Tracking micropollutants in Lake Geneva

October 25, 2013
Tracking micropollutants in Lake Geneva
Credit: © 2013 EPFL – Herzog

Antibiotics, urban pesticides, and other contaminants accumulate where wastewater is released into Lake Geneva. Using computer simulations, EPFL researchers have shown that the risk they pose is highest during summer and that they degrade most efficiently during the winter.

Once taken, antibiotics do not simply disappear. Released from the body by urine, some escape treatment plants unscathed. Traces of antibiotics and a variety of other molecular contaminants such as pesticides and pharmaceutical drugs are consistently measured in Vidy Bay, Lake Geneva, above the site where processed wastewater is released. The impact of micropollutants is receiving ever more attention from the scientific community, but it is still unclear how long they persist and to what extent they pose a threat to organisms. In an article published in late July, researchers at EPFL present findings on the dilution of the micropollutants by currents. Their results showed that the pollutants are most persistent during summer, several meters under water. In the winter, they rise to the lake surface and are most effectively degraded by sunlight

Every second, between one and three tons of treated wastewater from the city of Lausanne gush out into Lake Geneva from a pipe that ends 700 meters offshore and 30 meters below the lake's surface. Lake currents disperse and dilute this water in the same way that wind disperses the smoke from a chimney, with a persistent underwater pollutant plume emanating from the discharge pipe.

Florence Bonvin from EPFL's Environmental Chemistry Lab has been studying the dynamics of this plume and the pollutants it contains. "We consistently detect a range of micropollutants – antibiotics, anti-inflammatory and anti-epileptic drugs and urban pesticides in particular in the vicinity of the wastewater treatment plant outfall," she says. Although the concentrations are very small, she says, they are frequently well above threshold concentrations that are considered to have no effect on organisms.

Driven by lake currents
Actually getting on a boat and sampling for micropollutants is time-intensive, expensive, and only provides a partial snapshot of the situation at any given time. A computer model gives a more complete image. "We would have to sample hundreds of sites simultaneously – which is basically impossible – to come close to getting as much detail as we can using a computer model," says Bonvin. This way, she can study two simultaneously occurring processes in great detail: dilution of the micropollutants by mixing with the surrounding lake water, and their degradation by sunlight.

The dynamics of the pollutant plume are mostly driven by lake currents, which vary with the season and the weather. Summers see the lake's warm up, while the depths remain cool. The interface between warm and cold water – the thermocline – acts as a cap, trapping currents and the pollutant plume several meters under water. But as the lake's surface waters cool down during the winter, the thermocline disappears and the plume rises to the surface. With increased exposure to surface winds and sunlight, pollutants are diluted and degraded much more effectively.

The computer model lets Bonvin quantify the ecotoxicological threat posed by 24 different micropollutants as they enter the lake, disperse into the surrounding water, and eventually degrade. To study two different lake circulation patterns driven by the most prevalent winds, such as the bise, both with and without the summer thermocline, Bonvin used results from a 3D lake circulation model developed by Amir Razmi from EPFL's Ecological Engineering Laboratory, which reproduced lake currents measured during an experimental campaign carried out in Vidy Bay. The degradation of the pollutants by the sun were based on measurements she performed herself in the lab.

"Using our simulation results, we can draw a map of where the pollutants are depending on the season, the winds, and the cloud cover," says Bonvin. And they offer a way to evaluate the potential risk they pose. "The concentrations of each pollutant are very low, but since we don't know how living organisms respond to being exposed to these pollutants over long time periods, this isn't an issue we can simply ignore."

Bonvin previously showed that even that have been processed and inactivated in our body and then released into the lake could be revived when exposed to , returning to their original, active form. This adds another source of micropollutants to the picture. Her findings, which add an additional twist to this story, were published earlier this year in Environmental Science & Technology.

Explore further: Analyzing Lake Geneva from the air: A second wind for elemo

More information:

Related Stories

Analyzing Lake Geneva from the air: A second wind for elemo

November 20, 2012

One year after the MIR submersibles dove into the depths of Lake Geneva, the elemo program is delivering its first scientific results. And with the support of Ferring Pharmaceuticals, the operation will be extended with a ...

Lake Erie's thermal structure and circulation are backward

April 23, 2012

A series of high-resolution measurements has shown that Lake Erie, one of the North American Great Lakes, is, in some respects, backward. In the majority of thermally stratified lakes, the thermocline, a thin subsurface layer ...

Microplastic pollution prevalent in lakes too

May 28, 2013

EPFL researchers have detected microplastic pollution in one of Western Europe's largest lakes, Lake Geneva, in large enough quantities to raise concern. While studies in the ocean have shown that these small bits of plastic ...

Mini-submarines to gauge Lake Geneva pollution

June 14, 2011

Two mini-submarines that have filmed the wreckage of the doomed luxury cruise liner Titanic will dive into Lake Geneva to gauge its pollution levels, Swiss researchers said Tuesday.

A scientific adventure from Lake Geneva to Lake Baikal

May 15, 2013

Ultra-light aircraft are being deployed in both Switzerland and Russia as part of the Léman-Baïkal project. Lake Geneva, the largest lake in the Alps, and Lake Baikal, the world's largest lake, will be studied using new ...

Recommended for you

Caves in central China show history of natural flood patterns

January 19, 2017

Researchers at the University of Minnesota have found that major flooding and large amounts of precipitation occur on 500-year cycles in central China. These findings shed light on the forecasting of future floods and improve ...

New England's 1816 'Mackerel Year' and climate change today

January 18, 2017

Hundreds of articles have been written about the largest volcanic eruption in recorded history, at Indonesia's Mt. Tambora just over 200 years ago. But for a small group of New England-based researchers, one more Tambora ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.