Distribution of heavy metals in the river yare and its associated broads III. Lead and zinc

Abstract

The magnitude and spatial distribution of lead and zinc within the bottom sediments of the River Yare, Norfolk, have been investigated, together with effects imposed by grain size, sediment organic content and channel morphological controls. Metal concentrations varied widely, ranging between 10.7 and 728 mg Pb kg −1 and between 23 and 1190 mg Zn kg −1, producing maximum enrichments of 22 and 27 times the Yare's natural background levels of lead and zinc, respectively. Non-point source metal inputs appeared to be the dominant mode of entry into the system. Sediment texture, particularly its organic content, appeared to be the principal factor controlling metal accumulation. Mean lead and zinc concentrations increased by a factor of four as the proportion of volatile solids increased from 0.5 to 25%. The influence of grain size appeared to be limited to its effects upon sediment transport and sediment organic content rather than to the adsorptive capacity of different particle sizes. Lead and zinc concentrations did not increase with decreasing particle size, but showed elevated loadings in the 1000-250 and 250-64 μm particle size ranges. A shift in dominant metal/particulate associations was observed towards the finer size fractions with increased distance downstream. This probably reflected disaggregation effects during sediment transport and the contrasting nature of metallic inputs, from the coarser runoff derivatives in the upper urban stretch to the finer, highly organic particles released in STW final effluent further downstream.