Summing up Fluxes of Cr Cu Ni and Zn from Society to the Environment

The many studies of trace metal fluxes from society to the environment in various countries have provided data that made it possible to close the input-output balance for at least some important trace metals, if the calculations are based upon "total" metal (all chemical species lumped together). However, there is still very insufficient information on the actual metal species in most studied metal flows. Even less knowledge is available on the various transformations, from one set of species to another, that may take place during transport from the source to the final environmental sink for the metal. Such information is crucial for a proper assessment of the environmental risks associated with the spectrum of current metal uses in society.

In chapter 4, some critical steps in these large-scale metal fluxes have been discussed and a few important examples of recent advances in metal-flux research have been presented. These examples covered:

• a careful quantification of environmentally induced variations in metal release following corrosion of metal surfaces in the urban environment, and a description of changes in metal speciation in the runoff liquid during transport away from the source;

• a demonstration of how difficult it may be to establish relationships between metal releases from a certain road traffic intensity and biological uptake and effects in the receiving aquatic environment;

• a survey of the sources of trace metal fluxes to municipal sewage treatment plants and from there to sludge and agricultural land, as well as a discussion about metal speciation, uptake and biological effects in sludge-amended soil ecosystem;

• an assessment of ecosystems exposed to heavy and extremely long-term metal pollution, caused by mining activities for more than a millennium, including a discussion of why today's impacts on biota appear to be relatively limited.

One of the remaining central problems, when interpreting results from field studies regarding impacts of trace metals on aquatic biota and on predators (including man) consuming fish and other seafood, is how to separate the risks associated with current polluting activities from the risks caused by exposure to the cumulative mass (e.g. in bottom sediments) of a long history of metal pollution. Some recent examples from Stockholm provide a possible way of solving this problem, which is partly related to the difficulty in finding good estimates of the natural regional background concentration of trace metals in aquatic sediments.

Although some important advances have been made recently, it can still be concluded that our knowledge about the quantitative aspects of trace metal fluxes from society to the environment is insufficient and scattered. In particular, our knowledge about the exact quantities flowing from a specific source to different environmental compartments is far from complete. Furthermore, the precise composition of a certain metal flow with respect to the distribution between different metal species is usually not well described. Even less so are the various transformations from one set of species to another during the transport of metals from the source to the final environmental compartment, under the successive influence of different environmental conditions along the transportation route.

Nonetheless, it is extremely important to have at least a rough idea about the speciation and the factors governing the bioavailability of the metals released from society and their potential for causing toxic effects in terrestrial and aquatic ecosystems as well as on man.

Even if, today, we are able to close the input-output balance of the most investigated among the trace metals, taken as "total" metal (all species combined), there is very insufficient information on the flows of, for instance, the bioavailable fraction of a certain metal. Researchers in Sweden have devoted a great deal of interest to quantify the fluxes of many metals from society to the environment, and a few among them have had the ambition to separate the bioavailable fraction from the rest and to quantify that fraction. But still today, we are not in a position to draw a complete picture of the flows of even the most relevant metal species and based on that, carry out a comprehensive environmental risk assessment of metals such as chromium, copper, nickel or zinc.

One of the obvious problems involved is that most studies published in the literature are focusing on just one or a few aspects of the complex problem: for example, one study may be providing a very complete account of the sources of a certain metal and even quantify the amounts emitted per unit time, while another study contributes with a complete picture of the different metals entering into an STP and even of the distribution of the metals between sludge and effluent. However, integrating the different parts of the complex reality is still very rare and, particularly, reports describing the dynamics of the processes with the likely transformations between the most important chemical species are almost non-existing.

From what has been described in this chapter on the different approaches taken by Swedish researchers over the past few years, it can be concluded that:

• Researchers who have quantified the metal fluxes caused by corrosion and runoff from roofs and other constructions are among the very few (in Sweden) who try to make a serious description and quantification of the metal species involved, especially of the fraction that is bioavailable. However, although we have today a fairly good idea about the situation at the edge of a roof or just below a metal construction, we still need a good description of how the composition of metal species change during transport of the runoff water: How much of each species (in a city like Stockholm) goes via the combined sewer system to the STP, how much via the separate storm-water sewer, and how much would remain in the near-field environment due to chemical binding, adsorption, etc ? During this transport process, how much of the original metal species will be transformed into less (or more) mobile species ? Research is in progress in this field, and in a few years, the understanding of these processes is supposed to be considerably improved.

• To follow the metals released from road traffic related activities via storm-water flows to receiving lakes or rivers and there, to describe their uptake and possible effects on biota is an extremely difficult task, because the systems and processes involved are so very complex. It is seldom possible to separate out a certain quantity of a metal occurring in a storm-water sewer - and even less one occurring in a lake sediment - from all the rest and label it as a "traffic-generated" pollutant. Only when dealing with metals that are very strongly linked to the traffic sector, such as tungsten or lead, it would be possible to attribute the body burden of e.g. tungsten in an aquatic animal to its origin, the tyre studs of a car. For metals with a more diverse use in society, it might be almost impossible to base any environmental risk assessment of a certain traffic activity causing metal releases to the environment on biological studies in receiving waters.

• In many respects, it seems a lot easier to follow the chain of mobilised metals from the (relatively) closed system of wastewater disposal in domestic, commercial or small-industrial areas to the public sewer and the STP. From there it is relatively uncomplicated to describe the distribution of a certain metal between the sludge and the effluent from the STP to the receiving environment. As far as the sludge is concerned, the key question is, of course, whether or not the sludge would be recycled back to agricultural land and thus back to the crop-food cycle. When sorting out the relevant issues in this metal flux, it seems to be most important to focus the metal speciation efforts on the agricultural soil compartment and answer questions, such as:

- what is the ability of the metal to be taken up by the roots of the actual crops?

- what metal species can affect the soil micro-organisms?

- what is the potential for the metal to be leached out to the groundwater?

• Even easier might be to describe the consequences of metal release from mining wastes, since in this case, the metals being released from the waste heaps usually are so dominant in the system under investigation that all other sources would be totally marginal. If it is then possible to conduct biological studies of both metal uptake and effects on sensitive components of the aquatic ecosystem, the results obtained, e.g. concerning established relationships between a metal concentration, the prevalent metal species and the biological effect, would be rather straight-forward and unambiguous. In the case of the Falun Mine, it was possible to undertake studies of all the relevant links in the chain and, even if a detailed speciation of the metals involved was not carried out in the rivers or lakes studied, the relationship between exposure and effect could be sufficiently well described to draw important conclusions. Nonetheless, there are many remaining problems to solve in relation to the Falun Mine, for example to give more comprehensive and precise explanations of the relative lack of biological effects that would be expected at the concentrations of total metal recorded.

One of the most fundamental problems of metal research conducted in the field, if the results should be used to provide a basis for decisions on necessary administrative or technical measures to protect human health and the environment, is how to differentiate between the risks associated with present-day activities and risks caused by exposure to the cumulative results of a long history of metal pollution. In fact, this is an aspect that has not been given sufficient attention, but which is crucial for the correct understanding of the impacts of today's activities. In particular when conclusions are drawn on the basis of findings from sediments or sediment-dwelling organisms, i.e. when information is based on the natural archive accumulating the remains of many years of metal emissions, it is usually very difficult to separate the traces of the past from those of the present.

A relatively recent example from Stockholm can show how difficult it might be for the city authorities to arrive at the correct conclusions on whether or not there is a need for urgent measures to limit the use of certain metals, when the research report does not, to a sufficient extent, differentiate between effects of the past and effects of the present.

In the final report from a sub-project, being a part of the environmental monitoring in Stockholm's inner waterways, Broman et al. (2001) gave an account of the amounts of metals and organic pollutants that are deposited in "fresh" sediments. Fresh sediments were collected in sediment traps, which were kept at a water depth of 15-20 m for about one year, during each of the years 1996-97, 1997-98 and 1998-99. The traps were fixed at three different sites selected to illustrate the impact of the city on the current pollution load. One site was upstream of central Stockholm, Klubben, one in Lake Malaren (Riddarfjarden) in central Stockholm, and the last one in Saltsjon (Kastellholmen), the brackish-water area just downstream of the central city. The authors specified that it had previously been demonstrated that in the sediment traps used, a certain fraction of the collected sediments originates from resuspended bottom sediments. At the two sites in Lake Malaren, about 50% of the collected sediments can be estimated to be resuspended bottom sediments, while at Kastellholmen, this fraction would be 75%. However, in the evaluation of the data and presentation of the conclusions, no mentioning was made of how this difference would affect the results.

In order to illustrate the risk of misinterpreting this kind of data, we are presenting an overview of the most relevant results from the sediment traps and from analyses of the superficial sediment at the same sites, made by other researchers (Table 4.16). In the table, only the results for the two metals copper and zinc are presented. One reason for not displaying the data on chromium and nickel in this table is that no clear difference was found between the concentrations of these metals in the material collected in the sediment traps and in the superficial bottom sediments, which probably is due to the very small emissions of these metals from the anthroposphere in Stockholm.

The data in Table 4.16 regarding metals in sediment traps is averages based on four samples, (three consecutive years of sediment collection by Broman et al., 2001, and one sample reported by Lithner et al., 2003). Metals in superficial bottom sediments are averages from three samples at each site, two reported by Ostlund et al., 1998, and one by Lithner et al., 2003). Since the copper and zinc concentrations in the trapped sediments were always much lower than in the sediments from the bed, it may be concluded that freshly sedimenting material is less contaminated with these metals. The question is how much less. Since the material in the sediment traps is reported to be a mixture of freshly sedimenting material and resuspended bottom sediments, the new material must be much lower in metal content than what is reflected in the sediment traps.

In Table 4.16, a plausible concentration of copper and zinc has been given, based on calculations assuming that 50% resuspended material was found at the two sites in Lake Malaren, while 75% old material occurred in the sediment trap in Saltsjon. For comparative purposes, copper and zinc levels in zebra mussels exposed for six weeks at the same sites are also presented in the table and so are the regional background concentrations in sediments, estimated by Landner (section 5.7 of this report), and by Lithner et al., 2003, both as "unaffected superficial sediments upstream Stockholm" and as "preindustrial sediments from the Strangnas area". Finally, the "background" levels in superficial, oxidised sediments in the central part of the Baltic Sea are also given as a reference.

Table 4.16. Overview of copper and zinc concentrations in sediments collected in sediment traps and in superficial bottom sediment from three sites in the inner waterways of Stockholm. The table also displays calculated concentrations of copper and zinc in freshly settling material. For comparison, estimated regional background concentrations in sediments and copper and zinc levels in transplanted zebra mussels are given. All concentrations expressed as ^g/g DM. After Broman et al., 2001; Östlund et al., 1998; Lithner et al., 2003.

Matrix Klubben Riddarfjärden Kastellholmen

Cu Zn Cu Zn Cu Zn

Sediment traps (4)*

100

280

140

430

320

650

Surf. bottom sedim, (3)*

170

380

220

560

370

700

Difference

70

100

80

130

50

50

Freshly settling material (calculated) Background (Landner) (L. Malaren) Background (Lithner)

- recent sediments

245 130

~60

~290

~140

~490

Central Baltic Sea - oxidised surf. sediment

45

190

Zebra-mussels (sect. 4.2.3)

11

180

12

320

10

240

* number of samples

* number of samples

Due to the geological background in the Lake Malaren catchment, the pre-industrial background of copper in sediments is relatively high (35-37 ^g/g DM) as compared with the median value in pre-industrial sediment layers in forest lakes (about 15 ^g/g DM).

The table also shows that concentrations of copper and zinc in surficial bottom sediments are higher than in the material collected in the sediment traps. The difference was greatest at the site in Riddarfjarden in central Stockholm. Considering that the traps in Lake Malaren did contain about 50% resuspended bottom sediments, the freshly settling material would have a copper concentration close to the regional background level (35-50 ^g/g DM) at the site Klubben, and about twice as much at the site Riddarfjarden. However, at Kastellholmen, the freshly settling material appears to be clearly contaminated by anthropogenic copper. A similar picture emerges for zinc: no or very low contamination at Klubben, a certain impact in Riddarfjjarden, and a clear enrichment, probably caused by anthropogenic zinc at Kastell-holmen.

In a follow-up study in May and June 2002, IVL carried out a comprehensive sampling of bottom sediments from water-ways and lakes in Stockholm and in the adjacent Svealand coastal region, in order to determine the concentrations of 32 Water Framework Directive (WFD) priority substances (Sternbeck et al., 2003). Among the analysed substances were 10 metals, including Cr, Cu, Ni and Zn. The sediment samples were taken from the top (0-2 cm) layer and each sample was a mixture from at least 8 sediment cores. Samples were digested with concentrated nitric acid in the autoclave before chemical analysis. Thus, no attempt was made to separate different chemical species, such as the bioavailable fraction.

When the trace metal levels in the sediments sampled in central Stockholm were compared with preindustrial levels, or upstream areas, a slight enrichment was noted for Cu and Zn. But when levels in central Stockholm were compared with those in small lakes in the region, no enrichment was demonstrated, except for a small enhancement of Cr. In the coastal region, just downstream of Stockholm, there was a fairly low impact of trace metals, with Cr and Ni being almost identical to the preindustrial levels, while Cu and Zn were slightly elevated in the archipelago of Stockholm, but levels were lower than in surface sediments from the open Baltic proper (Sternbeck et al., 2003).

The actual metal concentrations in central Stockholm sediments were also compared with the levels found in 1997, and it turned out that there was a general decline in the concentrations of Cu and Zn, while those of Cr and Ni did not show any change. The decreasing trend for sediment metals over the last 5 years was found to be in agreement with general long-term trends in the region (Sternbeck et al., 2003).

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