Urban areas, lead soil concentrations

A proportion of the atmospheric content of trace elements produced in urban areas must be deposited in areas downwind, so that there must also be a general background of contamination at a much lower level in soils in rural areas in industrialised and densely- populated countries. Some fallout can also be expected in nearby seas of even in other countries beyond, and Tyler [167] states that the high concentrations of cadmium and lead in the moss carpets of southern Norway and south-western Sweden, reflect an atmospheric deposition ofithese elements transported from remote sources, probably from continental Europe and the UK. Fallout from atmospheric pollution into the sea is unlikely to produce high local concentrations of any potentially toxic element. Dispersion of trace elements in the environment by this particular route is more efficient than by any other and no pollution problems have been reported as having arisen in this way. 

An extended series of pot experiments in the greenhouse has been carried out at the Edinburgh School of Agriculture since 1964, with a view to providing information (i) on the relationship between the available levels of the elements, boron, copper, lead, zinc and cadmium in the soil and uptake of these elements by oats, clover, radishes and lettuces, and (ii) on the effects on yield of these plants of increasing soil concentrations of these elements. Since the levels of all these elements are markedly enhanced in urban and industrial areas, it has become important to determine the level of contamination above which plant growth is restricted, and the threshold toxicity levels above which no effective growth is possible. 

Values of AAF are plant dependent, due to differences in surface characteristics, but values of 5 0 are typical.Thus, a plant grown on an agricultural soil with 50 mg kg lead will derive 0.25 mg kg dry weight lead from the soil (CF = 5 x 10 ), while airborne lead of 0.1 pg m will contribute 2.0 pg g (=mg kg ) of lead (AAF = 20 m g ). Thus, in this instance airborne lead deposition is dominant. The air lead concentration of 0.1 pg m was typical of rural areas of the UK until 1985. Since that time, the drastic reduction of lead in gasoline has led to appreciably reduced lead-in-air concentrations in both urban and rural localities. 

Oxides and hydroxides of Fe and Mn are ordinary components of black soils, but their impact on the behaviour of microelements is very important. These compounds can absorb microelements, because they form membranes in soil (Kabata-Pendias et al., 2003). High content of Fe and Mn oxides and hydroxides in soil may lead to significant changes in the geochemical balance. However, the environmental impacts are not equally distributed over the territory. It is a well-known fact, that impacts in certain industrial areas are higher than in others. Table 3 shows the concentrations of 15 elements in urban soils. 

The various national and international surveys included food crop data for food crop components produced in uncontaminated soils. Soils in mban areas or areas impacted by lead smelters not only have elevated Pb content, as is to be expected, but crops grown there also show Pb concentrations well above samples produced in uncontaminated soils. Table 6.48 depicts food crops with high Pb content under these conditions. Finster et al. (2004) reported that leafy and root vegetables grown in urban gardens in Chicago, IL, had Pb levels well above 10 ppm. Swiss chard was found at 22—24 ppm... 

---