Example 3 Urban metal flows Stockholm Cr Cu Ni and Zn

After the preparation (by MFG) of the three monographs on metals in society and the environment in Sweden, in which preliminary mass balances of Cr, Cu, Ni and Zn were included, more detailed data was compiled on metal stocks and fluxes in the city of Stockholm and comprehensive mass balances for metals were established for the Stockholm area. This work, published in 2001, was part of the project "Metals in the Urban and Forest Environment", financed by the Swedish EPA.

The total stock of Cu in Stockholm was slightly greater than 120 kt with an annual net growth rate of about 1.6%. The stock of Zn was less than a fourth of the Cu stock (28 kt) with a net growth of 4.3%. Ni showed that fastest growth (6.4%), but from a low stock level (2.5 kt).

Metal emissions, except for the flows to the solid waste compartment, were twice as high for Zn (24 t/y) as for Cu (12 t/y), while known emissions of Cr and Ni were low (0.8 and 0.6 t/y). Considerable fractions of these amounts of metals are channelled through sewage treatment plants, where most of the incoming mass of metals is incorporated into the sludge. However, a substantial part of the total releases of metals to water from the city of Stockholm ends up in the Stockholm Stream: compared to the large-scale flux of metals from Lake Malaren to the Baltic Sea, the contribution from Stockholm city is 15-25% for Cu and Ni, 30-40% for Cr and 40-55% for Zn.

The types of goods causing the greatest releases of metals in Stockholm were found to be:

- for Cu: water pipes in buildings, followed by motor vehicle brakes;

- for Zn: motor vehicle tyres, followed by various galvanized materials;

- for Cr and Ni: road pavements, followed by tyres.

Based on a comparison between the estimated metal emissions from various goods and the measured quantities of metals entering the biggest sewage treatment plant in Stockholm, it was concluded that virtually all existing sources of Zn had been identified, while the total emissions of Cu from possible sources were somewhat overestimated. By contrast, the correspondence between measured quantities and estimated emissions was poor to very poor for Ni and Cr. A closer look at the estimates of Zn and Cu emissions, however, tells us that also for these metals, there is a need to revise the figures published. In particular, it was found that a better correspondence between total fluxes from identified sources and measured inflows to sewage treatment plants can be obtained, if a certain retention in soil and concrete of Cu and Zn released from roofs and other infrastructure materials is taken into account.

The recently published material also allows a more accurate picture to be drawn of the pools and fluxes of the trace metals in the groundwater, soils and lake sediments in Stockholm. The total storage of metals in the aquifer of Stockholm was estimated at about 2.5 t for Zn, 0.7 t for Cu, 0.6 t for Ni, while less than 70 kg of Cr were stored in the groundwater. The total fluxes of the same metals with groundwater were estimated at 0.41 t, 0.12 t, 0.09 t and 0.01 t, respectively. If the entire metal transport with groundwater in Stockholm is compared to the total flux of metals from the Stockholm anthroposphere to the biosphere, the groundwater fraction constitutes 0.81.8%. In fact, metal fluxes with the Stockholm groundwater are much smaller than those in forest ecosystems, due to lower groundwater recharge in urban areas.

The comprehensive studies of metals in urban soils in Stockholm showed that the mean levels of Cu and Zn were enhanced 2-3 times in the city centre, compared to reference levels in regional arable soils and undisturbed soils within the city. No enhancement was found for Cr and Ni. The only soils showing considerably increased levels of Cu and Zn were those where former industrial activities had taken place. Here, 10-15 times higher levels of Cu and about 5 times higher levels of Zn, compared to reference values, were recorded.

Investigations of metals in the aquatic sediments in the Stockholm area, carried out as part of the programme "Metals in the Urban and Forest Environment", have been used to estimate the relative contribution from the city to the annual load of metals on the sediments. It was found that Stockholm every year contributed about 1 t of Cr, 1.7 t of Cu, 4 t of Zn, but no Ni to the regional aquatic sediments. Compared to the total fluxes from the city, via sewage treatment plants, storm water and groundwater, 0.4 t/y of Cr, 2.6 t/y of Cu, 9.1 t/y of Zn and 1.5 t/y of Ni, it might be concluded that there must be some still unidentified sources of Cr in the city, while a relatively small amount of Cu, (2.6 - 1.7) = 0.9 t/y, escapes deposition in the innermost archipelago and large to very large relative amounts of Zn, (9.1 -4.0) = 5.1 t/y, and Ni are transported further out across the archipelago into the Baltic Sea.

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