Chemical Pollution of the Aquatic Environment by Priority Pollutants and its Control

B. CRATHORNE, Y. J. REES AND S. FRANCE

1.1 INTRODUCTION

It is difficult to imagine a modern society without the benefits of chemicals and the chemical industry. Pharmaceuticals, petrochemicals, agrochemicals, industrial and consumer chemicals all contribute to our modern lifestyles. However, with the rise of chemical manufacture and use has come increasing public awareness and concern regarding the presence of chemicals in the environment.

There is an important distinction between the presence of chemicals in the environment (contamination) and pollution. Although these terms tend to be used in similar ways in everyday speech and journalism, in scientific areas there is a broad consensus that the term 'contamination' should be used where a chemical is present in a given sample with no evidence of harm and 'pollution' used in cases where the presence of the chemical is causing harm. Pollutants therefore are chemicals causing environmental harm.

The effects of water pollution can be summarized as:

• aesthetic: visual nuisance caused, e.g. litter, discoloration and smells

• temperature: usually heat

• deoxygenation: lack of oxygen in the water

• toxicity: acute or chronic toxicity causing damage to aquatic or human life

• sublethal toxicity: such as endocrine disruption or changes in biodiversity

• acidity/alkalinity: disturbance of the pH regime

• eutrophication: nutrients giving rise to excessive growths of some organisms

Any chemical can become a pollutant in water causing one or more of these effects if it is present at a high enough concentration. For example, serious pollution incidents result from spills of sugar and milk, substances which contain a high organic content. In fact, the majority of pollution incidents in the UK continue to be due to gross organic pollution. In 1998 sewage accounted for 24% of the l7 863 substantiated pollution incidents in England and Wales with oil being the single most frequent cause, accounting for 30%. Such organic pollution is caused by effluents containing biodegradable organic chemicals which generally act as pollutants, not because they contain chemicals at concentrations that are toxic, but rather the reverse. They contain chemicals that provide food for microorganisms which multiply rapidly as a result of the increased food input. The microorganisms, in the process of growing and oxidizing the organic chemical foodstuff, use up the dissolved oxygen rapidly which in some cases leads to the death of higher organisms, like fish.

Despite the fact that any chemical can be a pollutant, certain chemicals have been identified in regulation or by international agreement as being 'priority chemicals for control'. Such chemicals have generally been selected based on the following criteria:

• the chemicals are frequently found by monitoring programmes

• they are toxic at low concentrations

• they bioaccumulate

• they are persistent

• they are carcinogens

While a few years ago it would have been possible to quote the List I substances in the Dangerous Substances Directive (see Section 1.3.1) as being the priority pollutants, it is no longer possible to identify a single list. Different chemicals are priority pollutants in different contexts.

For many of these priority pollutants, the elimination of pollution is not considered sufficient by many and, applying the precautionary principle, targets of 'no contamination' have been set. This is demonstrated by a recent agreement between the North Sea countries, through the Oslo and Paris Commission, to reduce discharges, emissions and losses of specific hazardous substances continuously, with the ultimate aim of reducing concentrations in the marine environment to near background values for naturally occurring substances and close to zero for man-made synthetic substances.

1.2 POLLUTION CONTROL PHILOSOPHY

Preventing pollution of the environment by priority chemicals is very complex as chemicals are released and can gain entry to the environment at any stage in their life cycle (Figure 1.1), from development and testing, through manufacture, storage and distribution through to use and finally disposal.

Releases to the environment can broadly be categorized into point source releases, i.e. specific inputs from an industrial site or sewage works, etc., and diffuse or non-point source releases. Controls have tended in the past to concentrate on tackling the largest point sources and introducing strict requirements on discharges to water and sewer. A great deal has been achieved but, whilst the source and single media based controls have been effective, we are left with other problems. In the last decade or so the emphasis on priority pollutant control has shifted away from control of point sources towards control of diffuse sources. The main reason for this is the success achieved by control of point source pollution. A combination of factors has caused this, principally:

• new technology enabling priority pollutant use to be avoided, e.g. new pesticide discovery and new production processes

• improved pollution control technology

Given the success in this area, the relative contribution of diffuse sources has risen and attention has come to focus on these areas to facilitate further improvements. By their nature, diffuse sources...