Mercury pollution still high despite decrease

7 August 2009 by Sara Coelho

Scientists have found that atmospheric levels of mercury are declining since they peaked in the 1960s, but levels are still not as low as before the industrial revolution. The findings come from the study of cores collected from Scottish peat bogs.

Peat landscape

"Mercury is well established as a neurotoxic poison," says Professor John Farmer, from the University of Edinburgh and lead author of the study. To understand how the element moves up and down the food chain, it's important to understand its global biogeochemical cycle. But to do this, "we need an historical perspective," Farmer adds.

One way to figure out how mercury accumulation rates changed in the past is to look at peat bogs. Farmer explains: "bogs are like ice cores: they preserve a record of environmental pollution just as ice holds information about climate change."

Farmer and his team focused on the so-called ombrotrophic bogs, which get all their nutrients and water from rainfall, without input from rivers or groundwater. This means that, for some chemical elements, changes in ombrotrophic bog profiles largely reflect the changing composition of the atmosphere.

The team collected cores from four bogs distributed across Scotland and dated the peat using radioactive lead and carbon-14 isotopes. The four cores, which span 2,000 years, were then analysed for mercury. The results, published this week in the Science of the Total Environment journal, show how atmospheric mercury accumulated in Scottish bogs during 2,000 years of deposition.

Farmer and colleagues found that mercury accumulation rates remained relatively stable for the better part of two millennia until a dramatic increase in the mid-to-late 19th century. The effect was probably due to a sharp increase in coal burning to fuel the fast-paced development of the Scottish economy during the industrial revolution.

"At its peak during the 1950s and 1960s, mercury was being accumulated in peat 10-20 times faster than in pre-industrial times," says Farmer.

Mercury accumulation rates started to decline in the late 20th century, as the main source of energy switched from coal to natural gas and pollution control measures improved. The decrease has been steady but the "rates are still five to six times higher than in before the industrial revolution," says Farmer. "This is similar to trends observed in Western Europe and North America," he adds.

If emissions in the UK have dropped, why are Scottish bogs are still accumulating more mercury than before the industrial revolution? The reasons may be found in mercury's complex global cycle.

"Mercury is highly volatile and can travel very far in the atmosphere," says Farmer. Further decreases in accumulation rate may be hindered by emissions elsewhere - for example, many Asian economies are still heavily dependent on coal and it's possible that some of the mercury emitted there is being deposited elsewhere, including Europe, he suggests.

Farmer argues that this research reaffirms the reliability of peat bogs as records of past atmospheric conditions, despite previous suggestions that "recent accumulation rates for mercury were being overestimated due to weaknesses in the dating by radioactive lead."

"We found that the stable lead isotope signature, characteristic of the changing sources of lead pollution, in the bogs is similar to the trends recorded in Scottish herbarium moss samples of known age ," he says. "This means that the radioactive lead-dating is reliable and that the accumulation rates are accurate."

'Historical accumulation rates of mercury in four Scottish ombrotrophic peat bogs over the past 2000 years' - JG Farmer et al, Science of the Total Environment (2009), doi:10.1016/j.scitotenv.2009.06.014