Wood deterioration can be caused in a number of ways;

• fungal attack;
• insect attack;
• weathering;
• mechanical wear and fire.

There are numerous active ingredients (in excess of 50) registered for use in the UK (PSD/HSE 1998) which can be broadly grouped into three categories:

• organic solvents;
• water borne; and
• creosote.

Organic solvent based preservatives most commonly comprise either one or a mixture of some of the following preservatives;

• pentachlorophenol;
• pentachlorophenol laurate;
• 2-phenyl phenol;
• lindane;
• metal naphthenates and bis (tributyltin)oxide (TBTO), dissolved in an organic solvent usually of the white spirit or kerosene type.

In addition to the active ingredients, additives such as resins, colouring agents and water repellents may also be added (HMIP 1992).

Water borne preservatives usually comprise a mixture of inorganic salts dissolved in water. The water borne preservative group is dominated by the copper-chromium-arsenic (CCA) preservatives which frequently consist of a mixture of copper sulphate, sodium dichromate and arsenic pentoxide. Copper (see Section B7) and arsenic (see Section B9) are the principal pesticidal agents while the dichromate fixes the copper and arsenic in the wood. The preservative is mixed with water at the treatment site. The advantage of the CCA wood preservative is that they bind very tightly to the timber, giving the treated timber a long life even when immersed in water. Other compounds used in water borne preservative include sodium arsenate; disodium octaborate and sodium salts of chlorinated phenols, boron based compounds and quaternary ammonium compounds (Williams 1994).

The third type of preservative is creosote, a blend of distillate oils, mainly from coal tar with boiling points ranging from 200 - 400^oC. It contains a high proportion of polyaromatic hydrocarbons (PAHs), together with a few percent of tar acids (phenolic derivative). While the use of creosote has fallen since the early 1970s, it still accounts for a significant proportion of the UK market (Williams 1994).

The use of timber treatment chemicals falls into three main categories:

• industrial pre-treatment of timber (pressure and vacuum plants and non pressure processes such as immersion plants);
• remedial treatment (so called professional use);
• domestic (DIY) use by retail customers.

Inputs of timber treatment chemicals into the aquatic environment can emanate from point and diffuse sources.

Point sources include wastewater arising during the manufacture of active ingredients and formulations; where treatment plants use formulation with volatile ingredients, release to air may occur, and these may enter the aquatic environment through atmospheric deposition. There is the potential for spillages or releases, resulting from either bad on-site housekeeping or through accidents.

Of the three main use categories, industrial pre-treatment of timber represents the most likely point source of timber treatment chemicals due to the large quantities of chemicals handled at the pre-treatment sites.

Diffuse sources may also be significant for some preservatives, such as creosote used in remedial and domestic situations.

Williams (1994) reviewed timber treatment chemicals in order to identify chemicals with a high to medium risk to the aquatic environment for the purpose of EQS development. From the list of over 50 active ingredients registered for use in the UK, 20 were identified for the risk assessment.

Williams (1994) concluded that the substances of greatest concern in terms of their usage, aquatic toxicity, persistence and bioaccumulation were CCAs and tributyltin naphthenate (TBTN) whilst creosote, zinc versatate, zinc octoate, acypetacs zinc and copper naphthenate were of medium concern.

CCAs and copper naphthenate are likely to exert their toxic effect due to the dissociation of copper and, therefore, the existing standards for copper (EQS) were considered sufficient for the protection of the aquatic environment (see Section B7). Similarly, the existing EQSs for TBTO and zinc were considered sufficient for the protection of aquatic life from TBTN and zinc compounds.

Creosote was the only remaining substance of concern with no existing EQS. The remainder of this profile concentrates on creosote using information from Williams (1994).

Creosote typically comprises 85% PAHs, 10% phenols (or tar acids) and 5% tar bases. Coal tar creosote usually consists of liquid and solid aromatic hydrocarbons, such as guaicol, phenol, cresols, pyrol and pyridine. The major PAH constituents are acenaphthene, anthracene, benzanthracene, fluoranthene, fluorene, phenanthrene and pyrene, although as many as 36 have been identified.

Creosote is not monitored under any national monitoring programme and would probably only be measured close to known sources. Concentrations of individual PAHs are occasionally measured in sediments and biota (see Appendix D). Contributions from creosote may add to these close to known sources.

Williams (1994) reported that creosote oil had been recorded in biota (molluscs and crustaceans) and that lipid concentrations of 1,046 (mussels), 459 - 3,254 (periwinkles), 202 - 354 (whelks) and 459 ppm (clams) had been measured.

Creosote is of low solubility and stable in water. Only about 9% may dissolve and be transported away from point sources (Williams 1994). Some of the constituent PAHs have been shown to accumulate in sediments. The major fate processes are believed to be biodegradation and photolysis, depending on light availability. Complete breakdown may take between 3-12 months.

An exhaustive literature review on the toxicity of creosote to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Williams 1994). The most sensitive organisms have been identified.

Creosote appears to be of moderate to high toxicity to marine fish and of high toxicity to marine invertebrates with 96-hour LC50s of 0.56 - 4.42 mg l^-1 and 0.018 - 0.24 mg l^-1 respectively.

An EQS for creosote has not been developed because of the heterogenous nature of the mixture and associated analytical difficulties.

Williams (1994) report a BCF of 0.6 for the dissolved fraction in a water body, indicating that it is not likely to be bioaccumulated in aquatic organisms.

Potential effects include:

• toxicity of creosote in the water column to marine invertebrates and fish;
• physical effects of a spillage of large quantities of creosote may be similar to the effect observed for oil and petrochemicals.

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