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Table 11 Inputs and outputs of cadmium to agricultural soils, in tons per year
|Run-off||-6.8||-4.2||-2.6||-1 .7||-1 .4|
APPLICATION OF AGROCHEMICALS. Three major inputs of cadmium to agricultural lands are via atmospheric deposition, application of phosphate fertilizer, and the spreading of sewage sludge. Once the cadmium is in the soil, it can be transported out of the soil by plant uptake, surface run-off, and erosion. The net accumulation of cadmium in agricultural soils can be determined by accounting for inputs and outputs, as shown in table 11.
In the Rhine Basin, net cadmium inputs to the soil have been reduced by more than 50 per cent between the early 1970s and the late 1980s. The most significant reduction has been in atmospheric deposition, which decreased by nearly 50 tons per year (80 per cent) over this time period. In addition, there were more moderate reductions in inputs from phosphate fertilizer (25 per cent reduction) and sewage sludge (about 50 per cent reduction). Whereas atmospheric deposition was the largest source of cadmium to agricultural lands in 1970, by the mid-1970s phosphate fertilizer became the dominant source. By the late 1980s it accounted for more than 70 per cent of the total inputs.
Plant uptake of cadmium increased mainly because of increases in crop yields over the 18-year period. (The yield of cereal crops in the basin is estimated to have increased from about 2.7 tons per year in 1970 to about 5.6 tons per year in the late 1980s; WRI, 1988.) Surface run-off of cadmium decreased because of decreases in the concentration of cadmium in wet deposition. Erosion of cadmium increased slightly, reflecting the slow increase in the total soil cadmium content over time.
An important question is whether the cumulative cadmium inputs to agricultural soils are causing significant increases in the soil concentrations of cadmium above background levels, and, if so, whether these increases could lead to unacceptably high levels of cadmium in the crops grown in the basin. With this question in mind, the net inputs of cadmium to agricultural soils since 1950 were estimated, using available information on historical phosphate use (Behrendt, 1988) and calculating atmospheric deposition.³ The net inputs were converted into soil concentration units, and the calculated increase in concentration was plotted over time as shown in figure 7. Under the assumptions of the model, the average cadmium concentration in agricultural soils has risen from approximately 360 g/ha in 1950 to about 700 g/ha in 1988, corresponding to an increase of 94 per cent in the 38-year period. (A later section discusses the possible implications of this increase for crop uptake of cadmium and human health.)
Fig. 7 Estimated build-up of cadmium concentration in agricultural sons in the Rhine Basin, 1950 1988 (Source: Stigliani and Jaffe, 1992)
Eventually, a steady state will be reached between inputs and outputs, and soil concentrations of cadmium will level off. Hutton (1982) has calculated a likely steady state concentration for the EC agricultural lands to be between 1,400 g/ha and 2,900 g/ha, for annual inputs of 8 g/ha of cadmium. If this value pertains to the Rhine Basin as well, then the current concentrations are still far from equilibrium, and soil concentrations will continue to rise well into the next century if current annual inputs continue in the future.
Diffuse sources of aqueous emissions
Diffuse sources of emissions may be differentiated from point sources in at least three ways. First, the emissions occur at random, intermittent intervals; secondly, emissions can vary by several orders of magnitude from one event to another; and, thirdly, the emissions are usually closely related to meteorological variables such as precipitation. The load of diffuse sources to the River Rhine was estimated in two ways. The analysis of extensive rivermonitoring data by Behrendt and Boehme (1992) provided annual estimates of the diffuse load during the 1970s and 1980s. Their results were shown graphically in figure 6. As a second approach, annual loads from the sources of diffuse emissions were estimated independently of the monitoring data. The results of the two approaches were then compared.
As was shown in figure 2, in urban areas four major sources of diffuse emissions have been identified: atmospheric deposition, corrosion, traffic in paved areas, and atmospheric deposition and landfills in unpaved areas.
Given that most of the population in the Rhine Basin is concentrated close to the river or its tributaries, it may be expected that a relatively high percentage of the cadmium deposited from these sources will enter the river. Atmospheric deposition (wet and dry) on paved surfaces accumulates as street dust, is washed off during storm events, and is conducted to the river through storm sewers or municipal sewage-treatment plants. Cadmium emissions from corrosion occur mainly via the corrosion of galvanized zinc, in which cadmium is present as an unwanted impurity. The corroded material accumulates in street dust and is washed out through the same pathway as atmospheric deposition. The main source of cadmium from traffic is automobile tyres. Zinc oxide is used in tyre manufacturing, and a small fraction of cadmium is present with the zinc as an impurity. The fragments from tyre wear collect in street dusts and are transported to surface waters along with other cadmium dusts.
Unpaved urban areas have been the recipient of high loads of cadmium from both short-range and long-range atmospheric deposition. The cumulative load to all unpaved urban soils over the period 195-1988 is calculated at about 830 tons. In addition, solid waste landfills are concentrated in unpaved urban areas. As was shown in table 10, the cumulative leaching potential during the 1970s and 1980s is estimated to have been between 200 and 300 tons of cadmium per year. During storm events, cadmium stored in unpaved areas may be transported horizontally by surface run-off or vertically through seepage to ground waters, where it may be transported to the river via subsurface flow. Leaching from landfills is more problematic for the older sites, which were not constructed to contain the wastes. In the case of the new, safer landfills, drainage waters are collected and treated at municipal sewage treatment plants. Even so, the plant may only remove 50-70 per cent of the cadmium. The balance of the cadmium leaves the plant in the aqueous effluent.
There are three major sources of diffuse emissions from agricultural areas. One is surface run-off during heavy storm events. It is estimated that 10 per cent of the river flow is contributed by such run-off. Cadmium, as wet deposition, is deposited during the storm and enters the river as a component of run-off. The second source is erosion of a thin layer of the surface soil. Cadmium contained in the layer is transported with the eroded soil. The third source is through subsurface groundwater flow. In this case, the source of the cadmium is from natural geological deposits. Most of the cadmium enters the river during heavy storm events, when run-off and erosion are accelerated.
Table 12 gives estimates of the diffuse loads of cadmium to the River Rhine from urban and agricultural areas during the 1970s and 1980s. The total load decreased by more than 60 per cent during the two decades. The most important factor determining this trend was the reduction in atmospheric emissions, which led directly to reductions of atmospheric cadmium inputs to paved urban surfaces and agricultural run-off. Moreover, the reduction of SO2 concentrations in urban centres greatly reduced the corrosion rate of galvanized zinc and its cadmium impurity (Hrehoruk et al., 1992).
During the early 1970s, paved urban areas are estimated to have been the most important source of diffuse cadmium emissions, responsible for more than 40 per cent of the total load. In subsequent years this share declined sharply - 36 per cent in 1975, 27 per cent in 1980, 22 per cent in 1985, and 20 per cent in 1988. In addition to reductions in air emissions, another mitigating factor was the reduction in the concentration of cadmium impurity in zinc owing to the increase in the production of zinc from electrolytic refineries. The latter produce a much purer zinc than do the thermal refineries. Despite this factor, cadmium emissions from traffic increased slightly because of the large increase in traffic in the 1970s and 1980s.
Table 12 Diffuse of aqueous emissions of cadmium from urban and agricultural areas, in tons per year
|Run-off and leachinga||8.5||7.3||6.2||4.8||3.6|
|Total calculated diffuse|
|Estimates of Behrendt|
|and Boehme (1992)||73-77||78-82||83-87|
a. Determined as difference between sum of paved and agricultural areas and mean values determined by Behrendt and Boehme (1992).
The estimated cadmium loads from unpaved surfaces are highly uncertain because no data are available for making direct calculations. Nevertheless, the estimates appear to be reasonable. As noted by Stigliani and Jaffe (1992), the vertical leaching velocity of cadmium is very much influenced by soil acidity. At a pH of 6, cadmium leaches at a rate of about 0.3 cm/yr. The rate increases to about 1.2 cm/yr at a pH of 5, and about 5.7 cm/yr at a pH of 4. Given that urban soils have received high loads of acidic deposition for many decades, a substantial fraction probably have pH values of between 4 and 5. Thus, it is most likely that shallow urban groundwaters have been contaminated to some degree, and that transport of these groundwaters to the Rhine is a significant source of cadmium pollution in the river.
Assuming that half of the cadmium originates from the cumulative loading of atmospheric deposition on unpaved areas and half from cadmiumcontaining solid wastes, each source contributed between 2 and 4 tons/yr. With respect to solid wastes, this amounts to about 1 per cent of the total cumulative annual availability given in table 10. The fact that the load from unpaved areas appears to be decreasing may be due to both reduced atmospheric deposition of cadmium and better containment of solid wastes in new landfills. Because of the cadmium already present in unpaved urban soils, however, it is possible that leaching to the River Rhine will continue for decades into the future.
With regard to agricultural soils, run-off of cadmium decreased because of reduced cadmium concentrations in wet deposition. Erosion of cadmium increased slightly, because the soil concentration of cadmium continued to increase over this time period, albeit at a slower rate than in previous decades. Cadmium inputs from groundwaters were assumed to be constant, because their origin is from natural geochemical sources unaffected by anthropogenic activities.
Step 3: Construction of a basin-wide pollution model for assessment of proposed emission reduction polices, environmental impacts, or other issues related to the chemical in question (in our case, cadmium)
The result of completion of steps 1 and 2 is a basin-wide pollution model which provides an enhanced understanding of the "industrial metabolism" of cadmium in the Rhine Basin. Inputs and outputs have been quantified over time, and changing trends in the sources of pollution have been identified. The pathways by which cadmium moves from its introduction into the economy of the Rhine Basin to its final destination in the environment have been delineated to the extent possible from available data.
Impacts of cumulative loading
One use of the model is to determine the long-term environmental impacts of the cumulative loading of toxic chemicals in the soils and sediments of the Rhine Basin. From 1950 to 1988, it is estimated that cumulative inputs of cadmium in unpaved urban areas, agricultural lands, and forests in the basin have been 830 tons, nearly 4,000 tons, and about 1,500 tons, respectively. Stigliani and Jaffe (1992) have examined the effects of cadmium accumulation in agricultural soils. According to this research, crop lands are particularly vulnerable to cumulative loading for two reasons. First, inputs of toxic materials from application of agrochemicals and atmospheric deposition have been historically high. Secondly, the pH of the soils has been artificially maintained at around 6 by the addition of lime. Soils at a pH of 6 have a much higher capacity to adsorb heavy metals and pesticides than do more acid soils. Thus, in contrast to the situation for acidified urban soils in which cadmium is rapidly leached out, the metal essentially accumulates in agricultural soils. (The history of this accumulation was discussed previously with regard to figure 7.)
Table 13 Estimated average intake of cadmium (miug/week) at different values of pH in agricultural soils in the Rhine Basin
|Year||pH = 6.0||pH = 5.5||pH = 5.0|
Source: Stigliani and Jaffe, 1992.
The next step in the analysis is to estimate the impact of the increased soil concentrations on plant uptake and food consumption. These aspects are discussed in detail in Stigliani and Jaffe (1992). Table 13 presents estimates of the average cadmium intake in ma/ week per capita in the Rhine Basin, assuming that all of the diet is obtained from food grown in the basin. The World Health Organization (WHO) recommends that maximum cadmium intake should not exceed 400 to 500 mg/week. The table shows that cadmium intake is still below the recommended maximum, but only under the condition that the pH of the soils is maintained at 6 or above. If the pH were to shift from 6 to 5.5 (more acid), the increased uptake of cadmium in the food supply could lead to levels that exceed the WHO-recommended threshold standard for human consumption. Such a shift is quite feasible, particularly in areas affected by acid deposition and given the fact that lime is often not applied continuously on an annual basis.
The most important point of the analysis is that in 1960 a shift to a soil pH level of 5.5 would have resulted in consumption levels still well below the WHO standard. Thus, the major impact of the cumulative loading of cadmium in agricultural soils since 1950 has been a loss of resilience with respect to cadmium uptake during random fluctuations in pH. The safe functioning of the agricultural system now requires tighter constraints on the range over which such fluctuations may occur.
Testing the effectiveness of proposed pollution reduction policies
One important application of the "industrial metabolism" approach is testing possible pollution-reduction strategies within the context of systems analysis. Because this approach is based on mass balance, policies that do not reduce overall emissions but, rather, merely shift them from one pollution pathway to another can be readily exposed. By way of example, a policy for reducing inputs of cadmium to the environment via the banning of cadmium-containing products will be discussed here. (Although no such ban is yet in effect, perhaps the anticipation that it will be implemented has already resulted in a marked decline, since the early 1980s, in the production and use of all cadmium-containing products, with the exception of Ni-Cd batteries.)
Let us assume that the policy under consideration would be the banning of all cadmium products in the Rhine Basin except for Ni-Cd batteries, for which there is a growing market and for which suitable substitutes have not yet been developed. Furthermore, it is proposed that by the late 1990s at least 50 per cent of the small sealed consumer Ni-Cd batteries entering the domestic disposal stream will be recycled. To what extent would this policy serve the purpose of reducing cadmium emissions?
The first step in the analysis is to determine the flow in the production, use, and disposal of cadmium products in the basin. Figure 8 depicts this flow for the mid-1980s.4 Also shown are the emissions of cadmium to air, water, and solid wastes. The numbers in brackets beside the solid wastes are the estimated availabilities, as previously defined. The sum of the availabilities of cadmium provides an approximate index by which the effectiveness of control strategies can be appraised. In the mid-1980s, this sum was 204 tons, with waste disposal the largest contributor at 195 tons.
Figure 9 is a flow diagram depicting the situation in the late 1990s, assuming that the proposed mitigation strategy has been implemented. It is assumed that the use of small, sealed consumer batteries will double relative to the mid-1980s. This represents a moderately reduced growth rate relative to the 1980s, when consumer use doubled within ten years.
One positive effect of the product ban would be a drastic decrease in availability from solid waste disposal, from 195 tons in the mid1980s to 49 tons in the late 1990s. A problem still remains, however, because the supply of cadmium entering the basin is essentially inelastic, since it is a by-product of zinc production. The production of zinc in the basin has been quite stable, averaging about 185,000 tons per year since the late 1960s. It is not likely that there will be any drastic reductions or increases in zinc production in the late 1990s. Thus, it seems reasonable to assume that about 600 tons of cadmium will be separated from zinc at the refineries. In contrast to the mid-1980s, with an import of 570 tons of cadmium metal, according to the scenario only 118 tons of cadmium will be required for battery production, resulting in a surplus of about 425 tons of cadmium at the zinc refineries. These will be in the form of flue dusts at the thermal refinery and leaching residues at the electrolytic refinery. There are no data on the availabilities of cadmium in these wastes. It appears that they may be considerable, however, since cadmium in most solid wastes is appreciably soluble in acids, as well as in the presence of commonly occurring complexing agents such as ammonia (Rauhut, 1978b). In recent decades the flue dusts from the thermal smelter have been sent to the electrolytic refinery for processing into cadmium metal. (As noted earlier in the discussion, until the late 1970s these flue dusts were discharged into the Rhine, and constituted the largest source of water pollution.) About 95 per cent of the cadmium in the leaching residues generated at the electrolytic refinery were refined to cadmium metal in the mid-1980s. Moreover, the iron and steel industry sent solid wastes containing about 35 tons of cadmium to the electrolytic refinery.
Fig. 8 Balance of cadmium product use and disposal in the Rhine Basin in the mid-1980s
Fig. 9 Scenario for balance of cadmium product use and disposal in the Rhine Basin in the late 1990s
Under the scenario shown in figure 9, there will be a very low demand for recycled cadmium-containing wastes from the metallurgical industries. EC regulations regarding the disposal of solid wastes will require that they be stabilized, i.e. they must be treated to ensure a low cadmium availability. Stabilization of wastes is expensive and not always effective in reducing availability (Kosson et al., 1991). So, the ultimate effect of banning cadmium products and recycling 50 per cent of the disposed consumer batteries may be to shift the pollution load from the product disposal phase to the zinc/cadmium production phase. This does not imply that banning cadmium-containing products is not a wise strategy; rather, it indicates that if such a ban were to be implemented, special provisions would have to be made for the safe handling of surplus cadmium wastes generated at the zinc refineries!
One possible option that would reduce the volume of surplus cadmium at the refinery would be to allow the production and use of cadmium-containing products with inherently low availability for leaching. The other option, depositing the cadmium-containing refinery wastes in safely contained landfills, has other associated risks, as noted above. What is certain is that as long as zinc is produced in the Rhine Basin, cadmium will be introduced as either a desired or an unwanted by-product.
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