Mercury Accumulation in Tree Rings: Observed Trends in Quantity and Isotopic Composition in Shenandoah National Park, Virginia

Recent studies have shown that mercury (Hg) concentrations in tree rings have the potential to archive historical Hg exposure from local, regional, and global sources. The southeastern United States has received elevated Hg deposition, yet no studies have evaluated tree ring Hg in this region. Here, we quantify Hg accumulation and isotopic composition in tree rings collected in Shenandoah National Park, Virginia. Cores were collected from three individuals of three tree species-white oak (Quercus alba), northern red oak (Quercus rubra), and pitch pine (Pinus rigida)-within the northern, central, and southern areas of the Park (n = 27 cores). The cores were analyzed for Hg content in 10-year increments, with some cores dating back to the early 1800s. Overall, tree ring Hg concentrations (ranging from below detection to 4.4 ng/g) were similar to other studies and varied between species, with pitch pine having higher concentrations than the deciduous species. The most notable feature of the tree ring Hg time series was a peak that occurred during the 1930s through 1950s, coinciding with the use of Hg at a local industrial facility. Atmospheric modeling indicates that potential emissions from the plant likely had a stronger impact on the southern region of the Park, consistent with the latitudinal gradient in tree ring Hg concentrations. Mass-dependent and mass-independent fractionation of Hg isotopes suggests contributions from both regional anthropogenic and local industrial sources during this period. This study demonstrates the potential usefulness of tree ring dendrochemistry for identifying historical sources of atmospheric Hg exposure. Key Points Tree ring mercury levels in Shenandoah National Park, Virginia, were similar to those measured in other North American forests Tree ring mercury peaked during the 1930s to 1950s, coinciding with mercury use at an industrial facility near the southern end of the Park >Mercury isotopes suggest a local source at this time, demonstrating the potential of dendrochemistry to identify historical sources Plain Language Summary For many years scientists have used tree rings to reconstruct past climate. Increasingly, tree rings are being used to document the historical exposure of trees to pollutants. In this study, we cored trees in Shenandoah National Park, Virginia, dated the tree rings, and then measured the amount of mercury stored within decadal core increments. We were surprised to find that mercury levels peaked in the 1930s to 1950s, even though global mercury emissions continued to rise throughout the past century, mostly as a by-product of energy production. Our findings suggest that the trees were exposed to a local pollutant source during this earlier time period, perhaps from a nearby industrial plant that used mercury in the production of rayon. By examining the chemistry of wood within tree rings, we can get a clearer picture of when and where human activities have affected air pollution over recent centuries.