Lime Dosing to Restore the Upper Towy © EA

This text of an Environment Agency 1997 publication is wholly reproduced apart from minor grammatical and spelling corrections. Since publication, Hyder has agreed to continue funding the cost of the lime in a colaborative initiative, and CFF has paid for the purchase and installation of wind power generators.

The Camddwr Lime Doser

The effects of the acidification of freshwaters were first identified in upland areas of Britain in the early 1980's. The problem is caused by the release of sulphur and nitrogen gases into the atmosphere from the burning of fossil fuels which are converted in the atmosphere to sulphuric and nitric acids. The increased acidity of rainwater results in raised concentrations of toxic metals in rivers and streams particularly aluminium which can be poisonous to fish and other aquatic life.

The headwaters of the river Tywi which feed into Llyn Brianne (the principal water supply reservoir in West Wales) were found to be particularly badly affected by this type of pollution. The Tywi is a very important salmon and sea trout fishery but surveys in the 1980s showed that young stages of salmon were unable to survive the acidic conditions that existed in the 17 km of river downstream of Llyn Brianne reservoir. Trout were also severely affected.

To counter the effects of acidification in the Tywi, the predecessor organisation of the Environment Agency Wales decided on a programme of adding lime in the form of powdered limestone to the waters of the Tywi. This project commenced in 1991 and initially involved distributing 500 tonnes of lime every 6 months by boat to the waters of Llyn Brianne reservoir. Subsequently in 1996 a more efficient and more economic method of liming was developed by Environment Agency Wales.

This involved adding lime from dosing silos to the two main feeder streams to the reservoir. These are the first units of their kind to be installed in the UK. Purchase of the lime dosers, which cost almost £90,000, followed extensive consultations with experts KEMIRA KEMWATER (per.ostensson@kemira.com) from Sweden, where extensive Government funded liming operations are undertaken to combat the effects of acidification.

The project has been very successful in re-establishing substantial populations of young salmon in the upper Tywi and numbers of trout have markedly increased. It is estimated that the project could result in an increase of about 600 adult salmon and 2400 sea trout returning to the river Tywi each year. There will also be benefits to animals which rely on aquatic life as a source of food such as herons, kingfishers and otters.

The Environment Agency Wales has been generously supported in this venture by Hyder PLC, who have made a substantial capital contribution of £20,000 towards the construction costs of the lime dosing plant in addition to funding limestone for the first two years of operation and assisting in weekly maintenance checks. The forestry company Forest Enterprise is also providing valuable support with doser maintenance.

Acidification - the issue

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 poor base 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.

Acidification results in lowered pH (a measure of water acidity or alkalinity) and elevated aluminium levels which are very toxic to fish. Salmonids are particularly susceptible, especially in 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.

Lime Treatments

The river Towy is recognised as the best seatrout 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 Towy downstream fro Llyn Brianne were either fishless or supported very low numbers of fish. This was a direct result of the acidic conditions in the river and the associated high levels of toxic metals, particularly aluminium.

In 1990. a project was initiated by one of the Agency’s predecessor bodies, the National Rivers Authority to investigate the effectiveness of boat-liming Llyn Brianne reservoir to improve water quality in the Towy downstream. Experimental liming began in 1991, and 500 tonnes of powdered limestone has been applied every 6 months for the past 5 years.

Before liming, the average pH in the main river was 5.1 units. Post-liming pH’s increased, and generally have been maintained above pH6. Improvements in the water quality have been mirrored by improvements in the biota. Before liming only one site, 17 km downstream from the dam supported salmon fry. Following liming there has been an increase in fry densities at all sites, with the exception of the site immediately below the reservoir. Similar improvements in salmon parr, and trout/parr densities have been recorded. Juvenile salmon and trout numbers have increased to levels which indicate that successful spawning is occurring in the improved river reach. It is predicted that the improvements in water quality could result in an increase of about 600 adult and 2400 adult seatrout returning to the river Towy each year.

Lime Dosers

The Environment Agency is eager to continue with liming of the reservoir to sustain the benefits to the river. A review of liming techniques, and a modelling exercise, indicated that changing from boat liming to liming the streams flowing into the reservoir using automatic lime dosers, would provide a more consistent water quality through the year at substantially lower dosages, and consequently lower cost of lime.

Two purpose built lime dosing units were installed on the Towy and Camddwr upstream from the reservoir in October 1996. These are the first dosing units of their kind to be installed in the UK. Powdered limestone, which is produced as a by-product of the road construction industry, is supplied by tanker to the doser. The dosers measure the flow of the river and automatically calculate the amount of lime required to neutralise the acidic waters flowing into the reservoir. Both dosers have telemetry links to the Agency’s office in Llanelli which allows the dosing rate and flow to be monitored, and signals when to re-order lime or if there are any mechanical or intruder problems.

The performance of the dosers is being monitored using data logging instruments, deployed upstream and downstream from the dosers, which record the pH of the water every 15 minutes.

The Towy Lime Doser

The Camddwr Lime Doser