POP surprise: alpine lake levels on the increase
The manufacture, production and use of persistent organic pollutants (POPs) have all decreased since their adverse health properties and environmental persistence were recognised but they remain an established and unwanted legacy. Their resistance to degradation saw levels of POPs rise in the environment during the 20th Century until they stabilised and began to fall slowly following their ban.
However, this steady but gentle decrease in measured concentrations has not been universally observed. A study published in 2009 by scientists in Switzerland found that the levels of all POP compound classes in the sediment from a Swiss lake had risen unexpectedly since the 1990s. In fact, the amounts flowing into the lake were higher than those of the 1960s and 1970s, while POP use was still high.
This surprising observation was attributed to the effects of climate change. The lake in question, Lake Oberaar, is fed directly by meltwater from a glacier. Accelerated melting of the glacier due to rising European temperatures has released POPs that had been deposited and stored over the years, causing environmental levels to rise.
This glacier hypothesis, as it was dubbed, was derived from the data measured in just one lake and included no sediment measurements beyond 2005. The Swiss research team decided to carry out further tests, to see whether advanced glacial retreat and the associated release of POPs really is occurring.
POPs in alpine lakes: a double-barrelled approach
Peter Schmid, Christian Bogdal, Nancy Bluthgen, Flavio Anselmetti, Alois Zwyssig, and Konrad Hungerbuhler from the Swiss Federal Laboratories for Materials Testing and Research (Empa), Dubendorf, the Swiss Federal Institute of Aquatic Science and Technology (Eawag), ETH Zurich and the University of Applied Sciences Northwestern Switzerland, Muttenz, examined the sediments from two high altitude alpine lakes.
They were located within 8 km of each other at similar altitudes, so were deemed to have experienced comparable levels of atmospheric exposure to POPs. However, their catchment areas had notable differences.
Lake Stein is fed mainly by meltwater from the Stein Glacier, which has experienced notable melting since the 1980s. On the other hand, Lake Engstlen is entirely non-glacial, being fed from surface runoff and inflows from the karst-type rocks around it, which are riddled with caves and underground channels.
Sediment cores were drilled from both lakes. That from Lake Stein was 88 cm long, covering the period 1973-2008, and it was supplemented by an earlier core drilled from the same location that covered 1960-2000. The core from Lake Engstlen covered 1963-2008. The dates were established by caesium-137 measurements which marked 1963 and 1986 by historical nuclear bomb testing and the Chernobyl accident, respectively.
The POPs were extracted from the sediment and analysed by gas chromatography coupled to high-resolution mass spectrometry with electron ionisation, using isotope-labelled internal standards. The compound classes measured were PCBs and total DDT, which included DDT itself and its dichlorinated breakdown products DDD and DDE. The measured concentrations were converted to input fluxes using the sedimentation rates of each lake and presented as graphs of fluxes versus date.
Alpine lake POP levels support glacial influence
The PCB and DDT input fluxes to Lake Engstlen, the non-glacial lake, increased during the 1970s-1990s, coinciding with their high emissions before national and international bans came into force. After 1990, the fluxes of both classes fell gradually. This trend is similar to those reported for low-altitude non-glacial lakes in the Swiss plateaus.
The picture was much different for the glacial lake, Lake Stein. Here, the input fluxes of PCBs and DDT fell until 1984 and 1990, respectively, before increasing again to peak in 2006. Later values showed decreases, although the input fluxes remained higher than the 1980-1990 minima.
This finding correlates with the accelerated melting of the Stein Glacier over the recent period. Further evidence to support this was found by comparing the annual length variation of the glacier with the PCB and DDT fluxes. The year-by-year curves for glacier length variation and the pollutant influxes displayed parallel behaviour, matching even short-term changes.
For instance, the most rapid retreat of the glacier, which occurred in 2004, was matched by the highest influxes of PCBs and DDT. Then from 2005-2008, which experienced slower glacier melting, the influxes decreased noticeably.
The research team noted that melting of the ice itself was not necessarily an indication of higher pollutant influxes to the lakes. The key criterion for high influxes is melting of a portion of the glacier that was formed when atmospheric pollution by the POPs was high.
The new data for these two lakes support the team's earlier glacier hypothesis and the model will be applicable to other glaciers that were formed during periods of high POP use.
This work "calls for an extended understanding of the fate of POPs in mountain areas" and highlights the lack of knowledge about the transport of POPs throughout Alpine glaciers. The effects of climate change on the accelerated release of POPs from glaciers should also be considered in future climate models.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
|
