Stockholm

The possibility of using bottom sediments in lakes and inlets of the Baltic Sea to quantify the annual loads of metals from the Stockholm area, in particular metal fluxes resulting from diffuse urban emissions, was investigated by Lindström (2001) and by Lindström et al. (2001).

In the study forming part of the collective project "Metals in the Urban and Forest Environment" (Lindström et al., 2001), the Stockholm archipelago area and the eastern-most parts of Lake Mälaren were divided into 14 subareas, mainly along a transect corresponding to the principal flow of water, to investigate the metal load on the sediments. During two sampling cruises in August 1997 and June 1988, 5-7 sediment samples from accumulation bottoms in each of the defined sub-areas were collected. Two or three cores per sub-area were dated using the 137Cs technique.

Sediment samples (<0.5 g) for metal analyses were digested in 20 ml of 7 M HNO3 at 120 °C for 30 min. (according to Swedish Standard), and then analyzed using ICP-MS. Average annual deposition of metals in each sub-area was determined by multiplying surface sediment (0-2 cm) metal concentrations (^g/g dry sediment, ds) with mean sediment deposition rates (g ds/m2, y) from the dated cores and areas of accumulation bottoms (m2).

The estimated total metal deposition in the combined investigated sub-areas situated in Lake Mälaren close to the city of Stockholm as well as in the combined sub-areas in the archipelago (Baltic Sea) close to the city are presented in Table 3.16, together with the estimated range of current metal sediment load from Stockholm and the average metal fluxes with water flowing from Lake Mälaren to the sea. All fluxes are given in t/y, after Lindström et al. (2001).

In order to estimate the metal load on the sediments from the city of Stockholm, the deposition had to be corrected for the background deposition. The approach chosen to estimate the background deposition was to utilize the metal concentrations from the outermost sampling points as reference values, and then correct these concentrations to account for processes influencing the natural metal concentration (reduction/oxidation reactions and desorption). Therefore, the reference values were normalised to an element that has only (or almost only) natural sources and similar sedimentological properties as the elements of interest, such as Sc, Al or Ti. It turned out that also Ni could be used for normalizing purposes, since it was not possible to detect any enrichment of Ni in central Stockholm compared to Sc. The ranges of the Stockhom metal load on the sediment were then calculated from the 95% confidence intervals of the ratios of Cr, Cu and Zn to Ni (Lindström et al., 2001).

Table 3.16. Estimates of total metal deposition to sediments in Lake Mälaren and in the Baltic Sea archipelago close to the city of Stockholm, and of the sediment metal loads originating from Stockholm (max and min), as well as the average metal flux from Lake Mälaren to the Baltic Sea in the period 1995-1997. All fluxes in t/y. After Lindström et al., 2001.

Table 3.16. Estimates of total metal deposition to sediments in Lake Mälaren and in the Baltic Sea archipelago close to the city of Stockholm, and of the sediment metal loads originating from Stockholm (max and min), as well as the average metal flux from Lake Mälaren to the Baltic Sea in the period 1995-1997. All fluxes in t/y. After Lindström et al., 2001.

Type of flux

Areas

Cr

Cu

Ni

Zn

Total deposition to sed.

L.Malaren

1.13

1.54

1.18

5.91

Archipel.

3.55

5.56

2.15

16.8

Combined

4.7

7.1

3.3

23

Load to sed. from Sthlm

Min

0.3

1.5

__

3.6

Max

1.2

4.5

--

12

Flux with outflow from L.Malaren

1.2

14

11

21

If the metal amounts deposited in the innermost archipelago exceed the outflow from Lake Mälaren (Cr is an example of this), there must exist other sources of metals than the surface water outflow to explain the deposition. The authors discuss such potential sources, such as metal fluxes with ground water and with effluents from sewage treatment plants, but conclude that these are very small to small compared to the outflow from Lake Mälaren (Lindström et al., 2001). Also the direct atmospheric deposition to the water surface appears to be low (<20% of the metal fluxes with the Lake Mälaren outflow, according to the analytical laboratory, SLB, 1998).

The possible explanation to the above phenomenon, put forward by Lindström et al. (2001), is that there might be a large inflow of Baltic Sea water into the Stockholm archipelago. To support this hypothesis, they mention the observed metal depositon pattern and the surprisingly high 137Cs inventories (10-40 kBq/m2) in these archipelago areas, in spite of the low direct fallout (0-2 kBq/m2), as pointed out by Meili et al. (2002).

As can be seen in Table 3.16, the estimates of the metal loads to the sediments originating from the city of Stockholm show a great uncertainty, with a range of 3-4 times between the minimun and maximum value. There may be a different approach to determining the contribution from Stockholm city to the metal burden of the sediments. This approach is based on the rather simplistic assumption that the increase in relative metal deposition between Lake Mälaren and the innermost Baltic Sea archipelago would very roughly reflect the contribution from the city of Stockholm. However, the total metal deposition values for the two areas, uptream and downstream of

Stockholm, given in Table 3.16 have to be corrected for difference in total surface area and difference in the average percentage of accumulation bottoms in the two regions, acccording to data from Lindström et al. (2001). Normalisation of the metal deposition data has been made as follows:

The combined surface area of the innermost archipelago, used to calculate total metal deposition is 123.5 km2, i.e. 1.49 times larger than the corresponding surface area in Lake Mälaren, 83 km2. The average percentage of accumulation bottoms is 1.14 times greater in the archipelago region than in the Lake Mälaren regions under investigation. The product of these relations is 1.7.

- Cr: 3.55 / 1.7 = 2.09 (normalised deposition in innermost archipelago); thus, the approximate contribution from Stockholm would be: 2.09 -1.13 = 0.96 t/y;

- Cu: 5.56 / 1.7 = 3.27 ; thus the contribution from Stockholm would be about 3.27 - 1.54 = 1.73 t/y;

- Ni: 2.15 / 1.7 = 1.26; thus Stockholm would give: 1.26 - 1.18 = 0.08, i.e. close to zero contribution;

- Zn: 16.8 / 1.7 = 9.88; thus Stockholm would contribute 9.88 - 5.91 = 3.97 t/y.

Using this simplified method to roughly estimate the contribution from Stockholm to the total deposition of metals to the sediments in the innermost archipelago of the Baltic Sea, we arrive at almost the same conclusions as did Lindström et al. (2001). The city of Stockholm seems to be insignificant for the sediment loading with nickel, which is in accordance with the view expressed by Lindström et al. (2001). The contribution to the chromium load to sediments (about 1 t/y) is close to the maximum value calculated earlier, while the now calculated loads of copper and zinc to sediments (1.7 and 4.0 t/y, respectively) are close to the minimum levels calculated by Lindström et al. (2001). Of the total amount of metals deposited in the sediments in the innermost archipelago, it can be estimated that about 30% of the chromium and copper originated from Stockholm, while less than 25% of the zinc had the same origin. The contribution of the city to the nickel deposition in the sediments was almost nil.

If we compare the above calculated contributions from Stockholm to the metal loads on sediments in the innermost archipelago with the combined measured metal fluxes from STPs, with storm water and ground water (i.e. the waterborne fluxes from the anthroposphere), see Table 3.10, the following picture emerges:

- Cr: 0.4 t/y (measured efflux) to be compared with 1.0 t/y (from Stockholm to sediment) - additional sources existing, to explain higher depositon load;

- Cu: 2.63 t/y (measured efflux) to be compared with 1.7 t/y being deposited - a relatively small net amount of the city's emissions seems to be transported further out to the sea;

- Ni: 1.53 t/y (measured efflux) to be compared with 0.08 t/y being deposited - indicating a considerable amount being transported towards the sea;

- Zn: 9.12 t/y (measued efflux) to be compared with 4.0 t/y being deposited - a relatively large net amount appears to be transported towards the sea.

Finally, the relation between the total metal deposition on sediments in the innermost archipelago and the total metal flux through the Stockholm Stream shows that the most mobile element of the four being in focus here is nickel (only 20% of the flux is deposited), copper has an intermediate mobility (40% deposited), and zinc is the least mobile of the four elements with 80% of the flux being deposited. For chromium, the deposition is greater than the flux through the Stockholm Stream. The indicated relative mobilities of the metals are not in accordance with conventional knowledge, according to which zinc usually is considered a mobile element. These unexpected relationships emphasize the risks involved in using relatively uncertain data for far-reaching conclusions, and brings attention to the fact that the establishment of mass balances of environmental elements based on sedimentological studies is very difficult and calls for a thorough scrutiny of methods used. Just to mention one limitation in the possibility to use Lindstrom's et al. (2001) work for calculating the relative contribution of the city of Stockholm to the total metal loads to sediments, it should be mentioned that some of the investigated sub-areas in the archipelago (e.g. sub-areas 3, 4 and 5) are influenced not only by the emissions from the city of Stockholm, but also by STP discharges (e.g. the Kappala STP), treating sewage from other towns. Therefore, the amount of metals in Baltic Sea sediments that were estimated to originate from the city is, in reality, an overestimate of the contribution from Stockholm.

Nonetheless, Lindstrom et al. (2001) have provided a much more reliable picture of the real impact of the infrastructure and activities in Stockholm City on sediment ecosystems in the surrounding waters than has been given by some other authors in their recent reports. Thus, unfortunately, certain reports, from studies of metals and organic contaminants in sediments of waters surrounding Stockholm (e.g. Ostlund et al., 1998; Sternbeck and Ostlund, 2001; Broman et al., 2001), have presented rather misleading conclusions on the actual role of the city in explaining the total load on nearby sediments. However, in a later raport by Sternbeck et al. (2003), some of the previous overestimates have been rectified (cf. section 4.5).

In none of the above-mentioned studies from 1998-2002, there was any serious attempt to estimate the natural background concentration of metals in the region, which makes it almost impossible to assess the relative contribution from the city. Moreover, in the latter of the three earlier studies (Broman et al., 2001), the deposition of contaminants to sediments in the innermost Baltic Sea archipelago was overestimated about three times due to a calculation error.

Because of these shortcomings, an approach is proposed in this report to how the regional, natural background concentrations of trace metals in aquatic sediments may be established. This proposal is included as section 5.7, after a thorough discussion about speciation and bioavailability of trace metals in the environment.

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