The fate and behaviour of a substance entering the water column is determined to a large extent by its physical-chemical properties. Properties, such as solubility, volatility and its affinity for organic matter determine whether a substance remains dissolved in the water column, evaporates into the atmosphere or becomes associated with suspended or deposited organic material or sediment. Affinity for organic material or sediment is measured for a substance in the laboratory, or estimated by modelling, by determining the extent to which the substance partitions between water and octanol (an organic solvent) and is expressed as a partition coefficient (log Kow). Log Kow values of greater than 4 indicate that a substance is likely to sorb strongly to sediments/suspended solids. As such, these substances may disappear relatively quickly from the water column, but persist in sediments where they may continue to be available to sediment fauna, or risk re-suspension when sediment is disturbed.

The fate and behaviour of non-synthetic substances, such as metals, are influenced by the chemical form of the metal. Certain forms are soluble in water and are likely to remain in the water column. Others are insoluble and are likely to be deposited and incorporated into sediments. Soluble forms might be transformed into insoluble forms, and vice versa, by chemical or biological processes such that metals can cycle between sediments and the water column over a period of many years. For these substances, fate and behaviour is dependent on its chemical form and environmental conditions and cannot be classified with respect to a partition coefficient.

Many of the synthetic organic substances are broken down by natural processes, such as photolysis (reaction with light), hydrolysis (reaction with water) or by biological processes (biodegradation). The rate at which this occurs is measured in terms of the half-life (the time taken for the concentration of the substance to reduce by half). Persistence in water or sediments can be measured in the laboratory. The following criteria have been used:

• half life <10 days - low persistence
• half-life >10 and <100 days - moderate persistence
• half-life >100 days - high persistence

Non-synthetic (naturally occurring) toxic substances, such as metals, do not break down but undergo chemical transformations and cycle between sediment, water column and organisms. Persistence in any one environmental medium is dependent on the chemical form and prevailing conditions. Such substances cannot be classified with respect to a half-life.

Bioaccumulation of a substance occurs when it is taken up by an organism and not metabolised or excreted to a large extent such that the concentration of the substances in the body tissues increases over time. The extent to which a substance bioaccumulates in an organism is expressed as a bio-concentration factor (BCF) which is a ratio of the concentration in an organism after a period of exposure relative to the concentration in the environment. The higher the concentration in the organism relative to the surrounding environment, the greater the BCF. Many substances that bioaccumulate are stored in fatty tissues and so a substance that has an affinity for organic matter is more likely to bioaccumulate that one that does not. Consequently, expectation to bioaccumulate is assessed by a combination of BCF and log Kow. The following criteria have been used:

• CF value of <100 or log Kow <3 - not expected to bioaccumulate
• CF value of >100 but <1,000 or log Kow >=3 and <=4 - has the potential to bioaccumulate
• CF value of >1,000 or log Kow >4 - has the potential to bioaccumulate significantly.

These criteria do not apply to ionic chemicals that are generally metabolised or excreted or to organic chemicals with molecular weights greater than 700 as these molecules are too large to pass through cell walls.

Toxicity has been classified as follows for major groups of organisms using available information. The criteria used for the classification are those currently being used in the development of the Environment Agency Substance Information System (EA-SIS) which will be used by EA staff to readily access information on substances of concern.

The following criteria have been applied:

• Very toxic to aquatic organisms - acute effect data £1 mg l^-1 or chronic effect data £0.1 mg l^-1
• Toxic to aquatic organisms - acute effect data >1 mgl^-1 and £10 mgl^-1 or chronic effect data >0.1 mgl^-1 and £1 mgl^-1
• Unlikely to be toxic to aquatic organisms - acute effect data >10 mgl^-1 or chronic effect data >1 mgl^-1

Very little is known about interactive effects of many of the substances listed in Appendices B and C, although some, such as effects of pH on toxicity of ammonia, are well understood. Where interactions such as this are well known, they have generally been referred to in the Appendices, but, in most cases, the toxicity of individual substances has been considered in isolation.

This whole area is one of great uncertainty so it is difficult to provide advice on particular risks. The Environment Agency is currently considering the applicability of Toxicity-Based Consents for discharges where toxicity of contaminants is uncertain; this approach may be useful for some Natura 2000 sites.

References