The intake of lead and associated metals by sheep grazing mining-contaminated floodplain pastures in mid-Wales, UK: I. Soil ingestion, soil–metal partitioning and potential availability to pasture herbage and livestock
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Abstract
This paper first evaluates the relative importance of the soil-plant-animal and soil-animal
pathways of Zn, Cu and (especially) Pb investigated over a 15-month study period at
12 floodplain sites located within and downstream of the mineralised and historic
mining area of mid-Wales, and secondly considers the implications of a sequential
extraction procedure (SEP) undertaken on soils of varying particle size sampled from
the study locations. Generally, very good agreement was found between the chemical
partitioning of the three metals for each of the physical soil fractions subjected
to the SEP. The availability of Pb to pasture vegetation, especially at the contaminated
sites, is indicated with its associations with the more soluble (i.e. exchangeable
and Fe/Mn oxide) soil phases, yet soil and/or plant barriers effectively restrict
above-ground herbage concentrations of this metal. Consequently, with sheep ingesting
soil at rates varying according to season from 0.1% to 44% or more of dry matter intake,
the soil-animal pathway accounts for the majority of Pb consumption through most of
the year, and at moderately and highly contaminated sites significant quantities of
relatively soluble soil-Pb can be ingested at rates exceeding safety threshold limits.