(introduction...)

William M. Stigliani and Stefan Anderberg

Introduction

In November 1986 an accident at a pharmaceutical company located on the River Rhine at Basel, Switzerland, resulted in the inadvertent release of 33 tons of toxic materials.' The effects were immediate and dramatic: half a million fish and eels died and local residents could not use the river as a source of drinking water for about a month. This accident, highly publicized in the world press, raised a major public outcry calling for an action plan for reducing the risks of such accidents in the future.

Historically, however, the impact of chemical accidents on the overall pollutant load to the river has been relatively minor. For example, in 1980 about 27 tons of toxic materials daily (10,000 tons per year) were transported by the Rhine into the Dutch Delta and the North Sea. This toxic load was the result not of accidents but, rather, of normal industrial, commercial, agricultural, and urban activities conducted within the Rhine Basin on a routine basis.

The effects of such chronic pollution are not as obvious or spectacular as those occurring after industrial accidents. Much of the daily input ends up in sediments of the Dutch Delta, and the rest is washed out to the North Sea. Even today the sediments in the delta are so polluted that the spoils, collected during dredging operations to keep navigation lanes open, are too toxic to be applied to polder lands in the Netherlands, as was the practice in the past. On the other hand, the River Rhine today transports far fewer pollutants to the Netherlands than it did in 1980, even though the level of economic activities in the basin has not changed very much since then.

Analysing the history of pollution in the Rhine Basin, including the recent clean-up, can provide valuable insights into the linkages between economic activities and chemical pollution, and the opportunities for decoupling economic growth from environmental degradation. The research described in this chapter, while not addressing all possible aspects of this history, will, we hope, provide a basis for improved policy-making.

Geographic features of the Rhine basin

The Rhine Basin extends over five European nations (fig. 1). Included are most of Switzerland, the north-east corner of France, Luxembourg, most of the south-western Lander (provinces) of Germany, and most of the Netherlands. The population of the basin is about 50 million and the area is about 220,000 km². About half of the land is used for agriculture, one third is forests, and the remainder is urban and suburban areas.

The basin is perhaps the most heavily industrialized in the world. Although the stream flow of the Rhine comprises only about 0.2 per cent of the flow of all rivers, about 10 to 20 per cent of the total Western chemical industry (OECD countries) is located in its basin. Industry is particularly concentrated in the catchment areas of the Ruhr, Neckar, Main, and Saar tributaries. Little net sedimentation of heavy metals occurs until the flow reaches the Dutch Delta, which extends from the German-Dutch border to the North Sea. About 75 per cent of the metals are deposited in the sediments of the delta, and the remainder disperses into the North Sea.

Methodology

Our study analyses the entire system by which resource inputs to the industrial economy are converted into outputs that must be absorbed and processed by the environment. For analysing a given chemical, this systems approach can be divided into three steps:

1. Identification of materials in which the chemical is embodied, and the pathways by which they flow through the industrial economy.
2. Estimation of emissions and deposition to air, water, and soil for each material at each stage of its life cycle.
3. Construction of a basin-wide pollution model for assessment of proposed emission reduction policies, environmental impacts, or other relevant issues related to the chemical in question.

Fig. 1 The Rhine Basin. Place-names in boxes signify locations of monitoring stations of the International Commission for the Protection of the Rhine

In step 1, it is essential not to miss any important source of pollution. In this regard, it should be noted that many chemicals enter the industrial economy inadvertently as trace impurities of high-volume raw materials such as fossil fuels and iron and non-ferrous ores. Moreover, a full accounting should be made of all stages of the material's life cycle. These include not only the stage of production, but also the later stages of use and disposal.

Overlooking important sources of emissions can be costly. For example, Tschinkel (1989) has noted that billions of dollars have been spent in the US on the construction of secondary sewage treatment plants. Many of the benefits gained from this technology, however, have been nullified because discharges of untreated storm waters containing toxic urban street dust continue to flow into lakes, rivers, and estuaries. Such an omission may not have happened had planners been more aware of the significance of street dust as a major source of toxic materials.

In step 2, emissions and deposition are estimated quantitatively. Emissions may be classified broadly into two categories: point source and diffuse.. Point sources include electric power plants, industry, incinerators, sewage treatment plants, and others. Their emissions are typically highly concentrated and confined to a specific location, usually within an urban area. For each type of point source, emission factors, generally expressed as weight of pollutant per unit weight of material consumed or produced, are assigned for emissions to air, water, and land. Emission factors may change over time, decreasing as cleaner technologies are implemented. Total emissions are calculated as the product of the emission factor and the weight of material consumed or produced.

Particularly in the case of atmospheric pollution, it is important to make a distinction between emissions and deposition (or immissions, as it is called in other languages). Via the mechanism of long-range atmospheric transport, emissions may be deposited hundreds or even thousands of kilometres from their sources. Thus, some emissions generated in the basin are transported and deposited outside the basin, and some emissions from outside the basin are transported into it. A long-range atmospheric deposition model has been developed at IIASA (Alcamo et al., 1992) for estimating deposition in the basin.

In contrast to point-source emissions, diffuse emissions are generally less concentrated, more dispersed spatially, and dependent on land use, which can be broadly categorized as forests, agricultural lands (both arable and grassland), and urban areas. The only inputs to forested lands are assumed to be atmospheric deposition via long-range transport. Chemical inputs to agricultural soils include not only longrange atmospheric deposition, but also agrochemicals, manure, and sewage sludge. Diffuse emissions from these two land uses are determined using a runoff export model (Jolankai et al., 1991).

Transport of pollutants to surface and ground waters is much greater from agricultural lands than from forested areas. Transport occurs via storm runoff, erosion, and vertical seepage. The relevant parameters to be determined are the rates of applications of particular chemicals, expressed as weight per hectare, and the partition coefficient, which determines the fraction of chemical that is mobilized and transported and the fraction that remains bound in the soil. Even if only a small percentage of the chemicals is mobilized, total emissions can be significant because of the enormous chemical inputs and the large spatial coverage of agricultural lands.

Another important source of diffuse emissions is transport of pollutants from paved urban areas to surface waters. This occurs by the build-up of toxic materials in street dust during dry periods, and the washing out of the dust during storm events. The pathways by which the transport may occur are shown in figure 2. There are three main sources of toxic materials in urban dust: corrosion of building materials (particularly for heavy metals such as zinc, used in galvanizing and surface materials), exhausts and lyre wear from automobiles and other road vehicles (important for lead and zinc), and local and longrange atmospheric deposition (a dominant source of cadmium).

When storm sewers are separate from municipal sewers (path SSS in figure 2), the pollutants are transported directly to surface waters. Alternatively, storm sewers may be connected to municipal sewers that discharge to surface waters without treatment (path CSSW in figure 2), or they may be connected to municipal sewers in which the effluents are treated (path WWTP in figure 2). In sewage treatment plants with primary and secondary treatment, typically 50 per cent or more of input heavy metals are trapped in sewage sludges. Even when the storm sewers are connected to sewage treatment plants, however, some fraction of the polluted street dust may be transported to the river unabated if the volume of storm flow exceeds the flow capacity of the sewage treatment plant (path CSO in figure 2), which is often the case. Another important source of pollution, also indicated in the figure, is the atmospheric deposition on unpaved urban areas, with subsequent seepage to ground waters and transport to the river.

Fig. 2 Pathways by which pollutants in urban areas are transported to surface waters (Source: Behrendt and Boehme, 1992)

To calculate emissions from corrosion, it is necessary to determine rates of corrosion per unit surface of the corroded material and the total surface coverage of the material in question. For instance, rates of zinc corrosion are strongly linked to urban SO₂ concentrations and will decrease over time as SO₂ levels are lowered. The following equation (ECE, 1984) shows the empirical relationship between SO₂ concentration and the rate of zinc corrosion from galvanized steel:

Y = 0.45* [SO₂] + 0.7.

where Y = annual corrosion rate of galvanized steel (g/m²/yr), and [SO₂] = concentration of SO₂ in air (mg/m³)

Emissions from road traffic owing to tyre wear may be estimated by determining emission rates per vehicle km, and multiplying this rate by vehicle km per year and the number of vehicles per year.

Lead emissions from combustion of gasoline may be calculated by multiplying lead emitted per unit of gasoline burned and multiplying by annual gasoline consumption. The emissions are allocated spatially by apportioning them according to traffic density.

In urban areas, local atmospheric emissions are particularly important, since a significant fraction, typically around 10 per cent of the total emissions, are deposited within 10 to 20 kilometres of the source. Factors affecting the proportion of local to long-range emissions include smokestack height, velocity of the gases and particulate matter leaving the stack, meteorological conditions, and particle size of emitted pollutants. The IIASA study includes an analysis of trends in local emissions as affected by changes in the abovementioned factors (Hrehoruk et al., 1992).

Lastly, it is necessary to employ an urban hydrology model that estimates the fraction of street dust that flows to the river. Even though urban and suburban areas occupy only about 15 per cent of total surface area in the basin, their contribution to the total diffuse load of aqueous emissions is significantly higher. This is because of the prevalence of hard, impermeable surfaces in urban lands (typically around 33 per cent of total urban area), from which run-off and transport of pollutants can be as high as 90 per cent, compared to a maximum of about 25 per cent for agricultural lands (Ayres and Rod, 1986).

Completion of steps 1 and 2 for a given chemical provides a pollution model of the basin, including inputs and outputs for the chemical, its flows through the industrial economy over time, and its spatial allocation for each time period of interest. The model can be used for various purposes. For example, a historical analysis of pollution can provide information on changing trends in pollution sources. In the case of pollution in the River Rhine, the IIASA analysis indicates that since the mid-1970s diffuse sources of emissions of heavy metals have become increasingly important relative to point sources. Another useful application of the historical analysis is the possibility for estimating the cumulative build-up of toxic materials in soils and sediments. Currently, hardly any information exists on the rates of accumulation of toxic chemicals over wide spatial regions, or on the evaluation of resulting impacts to the environment and human health. (For a comprehensive discussion of cumulative chemical loading and potential environmental impacts see Stigliani, 1988, and Stigliani et al., 1991.)

The pollution model can also be used to test the effectiveness of proposed policy options for reducing emissions of toxic chemicals. Because the model is based on mass balance analysis, all material flows to air, water, and land within the basin must be taken into account. The model will thus expose options that would not reduce overall emissions in the basin, but, rather, would transfer them from one pollution pathway to another.

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The example of cadmium

Step 1: Identification of materials containing cadmium and their pathways through the industrial economy

As an example of the approach taken in our study, the discussion focuses on the industrial metabolism of cadmium (Cd). Primary materials containing cadmium and its range of concentrations are shown in table 1.

Table 1 Natural occurrence of cadmium

Material	Typical range (ppm)
Soils, global average	0.01-0.7
Zinc ore concentrates	1,000-12,000
Lead ore concentrates	3-500
Copper ore concentrates	30-1,200
Iron ore	0.12-0.30
Hard coal	0.50-10.00
Heavy oil	0.01-0.10
Phosphate ore	0.25-80

Source: Boehm and Schaefers, 1990.

Cadmium enters the industrial economy inadvertently as a trace impurity of high-volume raw materials. The most important of these are phosphate ores, coal, oil, and iron ore. The production of phosphate fertilizer is a major source of aqueous cadmium pollution in the Rhine Basin (Elgersma et al., 1991), and fertilizer application is now the major source of cadmium pollution in agricultural soils. Combustion of coal and oil are major sources of atmospheric cadmium pollution. Iron and steel production results in the generation of large volumes of solid wastes contaminated with cadmium, as well as atmospheric and aqueous cadmium emissions. An added source of cadmium pollution in steel production is the input of steel scrap treated with cadmium as a surface coating.

The cadmium contained in zinc ores is generally of sufficiently high concentration that separating and refining it as a by-product of zinc refining is economically feasible. In fact, it is via this route that all primary cadmium is produced. There is some mining of Zn/Cd ores within the basin, but most of the ores, in the form of zinc concentrates, are imported. Inputs of cadmium to the Zn/Cd refinery are transformed into three outputs: refined cadmium metal; refinery emissions (to air, water, and soil); and a trace component in refined zinc metal (0.15-0.50 per cent in zinc produced at thermal refineries, and 0.02 per cent or less in zinc produced at electrolytic refineries). Cadmium is also present in lead and copper ores but, as shown in table 1, the concentrations are much lower than for zinc, and the production and use of lead and copper in the Rhine Basin is not an important source of cadmium pollution.

As shown in figure 3, cadmium and zinc pollution are linked for at least two zinc-containing products, automobile tyres and galvanized metals, which are significant sources of emissions, particularly in urban areas. Tyre wear and corrosion of galvanized zinc cause the release of zinc and associated cadmium. The deposited metals accumulate in street dust and may be transported as aqueous emissions during storm run-off or dispersed as windblown dust. The amount of cadmium contained as an impurity in finished zinc metal has been decreasing since the 1960s, as electrolytic refineries have accounted for an increasingly greater share of zinc production. Nevertheless, even in recent decades significant quantities of cadmium have entered the economy by this route.

For the Federal Republic of Germany in the period 1973-1986, it is estimated that 40 tons of cadmium per year on average (560 tons over the entire period) were contained in zinc products for domestic consumption (Rauhut and Balzer, 1976; Rauhut, 1978a, 1981, 1983, 1990; Balzer and Rauhut, 1987). Since the population of the West German part of the Rhine Basin comprises about one-half of the total West German population, and assuming an equal per capita distribution of zinc products, it is estimated that about 280 tons of cadmium entered the basin by this route from the Federal Republic of Germany alone. When account is taken of the rest of the population in the basin, a total of about 467 tons of cadmium is estimated to have been associated with zinc products over the 14-year period.

Fig. 3 The coupling of zinc and cadmium pollution caused by the presence of cadmium as a trace impurity in zinc-containing products

The largest inputs of cadmium to the Rhine Basin are not from inadvertent trace impurities, but rather through the refining of cadmium metal, and the production, use, and disposal of cadmium products. As shown in figure 4, cadmium metal, some of which is produced at the basin's zinc refineries and some of which is imported, is the input to plants that manufacture cadmium-containing products. The four major products are pigments (mostly for plastics), nickel-cadmium (Ni-Cd) batteries, plate (for surface protection of steel and other metals), and stabilizers (in PVC plastic). Emissions of cadmium occur for each of these manufacturing sectors.

Fig. 4 The flow of cadmium-containing products through the industrial economy

The next stage in the material flow is the use of cadmium in the basin. Ayres et al. (1988) have noted the importance of dissipative emissions, referring to emissions that may occur during the normal use of products. Such emissions are important, however, only when the chemical is easily mobilized during use. Of the four major cadmiumcontaining products, only cadmium plate might be expected to generate emissions to the environment via corrosion when exposed to polluted atmospheres during use (Carter, 1977). In the other three products, the cadmium is tightly embedded in the matrix of the product, and emissions during normal usage would be expected to be low or even negligible.

As shown in figure 4, recycling so far is important for only two cadmium-containing products, large industrial batteries and cadmiumplated steel. Recycling of batteries back to the battery manufacturers by the industrial users, mainly the railroad industry and operations requiring emergency power supplies, is a long-established practice. This is not the case, however, for small, sealed batteries, used by consumers in personal computers and other light electronic equipment. Most of these batteries still end up in municipal landfills or incinerators.

An estimated 30 to 40 per cent of cadmium-coated steel is recycled back to iron and steel producers as scrap. As noted earlier, this practice has been a major source of cadmium emissions in steel production. The remainder of the cadmium-plated metals ends up in landfills where most of the cadmium is corroded over time. Surface coatings for steel and other metallic substances were once the dominant use of cadmium in the basin. Currently, however, it is the smallest use, accounting for only about 20 tons in 1988, compared to about 250 tons in 1970.

Cadmium as a pigment in plastics is a major source of cadmium in the consumer waste stream (Schulte-Schrepping, 1981). Since the cadmium is tightly bound in the plastic matrix, however, it is expected that very little of it would be mobilized if the wastes were directly landfilled (Raede and Dornemann, 1981). When the plastics are incinerated, however, they constitute a major source of atmospheric cadmium emissions, as well as cadmium concentrated in the residue incinerator ashes.

Cadmium as a stabilizer in PVCs is used particularly in outdoor window frames. AS with cadmium-pigmented plastics, cadmium in this form is not likely to be appreciably mobilized, either during use or after disposal to landfills. Very little of the disposed PVC is incinerated, since most of it ends up in landfills with other debris from demolition of old buildings.

Step 2: Estimation of cadmium emissions and deposition to air, water, and solid wastes

Atmospheric emissions and deposition

ATMOSPHERIC EMISSIONS WITHIN THE RHINE BASIN. Because atmospheric pollutants may be transported over long distances, a substantial fraction of emissions generated in the basin are transported and deposited out of the basin and, conversely, some fraction of emissions generated out of the basin are transported and deposited in the basin. Therefore, the calculation of cadmium deposited in the basin from the atmosphere requires the incorporation of emission sources both inside and outside the basin.

Fig. 5 In-basin atmospheric emissions of cadmium

Such a European-wide database was provided by Pacyna (1988) and Pacyna and Munch (1988) for the early 1980s. Historical emissions were calculated using available production statistics for the relevant sectors generating the emissions, and estimations of the evolution of emission factors per sector since the 1950s (Pacyna, 1991). Deposition was calculated using the TRACE 2 model developed at IIASA and described in detail in Alcamo et al. (1992). The model employs "transfer matrices" which convert emission inputs into deposition outputs.

Figure 5 shows the in-basin atmospheric emissions of cadmium for selected years in the 1970s and 1980s. The Federal Republic of Germany has been the predominant source of emissions, accounting for between 75 and 80 per cent of total emissions over the entire period. Table 2 shows the distribution of emissions by sector for 1970 and 1988. One may observe that over the 18year period there was an overall reduction in air emissions of 87 per cent. This decrease occurred mostly from the implementation of emission-control technologies.

Additional factors, however, were also important. Emissions from coal and oil combustion declined because of the adoption of energy conservation measures, and the increased use of nuclear power. Emissions from iron and steel production declined in part because of the stagnation of production in the basin. The very large reductions in non-ferrous metal production (zinc, copper, and others) were in part the result of the closing down of large pyrometallurgical smelters. Another significant trend was the increase in the relative share of emissions from incinerator wastes. In 1970 these emissions only accounted for about 5 per cent of the total; by 1988 they already accounted for 14 per cent of total emissions.

Table 2 Atmospheric emissions of cadmium in the Rhine Basin by industrial sector, in tons per year and percentages

Process	1970 (%)	1988 (%)
Hard coalcombustion	26.2 (15.3)	10.1 (30.2)
Oil combustion	14.1 (8.2)	4.6 (13 8)
Other fossil fuel combustion	4.7 (2.8)	1.5 (4 5)
Zinc refining	31.6 (18.5)	3.5 (10.5)
Primary copper refining	25.2 (14.7)	0.4 (1 2)
Other non-ferrous metal refining	4.1 (2.4)	1.1 (3 3)
Iron and steel production	44.9 (26.3)	6.3 (18.9)
Coke production	3.9 (2.3)	0.5 (1 5)
Cement manufacturing	7.8 (4 6)	0.8 (2.4)
Waste incineration	8.4 (4 9)	4.6 (13.8)
Total	171.0 (100)	33.5 (100)

ATMOSPHERIC DEPOSITTON IN THE RHINE BASIN. Table 3 lists total atmospheric deposition in the basin, and the calculated contributions to the deposition by countries inside and outside the basin. One may observe that there was a 79 per cent decline in deposition over the 18year period, which is obviously strongly related to the decline in emissions within the Rhine Basin, as well as in the Western European nations in close proximity to it. Deposition in the basin contributed by the formerly socialist countries of Eastern Europe also declined during this period, although the decrease was not nearly as large as in the Western European countries.

Table 3 Atmospheric deposition of cadmium in the Rhine Basin - contribution by country, in (tons per year and percentages)^a

Country	1970	1975	1980	1985	1988
Federal Republic of Germany	55.3 (39.4)	41.2 (46.8)	23.6 (43.4)	13.9 (40 3)	11.7 (39 3)
France	8.5 (6.1)	5.7 (6.4)	3.9 (7.1)	2.4 (6 9)	1.8 (6.2)
Netherlands	14.6 (10.4)	2.5 (2.9)	1.9 (3.5)	1.2 (34)	0.9 (2.3)
Switzerland	1.4 (1.0)	1.1 (1.2)	0.9 (1.7)	0.7 (1 9)	0.5 (1.6)
Luxembourg	1.2 (0.9)	0.8 (0.9)	0.4 (0.7)	0.2 (0.6)	0.2 (0.6)
Belgium	35.5 (25.3)	15.3 (17.4)	7.1 (13.1)	3.6 (10.5)	3.5 (11.8)
United Kingdom	6.0 (4.3)	3.8 (43)	2.5 (46)	1.7 (5.0)	1.6 (5.3)
Italy	1.4 (1.0)	1.3 (1.4)	0.9 (1.6)	0.7 (2.0)	0.6 (2.1)
German Democratic Republic	5.1 (3.6)	5.4 (6.2)	4.8 (8.8)	3.9 (11.3)	3.5 (11.7)
Poland	4.2 (3.0)	5.3 (6 1)	4.1 (7.5)	3.1 (9.0)	2.8 (9.4)
Czechoslovakia	0.9 (0.7)	1.0 (1 1)	0.8 (1.5)	0.6 (1.9)	0.6 (2.0)
Soviet Union	1.8 (1.3)	2.1 (24)	1.8 (34)	1.4 (4 0)	1.2 (3.9)
Other	4.2 (3 0)	2.7 (3.0)	1.6 (3.0)	1.1 (3.1)	0.9 (3.1)
Total	140.3 (100)	87.9 (100)	54.4 (100)	34.6 (100)	29.8 (100)

a. Percentages in parentheses.

The share of total deposition attributed to the five Rhine Basin countries showed a slight but continuous decline, beginning in 1975, when they accounted for about 58 per cent of the total deposition, until 1988, when they comprised about 50 per cent. The share of total deposition from the three Western European nations Belgium, the United Kingdom, and Italy dropped by about 37 per cent between 1970 and 1988. In fact this trend was dominated by large decreases in Belgium's contribution, which was reduced from 25 per cent in 1970 to about 12 per cent in 1988. Belgium is one of the leading producers of zinc/cadmium in the world. Until the early 1970s, cadmium was produced solely at thermal zinc refineries with large atmospheric emissions of cadmium. During the 1970s, the thermal smelters were phased out, in some cases by closures and in others by a switch to electrolytic zinc/cadmium production with greatly reduced air emissions.

The opposite trend is shown for the Eastern European countries. Emissions from the German Democratic Republic, Poland, Czechoslovakia, and the Soviet Union contributed only about 9 per cent of the total deposition in 1970, but the share increased to about 27 per cent in 1988. This occurred not because emissions from Eastern Europe increased so much, but rather because total deposition in the basin decreased so rapidly, thus increasing the shares of contributions from Eastern Europe. These shifts in percentage shares reflect regional differences in efforts to limit air pollution. While Western European nations, beginning in the 1970s, were able to reduce their emissions mainly through the extensive application of air pollution control equipment, emissions in Eastern Europe remained largely unchanged during this time.

The estimated distribution of cadmium deposition among the three major land uses (agriculture, forests, and urban areas) in the basin is given in table 4. About 44 per cent of the total deposition goes to agricultural lands, 31 per cent to forests, and about 25 per cent to urban areas. Relative to their spatial coverage in the basin (about 15 per cent of the total land), urban areas receive more cadmium than forests or agricultural lands. This is particularly so because most point sources are located in urban areas, and about 10 to 15 per cent of atmospheric emissions are deposited locally within a radius of up to 20 km from the point source. (As will be discussed below, urban atmospheric deposition constitutes a major source of diffuse cadmium loading to surface waters in the basin.)

Table 4 Distribution of atmospheric deposition of cadmium in the Rhine Basin according to land use, in tons per year

Year	Agriculture	Forests	Urban areas	Total
1970	61.6	43.1	35.6	140.3
1975	38.0	26.6	23.2	87.8
1980	24.1	16.9	13.4	54.4
1985	15.4	10.7	8.5	34.6
1988	13.2	9.3	7.3	29.8

Figure 6 shows the estimated annual loads of cadmium for the time periods 1973-1977, 1978-1982, and 1983-1987 at various monitoring stations on the Rhine and some of its tributaries. The station at Lobith is on the GermanDutch border. Since there is no net sedimentation of cadmium on an annual basis in the River Rhine before the Netherlands border, the load represents the total inputs to the river from all upstream sources. The analysis was conducted by Behrendt and Boehme (1992) and is based on monitoring data provided by the International Commission for the Protection of the Rhine in Koblenz, Germany. The analysis also included a disaggregation of the total load into point and non-point (diffuse) loads by a methodology developed by Behrendt (1992).

One may observe the emergence of two major trends during this 14-year time period. Firstly, the cadmium load at Lobith decreased significantly over time, from 145 tons per year during the first period, to 96 tons per year in the second period, to 26 tons per year in the third period. Secondly, the relative contribution to the total load from point sources decreased from 82 per cent in the first period, to 79 per cent in the second period, to 42 per cent in the third period, while the contribution from diffuse sources increased from 18 to 21 to 58 per cent.

Aqueous emissions

INDUSTRIAL SOURCES. Table 5 lists estimated aqueous emissions of cadmium in the Rhine Basin during the 1970s and 1980s according to industrial sector (Elgersma et al., 1991). The estimates for a given point source were calculated by multiplying plant production by an assumed emission factor. Historical emissions were derived from production statistics for the various industrial sectors, together with an appraisal of changes in emission factors over time owing to the implementation of regulatory standards and the adoption of waterpollution control technologies. The assumptions applied in these estimations were calibrated by comparison with the estimates shown in figure 6.

Table 5 Aqueous emissions from industrial point sources in the Rhine Basin^a.

Activity	1970-72	1973-77	1978-82	1983-87	1988
Zinc production (primary and secondary)	74.0	63.9	40.4	2.8	0.1
Cadmium production (secondary)	3.0	3.0	1.2	0.0	0.0
Lead production (primary and secondary)	3.7	4.3	1.6	0.1	0.0
Coke production	10.7	9.5	4.3	1.0	0.9
Iron and steel production	21.4	21.4	19.5	8.5	2.1
Pigment manufacturing	13.4	7.0	1.3	0.3	0.1
Stabilizer manufacturing	1.3	0.6	0.0	0.0	0.0
Battery manufacturing	2.4	1.5	1.1	0.6	0.3
PVC manufacturing	1.4	1.2	0.9	0.4	0.1
Phosphate manufacturing	27.9	26.9	15.2	15.1	9.8
Total	159.2	139.3	85.5	28.8	13.4

Source: Elgersma et al., 1991.

a. The emission values listed here are somewhat higher than the values for point sources shown in figure 6 because not all the aqueous emissions within the basin end up in the River Rhine. Some are trapped in sediments of tributaries. Also, this table includes point source emissions in the Netherlands, while figure 6 includes emissions only up to the German-Netherlands border.)

One may observe that during the 1970s primary and secondary production of zinc was by far the most important source of aqueous cadmium pollution, accounting for more than 45 per cent of all emissions. Most of the pollution from this sector can be attributed to two thermal zinc smelters that did not refine cadmium, but, rather, treated it as a disposable waste. Reductions in emissions from these plants in the late 1970s and 1980s were achieved in part by recycling the wastes to an electrolytic zinc/cadmium refinery located in the basin. Further reductions were achieved when one of the smelters ceased refining zinc ore in the early 1980s.

Fig. 6 Estimated annual loads of cadmium at various stations on the River Rhine and its tributaries (Source: Behrendt and Boehme, 1992)

The second-largest polluting sector was the iron and steel industry, including coke production, which accounted for between 20 and 33 per cent of the emissions during the 1970s and 1980s. As in the case of the thermal zinc-refining sector, reductions in aqueous cadmium emissions were in part achieved by recycling the cadmium-containing wastes to electrolytic zinc/cadmium refineries. In fact, recycling of industrial sludges and solid wastes increased significantly during the 1970s and 1980s for most industrial sectors in the basin. The wastes were either recycled as a feed stock to other industrial sectors, or used as a filler in construction materials such as cement or asphalt. Another significant factor in reducing aqueous cadmium emissions in steel production was the reduction in cadmium-plated steel as a component of recycled steel scrap. Annual use of cadmium plate in the basin averaged about 300 tons per year throughout the 1960s; by the early 1980s use had dropped to around 80 tons per year, and by the late 1980s it was less than 30 tons per year.

Historically, among the industries manufacturing cadmiumcontaining products, the production of cadmium pigments was a major source of water pollution. In the early 1970s the pollution load from this source comprised about 8 per cent of the total from all point sources. By the early 1980s, however' pigment manufacturers accounted for less than 2 per cent, and in the late 1980s less than 1 per cent of the total load. These decreases occurred in part because of the implementation of more efficient pollution-control technologies (thus lowering emission factors), and in part because of the decrease in production of cadmium-containing pigments. (Nearly 900 tons per year of cadmium were used in pigment manufacturing in the early 1970s; by the late 1980s cadmium use had been reduced to under 400 tons per year.)

Another major source of industrial aqueous cadmium emissions is the phosphate fertilizer manufacturing industry. This accounted for about 17-19 per cent of total industrial pollution in the 1970s, and in the 1980s was the dominant source of pollution, responsible for about 50 per cent in the mid 1980s and 75 per cent in the late 1980s. During the conversion of phosphate ore to phosphoric acid, gypsum, containing about 30 per cent of the cadmium contained in the ore, is produced as a waste product. The largest polluters have been two fertilizer companies located in the Netherlands, both of which discharged the gypsum directly to the Rhine. It is the intention of these companies, however, to limit their combined emissions to less than one ton by 1993 (Elgersma et al., 1991).

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Conclusions

Analysis of the chemical "metabolism" of modern industrialized societies is very complex, requiring a sophisticated systems approach. Nevertheless, studies tracking the flow of toxic materials through the economy and into the environment can be an indispensable tool for environmental management.

Studies of the Rhine Basin are of particular interest because the region has passed through the phase in which high productivity is associated with correspondingly high levels of polluting emissions, into a phase where production has been partially decoupled from pollution. Understanding the societal, economic, and technological driving forces that stimulated this transition could provide a blueprint for the clean-up of river basins that are currently trapped in the coupled production-pollution syndrome - and there are many of them in the world today. The insights provided by our study, together with studies of the industrial metabolism of highly polluted regions such as Eastern Europe, could provide a basis for the rational prioritization of actions required for restoring the environment to an acceptable level.

Furthermore, the Rhine Basin study has demonstrated that even regions in a more advanced stage of environmental management could benefit from such studies. Indeed, "input management" (e.g. Odum, 1989), rather than "fire brigade actions," will be of increasing importance in formulating action alternatives that are better integrated and more directed toward the goals of long-term economic and ecological sustainability.

Finally, it should be noted that one of the most useful functions of soils and sediments is their ability to serve as a "sink" for the retention of toxic chemicals. This sink, however, usually has a finite sorption capacity, the size of which is governed by fundamental chemical properties such as pH, organic matter content, etc. So far, hardly any information is available regarding the long-term. broad-scale cumulative loadings of toxic chemicals into the environmental sinks, and on whether, and under what conditions, the sorption capacity might be transgressed beyond a threshold value considered safe for the environment and for human health. We believe that the analysis presensed in this chapter illustrates the usefulness of our approach in evaluating long-term environmental impacts.

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