This Section summarises the key variables that must be considered when assessing projects affecting water quality. Their role and significance are considered only briefly here, for a fuller account staff should refer to the separate reports for nutrients, metals and turbidity:

Impact of Nutrients in Estuaries - Phase 2 (CCRU and CEFAS, 1999).

Turbidity in England and Welsh Tidal Waters (WRC, 1999).

Assessment of Risks Posed by List II Metals to >Sensitive Marine Areas= and Adequacy of Existing Environmental Quality Standards (EQSs) for SMA Protection (WRC, 1999).

Chemicals introduced to the marine environment as a result of an activity or a discharge include toxic and non-toxic substances. Toxic substances generally exert their effects on organisms through direct toxicity and the extent of the effect is dependent on the chemical form of the substance (e.g. dissolved forms of metals are most toxic), the concentration and the period of exposure. Non-toxic substances generally exert their effects on organisms through direct or indirect means by changing the natural balance of the physico-chemical environment through a series of chemical and biological transformations. For example, nutrients stimulate growth of planktonic algae which, in turn, affects the light regime in the water column.

The fate and behaviour of a substance once discharged to the marine environment is determined to a large extent by its physico-chemical properties. However, the concentration and period of exposure of toxic substances and the chemical and biological transformations of non-toxic substances are affected by a number of important processes in the receiving environment. These include:

• Dilution (including the concept of the mixing zone);
• flushing time;
• stratification;
• sediment type.

The effects of each of these on the behaviour and hazard posed by chemicals introduced into the marine environment are considered in turn.

References