Water quality


The European Water Framework Directive (WFD) promotes the assessment of our rivers based on many different indicators of ecosystem health. As a result, the aquatic environment is now assessed, not just by its chemical composition but also by the river’s physical characteristics and the quality of associated flora and fauna. The outcomes of this classification system enable the protection and improvement of water quality and dependent ecosystems, and allows us to set and achieve ecological objectives for all surface and groundwaters.

Monitoring results show that around 50% of the riverine water bodies within the Tweed catchment have good or high water quality, with the remainder impacted by various pressures including nutrient enrichment, sediment load, pesticides and other pollutants. The SEPA RBMP data tool has detailed information on the current classification of water bodies in the Tweed catchment. Throughout the Tweed catchment, sampling for chemical analysis of river water, sewage and trade effluent, as well as biological sampling and assessment, is carried out, along with regular inspections and investigations. The two bodies responsible for monitoring our water environment are SEPA (Scotland) and the Environment Agency (England). This page describes the principal types of pollution affecting our rivers, the processes of pollution prevention and various means of monitoring water quality.

Types of pollution

Pollution by organic materials

Pollution of the river by organic materials (e.g., sewage, silage effluent, residues from food manufacturing etc.) can result in a reduction of the amount of dissolved oxygen contained in the water. If the discharge of these materials exceeds the river’s capacity to absorb them, the dissolved oxygen level in the river can be depleted to such an extent that the whole natural balance of the river can be disturbed. In extreme cases this can result in complete deoxygenation of the river, leading to fish mortalities.

Pollution by directly toxic materials

Many modern chemical compounds ranging, for example, from pesticides applied in the farming industry and solvents utilised in the electronics industry, are directly toxic to river life. Some of these, such as organo-, chloro- and phosphorus compounds, heavy metals and phenols, are extremely toxic even in minute concentrations. Additionally, their effects can accumulate over many years before becoming a problem and can become more acute if other pollutants (e.g., organic effluents) are affecting the river.

Pollution by suspended solids

Suspended solids in the water column cause turbidity (cloudiness). Whilst this occurs naturally at frequent intervals during normal floods, man-made turbidity can cause problems to amenity and fishing interests. If this occurs persistently, the entire biology of the river can be affected.

Processes of pollution prevention

Prevention of organic pollution

Treatment of organic material (e.g., sewage, and other organic wastes) is based on the oxidation of organic matter before discharge into the river. Whilst a river, depending on its rate of re-aeration as it passes through streams and rapids, always has some capacity to purify polluting matter naturally, it is generally necessary to provide treatment for the oxidation of organic matter before discharge into the river. The specific methods used in effluent treatment are not covered here, but, basically, water is processed through settlement (removal of settleable solids) and biological treatment (aeration or biological filter treatment) to remove the remaining polluted matter in solution.

Prevention of pollution by toxic materials

Effective treatment of toxic materials, although available, is generally expensive. The development of new technologies often means the arrival of new pollutants and these present an increasing hazard to the water environment and will require a high degree of vigilance if pollution prevention is to be successful. The advent of the “polluter pays” principle may result in pollution control becoming more effective.

Prevention of pollution by suspended solids

Treatment of suspended solids arising from human activities, such as sand and gravel quarrying and other mineral workings, is normally affected by simple settlement in ponds/lagoons but, if necessary, it can be assisted by the addition of flocculating agents. If there is potential for suspended solids from riverworks, settlement is generally the treatment adopted, although depending on local conditions this may be altered to include the method and timing of any in-river operations.

Monitoring of chemical and biological quality

To monitor chemical quality many tens of thousands of tests are analysed each year for pollution control purposes. Analysis may range from simple tests, such as the determination of the pH value or suspended solids, to sophisticated techniques for measuring the most minute amounts of, for instance, heavy metal or organo-phosphorous compounds. Biological monitoring on the Tweed generates a large amount of data, which can be used to identify and interpret the nature of any problems occurring within the system. Data are currently collected for phytobenthos, macrophytes, benthic invertebrates, fish and non-native invasive species. The macroinvertebrate data affords the use of biotic indices, which are calculated by scoring species of invertebrates in terms of their sensitivity to pollution. The highest scoring species are more sensitive to pollution and occur only in very clean waters and, consequently, the higher the index the higher the quality of the water. The relative abundance of taxonomic groups of macroinvertebrates is also important.