I have been researching polycyclic aromatic hydrocarbons (PAHs) for the past four years for my PhD, and I’ve been surprised by the complexity of their story in the environment. PAHs are a concern in urban water bodies due to their toxicity and prevalence. While these compounds are naturally occurring, they are also found in high concentrations in common infrastructure materials, such as coal tar and asphalt sealcoat, and in vehicular soot. Coal tar and asphalt can enter an urban lake when sealcoat particles are eroded from pavements due to abrasion from tires and weathering and be transported to lakes by runoff and wind. Soot particles deposited on roadways from vehicle combustion are similarly transported.
Previous research conducted by the U.S. Geological Survey (USGS) and the University of Illinois found that coal tar, asphalt and soot contribute strongly to PAH loading in urban lake sediments. The goal of my research is to determine the fate of PAHs associated with these materials after they enter the lake. Once we know how these compounds behave, we can make informed decisions on managing PAH contamination in urban lake sediments.
With support from a USGS-National Institutes of Water Resources National Competitive Grant, I conducted a two-year field study at Whitnall Park Pond near Milwaukee, Wisconsin to measure the fate and transport of PAHs originating from coal tar and asphalt sealcoat, soot and charcoal. Sediment cores taken from the lake and particles of coal tar, asphalt and soot were added before placing the cores back in the lake to see whether the PAHs would unbind from some materials and rebind to others. The cores were retrieved over the course of two years and the exchange of PAHs between the different materials and the sediment was quantified.
Analysis of the samples and interpretation of the data is now close to completion. But my initial results have shown that PAHs associated with coal tar and asphalt sealcoat are the most mobile, moving in significant quantities to soot, charcoal and sediment. However, even though PAHs redistributed among the different materials, the majority of the PAHs did not leave their original material. This means that not only was there limited transport of PAHs but there was also limited degradation.
For those charged with managing environmental contamination, these findings are both positive and negative. On one side, my results indicate PAHs are strongly bound to these materials, meaning they are less biologically available to aquatic life. On the other side, I did not measure any significant degradation of PAHs in my samples over two years. So although concentrations may be low, organisms in the sediment are being exposed to PAHs for extended periods of time. Considering that these compounds are carcinogenic to aquatic life, long-term exposure, even in low doses, may be an unacceptable risk.
As I prepare to graduate in May of 2017, I’m excited to apply what I’ve learned measuring contamination at my field site to a career in environmental engineering and look forward to gaining experience designing and implementing solutions.