Acid rain gains are positive but limited

The UK Acid Waters Monitoring Network says that recovery from acid rain in the uplands since the 1970s has been limited and is by no means guaranteed to continue.

A report from the UK Acid Waters Monitoring Network (AWMN) argues that the recovery of UK upland surface waters has been only partial.1  It suggests they are still subject to stress from nitrogen deposition and climate change, while continued improvement is far from guaranteed.

The AWMN was founded by the government in 1988 to monitor how acidified waters responded to reductions in emissions of sulphur and nitrogen air pollutants, as they were reduced through environmental legislation. The network comprises 22 headwater lakes and streams in upland northern and western Britain and includes ‘control sites’ in low-acid deposition areas.

The high rainfall and geology of the sites makes them vulnerable to acidification. The slow weathering of rocks such as granite and slate means that base metals, such as calcium and magnesium, washed out of soils by acid deposition, are not readily replaced. The result is that waters become more acidic and soils may release aluminium, which is highly toxic to aquatic life.

Pollution sources

Key air pollution sources include sulphur and nitrogen oxides from power stations, industrial boilers and vehicles. But ammonia emissions from farming are also significant. Although apparently alkaline, the compound has an acidifying effect on soils.

“After 20 years there is consistent and compelling evidence for both chemical and biological improvement of the acidified sites across the network,” the report says. Chemical evidence includes a decline in sulphur deposition. UK levels fell by 80% between 1986 and 2006, the report concludes, but finds little change since 2000. Levels at the most acidified sites remain three to six times higher than those at remote control sites in north-west Scotland.

There has been a decline in the acidity of waters and frequency and severity of ‘acid episodes’ – the flushes of acidic, aluminium-rich water likely to harm wildlife.

In terms of biology, changing diatom (algae) and invertebrate populations suggest  water acidity is decreasing. Acid-sensitive aquatic plants are returning to some sites and brown trout populations are improving, although densities often stay low.

But the report cautions that while the bio­­logical recovery is “positive”, it is also “very limited”. This is confirmed by palaeoecological studies that compare past diatom populations in sediments with current ones. It is not alone in warning that the effects of acidification are still apparent (ENDS Reports 392, p 17 and 396, p 14).

“The central concern is whether the im­provement can be sustained and whether the targets for recovery can be achieved,” the report concludes.

Reasons for the uncertainty include a trend in nitrate deposition, the second most important acidifying pollutant, which has varied from year to year. Only seven sites show a slight, but statistically significant, long-term decline.

And acid deposition continues to exceed rock weathering rates at some sites with levels of base metals in soils not increasing. The acidity of waters is often not declining and soils are continuing to release remnants of past pollution, including sulphur and toxic metals such as lead and mercury.

Goals for recovery are set as ‘critical loads’, defined as the level of acid in rain, mist and snow that soils or waters can stand without a rise in acidity. Acidified sites have been exceeding critical loads for decades.

Modelling suggests 12-20 of the AWMN sites continue to exceed critical loads, and the predictions are that 5-15 will continue to do so by 2020. Natural England and the Countryside Council for Wales (CCW) also argue that almost 40% of the UK’s most sensitive ecosystems will still exceed critical loads by 2020.

The AWMN itself faces an uncertain future. In 2007 the previous government cut its funding by 78% as air quality was put on the backburner. A reduced network of sites has survived thanks to contributions from benefactors including NGOs and the devolved administrations. But this situation is considered unsustainable.

The network was originally set up to focus on acid deposition and has been funded by DEFRA’s air-quality division. Dr Chris Curtis, principle research associate at the Environmental Change Research Centre at University College London and a lead author of the AWMN report, told ENDS that the interdisciplinary applications of its findings are becoming increasingly evident:

“It can tell us almost as much about biodiversity, water quality and climate change as it can about acidification. For example, upland waters provide drinking water for the majority of the UK population, but the AWMN provides the only reliable source of information on how the quality of them is changing.”

DEFRA funding is committed until March 2011, but beyond that there is further uncertainty, especially given the current funding cuts. “You just have to try not to be too pessimistic and hope that the powers that be see the future potential of the data. It stretches far beyond telling us about acidification,” said Dr Curtis.

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