- Email: [email protected]
The containment of toxic wastes: II. Metal movement in leachate and drainage at Parc lead-zinc mine, North Wales
Environmental Pollution, Vol. 90, No. 3, pp. 379-382, 1995
0269-7491(95)00011-9
ELSEVIER
Elsevier Science Ltd Printed in Great Britain 0269-7491/95 $09.50 + 0.00
THE CONTAINMENT OF TOXIC WASTES: II. METAL MOVEMENT IN LEACHATE A N D D R A I N A G E AT PARC LEAD-ZINC MINE, NORTH WALES Yan Gao* & A. D. Bradshaw Department of Environmental and Evolutionary Biology, University of Liverpool, Liverpool L69 3BX, UK (Received 15 December 1993; accepted 14 February 1995)
Gwydyr, was a lead and zinc mine. When it was finally closed in 1954, it left behind a large mine tailings heap, about 31 m in height and 100 m × 400 m in extent, occupying 2.2 ha, which had a flat top with steeply sloping sides. The waste is composed of very fine calcareous silt-sized material with high lead and zinc content (Smith, 1973; Smith & Bradshaw, 1979; Johnson & Eaton, 1980). Gully erosion by surface drainage, bulk erosion by the stream of Nant Gwydyr, and leaching of the waste heap, resulted in heavy metal contamination on the agricultural land of the flood plain and in the River Conwy itself. Most seriously, in 1964 during a storm, a large quantity of waste was washed into the valley of the River Conwy. Serious loss of polluted material from the heap continued to occur until 1977, when a reclamation programme designed to stabilise the tailings and prevent further transference of heavy-metal laden spoil beyond the mine boundaries was carried out. Preliminary stabilisation works were accomplished in August 1977, and the main drainage engineering and recontouring operations were completed between November 1977 and June 1978. The method of stabilisation adopted, discussed by Shu and Bradshaw (1995), was to cover the whole heap with a layer of coarse quarry waste material, which was then grassed. This technique has allowed excellent control of upward metal movement, but does not prevent precipitation passing down into the heap and removing soluble toxic metals by leaching.
Abstract Nant Gwydyr, a tributary of the River Conwy in North Wales, has been affected by metal wastes, from a lead and zinc mine, Parc Mine, through contaminated mine drainage waters and episodal erosion of an unstable tailings heap. From 1954 when the mining operation was discontinued to 1978 when a reclamation programme aimed to stabilise the tailings was accomplished, 13 000 tonnes of metalliferous spoil, containing 43 tonne Pb, 104 tonne Zn, and 1 tonne Cd was eroded from the main tailings dam. Dispersal and redeposition during flood events caused extensive pollution of the agricultural land of the flood plain. Analysis of the present water quality of the Nant Gwydyr, 14 years after the stabilisation work, shows that although there has been a marked improvement and no particulate matter is released, the Nant Gwydyr is still a polluted stream. Under normal discharge conditions, it contributes approximately 1 tonne of Zn, 0.2 tonne of Pb and 0.05 tonne of Cd per year to the River Conwy. Most of this originates from water issuing from the mine adit which has been impossible to control There is still, however, a major contribution by the leachate from the tailings heap, because the stabilisation method used does not prevent this. The control of pollution in mine drainage is discussed.
Keywords: Metalliferous waste, mine drainage, toxic leachates, heavy metal pollution, contaminated land. INTRODUCTION
Before the main reclamation work started, a detailed baseline environmental assessment was conducted by the University of Liverpool and the Welsh National Water Authority. During and subsequent to the reclamation, from November 1977 to December 1978, regular sampling and analysis of the Nant Gwydyr was continued (Environmental Unit, 1978; Johnson & Eaton, 1980). After this no records of the water quality have been published. Parc Mine provides an excellent opportunity to evaluate the effects of stabilisation of mine spoils by surface covering 14 years after treatment. A brief re-examination of the water quality of effluents from the heap has therefore been carried out. This has concentrated on
Lead mining in Wales originated before the Roman occupation. The main period of activity was from 1750 to 1900, when zinc and copper were also mined. During this period only simple and inefficient ore mining and processing methods were available. Consequently large amounts of lead, zinc and copper compounds were lost to the environment and serious pollution was caused, particularly of rivers and streams (Griffith, 1919; Davies, 1987). The same effects can be found in many other areas (e.g. Say & Whitton, 1983). Parc Mine, located in North Wales in the valley of Nant *Present address: Qingdao Research Institute of Environmental Science, 39 Yan An Yi Road, Qingdao, China. 379
380
Yan Gao, A. D. Bradshaw To River Conwy
heavy metals in the leachate and the stream of Nant Gwydyr. S A M P L I N G AND ANALYSIS
30 m
Figure 1 shows the sampling points. Samples were taken on a single occasion in June 1992 after a period of average rainfall. The discharge at each point was determined by means of measures of the cross sectional area and the flow rate. Water samples were collected in glass bottles. 100 ml of each water sample was placed in a conical flask, acidified with 1 ml concentrated nitric acid and digested in an autoclave at 121°C for one hour. After cooling, they were made up to 100 ml and metal concentrations determined by atomic absorption spectrophotometer directly. R E S U L T S AND DISCUSSION Figure 1 shows that the tailings heap lies beside a stream (2) which flows down the valley in which the mine is situated. In the section close to the tailings the stream has been canalised. As it reaches the mine it is joined by a substantial flow from the main adit of the mine (1), but it also receives water from a mine shaft at the foot of the heap (4), and leachate from the base of the heap (5 & 6). There are surface drains over the heap but there are no signs that these ever collect water. Table 1 shows the results of the metal analysis of the different water samples. Zinc and lead were detected in all samples, but iron in none. Cadmium was detected in only three sampling points. The highest concentration of zinc and lead occurred at point 6, the leachate coming from the heap. Table 2 shows a comparison with the previous monitoring results. Needless to say the most marked change in water quality consequent on the reclamation has been the reduction of total suspended solid (TSS) burden (not able to be analysed). This improvement has been maintained. The grass is now growing very well apart from small areas of dieback because of the existence of leachate (Shu & Bradshaw, 1995). A vegetation cap has formed and completely covered the waste heap, which has effectively reduced erosion of the heap to zero. The water in the stream both before it reaches the mine area and after it leaves is clear and must con-
560 m
Fig. 1. Map of Parc Mine in relation to Nant Gwynant and its surroundings, indicating water sampling points and contours (heights in m).
tain a very low concentration of TSS. Previous work showed that as much as 87% (W/W), and normally 20-35%, of the total non-ferrous metal load was present as suspended particulates (Johnson & Eaton, 1980). The reduction in the concentration of TSS has resulted in the reduction of total metal burden transferred to Nant Gwydyr not only in flood conditions but also in normal conditions. However, there has been effectively no improvement after 14 years in the dissolved metal concentrations of the water leaving the mine area. Two problems remain. The first is the leachate produced by the rainfall which penetrates the coarse covering materials and passes through the waste. It contains elevated levels of heavy metals especially zinc. The very great difference
Table 1. Metal concentrations at different sampling points around Parc Mine (mg litre-I)
Zn
Pb
Cd
Fe
2.17 1.20 0-38 1.10 1.62 2-60 1.55
0.27 0.32 0.30 0-34 0.32 0.34 0-29
b 0.073 --
--
Discharges ~
Sampling points 1 Adit 2 Main stream 3 Road side seepage 4 Mine shaft 5 Leachate drain 6 Leachate pipe 7 Stream below mine " Discharges in m 3 sec ~. h Below limits of detection.
-0-27 0.073
----
0.017 0.003 -0.001 0.001 0.003 0.022
381
The containment of toxic wastes: H
Table 2. Comparison of present concentrations of zinc and lead with concentrations before and after reclamation work (from Environmental Unit 1978) at point 7 immediately below the waste heap (rag litre -l)
Monitoring times Pre-reclamation 1977 Post-reclamation 1978 Present 1992
Zn
Pb
Discharges a
3.2(1.6-4.6) b 2.8(0.4-4.2) 1.55
0.04(<0.01-0.2) 0.04(0.01-0.3) 0.30
0.05(0.022-0.2) 0.05(0.017-0.3) 0.022
Discharge in m 3 s e c . h Range under normal conditions and complete range in parentheses.
between the concentrations of zinc and lead reflect their different solubility. The low concentration of cadmium reflects its low concentration in the waste. Although the reclamation work is successful in preventing the upward movement of water (Shu & Bradshaw, 1995), it has failed to prevent water from moving down and sideways in the heap. This leachate could cause the pollution of groundwater, but it seems likely that impervious rocks beneath the heap cause it to come out at sampling points 5 and 6. This leachate emanating from the waste continues to influence surface water. 0.32 tonne of zinc and 0-04 tonne of lead are released from this source annually. It is unlikely that the concentrations of heavy metals will be reduced further, unless measures are taken to treat the pollutants. However the stabilisation works, consisting of drainage engineering, re-contouring, covering, and vegetation establishment, have contributed considerably to the reduction of pollution from the heap in several different ways. Firstly, the drainage engineering and the re-contouring has reduced the mass flow of water into the heap. Secondly the grass grown on the waste heap will have increased the evaporation:rainfall ratio substantially. It is likely that now 50% of the rain falling on the grass surface is evaporated. These two factors will have greatly decreased the amount of water penetrating through the wastes, and therefore reduced the total volume of leachate. Unfortunately no measures of the amount of leachate produced by the heap before 1978 are available. Thirdly, the cover and the vegetation cap will have reduced the degree to which the surface of unweathered minerals are exposed to the atmosphere and oxidised, a process that could make insoluble metals become soluble. The residual metal load emanating from the adit remains a second major problem. Flow of water from
the adit, and other sections of the catchment is considerable. The lead concentrations, although low, are higher than previously. Table 3 shows the pollution loads and pollution load ratios (PLR) of the different sources. From this it can be seen that the adit is the main pollution source with a PLR of 70%. So although the metal concentration in the leachate is slightly higher than the adit, its PLR is only 20%, a small proportion of the total pollution load, because the discharge of the leachate is smaller compared with the discharge of the adit. Apart from the adit, the original metal concentrations of the main stream are also high, with a PLR of 8%. The stream receives water from other mine workings further up the valley. It also drains an area of scattered mineralisation, where small metalliferous veins are distributed through the catchment. Even in the road-side seepage water, above the waste heap (3), lead and zinc can be detected. So the heavy metals are coming from the whole catchment in a manner which is almost impossible to prevent. CONCLUSIONS This straightforward piece of work shows clearly that the Nant Gwydyr is a polluted stream and that it will probably remain so for centuries. The concentration of Pb and Cd at sampling point 6, the leachate from the mine, exceeds the WHO and EC water quality standards. Pb is 3.4 times higher than the WHO standard, 7 times higher than the EC standard and Cd 27 and 54 times higher, respectively. Zn is the main pollutant in terms of toxicity to plants and animals. A concentration of > 1 mg litre I Zn will reduce growth and survival of aquatic organisms (Besser & Rabeni, 1987). At present the stream contributes approximately 1 tonne of Zn, 0.2 tonne of Pb and 0.05 tonne of Cd annually
Table 3. The pollution loads and pollution load ratios (PLR) of different sources
Pollution loa&
Sampling point
1 Adit 2 Main stream 4 Mine shaft 5 Leachate drain 6 Leachate pipe Total
PLR b
Zn
Pb
Cd
Total
132.8 13-0 4-0 5.8 28.1 183.7
16.5 3.5
-0.8
1-2
--
1.2 3.7 26.1
-2.9 3.7
149.3 17-3 5-2 7.0 34.7 213.5
a Pollution loads derived from data in Table 1 in g h-I. b Pollution loads as % of total.
70.0 8.0 2.4 3.3 16.3 100-0
382
Yan Gao, A. D. Bradshaw
to the River Conwy. This represents a reduction in heavy metal burden in the water of only 20-30% compared with the previous situation. But of course, a great reduction in movement of particulates, which contained insoluble heavy metals, has been achieved, and wholesale movement in times of flood eliminated. A considerable reduction in total metals transferred to the River Conwy has therefore resulted from the stabilisation works. The remaining pollutants are diluted by the river water when they flow into the River Conwy; however they can accumulate biologically or physico-chemically in ecosystems below the mine area. The tailings heap was covered with coarse material which does not restrict the flow of leachate, containing metals at a relatively high concentration. This could have been prevented if the heap had been given an impermeable cap under the inert covering. This would have been expensive and would only have contributed a further reduction of 20% of the total pollution load. To conclude, fourteen years after establishment, the reclamation works at Parc Mine are successful in that surface stabilisation has been achieved, with a vegetation cover sufficiently productive and free of heavy metals to be able to be grazed upon. The technique provides a practical and economical method of reducing pollution from mine wastes, sufficient to remove the serious threat to the catchment previously posed by the mine. The same technique has been used successfully elsewhere (Norris, 1986). It is not a perfect solution; some problems will remain in perpetuity from the wastes as well as from the mine workings. The emissions from the wastes could be controlled by further major engineering works but these would be extremely costly. The major emissions from the mine adit are effectively uncontrollable. The situation illustrates the great difficulties that can arise when ore bodies are disturbed in the search for metals. Heavy metals mining in particular leads to serious problems, because toxic elements previously locked up in ore bodies are irrevocably freed, and exposed to
release by weathering processes. Mining of heavy metals therefore produces a number of serious long-term pollution problems, some of which are so difficult to treat that they should be a very important consideration in the planning and environment impact assessment of any proposed mining development.
ACKNOWLEDGEMENT This work was carried out under the auspices of the British Council under an Academic Links with China Scheme.
REFERENCES Besser, J. M. & Rabeni, C. F. (1987). Bioavailability and toxicity of metals leached from lead-mine tailings to aquatic invertebrates. Environ. Toxicol. Chem., 11, 879-90. Davies, B. E. (1987). Consequences of environmental contamination by lead mine in Wales. Hydrobiologia, 1, 149-62. Griffith, J. J. (1919). Influence of mines upon land and livestock in Cardiganshire. J. Agric. Sci. Camb., 9, 366-95. Environmental Unit (University of Liverpool) (1978). Chemical and Biological Changes in the Nant Gwydyr Following Rehabilitation Works. Welsh Development Agency, Cardiff. Johnson, M. S. & Eaton, J. W. (1980). Environmental contamination through residual trace metal dispersal from a derelict lead-zinc mine. Environ. Qual., 9, 175-9. Norris, R. H. (1986). Mine waste pollution of the Molonglo River, New South Wales and the Australian Capital Territory: Effectiveness of remedial works at Captains Flat mine area. Aust. J. Mar. Freshwat. Res., 2, 147-57. Say, P. J. & Whitton, B. A. (1983). Impact of heavy metal waste leachates on a stream in south-west France. Conserv. Recycling, 7, 2-4. Shu, J & Bradshaw, A. D. (1995). The containment of toxic wastes: I. Long term metal movement in soils over a covered metalliferous waste heap at Parc lead-zinc mine, North Wales. Environ. Pollut., 90, 371-7. Smith, R. A. H, (1973). The reclamation of metalliferous Mine workings using tolerant plant populations. PhD Thesis, University of Liverpool. Smith, R. A. H. & Bradshaw, A. D. (1979). The use of metal tolerant populations for the reclamation of metelliferous wastes. J. Appl. Ecol., 6, 595-612.
0269-7491(95)00011-9
ELSEVIER
Elsevier Science Ltd Printed in Great Britain 0269-7491/95 $09.50 + 0.00
THE CONTAINMENT OF TOXIC WASTES: II. METAL MOVEMENT IN LEACHATE A N D D R A I N A G E AT PARC LEAD-ZINC MINE, NORTH WALES Yan Gao* & A. D. Bradshaw Department of Environmental and Evolutionary Biology, University of Liverpool, Liverpool L69 3BX, UK (Received 15 December 1993; accepted 14 February 1995)
Gwydyr, was a lead and zinc mine. When it was finally closed in 1954, it left behind a large mine tailings heap, about 31 m in height and 100 m × 400 m in extent, occupying 2.2 ha, which had a flat top with steeply sloping sides. The waste is composed of very fine calcareous silt-sized material with high lead and zinc content (Smith, 1973; Smith & Bradshaw, 1979; Johnson & Eaton, 1980). Gully erosion by surface drainage, bulk erosion by the stream of Nant Gwydyr, and leaching of the waste heap, resulted in heavy metal contamination on the agricultural land of the flood plain and in the River Conwy itself. Most seriously, in 1964 during a storm, a large quantity of waste was washed into the valley of the River Conwy. Serious loss of polluted material from the heap continued to occur until 1977, when a reclamation programme designed to stabilise the tailings and prevent further transference of heavy-metal laden spoil beyond the mine boundaries was carried out. Preliminary stabilisation works were accomplished in August 1977, and the main drainage engineering and recontouring operations were completed between November 1977 and June 1978. The method of stabilisation adopted, discussed by Shu and Bradshaw (1995), was to cover the whole heap with a layer of coarse quarry waste material, which was then grassed. This technique has allowed excellent control of upward metal movement, but does not prevent precipitation passing down into the heap and removing soluble toxic metals by leaching.
Abstract Nant Gwydyr, a tributary of the River Conwy in North Wales, has been affected by metal wastes, from a lead and zinc mine, Parc Mine, through contaminated mine drainage waters and episodal erosion of an unstable tailings heap. From 1954 when the mining operation was discontinued to 1978 when a reclamation programme aimed to stabilise the tailings was accomplished, 13 000 tonnes of metalliferous spoil, containing 43 tonne Pb, 104 tonne Zn, and 1 tonne Cd was eroded from the main tailings dam. Dispersal and redeposition during flood events caused extensive pollution of the agricultural land of the flood plain. Analysis of the present water quality of the Nant Gwydyr, 14 years after the stabilisation work, shows that although there has been a marked improvement and no particulate matter is released, the Nant Gwydyr is still a polluted stream. Under normal discharge conditions, it contributes approximately 1 tonne of Zn, 0.2 tonne of Pb and 0.05 tonne of Cd per year to the River Conwy. Most of this originates from water issuing from the mine adit which has been impossible to control There is still, however, a major contribution by the leachate from the tailings heap, because the stabilisation method used does not prevent this. The control of pollution in mine drainage is discussed.
Keywords: Metalliferous waste, mine drainage, toxic leachates, heavy metal pollution, contaminated land. INTRODUCTION
Before the main reclamation work started, a detailed baseline environmental assessment was conducted by the University of Liverpool and the Welsh National Water Authority. During and subsequent to the reclamation, from November 1977 to December 1978, regular sampling and analysis of the Nant Gwydyr was continued (Environmental Unit, 1978; Johnson & Eaton, 1980). After this no records of the water quality have been published. Parc Mine provides an excellent opportunity to evaluate the effects of stabilisation of mine spoils by surface covering 14 years after treatment. A brief re-examination of the water quality of effluents from the heap has therefore been carried out. This has concentrated on
Lead mining in Wales originated before the Roman occupation. The main period of activity was from 1750 to 1900, when zinc and copper were also mined. During this period only simple and inefficient ore mining and processing methods were available. Consequently large amounts of lead, zinc and copper compounds were lost to the environment and serious pollution was caused, particularly of rivers and streams (Griffith, 1919; Davies, 1987). The same effects can be found in many other areas (e.g. Say & Whitton, 1983). Parc Mine, located in North Wales in the valley of Nant *Present address: Qingdao Research Institute of Environmental Science, 39 Yan An Yi Road, Qingdao, China. 379
380
Yan Gao, A. D. Bradshaw To River Conwy
heavy metals in the leachate and the stream of Nant Gwydyr. S A M P L I N G AND ANALYSIS
30 m
Figure 1 shows the sampling points. Samples were taken on a single occasion in June 1992 after a period of average rainfall. The discharge at each point was determined by means of measures of the cross sectional area and the flow rate. Water samples were collected in glass bottles. 100 ml of each water sample was placed in a conical flask, acidified with 1 ml concentrated nitric acid and digested in an autoclave at 121°C for one hour. After cooling, they were made up to 100 ml and metal concentrations determined by atomic absorption spectrophotometer directly. R E S U L T S AND DISCUSSION Figure 1 shows that the tailings heap lies beside a stream (2) which flows down the valley in which the mine is situated. In the section close to the tailings the stream has been canalised. As it reaches the mine it is joined by a substantial flow from the main adit of the mine (1), but it also receives water from a mine shaft at the foot of the heap (4), and leachate from the base of the heap (5 & 6). There are surface drains over the heap but there are no signs that these ever collect water. Table 1 shows the results of the metal analysis of the different water samples. Zinc and lead were detected in all samples, but iron in none. Cadmium was detected in only three sampling points. The highest concentration of zinc and lead occurred at point 6, the leachate coming from the heap. Table 2 shows a comparison with the previous monitoring results. Needless to say the most marked change in water quality consequent on the reclamation has been the reduction of total suspended solid (TSS) burden (not able to be analysed). This improvement has been maintained. The grass is now growing very well apart from small areas of dieback because of the existence of leachate (Shu & Bradshaw, 1995). A vegetation cap has formed and completely covered the waste heap, which has effectively reduced erosion of the heap to zero. The water in the stream both before it reaches the mine area and after it leaves is clear and must con-
560 m
Fig. 1. Map of Parc Mine in relation to Nant Gwynant and its surroundings, indicating water sampling points and contours (heights in m).
tain a very low concentration of TSS. Previous work showed that as much as 87% (W/W), and normally 20-35%, of the total non-ferrous metal load was present as suspended particulates (Johnson & Eaton, 1980). The reduction in the concentration of TSS has resulted in the reduction of total metal burden transferred to Nant Gwydyr not only in flood conditions but also in normal conditions. However, there has been effectively no improvement after 14 years in the dissolved metal concentrations of the water leaving the mine area. Two problems remain. The first is the leachate produced by the rainfall which penetrates the coarse covering materials and passes through the waste. It contains elevated levels of heavy metals especially zinc. The very great difference
Table 1. Metal concentrations at different sampling points around Parc Mine (mg litre-I)
Zn
Pb
Cd
Fe
2.17 1.20 0-38 1.10 1.62 2-60 1.55
0.27 0.32 0.30 0-34 0.32 0.34 0-29
b 0.073 --
--
Discharges ~
Sampling points 1 Adit 2 Main stream 3 Road side seepage 4 Mine shaft 5 Leachate drain 6 Leachate pipe 7 Stream below mine " Discharges in m 3 sec ~. h Below limits of detection.
-0-27 0.073
----
0.017 0.003 -0.001 0.001 0.003 0.022
381
The containment of toxic wastes: H
Table 2. Comparison of present concentrations of zinc and lead with concentrations before and after reclamation work (from Environmental Unit 1978) at point 7 immediately below the waste heap (rag litre -l)
Monitoring times Pre-reclamation 1977 Post-reclamation 1978 Present 1992
Zn
Pb
Discharges a
3.2(1.6-4.6) b 2.8(0.4-4.2) 1.55
0.04(<0.01-0.2) 0.04(0.01-0.3) 0.30
0.05(0.022-0.2) 0.05(0.017-0.3) 0.022
Discharge in m 3 s e c . h Range under normal conditions and complete range in parentheses.
between the concentrations of zinc and lead reflect their different solubility. The low concentration of cadmium reflects its low concentration in the waste. Although the reclamation work is successful in preventing the upward movement of water (Shu & Bradshaw, 1995), it has failed to prevent water from moving down and sideways in the heap. This leachate could cause the pollution of groundwater, but it seems likely that impervious rocks beneath the heap cause it to come out at sampling points 5 and 6. This leachate emanating from the waste continues to influence surface water. 0.32 tonne of zinc and 0-04 tonne of lead are released from this source annually. It is unlikely that the concentrations of heavy metals will be reduced further, unless measures are taken to treat the pollutants. However the stabilisation works, consisting of drainage engineering, re-contouring, covering, and vegetation establishment, have contributed considerably to the reduction of pollution from the heap in several different ways. Firstly, the drainage engineering and the re-contouring has reduced the mass flow of water into the heap. Secondly the grass grown on the waste heap will have increased the evaporation:rainfall ratio substantially. It is likely that now 50% of the rain falling on the grass surface is evaporated. These two factors will have greatly decreased the amount of water penetrating through the wastes, and therefore reduced the total volume of leachate. Unfortunately no measures of the amount of leachate produced by the heap before 1978 are available. Thirdly, the cover and the vegetation cap will have reduced the degree to which the surface of unweathered minerals are exposed to the atmosphere and oxidised, a process that could make insoluble metals become soluble. The residual metal load emanating from the adit remains a second major problem. Flow of water from
the adit, and other sections of the catchment is considerable. The lead concentrations, although low, are higher than previously. Table 3 shows the pollution loads and pollution load ratios (PLR) of the different sources. From this it can be seen that the adit is the main pollution source with a PLR of 70%. So although the metal concentration in the leachate is slightly higher than the adit, its PLR is only 20%, a small proportion of the total pollution load, because the discharge of the leachate is smaller compared with the discharge of the adit. Apart from the adit, the original metal concentrations of the main stream are also high, with a PLR of 8%. The stream receives water from other mine workings further up the valley. It also drains an area of scattered mineralisation, where small metalliferous veins are distributed through the catchment. Even in the road-side seepage water, above the waste heap (3), lead and zinc can be detected. So the heavy metals are coming from the whole catchment in a manner which is almost impossible to prevent. CONCLUSIONS This straightforward piece of work shows clearly that the Nant Gwydyr is a polluted stream and that it will probably remain so for centuries. The concentration of Pb and Cd at sampling point 6, the leachate from the mine, exceeds the WHO and EC water quality standards. Pb is 3.4 times higher than the WHO standard, 7 times higher than the EC standard and Cd 27 and 54 times higher, respectively. Zn is the main pollutant in terms of toxicity to plants and animals. A concentration of > 1 mg litre I Zn will reduce growth and survival of aquatic organisms (Besser & Rabeni, 1987). At present the stream contributes approximately 1 tonne of Zn, 0.2 tonne of Pb and 0.05 tonne of Cd annually
Table 3. The pollution loads and pollution load ratios (PLR) of different sources
Pollution loa&
Sampling point
1 Adit 2 Main stream 4 Mine shaft 5 Leachate drain 6 Leachate pipe Total
PLR b
Zn
Pb
Cd
Total
132.8 13-0 4-0 5.8 28.1 183.7
16.5 3.5
-0.8
1-2
--
1.2 3.7 26.1
-2.9 3.7
149.3 17-3 5-2 7.0 34.7 213.5
a Pollution loads derived from data in Table 1 in g h-I. b Pollution loads as % of total.
70.0 8.0 2.4 3.3 16.3 100-0
382
Yan Gao, A. D. Bradshaw
to the River Conwy. This represents a reduction in heavy metal burden in the water of only 20-30% compared with the previous situation. But of course, a great reduction in movement of particulates, which contained insoluble heavy metals, has been achieved, and wholesale movement in times of flood eliminated. A considerable reduction in total metals transferred to the River Conwy has therefore resulted from the stabilisation works. The remaining pollutants are diluted by the river water when they flow into the River Conwy; however they can accumulate biologically or physico-chemically in ecosystems below the mine area. The tailings heap was covered with coarse material which does not restrict the flow of leachate, containing metals at a relatively high concentration. This could have been prevented if the heap had been given an impermeable cap under the inert covering. This would have been expensive and would only have contributed a further reduction of 20% of the total pollution load. To conclude, fourteen years after establishment, the reclamation works at Parc Mine are successful in that surface stabilisation has been achieved, with a vegetation cover sufficiently productive and free of heavy metals to be able to be grazed upon. The technique provides a practical and economical method of reducing pollution from mine wastes, sufficient to remove the serious threat to the catchment previously posed by the mine. The same technique has been used successfully elsewhere (Norris, 1986). It is not a perfect solution; some problems will remain in perpetuity from the wastes as well as from the mine workings. The emissions from the wastes could be controlled by further major engineering works but these would be extremely costly. The major emissions from the mine adit are effectively uncontrollable. The situation illustrates the great difficulties that can arise when ore bodies are disturbed in the search for metals. Heavy metals mining in particular leads to serious problems, because toxic elements previously locked up in ore bodies are irrevocably freed, and exposed to
release by weathering processes. Mining of heavy metals therefore produces a number of serious long-term pollution problems, some of which are so difficult to treat that they should be a very important consideration in the planning and environment impact assessment of any proposed mining development.
ACKNOWLEDGEMENT This work was carried out under the auspices of the British Council under an Academic Links with China Scheme.
REFERENCES Besser, J. M. & Rabeni, C. F. (1987). Bioavailability and toxicity of metals leached from lead-mine tailings to aquatic invertebrates. Environ. Toxicol. Chem., 11, 879-90. Davies, B. E. (1987). Consequences of environmental contamination by lead mine in Wales. Hydrobiologia, 1, 149-62. Griffith, J. J. (1919). Influence of mines upon land and livestock in Cardiganshire. J. Agric. Sci. Camb., 9, 366-95. Environmental Unit (University of Liverpool) (1978). Chemical and Biological Changes in the Nant Gwydyr Following Rehabilitation Works. Welsh Development Agency, Cardiff. Johnson, M. S. & Eaton, J. W. (1980). Environmental contamination through residual trace metal dispersal from a derelict lead-zinc mine. Environ. Qual., 9, 175-9. Norris, R. H. (1986). Mine waste pollution of the Molonglo River, New South Wales and the Australian Capital Territory: Effectiveness of remedial works at Captains Flat mine area. Aust. J. Mar. Freshwat. Res., 2, 147-57. Say, P. J. & Whitton, B. A. (1983). Impact of heavy metal waste leachates on a stream in south-west France. Conserv. Recycling, 7, 2-4. Shu, J & Bradshaw, A. D. (1995). The containment of toxic wastes: I. Long term metal movement in soils over a covered metalliferous waste heap at Parc lead-zinc mine, North Wales. Environ. Pollut., 90, 371-7. Smith, R. A. H, (1973). The reclamation of metalliferous Mine workings using tolerant plant populations. PhD Thesis, University of Liverpool. Smith, R. A. H. & Bradshaw, A. D. (1979). The use of metal tolerant populations for the reclamation of metelliferous wastes. J. Appl. Ecol., 6, 595-612.