Stockholm

As a part of the multifacetted project "Metals in the Urban and Forest Environment", a special study was conducted with the aim of quantifying the concentrations and pools of metals in urban soils of Stockholm (Linde et al., 2001). In addition to determing the metal concentrations in soils in different parts of the municipality, these concentrations were related to the distance from the city centre and to various types of land use. The amounts of metals per unit surface area were calculated as well as the total accumulated metal pool in the soils of the central city and this was compared with the estimated recent annual input.

The soil types were classified as "city centre soils" (public parks with lawns etc. in the city centre), "undisturbed soils" (areas with semi-natural vegetation), "public park soils" (green areas, excluding those with semi-natural vegetation), "waste land" (mainly former industrial areas), and "roadside soils" (situated within a few metres from the roadway). It is understood that all soil samples except those from soils classified as "city centre soils" were taken from areas outside the immediate city centre. Altogether samples (5-10 from each site) were taken from 42 sites, and data are presented for the surface layer (0-5 cm) and for subsurface soil (30 cm depth); however, data from the subsurface sampling were reported only for 40 sites. A number of soil characteristics were determined and metals were analysed by means of AAS after sample digestion in boiling 7 M HNO3, according to Swedish Standard (see further details in Linde et al., 2001).

The authors have emphasized that "In order to quantify heavy metal accumulation in urban soils it is of vital importance to establish the background levels originating from the parent material. Rural (arable) soils in the region as well as park soils outside the city centre were found to be useful." (as reference material). Data on arable soils from the Stockholm region were derived from reports on the metal content in the plough layer (020 cm) of 226 samples given by Eriksson et al., (1997; 1999). Concentrations of chromium, copper, nickel and zinc in the different soil types in Stockholm as well as the arable soils in the surroundings of Stockholm are pesented as mean values in Table 3.14.

A certain number of interesting conclusions can be made on the basis of these results:

1. Concentrations of chromium and nickel were low and uniform in all soil types, i.e. there was no tendency to enhanced concentrations of these metals in the city centre soils, when compared to the arable or undisturbed soils.

2. Mean concentrations of copper and zinc were enhanced about 2-3 times in the city centre soils, in comparison with the reference levels in regional arable soils and undisturbed soils within the city.

3. No clear enhancement of the mean copper levels were found in the roadside soils, compared wto the reference material, while zinc levels were about twice as high as zinc concentrations in reference soils. Thus, road traffic, although being an important source of diffuse emission of copper today, obviously has not yet caused a measurable increase in the

5-cm thick layer of soil along the roads, perhaps due to the relatively short time period when copper-lined brakes have been in use.

4. The only soils types showing considerably increased concentrations of copper and zinc were those characterized by former industrial activities (waste land soils), where the copper levels were 10-15 times higher and the zinc levels about 5 times higher than the reference levels. Thus, these results indicate that copper contamination of urban soils appears to be more directly linked to pure industrial activities in the past, and not primarily a result of a general diffuse release from various goods in use.

Table 3.14. Concentrations of chromium, copper, nickel and zinc (as mean values of total metal in mg/kg dry weight) in various soil types in the city of Stockholm and in arable soils in the Stockholm region. After Linde et al., 2001 and Eriksson et al., 1997 and 1999.

Type of soil

Depth

Cr

Cu

Ni

Zn

City centre soils

0-5 cm

27

47

9.0

157

30 cm

27

67

10.6

193

Undisturbed soils

0-5 cm

49

23

14

76

30 cm

35

23

16

80

Park soils

0-5 cm

35

30

17

140

30 cm

40

30

22

100

Waste land soils

0-5 cm

44

290

15

400

30 cm

36

140

18

130

Roadside soils

0-5 cm

25

27

11

130

30 cm

36

28

19

150

Arable soils

0-20 cm

29

19

15

72

Linde et al. (2001) have also calculated the average pools of metals in the various soil types in Stockholm. Calculations were made for the upper 5 cm of city centre soils, park soils and arable soils, and for the upper 30 cm of city centre and park soils. Moreover, the total pools of metals in city centre soils were computed (based on a total area of 4.5 million m2) together with the difference in total pools between city centre soils and park soils, in order to provide a rough estimate of the amount of metals accumulated in the city centre soils due to local urban activities. The results for the 0-30 cm depth layer are presented in Table 3.15. The table does not include data for chromium and nickel, because the pools of these metals were smaller in the city centre soils than in park soils and in arable soils. In contrast, data on lead are included for the sake of comparison.

Table 3.15. Calculated average pools of copper, lead and zinc (g/m2) in the top 30 cm of soils in the city centre of Stockholm as well as in parks outside the city centre. Data are also given on total pools, together with estimates of amounts of metals accumulated due to local emissions (given in tonnes). After Linde et al., 2001.

Table 3.15. Calculated average pools of copper, lead and zinc (g/m2) in the top 30 cm of soils in the city centre of Stockholm as well as in parks outside the city centre. Data are also given on total pools, together with estimates of amounts of metals accumulated due to local emissions (given in tonnes). After Linde et al., 2001.

Metal

A v e r a g e Park soils

p o o l s (g/m2) City centre soils

Metal pools in city centre soils (tonnes) From local emissions T o t a l

Copper

12

21

40

93

Lead

12

38

120

172

Zinc

40

58

83

260

The average pools in city centre soils (0-30 cm) of lead was 3-4 times higher and of copper and zinc about 1.5-2 times higher than the background level.

The authors (Linde et al., 2001) also concluded that the estimates of annual emissions of copper and zinc to the Stockholm soil environment, 4 tonnes and 15 tonnes, respectively, made by Sörme et al. (2001b) and Bergbäck (1998), seem to be rather unlikely. This is because the accumulated metal amounts in the city centre would be the result of only 10 years of emissions in the case of copper and about 6 years of emissions in the case of zinc. The explanation of this difference might be either that only small fractions of the copper and zinc released from the technosphere end up in the city centre soils or that the total diffuse emissions in reality are smaller than those estimated by Sörme et al. (2001b) and by Bergbäck (1998).

As general conclusions, Linde et al. (2001) noted that their study showed:

• that more homogeneous soil groups - with regard to metal contamination - were obtained based on present land use than on geographic distance to the city centre;

• the importance of establishing a background level of metal concentrations in order to quantify the degree of local contamination; and

• that soil samples have to be taken below the surface layer (and deeper than 30 cm) in order to quantify the accumulated metal pools in urban soils.

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