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

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<ul><li><p>Land Degrad. Develop. 9, 6779 (1998)</p><p>EVALUATION OF THE REHABILITATION PROCEDURE OFA PYRITIC MINE TAILINGS POND IN AVOCA,</p><p>SOUTHEAST IRELAND</p><p>C. ONEILL,1 N. F. GRAY2 AND M. WILLIAMS31Department of Mechanical and Manufacturing Engineering, University of Glamorgan, Pontypridd, Wales</p><p>2Department of Civil, Structural and Environmental Engineering, Trinity College, University of Dublin, Dublin 2, Ireland3Department of Botany, Trinity College, University of Dublin, Dublin 2, Ireland</p><p>Received 25 June 1996; Accepted 27 February 1997</p><p>ABSTRACT</p><p>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 eect of the tailings. A number of floristic habitats wereidentified 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 2530 cm deep and contained levels of nutrients and metals comparable to those found in unpolluted soils. It wasfollowed by a 2025 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 significantly 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 &amp; Sons, Ltd.</p><p>KEY WORDS: metals; mine management; mine waste; soil contamination; tailings</p><p>INTRODUCTION</p><p>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 northsouthAvoca 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,</p><p>the disposal of the tailings was seen as the most dicult 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</p><p>LAND DEGRADATION &amp; DEVELOPMENT</p><p>Correspondence to: Professor Nick Gray, Department of Civil, Structural and Environmental Engineering, Trinity College, Universityof Dublin, Dublin 2, Ireland</p><p>CCC 10853278/98/01006713$17.50</p><p># 1998 John Wiley &amp; Sons, Ltd.</p></li><li><p>locality (Figure 2). The tailings pond was separated into two distinct lagoons divided by an earth bund, butoperated in parallel with a single influent but two euent 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</p><p>Figure 1. Map showing the location of the Avoca Mining Strip</p><p>68 C. ONEILL, N. F. GRAY AND M. WILLIAMS</p><p># 1998 John Wiley &amp; Sons, Ltd. LAND DEGRADATION &amp; DEVELOPMENT, 9, 6779 (1998)</p></li><li><p>Figure 2. Plan of the tailings pond showing the dierent habitats outside the site</p><p>REHABILITATION OF MINE TAILINGS POND IN IRELAND 69</p><p># 1998 John Wiley &amp; Sons, Ltd. LAND DEGRADATION &amp; DEVELOPMENT, 9, 6779 (1998)</p></li><li><p>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</p><p>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 2030 cm of shale being placed on the surface of the tailings followed by 710 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 Scientific 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 dierence between the north and south of the siteincluding drainage and seed dispersal. Therefore habitat dierences within the site cannot simply be linkedto dierences 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 ha1</p><p>and fertilizer was applied at 250 kg ha1 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 cvParys. The Parys cultivar was developed specifically 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 to</p><p>examine the current status of the reclaimed tailings pond eight years after rehabilitation.</p><p>METHODS</p><p>Vegetation surveys were carried out on three occasions, winter 1992, spring 1993 and summer 1993, toidentify the presence of dierent 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 stratified 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 a</p><p>depth of 60 cm in order to estimate soil parameters directly aecting 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 by</p><p>measuring 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. Hesses method was employed usinga more dilute soil solution to avoid damage to the electrode (Hesse, 1971). X-ray fluorescence 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 flow injection analysis (Allen, 1989; Tecator, 1984). Phosphorus content was determined by means of</p><p>70 C. ONEILL, N. F. GRAY AND M. WILLIAMS</p><p># 1998 John Wiley &amp; Sons, Ltd. LAND DEGRADATION &amp; DEVELOPMENT, 9, 6779 (1998)</p></li><li><p>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.</p><p>RESULTS AND DISCUSSION</p><p>Vegetation Analysis</p><p>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 significant compaction problems leading to surface-waterretention. Additionally, the drainage channels at the perimeter of the site were flooded 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</p><p>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 verified 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 flooded after heavy rainfall.The leguminous habitat (14 per cent coverage) was bright green in appearance due to the presence of</p><p>large 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 firm although someareas became waterlogged occasionally.The grass habitat was generally straw-coloured in appearance. The predominant species in this habitat</p><p>were 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 relatively</p><p>large 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 within</p><p>the 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</p><p>REHABILITATION OF MINE TAILINGS POND IN IRELAND 71</p><p># 1998 John Wiley &amp; Sons, Ltd. LAND DEGRADATION &amp; DEVELOPMENT, 9, 6779 (1998)</p></li><li><p>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 dominancediversity curves (rank abundance plots) for the summer vegetation taken from</p><p>each habitat type. Species sequence is plotted against abundance (as log10 percentage frequency). Speciesnumber and composition can easily be determined and clearly...</p></li></ul>