Organohalogens of natural or unknown origin in surface water and precipitation

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Abstract
Information about the occurrence and chemical character of adsorbable organic halogens (AOX) in surface water and precipitation is reviewed and updated with results from on‐going studies. There is now very strong evidence that the widespread occurrence of AOX in unpolluted aquatic environments is primarily caused by naturally halogenated humic substances; and, recently, the first chlorinated structural elements in aquatic fulvic acids were identified. Despite the considerable amounts of high‐molecular‐weight organohalogens in humus‐rich surface waters, the concentrations of specific low‐molecular‐weight organohalogens are generally low. This is illustrated by chromatograms obtained by combining different enrichment procedures for trace organics in water with GC‐AED analysis (gas chromatography with atomic emission detection) of chlorinated and brominated compounds. On‐going studies of the chemical character of AOX in precipitation have shown that the compounds responsible for the major fraction of the AOX are more lipophilic than those in surface water. In addition, it has been shown that the AOX content in precipitation is primarily comprised of semi‐volatile to non‐volatile compounds. GC‐AED chromatograms of trace organics in rainwater samples demonstrate that, except for chloroacetic acids, the concentrations of specific, compounds amenable to gas chromatographic analysis are normally at least three orders of magnitude lower than the AOX concentration. Enzyme‐mediated chlorination of soil organic matter has been identified as a potentially important natural halogenation process. Further studies are required to determine the significance of photochemical halogenation processes.