Geochemistry of Cd in the secondary environment near abandoned metalliferous mines, Wales

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App/kd Gtoehemi.stry, Suppl. Issue No.2, pp. 29-3S, 1993 Printedin Great Britain

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Geochemistry of Cd in the secondary environment near abandoned metalliferous mines, Wales M. PEARCE, NICHOLAS J. G. PEARCE and WILUAM T. PERKINS Geochemistry and Hydrology Research Group, Institute of Earth Studies, University College of Wales, Aberystwyth, Dyfed SY23 3DB , Wales

RONALD FUGE, FIONA

Abstract-Wales, with its long history of metalliferous mining, has many abandonded mines and ore-rich spoil tips, several of which are very extensive. Most of the ore minerals are sulphides, Zn occurring as sphalerite in which Cd is ubiquitous. Soils in the vicinity of the tips contain up to 1000ppm Cd. Analyses of the ZnS reveal Cd values up to 0.52% with ZnlCd ratios ranging from 120 to 665, mean 306. Fine spoil from the area contains up to 1050ppm Cd, the mean ZnlCd ratio of the most metal-rich samples being 75. Surface samples of tip contain high levels of water-soluble Cd but ZnlCd ratios are much higher than those for the whole tip material , suggesting that Zn is preferentially leached out. Adit and spoil drainage waters have Cd levels up to 2.5 mgll while ground waters from the tips contain up to 2.7 mgll. The ZnlCd ratios of these waters are very variable ranging from -20 to > 1000. The ZnlCd ratio in the ground waters is generally <100, while surface and adit drainage waters are mostly in the range 100-250. The differing ratios in surface and subsurface waters probably reflect variable Eh and pH conditions. In addition , it has been found that adit and tip waters which are rich in Fe and tend to precipitate ochre have relatively high ZnlCd ratios , typically between 300 and BOO, while the ochreous precipitates have ratios between 140 and 250. It is suggested that the Cd content of these waters is controlled by oxidation with consequent precipitation of Fe which scavenges Cd. The Cd content of tip drainage waters shows marked seasonal variation with highest values recorded in winter and after summer droughts. However, the ZnlCd ratio of individual streams shows remarkable temporal constancy .

levels would be expected. Several potential contaminant sources of Cd in soils are listed by ALLOWAY (1990) including the application of phosphate fertilisers and sewage sludges; he also listed the effects of present and past Zn and Pb mining and processing as a serious source of soil Cd pollution. There is little data on Cd in the aquatic environment but levels are generally <1 pg/l (MARTIN and WHITFIELD, 1983; CHESTER, 1990); where levels are markedly above this an anthropogenic source is generally indicated. Wales has a long history of metalliferous mining stretching as far back as the Bronze age. The most extensively mined metals were Pb, Zn and Cu , but Au, Ag , Ba , Mn and Fe have also been extracted. While only limited metal extraction has occurred over the last half century, Wales was a major ore producer between the 17th and early 20th centuries. As a result many abandoned mines with their attendant spoil tips litter the old ore-fields and are a continuing source of environmental heavy metal contamination (Fig. 1). Most of the mine workings occurred on steep river valley sides and on the valley floors, and as a result water courses, stream sediments and alluvial plains have been and continue to be contaminated with metals. The chief Zncontaining mineral in the Welsh ore-fields is sphalerite and this provides a major source of Cd pollution in the Principality. The present study sought to outline the behaviour of contaminant Cd from the mine sites in the secondary environment. To do this soils in the vicinity of the mines and waters draining the sites were analysed.

INTRODUCTION

CADMIUM ALONG with Zn and Hg is a member of group 1m of the Periodic Table. The general chemistry of Cd is very like that of Zn , both having very similar electronic structures, electronegativities and ionisation energies (Table 1). Not surprisingly, then, the geochemistries of Cd and Zn are very closely linked (LEVINSON, 1980; KABATA-PENDIAS and PENDlAS, 1984). In general terms the abundance of Cd in crustal rocks is low with only carbonaceous shales and phosphorites showing any marked enrichments (Table 2); Cd is the 63rd element in order of crustal abundance (GREENWOOD and EARNSHAW, 1984). Cadmium, like Zn, is a chalcophile element but GOLDSCHMIDT (1954) suggested that Cd is more so than is Zn. Cadmium forms the sulphide greenockite (CdS), but its main occurrence in a sulphide phase is within sphalerite (ZnS) where it commonly occurs in the range 0.1~.5% (WAKITA and SCHMITT, 1970) but ranges up to 4.4% (LEVINSON , 1980). According to LEVINSON (1980) the levels of Cd in sphalerite within individual deposits appear to be temperature dependent, the highest levels being found in the lowest temperature zones of the deposit. However, IVANOV (1964) suggested that, in addition to temperature, the type of deposit controls the ZnlCd ratio of sphalerite. KABATA-PENDIAS and PENDIAS (1984) quoted the background range of Cd in soils as 0.07-1.1 ppm and suggested that levels >0.5 ppm reflect anthropogenic input. ALLOWAY (1990) suggested that uncontaminated soils should contain <1 ppm Cd except where they are derived from black shales where elevated 29

R. Fuge et al.

30

Table 1. Some chemical properties of Zn and Cd Zn

Property Atomic number Atomic mass Electronic configuration First ionisation energy (kJ/mol) Electronegativity (eV) Ionic radius: 2+ (pm)

Cd

30

65.39 (Ar)3d 104s2 906.1

1.6 74

48 112.411 (Kr)4d105s2 876.5 1.7 95

Data from GREENWOOD and EARNSHAW (1984) and WEAST (19811982). Sphalerite samples from the mid Wales ore field were also analysed and in the case of the Fan site (Fig. 1), the extensive tailings and other fine spoil were analysed. This mine, in common with many others in Wales, was essentially worked as a Pb mine and Zn ore was treated as a waste product, consequently the tips are very rich in Zn and Cd.

METHODOLOGY

Water samples were collected in acid-washed polypropylene bottles and stored in the refrigerator until analysed, analyses always being carried out within 48 h of collection. Initially duplicate samples were collected with one sample being stored as outlined and the other being filtered through a 0.45.um filter and acidified with HN03 . Analytical results revealed data for filtered and unfiltered samples to be identical, within analytical error (PEARCE, 1991). Subsequently, samples were not prepared prior to analysis. Soil samples were collected by screw auger or trowel to a depth of 15 cm by sampling around concentric circles of 1-2 m diameter. Spoil tip material was collected from surface samples using a trowel and also from pits dug to a depth of up to 1 m, Suitable sphalerite samples were collected from the spoil tips. After drying, disaggregation and sieving through a 2 mm mesh, the samples of soil and spoil were evaporated to dryness with a 4:1 mixture of HN0 3 and HCl04 and solubilised material was taken up in 10% HN0 3 • The sphalerite samples were crushed in a tungsten carbide ball mill and were then treated in the same way as the spoil samples. Analyses were performed by atomic absorption spectrophotometry and by inductively coupled plasma-mass spectrometry .

RESULTS AND DISCUSSION

Soils Much data has been published on Cd levels in Welsh soils (Table 3). PAVELEY (1988) showed that the background Cd values for Welsh soils range up to 2.0 ppm with a geometric mean of 0.32, and states that anomalously high values are generally due to previous metalliferous mining and processing. It is apparent from the data in Table 3 that many of the samples from the mineralised areas and the valleys draining them contain anomalously high Cd. Much of this contamination, particularly in the alluvial valley soils, was derived during extraction because much of the mining and milling was carried out during a period when little or no environmental restrictions were imposed. Finely ground ore and gangue were allowed to escape into rivers with the result that large areas of alluvial plains were contaminated, which today provide a vast reservoir of potential heavy metal pollution (DAVIES and LEWIN, 1974; FUGE et al., 1989). Present day contamination of soils is generally due to fine ore-rich tip material (Table 4) being blown and washed out into surrounding areas (DAVIES and WHITE, 1981; JOHNSON et al., 1978), resulting in some extremely high Cd values in the immediate vicinity of the tips (Table 3). While it is likely that appreciable Cd in the contaminated soils is present in sphalerite grains, the range of Zn/Cd ratios in some Welsh soils is markedly

Table 2. General geochemistry of Zn and Cd (all values in ppm)

Cosmic abundance" Crustal abundance Igneous rocks: Felsic Intermediate Mafic Sedimentary rocks: Shales (black shales) Sandstones Carbonates

Zn

Cd

1260 76

1.55 0.16

50 70 100

0.2 0.13 0.15

100 (up to 15(0) 16 25

0.25 (up to 2(0) 0.05 0.07

" Cosmic abundance in atoms per Ilfi atoms Si. Sources of data: ADRIANO (1986), ALLOWAY (1990), GREENWOOD and EARNSHAW (1984), KABATA-PENDIAS and PENDIAS (1984), LEVINSON (1980), MASON and MOORE (1982) and WEDEPOHL (1972).

Cd near abandoned metaJliferous mines, Wales

different from that generally found in sphalerite (Table 4). DAVIES and ROBERTS (1978) suggested that the low ZnJCd ratios (mean 120) of the soils from the NE Clwyd mining district reflect the high Cd content reported in sphalerites of that area (EL SHAZLY et al., 1957). However, it seems likely that the relatively low ratios in other areas of Wales are due to the release of Zn and Cd from sphalerite, with Cd more prone to be adsorbed by the soil. ALLOWAY (1990) discussed Cd adsorbtion in soils and suggested that it is greatest in low acid to slightly alkaline conditions. In addition, Cd has been described by GOLDSCHMIDT (1954) as being more chalcophile than Zn and therefore survives as a sulphide when ZnS is being weathered out. Generally, in the present study, it was found that Cd was more concentrated in surface soils, but in some acidic soils (pH 3.8--4.5) it was concentrated in the B horizon.

Ore and spoil As stated previously the Cd content of sphalerite is commonly within the range 0.1-0.5% (WAKITA and SCHMITT, 1970) and data for sphalerites from the midWales are field show levels to be generally within this range (Table 4). The mean ZnJCd ratio of these sphalerites is close to 300. The corresponding ratio for the Fan spoil tip material analysed is, however, much lower (92), while the most metal-rich samples (>0.5% Zn) have an average ZnJCd ratio of only 75, lower than any ratio for sphalerite recorded in this study. Only two sphalerite samples from the spoil were analysed and these gave ratios of 450 and 310, respectively, which suggests that the Cd content of the Fan are is similar to that found in the rest of the

31

mid-Wales ore field. The relatively high levels of Cd in the tip material are, therefore, somewhat surprising. Most of the spoil samples analysed are from the surface or near surface of the tip and this could be a contributory factor to the low ZnJCd ratio. The tip area in question is entirely composed of fine waste left after mineral processing. During periods of dry weather this tip complex develops a white coating, described by JOHNSON et al. (1978) as crystalline zinc sulphate. Analysis of this material showed it to contain 35% Zn and 0.1% Cd in a water-soluble form. Indeed, in the immediate surface layer of the tip most of the Zn appears to be in a water-soluble form, while <40% of the total Cd is water soluble (Table 4). This suggests that the ZnS is being preferentially weathered to the very soluble sulphate, while CdS is being weathered to a lesser extent, which is in accordance with the observation by GOLDSCHMIDT (1954) that Cd is more chalcophile than Zn. The same author comments that greenockite tends to form a coating on sphalerite during weathering. In this context spacial analyses were carried out, using laser ablation inductively coupled plasma-mass spectrometry, of a sphalerite sample from the tip. This reveals that while the ZnJCd ratio (~520) is fairly constant in the unweathered main mass of the sample, the ratio drops markedly to 320 in the outer regions, while on the surface the ratio was 230. The Fan mine began to produce are in 1866and production peaked between 1870 and 1880, the mine being abandoned in 1921 (MORRISON, 1971). Thus the very fine tip material, still totally non-vegetated, has been exposed to weathering for >70 a. It is likely, then, that during this time Zn has been preferentially weathered out leaving the surface area enriched in Cd. In this context it was also found that sphalerite contains small amounts of water-leachable Zn and Cd.

Waters 1. Rheidol veney 2. Vltwyth veney 3. Fen 4. Llengynog 5. Gwynfynedd

~ARLECH DOME ·Au + Cu. Pb. Zn

o

30

>-------< km

WAL•• MAIN 0... PI.LDS

FIG. 1. The major ore fields of Wales.

ABDULLAH and ROYLE (1972) and FUGE et al. (1991) showed that rivers draining mineralised areas of Wales contain elevated levels of Cd. A background level of 0.41 .ug/l for uncontaminated river waters from the region was proposed by ABDULLAH and ROYLE (1972). All of the samples analysed in the present study exceed this value. Data have been collected for many mineralised sites in Wales (Fig. 1) with Cd and Zn levels in tip runoff being determined at monthly intervals for several of them. The data for two Pb/Zn mines are shown in Fig. 2; one, Llangynog, in the Berwyn Dome is in an area of lower Palaeozoic clastic sedimentary and volcanic rocks; the other in NE Clwyd is hosted by Carboniferous Limestone. Data from a Au mine, Gwynfynedd, in the Harlech Dome are also presented in Fig. 2. The data for Cd and Zn from samples collected on a monthly basis showed very constant ZnJCd ratios over time, the levels being of

R. Fuge et al.

32

Table 3. Zn and Cd in surface soils, Wales Zn Cd Range and mean (ppm)

Area (source of data) Whole of Wales (PAVELEY, 1988)

4.7-2119 83 (n = 824)

0-15 0.60

Alluvial soils, Ystwyth valley (ALLOWAY and DAVIES, 1971)

95-9810 455 (n = 17)

1.2-4.0 2.5

Alluvial soils, Ystwyth valley (present study)

120-7050 571 (n = 42)

0.5-8.8 2.6

Alluvial soils, Rheidol Valley (DAVIES and LEWIN, 1974)

242-630 372 (n = 16)

0.1-3.5 0.83

Alluvial soils, Rheidol valley (present study)

90-1050 231 (n = 36)

0.1-5.2 1.7

Alluvial soils, Conwy valley (ALLOWAY and DAVIES, 1971)

114-400 226 (n = 8)

0.8-2.5 1.8

Alluvial soils, Llangynog area (FuGE et al., 1989)

28-4800 445 (n = 67)

0.1-450 7.6

NE Clwyd mining district (DAVIES and ROBERTS, 1978)

10-49393 728 (n = 260)

0.4-540 6.1

Heavily contaminated soils around tip areas, mid Wales (present study)

200-12100 1250 (n = 41)

1.0-980 11.6

the order of 150 for the Berwyn Dome site, 180 for the NE Clwyd site, and 220 for the Harlech Dome site. The data show that there are marked peaks of Cd and Zn concentration during winter, which seem to be related to greater flushing effects. Summer highsfollowperiods of dry weather with resultant ore mineral oxidation and subsequent washout. These observations are in line with those found for Zn by GRIMSHAW et al. (1976) for the Yswyth River in mid Wales. The constancy of the Zn/Cd ratio would not be expected in view of the relatively high content of soluble Zn found in the surface of the Fan tips; it might be anticipated that the ratio would vary greatly, particularly after periods of drought when

much Zn would be expected to be solubilised. While data for Fan coveringthe same period of time are not available, data collected on a regular basis from February to July 1991 show that the main drainage stream at this site exhibits a narrow range of ratios, 111-153. Similarly, the ratios in surface tip and adit drainage at this mine seem to be fairly constant through time. The lower than expected Zn/Cd ratio of the surface drainage may reflect the sampling scheme. GRIMSHAW et al. (1976) found that during storm events followingdrought, the initial runoff causes a flushing effect of just a few hours duration, followed by a dilution effect. Therefore, the flushing of the more soluble Zn could occur during the initial stages of the

Table 4. Zn and Cd in ore and tip material Ratio ZnlCd

Material (No. of samples)

Zn Cd Range and mean (ppm)

Sphalerite, mid Wales ore field (48)·

-63%

950-5200 2060

120-665 306

400-10500 6500

0.25-1050 72

45-1200 92

5000-105000 20550

62-1050 275

45-110 75

9500

26

365

Surface layer of tip, top 2 ern (2)

26100, 26300 26200

312,376 344

70,84 76

Surface layer, cold water soluble (2)

23200, 25000 24100

126,137 132

182, 184 183

White powdery material coating surface of tip, cold water soluble

350000

1020

344

Sphalerite from tip, cold water soluble (2)

51, 162

0.2,0.64

253,254

Tailings and waste, Fan mine, mid Wales (120) Most contaminated material from above (39) Sandy layer, base of tip

"Includes data from KAKAR (1971).

Cd near abandoned metalliferous mines, Wales

rainfall when there would also be a high Zn/Cd ratio which would subsequently drop fairly rapidly. Thus to observe any large Zn Bush would require samples to be collected at short intervals during a rainfall event. While all surface and adit drainage waters analysed in the present study, including those from Fan, have Zn/Cd ratios> 100, samples of subsurface tip drainage from Fan are relatively rich in Cd (Table 5) with an average ratio (77) similar to that found in the spoil. The subsurface waters are more acid than the surface waters and this may cause greater mobility of Cd. In addition, it is likely that in the fine-grained, sulphiderich tip, Eh conditions are much less oxidizing than on the surface. The solubility of CdS is 1.3 mg/l, +

double that of ZnS (0.65 mg/l; WEAST, 1981-1982), and it is possible that at least some of these metals may be in sulphide form in solution. Soit appears that the solution chemistry of Cd in the subsurface waters may reflect control by Eh and pH. Another feature of Cd chemistry in mine and spoil drainage is demonstrated in Table 6. In waters from mines and spoil where ochre is precipitating, the Cd levels are much lower than in non-ochreous waters with Zn/Cd ratios up to 760. The ochre precipitates found in these streams have Zn/Cd ratios that are much lower than those in the waters, ranging from 140 to 250; it seems likely that the Cd levels of these waters are controlled by the precipitation of the ochre. While many of the ochre-precipitating streams

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R. Fuge et al.

34

Table 5. Zn and Cd in drainage, Fan mine Zn Locality (No. of samples)

Cd (ugll)

ZnlCd ratio

pH

Adit 1 (4)

22000-50600 32530

160-198 177

138-205 183

5.4-5.6 5.5

Adit2 (4)

58000-146 000 90000

451-831 587

129-176 153

4.7-5.2 5.0

Surface tip drainage (8)"

13470-128 820 48366

96-845 351

108-152 138

5.8-6.1 6.0

Subsurface tip drainage (17)"

137-297000 40070

2-2700 518

12-356 77

3.2-5.6 4.9

Main drainage stream 0.5 km below tips (8)

3051-9025 5900

23-59 46

111-153 128

6.2~.5

6.3

"Subsurface data are for samples collected from 17 different sites over a 2 d period. Surface data are for 2 sites collected at 4 different times of the year. All other data are for single sample sites collected at different times of the year.

the very Fe-rich waters were released, with the amount subsequently declining with time.

are very acid, with pH values as low as 2.7, some are only mildly acid and it is unlikely that pH is a controlling influence on the Cd content of these waters. It is probable that a relatively high Eh causes precipitation of the ochre and it is this which controls the Cd content of the waters. In the case of the Gwynfynedd Gold mine adit (Fig. 1), mining was carried out until comparatively recently. After cessation of this activity late in 1989, there appears to have been a build up of highly acid ochreous water in the blocked up mine. This resulted in highly contaminated water bursting from the mine during spring, 1990. Samples of this water were analysed soon after the burst occurred and monthly samples have been collected since this time (Table 7). The Cd content of these waters decreased by -50% during the course of a year while Zn contents decreased by -80% during the same period. The Zn/Cd ratio decreased markedly during this time from > 1000 to -400. Two samples of ochre from this stream bed had Zn/Cd ratios of 160and 140, respectively. It seems likely that the decreasing Zn/Cd ratio through time in this adit drainage is the result of the initial precipitation of a large amount of ochre when

CONCLUSIONS

1. Sphalerite in old mine tips is a major source of environmental contamination in Wales. Soils in the vicinity of these tips are heavily polluted by wind and water-bourne ore material. 2. Because Cd is more chalcophile than Zn it tends to persist as a sulphide while ZnS is weathering to the very soluble sulphate. This results in Cd being concentrated in the outer regions of weathered sphalerite fragments and similarlyleads to Cd being enriched relative to Zn in fine-grained tip material. In soils the greater chalcophile nature of Cd could, in part, explain its relative immobility in surface soils contaminated by Zn mine waste. 3. Despite its chalcophile nature, some Cd is released from sphalerite on weathering, probably as CdS04 • Thus surface tip and adit drainage is rich in Cd, with ZnlCd ratios generally much higher than those found in the spoil. Surface tip run off shows a

Table 6. Zn and Cd in acid mine and spoil waters, where ochre is precipitating Zn Locality (No. of samples)

Cd (ugll)

ZnlCd

pH

Ystwyth valley Adit (4) Spoil drainage (4)

21100 5836

28 14

756 417

6.1 4.9

12730 82830 777000

33 109 2530

386 760 307

3.5 2.9 2.7

25700

38

676

5.5

Rheidol valley Adit 1 (4) Adit2 (4) Spoil drainage (3)

Fanmine Ochreous pool (2)

Cd near abandoned metalliferous mines, Wales Table 7. Temporal variation of Zn and Cd in drainage, Gwynfynedd gold mine Zn Date May 1990 June 1990 July 1990 August 1990 September 1990 October 1990 November 1990 December 1990 January 1991 February 1991 March 1991 April 1991 May 1991

Cd

(ugll) 15750 15030 9950 11190 10 920 3785 4970 4790 4420 2980 2910 3040 3450

11.4 15.5 12.0 11.9 11.9 9.5 13.7 15.6 12.4 6.9 6.5 6.7 8.4

Zn/Cd

pH

1381 970 829 940 918 398 363 307 356 432

3.4 3.6

448 454 411

4.3 3.6 3.3 3.7 3.7 4.4 3.9 4.2 4.3 3.3

marked temporal constancy of ZnJCd ratios for individual sites. 4. Subsurface drainage from fine-grained tip material has a markedly lower ZnJCd ratio than surface and adit drainage in the area, due probably to increased acidity and decreased Eh. In these waters the greater solubility of CdS is likely to be an important factor. 5. Eh appears also to be controlling the Cd content of ochre-precipitating adit and tip drainage waters. Higher Eh results in the precipitation of Fe with consequent scavenging of Cd. Acknowledgements-The authors are grateful to Mr Andrew Nash for providing some of the analytical data for the Fan mine spoil material and to Mr M. Ap Owain Ingram for providing invaluable background information on the Fan area. Editorial handling: Brian Hitchon.

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35

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