HEAVY-METALS IN THE HYDROLOGICAL CYCLE - TRENDS AND EXPLANATION

This paper reviews the major sources and transport characteristics of heavy metals in the hydrological cycle. It is demonstrated that heavy metal releases to the environment have changed from 19th and early 20t h century production-related activities to consumption-oriented factor s in more recent times. The relative roles of particle size, sorption and desorption processes, partitioning and the chemical speciation of heavy metals on fine sediments are identified to understand the likely fate of heavy metals released into fluvial systems. It is argued that the spatial and temporal distribution of heavy metals in the river co rridor depends not only on an understanding of metal solubility and sp eciation, but also on an understanding of sediment dynamics which cont rol, for example, floodplain alluviation and the accumulation of metal s in the bottom sediments of contaminated rivers, lakes and reservoirs . Existing long- and short-term records are examined to identify the c urrent state of knowledge about the factors which affect heavy metal r eleases into aquatic environments. With limited exceptions, it is show n that few long-term studies of trends in heavy metal transport are av ailable although, for some major rivers, limited data on trends in met al concentration exists. Palaeolimnological reconstruction techniques, based on an analysis of lake and reservoir sediments, are identified as a possible means of supplementing monitored records of heavy metal transport. Although numerous studies have suggested that trends in atm ospheric contamination, mining and urbanization may be identified in t he bottom sediment record, other research has shown that the radionucl ide-based chronology and the heavy metal distribution within the sedim ent are more likely to be a function of post-depositional remobilizati on than the history of metal loading to the basin. Despite these limit ations, it is shown that the incorporation of reservoir bottom sedimen t analysis into a heavy metal research programme, based in river corri dors of Midland England, provides an opportunity to identify and quant ify the relative contribution of point and non-point contributions to the heavy metal budget and to relate trends in metal contamination to specific periods of catchment disturbance.