Water quality generally means the state of the water in the natural environment or used as a potable water supply or in industrial processes. Effluent quality is a term used to describe water that is discharged into the environment after being used in a process. Effluent properties can have significant impact upon water quality, and in turn, affect all surface waters, groundwaters and seas. This is reflected within the legislation passed to regulate this which includes the Control of Pollution Act (COPA) 1974, the Water Act 1989, the Water Resources Act 1991 and the Water Industry Act 1991. Also, within EPA 1990 and EA 1995, there are also important provisions for the control of effluent discharges and protection of water quality.

The only representation of the scope of water pollution in England and Wales available is through using 1996 figures.

Over 90 per cent of all substantiated incidents in 1996 were 'minor' incidents. The number of substantiated incidents was lower in 1996 than in any previous year in the 1990s. Between 1991 and 1996 the number of 'major' incidents fell by 60 per cent and the number of 'significant' incidents by 78 per cent. The number of prosecutions fell by 61 per cent between 1991 and 1996. Over half the substantiated incidents in 1996 were of pollution by fuels and oil and by sewage.

Source: Environment Agency