Heavy metals in water

	Authors: Christa Pohl and Ursula Hennings Baltic Sea Research Institute, Warnemünde

Key message

In general the 2003 (February) results of lead (Pb), mercury (Hg), and cadmium (Cd) concentrations in Baltic Sea surface and deep waters revealed no pronounced changes compared to previous years. Regional and temporal limited variations of trace metal concentrations, which have been observed periodically in the western Baltic, have been attributed to the wind induced mixing of the water column coupled to a more or less enrichment of suspended matter and associated trace metals in the water column.

Introduction

Lead, Mercury, and Cadmium are hazardous metals in the Baltic marine environment. Because of their large impact by rivers and the atmosphere (HELCOM 1997; 1998), their toxic effects on the ecosystem and their complex biogeochemical behaviour, it was recommended by HELCOM to monitor these contaminants isochronous.

The enrichment of metals in biota is dependent on their bioavailability. In this context it is useful to differentiate between dissolved and particle bounded (volume related) metal species as also implemented by OSPAR in the North Sea.

To study the development of trace metal trends in the water column and to establish comparable time series, it was recommended to perform the sampling of these contaminants once a year in wintertime (February), when the biological activities are limited. 

Background concentrations

The valuation of background concentrations for trace metals in Baltic waters to centuries, when anthropogenical pollution was reduced to a minimum, is limited. One possibility is the use of comparable data from areas which are less influenced by human activities e.g. the northern Atlantic waters (Table 1).

The trace metal concentrations in Baltic waters are still higher than in Atlantic waters, although a decreasing trend of 6 % per year for Cd and Cu was revealed between 1980 and 1993 (HELCOM 1996) in Baltic surface waters.

Table 1.Concentrations of dissolved trace metals (ng/kg) from theNorth Atlanticand theBaltic Sea.⁽¹⁾Kremling, K. & Streu, P. (2001); ⁽²⁾Pohl, C. et al.(1993); ⁽³⁾Pohl, C. & Hennings, U. (1999); ⁽⁴⁾Dalziel, J. A. (1995)

Geographical distribution

In 2003 regional differences between the surface waters of the western Baltic and the central Baltic Sea were observed for the lead (Fig. 1) and mercury (Fig. 2) concentrations, while for cadmium (Fig. 1), the concentrations were quite uniform in both regions. One explanation is, that in the 1960^th a dumping area highly contaminated in trace metals was established in the Bay of Lübeck. In combination with wind induced mixing and bioturbation of marine organism this area is still more or less a source for SPM and trace metal (Pb) enrichment in the water column.

Elevated mercury concentrations in the Baltic Proper were possible a result of atmospheric input, because the Hg emissions from Germany, Poland and Estonia are still high.

A new awareness has been observed in connection with the saltwater inflow event in January 2003. Not only the rivers and the atmosphere are potential input sources for trace metals into the Baltic Sea. Also the higher Cadmium and Copper concentration of the inflowing North Sea water contributed to an increase of the concentrations in the deeper basins in May 2003. It was calculated, that with the inflow of 200 km³ of North Sea water, an additional input of 3000 kg (3t) of Cd and 60000 kg (60t) of Cu entered the Baltic Sea.

 Figure 1. Distribution of lead and cadmium in the dissolved (diss) and particulate (SPM) phases in surface waters of the western and central Baltic Sea in 2003

 Figure 2. Distribution of total mercury in surface waters of the western and central Baltic Sea in 2003.

Long term development

Western Baltic

Inter annual differences at the sampling stations in the western Baltic for the period 1993-2003 demonstrated that not only salinity but also heavy metal concentrations are subject to fluctuations especially in the transition area between the North Sea and Baltic Sea (Fig. 3 - Fig. 5).

The observation period was perhaps too short for the detection of clear temporal trends in this transition area, especially because the Baltic was influenced by a major saltwater inflow in 1993/1994 and 2003.

Baltic Proper “Above Halocline”

Since 1999 we observed, that the water body “Above Halocline” is characterised by slightly decreasing trends (not significant) over the passed 5 years for the dissolved phases of Cd, Pb, and total Hg (Fig. 3 - Fig. 5). Now the question arise, if long-term trace metal trends in surface waters of the central Baltic are a result of reduced input to the Baltic Sea, or if they are related to a “feedback mechanism” including the stabilisation of the anoxic deep water regime and the trace metal export by diffusive exchange across the oxic-anoxic interface.

Baltic Proper “Below Halocline”

In the last years decreasing lead concentrations were also observed in the Baltic. Since 2000 they stabilised on a low level “Below Halocline” in the central Baltic. Reasons for that are the use of unleaded petrol, the fast vertical transport of lead bounded to particles and there enrichment in the sediments. Baltic Sea sediments are still high contaminated in trace metals.

Assuming that the Baltic deep water system changes to oxic conditions it is expected, that remobilisation of metals and their release back into the water column will take place.

 Figure 3. Trends for dissolved (diss) and particulate bounded (SPM) lead in the western Baltic Sea, and "Above" and "Below" halocline in the central Baltic Sea between 1993 – 2003

 Figure 4. Trends for dissolved (diss) and particulate bounded (SPM) cadmium in the western Baltic Sea, and "Above" and "Below" halocline in the central Baltic

Since 1995 /1996 a decrease of the Cd(dissolved) concentrations was observed for the waters “Below Halocline” in the central Baltic Sea, due to the precipitation of sulphidic Cd species under anoxic conditions in the deep water body (Fig. 4) and their enrichment in surface sediments. This example demonstrates, how variations in the trace metal concentrations are linked to eutrophication.

Figure 5. Trends for total mercury in the western Baltic Sea, and "Above" and "Below" halocline in the central Baltic Sea between 1993 – 2003

References

Dalziel, J. A. (1995): Reactive mercury in the eastern North Atlantic and southeast Atlantic. Mar. Chem., 49, 307-314.

HELCOM (1996): Third periodic assessment of the state of the marine environment of the Baltic Sea, 1989-1993, No. 64B, Background document, pp. 252.

HELCOM (1997): Airborn pollution load to the Baltic Sea 1991-1995. Balt. Sea Environ. Proc. No. 69.

HELCOM (1998): The third Baltic Sea Pollution Load compilation (PLC-3); Balt. Sea Environ. Proc., No. 70; Helsinki Commission; pp.133.

Kremling, K.; Streu, P. (2001): Behaviour of dissolved Cd, Co, Zn, and Pb in North Atlantic near-surface waters (30°N/60°W to 60°N/2°W). Deep Sea Research I; 48/12; 2541-2567.

Pohl, C.; Kattner, G.; Schulz-Baldes, M. (1993): Cadmium, copper, lead and zinc on transects through Arctic and Eastern Atlantic surface and deep waters. J. Mar. Syst.; 4; 17-29.

Pohl, C.; Hennings, U. (1999): Bericht zum Ostsee-Monitoring: Die Schwermetall-Situation in der Ostsee im Jahre 1999. Institut für Ostseeforschung, Warnemünde, Seestr. 15, 18119 Warnemünde, Germany.