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Possible Control of EU Priority Substances in Danish Waters

11 Assessment of tributyltin

• 11.1 Definition of the reference state
  • 11.1.1 Introduction
  • 11.1.2 Main uses and pollution sources
  • 11.1.3 Releases to and state of the aquatic environment
  • 11.1.4 Existing legislation/regulation and their impact
  • 11.1.5 Conclusion on the need for further regulation
• 11.2 Possible reduction/elimination measures
  • 11.2.1 Technical measures to reduce/eliminate TBT
  • 11.2.2 Possible synergies with other (priority) substances
• 11.3 Economic Assessment
• 11.4 Conclusion on TBT
• 11.5 References

11.1 Definition of the reference state

11.1.1 Introduction

Tributyltin compounds (TBT; CAS no. 56573-85-4) belong to the group of organotin compounds whose dominant technical function, as a group, is to act as stabilizer in PVC and as a biocide. However, the primary use of TBT is as antifouling compound.

Environmentally, TBT is moderately lipofilic, has a moderately high octanol-water partition coefficient and low water solubility. Bioconcentration factors up to above 100,000 have been reported for mussels and from 210 - 7,000 for fish and molluscs /13/. TBT will be degraded in the environment by dealkylation to di- and monobutyltin and then mineralised to inorganic tin. Abiotic degradation is very slow, and biodegradation is the most important route of degradation. In anaerobic environments degradation is slow. TBT half-life in water varies from few days to months and in sediments up to 1-5 years and even more in undisturbed anaerobic mud /13/.

11.1.2 Main uses and pollution sources

Historically, TBT has been widely used in Denmark as a biocide in wood preservatives, in antifouling paints and as a pesticide /1/. This use has, however, been regulated for a number of years, and today it is prohibited to apply new TBT-containing antifouling paint on ships in Denmark.

According to the national statistics on the sale and use of pesticides in Denmark, which is published yearly, use of TBT has not been registered in the statistics since 1999 /2/. These statistics further include paint primers for the impregnation of wood and products used for vacuum impregnation of wood. TBT has thus not been used for these applications: pesticide, paint primers for impregnation of wood and vacuum impregnation of wood since 1999.

TBT has also been used in coatings. According to information from the Trade Association for the Paints and Lacquer Industry (FDLF) a newly performed survey at their members showed that TBT is not used anymore in Denmark for this application /6/. FDLF organises around 90-95 % of the relevant Danish companies. Naturally, there is always the risk that TBT is  used in coatings by companies not organised under FDLF, but this is not believed to be the case. Even if it were the case, uses of TBT in this respect would be insignificant.

Organic tin compounds are used for stabilisation of PVC. Normally dioctyl- and dibutyltin compounds are used for this application, but other compounds such as TBT are often seen as impurities. TBT are for instance found in shower curtains, vinyl wallpaper, vinyl flooring, vinyl gloves etc. /7/

According to information from The Danish Plastics Federation organotin compounds are not used in Denmark /8/.

11.1.3 Releases to and state of the aquatic environment

Monitoring data for TBT in the environment can be seen in the table below.

Table 11-1 Monitoring data for TBT (mean values).
Sources:  /1/ /3/ /4/ /5/

*:            In total six wastewater treatment plants with two measuring rounds on three plants
**:         Marine water (inside harbour area)

Mathematical model calculations of water concentrations of TBT in harbours in 2000 showed concentrations of around 0.15-0.2 μg/l in a traffic harbour and around 4 times higher concentrations in a marina /9/.

Based on the median values in sewage and in stormwater runoff presented in table 11-1, the total Danish releases of TBT to the aquatic environment with sewage effluent can be estimated at less than 0.3 kg/year while no data are available to make a similar estimate for stormwater.

EQS proposal

The proposed EQS for TBT are AA-EQS = 0.0002 μg/l and MAC-EQS = 0.0015 μg/l. Monitoring data, which exceed the proposed EQS, are marked with bold in the table above.

11.1.3.1 Point sources

Vacuum impregnated wood has a lifetime of more than 30 years. In 1994, there was still a consumption of around 5-8 tonnes of TBT for this use in Denmark. TBT will thus be found in impregnated wood in Denmark many years from now, and TBT-impregnated wood thus constitutes a point source for introducing TBT to the aquatic environment. However, over time TBT will degrade to dibutyl and monobutyl compounds, which have less fungicidal toxic effects than TBT /1/. On the wood surface TBT are further expected to degrade, due to exposure to sunlight /1/.

Since July 2003, it has been prohibited to use TBT-containing antifouling paints in Denmark and the EU. The ban came into force in 1991 for boats below 25 metres and for boats used predominantly in inland waters. Ships painted before 2003 may, however, still release TBT to the aquatic environment as antifouling paints on ships are believed to last for 3-5 years /1/. Around 2/3 of the TBT is believed to be washed out/leach from the paint during navigation /1/.

In consequence, , larger Danish ships thus constitute a point source for introducing TBT to the marine environment for some years ahead. Further, foreign and Danish ships painted with TBT-containing antifouling paint can still pass through Danish waters until 2008. The highest TBT concentrations are found close to harbours and shipping routes.

The remainder of TBT in antifouling paints on ships will either be covered by a new paint surface or eventually be removed (scraped or flushed off) before re-painting. Therefore, repair yards might be point sources of TBT emissions to the environment still a few years ahead.

Due to the previous, very extensive use of TBT in antifouling paints for ships, the sediments in many harbours are today highly contaminated with TBT. TBT has a long lifetime in sediments and can thus be a source for release of TBT to the water phase in the harbours for a number of years.

11.1.3.2 Diffuse sources

Runoff from buildings and constructions may release TBT into the aquatic environment directly through separate rainwater outlets and via wastewater discharges and overflows.

11.1.4 Existing legislation/regulation and their impact

Statutory Order No. 926 of 18 November 2002 on limiting the sale and use of certain dangerous chemical substances and products for special purposes.

The Order bans the sale and use of organic tin compounds for paints where the tin compounds function as biocides, unless the compounds are chemically bound or polymerised with the other compounds in the paint. The order further bans the sale and use of organic tin compounds as a biocide for anti-fouling on all vessels, equipment and apparatus used in marine fish and shellfish farming, and other equipment and apparatus to be fully or partly immersed in water. Finally, the order bans the sale and use of organic tin compounds for industrial treatment of water.

Assessment

The  Order banned the sale and use of organic tin compounds, including TBT, for the above-mentioned applications from January 2003. In relation to vessels, the order had most impact on larger ships as the use of TBT-containing anti-fouling paints on smaller boats at that time had been prohibited for a number of years already (e.g. the use of TBT on ships smaller than 25 metres used predominantly in inland waters has been banned since 1991). The vessels, equipment and apparatus that had been painted with TBT-containing paint before 2003 could, however, still be used after that date implying that TBT could still be released to the aquatic environment for some years after 2003. These releases are believed to be low today and will be close to zero within a few years.

IMO International Convention on the Control of Harmful Anti-fouling Systems on Ships adopted 5 October 2001 and Regulation (EC) No 782/2003 of the European Parliament and of the Council of 14 April 2003. on the prohibition of organotin compounds on ships ...

In 2001 the International Maritime Organisation (IMO) adopted the International Convention on the Control of Harmful Anti-fouling Systems on Ships. This convention prohibited application of TBT-containing antifouling paints on all boats from 2003 and prohibits navigation of ships with active TBT-containing paint from 2008. The Convention was ratified by Denmark in 2002, but the Convention is not in force yet. The EC implemented the convention through EC regulation 782/03 of 14 April 2003 and Directive 2002/62/EC of 9 July 2002..

Assessment:

The IMO Convention and the related EC Regulation prohibit the application of new TBT-containing antifouling paints on all EC ships and ships from other countries, which have ratified the IMO Convention. These regulatory steps have significantly reduced the release of TBT to the marine environment in Denmark. TBT from antifouling paints on ships is, however, still emitted to the marine environment from ships painted before 2003 and from foreign ships entering Danish waters.

From 2008, release to the marine environment in Danish and EU waters of TBT from antifouling paints should completely cease because of the regulation. The status regarding TBT for non-EU-ships entering EU waters will be decided upon around January 1, 2007 depending on the status of the IMO Convention. It is assessed that some foreign ship owners could possibly try to avoid the prohibition and still apply TBT-containing antifouling paint for a few years as the known alternatives are not as effective as the traditional TBT-containing products. The release of TBT from ships will, however, be reduced after 2008.

Due to the extensive TBT-pollution of marine sediments in Denmark, TBT will be released from sediments to the water phase for a number of years from now.

The prohibition list on Consolidated Act No 21 of 16 January 1996 from the Ministry of the Environment on chemical substances and products

Bis-tributyltin oxide is included in the prohibition list of active substance which may not be used in pesticides in Denmark.

Assessment:

Inclusion in the prohibition list has implied that bis-tributyltin oxide has not been used in pesticides in Denmark since 1999 /2/.

11.1.5 Conclusion on the need for further regulation

Assessment of the environmental concentration of TBT in relation to the proposed EQS is complicated by the fact that the analytical detection limit for the TBT-analyses is higher than the proposed AA-EQS. However, even a level corresponding to the detection limit will be acceptable for discharges of sewage effluent and stormwater from separate systems (no data exist for the latter).

TBT is classified as a priority hazardous substance, which implies that in Scenario B "discharges, emissions and losses" of TBT to the aquatic environment must cease/be phased out within 20 years from the date of entry into force of the directive. Scenario A implies only an obligation to consider the technical reduction measures that can be applied in order to achieve the goal, and there is no specified time frame.

11.1.5.1 Wastewater

The highest concentration measured in a sewage effluent is 10 times the AA-EQS while the average value is significantly lower. Thus, compliance of such discharges with the EQS requirement is regarded as having been achieved already. However, generation of more data to support this conclusion is recommended. If possible, an analytical analysis method with a detection limit below the AA-EQS should be used for this.

Thus, there is no need for further national progressive reduction measures in Scenario A while Scenario B implies an obligation to continue the progressive reduction of emissions, discharges and losses of TBT. It is, however, the interpretation that the existing regulation will fulfil this obligation.

As TBT is a prioritised hazardous substance, Scenario B implies an obligation to eliminate TBT in emissions and discharges including wastewater outlets while in scenario A the obligation only goes as far as to consider all technical reduction measures that can contribute to achieve this goal. A main source for TBT in wastewater could be the tear and wear of PVC-products stabilised with organotin compounds.

To cease/phase out TBT emissions/discharges in Scenario B it seems necessary to regulate (prohibit) the import and use of organotin-stabilised PVC-products to Denmark (according to information /8/ organotin-stabilised PVC is only seen in imported products). Such regulation, which can also be regarded as a progressive reduction measure in Scenario B, can, however, not only be adopted at the national level but requires action at EU level.

11.1.5.2 Marine waters

Most of the very limited data on TBT-concentration in marine waters (harbours) exceed the proposed AA-EQS significantly (by a factor 200-44,000). However, it is believed that less stringent environmental quality objectives will be defined for the waters inside the harbour boundaries (WFD article 4.5), i.e. the AA-EQS does not apply until outside the breakwaters.

The concentration of TBT in marine waters has been and will be further reduced over the coming years because of the prohibition of the use of TBT-containing antifouling paints. TBT will, however, continue to be lost to the marine water phase from the pool of TBT in the marine sediments. This process will go on for a number of years due to the long lifetime of TBT in sediments.

If such release of TBT from sediments to the water phase is covered by the Directive's requirement in scenario B for ceasing "discharges, emissions and losses" within 20 years, the most heavily TBT-contaminated marine sediments could be demanded removed and deposited on land.

Such contaminated sediments will be found in harbours or marinas with limited natural sediment exchange. Within the next 20 years, dredging and subsequent disposal of harbour sediments at sea ("klapning") or on land will, however, significantly reduce the volume of remaining contaminated sediments and lower TBT concentrations in the harbours in general. It is not believed that the Daughter Directive will prevent future marine disposal of dredged sediments that today are allowed for disposal at sea according to the existing guideline for marine disposal /16/.

Marine disposal of sediment containing up to 200 μg TBT/kg (dry weight) (under normal conditions the maximum TBT-concentration in sediments for marine disposal according to the guideline) is thus not estimated to result in TBT concentrations higher than the AA-EQS in the water outside the disposal site. This estimation is based on a calculation of the corresponding TBT-concentrations in the water phase and sediment phase (0.5 μg/l and 2,000 μg/kg dw respectively) for a marina /17/, combined with the much larger water exchange at most marine disposal sites /15/.

In Scenario A, all technical reduction options to achieve the goal of ceasing/phasing out of emissions, discharges and losses must be considered, however, without a fixed time frame. It is considered unlikely that this obligation should include disposal on land of the most contaminated fractions as a purely national measure, but only if this is introduced as a common measure in the EU.

11.2 Possible reduction/elimination measures

11.2.1 Technical measures to reduce/eliminate TBT

There are a number of substitutes for stabilisation of PVC, including calcium/zinc, lead (not a recommendable substitute from an environmental point of view), antimony etc.

The fact that TBT is a prioritised hazardous substance in scenario B will possibly in 20 years demand dredging and disposal on land of sediments from some harbours to fulfil the obligation to eliminate “discharges, emissions and losses" of such substances to the marine aquatic environment.

The present mean dredging volume of sediments from Danish harbours is around 3-4 million m³/year of which potentially up to 675,000 m³/year is so contaminated with TBT that it could be required to dispose of it on land /10/. In 20 years this volume will, however, be reduced compared to now due to the significant reduction in future TBT-releases to Danish harbours, the environmental degradation of TBT etc.

11.2.2 Possible synergies with other (priority) substances

A general action against suspended matter/pollutants in stormwater runoff will not only lead to a significant reduction in TBT releases to the aquatic environment but also bring down releases of most of the other PS/PHS.

11.3 Economic Assessment

There are a number of replacements for stabilisation of PVC, but since the relevant products are not produced in Denmark, it is not assessed as a viable Danish policy option to enforce substitution. Since TBT is a prioritised hazardous substance, there is, however, a need for further action and the cost of dredging TBT-contaminated sediments and disposing of the sediments on land is estimated here.

The price of dredging harbour sediments and disposing of it on land is around 145-515 DKK/m³ sediment /10/ depending on the kind of disposal facility on land (disposal area ("spulefelt"), landfill etc.). This is an estimate of the financial cost, and the corresponding welfare-economic cost to society is thus around 200-700 DKK/m³ sediment.

The cost estimates are unit costs that include both marginal investment cost and operating and maintenance costs.^[20]

The total number of harbours in Denmark is around 400 of which around 60 are industrial harbours /14/. The number of harbours that in 20 years will contain sediments being so contaminated with TBT that removal and safe disposal will be required will be considerably lower. This is a result of the stop for introduction of new TBT to the harbour sediments after 2008, degradation of existing TBT in the sediments, and of dredging carried out between today and 2025.

Especially the dredging anticipated to be carried out between today and 2025 will considerably reduce the number of harbours from which the sediments must be dredged away in 20 years. In most harbours dredging will have taken place several times within that period /14/ /18/. The number of harbours, which have not been cleaned during the 20 year period, will most likely be delimited to few marinas on Funen, Zealand (except harbours at the Sound) and the southern part of the east coast of Jutland. Sediments from harbours on the west coast and northern part of Jutland are removed very frequently (every year or every second year), the same goes for industrial harbours in general /14/ /18/.

In 2001, a survey was conducted to examine the present and future need for marine disposal of harbour sediments in the period 1998 to 2003 /15/. The survey found that 110 out of 121 harbours were in need of removal and marine disposal of (some of their) harbour sediments /15/. In other words, 90-95 % of the harbours needed dredging and disposing of sediments within the six-year period. However, only around 30 % of the Danish harbours were included in the survey.

Based on the 2001 survey and discussions with relevant actors (/14/ /18/ /19/), it is conservatively estimated that the number of places with “untouched” sediment in 20 years will correspond to 10-25 % of the Danish marinas. A “high mean value” per dredging in marinas in 2001 is estimated to be around 3,000 m³ /15/. Based on these numbers, the volume of TBT-contaminated sediments to be dredged from harbours in 20 years and disposed of on land (as a means of eliminating unacceptable “losses" to the marine aquatic environment) can be roughly calculated to about 100,000-250,000 m³.

In Scenario A/C, considering the non-legally binding character of the reduction/cessation targets of the WFD (Article 4), any future clean-up and land disposal of the low-contaminated sediments at the marine disposal sites with the aim to eliminate "losses" of TBT is assessed by the DEPA to be out of the question as the environmental benefits reaped will be insignificant compared to the disadvantages, and the costs of such action will thus become disproportionally high. Therefore, in this assessment of TBT no cost is associated with scenario A/C.

Figure 11-1
Amount of TBT-contaminated sediment for dredging and disposal on land in 20 years - High and low estimate for Scenario B.

The financial and welfare-economic costs can be calculated as the discounted net present value (NPV), which is the expression of the cost today of expenses over a period of time. The discount rate used by the Ministry of the Environment is 6 % and for the financial analysis and 3 % for the welfare-economic calculation. The results of the estimation are presented in the tables below. The welfare-economic cost is the cost which is relevant to this study as it expresses the total cost to society.

Table 11-2 Financial and welfare economic cost, dredging and disposal of TBT-contaminated sediment on land in 20 years - High and low estimates

*) The minimum cost is 145 DKK/m³ and the maximum cost is 515 DKK/m³

Note: With 6 % discount rate, the welfare-economic estimates are DKK 6-22 million for the low estimate of sediment disposal and DKK 15-55 million for the high estimate.

The extra financial cost of Scenario B is estimated to be between DKK 5-16 and DKK 11-40 million in net present value depending on whether the low estimate of 100,000 m³ or the high estimate of 250,000 m³ is used. The welfare-economic cost is estimated to be between DKK 11-39 million and DKK 27-98 million in net present value. It should be emphasized that this is a very tentative estimate.

Another possible technical measure for the purpose of ceasing/phasing out TBT emissions/discharges is regulation (prohibition) of the sale and use of organotin-stabilised PVC-products in Denmark. Organotin- stabilised PVC is only seen in imported products. If substitute additives or products were to be used instead, the functionality, environmental effects and price would have to be mapped to in order to assess the economic consequences of such a policy. Finally, a ban on imports of certain products due to environmental concerns would conflict with international trade agreements and the EU common market. However, as mentioned above, an EU-wide measure would probably be more cost-effective than a purely Danish measure.

11.4 Conclusion on TBT

Compliance with the EQS is not a problem with respect to the present discharges into surface waters. However, the concentrations in harbours, especially marinas, may reach levels way above the AA-EQS for coastal waters, and consequently concentrations in discharge of surplus water from land based disposal of dredged material could also exceed the EQS.

As TBT is classified as a priority hazardous substance, also losses originating from the historical use of TBT containing ship paints must cease within 20 years in Scenario B. In Scenario A, Denmark is only obliged to pursue the aim to the extent it can be achieved by considering technical reduction options.

Some imported products still appear to contain organotin compounds to stabilize certain PVC products. For these products it is assessed that action (in relation to the progressive reduction required in Scenario B) needs to be taken at EU level to be effective. The most important release of TBT to the aquatic environment appears, however, to be the release from contaminated harbour sediments.

The cost in Scenario B of dredging remaining, TBT-contaminated sediments and disposing of them on land in 2025 (as a means of eliminating unacceptable “losses" to the marine aquatic environment) was calculated. This results in a total extra welfare-economic cost of between DKK 11-39 and SKK 27-98 million in net present value depending on whether the low estimate of 100,000 m³ or the high estimate of 250,000 m³ is used. The corresponding financial cost is estimated to be between DKK 5-16 and DKK 11-40 million in net present value.

It is considered unlikely that the obligation in Scenario A/C should include disposal on land of the most contaminated fractions as a purely national measure, but only if this is introduced as a common measure in the EU. Hence, there is no cost associated with Scenario A/C.

11.5 References

/1/ Massestrømsanalyse for tin med særligt fokus på organotinforbindelser. Arbejdsrapport fra Miljøstyrelsen, nr. 7, 1997. Udført for Miljøstyrelsen af C. Lassen, S. Vaaben og E. Hansen, COWI A/S.

/2/ Bekæmpelsesmiddelstatistik 1997 - 2004. I alt 8 rapporter udgivet af Miljøstyrelsen (orientering fra Miljøstyrelsen) fundet på www.mst.dk

/3/ Sanne Hjorth Andersen, 2005, Effects of the Antifouling Compounds Tributyltin and Zincpyrithione on Marine Macroalgae, Master thesis, Department of Phycology, University of Copenhagen/Department of marine Ecology, National Environmental Research Institute.

/4/ Foreløbige analyseresultater af NOVANA screeningsundersøgelse for organotin i spildevand 2004. Modtaget via mail fra Lis Morthorst Munk, Miljøstyrelsen.

/5/ Tributyltin (TBT) i det marine miljø og misdannelser af marine snegle i Århus Amt 1998 - 1999. Århus Amt, Natur og Miljø. Juli 2000.

/6/ Personal communication with Vibeke Plambech, the Danish Trade Organisation for the Paints and Lacquer Industry. December 2005.

/7/ Resumé af "Analyserapport, Phthalater og organiske tinforbindelser i produkter med PVC". Miljøstyrelsen, 14. august 2002. www.mst.dk.

/8/ Personal communication with The Danish Plastics Federation. December 2005.

/9/ Belastning af danske havne med antibegroningsmidler - modelberegninger af koncentrationer i vand og sedimenter. Arbejdsrapport fra Miljøstyrelsen Nr. 23 2005.

/10/       Klapning - konsekvenser af ændring af klapningsstrategi med indførelse af koncentrationsgrænse og øvre mængdegrænse. Note prepared by COWI for DEPA, November 2004. Document no.: 59129-A-1 rev. 01.

/11/       Omfang og konsekvenser af forskellige strategier for håndtering af forurenede sedimenter. Arbejdsrapport fra Miljøstyrelsen nr. 34 2005.

/12/       Kjølholt J, Winther Ringgaard K, Skårup S (2006). Kilder og miljøtilstand for prioriterede stoffer under Vandrammedirektivet. Miljøprojekt nr. XXXX (in press). Danish Environmental Protection Agency.

/13/       Tributyltin. Environmental Project No. 451, 1999. Ministry of Environment and Energy, Denmark.

/14/       Personal communication with Danish Ports. September 2006

/15/       Jensen A og Gustavson K (2001). Havnesedimenters indhold af miljøfremmede organiske stoffer. Kortlægning af nuværende og fremtidige behov for klapning og deponering. Miljøprojekt Nr. 627, 2001. Danish Environmental Protection Agency

/16/       Vejledning nr 9607 af 01/10/2005 fra Miljøstyrelsen. Dumpning af optaget havbundsmateriale – klapning

/17/       Gustavson K, Møhlenberg F og Erichsen A. Belastning af danske havne med antibegroningsmidler – modelberegninger af koncentrationer i vand og sedimenter. Arbejdsrapport fra Miljøstyrelsen Nr. 23 2005. Danish Environmental Protection Agency

/18/       Personal communication with Foreningen af Lystbådehavne i Danmark. September 2006

/19/       Personal communication with Danish Environmental Protection Agency, Water. September 2006.

Footnotes

^[20] This means that the scrap values are not included in the estimation.

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