Contaminant contents, soil/crops

Use has also been made of the metal content of crop plants in the assessment of contaminated soils. Kabata-Pendias et al. (1993) suggest that legumes are promising as bioindicators of metal pollution since they have in general a relatively higher tolerance to and uptake of metal than monocotyledons. Kovacs et al. (1993) have... 

It has been shown repeatedly that an increase in soil Cd content results in an increased plant uptake of the metal. This has been demonstrated for soils with naturally elevated Cd levels (Lund et al., 1981), soils contaminated by non-ferrous metal mining (Alloway et al., 1988), and soils that have received Cd via sewage sludge application (Davis and Coker, 1980). It is this basic relationship that makes the soil-crop pathway of human exposure susceptible to increased levels of soil Cd. 

In addition, John and Laerhoven [374] have reported results indicating that in lettuces, the total cadmium uptake and translocation of cadmium to the edible leaf portion are dependent on the variety of lettuce. If other plants behave similarly, the effect of cadmium contamination of soils on the cadmium content of crops will be practically unpredictable. 

Localised problems are also possible as a result of the contamination of soils by arsenic and mercury present in compounds used as agricultural pesticides. Bowen [2] (p. 166) has stated that the arsenic content of arable fields in some parts of the world has risen to such an extent that crops taken from them will continue to contain excessive levels of arsenic for many years. This may also be true of the area around Manfredonia in south-east Italy, contaminated with arsenic as a result of an explosion in a chemical plant in September 1976 (Chapter 3.1). 

The uptake of heavy metals from soils is also determined by their contents in soil and by plant species. If these crops are used for anaerobic digestion one has to consider that there is a remarkable risk of an accumulation of heavy metals in soil. In the case of combustion, most heavy metals can be removed by filtering the ash and it can be used safely as fertiliser. Thus cultivation and combustion of short rotation coppice is a smart scheme of removing heavy metals from contaminated soils. 

The assessment of plant-available soil contents can frequently be achieved and validated by field experiments for nutritionally essential elements, and, for a few potentially toxic elements such as chromium, nickel and molybdenum, at the moderately elevated concentrations that can occur in agricultural situations. The validation of extraction methods, devised for agricultural and nutritional purposes, is much less easy to achieve when they are applied to heavy metals and other potentially toxic elements, especially at the higher concentrations obtained in industrially contaminated land. This is not surprising in view of the fact that for some heavy metals, for example lead, there is an effective root barrier, in many food crop plants, to their uptake and much of the metal enters plants not from the root but by deposition from the atmosphere on to leaves. In these circumstances little direct correlation would be expected between soil extractable contents and plant contents. For heavy metals and other potentially toxic elements, therefore, extraction methods are mainly of value for the assessment of the mobile and potentially mobile species rather than plant-available species. This assessment of mobile species contents may well, however, indicate the risk of plant availability in changing environmental conditions or changes in land use. 

The measured activity concentrations of tritium for crops grown in uncontaminated soil were generally very low, many being below the limit of detection. The tritium content of all crops grown in contaminated soil were measurable but low. Generally however, where analyses were carried out on the same crop and soil type in two separate containers, the results were not significantly different from each other. While the dominant form of tritium in soil was as OBT, the tritium in crops was nearly all present as tritiated water. 

Due to the wide use of cadmium-based products, cadmium is widely distributed in the environment. The cadmium content in soil and water has been increasing as a result of disposal of cadmium-contaminated waste and the use of cadmium-containing fertilizers (particularly on cereal crops). Commercial sludge, contaminated with cadmium, has been used to fertilize agricultural fields. Cadmium concentrations in urban air are quite low, because of regulation of industrial air emissions. Lead and zinc smelters and waste incineration account for the majority of cadmium present in ambient air. 

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