Historical Mercury Pollution: Tree Rings Have the Receipts!

Article: Scanlon, T. M., Riscassi, A. L., Demers, J. D., Camper, T. D., Lee, T. R., & Druckenbrod, D. L. Mercury Accumulation in Tree Rings: Observed Trends in Quantity and Isotopic Composition in Shenandoah National Park, Virginia. Journal of Geophysical Research: Biogeosciences, e2019JG005445. https://doi.org/10.1029/2019JG005445

Mercury Pollution 101

Mercury is a particularly problematic environmental pollutant. What makes it troublesome is that it is both toxic, causing a range of health issues, and persistent, such that when it gets into an environment it tends to stick around for a long time. You may have heard warnings about mercury in fish, but people can also be exposed to mercury through the air they breathe and the water they drink. Mercury can affect the nervous system, digestive system, and immune system and damage lungs, kidneys, skin, and eyes. Human-caused sources of mercury pollution include industrial processes that use mercury compounds and fossil-fuel use, especially the burning of coal for energy production.

Tree Ring Time Machine

Since the human activities that largely drive mercury pollution include industrial processes and energy production, we know that there has been a large increase in mercury pollution since the industrial revolution. However, we don’t have a great idea of exactly how levels of mercury pollution have increased and how the amount has varied from location to location throughout history. This information can give us better insight into how humans have impacted the environment, which is important since there can often be a lasting legacy of this pollution on ecosystems and communities, even after the polluter has stopped.

Tree rings are sampled by taking “cores” from tree trunks, which we can then use to sample and count the rings. Image Credit: JV Wilkening

In a new study led by Todd Scanlon from the University of Virginia, scientists used tree rings as a way to study past mercury pollution in Shenandoah National Park in Virginia. This area has likely experienced considerable mercury pollution, since it is along the Appalachian Mountains where the majority of coal mining in the US is located and is also downwind of the Ohio River Valley, where there are a number of coal-fired power plants. Tree rings are a valuable tool that have been used by a lot of scientists to study the past since trees regularly form a new ring of wood in their trunk each year, creating a timeline of the tree’s life. Each ring acts as a sort of time capsule of the conditions that the tree experienced that year, including any mercury that may have been in the air. Mercury from the air can come into the tree through pores in the leaves and gets stored in the wood. If there was more mercury in the air during a given year, more mercury ends up in the tree ring for that year.

The Highs and Lows of Mercury Pollution

Scanlon and the other scientists in the study collected samples of tree rings from trees in Shenandoah Valley, some of which were more than 200 years old! They then extracted the mercury from the tree rings, and looked at both the amount of mercury in the tree rings, as an indication of the overall level of mercury pollution, and the type of mercury present, as an indication of where that mercury may have been coming from. For the latter, they looked at the isotopes of mercury that were present – slight variations in the number of neutrons in the mercury atoms. This means that there are both “heavy” mercury atoms and “light” mercury atoms. Human sources tend to have a higher number of “light” atoms, so by looking at the ratio of the different masses of mercury atoms, scientists can get a further idea of where the mercury is coming from.

Shenandoah Valley National Park where the study was conducted. Image Credit: Shutter Runner (CC BY-NC 2.0)

When the scientists analyzed the long record of tree rings, they found that there was more mercury in the tree rings during the 20th century, corresponding to the increasing industrialization over that era. However, there was a difference at this site in the Shenandoah Valley than compared to the overall global trends. Overall across the globe, mercury pollution has continued to steadily increase, largely due to increasing energy production. However, at this site, the peak of mercury pollution occurred in the 1930s-1950s. After consulting historical records of the surrounding areas, the scientists concluded that this earlier peak was likely due to a nearby manufacturing plant that operated during that time. The plant made rayon fabric, and used a mercury-containing compound in the manufacturing process. With this result, this study shows how tree rings can be a valuable tool for study local historical mercury pollution, since it can vary drastically from global trends.

Study Historically, Think Globally, Act Locally

There has been a lot of actions taken to help prevent and limit mercury pollution. Even though there have been improvements, there are still a number of things you can do to further help. Some of those actions include:

  • Find out where your electricity is coming from. If you live somewhere that it comes from burning coal, conserve electricity as much as you can and push for changing to cleaner energy sources that don’t emit as much mercury.
  • For products like compact fluorescent lightbulbs (CFLs) which contain mercury but are still great because they conserve a lot of energy, be sure to find where to properly recycle them rather than throwing them in your normal trash or recycling bin.
  • Whenever you can, choose to purchase mercury-free products (especially thermometers, thermostats, etc). For disposing of products you already have that contain mercury, check with your local hazardous waste facilities for safe disposal options.

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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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