Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater

Abstract

A new approach to estuarine studies (termed the product approach) is presented which establishes 1. (1) the extent and salinity dependence of non-conservative behaviour; 2. (2) composition and chemical form of removal products; and 3. (3) abiological removal mechanisms. In the product approach the composition of removal products (i.e. flocculants), which result from mixing of river waters (of Scotland) and seawater at varying salinities, are determined using laboratory experiments designed with the minimum of constraints. Rapid flocculation of Fe, Mn, Al, P, organic carbon and humic substances occurs when filtered river waters are mixed with filtered seawater. The amounts of flocculated constituents increase as salinity increases from 0 to 15–20%., above which little additional removal occurs. The extent of flocculation is very salinity-dependent indicating the destabilization of river-introduced colloidal humic substances during the mixing with sea water. The close association of Fe, Mn, Al and P with both river-dissolved humic substances and the sea water-flocculated humates demonstrates the important role of dissolved organic matter in controlling the non-conservative behaviour of inorganic constituents. The extent of removal, relative to the river water, ranges between 75 and 115% for Fe, 90 and 100% for Mn, 50 and 200% for P and 10 and 70% for Al. Although only 3–11% of the river DOM is flocculated, the organic component (i.e. humates) outweigh the sum total of the inorganic trace elements. Silica shows a lack of reactivity (3–6% removal). Removal processes, described in this paper, are applicable to the major rivers of the world where they may play an important role in the chemical mass-balance between the rivers and the ocean.