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Volume 15, Number 3 |
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Acid deposition causes spikes in aquatic pH during spring
As the prevailing winds in Europe blow from the west, they carry airborne pollutants from Britain and Eastern Europe to be deposited over Scandinavia as acid precipitation. When the snow pack in Sweden melts each spring, the rush of water with accumulated pollutants can abruptly acidify streams and lakes, and affect the aquatic life. The Swedish government has responded by pouring lime in affected areas to buffer the acid, but some scientists have doubts about whether this costly program is effective or even helpful. A Wallenberg Fellow in Environmental Sustainability at MIT, postdoc Hjalmar Laudon recently finished a year with Prof. Harry Hemond in Parsons Lab to study aquatic systems. While the airborne pollutants have been deposited over Sweden for decades, the levels seem to have peaked around 1970, he says. After the provisions of the UN Economic Commission for Europe and the Clean Air Act in the US took effect, "emissions in Europe have declined by about 75 to 80%. The corresponding decline in emissions in the US has been around 50%." He attributes Europe's greater drop in pollutants to a larger general concern for the environment, and to the collapse of communism in Eastern Europe with subsequent economic changes. Many inefficient, polluting industries lost their state subsidies and closed down, while others modernized and cleaned up dramatically. For his PhD research at the Uppsala campus of the Swedish Univ. of Agricultural Sciences, Laudon studied how snow melt, and to some extent rainstorms, increase the discharge of water into streams and often lead to very dramatic drops in pH. Among the causes for the lower pH are the dilution in the stream's buffer capacity, and an increase in acidity from the rain or natural sources. The natural breakdown of plant detritus in the forests yields carbon dioxide and water, and sometimes dissolved organic carbon (DOC), which is acid. Boreal forests in northern regions have a high productivity but a slow breakdown rate of the organic material, resulting in very high concentrations of DOC in the streams. That concentration often increases during the spring flood and rainstorms.
In northern Sweden, about half of the annual runoff occurs during the three to four weeks of the spring flood. When six months of accumulated snowpack suddenly melts, pH levels in water can drop from 6 to an acidic 4.5. "We wanted to know what drives this pH decline," says Laudon. "We found out that as the deposition of airborne pollutants has declined, streams have recovered from these acid episodes very rapidly. There is a direct link between how much of the pH decline is due to anthropogenic sources and the concentration of the acid deposition." Brown trout and salmon are very sensitive to pH changes, and the Swedish government has spent about $100 million in the past 10 years liming surface waters in the country's north to increase the pH. "I argue that most of this is a waste of money, because many of these streams are naturally acid. If you artificially raise the pH in the streams, you will introduce new species that shouldn't be there," warns Laudon. Coincidentally, the Swedish equivalent of the American idiom, "throwing money down the drain," is "throwing money in the lakes." Laudon developed a model to predict how pH will decline due to the acid neutralization capacity of surface waters. "We try to project how acid deposition will influence the decline in pH. At MIT I applied these tools to surface waters in Nova Scotia and Ontario, Canada, and some sites in the Northeastern US where long-term monitoring programs exist. I wanted to see if we can use the same predictions here as in Sweden, and how North American streams respond to the decline of acid deposition." Acidification can be divided into two sets of problems, specifies Laudon. "With chronic acidification, there is so much acidity in the soils that the streams are also acidified, as in the Adirondacks in New York. I'm working with episodic acidification, which only occurs when a very high outflow of water contains acidity either stored in the catchment or in the rain or snow. "The big interest today is recovery: when will catchments return to normal? How long before streams will have a pH where fish can thrive? Chronic acidification sites will take at least 50 to 100 years to recover, if they ever do. But recovery is very rapid with episodic systems during snow melt-as soon as the deposition declines, the surface water chemistry improves." Over the long term, streams will always experience a natural fluctuation in pH levels. "That's an important part of the stream ecology. Without that pH decline, other species will invade," says Laudon. Since liming smooths out the pH over the entire year, it interferes with the natural ecology. Superimposed on the normal variability are the changes in airborne pollutants. Laudon is looking at how this anthropogenic effect has declined over time as the deposition is reduced in North America and Europe. Politically active to try to stop the Swedish government from pouring so much money in the lakes, Laudon has stated his case numerous times on radio, TV, and in print. "Of course the liming works--it increases the pH and you'll see a lot of fish. But are you replicating the natural chemistry in these waters? Liming is intended to restore the natural biodiversity. However, nobody really knows what the original biodiversity was in terms of numbers or species, because nobody did studies before acidification started." Long before acid rain or natural diversity were well publicized issues, many Swedish rivers were substantially altered by human activity. Nobody compiled environmental impact statements before excavating boulders from major waterways to make an unobstructed passage for loggers to float timber downstream to mills. Salmon and brown trout use the eddies around boulders for spawning, so the removal of boulders wiped out traditional fish nurseries. Laudon's model was commissioned by and developed for the Swedish Environmental Protection Agency to enable policy makers to make better predictions of how waters behave due to deposition. "Now we have to see whether it will be implemented," since it will show that the government liming policy is not necessarily useful or beneficial, he says. With acid deposition down very sharply thanks to improved air quality, Laudon agrees that the government should continue the liming in waters "where it makes sense, and then open up the liming budget so that we can use the money for other projects which will improve the aquatic ecosystem." Using his model, one can locate the diminishing number of rivers where acidification still kills fish, and apply lime where it would make a difference, rather than indiscriminately throughout a region. "We have to find some way to give priority to waters that need it, and to do other work to systems that need other help to improve," acknowledges Laudon. "Deposition was so much higher in the past, and it takes 20 or 30 years for the populations of aquatic organisms to recover." In addition Laudon describes inadvertent complications, such as building a forest road which has a culvert to allow water to flow, but which also prevents fish from migrating. Considering that the fish are already stressed during the spring flood, "sometimes this little anthropogenic effect imposed on the natural variability can be enough to kill the fish." During his year at MIT, Laudon personally encountered the legendary American ignorance of geography. "I don't know how many people mix up Sweden and Switzerland. If they know anything about Sweden, it's Ingmar Bergman, hockey players, Björn Borg, and tall blond women. In general people here have a fairly limited knowledge of the rest of the world. But on the other hand, when I say that I'm from Sweden, people are genuinely interested." Thanks to his advisor, Prof. Harry Hemond, Laudon has met other people working on the natural chemistry of waters, as well as related fields in chemistry and environmental science. He has also bonded with the other Wallenberg Fellows for Environmental Sustainability. The Swedish foundation sends about seven or eight postdocs to MIT each year to study anything which can fit under the broad umbrella of environmental sustainability, such as chemistry, physics, social and political science. After they return to Sweden and disperse among the different universities, they remain in contact as they develop their different specialties in environmental sustainability. |
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"Civil and Environmental Engineering at
MIT"
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