Natural AOX in the river Rhine: modelling and trace analysis

A new, simple model is presented to estimate natural AOX concentrations of anthropogenically polluted river systems. The model is based on the measurement of AOX/DOC ratios of relatively natural tributaries of the river system under investigation. An average natural AOX/DOC ratio of 0.35% was determined for the Rhine basin. The natural AOX concentration of a river, in a specific anthropogenically polluted part, can be calculated by multiplying the average natural AOX/DOC ratio with the DOC value at the polluted site. It appears that, in the Dutch part of the Rhine, ca. 12 ± 4 µg Cl/l (55 ± 17% of Rhine AOX) is of natural origin, that ca. 2 µg Cl/l (9% of Rhine AOX) is introduced by pulp mills and that ca. 8 µg Cl/l (36% of Rhine AOX) is of unknown anthropogenic origin. It is postulated that sewage works may introduce a large part of the unidentified AOX fraction. Pyrolysis-gas chromatography-mass spectrometry-single ion monitoring (Py-GC-MS-SIM) has been used to identify and quantify chlorolignosulphonic acids in the river Rhine, using the 5-chloro-2-methoxyphenol substructure. It appears that, by exception, the 5-chloro-2-methoxyphenol substructure can be identified in aquatic humic substances isolated from a single Rhine tributary.