Particulate metal speciation in surficial sediments from Loch Dee, southwest Scotland, U.K.

The geochemical partitioning of ten elements in stratified Holocene sediments from Loch Dee, southwest Scotland, has been established by use of a five-stage sequential extraction procedure. Samples from below 15 cm sediment depth show minimal evidence of modification by anthropogenic contamination or active diagenesis and hold Fe, Mg, Cu, Cd, Co, Pb, and Ni primarily in detrital silicates or organic complexes, while Mn, Ca, and Zn reside largely in adsorbed and reducible oxide phases. In the uppermost ca 15 cm of sediment, enhanced total concentrations of Zn, Cu, and Pb reflect increased atmospheric deposition during the postindustrial period. Of these metals, only Pb displays any notable adjustment of partitioning in the enriched zone, showing disproportionate accumulation in labile oxides and organic-Pb phases. The lack of Pb and Zn carbonates in the contaminated horizon may reflect inherent thermodynamic instability under the acid surface and pore-water conditions of Loch Dee. Increments to total Mn and Co in the surficial ca 5 cm of sediment are attributable to the accumulation of secondary oxides and adsorbed species, consistent with precipitation from the interstitial pore-waters across a sedimentary redox front. The presence of metals such as Zn and Cd in soluble or acid-volatile phases in the interfacial sediment has implications for the future management of the Loch Dee basin, with leaching into the overlying waters likely, given the continuation of current trends of lake acidification.