Cadmium

Cadmium (Cd) is a relatively rare metal and, for uncontaminated soils, 'total' contents of <1mg/kg would be expected. Its geochemistry closely parallels that of Zn and, indeed, one of the major sources of Cd to the wider environment is through Zn mining and smelting. In an agricultural context, a major source of Cd is phosphatic fertilisers and almost all soils used for arable agriculture will have elevated levels of cadmium from that source. In contrast to a number of other metals, Cd is not known to have any metabolic role in plants or animals. It is toxic to animals at quite low concentrations and this toxicity is exacerbated by its accumulation in the kidneys of humans. Plant uptake may act as a significant pathway to the human food chain although it is not the only one - smoking, for example, can result in a daily intake of approximately one third of the recommended maximum daily intake. For transect 1 the mean cadmium content is 0.5mg/kg, for transect 2 it is 0.8mg/kg, for transect 3 it is 1.1mg/kg while for transect 4 it is 1.3mg/kg. This pattern is different from that obtained for the other metals where the mean for transect 3 is the highest but, for Cd, within transect 3 there is considerable variability from site to site and in transect 4 the mean is skewed by one very high value. As with the other metals, the values for soils receiving sewage sludge tend to have been adopted as environmental standards for soils but the values recorded here are below the maximum permissable value of 3mg/kg although a number are above the recommended value of 1mg/kg.