Cadmium dispersal on a raised heathland in the Peak District National Park adjacent to a major trunk road

Abstract

Heavy metals are naturally present in soils as trace elements but deposition from vehicle wear and tear increases concentrations found adjacent to highways and has been shown to disperse further in relation to traffic volume. The heavy metal cadmium (Cd), used in the manufacture of tyres, has toxic effects on some plant species, with soil acidity being a major factor in plant Cd uptake. In this study, levels of Cd in soil and root material from Calluna vulgaris were investigated along with soil pH on wet heather moorland in the Peak District National Park. In December 2009, samples were collected from 10 transects extending up to 125 m from a trunk road that has a daily vehicle use >23 000. The peat substrate of the study site was found to be highly acidic (mean pH ± S.D.: 3.44 ± 0.119). Reported Cd concentrations in peat core samples (50–200 mm depth) are within the UK rural soil distribution range (0.1–1.8 mg kg −1 ), but increase progressively up to 85 and 125 m on either side of the road probably assisted by the wind. Root samples from C. vulgaris showed a degree of Cd accumulation (mean ± S.D.: 17.78 µg g −1 ± 9.338) compared with normal concentrations in plants from unpolluted soils (0.1 µg g −1 ). Coupled with data from previous research, results from this study suggest that increased soil Cd concentrations could affect the competitive balance between C. vulgaris and other moorland plants such as Molinia caerulea. Advances in analytical techniques allowing a better understanding of plant responses to metal toxicity are also discussed.