*This will be reduced to less than 3kms once Eccles Sewage works improvements are completed.
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, throughout the entire catchment by the respective river authorities. This sheet describes the principal types of pollution affecting our rivers, the processes of pollution prevention and the means of monitoring the quality of the water.
Types of pollution
Pollution by organic materials
Pollution of the river by organic materials (eg 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 the 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, of course, can be more acute if pollution by other agencies (eg organic effluents) is affecting the river.
Pollution by suspended solids:
Suspended solids in the water column cause turbidity of the water. 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 (eg sewage, and other organic wastes) is based upon 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 (self-purification), 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 in this fact sheet, 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. These present an increasing hazard to rivers 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 at the production process becoming more effective.
Prevention of pollution by suspended solids:
Treatment of suspended solids arising from man made activities such as sand and gravel quarrying and other mineral workings is normally affected by simple settlement in ponds/lagoons but if necessary can be assisted by the addition of flocculating agents. When there is potential for suspended solids from riverworks such as realignment, again, 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 a total of upwards of 60,000 tests are analysed each year for pollution control purposes. The whole gamut of analysis ranging from such simple tests as the determination of the pH value or suspended solids to the - most sophisticated instrumentation techniques for measuring the most minute amounts of, for instance, heavy metal or organo - phosphorous compounds is available within the respective river authority's laboratories.
The biological quality of the Tweed is monitored on a routine basis by looking at populations of macroinvertebrates (small animals such as insects, worms and snails) because of their ease of sampling, identification and importantly, susceptibility to change caused by pollution. Although the Tweed retains a “Class 1” water quality throughout the catchment it does change in nutrient status from the headwaters to the lower reaches. The headwaters are relatively low in nutrients (oligotrophic) whilst the lower reaches are relatively high in nutrients (eutrophic). The macroinvertebrate populations reflect this change in nutrient status. Whilst this pattern of change occurs naturally in rivers, the effect can be greatly amplified by man's activities.
Biological monitoring on the Tweed generates a large amount of data, which can be used to identify and interpret the nature of any pollution occurring within the system. The data collected 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.
