The distribution of micro-organic contaminants in river bed-sediment cores

There is little information available on the distributions of micro-organic contaminants in river bed-sediments below a depth of 5 cm. The aim of this study is to determine the concentrations and distributions of contaminants in river bed-sediments up to 1 m depth and to make an assessment of the results. The approach taken was to collect five cores from each of two lowland rivers in southern England, the first a rural river and the second a dominantly urban catchment. The cores were analysed for micro-organic compounds and sediment/sediment porewater characteristics. Compounds detected were polyaromatic hydrocarbons (naphthalene, fluoranthene and pyrene), pesticides (carbaryl, linuron, fenpropimorph, the synthetic pyrethroids, and prometryn), and non-ionic surfactant residue (nonylphenol). A particularly important finding was that some micro-organic contaminants penetrated to depths of 1 m, and in one of the rivers they were detected in undisturbed Eocene substrata. The more hydrophobic contaminants showed a clear depth distribution with higher concentrations towards the top of the cores. The less hydrophobic contaminants demonstrated no systematic trend suggesting they had become soluble in porewaters and subsequently travelled within the cores. Partition between the porewater and sediment appeared to be controlling the distribution of the compounds with depth.