Metal dispersion and transportational activities using food crops as biomonitors

Abstract

The multielement (Al, Ca, Cd, Ce, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Si, and Zn) levels of various common vegetables (bean, broccoli, cabbage, cauliflower, lettuce, marrow, onion, parsnip, spinach, sprouts, sweet corn, and tomato); fruits (grape and strawberry); herbs (garlic, lemon balm, marjoram, mint, rosemary and tarragon); local pasture species and surface soils collected from a commercial garden centre located within a distance of 30 m of the London Orbital Motorway (M25) is presented. Comparative values are given from a background area, namely a domestic garden located in the North Yorkshire Dales National Park area. Analysis was undertaken by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma-source mass spectrometry (ICP-MS) with quality control assessment using four international biological reference materials; BCR:CRM 62 Olive Leaves, NIST 1575 Pine Needles, NIST 1573 Tomato Leaves, and NIST 1572 Citrus Leaves. Inter-analytical method comparison is given using two methods of ICP-MS; namely conventional pneumatic nebulisation of sample solution, and direct solids analysis by laser ablation; and neutron activation analysis methods (NAA). For the elements listed there is a good precision obtained by ICP-MS and NAA. In particular levels of < ± 1–10% (rsd) are obtained. Comparison of data with certified values and other analytical methods are generally of very good agreement. Lead levels in background areas ranged from 0.0008 to 0.340 μg/g (fresh weight) for plant material; with the lead magnitude greater for grasses > herbs > vegetables > cereals > fruits. Measured values are in good agreement with reported literature values. The lowest Pb values are for marrow, lettuce, tomato and sweet corn samples (∼ 0.001–0.021 μg/g). ‘Green’ leaf material levels were ∼ 0.02–0.10 μg/g (i.e. sprouts and cabbage). Root vegetables contain higher levels, ∼0.02–0.125 μg/g (especially carrot), reflecting possible metal uptake from soil. The highest vegetable Pb values are for leek and onion (∼ 0.35 μg/g). Background values are also provided for nineteen elements (Al, As, B, Ba, Br, Cd, Co, Cr, Cu, Fe, Li, Mn, Mo, Ni, Rb, Se, Sr, V, and Zn). Exposure to motor vehicle activities at a site some 30 m from the M25 shows only significant increases in Pb for unwashed plant material and surfaces soils. Typically Pb levels of 40–80% can be removed by washing plant surfaces resulting in metal levels similar to background areas. The importance of washing fruit is shown in 82% and 88% of the Pb content of grapes and strawberries being removed by washing, reflecting both airborne dusts, and soil particle (probably via ‘splash’ during periods of precipitation) as sources of Pb. All edible plant portions had Pb levels below the EEC guidelines of 1 μg/g (fresh weight). Enhanced levels of other metals (Cd, Cr, Cu, Mn, Ni, and Zn) which are possibly related to the wear-and-tear of motor vehicles, are observed only in surface soils at sites along the M25. There is no evidence of other metals contaminating foodcrops.