PHASING OUT CADMIUM AND LEAD – EMISSIONS AND SEDIMENT LOADS IN AN URBAN AREA.
Arne Jonsson* (Dept. of Water and
Environmental Studies, Linköping University, S-581 83 Linköping, Sweden, arnjo@tema.liu.se,
phone: +46 13 28 29 59, fax: +46 13 13 36 30),
Martin Lindström (Dept. of Earth Sciences,
Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden)
This paper
examines the effects of the reduced use of cadmium (Cd) and lead (Pb) in
Stockholm, Sweden between 1975 and 1995. By applying emission factors on
estimates on the use of cadmium and of leaded gasoline, emissions to the
environment during the phase-out period have been reconstructed. The fluxes of
Cd and Pb at sewage treatment plants, in storm water, and in atmospheric
deposition was studied using literature data.
As an independent
record, the sediment deposition of cadmium and lead in the waters around
Stockholm was investigated, which facilitated reconstructions of annual metal
deposition in the sediments.
The data set indicates a reduced load of cadmium and lead on the aquatic surroundings of Stockholm. The reduction is, however, not as pronounced in the sediment deposition as in the calculated emissions or the turnover at sewage treatment plants and atmospheric deposition. This indicates that emissions may be delayed, or that there are other sources, e.g. resuspension of older sediments.
Cadmium (Cd) and lead (Pb) are potentially very toxic metals. They are intensively used, which in urban areas gives rise to the accumulation and possible relase of large amounts. Due to the potential hazards, the uses have, however, during the last centuries been restricted by legislation. In this context, it is the aim of this study to investigate the effects of the reduced usage on the technosphere and environmental fluxes in the city of Stockholm.
Stockholm, the capital of Sweden with approximately 1.5 million inhabitants, is sometimes called 'Beauty on water'. The main surface waters of Stockholm are the eastern parts of the large Lake Mälaren (area 1140 km2), which in the central city passes one main stream (Norrström) and enters the archipelago leading towards the brackish Baltic Sea.
The use of cadmium in Stockholm between 1940 and 1995 has been described by Lohm et al. (1997). These calculations are, in turn, largely downscaled from figures presented on a national level by Bergbäck et al. (1994) and by the Swedish Environmental Protection Agency. The downscaling was based on population statistics.
The total turnover of lead in petrol since its introduction in the 1940s was also described in Lohm et al. (1997). This reconstruction was based on data on the distribution of petrol and the lead concentration in petrol from Statistics Sweden and the Swedish Petroleum Institute.
Emission coefficients for different uses of metals and metal-containing products have been presented by Ayres and Ayres (1994). The coefficient gives the “fraction of material in question that is released in mobile form (to the air or water) within a certain period (a decade, more or less)”. “Consumption” in this context does not refer to final use, as in traditional economy, but to dissipative use, including intermediate consumptive uses, such as fuel additives and pigments. Also note that emissions calculated in this way includes atmospheric emissions, whereas the discussion below is mainly focussed on aquatic metal fluxes.
The coefficients used in the present study are: Cd in plating 0,15; pigments 0,5; batteries 0,02; stabilisers 0,15; Pb in petrol 0,75.
The turnover of cadmium and lead at the sewage treatment plants of Stockholm has been studied using the annual reports of the plants. Östlund and Jonsson (1998) present metal fluxes for the period 1975-96. The sum of metal emissions to water and sludge was assumed to represent the incoming amounts of metals to the plant.
A number of different studies on atmospheric deposition of metals in and around Stockholm have been used to calculate both the regional background deposition representing the long-range transported metals, and the local deposition in Stockholm, which is thought to include metal sources in the city, such as fuel additive lead (Rühling et al., 1996; Statistics Sweden, 1996; Johansson and Burman, 1998).
The fluxes of metals in storm water to the aquatic recipients of Stockholm in 1984 and 1994 have been presented by the Stockholm environment and health protection administration. It should be noted that these figures include the fluxes to a number of smaller lakes that have not been investigated in this study.
The sediment investigations used in this study are presented in Lindström et al. (submitted) and Jonsson (submitted). The annual deposition of Cd and Pb on the soft areas of fine sediment accumulation (A-areas) was calculated by multiplying the metal concentration in the sediment samples by the deposition of dry matter. This was then multiplied by the total area of accumulation to get the annual deposition.
The historical development of cadmium and lead fluxes in Stockholm is presented in table 1. Note that the methodology used does not allow too much confidence in the numbers, which are to be seen as semi-quantitative indications of the orders of magnitude of different fluxes, and of the different temporal trends.
|
|
Cd |
Pb |
||||
|
|
1975 |
1985 |
1995 |
1975 |
1985 |
1995 |
|
Use |
9 |
4,5 |
8 |
100 |
61 |
5 |
|
Emissions |
1,6 |
0,2 |
0,2 |
80 |
45 |
4 |
|
To sewage treatment |
0,4-0,61) |
0,1 |
0,05 |
6-101) |
2,5 |
1,5 |
|
From sewage treatment |
0,1-0,31) |
0,03 |
<0,001 |
1-21) |
0,42 |
<0,04 |
|
Storm water |
|
0,0082) |
0,0083) |
|
1,72) |
0,53) |
Atmospheric deposition
|
0,14) |
0,045) |
0,01-0,027) |
124) |
2-4,26) |
0,4-0,77) |
|
Of which regional
background |
0,033 |
0,021 |
0,01 |
1,2 |
0,6 |
0,1 |
|
Sediment deposition |
0,25-0,30 |
0,20 |
0,15 |
25-30 |
20 |
15 |
|
Of which from local sources
8) |
0,12-0,15 |
0,1 |
0,075 |
12,5-15 |
10 |
7,5 |
Table ¤. The fluxes of cadmium and lead in Stockholm 1975-1995 (tons/year), note that the lead use and emissions refer to only petrol lead.
1)
Because of considerable year to year variations between 1975 and 1980, the
highest and lowest values from this period are given. 2) 1984; 3) 1994; 4) 1970;
5) 1988; 6) 1980 (upper
limit) and 1988 (lower limit); 7) precipitation
samples from the inner city (upper limit) and moss analyses from the whole
municipality (lower limit); 8) As calculated for present day in Lindström et al. (submitted), the same proportions have been assumed for the whole
period.
The effect of the cadmium ban from 1982 is obvious in the estimation of cadmium turnover. In the early 1990s the wider use of NiCd-batteries reversed the trend of decreasing Cd turnover, and in 1995 the total use of cadmium in Stockholm was nearly as large as before the ban. Since the emissions from the batteries are very limited compared to the applications affected by the cadmium ban, the calculated emissions have decreased dramatically during the studied period without any significant changes due to the increased turnover in the 1990s.
The sewage treatment plants (STPs) were loaded with substantial amounts of cadmium in the late 1970s. Between 1979 and 1980 the load of cadmium was dramatically reduced at all STPs. Apart from the cadmium ban, which was already being implemented, installation of separate sewage treatment at industries is a possible explanation to this. During the 1990s it was possible to reduce the Cd concentration of emitted water further to get below detection limits in most analyses.
The cadmium load from storm water has been constant since the mid 1980s. Thus, it has changed from being marginal to being several times higher, compared to the contribution from STPs. It should be noted that the analysis of metal fluxes in storm water is a difficult task, since the water fluxes and metal concentrations are very irregular, so the figures should be used with great consideration.
The atmospheric deposition of Cd has been reduced over the whole period. This seems to have been due mostly to a reduction of local sources, since the regional background deposition has been reduced much slower.
The water surface of the studied area is approximately the same as the land area of the municipality, i.e. about 200 km2. But since the environment of the archipelago varies from urban to rural, the direct deposition on the studied water areas should be somewhere between the estimated total deposition in Stockholm, and what has been estimated as the regional background contribution of this amount.
The sediment load of Cd in the studied area has decreased since the 1970s, but not as much as the other flows. This indicates that there are sources of Cd to the sediment that have not decreased as much as the ones described above. The fact that the sediment load exceeds the calculated aquatic emissions from Stockholm points in the same direction. One such possible source is internal load, i.e. Cd may be resuspended from relatively shallow areas and gradually focused into deep areas of continuous sediment deposition.
The metal fluxes through Norrström in the central Stockholm, where the fresh-water from Mälaren meets the brackish water of the archipelago, have been monitored since 1986. Contrary to the other Cd fluxes discussed in this study, this one has increased during the studied period, from 50 kg/year in the late 1980s to 100-200 kg/year in the mid 1990s. The reason could be either an increased flow of Cd from the inner parts of Mälaren, or changed local conditions at the sampling site. Note that this flux includes Cd from up-stream sources as well as some of the sources within Stockholm. It may therefore not be easily compared to other fluxes. However, the increase in the Cd fluxes through Norrström is larger than the reduction in the other fluxes.
Both in 1985 and 1995 the total sediment deposition of Cd is in the same order of magnitude as the outflow through Norrström plus the other sources described above, indicating that the Cd reaching the archipelago from Mälaren is distributed over a larger area than Pb (c.f. below).
In 1975 the lead concentration of petrol was 0,34 g/l and the consumption in Stockholm was 317000 m3. In 1985, the consumption had increased to 400000 m3, but since the lead content was more than halved during the same period, the total turnover of lead in petrol was reduced significantly. Since 1995 lead additives in petrol are prohibited, which almost totally eliminated the lead emissions from road traffic (Lohm et al., 1997). The trend during the studied period has been a more or less linear reduction of the lead emissions from a maximum of 120 tons/year in the late 1960s.
Thus it is not surprising that all lead fluxes have decreased, including the outflow from Mälaren, although this shows considerable year-to-year variations: in the second half of the 1980s it was between 2,7 and 5,3 tons/year – in the mid 1990s 1,5-2,5 tons/year.
The lead emissions from STPs were dramatically reduced between 1975 and 1985, as was the load to STPs. After 1985, however, the emissions continued to decrease, whereas the load was practically constant. The fact that the load to the STPs is not correlated to the emissions from fuel indicates that other sources were, or became, more important.
The Pb in storm water was reduced between 1984 and 1994. Although quite significant this reduction does not correspond to the 90 % decrease in emissions from fuels. This indicates that there might have been other sources of Pb, or that the effect in storm water of reduced emissions lead was delayed. Compared to emissions from STPs, storm water fluxes of Pb have been much higher during all of the studied period. It also needs repeating that the data on storm water fluxes are subject to great uncertainties.
As for atmospheric deposition the local contribution seems to have been much more important than for Cd. Local and long-distant sources also seem to have followed approximately the same trend, so that, contrary to what was the case for Cd, the local sources are still more important than the regional background. Between 1975 and 1985 the atmospheric deposition of lead decreased more rapidly than the emissions from fuel combustion. In the following ten years the trends of emissions and deposition agreed, or the emissions decreased faster. It may also be noted that only a small fraction, about 10 %, of the Pb that was emitted from vehicles in Stockholm was deposited within the city limits (c.f. table 1). This is a bit surprising since it seems to indicate that most Pb is transported out of the city, whereas lead is usually thought to be deposited in the very vicinity of the roads. Another explanation is that the samples for atmospheric deposition measurements were taken in such a way that they missed the Pb that was deposited closest to the roads, i.e. within the first few meters.
While there seems to be a delay in the response of reduced lead emissions in most fluxes, for the deposition of Pb to sediments this is even more so. Whereas all other fluxes, including regional background atmospheric deposition, have been reduced by a factor 10 or 20 during the period 1975-95, the sediment load was only reduced by half, or even less. This trend is more or less the same as for Cd.
We have found that approximately 50 % of the present day sediment deposition of Pb originates from Stockholm (Lindström et al., submitted). This is much more than the sum of the emissions discussed above, including the flux through Norrström. One explanation to this could be that the large amounts of lead emitted during the 70’s and 80’s have not yet reached the sediments. The retention of Pb in soil is rather strong (Blais and Kalff, 1993), so polluted soils can still be acting as anthropogenic sources of lead to the aquatic recipients, long after the actual emission. The same is valid for lead that has already been deposited in sediments, as has been described for Cd. This illustrates that a rapid decrease in the emissions of metals can not be expected to be immediately reflected in the sediment deposition. Thus, the problem of metal pollution will continue for a long time.
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