Introduction:
Over the past five weeks, my team and I conducted research on pollutants in local San Diego waterways. More specifically, we were testing for the concentrations of PFAS molecules in the sediments (basically dirt that is underwater). PFAS stands for polyfluoroalkyl substances and are molecules comprised of a long carbon chain, surrounded by fluorine atoms. The molecules we looked for also contained either a carboxylic acid group (the same thing that makes vinegar an acid) or a sulfur group (what gives garlic its great scent) at the end (Figure 1). These carbon-fluorine bonds are extremely strong, so strong in fact that there is no known natural process to break them.
In our ecosystems, this presents a problem as the chemicals can easily build up in the environment. Unfortunately, PFAS molecules were extensively used in the latter half of the twentieth century in industrial products. These products vary from fabric protectors like Scotch-Guard to Teflon due to PFAS’s ability to repel both oil and water. Due to their heavy use, they are discovering build-ups of these molecules in our local environment. This is concerning as recent studies are showing links between high concentrations of PFAS and a variety of health complications. These include increased cholesterol levels, decreased vaccine response in children, increased blood pressure, pre-eclampsia in pregnant women, and even increased risk of kidney or testicular cancer. To find out more about this emerging environmental and public health issue please read the attached article below.
Figure 1: The above picture on the left depicts the structure of a PFAS molecule with a carboxylic acid group on the end. The above picture on the right depicts the structure of a PFAS molecule with a sulfonic acid group on the end.
Hypothesis:
In our study, we addressed the question of how do these different sizes of PFAS molecules travel through water? Resultantly, our hypothesis was that long PFAS molecules (those with more than seven carbons in them) will not travel as far from their source as short PFAS molecules (those with less than seven carbons in them). We believed this because long-chain molecules are better able to stick to sediment surfaces than short-chain molecules. In our study, the source of PFAS compounds was the Miramar landfill in San Diego, CA.
This hypothesis is important as it will allow us to see how these pollutants begin to spread in our environment. By learning how they spread we can further learn how to safely remove or clean up the areas that contain PFAS. Not only will this allow us to protect our environment but also our public health as these substances are toxic to humans. Furthermore, knowing how these substances travel will allow us to predict where we can find potentially unknown areas that contain PFAS.
Experiment:
To test our hypothesis, we took twenty different samples of sediment from Rose Creek, Tecolote Canyon, and Mission Bay. Each sample weighed about two grams and once collected we stored the samples in a refrigerator for preservation. The next week we extracted the PFAS molecules from our samples using a technique called liquid chromatography and mass spectrometry (LC/MS). Essentially this technique separates the sediment into all its individual parts. After, the mass spectroscopy can tell us what those parts are using their mass. Using this data and the known mass of PFAS compounds we determined the concentration of PFAS molecules in our samples. By comparing the results based on location (Rose, Tecolote, or Mission Bay) we saw how these molecules travel. This was possible as Mission Bay is the end of the line for water from both Rose and Tecolote Canyons.
Results:
Figure 2: The above grouped bar graph shows the average PFAS concentration found in each of the sampling locations. The height of the bars depicts the average concentration in part per billion (PPB). The error bars show represent plus and minus one standard deviation from the mean.
Looking at our results from Rose Canyon (the closest sampling location to the landfill), we saw that long-chain PFAS had the greatest concentration. Looking at Mission Bay (the end of the line for water in Rose Canyon) short-chain PFAS had the greatest concentration. This was similar in Tecolote Canyon, where more long-chain PFAS were found upstream but in Mission Bay (also the end of the line for water in Tecolote Canyon), mostly short-chain PFAS molecules were found.
Conclusion:
Analyzing these results, they support our hypothesis that long-chain PFAS molecules will remain closer to their source. Furthermore, they also show that short-chain PFAS molecules will travel further from their source. This is because Mission Bay is the end of the line for water in both Tecolote and Rose Canyons and we expect to find more short-chain PFAS in the bay than in either canyon. The results described above show this. This also suggests that long-chain PFAS were trapped in the canyons and remain closer to the source. This was also supported by our data that found more long-chain PFAS in the canyons than in the bay.
Looking at the results of the study in general, we can see that our hypothesis was supported. Long-chain PFAS molecules cannot travel as far as short-chain PFAS molecules. This is important as it can give us an idea of how to clean up contaminated sites. In particular, this shows that cleanup efforts, such as covering contaminated soil with clean soil, could work in areas near the source of long-chain PFAS molecules. However, the answer to clean up areas contaminated by short-chain PFAS molecules is still unknown. This is because short-chain molecules can travel far distances that make covering them not realistic. Further research into clean-up methods is needed to determine how to clean up areas contaminated by these short-chain PFAS molecules.