Biogeochemistry of Aromatic Surfactants in Microtidal Estuaries

Abstract

This paper summarizes several studies on input, distribution, and fate of two different types of aromatic surfactants, the anionic linear alkylbenzene sulphonates (LAS) and nonionic alkylphenol polyethoxylates (APnEO), in a karstic estuary characterized by sharp salinity gradients. A combination of reversed-phase and normal-phase HPLC analysis followed by spectrofluorimetric detection allowed detailed characterization of homologue and oligomer compositions of aromatic surfactants in wastewater and estuarine waters. The distribution patterns of homologues and/or oligomers in the wastewater plume reveal a significant alteration of the original composition of both surfactants by biotransformation and physico-chemical partitioning. The vertical distribution of surfactant residues in the estuarine water column is characterized by pronounced maxima at the air–brackish water and brackish water–sea water interface. The brackish water–sea water interface is a very efficient barrier, which prevents spreading of surfactants into the deeper layers. In addition, the interface seems to be a site of high biological activity, including biotransformation of surfactant residues. As a consequence, this layer can contain enhanced levels of lipophilic metabolites of APnEO. Biotransformation is the main mechanism for the removal of both surfactant classes from the water column. However, our studies suggest that a significant percentage of surfactant residues discharged into the stratified estuaries reaches the coastal sea due to the incomplete removal in the estuary. This situation is a consequence of a rather fast flushing of the brackish layer and a very slow biotransformation kinetics in the saline layer, especially in the winter period.