Straight Lake Segment of Ice Age National Scenic Trail / NPS

The largest study of its kind ever conducted has found evidence of trace organic compound pollutants in bodies of water at 246 sample sites across 46 national park units between 2009 and 2019. Not all of the sites contained levels of the pollutants that would necessarily be considered worrisome, other than a concern that they're present at all. DEET, the camper and hiker's favorite mosquito-repellent which is, nevertheless, officially a pesticide, was found in more than 20% of the samples. So was theobromine, a chemical compound found in coffee that is often present in human urine. Meaning this was likely from people urinating near bodies of water.

Having said that, results from the study showed that in some cases, particularly when it comes to agricultural pesticides, these substances weren't used in the immediate vicinity of the sample sites, but flowed there as part of natural water movement. The study, conducted by National Park Service scientists, working with experts from outside the agency as well, was published in the December 2024 issue of the journal "Environmental Pollution."

The study is notable not just because it illustrates how pervasive chemical pollutants can be, even finding their way into protected bodies of water inside national parks, but because it's now the benchmark for future studies of chemical levels in national park waterways. "These data provide a baseline that can be used to inform future monitoring within the parks and to assess changes in water quality," the authors wrote in the report. 

NPS units in the Midwest were particularly prone to accumulating pesticide pollutants. According to the study, that likely coincides with the large amounts of agricultural operations in that region of the country, as well as wastewater treatment plants. Parks with the Great Lakes and Sierra Nevada regions also had elevated concentration of chemical pollutants. The good news is the concentrations of pollutants were generally below what it would take to represent a threat to aquatic biota, but, nevertheless, are chemicals that wouldn't be present at all if not for being emitted from human sources. 

And, it should go without saying, the study found it was waterways within national parks with more human activity than lesser visited parks that showed the highest concentration of pollutants. 

The study authors, who didn't respond to requests for comments in time for this article, point out the data collected can serve to set a baseline to see how these waters accumulate pollutants in the future. 

But for now, a good example and reminder that that sunscreen or DEET campers put on without thinking much about it, accumulates in the waters of our most precious places.