Climate change triggers peatland decline and hits water supplies

New data suggest that peatlands may be in decline because of climate change - helping to explain recent problems with discolouration and disinfection by-products in upland water supplies. Significant amounts of carbon stored in peat may already be escaping to the atmosphere.

Peatlands are globally significant stores of carbon which, if released, would double atmospheric carbon dioxide levels.

Research reported in Nature suggests that concentrations of dissolved organic carbon (DOC) - largely manifested as brown coloration in peaty upland waters - has increased by 65% over the past 12 years.1 The observations were made at 22 sites across the UK which were originally set up to monitor acid deposition on upland ecosystems.

Chris Freeman and colleagues at the University of Wales in Bangor attribute the rising level of DOC to increasing temperatures - even though these amounted to only a mean increase of 0.66oC in the 1990s relative to the three preceding decades. Changes in land use or acid inputs were ruled out because the effects were consistent across 20 of the sites, including both acidified and control areas.

DOC in waters results from bacterial breakdown of organic matter in peat and soils which proceeds most rapidly in warmer and drier conditions. The rising DOC levels suggest a steady change in upland conditions which could be linked to changing rainfall patterns.

The increase in colour from upland supplies has also been noted by water companies. Dr Chris Spray, environmental adviser at Northumbrian Water, said that the company had contracted Professor Tim Burt at Durham University to look into extensive colour records at one of its water treatment plants. The work has yielded data on colour in the river Tees at Darlington going back 30 years.

Professor Burt explained that there was an annual cycle of colour which peaks in August to October following drying and oxidation of the peat in summer. Droughts cause a temporary decline in colour, followed by an increase two or three years later when the peat rehydrates.

But Professor Burt said that the Tees data "showed a very linear increase in colour" over the 30-year period over and above that due to annual fluctuations and occasional droughts. "Rising DOC and colour seem to be something people have seen widely - in Belgium and Scandinavia, for example."

The Nature article makes the point that coloured upland waters transfer soil carbon reserves to the oceans. According to some oceanographers, it is likely that most of this will be oxidised, adding to atmospheric CO2, rather than being laid down in sediments.

But peat oxidation may also result in direct emissions to the air of carbon dioxide or the more potent greenhouse gas methane. The process is not understood and has not been included in the UK's greenhouse gas emission projections - except in the context of land use changes such as afforestation.

However, any significant breakdown of the five billion tonnes of carbon locked up in the UK's peatlands would make it difficult to achieve the national target of a 12.5% reduction in 1990 levels of greenhouse gas emissions by 2008-2012.

Increasing colour also represents a threat to drinking water quality and will increase water companies' treatment costs. Following the protracted drought of 1995, for example, companies noticed an increase in colour problems. In its 1998 annual report, the Drinking Water Inspectorate reported an increase in failures in 1997 and 1998 of the standard for trihalomethanes - chlorination by-products formed from DOC when upland waters are treated.

United Utilities - then North West Water - was worst affected. In 1998, 46 of the company's 306 water supply zones failed the trihalomethane standard, compared with only three in 1996. The company was required to improve water treatment to reduce trihalomethanes in 35 supply zones as a result.

Keith Osborn, United Utilities' public health scientist, commented that it was difficult to put a cost on colour removal because the remedy - improved treatment by chemical coagulation - was also needed for other reasons such as removing the pathogen cryptosporidium.

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