Natural Resources Wales (NRW) has monitored water quality monthly at 17 sites in the upper Tywi catchment, to assess the extent of acidification and the effects of amelioration measures such as liming. This monitoring is carried out for 3 main purposes:

1. To assess long-term trends in catchments with differing land use and management practices (9 sampling points).

2. To assess the effectiveness of lime dosing upstream of Llyn Brianne reservoir and to inform the management of these dosers (5 sampling points).

3. To assess the effectiveness of limestone sand applications in the Doethie catchment and to inform decisions on the further continuation of this operation (3 sampling points).

From 1984 to 1991, the Department of the Environment and Welsh Office funded research into the relationships between precipitation, geology, soils, vegetation, stream quality and land use. In particular these studies looked at the effect of direct application of neutralising compounds into stream catchments. Study catchments were located in the headwaters of the Afon Tywi and the Camddwr input streams of Llyn Brianne reservoir where a number of different land treatments and catchment liming experiments were carried out.

The National Rivers Authority (NRA) carried out further R&D studies from 1991 to 1994 to investigate the impacts of the catchment liming treatments and land use change on stream chemistry, and to investigate the management options for acidified surface waters. These studies found that catchment liming of acidified streams can create water quality suitable for supporting acid sensitive biota such as salmonid fisheries for a period of more than six years. Monitoring has continued at a number of these study sites to detect long term trends in water quality.

The data from monitoring upstream of Llyn Brianne and in the Doethie catchment have been used to inform NRW policy on liming and have also been used in numerous research projects by several universities. These sites are of critical importance locally to NRW, nationally to the UK's resource management, and globally to efforts to understand stressors affecting stream ecosystems.

Details of the study sites can be found in Sampling point map and Monitoring reports & data.