RESUMO
The major, minor and trace element chemistries at two freshwater tidal sites on the River Trent are described and compared with the non-tidal river, to demonstrate the variability in chemical processes in a major chemically and hydrologically active part of the river system, which is often overlooked in freshwater hydrochemical studies. The study shows a chemical gradient along which concentrations increase downstream for suspended sediments and acid-available particulate (AAP; > 0.45 microm) trace element fractions and also the 'dissolved' (< 0.45 microm) fractions of those trace elements with low-solubility phases. The study highlights the importance of tidally induced suspended-sediment dynamics and the generation of microparticulates for trace element transport within the tidal reaches. The microparticulate and particulate fractions are associated with both anthropogenic and lithogenous sources, and the concentrations of these two fractions co-vary according to a two-component mixing model involving riverine and tidal endmembers, with the tidal endmember exhibiting a strong lithogenous component. In many cases the AAP trace element concentrations are highly linearly correlated. At the downstream tidal site, suspended sediment concentrations were particularly high and ranged from 44 to over 24,000 mg/l as a result of tidally induced sediment mobilisation. As a consequence of this, particulate metal concentrations are especially high, and here, for the first time within the east coast studies of UK rivers, particulate fractions were measurable for trace elements such as boron and molybdenum, previously reported above detection limits only in dissolved form. It is demonstrated that the microparticulate components are not simply related to suspended sediment concentrations or to the bulk composition of the AAP fraction. Rather, microparticulate generation is related to a more complex pattern, probably linked to hydrodynamic factors involving sediment resuspension and microparticulate transport mechanisms.
RESUMO
The water quality of a major river system, the River Trent, which flows into the Humber estuary and drains large centres of population and industry in central England, is examined in terms of trace element, nutrient, major-ion, suspended sediment and carbon concentrations and set within the context of the transition from non-tidal to the tidal freshwater reaches. Detailed investigation of the water quality variability in the non-tidal river Trent has revealed mainly simple patterns in dissolved chemical concentrations, controlled largely by hydrology and the mixing of baseflow and stormflow endmember chemistries. However, silicon, carbon and nitrate show much more complex behaviour, as concentrations of these chemical determinands are regulated by in-stream biological processes. Major increases in concentrations of suspended solids and acid-available particulate (AAP) trace elements are shown to be linked to tidal mobilization of sediment and the location of the 'turbidity maximum'. Reductions in the concentrations of AAP trace elements per gram of suspended sediment suggest the mobilization of a different sediment type with a lower trace element content, which may result from: (i) a secondary sediment source; or (ii) mobilization of coarser-grained sediment under the stronger tidal hydrodynamic forces. Increased correlations for AAP-trace element interrelationships downstream suggest increasing simplicity of mixing relationships.
