Distribution and biogeochemical cycling of trace metals in soils at an abandoned lead mining and smelting complex (Priddy, Somerset, England)
The spatial distribution of lead, zinc, cadmium, copper, iron and manganese was investigated in the soils of an abandoned lead mining and smelting complex near Priddy, in the Mendip Hills, Somerset. Soils were collected at 0-200 mm and 200-400 mm depths over the period February 1995 to April 1997. The study area was shown to be one of the most intensely Ph-enriched historic mining and smelting sites on a regional and national level. The concentration of Pb in the soils of the study area ranged from 1 394 to 138 800 J.lg g-1 at 0-200 mm, and from 180 to 171 688 J.lg g-1 at 200 to 400 mm. The concentration of Zn ranged from 28 to 10 349 J..tg g-1 at 0-200 mm, and from 36 to 8 089 J..tg g-1 at 200 to 400 mm. The concentration of Cd ranged from 1 to 12 J.lg g-1 at 0-200 mm, and from 1 to 1 1 J.lg g-1 at 200 to 400 mm. The concentration of Cu ranged from 7 to 182 J..tg g-1 at 0-200 mm, and from 5 to 206 J..tg g-1 at 200 to 400 mm. The concentration of Fe ranged from 5 060 to 59 754 J.lg g-1 at 0-200 mm, and from 10 465 to 67 035 J..tg g-1 at 200 to 400 mm. The concentration of Mn ranged from 2 500 to 3 769 J..tg g-1 at 0-200 mm, and from 66 to 3690 J..tg g-1 at 200 to 400 mm. The greatest concentrations ofPb, Zn, Cd, Cu, Fe and Mn were present in the study area in the wastes associated with the beneficiation processes carried out during the final stage of the Mendip lead industry ( 1857 -1908), when the wastes of Roman and Mediaeval operations were reworked. While most of the wastes from 1857-1908 were present as clearly identifiable tailings piles, previously unrecognised areas contaminated by beneficiation w;istes were identified. Based on the Greater London Council Contamination classification, 88.5% of the soil samples in the study area were contaminated to unusually heavily contaminated by Pb. Unusually heavy contamination by Pb was present in 44% of the soils (>10 000 J..tg g-1), and by Zn in 15% of the soils (>5 000 J..tg g-1). A ranking of the percentage of samples classified as contaminated showed that in terms of magnitude of contamination, the decreasing order for the study area was Pb>Zn>Cd>Mn>Cu. The five soil types present in the study site were identified as the Maesbury, Thrupe, Ashen and Nordrach Series and Tailings, the wastes associated with beneficiation processes. Representative soils of the five types at 0-200 mm and 200-400 mm depths were collected and analysed each month between September 1998 and August 1999 to assess the temporal variation in Pb, Zn, Cd, Cu, Fe and Mn concentration. The general trend for Pb, Zn, Cu, Fe and Mn in the Maesbury and Thrupe soils and the Tailings was an overall decrease in concentration over the sampling period. The concentration of soil Pb, Zn and surface Cu also decreased in the Ashen soil , and Pb, Fe and surface Cu in the Nordrach soil. A ranking o f the soil series by % Coefficient of Variance (% CV) in trace metal concentrations at 0-400 mm showed that the degree of variation ranged from 31% C V in the Maesbury series to 11.32% CV in the Nordrach Series, in the following order of magnitude : Maesbury>Ashen> Thrupe>Tailings>Nordrach. Variation in soil trace metal concentrations by % CV for all soil series in the surface horizons showed that the degree of variation ranged from 8.41% CV for Mn to 11.97% C V for Fe in the following order : Mn>Zn>Cd>Pb>Cu>Fe. The variation in soil trace metal concentrations by % C V for all soil series at depth showed that the degree o f variation ranged from 22.52% CV for C u to 14.05% C V for Fe i n the following order : Cu>Cd>Zn>Mn>Pb>Fe. When expressed as a percentage of the Total trace metal concentration, the percentage o f EDT A extractable metals were consistently higher in the surface horizons than at depth. The variation was thought to be related to the source of enrichment, the fraction of soil the trace metals were held in, soil pH and hydrology and the availability of trace metals in the surface horizons for plant uptake. The overall ranking of EDT A extractable trace metal concentration as a percentage of Total trace metal concentration for all soil series at both surface and depth was in the following order of magnitude : Pb>Cu>Cd>Mn>Zn.>Fe. The EDTA extractable Pb as a percentage ofthe Total Pb was remarkably high in the Thrupe, Ashen and Maesbury soils. In the surface soils, this ranged from 84% to 102%, and at depth (200-400 mm) from 65% to 101%. Possible reasons for anomalous results where EDTA extractable Pb as a percentage of Total Pb apparently exceeded 100% were discussed. In the Nordrach Series, EDTA extractable Pb ranged from 49% to 81% of Total Pb in the surface soil and from 52% to 69% of Total Pb at depth. The EDTA extractable Pb as a percentage of the Total Pb was lowest in Tailings at 20% to 41% at the surface to 14% to 38% at depth. The high EDTA extractability ofPb in the Thrupe, Ashen and Maesbury soils was thought to be related to the source of enrichment, the soil pH, soil redox reactions, mineral composition and texture and soil hydrology.