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Natural organochlorines in precipitation and surface watersPrecipitation
08/2008 Organic forms of halogens are ubiquitous in rain and snow. Measurements of the group parameter AOX (adsorbable organic halogens) in precipitation collected in both populous and rural areas in central and northern Europe have shown that, over broad areas, the average concentration of such substances is 5-10 mg/L. At more remote sites, such as northern Sweden and Finland and the Russian tundra, the average concentration is 1-5 mg/L. Measurements of individual halogens in organic matter derived from precipitation has revealed that most of the organically bound halogens detected as AOX are chlorinated compounds. Brominated compounds are widespread but less prevalent, and organically bound iodine has only been detected at sites close to the sea. Further characterisation of the organohalogens present in rain and snow has shown that the largest amounts of AOX are found in fractions of relatively polar and non-volatile to semi-volatile compounds, in particular organic bases and acids. Gas chromatographic analysis of specific compounds has revealed that, of the volatile chloroorganics found in precipitation, dichloronitromethane is often present in the highest concentration. Chlorobenzenes, in particular 1,4-dichlorobenzene, are also widespread. In the fractions of hexane- or ether extractable compounds, chlorinated alkyl phosphates are normally responsible for the largest peaks in chlorine-specific chromatograms, whereas chloroacetic acids are the most prevalent organochlorines in the fraction of low-molecular-weight organic acids. However, none of the specific compounds just mentioned can explain more than a very small fraction of the AOX values. The origin of the organochlorines in precipitation is still a matter of discussion. Known industrial pollutants, such as flame retardants (e.g. chlorinated alkyl phosphates) and pesticides (e.g. lindane), are typically present at ppt (ng/L) levels, i.e. in concentrations about three orders of magnitude less than the AOX concentration. Chloroacetates, that occasionally may explain several percent of the AOX value, were previously assumed to be almost exclusively of anthropogenic orgin. However, analysis of Antarctic snow that accumulated in the beginning of the 18th century has demonstrated that natural sources play a significant role in the global distribution of this group of compounds. Surface WatersOrganic halogen compounds, in particular chlorine compounds, are more widespread in surface waters than was previously assumed. Surveys of the group parameter AOX have shown that, even at remote sites, the total concentration of such compounds is normally between 5 and 50 mg Cl/L. Only a minor fraction of observed concentrations can be explained by known pollutants of industrial origin; in most lakes and rivers, a majority of the organically bound halogens can, in fact, be attributed to naturally produced compounds. Remarkably high AOX concentrations, up to 200 mg Cl/L, have been found in unpolluted waters with a high content of humic substances. In addition, there is often a positive correlation between the amounts of AOX and the total organic carbon in the investigated waters. This can be explained by natural chlorination of soil organic matter, which is then leached to surface water. Some of the halogenated structures in aquatic humus were identified by degrading the macromolecules to low-molecular weight compounds; the degradation products identified were almost identical to those of chlorolignins isolated from effluents of chlorine bleaching of paper pulp. In addition to the chlorine-containing macromolecules, which are responsible for the major fraction of AOX, surface waters normally contain minor amounts of low-molecular-weight organochlorines that can be produced in non-specific reactions between reactive inorganic chlorine species and naturally occurring organic matter. In the fraction of low-molecular-weight acids, chloroacetic acids are the most prevalent organohalogens. Dichloroacetic acid can occur in concentrations approaching 1mg Cl/L in seemingly unpolluted humus-rich lakes, and mono- and trichloroacetic acids have also been detected in the same type of waters. Chloroacetic acids may reach surface waters through atmospheric deposition, but observed concentrations of dichloroacetic acid indicate that there is also a natural production in soil. Of the phenolic compounds, 2,4,6-trichlorophenol seems to be the most widespread organohalogen. Surveys of unpolluted humus-rich waters have shown that this compound can occur in concentrations up to 10 ng/L. Its methylated analogue, 2,4,6-trichloroanisole, is also widespread in unpolluted environments, indicating that the two compounds have a common, natural source. |
