Air pollution

Surface water acidification

Loch Doilet, Scotland There is very little direct chemical evidence that shows that surface waters have acidified, both in the UK and worldwide. The problem of acid rain was only identified once emissions of acidifying pollutants were on the decline, and monitoring data largely fails to capture the trend of increasing acidity. However, evidence for the acidification of surface waters has been obtained from pH reconstructions based on diatoms in lake sediment cores. This palaeolimnological data has provided the key evidence linking historical emissions of acidifying pollutants with the acidification of surface waters.

In the early 1980s the link between acid deposition and and the acidification of surface waters was not universally accepted. Alternative hypotheses for the cause of the acidification were proposed:

  • natural acidification processes
  • changes in catchment management
  • afforestation of upland soils

A major project of palaeolimnological work was funded by the Surface Water Acidification Programme (SWAP) and later by the then Department of the Environment (now DEFRA) to evaluate these alternative hypotheses. Sites in Norway, Sweden and the UK were chosen to answer specific questions about the timing, extent and cause of surface water acidification. A range of techniques were employed to answer these specieific questions:

  • analysis of the remains of diatoms and other biological groups
  • trace metal analysis
  • fly-ash particle analysis

The overall conclusion was that acid deposition was the main cause of the recent acidification of surface waters, although natural acidification had increased the sensitivity of many sites to the effects of acid deposition. Afforestation had also led to stronger acidification at those site in areas receiving high levels of acid deposition.