Concentrations and deposition fluxes of polynuclear aromatic hydrocarbons and heavy metals in the dated sediments of a rural english lake

Abstract

Deposition fluxes have been estimated from concentrations of 12 polynuclear aromatic hydrocarbons (PAHs) and six heavy metals measured in sediment cores from Esthwaite Water, a rural English lake. Atmospheric deposition is a major source of PAHs and heavy metals (Cd, Cu, Pb, and Zn) to the lake, so trends in the core may indicate the long-term record of changes in the atmospheric loading of these pollutants. The cores were dated radiometrically and pollutant trends studied since about 1820. PAH fluxes began to increase from stable background levels around the turn of the century (4.1 μg ΣPAH per gram dry weight, 287 ng ΣPAH per centimeter squared per year), rising to maximum inputs between 1950 and 1970 (42.0 μg/g, 2,954 ng/cm2 per year). The deposition flux of PAHs has decreased steadily since then and is now less than half the maximum (18.0 μg/g, 1,238 ng/cm2 per year). In recent sediments (1900-1970), the relative proportions of individual PAHs making up the ΣPAH burden have remained virtually unchanged, with fluoranthene, pyrene, and benzo[ghi]perylene the most abundant compounds. However, perylene, a biogenically produced PAH, dominates the mixture in the sediments older than 1890. Cadmium, Cu, Ni, Pb, and Zn concentrations in the Esthwaite sediments broadly follow the same temporal trends as the PAHs, with varying peak inputs between 1925 and 1975. Concentrations in the modern (surface) sediments are now lower than in recent decades, with ΣPAH, Cd, and Pb showing the greatest reduction. However, fluxes of these contaminants to surface sediments are still greater than those deposited before 1840.