Metal distribution, bioavailability and isotope variations in polluted soils from Lower Swansea Valley, UK

Soils in the Lower Swansea Valley, (United Kingdom) contain elevated level of metals, enough to cause direct or indirect effects on human health. This study assesses the severity of soil pollution and bioavailability of Cu and other metals (Ni, Zn, Co, Pb and Cr) in soils with various distances from a Ni refinery. We compare Cu concentrations in operationally defined soil fractions (bioavailable, bound to Fe/Mn oxide and incorporated in organic matter) with other metals (Ni, Zn, Pb, Co, Cr) usually occurring in ores used in metallurgic processes and report their pollution and geoaccumulation indices (PI and I_geo). Further, we use Cu stable isotope ratios (δ⁶⁵Cu) to trace the fate and mobility of Cu in soils. Our data suggest a point source of contamination for some of the heavy metals including Ni (I_geo = 1.9), Zn (I_geo = 0.28) and Cu (I_geo = 3.6) near the Ni refinery. However, Co (I_geo = 0.15) and Pb (I_geo = 3.3) contaminations are likely to be linked to different sources. No elevated Cr levels (I_geo= -0.07) occur in any of the studied soils. All soil metals are predominantly associated with organic matter (>50%) which reduces their bioavailibility and thus their risk for ecological and human health. The Cu isotope data show that Cu in soil organic matter is enriched in ⁶⁵Cu, while the lighter isotopes (⁶³Cu) remain in the dissolved bioavailable Cu fraction (Δ⁶⁵Cu_{organic-bioavailable} is +0.12 ± 0.13‰). This suggests the preferential complexation of ⁶⁵Cu with soil organic matter after dissolution of Cu deposited to the soil. Thus, Cu isotope data can effectively indicate pathways of metal migration in polluted soils.