Studies of the behaviour of trace metals during mixing in some estuaries of the Solent region

Abstract

Measurements have been made in the Beaulieu Estuary of dissolved nutrients, dissolved organic carbon (DOC) (1.2 pun and 1 nm cut offs), trace metals (0.4 /jm and lnm) with determinations also of fractions taken up on C18 Sep-Pak material, and trace metals in suspended particulate material (SPM), including acetic acid leachable and residual fractions. Apart from DOC and the truly dissolved fractions, the same parameters were measured on samples from the Southampton Water system. Analytical chemical investigations were undertaken, particularly to evaluate the performance of the cross-flow filtration and Sep-Pak separations used in these studies. Localized anthropogenic influences on the Beaulieu Estuary generally have minor effects, but a marked addition of Cu within the estuary is attributed to input from the use of antifouling paints. Catchment processes produce high concentrations of dissolved (0.4 pm) Fe (1710-2750 nM), Mn (1265-1750 nM) and Co (6.1-20 nM) and leachable Fe (11-15%) in SPM in the river waters. Behaviour during estuarine mixing approximated to conservative for dissolved Mn, Ni and probably Zn; addition of dissolved Cd and Co, and removal of microcolloidal Fe, occurred. A marked decrease in colloidal Fe in the lower salinity (<5) region was broadly paralleled by the behaviour of Pb. The colloidal Mn and Ni were much less important than the truly dissolved fraction within the system. Colloidal fractions of Cd (52-46%), Co (43-51%) and Cu (32-46%) were significant in the river water; these fractions decreased for Cd and Co with increasing salinity but for Cu changes within the estuary were temporally variable. Significant proportions of dissolved (0.4 ^m) Cu (17-55%) and Zn (1-40%) were organically associated as defined by the Sep-Pak separation; for Cu these fractions were similar for the truly dissolved metal. Compositions of SPM in the Beaulieu Estuary were strongly influenced by the presence of iron-rich riverine material, with strong correlations between leachable Fe and other metals. Ratios of trace metals to Al in SPM, especially for Fe, Mn, Zn and Cd decreased markedly within the lower salinity region. Overall, changes in composition of estuarine SPM were probably largely attributable to physical mixing of riverine and marine materials. In Southampton Water and the Test and Itchen Estuaries the dissolved trace metals showed non-conservative behaviour, with evidence of inputs from benthic sources and waste effluents. The range of percentages of organically associated Cu (17-53%) was similar to that in the Beaulieu Estuary. Correlations between concentrations of leachable Fe and the other metals in SPM were weak but ocasional strong correlations were found between Mn and other metals. Results were broadly consistent with a primary control on composition of SPM by mixing of riverine and marine material, but with considerable variability in composition locally, attributed to in situ scavanging and localized direct input. The KD values for metals found in the Beaulieu river were similar to those from the Test river, but those for Co and Mn were slightly lower in the former. With the exception of Cd, Mn and probably Co, there were no systematic variations of the KD values with changes in salinity in the Beaulieu Estuary and Southampton Water system. The KD values for Cd showed a decrease with salinity in both systems, while those for Mn increased with increasing salinity, especially at high salinity. The KD values for Co showed a slight increase at high salinity but not as clearly as for Mn. Fisrt-order estimates of fluxes of metals from the Beaulieu Estuary were derived using extropolated zero salinity end member concentrations derived from concentration/salinity relationships. The factors identified made this approach of limited applicability to Southampton Water.

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