Metal fluxes with groundwater in Stockholm

In the present context, it is also of interest to examine the metal fluxes with groundwater to the water recipients in Stockholm. As part of the Swedish project "Metals in the Urban and Forest Environment", Aastrup and Thunholm (2001) carried out a great number of measurements of groundwater hydrology and quality. The objectives of this investigation were:

- to get a notion of the concentrations of certain metals in the groundwater of Stockholm; and

- to estimate the metal fluxes with groundwater, in order to reveal the importance of this route, compared with others, for the input of metals to Lake Malaren and the Baltic Sea.

Sampling of metals was carried out at 70 sites and based on this and other data, a mathematical simulation of groundwater flow and metal transport was conducted to obtain estimates of the total amount of metals stored in the Stockholm groundwater, the total metal transport and the annual leakage of metals into Lake Malaren and the Baltic Sea. The main results of the study are summarized in Table 3.13 (after Aastrup and Thunholm, 2001).

Table 3.13. Chromium, copper, nickel and zinc in the groundwater of Stockholm. Median and mean concentrations, degrees of elevation above the levels in groundwater in forest ecosystems (nation-wide), total amounts stored in the groundwater, estimates of total fluxes and direct contributions to Lake Malaren and the Baltic Sea, the latter expressed both as percent of total transport from Lake Malaren to the Baltic Sea and as percent of the total discharge to water from the city of Stockholm. After Aastrup and Thunholm, 2001.

Table 3.13. Chromium, copper, nickel and zinc in the groundwater of Stockholm. Median and mean concentrations, degrees of elevation above the levels in groundwater in forest ecosystems (nation-wide), total amounts stored in the groundwater, estimates of total fluxes and direct contributions to Lake Malaren and the Baltic Sea, the latter expressed both as percent of total transport from Lake Malaren to the Baltic Sea and as percent of the total discharge to water from the city of Stockholm. After Aastrup and Thunholm, 2001.

Parameter

Cr

Cu

Ni

Zn

Median concentration, ^g/l

0.79

8.63

7.03

30.7

Ratio median(sto)/median(forest)

3.6

10.3

3.2

1.8

Mean concentrations, ^g/l

1.06

12.6

9.59

56.1

Ratio mean(sto)/mean(forest)

2.1

3.3

1.5

1.6

Total storage in groundwater, kg

67

720

590

2570

Total flux with groundwater, kg/y

11

115

94

410

Outflow to L. Malar + Balt.Sea, kg/y

3.1

34

28

120

% of total flux from L. Malaren

0.3

0.3

0.3

0.7

% of flux from anthroposphere

0.8

1.3

1.8

1.3

The results of the groundwater studies show that the enhancement of the median or mean concentrations of zinc in the Stockholm aquifer is small (<2 times) compared to corresponding values in groundwaters from forest ecosystems. Also with regard to chromium and nickel, the enhancements of median or mean concentrations are small (<4 times). As far as copper is concerned, the median value from Stockholm, 8.6 ^g/l, is ten times higher than the nation-wide forest ecosystem median, but the mean value (12.6

^g/l) is only 3.3 times higher than the "natural" level in forested regions. It should be noted, however, that the bedrock and pre-industrial lake sediments in the Malaren region hold about twice as much copper as sediments in the great majority of small forest lakes in the country (L & L -Cu). Therefore, the natural background for copper in Stockholm's groundwater is most probably also twice as high as in the forest ecosystems used for comparison in the table. Thus, the "real" enhancement of the copper median value in Stockholm's groundwater due to man's activities might be set at about 5 times.

The total storage of metals in the aquifer was estimated at about 2.6 tonnes for zinc and about a quarter of that quantity for copper (0.7 tonnes) and nickel (0.6 tonnes), while less than 70 kg of chromium were stored in the groundwater.

The direct outflow of metals to Lake Malaren, the Baltic Sea, including the Brunnsviken Bay, with groundwater is merely a fraction (about 30%) of the amount totally transported with groundwater. Before it reaches the great bodies of water, it flows into streams and small lakes or seeps into tunnels (Aastrup and Thunholm, 2001). It is also clearly shown in Table 3.13 that the fraction of the total flux of metals from Lake Malaren to the Baltic Sea that originates from the groundwater is extremely small, 0.3% for chromium, copper and nickel, and 0.7% for zinc.

If the groundwater flux of metals is regarded as caused by man, it can be noted that of the total flux of metals from the anthroposphere in Stockholm to the biosphere, the fraction transported with groundwater constitutes 0.8 - 1.8%. As pointed out by Aastrup and Thunholm (2001), the fact that a great part of the precipitation (thus, the runoff) is diverted by land sealing, drainage systems, tunnels, etc. in urban areas, groundwater recharge is usually lower than in rural areas.

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