Environmental Geochemistry and Health, Vol. 12, page 291
Heavy metals in soils in north Somerset, England, with special reference to contamination from base metal mining in the Mendips Brian E. Davies 1 and Rhoda C. Ballinger 2 I Department of Environmental Science, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK 2 Present address: Department of lvlaritime Studies, University of Wales College of Cardiff, PO Box 907, Cardiff, CF1 3YP, UK Abstract
A 2 x 2 km grid survey of surface soils was conducted over 600 km of North Somerset, England, to investigate the role of pollution from former base metal mines on soil heavy metal content. Soil contents of Pb, Zn, Cu and Cd were determined by atomic absorption spectrometry after extraction in hot, concentrated nitric acid. Most of the soils were contaminated by lead, zinc and cadmium, isoline maps and perspective block diagrams showed this contamination to be most severe on the Mendip plateau, especially in the vicinity of Shipham, Wells and Priddy districts. Resurgences, polluted from mine drainage, also contaminated soils at the base of the Mendip slopes and, over the lowlands, trace element enriched Liassic shales caused significant local anomalies.
Introduction Lead mining in Great Britain is of ancient origin and dates back to at least the Roman Occupation. The methods used for the separation and concentration of lead and other non-ferrous metal ores before froth flotation was invented, earlier this century, were inefficient. They relied on a simple, gravity-based separation in water (jigging) and the effluents from the jigs were grossly contaminated with metal solutes and fine ore particles. Prior to the Rivers Pollution Act of 1876, these effluents were discharged to waterways without treatment. As a consequence of flooding alluvial soils became contaminated by lead, zinc and copper, the main ore metals and also cadmium and ores. Non-regulated dumping of mine wastes and fallout from smelter stacks also contributed to wide-spread soil contamination. In most parts of Great Britain lead mining had declined or ceased by the early decades of this century. But pollution still continues through the action of wind, or subsurface and surface drainage and groundwater, on old mine spoil and through discharge from mine drainage adits. It has been estimated that over 4,000 km2 of England and Wales, including over 1,000 km2 of north Somerset, have been affected (Thornton, 1983). Investigations have demonstrated enhanced levels of lead, zinc, cadmium and other heavy metals in the surface soils of the Tamar Valley, north east Clwyd and Derbyshire mining districts (Davies and Roberts, 1975, 1978; Thornton, 1983). In north Somerset, in the vicinity of Shipham, soils have been shown to contain over 20 mg g-l of cadmium and extremely elevated zinc concentrations (Thornton et al., 1980). This paper presents the results of a general survey of surface soils of north Somerset. The objectives were to
establish the concentrations of heavy metals in the soils of the area, to compare the soil metal contents with those of similar areas and to describe the extent of contamination arising from lead and zinc mining in the Mendips through the preparation of distribution plots. The paper reports results for pH, organic matter, Pb, Zn, Cu and Cd contents.
Tile Study Area The study area (Figure 1) covered 600 km2 of north Somerset and Avon and included part of the former Mendip lead/zinc mining district. Its boundary was defined by the Ordnance Survey (OS) grid coordinates (ST) 3400 3600, 3600 3600, 3400 3300 and 3600 3300. The area is underlain by a variety of sediments from Devonian to Recent age. The Carboniferous Limestone ridge of the Mendips is surrounded by undulating lowlands of Triassic dolomitic conglomerates, red marls and sandstones and, to the south, rising above the peaty Somerset Levels, are conspicuous hills, such as the Isle of Wedmore, composed of Liassic strata which include the Lower Liassic Shales (black shales). This facies is responsible tbr various trace element disorders in animals including molybdenum-induced bovine hypocuprosis (Le Riche, 1959; Thomson et al., 1970; Thornton et al., 1969). Detailed descriptkms and maps of the soils of the area have been published by Avery (1955) and Findlay (1965). Lodes of galena (PbS), the most widespread and principally mined lead ore, occur in the Old Red Sandstone, Carboniferous Limestone and Triassic Dolomitic Conglomerate. Particularly rich deposits were located near Priddy and Charterhouse. The zinc mineral smithsonite (ZnCO3), locally known as calamine, formed in the Dolomitic Conglomerate producing important ore
292
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Figure 1 The north Somerset study area. resources around Shipham and Rowberrow. The orefield is thought to have extended over 135 km2 and to have produced around 2 x 103 tonnes of lead and much zinc. Gough (1967) has written a detailed account of the history of the Mendip mining and smelting operations which have left a legacy of disturbed gruffy ground over the Mendip plateau. Sporadic exploitation of the ores has occurred since Roman times, with bursts of activity in the early seventeenth and the mid and late nineteenth centuries. These periods correspond to the prime of the British lead mining industry. There has also been extensive resmelting of old slag containing up to 20% lead. The lead industry was concentrated at four central mineries, namely Priddy, Charterhouse, Chewton--onMendip and East Harptree where there was a reliable supply of water for ore dressing; zinc extraction was confined to the environs of Shipham, Rowberrow, Burrington, Winscombe and East Harptree.
Methods For the general survey, 174 topsoils (0-15 cm) were
collected on an approximately regular 2 • 2 km grid at National Grid intersections (Figure 2a). Additional sampling (Figure 2b) was later undertaken over the former mining grounds and in localities where anomalous soil contents had been revealed from the interpretation of the data from the grid survey. Each composite sample of 1 kg weight, comprising core samples obtained with a mild steel auger, was stored in a labelled polythene bag. in the laboratory the soils were left to dry at room temperature for between 12 to 24 h, gently disaggregated using an acid-cleansed porcelain mortar and pestle and then sieved through a 2 mm square aperture nylon mesh to yield the 'fine earth fraction' which was stored lbr analysis. Soil reaction was estimated potentiometrically after equilibration with 0.01 M calcium chloride solution. The amount of organic matter in the soils was approximated by a gravimetric determination (loss-on-ignition) after ignition lot 12 h at 430~ extraction of 'total' lead, zinc and cadmium followed a procedure adapted from Warren and Delavault (1960). Duplicate samples of soils were digested in hot, concentrated nitric acid prior to dilution and analysis for 'total' metal contents by atomic
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294
Heavy metals in soils
absorptiony spectrophotometry using a double beam instrument ( P y e - U n i c a m SP2900) with deuterium background correction. Precision of the determinations was checked by calculating the mean coefficient of variation (relative standard deviation x 100). For pH this was 2.9%, for ignition loss it was 3.2% and for the metals Pb, Zn, Cu and Cd the values were 2.1%, 2.2%, 1.7% and 1.8%, respectively. Accuracy was checked by analysing samples of NBS fiver sediment. Mean recoveries for Pb, Zn, Cu and Cd were 98.5%, 102.9%, 80.0% and 78.0% The EEC calcareous loam (another standard reference material) yielded recoveries of Pb = 96.6%, Zn = !00.8%, Cu = 90.8% and Cd = 116.7%. Results and Discussion
Summary of the grid survey data The analytical data are summarised in Table 1. Inspection of the results shows that, except for pH, the arithmetic means are greater than the medians and, for Pb and Zn, markedly greater. The skewness statistics also demonstrate departures from symmetry: only for pH is the statistic close to zero. This positive skewing is a consequence of some high data values weighting the mean and is often seen where soils are contaminated. Parametric statistical tests r e q u i r e the data to be n o r m a l l y d i s t r i b u t e d and normalisation is often done by a log (common or natural) transformation of the data. Here, the data were log10 transformed (except, of course for pH which is a log value). The resulting geometric means are similar to or much nearer the medians and the skewness statistics have decreased in value indicating that the transformation was relatively successful. From the normalised data the 95% probability range was calculated. This is a more robust summary of the spread of the results since the distorting effects of the few very high values are minimised.
Soil pH and organic content T h e wide r a n g e of v a l u e s for soil r e a c t i o n and loss-on-ignition (Table 1) was expected because of the variety of parent materials and soils within the region. About a third (66/174) of the samples were neutral to slightly alkaline in soil reaction. These included the Brown Earths on the north Mendip slopes and the calcareous soils of the Liassic lowlands. Within the remaining large group of acidic soils were a number (23) with a pH below 5. These were the lowland peaty soils of the ~dgemoors (pH 3.9, Godney Moor: OS 3480 3440) and those derived from the quartzitic sandstones over the higher Mendip Plateau (pH 3.3, Priddy: OS 3560 3541). For low lying ground water gleys mad peaty gleyed podzols over the sandstone cores of the Mendip Plateau organic contents Iay between 20% and 30%. Soils containing more than 40% were those of the peaty, organic lowland sedgemoors. These simple pedological parameters are comparable with those quoted for the two available descriptions of the soils of the area (Avery, 1955; Find.lay, 1965), Except for soils occurring over the disturbed, gruffy, ground the soils sampled during this survey are, pedologically, typical of the area.
Heavy metals in survey soils and comparisons with other areas There are few reliable data for background metal concentrations in British soils. Most reports have simply summarised published values without any attempt at statistical evaluation. The most comprehensive of recent summaries is that of Ure and Berrow (1982). They calculated worldwide averages for metals; for the metals discussed here, Pb = 29.2 mg kg -I, Zn = 59.8 mg kg -1, Cu = 25.8 mg kg-1 and Cd = 0.62 mg kg-~. Reaves and Berrow (1984) summarised lead concentrations for 3,944 samples from 896 Scottish soil profiles. The (geometric) mean content of all samples was 14 mg Pb kg-1 and for
Table 1 Summary of data for 174 surface soil samples from a general survey in north Somerset.
Arithmetic values: Mean Median Standard deviation Minimum Maximum Skewness Loglo transformed values: Mean Standard deviation 95% probability (low) " (high) Skewness LOI = loss-on-ignition.
pH
LOI (%)
Pb
5.9 5.9 0,86 3.3 7.3 --0.3
16.2 11.6 15.0 3.0 83 2.71
183 52 798 8 (1.0%) 11.7
12.7 11.6 3.6 44 1.0
66 3.0 7.6 577 1.3
-
Zn Cu (rag kg-j dry soil) 267 132 668 14 8,344 10.5
149 2.5 24 915 0.83
15 12 13 2.8 145 6,7
12 [.8 3.9 40 0.18
Cd
3.2 2,1 9.6 0.5 127 12.5
2.3 t .7 0.8 6.6 2.9
B. E. Davies and R. C. Ballinger Table 2 Comparative data for 288 soil samples from north Wales (Davies and Roberts, 1978) and 97 samples from SE Missouri (Davies and Wixson, 1985).
Pb
Zn Cu (mg kg-1 dry soil)
Cd
North Wales: Mean Geonaetric mean Minimum Maximum
886 234 35 (4.8%)
728 153 10 (4.9%)
18 14 2.3 252
6.1 1.4 0.4 540
SE Missouri: Mean Geometric mean Minimum Maximum
141 60 10 2,200
54 42 14 142
13 12 1.5 29
0.4 0.4 0.3 3.1
456 A horizon samples it was 17 mg Pb kg -1. In Pembrokeshire, Wales, Wilkins (1978) reported a mean content of 39 nag Pb kg-t for some 500 samples of surface soil. Davies (1983) e s t i m a t e d b a c k g r o u n d lead concentrations in soils from England and Wales using the graphical approach of Sinclair (1974): the data used included values from north Somerset. It was concluded that lead contents were unlikely (p
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298
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B. E. Davies and R. C. Ballinger Table 4 Summary of the analytical data for soils from a follow-up survey.
Pb
Zn Cu (mg kg -1 dry soil)
Cd
Area 1: Mean Median Standard deviation Minimum Maximum Geometric mean
29,462 24,032 32,623 215 87,414 5,553
2,995 436 4,029 131 10,066 844
37 30 30.7 6.3 111 25
5.8 2.7 8.36 1.0 34 3.4
Area 2: Mean Median Standard deviation Minimum Maximum Geometric mean
14,722 513 23,186 85 61,210 1,441
2,947 793 3,771 107 10,369 1,058
36 19 31.4 5.3 98 24
t1 3.4 t7.2 0,6 75 4.6
Area 3: Mean Median Standard deviation Minimum Maximum Geometric mean
150 102 160 49 561 111
1,349 134 3,637 59 11,047 214
21 15 13.3 9 52 18
1.5 1.3 0.89 0.5 3.6 1.3
Number of samples in each area: Area I = 15, Area 2 = 21, Area 3 = 9.
discounted, the village being tiny and remote. Shapwick lies on the slopes of the Polden Hills and Avery (1955) mentions the area as being one where molybdenosis in cattle (teart) is endemic because of the high molybdenum content of the rock. Molybdenum is chalcophilic. The area of elevated soil copper values which are to be seen in the south eastern part of the study area (Figures 3c and 4d) overlie an extensive area of mineral soils with impeded drainage. Avery's (1955) soil map depicts part of this area as being occupied by the Charlton Bank and Fladbury series, both of which are developed in either lower Liassic rocks or in a parent material dominated by this material, and teart is endemic here where soil pH is neutral or alkaline because of naturally high molybdenum concentrations. It is suggested that where soils contain higher concentrations of molybdenum originating in the sulphide phases of the Liassic rocks they are also likely to contained elevated contents of other heavy metals.
Detailed Follow-up Sampling The computer graphics discussed above show strong anomalies in several parts of the study area. These could arise from a single metal-rich sample at one grid intersection. The software used for these plots attempts to minimise this possibility when forming the interpolated
299
regular grid from the input data prior to isolining. This is done by searching in the intersection area for other points and then interpolating by inverse weighting using distance-squared. Thus an isolated very low or very high point's effect will be minimised. Nonetheless it is wise to carry out further sampling in anomalous areas to confirm the anomaly. This was done and the distribution of the second set of samples is shown in Figure 2b. Three areas were then defined: (1) The Wells area. This includes the major lead anomaly seen in Figure 4b. The coordinates of the south west and north east corners of the subsidiary area are 3500 3460 and 3600 3530. (2) The Shipham and environs area. This covers the main cadmium area of the north west part of the study area. It is bounded by coordinates 3400 3500 and 3500 3600. (3) The lowlands. Samples were taken from the higher land within the lowland peats. The area is bounded by 3400 3300 and 3600 and 3400. Samples lying within each area were separated from the others and summarised statistically: results are given in Table 4. In area 1 (Wells) there were 15 soil samples and their composition confirms that this area is characterised by a major lead anomaly. Zinc concentrations are also elevated together with cadmium. But copper concentrations are not especially high. In area 2 (Shipham) soil lead concentrations are lower but zinc and cadmium contents are higher than in area 1. These data are consistent with literature reports that this subsidiary area is particularly contaminated with cadmium. Finally, area 3 (Lowlands) encloses minor hills where soils in many localities are formed on the Liassic black shales. Lead, zinc and cadmium contents are much lower than on the Mendip plateau. Nonetheless they are relatively high compared with values quoted earlier in the discussion for normal soils. It is concluded that a strong parent material effect is operating here and natural geochemical anomalies have been created.
Conclusions This survey of soils in north Somerset has provided data for pH and organic matter together with concentrations of lead, zinc, copper and cadmium. Overall, the metal contents are higher or much higher than would be expected for uncontaminated soils. Subsequent follow up sampling has confirmed the results from the general survey. Isoline plots of the soil metal concentrations and perspective block diagrams illustrated the distributions of lead, zinc and cadmium in this area. Soils over the Mendip Plateau generally possess enhanced lead, zinc and cadmium concentrations. These areas of high metal concentration are contiguous with the main areas of past mineral exploitation and are presumed to have arisen through industrial contamination. The area to the north of Wells has emerged as having a particular lead problem while the unusual cadmium pollution in and around Shipham has been confirmed. In the rest of the area contaminated resurgences on the flanks of the Mendips and
300
Heavy metals in soils
trace element enriched Liassic parent materials are responsible for smaller, but significant anomalies. The metal concentrations and distributional patterns are similar to those previously reported in lead mining/limestone areas of south-east Missouri and north--east Wales. The data suggest that the Mendips area is less severely contaminated than the Halkyn Mountain area of north Wales, although localised accumulations, especially of cadmium, are more intense. Similarly, the area is more contaminated than the Madison county mining area of Missouri. Acknowledgments The experimental work was carried out in the laboratories of the Department of Geography at the University College of Wales in Aberystwyth where Rhoda Ballinger (nee Ginnever) was a research student during 1977-1980 supported by a University of Wales studentship for which she was most grateful. Brian Davies was on the staff of that department until 1985, and thanks his then colleagues for their support. The interpretive work for this paper was done at the University of Bradford. References Anderson R.J., Davies B.E., Nunn J.H. and James P.M.C. 1979. The dental health of children from five villages in north Somerset with reference to environmental cadmium and lead. British DentalJ., 147, 159-161. Avery B.W. 1955. The Soils of the Glastonbury District of Somerset. HMSO, London.Davies B.E. and Ginnever R.C. 1979. Trace metal contamination of soils and vegetables in Shipham, Somerset. J. Agrieult. Sei. (Cambridge), 9d3,753-756. Davies B.E. 1983. A graphical estimation of the normal lead content of some British soils. Geoderma, 29, 67-75. Davies B.E. and Roberts L.J. 1978. The distribution of heavy metal contaminated soils in north east Clwyd, Wales. Water, Air Soil PoUut., 9, 507-518. Davies B.E. and Roberts L.J. 1975. Heavy metals in soils and radish in a mineralised area of Wales, Great Britain. Science Total Environ., d4, 249-261. Davies B.E. and Wixson B.G. 1985. Trace dements in surface soils
from the mJneralised area of Madison county, Missouri, USA,
J. Soil Sci., 36, 551-570. f;indlay D.C. 1965. The soils of the Mendip district of Somerset. ttarpenden: Soil Survey of England and Wales. Gough J.W. 1967. The Mines of Mendip. David and Charles, Newton Abbot. Green G.W. 1958. Mendip Lead-Zinc Orefield: Bulletin Geological Survey of Great Britain No. I4. HMSO, London. Le Riche H.H. 1959. Molybdenum in the lower liass of England and Wales in relation to the incidence of teart, J. Soil Sci., 10, 133. Reaves G.A. and Berrow M.L. 1984. Total lead concentrations in Scottish soils. Geoderma, 32, 1-8. Sherlock J.C., Smart G.A,, Walters B., Evans W.H., McWeeny D.L and Cassidy W. 1983, Dietary surveys on a population at Shipham, Somerset, United Kingdom. SeL Total Environ., 29, 121-142. Sinclair A.J. 1974. Selection of thresholds "in geochemical data using probability graphs. J. Geochem. Exploration, 3, 129--149. Stenner R.D. 1977. The concentration of some heavy metals in sediments in some Mendip caves. Proc. Seventh Intnl Speliological Conf., Sheffield (Septen~ber),383-384. Stenner R.D. 1978. The concentration of cadmium, copper, lead and zinc in sediments from caves and associated surface streams on Mendip, Somerset. Trans. British Cave Res. Assoc., 5, 113-120. Thomson, L, Thornton, I. and Webb, J.S. 1972. Molybdenum in black shales and the incidence of bovine hypocuprosis. J. Sci. Food Agriculture, 23, 879-891. Thornton, I. 1983. Geochemistry applied to agriculture. In: Thornton, I. (ed.), Applied Environmental Geochemistry. Chapter 8, pp.231-26@ Academic Press Geology Series, London, Thornton L, Moon R.N.B. and Webb J.S. 1969. Geochemical reconnaissance of the lower lias. Nature (London), 221, 457-459. Thornton I., Moorcroft J.S. and Watt J. 1980. Cadmium at Shipham a unique example of environmental geochemistry and health, in: Hemphill, D.D. (ed.), Trace Substances in Environmental Health. Vol.26, pp.27-37. Ure A.M. and Berrow M.L. 1982. The elemental constituents of soils. In: Bowen, H.J.M. (ed.), Environmental Chemistry, Vol.2. The Royal Society of Chemistry, London. Warren H.V. and Delavault R.E, 1960. Aqua regia extractable copper and zinc in plutonic rocks in relation to ore deposits. Trans. Inst. Mining and Metallurgy, 69, 495-504. Wilkins C. 1978. The distribution of lead in the soils and herbage of west Pembrokeshire. Environ. Pollut., 15, 23-30, (Manuscript No.227: received December 18, 1989; fully reviewed; accepted for publication after modification July 9, 1990.)
Heavy metals in soils in north Somerset, England, with special reference to contamination from base metal mining in the Mendips Brian E. Davies 1 and Rhoda C. Ballinger 2 I Department of Environmental Science, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK 2 Present address: Department of lvlaritime Studies, University of Wales College of Cardiff, PO Box 907, Cardiff, CF1 3YP, UK Abstract
A 2 x 2 km grid survey of surface soils was conducted over 600 km of North Somerset, England, to investigate the role of pollution from former base metal mines on soil heavy metal content. Soil contents of Pb, Zn, Cu and Cd were determined by atomic absorption spectrometry after extraction in hot, concentrated nitric acid. Most of the soils were contaminated by lead, zinc and cadmium, isoline maps and perspective block diagrams showed this contamination to be most severe on the Mendip plateau, especially in the vicinity of Shipham, Wells and Priddy districts. Resurgences, polluted from mine drainage, also contaminated soils at the base of the Mendip slopes and, over the lowlands, trace element enriched Liassic shales caused significant local anomalies.
Introduction Lead mining in Great Britain is of ancient origin and dates back to at least the Roman Occupation. The methods used for the separation and concentration of lead and other non-ferrous metal ores before froth flotation was invented, earlier this century, were inefficient. They relied on a simple, gravity-based separation in water (jigging) and the effluents from the jigs were grossly contaminated with metal solutes and fine ore particles. Prior to the Rivers Pollution Act of 1876, these effluents were discharged to waterways without treatment. As a consequence of flooding alluvial soils became contaminated by lead, zinc and copper, the main ore metals and also cadmium and ores. Non-regulated dumping of mine wastes and fallout from smelter stacks also contributed to wide-spread soil contamination. In most parts of Great Britain lead mining had declined or ceased by the early decades of this century. But pollution still continues through the action of wind, or subsurface and surface drainage and groundwater, on old mine spoil and through discharge from mine drainage adits. It has been estimated that over 4,000 km2 of England and Wales, including over 1,000 km2 of north Somerset, have been affected (Thornton, 1983). Investigations have demonstrated enhanced levels of lead, zinc, cadmium and other heavy metals in the surface soils of the Tamar Valley, north east Clwyd and Derbyshire mining districts (Davies and Roberts, 1975, 1978; Thornton, 1983). In north Somerset, in the vicinity of Shipham, soils have been shown to contain over 20 mg g-l of cadmium and extremely elevated zinc concentrations (Thornton et al., 1980). This paper presents the results of a general survey of surface soils of north Somerset. The objectives were to
establish the concentrations of heavy metals in the soils of the area, to compare the soil metal contents with those of similar areas and to describe the extent of contamination arising from lead and zinc mining in the Mendips through the preparation of distribution plots. The paper reports results for pH, organic matter, Pb, Zn, Cu and Cd contents.
Tile Study Area The study area (Figure 1) covered 600 km2 of north Somerset and Avon and included part of the former Mendip lead/zinc mining district. Its boundary was defined by the Ordnance Survey (OS) grid coordinates (ST) 3400 3600, 3600 3600, 3400 3300 and 3600 3300. The area is underlain by a variety of sediments from Devonian to Recent age. The Carboniferous Limestone ridge of the Mendips is surrounded by undulating lowlands of Triassic dolomitic conglomerates, red marls and sandstones and, to the south, rising above the peaty Somerset Levels, are conspicuous hills, such as the Isle of Wedmore, composed of Liassic strata which include the Lower Liassic Shales (black shales). This facies is responsible tbr various trace element disorders in animals including molybdenum-induced bovine hypocuprosis (Le Riche, 1959; Thomson et al., 1970; Thornton et al., 1969). Detailed descriptkms and maps of the soils of the area have been published by Avery (1955) and Findlay (1965). Lodes of galena (PbS), the most widespread and principally mined lead ore, occur in the Old Red Sandstone, Carboniferous Limestone and Triassic Dolomitic Conglomerate. Particularly rich deposits were located near Priddy and Charterhouse. The zinc mineral smithsonite (ZnCO3), locally known as calamine, formed in the Dolomitic Conglomerate producing important ore
292
Heavy metals in soils
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Figure 1 The north Somerset study area. resources around Shipham and Rowberrow. The orefield is thought to have extended over 135 km2 and to have produced around 2 x 103 tonnes of lead and much zinc. Gough (1967) has written a detailed account of the history of the Mendip mining and smelting operations which have left a legacy of disturbed gruffy ground over the Mendip plateau. Sporadic exploitation of the ores has occurred since Roman times, with bursts of activity in the early seventeenth and the mid and late nineteenth centuries. These periods correspond to the prime of the British lead mining industry. There has also been extensive resmelting of old slag containing up to 20% lead. The lead industry was concentrated at four central mineries, namely Priddy, Charterhouse, Chewton--onMendip and East Harptree where there was a reliable supply of water for ore dressing; zinc extraction was confined to the environs of Shipham, Rowberrow, Burrington, Winscombe and East Harptree.
Methods For the general survey, 174 topsoils (0-15 cm) were
collected on an approximately regular 2 • 2 km grid at National Grid intersections (Figure 2a). Additional sampling (Figure 2b) was later undertaken over the former mining grounds and in localities where anomalous soil contents had been revealed from the interpretation of the data from the grid survey. Each composite sample of 1 kg weight, comprising core samples obtained with a mild steel auger, was stored in a labelled polythene bag. in the laboratory the soils were left to dry at room temperature for between 12 to 24 h, gently disaggregated using an acid-cleansed porcelain mortar and pestle and then sieved through a 2 mm square aperture nylon mesh to yield the 'fine earth fraction' which was stored lbr analysis. Soil reaction was estimated potentiometrically after equilibration with 0.01 M calcium chloride solution. The amount of organic matter in the soils was approximated by a gravimetric determination (loss-on-ignition) after ignition lot 12 h at 430~ extraction of 'total' lead, zinc and cadmium followed a procedure adapted from Warren and Delavault (1960). Duplicate samples of soils were digested in hot, concentrated nitric acid prior to dilution and analysis for 'total' metal contents by atomic
B. E. Davies and R. C. Ballinger
293
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294
Heavy metals in soils
absorptiony spectrophotometry using a double beam instrument ( P y e - U n i c a m SP2900) with deuterium background correction. Precision of the determinations was checked by calculating the mean coefficient of variation (relative standard deviation x 100). For pH this was 2.9%, for ignition loss it was 3.2% and for the metals Pb, Zn, Cu and Cd the values were 2.1%, 2.2%, 1.7% and 1.8%, respectively. Accuracy was checked by analysing samples of NBS fiver sediment. Mean recoveries for Pb, Zn, Cu and Cd were 98.5%, 102.9%, 80.0% and 78.0% The EEC calcareous loam (another standard reference material) yielded recoveries of Pb = 96.6%, Zn = !00.8%, Cu = 90.8% and Cd = 116.7%. Results and Discussion
Summary of the grid survey data The analytical data are summarised in Table 1. Inspection of the results shows that, except for pH, the arithmetic means are greater than the medians and, for Pb and Zn, markedly greater. The skewness statistics also demonstrate departures from symmetry: only for pH is the statistic close to zero. This positive skewing is a consequence of some high data values weighting the mean and is often seen where soils are contaminated. Parametric statistical tests r e q u i r e the data to be n o r m a l l y d i s t r i b u t e d and normalisation is often done by a log (common or natural) transformation of the data. Here, the data were log10 transformed (except, of course for pH which is a log value). The resulting geometric means are similar to or much nearer the medians and the skewness statistics have decreased in value indicating that the transformation was relatively successful. From the normalised data the 95% probability range was calculated. This is a more robust summary of the spread of the results since the distorting effects of the few very high values are minimised.
Soil pH and organic content T h e wide r a n g e of v a l u e s for soil r e a c t i o n and loss-on-ignition (Table 1) was expected because of the variety of parent materials and soils within the region. About a third (66/174) of the samples were neutral to slightly alkaline in soil reaction. These included the Brown Earths on the north Mendip slopes and the calcareous soils of the Liassic lowlands. Within the remaining large group of acidic soils were a number (23) with a pH below 5. These were the lowland peaty soils of the ~dgemoors (pH 3.9, Godney Moor: OS 3480 3440) and those derived from the quartzitic sandstones over the higher Mendip Plateau (pH 3.3, Priddy: OS 3560 3541). For low lying ground water gleys mad peaty gleyed podzols over the sandstone cores of the Mendip Plateau organic contents Iay between 20% and 30%. Soils containing more than 40% were those of the peaty, organic lowland sedgemoors. These simple pedological parameters are comparable with those quoted for the two available descriptions of the soils of the area (Avery, 1955; Find.lay, 1965), Except for soils occurring over the disturbed, gruffy, ground the soils sampled during this survey are, pedologically, typical of the area.
Heavy metals in survey soils and comparisons with other areas There are few reliable data for background metal concentrations in British soils. Most reports have simply summarised published values without any attempt at statistical evaluation. The most comprehensive of recent summaries is that of Ure and Berrow (1982). They calculated worldwide averages for metals; for the metals discussed here, Pb = 29.2 mg kg -I, Zn = 59.8 mg kg -1, Cu = 25.8 mg kg-1 and Cd = 0.62 mg kg-~. Reaves and Berrow (1984) summarised lead concentrations for 3,944 samples from 896 Scottish soil profiles. The (geometric) mean content of all samples was 14 mg Pb kg-1 and for
Table 1 Summary of data for 174 surface soil samples from a general survey in north Somerset.
Arithmetic values: Mean Median Standard deviation Minimum Maximum Skewness Loglo transformed values: Mean Standard deviation 95% probability (low) " (high) Skewness LOI = loss-on-ignition.
pH
LOI (%)
Pb
5.9 5.9 0,86 3.3 7.3 --0.3
16.2 11.6 15.0 3.0 83 2.71
183 52 798 8 (1.0%) 11.7
12.7 11.6 3.6 44 1.0
66 3.0 7.6 577 1.3
-
Zn Cu (rag kg-j dry soil) 267 132 668 14 8,344 10.5
149 2.5 24 915 0.83
15 12 13 2.8 145 6,7
12 [.8 3.9 40 0.18
Cd
3.2 2,1 9.6 0.5 127 12.5
2.3 t .7 0.8 6.6 2.9
B. E. Davies and R. C. Ballinger Table 2 Comparative data for 288 soil samples from north Wales (Davies and Roberts, 1978) and 97 samples from SE Missouri (Davies and Wixson, 1985).
Pb
Zn Cu (mg kg-1 dry soil)
Cd
North Wales: Mean Geonaetric mean Minimum Maximum
886 234 35 (4.8%)
728 153 10 (4.9%)
18 14 2.3 252
6.1 1.4 0.4 540
SE Missouri: Mean Geometric mean Minimum Maximum
141 60 10 2,200
54 42 14 142
13 12 1.5 29
0.4 0.4 0.3 3.1
456 A horizon samples it was 17 mg Pb kg -1. In Pembrokeshire, Wales, Wilkins (1978) reported a mean content of 39 nag Pb kg-t for some 500 samples of surface soil. Davies (1983) e s t i m a t e d b a c k g r o u n d lead concentrations in soils from England and Wales using the graphical approach of Sinclair (1974): the data used included values from north Somerset. It was concluded that lead contents were unlikely (p
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9 Z
b9 O
I 340
~=
330 L ~ 34-0
y=,o
/-~.~---
t 9 350
OS Eostings
330 , "~,~0
360
3A
350
OS Eastings
Soil Copper
J60
.3B
Soil Cadmium 360
350 O3 C ..s
C
o _
0 Z Of') 0 340
~340
340
350
OS Eastings
360
ac
330 340
350
OS Eastings
360
3D
Figure 3 Isoline plots of heavy metal concentrations for north Somerset," a: lead," b." zinc," c: copper," d: cadmium.
298
Heavy metals in soils
c~ 7
,,',i.~ Z n / k . ~
so
~i
o
,'3
Figure 4 Perspective block diagrams of organic matter and metal concentrations in soils of north Somerset; a: organic matter; b: lead; c: zinc; d: copper; e: cadmium.
~~
\Y /',Z:v:
\
T
B. E. Davies and R. C. Ballinger Table 4 Summary of the analytical data for soils from a follow-up survey.
Pb
Zn Cu (mg kg -1 dry soil)
Cd
Area 1: Mean Median Standard deviation Minimum Maximum Geometric mean
29,462 24,032 32,623 215 87,414 5,553
2,995 436 4,029 131 10,066 844
37 30 30.7 6.3 111 25
5.8 2.7 8.36 1.0 34 3.4
Area 2: Mean Median Standard deviation Minimum Maximum Geometric mean
14,722 513 23,186 85 61,210 1,441
2,947 793 3,771 107 10,369 1,058
36 19 31.4 5.3 98 24
t1 3.4 t7.2 0,6 75 4.6
Area 3: Mean Median Standard deviation Minimum Maximum Geometric mean
150 102 160 49 561 111
1,349 134 3,637 59 11,047 214
21 15 13.3 9 52 18
1.5 1.3 0.89 0.5 3.6 1.3
Number of samples in each area: Area I = 15, Area 2 = 21, Area 3 = 9.
discounted, the village being tiny and remote. Shapwick lies on the slopes of the Polden Hills and Avery (1955) mentions the area as being one where molybdenosis in cattle (teart) is endemic because of the high molybdenum content of the rock. Molybdenum is chalcophilic. The area of elevated soil copper values which are to be seen in the south eastern part of the study area (Figures 3c and 4d) overlie an extensive area of mineral soils with impeded drainage. Avery's (1955) soil map depicts part of this area as being occupied by the Charlton Bank and Fladbury series, both of which are developed in either lower Liassic rocks or in a parent material dominated by this material, and teart is endemic here where soil pH is neutral or alkaline because of naturally high molybdenum concentrations. It is suggested that where soils contain higher concentrations of molybdenum originating in the sulphide phases of the Liassic rocks they are also likely to contained elevated contents of other heavy metals.
Detailed Follow-up Sampling The computer graphics discussed above show strong anomalies in several parts of the study area. These could arise from a single metal-rich sample at one grid intersection. The software used for these plots attempts to minimise this possibility when forming the interpolated
299
regular grid from the input data prior to isolining. This is done by searching in the intersection area for other points and then interpolating by inverse weighting using distance-squared. Thus an isolated very low or very high point's effect will be minimised. Nonetheless it is wise to carry out further sampling in anomalous areas to confirm the anomaly. This was done and the distribution of the second set of samples is shown in Figure 2b. Three areas were then defined: (1) The Wells area. This includes the major lead anomaly seen in Figure 4b. The coordinates of the south west and north east corners of the subsidiary area are 3500 3460 and 3600 3530. (2) The Shipham and environs area. This covers the main cadmium area of the north west part of the study area. It is bounded by coordinates 3400 3500 and 3500 3600. (3) The lowlands. Samples were taken from the higher land within the lowland peats. The area is bounded by 3400 3300 and 3600 and 3400. Samples lying within each area were separated from the others and summarised statistically: results are given in Table 4. In area 1 (Wells) there were 15 soil samples and their composition confirms that this area is characterised by a major lead anomaly. Zinc concentrations are also elevated together with cadmium. But copper concentrations are not especially high. In area 2 (Shipham) soil lead concentrations are lower but zinc and cadmium contents are higher than in area 1. These data are consistent with literature reports that this subsidiary area is particularly contaminated with cadmium. Finally, area 3 (Lowlands) encloses minor hills where soils in many localities are formed on the Liassic black shales. Lead, zinc and cadmium contents are much lower than on the Mendip plateau. Nonetheless they are relatively high compared with values quoted earlier in the discussion for normal soils. It is concluded that a strong parent material effect is operating here and natural geochemical anomalies have been created.
Conclusions This survey of soils in north Somerset has provided data for pH and organic matter together with concentrations of lead, zinc, copper and cadmium. Overall, the metal contents are higher or much higher than would be expected for uncontaminated soils. Subsequent follow up sampling has confirmed the results from the general survey. Isoline plots of the soil metal concentrations and perspective block diagrams illustrated the distributions of lead, zinc and cadmium in this area. Soils over the Mendip Plateau generally possess enhanced lead, zinc and cadmium concentrations. These areas of high metal concentration are contiguous with the main areas of past mineral exploitation and are presumed to have arisen through industrial contamination. The area to the north of Wells has emerged as having a particular lead problem while the unusual cadmium pollution in and around Shipham has been confirmed. In the rest of the area contaminated resurgences on the flanks of the Mendips and
300
Heavy metals in soils
trace element enriched Liassic parent materials are responsible for smaller, but significant anomalies. The metal concentrations and distributional patterns are similar to those previously reported in lead mining/limestone areas of south-east Missouri and north--east Wales. The data suggest that the Mendips area is less severely contaminated than the Halkyn Mountain area of north Wales, although localised accumulations, especially of cadmium, are more intense. Similarly, the area is more contaminated than the Madison county mining area of Missouri. Acknowledgments The experimental work was carried out in the laboratories of the Department of Geography at the University College of Wales in Aberystwyth where Rhoda Ballinger (nee Ginnever) was a research student during 1977-1980 supported by a University of Wales studentship for which she was most grateful. Brian Davies was on the staff of that department until 1985, and thanks his then colleagues for their support. The interpretive work for this paper was done at the University of Bradford. References Anderson R.J., Davies B.E., Nunn J.H. and James P.M.C. 1979. The dental health of children from five villages in north Somerset with reference to environmental cadmium and lead. British DentalJ., 147, 159-161. Avery B.W. 1955. The Soils of the Glastonbury District of Somerset. HMSO, London.Davies B.E. and Ginnever R.C. 1979. Trace metal contamination of soils and vegetables in Shipham, Somerset. J. Agrieult. Sei. (Cambridge), 9d3,753-756. Davies B.E. 1983. A graphical estimation of the normal lead content of some British soils. Geoderma, 29, 67-75. Davies B.E. and Roberts L.J. 1978. The distribution of heavy metal contaminated soils in north east Clwyd, Wales. Water, Air Soil PoUut., 9, 507-518. Davies B.E. and Roberts L.J. 1975. Heavy metals in soils and radish in a mineralised area of Wales, Great Britain. Science Total Environ., d4, 249-261. Davies B.E. and Wixson B.G. 1985. Trace dements in surface soils
from the mJneralised area of Madison county, Missouri, USA,
J. Soil Sci., 36, 551-570. f;indlay D.C. 1965. The soils of the Mendip district of Somerset. ttarpenden: Soil Survey of England and Wales. Gough J.W. 1967. The Mines of Mendip. David and Charles, Newton Abbot. Green G.W. 1958. Mendip Lead-Zinc Orefield: Bulletin Geological Survey of Great Britain No. I4. HMSO, London. Le Riche H.H. 1959. Molybdenum in the lower liass of England and Wales in relation to the incidence of teart, J. Soil Sci., 10, 133. Reaves G.A. and Berrow M.L. 1984. Total lead concentrations in Scottish soils. Geoderma, 32, 1-8. Sherlock J.C., Smart G.A,, Walters B., Evans W.H., McWeeny D.L and Cassidy W. 1983, Dietary surveys on a population at Shipham, Somerset, United Kingdom. SeL Total Environ., 29, 121-142. Sinclair A.J. 1974. Selection of thresholds "in geochemical data using probability graphs. J. Geochem. Exploration, 3, 129--149. Stenner R.D. 1977. The concentration of some heavy metals in sediments in some Mendip caves. Proc. Seventh Intnl Speliological Conf., Sheffield (Septen~ber),383-384. Stenner R.D. 1978. The concentration of cadmium, copper, lead and zinc in sediments from caves and associated surface streams on Mendip, Somerset. Trans. British Cave Res. Assoc., 5, 113-120. Thomson, L, Thornton, I. and Webb, J.S. 1972. Molybdenum in black shales and the incidence of bovine hypocuprosis. J. Sci. Food Agriculture, 23, 879-891. Thornton, I. 1983. Geochemistry applied to agriculture. In: Thornton, I. (ed.), Applied Environmental Geochemistry. Chapter 8, pp.231-26@ Academic Press Geology Series, London, Thornton L, Moon R.N.B. and Webb J.S. 1969. Geochemical reconnaissance of the lower lias. Nature (London), 221, 457-459. Thornton I., Moorcroft J.S. and Watt J. 1980. Cadmium at Shipham a unique example of environmental geochemistry and health, in: Hemphill, D.D. (ed.), Trace Substances in Environmental Health. Vol.26, pp.27-37. Ure A.M. and Berrow M.L. 1982. The elemental constituents of soils. In: Bowen, H.J.M. (ed.), Environmental Chemistry, Vol.2. The Royal Society of Chemistry, London. Warren H.V. and Delavault R.E, 1960. Aqua regia extractable copper and zinc in plutonic rocks in relation to ore deposits. Trans. Inst. Mining and Metallurgy, 69, 495-504. Wilkins C. 1978. The distribution of lead in the soils and herbage of west Pembrokeshire. Environ. Pollut., 15, 23-30, (Manuscript No.227: received December 18, 1989; fully reviewed; accepted for publication after modification July 9, 1990.)
