Persistent contaminant baselines in dated sediments from a protected coastal lagoon in southeastern Uruguay

Deep beneath the surface of lakes, rivers, and coastal lagoons, layers of mud accumulate year after year, acting as a silent, natural archive of the environment. Just as tree rings record the history of a forest, these sediment layers trap chemicals, particles, and biological signals that wash in from the surrounding land and air. When scientists carefully extract a long, cylindrical sample of this mud, they can read the timeline of human impact, seeing when pollutants first arrived and how their presence has changed over decades. This approach is especially vital in places where we lack continuous monitoring equipment, allowing researchers to reconstruct the history of contamination long after the events occurred. The focus of this history often falls on persistent organic pollutants—chemicals that do not break down easily, cling tightly to soil and mud, and can travel vast distances through the atmosphere. Among these are polychlorinated biphenyls, once widely used in electrical equipment; polybrominated diphenyl ethers, common in flame retardants; and mercury, a heavy metal released by industrial processes and natural sources. Even in protected areas far from factories, these substances can arrive via wind and rain, making it difficult to know if a location is truly pristine or merely receiving a low-level, diffuse background of contamination.

In southeastern Uruguay, within a protected landscape known as the Laguna de Rocha, lies a small, shallow coastal lagoon called Nutrias Lagoon. Surrounded by natural grasslands and scattered forests with no nearby cities or industries, this body of water is widely considered one of the few remaining, low-impact environments in the South Atlantic region. It has long been assumed to be a clean reference point, a place where nature remains largely untouched by human pollution. However, assumptions are not data. To test whether this lagoon was truly free of these persistent contaminants, a team of researchers from Uruguay and the International Atomic Energy Agency decided to look directly into the mud. They collected a sixty-five-centimeter core of sediment from the deepest part of the lagoon, a sample that represented a timeline stretching back nearly a century. By dating the layers using radioactive isotopes that act as natural clocks, and by analyzing the chemical composition of each slice of mud, they sought to determine if the lagoon was truly pristine or if it carried a subtle, hidden record of global pollution.

The researchers approached the sediment core with a specific set of questions, looking for traces of mercury, polychlorinated biphenyls, and polybrominated diphenyl ethers. They used a specialized extraction method to pull these chemicals out of the mud and then analyzed them with high-precision instruments capable of detecting even the tiniest amounts. The results revealed a story that was more complex than a simple "clean" or "dirty" label. The levels of total mercury in the sediment were consistently low, ranging from 56 to 85 micrograms per kilogram of dry weight. These values remained well below the threshold where scientists expect to see harmful effects on the creatures living in the mud. The mercury profile showed a slight increase starting in the mid-twentieth century, a pattern seen in many parts of the world, but it lacked any sharp spikes that would suggest a major local spill or industrial discharge. Instead, the mercury appeared to be a steady, low-level background presence, likely arriving through the atmosphere and settling gently into the lagoon over time.

The search for the other two groups of chemicals yielded even more subtle findings. When the team looked for polybrominated diphenyl ethers, they found none at all. Not a single trace of the seven types they tested was detected in any layer of the sediment. This absence aligns with the lagoon's remote, rural setting, where the sources of these flame retardants—urban centers and industrial sites—are virtually non-existent. The story for polychlorinated biphenyls was slightly different but still pointed to a very low impact. None of the seven specific types of these chemicals were found in high enough quantities to be measured precisely. However, one specific type, known as PCB 52, was detected in nine different layers of the core, though always at levels too low to be quantified. These faint signals appeared sporadically, with a cluster of detections occurring in sediments deposited between 2001 and 2015. Because these chemicals are volatile, they can travel long distances through the air, and their presence here suggests that even in a protected area, the lagoon is not entirely isolated from the global circulation of legacy pollutants.

When the researchers compared these new findings with previous data from the same core regarding polycyclic aromatic hydrocarbons, a different picture emerged. While the mercury and the brominated chemicals were barely present, some of the hydrocarbons showed localized spikes that exceeded safety benchmarks in specific layers. This contrast is crucial: it demonstrates that different pollutants behave in unique ways depending on their chemical nature, how they travel, and how they settle in the sediment. The lagoon did not show a uniform blanket of contamination from a single source. Instead, it displayed a patchwork of signals, shaped by the specific behavior of each chemical and the natural movements of the water and mud. The sediment accumulation rate in the lagoon had also changed over time, particularly around the mid-1980s, which would have diluted or concentrated these chemical signals regardless of whether the amount of pollution entering the system had changed.

The study concludes that Nutrias Lagoon is not a pristine, untouched wilderness, but it is a low-impact system that serves as a valuable baseline for the region. It is a place where the signals of human activity are faint and diffuse, arriving through the air and rain rather than from a nearby factory or sewage pipe. The researchers emphasize that calling a place "pristine" can be misleading; even protected areas receive a background dose of global contaminants. However, the levels found here are low enough that they do not suggest widespread ecological harm from the chemicals tested. The true value of this work lies in providing a dated, chemical record that other scientists can use as a reference. When studying more polluted coastal lagoons in the future, researchers can compare their data against this baseline to understand just how much human pressure has altered those environments. The lagoon stands as a quiet, muddy witness to the subtle, global reach of pollution, proving that even in the most remote corners of the coast, the environment is never entirely disconnected from the rest of the world.