Diagnosis of Chemical Reactivity and Pollution Sources from Particulate Trace Metal Distributions in Estuaries
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Abstract A simple approach is outlined for the diagnosis of chemical reactivity and pollution sources from distributions of suspended particulate constituents in estuaries of low turbidity and limited bed-water column particle exchange. The approach is demonstrated using acetic acid-extractable particulate trace metal data for the Clyde Estuary, where water and particle mixing in the vertical is restricted by a strong pycnocline, resuspension is limited by slow subsurface currents and internal sediment cycling is inhibited by the topography of the outer estuary. The salinity distributions of particulate Fe and Mn result from end-member particle mixing, modified by the geochemical mechanisms controlling their particle–water exchange (salt-induced flocculation of riverine, Fe-bearing colloidal material and autocatalytic oxidation-sorption of dissolved Mn). The axial distributions of particulate Cu, Cr, Pb and Zn are, additionally, affected by external inputs to the tidal estuary. The magnitudes of internal (e.g. particle–water exchange) or external (e.g. pollution) sources are calculated from the deviation of metal concentrations from a theoretical dilution line adjoining estuarine end-members, via chemical mass balances and empirical equations defining particle–water partitioning as a function of salinity. Calculated particle–water exchanges of Fe and Mn are compatible with independent dissolved metal measurements in the Clyde Estuary. Calculated external sources of Cu and Pb are in reasonable agreement with monitored trace metal input data to the estuary, but discrepancies exist between calculated and monitored external sources of Cr and Zn because of the significance of unmonitored inputs and chemical reactivity for these metals. Although suspended particulate trace metal concentrations in the Clyde are among the highest of industrialized estuaries in the U.K., it is argued that this, in part, reflects the inability of the Clyde to buffer pollution inputs because of the restricted internal cycling and bed-suspension exchange of particles in the system. A more general implication is that the mobility of the sediment reservoir and standing stock of suspended particles, in addition to concentration of particulate constituents (w/w), should be accounted for in any estuarine pollution impact evaluation.