Arbejdsrapport fra Miljøstyrelsen, 33/2005 – Undersøgelse af eksisterende viden om tilbageholdelse og nedbrydning af PAH og TBT samt tilbageholdelse af sporelementer/tungmetaller til brug ved risikovurdering af kystnære depoter

Undersøgelse af eksisterende viden om tilbageholdelse og nedbrydning af PAH og TBT samt tilbageholdelse af sporelementer/tungmetaller til brug ved risikovurdering af kystnære depoter

Summary and conclusions

An investigation of existing Danish and international knowledge on sorption and degradation of the organic compounds PAH and TBT and on the sorption of trace elements/metals i polluted marine sediment and in initially unpolluted materials has been carried out. The purpose of the investigation has been to produce the best possible parameter-estimates for risk assessments of leaching from near-coastal deposits. Furthermore, it should be determined whether these parameter-estimates were well founded enough to used for such risk assessments or if supplementing informations should be gathered/produced.

The investigation is carried out partly as a litterature survey and partly through contact with the Danish ports and counties with the purpose of procuring information from existing deposits for contaminated marine sediments.

It is concluded, that for all the investigated substances a large variation in leaching is found, and that currently, this variation can not be explained satisfactorily. Since the variation is at a minimum of a factor of 10, the variation can have a significant influence on an actual risk assessment. Based on the investigation made, it is concluded that in an actual risk assessment, the leaching from the polluted sediment should be tested. Such tests can be carried out in such a way that they represent the local conditions in the best possible way and hence produce results that are as close to the actual leaching from the deposit as possible.

To this point methods for determining leaching from marine sediments has yet to be determined. Standardised tests exist for determining leaching of inorganic compounds from granular waste, soil and sludge, but these methods have not been verified for use with marine sediments. Regarding testing of leaching of organic compounds, methods are under development for polluted soils. Such methods could most probably be adjusted for use with marine sediments. Porewater extraction could also be a viable method for quantification of the partitioning of compounds between solid and liquid phase and thereby for quantification of leaching, especially if preservation of natural redox conditions is important to achive correct test results. Regardless of choice of the above test methods, tetsting of leaching can be performed realtively quickly (the test itself will take app. one day plus time for chemical analysis of the eluate) and relatively inexpensively. Performance of such tests can be used for building up the data material on the leaching of the substances in question.

With regard to sorption in connection with leaching through the dams surrounding (some) near-coastal deposits testing in connection to actual risk assessments kan be more demanding. This is because the amounts of pollutants in the leaching water are relatively small and that most of the compounds in question sorb so strongly, that correct testing can take very long. For now, it is recommended to use existing knowledge on sorption to soil in risk assessments. At the same time, it is recommended that the data material on on sorption of both trace elements and organic compounds is enlarged.

Assessment of degradation of TBT and PAH is actual risk assessments through testing is more difficult because of the relatively slow degradation for TBT and PAH. When including degradation in the risk assessment one should therefore use existing data on degradation rates, which in return should be strengthened for PAH.

Below, the most impotant conclusions regarding the three groups of substances investigated in this report are summarised.

TBT:

Sorption: Most of the given log(K_oc)-values are within the interval 5-6. It should be noted, however, that the results from polluted sediments in three Danish harbours (Miljøstyrelsen, 2003) are in the low end of the reported values (log K_oc 3,8-5,2), while the values found for Esbjerg Harbour (DHI, 2003) are in the high end of the interval (log K_oc 5,3-6,6). No explanation can be offered to the apparent difference between the Danish and the international investigations reported except possible differences in test methods and in the composistion of the exmined sediments.

Regarding sorption of TBT in matierials with a low content of organic carbon (f_oc < 0,3 %), the reported K_d-values are within a range from 2 to 200 l/kg and thus significantly lower than sorption to the sediments. This will be relevant when assessing soprtion of TBT in dam-materials surrounding near-coastal deposits.

Degradation: With regards to degradation of TBT in sediments, the investigation shows that for the conditions that can be expected in deposits for contaminated sediments, the half-life can be expected to be in the interval 1-10 years with an average value of around 3 years. Investigations from two Danish deposits for contaminated sediments show, given certain assumptions, half-lifes of 5 and 3,5 years. The data material concerning degradation of TBT in sediments is relatively strong and supplementing investigations are not likely to further limit the variation in parameter estimates for use in risk assessments. No investigations have been found, from which the results could be seen as driectly representative of the conditions that would exist during leaching from the deposits and through the surrounding dams.

With respect to degradation in surface waters, investigations performed in lighted condtions half-lives of 1-50 days, with an average value of approximately 10 days, whereas investigations performed under condtions of no light showed half-lives of 7-245 days.

The reported half-lives for TBT where determined under varying temparature conditions. Regarding degradation in surface waters, the half-lives were determined under relatively high temperatures compared to Danish conditions. Half-lives for degradation of TBT in surface waters under Danish conditions can therefore be expected to be in the high end of the reported interval. One should, however, also take into account the influence of the presence of light on the degradation of TBT. Regarding degradation of TBT in sediments, the reported intervals for half-lives are thought to also be representative for Danish conditions. Especially, since the results from the two Danish deposits are in the middle of the given interval.

PAH:

Sorption: The variation in the reported values for log (K_oc) for the PAH'es is large, typically 2-3 orders of magnitude, and the data material is limited for a portion of the individual PAH'es. By ruling out the results of two of the investigations reported, the variation in the remaining datamaterial was within 1-2 orders of magnitude. There was no clear difference between K_oc-values determined for fresh and marine sediments and the reported K_oc-values for the sedimants overlapped the values found for soil and groundwater.

Degradation: The data material for assessing the degradation of PAH'es is very modest. By comparing reported values for degradation in marine sediment to values reported for soil and groundwater, where the material is also very modest, there are, however, indications that the degradation proceeds faster in marine sediments than in soils. This faster degradation could be caused by differences in bacterial cultures and bacterial activity, but no certain explanation for the observed differences in degradation rates can be given.

Trace elements/metals:

The data material for assessment of sorption of metals to marine sediments and dam material is modest and the variation in the reported values is large; 2-3 orders of magnitude. The datamaterial for soprtion of metals in marine sediments did not show any clear deviations from sorption in soils/groundwater. This is supported by (Kuwabara et al. 1989), where sorption of three metals was found not to differ significantly between two different salinities.