Volatile organic compounds in the surface waters of a British estuary. Part 2. Fate processes

The fate of volatile organic compounds (VOC) in the Southampton Water estuary, presented in Part 1 of this paper, are described. The principal processes responsible for removing VOC from the water column include dilution and net transport, volatilisation and adsorption onto sediments. Volatilisation is the most important removal mechanism, and is significantly influenced by temporal and spatial changes in aqueous concentration and by the proximity of sources. Using surface renewal models, volatilisation rates ranging from 300 kg day−1 in summer months to 2000 kg day−1 in winter months were estimated. Within the water column, net transport rates were 3 to 5 times higher in winter relative to summer, reflecting the influence of enhanced input rates and increased dilution flows. Adsorption processes varied widely in their efficiency, and were dependent upon the availability of adsorbing solid surfaces within the water column and VOC affinity for sorption processes. The highest levels of adsorption were identified for chloroform in wastewater effluents where the organic matter content of suspended solids approached 100%. Within the estuary as a whole, not more than 5% of VOC such as aromatics and organohalogens was predicted to be adsorbed under the most favourable environmental conditions.