Monday, October 11, 2010

Reservoirs: a neglected source of methane emissions

Reservoirs: a neglected source of methane emissions

EAWAG bulletin, 11 October 2010

*Substantial amounts of the greenhouse gas methane are released not only
from large tropical reservoirs but also from run-of-the-river reservoirs
in **Switzerland**, especially in the summer, when water temperatures
are higher. This was demonstrated by Eawag scientists at **Lake**
**Wohlen**, near **Bern** – a finding which slightly tarnishes the
reputation of hydropower as a climate-neutral way of generating

When they first saw the data, environmental chemist Tonya Del Sontro and
her PhD supervisor Professor Bernhard Wehrli were sceptical. But the
unexpectedly high values stood up to careful analysis: on average, daily
emissions of methane (CH4) from Lake Wohlen amount to more than 150 mg
per m2 surface area. This is by far the highest emission rate recorded
to date for a temperate reservoir. At a water temperature of 17°C the
rate is twice as high, which makes it comparable to the emission rates
observed for tropical reservoirs.

Equivalent to 25 million car kilometres

Overall, the reservoir on the Aare produces 150 tonnes of methane a
year. This is about the same amount as is emitted annually by around
2000 cows; in terms of global warming potential, it is equivalent to the
carbon dioxide emissions from 25 million car kilometres, since methane
is about 25 times as potent a greenhouse gas as CO2. "So hydropower
isn't quite as climate-neutral as people have assumed in the past," says
Del Sontro. At the same time, she does not wish to dramatize the
findings of her research: even if the Aare hydropower plant is taken to
be responsible for all the methane emissions from the reservoir, and
these are expressed in CO2-equivalents, a coal-fired power station with
the same output produces 40 times as much CO2. However, the emissions
from Lake Wohlen do indicate that run-of-the-river reservoirs can also
be significant sources of methane emissions in temperate regions. As
Wehrli notes, "That's something which has previously been overlooked in
greenhouse gas budgets."

Bubbling sediments

Methane production in Lake Wohlen is attributable to organic matter
transported by the Aare, e.g. from Lake Thun. In the reservoir, the
organic matter settles rapidly, undergoing microbial fermentation in the
sediments. "In the summer," says Del Sontro, "the water in Lake Wohlen
sometimes looks like champagne, with masses of gas bubbles rising to the
surface." To study these bubbles, the Eawag scientists used custom-made
gas traps in the form of inverted funnels. The analyses revealed that
they were mainly composed of methane.

During the winter, on account of the cold temperatures, methane
emissions are minimal. According to Wehrli, this explains why climate
researchers have previously focused largely on the huge reservoirs lying
in tropical regions: here, temperatures are warm throughout the year,
and inundated forest areas provide a large supply of nutrients for
microbes. Likewise, low water temperatures and low nutrient inputs mean
that large Alpine reservoirs do not give rise to significant methane
emissions. The researchers now plan to study other run-of-the-river
reservoirs on the Central Plateau so as to determine whether Lake Wohlen
is a special case, or whether methane budgets need to be revised for
Switzerland as a whole.

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