The possibility of UPA production presenting a hazard to natural waters depends upon the kind of management, and also the local ‘environmental vulnerability,’ of natural waters. For example, significant areas where an economically major aquifer is overlain by soils of high or intermediate permeability occurs within both Greater London (NRA, 1994) and throughout north and east Kent (NRA, 1994). Allotment sites in the Ashford area are over aquifers from which public water supplies are drawn. Whilst the risk from heavy metals is discussed below, major risks to waters (both groundwater and from surface runoff) could arise from excessive use of pesticides and from nitrogen fertilisers, both artificial and organic. The polluting effects of urban cultivated areas adjacent to watercourses and effects on the underlying hydrogeology are potentially important, but largely ignored.
Table 20.1. Species, varieties of vegetables and their yields obtained from allotment holders in Ashford and Wye, Kent
Table 20.1 (continued)
1lb = 0.45359 kg; * = estimated yield available only.
Contaminated land is usually defined as ‘land that contains substances which, when present in sufficient quantities or concentrations, are likely to cause harm, directly or indirectly, to people, to the environment, or, on occasion, to other targets’ (Garnett, 1996c). If land is sufficiently contaminated, handling soil, or breathing vapours emitted from such land, or eating contaminated food grown on such land can pose significant health risks. The key issue is that much urban land remains unsurveyed and potential practitioners of UPA are therefore unaware of risks, real or potential. In some situations, concerns about land contamination may dissuade people from starting UPA. Some surveys have already suggested that lead (Pb) concentrations in garden-grown crops can be higher than in rural areas in England (Davies et al., 1983), and in Germany (Alt et al., 1982). However, not all results are alarmist: a study in the Netherlands suggested that cadmium (Cd) and Pb levels in soils and crops from allotments were more typical of national levels (van Lune, 1987). Moir (1985) found that seven out of eight allotment sites in Greater London were contaminated with Pb, that the soil metal content decreased with distance from central London and that spinach accumulated significantly higher Pb, zinc (Zn) and Cd levels than lettuce and radish. In Britain, legislation from 1 January 1986 to reduce permitted Pb concentrations in petrol from 0.40 to 0.15 g/l, has clearly led to a reduction in Pb concentrations in air (Denton, 1988). However, there appears to have been limited assessment of levels in urban soils and crops since then. A programme of urban land assessment therefore needs to be organised to quantify both the risk of land contamination and that of drainage water.