(Le Bolloch, 1993), indicating that the downwards leaching process is either complete or occurring at verylow levels. It was also found that the concentrations in the tailing were signi®cantly higher than those in theoverlying soil (p < 0:001), con®rming that upwards leaching of the elements has not occurred.

Some waterlogging of the site was observed throughout the year. This appeared to be due to compactionby vehicles. The shallowness of the soil combined with the ®ne size of the tailings may also be contributingfactors. The waterlogging impedes access to parts of the site in wet weather. Therefore drainage may berequired if the site is to be developed.

CONCLUSIONS

The Shelton Abbey tailings pond has now been revegetated for nearly a decade and had been left to dry outand stabilize for a number of years before that. The rehabilitation strategy used appears to have beensuccessful as there is no evidence of upward leaching of metals. However, some leaching of metals from thesite into either the groundwater or the river has occurred. This is con®rmed by the lower concentrations ofzinc contained in the samples analysed as compared to the original tailings. The concentrations of the otherelements are similar to the original concentrations.

The vegetative composition of Shelton Abbey di�ers from that of the seed mix originally sown on thesite. There are now three main habitats present: gorse, grass and leguminous. There are also somesmaller habitats, including one with a low canopy. This latter habitat appears to be grazed intensively byrabbits which may have prevented succession from taking place, generating a plant community di�erent tothe remainder of the site. The presence of di�erent habitats is linked to a number of factors including drainageand seed dispersal. Habitat distribution cannot simply be linked to di�erences in the cover soil used.

Di�ering conditions in the habitats examined may be contributing to the vegetation di�erences observed.When soil parameters were examined some signi®cant di�erences between habitats were found, principallywater, organic matter and manganese content. These parameters may have in¯uenced habitat formation.

The concentrations of iron and manganese were found to be higher in vegetation growing at SheltonAbbey than in vegetation growing in unpolluted soil. This may be due to the presence of tolerant cultivars ofthe species which accumulated the metals. Alternatively the species may contain higher metal concentrations



due to higher availability at the site. The concentration of manganese at the 25±50 cm depth was analysedand found to di�er signi®cantly from those in the 0±25 cm layer ( p < 0�01) indicating that plants wereabsorbing manganese and depositing it on the surface of the soil by the processes of senescence and guttation.Tests would have to be done on animals grazed on the site to determine if metal absorption was at a levelsuitable for human consumption.

The particle size of tailings normally ranges from the size of sand to that of clay (Williamson et al., 1982).Fine tailings are considered to be those with a mean particle size of less than 20 mm, and intermediate tailingswith a size of 0.02±5 mm (Johnson, 1979). At Shelton Abbey the tailings were of an intermediate size, so thattheir water-holding capacity was not as high as that found in ®ne tailings. However, the walls of some of thesample cores dug during the sampling period begun to cave in due to the high quantity of water held withinthe tailings. Statistical analysis found water content to di�er signi®cantly with habitat type, and foundparticle size to be a major determinant of the amount of water held by material. Waterlogging, leading to alow redox, combined with the low pH of the tailings, appears to have led to leaching of elements, particularlyzinc, in the past. However, it seems that little leaching is now occurring and that the site has stabilized.

The depth of soil covering a site determines the vegetation growing. Many tree species su�er toxicity symp-toms once their roots reach tailings, and the same applies to other plant species. The only element found atelevated levels in the tailings in Shelton Abbey was copper. Therefore future succession on the site wouldrequire deep-rooted plant species to be tolerant to this element. The thicker the soil layer placed over the site,the greater the variety of plants that is able to grow.The vegetation growing at Shelton Abbey appeared to be healthy and diverse, although there did not

appear to be a climax community present. Therefore the rehabilitation procedure can be considered to besuccessful. Some form of management strategy is, however, required to protect the integrity of the site. Thevegetation growing is limited in species number and of an uneven distribution, but since rehabilitation anumber of habitats have become established. The dominant vegetation was found to be grass, particularlyFestuca rubra,Agrostis tenuis, andHolcus lanatus. Other abundant species includedUlex europaeus,Trifoliumrepens, Lolium perenne and Rumex acetosella. Three main habitats were distinguished throughout the site:leguminous, grass and gorse. A fourth habitat was also determined, characterized by a low grass canopy androcky soil. The grass and gorse habitats were the most abundant in terms of coverage. Due to the diversity ofthe vegetation surrounding the site the major factor a�ecting species composition within Shelton Abbey islikely to have been the seedbank present in the topsoil originally placed on the site, and seed distribution fromthe vegetation growing at the perimeter of the site. Betula pubescens is dominant at one edge of the tailingspond and is colonizing the perimeter. As Betula sp. is known to out-compete U. europaeus stands and is thenout-competed by other tree species, there is potential for formation of a woodland community on the site,provided the substrate is suited to tree species (Grimes, et al., 1988).

No ®nal use of the Shelton Abbey site appears to have been envisaged, although the possibility ofreforesting the upland parts of the mine was considered (Platt, 1974). The site is now owned by the Depart-ment of Energy, who have allowed natural succession to occur, and it is currently utilized by a Dublinshooting syndicate primarily for pheasant shooting. Poor drainage and the high concentrations of iron in thevegetation may cause problems if potential future uses of the site are to be investigated. Among the potentialuses for the site are agriculture, forestry and amenity. It is likely that further succession may occur within thesite possibly leading to the development of a woodland community. However, in the short term, the invasivenature of the gorse will reduce the existing opportunity for amenity use if not controlled.

RREEFFEERREENNCCEESS

Allen, S. E. 1989. Chemical Analysis of Ecological Materials. Blackwell Scienti®c, Oxford.Bradshaw, A. D. and Chadwick, M. J. 1980. The Restoration of Land, Blackwell Scienti®c, Oxford.Freedman, B. and Hutchinson, T. C. 1981. Sources of metal and elemental contamination of terrestrial environments, pp. 35±94. in

Lepp, N.W. (ed.). E�ect of Heavy Metal Pollution on Plants-2, Applied Science, London.Grimes, J. P., Hodgson, J. G. and Hung, R. 1988. Comparative Plant Ecology, Unwin Hyman, London.



Hesse, P. R. 1971. A Textbook of Soil Chemical Analysis, Murray, London.Je�eries, R. A. 1981. `Legumes for the reclamation of derelict and disturbed land', Landscape Design, 134, 39±40.Jenkins, R. 1988. X-ray ¯uoresence spectrometry, Wiley, New York.Johnson,M. S. 1991. Revegetation of metalliferous mine tailings: a review of techniques and achievements in western Europe, pp. 51±69.

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Le Bolloch, O. 1993. Water Quality in the Lower Avoca River and its implications for ®sh migration, unpublished M.Sc. Dissertation,Trinity College, Dublin.

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Documents

Evaluation of the rehabilitation procedure of a pyritic mine tailings pond in Avoca, Southeast Ireland