Healthy Looking Waterways

Looks can be deceiving. Over the past two decades, waters in many parts of the Northern Hemisphere have become more colored, due to increasing concentrations of dissolved organic carbon, or DOC. The color change, while not aesthetically pleasing, actually indicates the lakes, streams, and their catchments are becoming healthier.

Scientists who conducted an international study, co-led by EPA's Office of Research and Development, have concluded the reduction in acid rainfall over the past 20 years has caused darker rivers and lakes in areas affected by the pollution. Surprisingly, the change represents a return to more natural, pre-industrial conditions. The findings are reported in the Nov. 22 issue of the journal Nature.

"When scientists first began recording increases in DOC, they assumed it was the result of global climate change. The concern was that these stores of carbon were becoming unstable, probably due to increased temperatures and carbon dioxide in the atmosphere," said John Stoddard, EPA scientist and co-author of the study.

" What our study has found is that increasing DOC in lakes and streams isn't a symptom of climate change, it's a symptom of recovery from acid rain," he said.

The United States and other countries have implemented successful controls that have led to the reduction of sulphur emissions, the main culprit in the formation of acid rain. As acid levels have declined, rivers and lakes affected by the acidity have begun to return to a more natural chemical balance.

The declines in acidity also caused the release of organic carbon compounds, trapped for years in the soils of relatively remote and pristine watersheds. This carbon is primarily in the form of organic acids that are easily dissolved in water; they make up the chemical stew that scientists call dissolved organic carbon. Scientists have observed their increase since 1990 in widespread areas of northern North America and northern Europe.

The organic carbon research involved the monitoring of more than 500 sites in six countries - Britain, the United States, Canada, Norway, Sweden and Finland. The monitoring was conducted between 1990-2004 by an international team of scientists.

Previously, many of these same areas were part of research, led by Stoddard, that showed control measures to reduce levels of acidity in lakes and rivers have been successful.

The research has resolved debate about the cause of the DOC trends. However, scientists are not clear about the consequences of the release of stored-up carbon from the soil. The carbon can be degraded by sunlight or released as carbon dioxide into the atmosphere. It can also travel to the oceans and be buried in sediments.

For more information about acid rain: http://www.epa.gov/acidrain/

Contact: Ann Brown

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