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Urban Geochemistry: A study of the influence of anthropogenic activity on the heavy metal content of soils in traditionally industrial and non-industrial areas of Britain
Vol. 1I, pp.363-370, 1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0883-2927/96$15.00+0.00
Applied Geoehemistr):
Pergamon
OfED-2927(95)00084-4
Urban Geochemistry: A study of the influence of anthropogenic activity on the heavy metal content of soils in traditionally industrial and nonindustrial areas of Britain J. Kelly and I. Thornton Environmental Geochemistry Research, Centre for Environmental Technology, Royal School of Mines, London SW7 2BP. U.K.
and P. R. Simpson British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
Abstract-Heavy metal concentrations have been determined in topsoils (0-15cm) in the London Borough of Richmond-upon-Thames, a non-industrial, mainly residential area of approximately 56 km’, and Wolverhampton an industrial city in the West Midlands of 70 km2. Soil samples were taken on a grid basis at a density of four per km’ and analysis for 25 elements was carried out by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Topsoils in Richmond were found to have significantly higher concentrations of heavy metals in developed locations compared to areas of open space, whilst in Wolverhampton topsoils a greater degree of contamination with Zn was found than with Pb. GIS-based mapping techniques used in conjunction with statistical analysis of the data have highlighted the influence of land-use on the heavy metal content of topsoils in these two urban areas. The highest concentrations of Pb in Richmond-upon-Thames ( > 1000 pg/g) tend to occur close to major road junctions on roads with high traffic densities. High levels of Pb (approx. 500 pg/g) also occur in the areas where the oldest housing is located (> 100 a). In Wolverhampton the highest concentrations of heavy metals, Zn in particular, are generally located to the east of the city in areas of both historical and contemporary industrial activity. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION The urban environment is affected by a wide variety of anthropogenic activities. Road networks, housing and the metal processing and manufacturing industries will tend to increase the heavy metal content of soils (Rodriguez-Flores and Rodriguez-Castellon, 1982; Davies and Thornton, 1987; Davies, 1990). Previous soil surveys have designated urban soils as ‘built-up’ (Hollis, 1991). Various soil ‘types’ are found within towns and cities (Bridges, 1991) ranging from relatively undisturbed soils, similar in some respects to their rural counterparts (Hollis, 1991), to completely man-made soils (Bockheim, 1974). Wolverhampton has been extensively redeveloped over the last century during a period of industrial decline, with made ground and imported topsoil a common feature within the city area. Investigations of heavy metal concentrations in urban soils commenced in the late 1960s (Purves, 1966, 1967; Purves and Mackenzie, 1969), and indicated that urban gardens had significantly higher levels of Cu and B than rural gardens (Purves, 1967) and greater heavy metal contamination in urban parks than in rural parks (Purves and Mackenzie, 1969). Later studies undertaken on urban soils include those by Warren et al. (1971). Wilkins (1978), Davies (1978)
and Gibson and Farmer (1986). The National Survey of Metals in Urban Dusts and Soils (Culbard et al., 1983, 1988; Thornton et al., 1985; Thornton, 1991) was based on 53 towns and cities in Britain. These were selected on the basis of geographical location, population size and the presence or absence of industrial activity, with 100 households being sampled in each. This comprehensive study showed that concentrations of Pb in garden soils from London are generally higher than for other towns and cities in Britain. The work presented in this study aims to assess the influence of urbanisation and industrial activity in Britain on the heavy metal content of topsoils. A similar study has been undertaken recently in Berlin (Birke and Rauche, 1994). A more comprehensive set of data are presented in the PhD thesis of the senior author (Kelly) and is held on open file at the British Geological Survey and Imperial College.
STUDY AREAS AND METHODS
Richmond-upon-Thames is one of 32 Boroughs within the Greater London area. It lies 17.5 km to the SW of the city centre, covering an area of approximately 56 km2. It is a residential suburb with a population of approximately 160,000 and has very 363
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364
little history of industrial activity. The majority of Richmond is underlain by River Terrace deposits which vary in age and thickness, and consist mainly of gravels and sands. Outcrops of London Clay occur mainly in Richmond Park. Richmond is unique as a London Borough since it has large areas of relatively undisturbed open space in Richmond Park, Bushy Park and Hampton Court. The amenity areas comprise 2/3 of the land area in the Borough. These open spaces have been used to establish baseline concentrations of heavy metals, with a view to comparing these with concentrations in soils in the built part of the Borough which are affected by anthropogenic contamination. Wolverhampton is located in the West Midlands approximately 72 km to the W of Birmingham, with an area of 70 km* and a population of 175,000. The city has a long history of industrial activity, with the Fe industry growing rapidly from the mid-18th century until it declined during the mid-19th century. Peak Fe production occurred from 1850 to 1860 (Gale, 1979). Figure la shows the locations of
et al.
London and Wolverhampton in Britain; both study areas are shown in detail in Figs lb and lc. The majority of Wolverhampton is covered with periglacial deposits, in the form of a widespread till sheet with subordinate sands, gravels and laminated clays. A systematic sampling strategy was developed in Richmond and then also applied to Wolverhampton. Samples were taken from household gardens, public and derelict land, and parkland. Four samples were collected per km2 at regular intervals, with topsoil samples (O-15 cm) comprising a composite of 9 subsamples collected from a 4 m2 grid. Sub-surface samples (30-45 cm) were also collected, but these results will be published elsewhere. Duplicates were collected at 10% of sites to enable an estimate of sampling and analytical precision. All soils were dried at 20°C disaggregated and sieved. Multi-element analysis of the <2 mm fraction was carried out by ICP-AES, following digestion in concentrated HN03 and HCL04 (4 : 1) and leaching with HCI (Thompson and Walsh, 1988). A complete sampling and analytical control scheme was implemented with sampling and
Wok’hamptocr
Metropolitan Borough
la
Fig. 1. (a) British location map of the two study areas. (b) The Metropolitan Borough of Wolverhampton. (c) The London Borough of Richmond-upon-Thames.
Heavy metal content of soils in industrial and non-industrial Britain
Fig. 2. Geochemical map of Pb concentrations (ppm) in Richmond-upon-Thames
topsoils (O-15 cm).
365
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et al.
Fig. 3. Geochemical map of Zn concentrations (ppm) in Wolverhampton topsoils (O-l 5 cm).
Heavy metal content of soils in industrial and non-industrial Britain analytical duplicates, reference materials and reagent blanks (Ramsey et al., 1987). The technical variance, a combination of sampling and analytical variance, was found to contribute less than 20% of the total variance for the heavy metals. This is within the suggested threshold, allowing a confident interpretation of the geochemical variability (Ramsey et al., 1992).
RESULTS AND DISCUSSION
The results from the urban soil surveys are shown as geochemical maps in Figs 2 and 3. These maps were generated at the British Geological Survey using an Interactive Surface Modelling griding algorithm for the Richmond-upon-Thames data. The map for Wolverhampton was produced by griding with a triangular algorithm on a G-MAP which is based on an Arc Info GIS. Both maps employ a colour classification applied to grey scale, density-sliced class intervals presented as percentiles. This facilitates the comparison between different elements in the two study areas and baseline values for the U.K. which are reported in geochemical atlases based on stream sediment samples (Webb et al., 1978 and British Geological Survey, 1991) and soil samples (McGrath and Loveland, 1992). The lowest concentrations of Pb (175 pg/g) for Richmond shown in Fig. 2 tend to occur in the 3 main areas of open space: Richmond Park, Hampton Park/ Bushy Park and Kew Gardens. This is clearly seen in Fig. lb which shows the layout of the Borough in detail. Soils at the centres of Richmond and Bushy Parks, over lkm from any major roads, have concentrations of Pb between 20 and 30 fig/g. Soils developed from individual geological units, in areas unaffected by development, show a range of metal concentrations. The concentrations of Pb found at the centre of the parks in Richmond are lower than the median value quoted in the Soil Geochemical Atlas of England and Wales of 40 pg/g (McGrath and Loveland, 1992) and are in the range quoted by Davies (1990) of 10-30 fig/g Pb for remote regions. The sites which show the highest concentrations of Pb generally occur close to major junctions or roundabouts on main roads with a high traffic density. This is particularly the case in the N of the Borough, both E and W of Richmond town centre. A busy road here carries 10,000 vehicles eastwards towards Mortlake (see Fig. 1b) and 20,000 westwards through Twickenham towards Heathrow airport each day. Most of the Pb deposited from traffic sources occurs where the traffic density is high and vehicles are required to stop and accelerate (Lyons et al.. 1990). Concentrations of lead are also seen to be high, from 342-581 pg/g, in locations parallel to busy roads, as seen in Fig. 2, along a major road in the N of the Borough. However, the levels of Pb found here are not as high as those found at the major road junctions. Similar patterns are also seen along the busier roads throughout the
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London Borough. Lead in roadside soils typically decreases exponentially, with most of the metal deposited within 30-50 m of the road (Davies, 1990). As many of the samples were taken from household gardens along busy roads, it is unlikely that vehicle emissions are the only source of Pb. The oldest housing in the Borough ( > 100 a old) tends to occur close to the town centre and along the road leading out to Mortlake (see Figs lb and 2). The houses are also of a similar age to the E of Kew gardens where Pb concentrations are comparable. A number of studies have shown the influence of house age on the Pb content of garden soil (Davies and Thornton, 1987; Davies et al., 1987). Lead was an important component ofpaint until recently (Rundle and Duggan, 1986; Schwar and Alexander, 1988; Davies, 1990). An additional source of heavy metals results from the once common practice of disposing of ash from open fires in garden soils (Thornton, 1991). The distribution of Zn and Cu is similar to that of Pb (Fig. 2). This is in agreement with a number of studies which show Zn and Cu to be present in elevated concentrations in roadside soils (Lagerwerff and Specht, 1970; Albasel and Cottenie, 1985; Muskett and Jones, 1990). Figure 3 shows Zn concentrations in Wolverhampton topsoils; these are generally higher than those for Pb in the city area (see Table 1). Highly anomalous levels of Zn, ranging from 586 to 2045 pg/g, are generally associated with areas of contemporary industry located to the E of the major road which bisects the city N-S. This is illustrated by the area around Monmore Green following a path S-eastwards towards Bilston (see Figs lc and 3). This region retains Fe-related industries which have been established for over a 100 a. To the NE, an anomalous area is attributed to metal workings, and to the NW an anomalous concentration of Zn can be found near the Goodyear plant associated with engineering work. Some soils in the NW and SW of the city have concentrations of Zn ranging from 586 pug/g to 2045 pg/g. These occur in industrial areas, although the largest of these, in the NW, coincides with a sewage works. The lowest Zn class shown in Fig. 3 (587 p/g). is most commonly present at the outskirts of the city which coincides with the change to a more rural environment, particularly in the W where farmland is several kilometres from the industrial centre.
COMPARISON OF THE TWO URBAN AREAS
Table 1 presents a comparison of the heavy metal concentrations in topsoils from Richmond-uponThames and Wolverhampton as geometric means, due to the log-normal distribution of the data. They have been calculated using all the topsoil samples taken in both locations. In Wolverhampton, topsoils generally have higher concentrations of Zn ( x 2), Cu (x 2) and Cd than those from Richmond. Seventy-
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J. Kelly et al. Table 1. Concentrations of metals in topsoils (O-l 5 cm) in Richmond and Wolverhampton @g/g)
Richmond2 Wolverhampton
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
CU range (g.m)
N
20-1840 (158) 16-14,900(106)
11.4-1810(108) 54.26740 (231)
<0.2-11.1 (<0.2) <0.2-54.7 (0.80)
3.8-l 130 (30) 9.7-2750 (62)
214 295
‘Geometric mean. 2Refers to results for the London Borough of Richmond-upon-Thames
Table 2. Concentrations of metals in topsoils (O-l 5 cm) relating to land-use in the London Borough of Richmondupon-Thames @g/g)
Residential’ Recreational3
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
CU range (g.m)
N
34-1840 (271) 20-1210 (93)
36.61810(179) 1I .4-657 (66)
<0.2-11.1 (<0.2) <0.2-l .2 (<0.2)
13.1-1130(48) 3.8-164(19)
106 108
‘Geometric mean. ‘Refers to samples taken in back gardens. 3Refers to samples taken elsewhere, i.e. parks and sports grounds.
Table 3. Concentrations of metals in topsoils (O-15 cm) relating to land-use in Wolverhampton (pg/g)
Residential’ Recreational3 Industrial4 Agricultural’ ‘Geometric mean. ‘Refers to samples ‘Refers to samples 4Refers to samples ‘Refers to samples
five percent concentrations
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
cu range (g.m)
N
1614,900 (111) 233687 (96) 28-1400 (144) 23-129 (51)
54.66740 (240) 54.2-2950 (205) 663040 (368) 80.8-200 (125)
<0.2-54.7 (0.7) <0.2-10.7 (0.7) <0.2-12.6 (1.2) <0.2-0.7 (0.3)
lo-841 (62) 12-387 (55) 18-2750(139) 20-60 (32)
178 58 25 15
taken taken taken taken
in back gardens. in areas of open space such as small parks or sports grounds. on industrial sites or within 50 m of industrial activity. on agricultural land.
of the soils in Richmond have Cd below the ICP-AES detection limit of
0.2 pg/g, and this is reflected in the calculation of the geometric means presented in Tables 1 and 2. Wolverhampton topsoils generally have lower concentrations of Pb than those in Richmond, see Table 1. This contrasts with other heavy metals listed in Table 1, although traffic densities are not higher and the houses not older in Richmond. It is suggested that this difference relates to the changes that have occurred in Wolverhampton over the last 100 a. As an industrially declining city, redevelopment has occurred with many industrial locations now being re-used for housing or leisure facilities. Much of the topsoil is now made ground or has been imported from elsewhere. The heavy metal concentrations reported here are probably much lower than in the recent past. It is considered that anthropogenic Pb has only recently accumulated in surface soils in Wolverhampton. The important infhrence of land-use in the 2 locations is shown in Tables 2 and 3. In Richmond
the sample sites are classified as either residential or open space, the latter including golf courses and sports grounds as well as the large parks. In contrast to developed areas, the soils in the middle of the parks in Richmond are over 1 km from major roads. The residential or garden soils have significantly higher concentrations of all the heavy metals than soils in areas of open space. These are shown to be statistically significant by means of a t-test, p
Heavy metal content of soils in industrial and non-industrial Britain
Fe and steel industry and Zn’s association with Fe ores and galvanizing techniques. Cadmium is also present in higher concentrations at industrial sites, at levels almost twice that found in soils in residential locations and areas of open space. Cadmium has a close geochemical association with Zn (Alloway, 1990), and in Wolverhampton topsoil concentrations of Zn and Cd are significantly correlated (r = > 0.90, p
CONCLUSIONS
The present study has shown that heavy metal concentrations in urban topsoils are strongly influenced by land-use. In Richmond-upon-Thames, a non-industrial London Borough, busy road junctions and old housing result in concentrations of Pb in topsoil in excess of 1000 pg/g. Concentrations of Pb in topsoils in general are higher than those in the more industrial city of Wolverhampton (a geometric mean for topsoils of 158 pg/g compared to 106 pg/g in Wolverhampton). The large areas of open space, which are isolated from major roads, result in concentrations of heavy metals at the centres which provide a local baseline of: Pb < 30 pg/g, Zn < 20 pg/g, Cu < 10 pg/g and Cd < 0.2 ,ug/g). In Wolverhampton contamination from the Fe industry has resulted in higher concentrations of Zn than Pb in topsoils. However, the redevelopment over the previous 3040 a has led to changes to the surface layer of soil which is now largely made ground or imported topsoil. This has been exposed to further heavy metal contamination during only a few years of declining industrial activity. Hence the concentrations of heavy metals reported here are probably now much lower than during the early 19th century when industry was at its peak. The highest concentrations of heavy metals, and Zn in particular (over 2000 pg/g), are found close to the much reduced industrial base that remains. Ackno,l,lengemenrs-P. R. S publishes with permission of the Director of the British Geological Survey (NERC). We are grateful to the staff of the British Geological Survey for extensive help with regard to sample collection in Wolverhampton, and for use of their excellent mapping facilities for both the Richmond-upon-Thames and Wolverhampton projects. We are particularly grateful to Malcolm Brown, Dr Chris McDermott. Alan Mackenzie, Dr Neil Breward and Mick Strutt. REFERENCES Albasel N. and Cottenie A. (1985) Heavy metal contamination near major highways, industrial and urban areas in
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Belgian grassland. Water, Air and Soil Pollut. 24, 103109. Alloway B. J. (1990) Cadmium. In Heavy Metals in Soils (ed. B. J. Alloway), Chap. 6, pp. 100-121. Blackie and Son. Birke M. and Rauche U. (1994) Geochemical investigation of the urban area of Berlin. Federal Institute of Geosciences and Natural Resources, Branch Office Berlin, Germany.
Bockheim J. G. (1974) Nature and properties of highly disturbed urban soils, Philadelphia, Pennsylvania. Paper presented before Div. S-5, Soil Science Society of America, Chicago, Illinois. Bridges E. M. (1991) Waste materials in urban soils. In Soils in the Urban Environment (ed. P. Bullock and P. J.
Gregory), Chap. 3, pp. 2846. Blackwell. British Geological Survey (1991) Regional geochemistry of the Easr Grampians area. (British Geological Survey, Keyworth, Nottingham). Culbard E., Thornton I., Watt J., Moorcroft S. Brooks K., and Thompson M. (1983) A nationwide reconnaissance survey to determine metal concentrations in urban dusts and soils. In Trace Substances in Environmental Health, XVII (ed. D. D. Hemphill), pp. 236-241. Univ. of Missouri, Columbia. Culbard E. B., Thornton I., Watt J., Wheatley M., Moorcroft S. and Thompson M. (1988) Metal contamination in British urban dusts and soils. J. Environ. Qual. 17(2), 226-234.
Davies B. E. (1978) Plant available lead and other metals in British garden soils. Sci. Tot. Environ. 9, 243-262. Davies B. E. (1990) Lead. In Heavy Metals in Soils (ed. B. J. Alloway), Chap. 9, pp. 177-194. Blackie and Son. Davies D. J. A. and Thornton I (1987) The influence of house age on lead levels in dusts and soils in Brighton, England. Environ. Geochem. Health. 9, 65-67. Davies D. J. A., Watt J. and Thornton I. (1987) Lead levels in Birmingham dusts and soils. Sci. Tot. Environ. 67, 177185.
Gale W. K. V. (1979) The Black Country Iron Industry: a Technical Hisrory. The Metals Society. Gibson M. J. and Farmer J. G. (1986) Multi-step sequential chemical extraction of heavy metals from urban soils. Environ. Pollut. Ser. B 11, 117-135. Hollis J. M. (1991) The classification of soils in urban areas. In Soils in the Urban Environmenf (eds P. Bullock and P. J. Greaorv). Chan. 2. DD. 5-27. Blackwell. Lagerwe& J.‘V. and Specht A. W. (1970) Contamination of roadside soil and vegetation with cadmium. nickel, lead and zinc. Environ. Sci. Technol. 4, 583-586. Lyons T. J., Kenworth J. R. and Newman P. W. G. (1990) Urban structure and air pollution. Atmos. Environ. Vol. 24B( I), 43-48. McGrath S. P. and Loveland P. J. (1992). The Soil Geochemical Atlas of England and Wales. London: Blackie Academic. Muskett C. J. and Jones M. P. (1990) The dispersal of lead, cadmium and nickel from motor vehicles and the effects on roadside invertebrate macrofauna. Environ. Pollut. ser. A 23, 231-242. Purves D. (1966) Contamination of urban garden soils with conuer and boron. Nature 210, 1077-1078. Purves D. (1967) Contamination of urban garden soils with copper, boron and lead. Plant and Soil 26, 380-382. Purves D. and Mackenzie E. J. (1969) Trace element contamination of parklands in urban areas. J. Soil Sci. 20, 288-290.
Ramsey M. H., Thompson M. and Banerjee E. K. (1987) Realistic assessment of analytical data quality from Inductively Coupled Plasma Atomic Emission Spectrometry. Anal. Proc. 24, 260-265. Ramsey M. H., Thompson M. and Hale M. (1992) Objective evaluation of precision requirements for geo-
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chemical analysis using robust analysis of variance. J. Geochem. Explor. 44, 23-36.
Rodriguez-Flores M. and Rodriguez-Castellon E. (1982) Lead and cadmium levels in soil and plants near highways and their correlation with traffic density. Environ. Pollut. ser. B. 4,281-290. Rundle S. A. and Duggan M. J. (1986) Lead pollution from the external redecoration of old buildings. Sci. To?. Environ. 57, 181-190. Schwar M. J. R. and Alexander D. J. (1988) Redecoration of external leaded paintwork and lead-in-dust concentrations in school playgrounds. Sci. Tot. Environ. 68, 45-59. Thompson M. and Walsh J. N. (1988) A Handbook of Inductively Coupled Plasma Spectrometry, 2nd Edn. Blackie, London.
Thornton I., Culbard E., Moorcroft 8, Watt J., Wheatley M., Thompson M and Thomas J. F. A. (1985) Metals in urban dusts and soils. Environ. Technol. Left. 6, 137-144. Thornton I. (1991) Metal contamination of soils in urban areas. In Soils in the Urban Environment (eds P. Bullock and P. J. Gregory), Chap. 4, pp. 47-75. Blackwell. Warren H. V., Delavault R. E. and Fletcher K. W. (1971) Metal pollution-a growing problem in industrial and urban areas. Can. Min. Metall. Bull. 64, 34-45. Webb J. S., Thornton I., Thompson M., Howarth R. J. and Lowenstein P. L. (1978) The Wolfon Geochemical Atlas of England and Wales. Oxford University Press. Wilkins C. (1978) The distribution of lead in the soils and herbage of west Pembrokeshire. Environ. Pollut. 15, 2330.
Applied Geoehemistr):
Pergamon
OfED-2927(95)00084-4
Urban Geochemistry: A study of the influence of anthropogenic activity on the heavy metal content of soils in traditionally industrial and nonindustrial areas of Britain J. Kelly and I. Thornton Environmental Geochemistry Research, Centre for Environmental Technology, Royal School of Mines, London SW7 2BP. U.K.
and P. R. Simpson British Geological Survey, Keyworth, Nottingham NG12 5GG, U.K.
Abstract-Heavy metal concentrations have been determined in topsoils (0-15cm) in the London Borough of Richmond-upon-Thames, a non-industrial, mainly residential area of approximately 56 km’, and Wolverhampton an industrial city in the West Midlands of 70 km2. Soil samples were taken on a grid basis at a density of four per km’ and analysis for 25 elements was carried out by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Topsoils in Richmond were found to have significantly higher concentrations of heavy metals in developed locations compared to areas of open space, whilst in Wolverhampton topsoils a greater degree of contamination with Zn was found than with Pb. GIS-based mapping techniques used in conjunction with statistical analysis of the data have highlighted the influence of land-use on the heavy metal content of topsoils in these two urban areas. The highest concentrations of Pb in Richmond-upon-Thames ( > 1000 pg/g) tend to occur close to major road junctions on roads with high traffic densities. High levels of Pb (approx. 500 pg/g) also occur in the areas where the oldest housing is located (> 100 a). In Wolverhampton the highest concentrations of heavy metals, Zn in particular, are generally located to the east of the city in areas of both historical and contemporary industrial activity. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION The urban environment is affected by a wide variety of anthropogenic activities. Road networks, housing and the metal processing and manufacturing industries will tend to increase the heavy metal content of soils (Rodriguez-Flores and Rodriguez-Castellon, 1982; Davies and Thornton, 1987; Davies, 1990). Previous soil surveys have designated urban soils as ‘built-up’ (Hollis, 1991). Various soil ‘types’ are found within towns and cities (Bridges, 1991) ranging from relatively undisturbed soils, similar in some respects to their rural counterparts (Hollis, 1991), to completely man-made soils (Bockheim, 1974). Wolverhampton has been extensively redeveloped over the last century during a period of industrial decline, with made ground and imported topsoil a common feature within the city area. Investigations of heavy metal concentrations in urban soils commenced in the late 1960s (Purves, 1966, 1967; Purves and Mackenzie, 1969), and indicated that urban gardens had significantly higher levels of Cu and B than rural gardens (Purves, 1967) and greater heavy metal contamination in urban parks than in rural parks (Purves and Mackenzie, 1969). Later studies undertaken on urban soils include those by Warren et al. (1971). Wilkins (1978), Davies (1978)
and Gibson and Farmer (1986). The National Survey of Metals in Urban Dusts and Soils (Culbard et al., 1983, 1988; Thornton et al., 1985; Thornton, 1991) was based on 53 towns and cities in Britain. These were selected on the basis of geographical location, population size and the presence or absence of industrial activity, with 100 households being sampled in each. This comprehensive study showed that concentrations of Pb in garden soils from London are generally higher than for other towns and cities in Britain. The work presented in this study aims to assess the influence of urbanisation and industrial activity in Britain on the heavy metal content of topsoils. A similar study has been undertaken recently in Berlin (Birke and Rauche, 1994). A more comprehensive set of data are presented in the PhD thesis of the senior author (Kelly) and is held on open file at the British Geological Survey and Imperial College.
STUDY AREAS AND METHODS
Richmond-upon-Thames is one of 32 Boroughs within the Greater London area. It lies 17.5 km to the SW of the city centre, covering an area of approximately 56 km2. It is a residential suburb with a population of approximately 160,000 and has very 363
J. Kelly
364
little history of industrial activity. The majority of Richmond is underlain by River Terrace deposits which vary in age and thickness, and consist mainly of gravels and sands. Outcrops of London Clay occur mainly in Richmond Park. Richmond is unique as a London Borough since it has large areas of relatively undisturbed open space in Richmond Park, Bushy Park and Hampton Court. The amenity areas comprise 2/3 of the land area in the Borough. These open spaces have been used to establish baseline concentrations of heavy metals, with a view to comparing these with concentrations in soils in the built part of the Borough which are affected by anthropogenic contamination. Wolverhampton is located in the West Midlands approximately 72 km to the W of Birmingham, with an area of 70 km* and a population of 175,000. The city has a long history of industrial activity, with the Fe industry growing rapidly from the mid-18th century until it declined during the mid-19th century. Peak Fe production occurred from 1850 to 1860 (Gale, 1979). Figure la shows the locations of
et al.
London and Wolverhampton in Britain; both study areas are shown in detail in Figs lb and lc. The majority of Wolverhampton is covered with periglacial deposits, in the form of a widespread till sheet with subordinate sands, gravels and laminated clays. A systematic sampling strategy was developed in Richmond and then also applied to Wolverhampton. Samples were taken from household gardens, public and derelict land, and parkland. Four samples were collected per km2 at regular intervals, with topsoil samples (O-15 cm) comprising a composite of 9 subsamples collected from a 4 m2 grid. Sub-surface samples (30-45 cm) were also collected, but these results will be published elsewhere. Duplicates were collected at 10% of sites to enable an estimate of sampling and analytical precision. All soils were dried at 20°C disaggregated and sieved. Multi-element analysis of the <2 mm fraction was carried out by ICP-AES, following digestion in concentrated HN03 and HCL04 (4 : 1) and leaching with HCI (Thompson and Walsh, 1988). A complete sampling and analytical control scheme was implemented with sampling and
Wok’hamptocr
Metropolitan Borough
la
Fig. 1. (a) British location map of the two study areas. (b) The Metropolitan Borough of Wolverhampton. (c) The London Borough of Richmond-upon-Thames.
Heavy metal content of soils in industrial and non-industrial Britain
Fig. 2. Geochemical map of Pb concentrations (ppm) in Richmond-upon-Thames
topsoils (O-15 cm).
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et al.
Fig. 3. Geochemical map of Zn concentrations (ppm) in Wolverhampton topsoils (O-l 5 cm).
Heavy metal content of soils in industrial and non-industrial Britain analytical duplicates, reference materials and reagent blanks (Ramsey et al., 1987). The technical variance, a combination of sampling and analytical variance, was found to contribute less than 20% of the total variance for the heavy metals. This is within the suggested threshold, allowing a confident interpretation of the geochemical variability (Ramsey et al., 1992).
RESULTS AND DISCUSSION
The results from the urban soil surveys are shown as geochemical maps in Figs 2 and 3. These maps were generated at the British Geological Survey using an Interactive Surface Modelling griding algorithm for the Richmond-upon-Thames data. The map for Wolverhampton was produced by griding with a triangular algorithm on a G-MAP which is based on an Arc Info GIS. Both maps employ a colour classification applied to grey scale, density-sliced class intervals presented as percentiles. This facilitates the comparison between different elements in the two study areas and baseline values for the U.K. which are reported in geochemical atlases based on stream sediment samples (Webb et al., 1978 and British Geological Survey, 1991) and soil samples (McGrath and Loveland, 1992). The lowest concentrations of Pb (175 pg/g) for Richmond shown in Fig. 2 tend to occur in the 3 main areas of open space: Richmond Park, Hampton Park/ Bushy Park and Kew Gardens. This is clearly seen in Fig. lb which shows the layout of the Borough in detail. Soils at the centres of Richmond and Bushy Parks, over lkm from any major roads, have concentrations of Pb between 20 and 30 fig/g. Soils developed from individual geological units, in areas unaffected by development, show a range of metal concentrations. The concentrations of Pb found at the centre of the parks in Richmond are lower than the median value quoted in the Soil Geochemical Atlas of England and Wales of 40 pg/g (McGrath and Loveland, 1992) and are in the range quoted by Davies (1990) of 10-30 fig/g Pb for remote regions. The sites which show the highest concentrations of Pb generally occur close to major junctions or roundabouts on main roads with a high traffic density. This is particularly the case in the N of the Borough, both E and W of Richmond town centre. A busy road here carries 10,000 vehicles eastwards towards Mortlake (see Fig. 1b) and 20,000 westwards through Twickenham towards Heathrow airport each day. Most of the Pb deposited from traffic sources occurs where the traffic density is high and vehicles are required to stop and accelerate (Lyons et al.. 1990). Concentrations of lead are also seen to be high, from 342-581 pg/g, in locations parallel to busy roads, as seen in Fig. 2, along a major road in the N of the Borough. However, the levels of Pb found here are not as high as those found at the major road junctions. Similar patterns are also seen along the busier roads throughout the
361
London Borough. Lead in roadside soils typically decreases exponentially, with most of the metal deposited within 30-50 m of the road (Davies, 1990). As many of the samples were taken from household gardens along busy roads, it is unlikely that vehicle emissions are the only source of Pb. The oldest housing in the Borough ( > 100 a old) tends to occur close to the town centre and along the road leading out to Mortlake (see Figs lb and 2). The houses are also of a similar age to the E of Kew gardens where Pb concentrations are comparable. A number of studies have shown the influence of house age on the Pb content of garden soil (Davies and Thornton, 1987; Davies et al., 1987). Lead was an important component ofpaint until recently (Rundle and Duggan, 1986; Schwar and Alexander, 1988; Davies, 1990). An additional source of heavy metals results from the once common practice of disposing of ash from open fires in garden soils (Thornton, 1991). The distribution of Zn and Cu is similar to that of Pb (Fig. 2). This is in agreement with a number of studies which show Zn and Cu to be present in elevated concentrations in roadside soils (Lagerwerff and Specht, 1970; Albasel and Cottenie, 1985; Muskett and Jones, 1990). Figure 3 shows Zn concentrations in Wolverhampton topsoils; these are generally higher than those for Pb in the city area (see Table 1). Highly anomalous levels of Zn, ranging from 586 to 2045 pg/g, are generally associated with areas of contemporary industry located to the E of the major road which bisects the city N-S. This is illustrated by the area around Monmore Green following a path S-eastwards towards Bilston (see Figs lc and 3). This region retains Fe-related industries which have been established for over a 100 a. To the NE, an anomalous area is attributed to metal workings, and to the NW an anomalous concentration of Zn can be found near the Goodyear plant associated with engineering work. Some soils in the NW and SW of the city have concentrations of Zn ranging from 586 pug/g to 2045 pg/g. These occur in industrial areas, although the largest of these, in the NW, coincides with a sewage works. The lowest Zn class shown in Fig. 3 (587 p/g). is most commonly present at the outskirts of the city which coincides with the change to a more rural environment, particularly in the W where farmland is several kilometres from the industrial centre.
COMPARISON OF THE TWO URBAN AREAS
Table 1 presents a comparison of the heavy metal concentrations in topsoils from Richmond-uponThames and Wolverhampton as geometric means, due to the log-normal distribution of the data. They have been calculated using all the topsoil samples taken in both locations. In Wolverhampton, topsoils generally have higher concentrations of Zn ( x 2), Cu (x 2) and Cd than those from Richmond. Seventy-
368
J. Kelly et al. Table 1. Concentrations of metals in topsoils (O-l 5 cm) in Richmond and Wolverhampton @g/g)
Richmond2 Wolverhampton
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
CU range (g.m)
N
20-1840 (158) 16-14,900(106)
11.4-1810(108) 54.26740 (231)
<0.2-11.1 (<0.2) <0.2-54.7 (0.80)
3.8-l 130 (30) 9.7-2750 (62)
214 295
‘Geometric mean. 2Refers to results for the London Borough of Richmond-upon-Thames
Table 2. Concentrations of metals in topsoils (O-l 5 cm) relating to land-use in the London Borough of Richmondupon-Thames @g/g)
Residential’ Recreational3
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
CU range (g.m)
N
34-1840 (271) 20-1210 (93)
36.61810(179) 1I .4-657 (66)
<0.2-11.1 (<0.2) <0.2-l .2 (<0.2)
13.1-1130(48) 3.8-164(19)
106 108
‘Geometric mean. ‘Refers to samples taken in back gardens. 3Refers to samples taken elsewhere, i.e. parks and sports grounds.
Table 3. Concentrations of metals in topsoils (O-15 cm) relating to land-use in Wolverhampton (pg/g)
Residential’ Recreational3 Industrial4 Agricultural’ ‘Geometric mean. ‘Refers to samples ‘Refers to samples 4Refers to samples ‘Refers to samples
five percent concentrations
Pb range (g.m)’
Zn range (g.m)
Cd range (g.m)
cu range (g.m)
N
1614,900 (111) 233687 (96) 28-1400 (144) 23-129 (51)
54.66740 (240) 54.2-2950 (205) 663040 (368) 80.8-200 (125)
<0.2-54.7 (0.7) <0.2-10.7 (0.7) <0.2-12.6 (1.2) <0.2-0.7 (0.3)
lo-841 (62) 12-387 (55) 18-2750(139) 20-60 (32)
178 58 25 15
taken taken taken taken
in back gardens. in areas of open space such as small parks or sports grounds. on industrial sites or within 50 m of industrial activity. on agricultural land.
of the soils in Richmond have Cd below the ICP-AES detection limit of
0.2 pg/g, and this is reflected in the calculation of the geometric means presented in Tables 1 and 2. Wolverhampton topsoils generally have lower concentrations of Pb than those in Richmond, see Table 1. This contrasts with other heavy metals listed in Table 1, although traffic densities are not higher and the houses not older in Richmond. It is suggested that this difference relates to the changes that have occurred in Wolverhampton over the last 100 a. As an industrially declining city, redevelopment has occurred with many industrial locations now being re-used for housing or leisure facilities. Much of the topsoil is now made ground or has been imported from elsewhere. The heavy metal concentrations reported here are probably much lower than in the recent past. It is considered that anthropogenic Pb has only recently accumulated in surface soils in Wolverhampton. The important infhrence of land-use in the 2 locations is shown in Tables 2 and 3. In Richmond
the sample sites are classified as either residential or open space, the latter including golf courses and sports grounds as well as the large parks. In contrast to developed areas, the soils in the middle of the parks in Richmond are over 1 km from major roads. The residential or garden soils have significantly higher concentrations of all the heavy metals than soils in areas of open space. These are shown to be statistically significant by means of a t-test, p
Heavy metal content of soils in industrial and non-industrial Britain
Fe and steel industry and Zn’s association with Fe ores and galvanizing techniques. Cadmium is also present in higher concentrations at industrial sites, at levels almost twice that found in soils in residential locations and areas of open space. Cadmium has a close geochemical association with Zn (Alloway, 1990), and in Wolverhampton topsoil concentrations of Zn and Cd are significantly correlated (r = > 0.90, p
CONCLUSIONS
The present study has shown that heavy metal concentrations in urban topsoils are strongly influenced by land-use. In Richmond-upon-Thames, a non-industrial London Borough, busy road junctions and old housing result in concentrations of Pb in topsoil in excess of 1000 pg/g. Concentrations of Pb in topsoils in general are higher than those in the more industrial city of Wolverhampton (a geometric mean for topsoils of 158 pg/g compared to 106 pg/g in Wolverhampton). The large areas of open space, which are isolated from major roads, result in concentrations of heavy metals at the centres which provide a local baseline of: Pb < 30 pg/g, Zn < 20 pg/g, Cu < 10 pg/g and Cd < 0.2 ,ug/g). In Wolverhampton contamination from the Fe industry has resulted in higher concentrations of Zn than Pb in topsoils. However, the redevelopment over the previous 3040 a has led to changes to the surface layer of soil which is now largely made ground or imported topsoil. This has been exposed to further heavy metal contamination during only a few years of declining industrial activity. Hence the concentrations of heavy metals reported here are probably now much lower than during the early 19th century when industry was at its peak. The highest concentrations of heavy metals, and Zn in particular (over 2000 pg/g), are found close to the much reduced industrial base that remains. Ackno,l,lengemenrs-P. R. S publishes with permission of the Director of the British Geological Survey (NERC). We are grateful to the staff of the British Geological Survey for extensive help with regard to sample collection in Wolverhampton, and for use of their excellent mapping facilities for both the Richmond-upon-Thames and Wolverhampton projects. We are particularly grateful to Malcolm Brown, Dr Chris McDermott. Alan Mackenzie, Dr Neil Breward and Mick Strutt. REFERENCES Albasel N. and Cottenie A. (1985) Heavy metal contamination near major highways, industrial and urban areas in
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