The Environment Agency has been generously supported in this venture by Hyder PLC, who have made a substantial capital contribution of 20,000 toward the construction costs of the lime dosing plant in addition to funding the supply of limestone for the first two years of operation and assisting with weekly maintenance checks. Acidification results from the increased emissions of gaseous sulphur and nitrogen oxides from the burning of fossil fuels, which are converted in the atmosphere to sulphuric and nitric acids. These compounds can be carried over large distances before being deposited as 'acid rain'. Large areas of upland Wales are particularly susceptible to acidification due to the area's high rainfall, and the local geology of hard weathering rocks and base poor soils, which offer little neutralising capacity to the acid rain. Extensive conifer afforestation in the area adds to the problem by scavenging pollutants and enhancing the naturally acidic conditions.
Salmonids are particularly susceptible, especially at the juvenile stage.
Insect life in acidified streams is also impoverished, resulting in lack of food for fish and bird life such as the dipper. Although reductions in pollution emissions are predicted for future years, extensive studies have shown that these will be insufficient to prevent continued acidification, and that it would take many years for the affected soils and the water flowing through them to recover naturally. Therefore alternative measures such as liming need to be considered, where appropriate, to protect fish stocks in the interim. The River Tywi is recognised as the best sea trout fishery in England and Wales and also has a sizeable salmon fishery, both of which have been under threat in the upper catchment from the effects of acidification. Fisheries and biological investigations undertaken in 1990 established that 17 km of very good salmonid spawning and juvenile nursery areas in the Tywi downstream of Llyn Brianne, were either fishless or supported very low numbers of fish.