Organic matter and the surface charge of suspended particles in estuarine waters1

Abstract

The surface electrical charge on suspended particles in four estuaries of the U.K. has been measured as a function of salinity by the technique of particle microelectrophoresis. Two characteristic types of behavior were found. In rivers low in dissolved cations, especially Ca 2+ (Conwy, Beaulieu), the electrophoretic mobility u E was negative in sign at all salinities, increasing slightly in magnitude from the seawater end member to lower salinities of 5–10‰, with a more pronounced increase toward the river water end member. In rivers draining calcareous terrain and having relatively high concentrations of Ca 2+ (Alde, Orwell) u E showed a similar dependence on salinity above 5–10‰ but no marked increase in magnitude at lower salinities. Ionic composition of the water appears to be the major factor controlling changes in uE with salinity. Positively charged particles were entirely absent. The charge distribution of all samples was highly uniform, in spite of the mixed nature of the suspended matter, indicating a dominant control of surface properties by adsorbed organic matter, metallic oxides, or both. This implies that differential flocculation of different suspended minerals is largely suppressed in the estuarine zone. Measurements of dissolved organic carbon (DOC) and surface‐active substances (by suppression of polarographic maximum) in the same estuaries indicate a sufficient supply of organic matter for the adsorption process. No evidence for nonconservative removal of DOC or surface‐active substances was found. Sewage inputs into some of the estuaries are clearly seen by the measurements of surface‐active substances. Significant quantities of surface‐active materials are injected into the Alde estuary through tidal flushing of a salt marsh area.