A Study of the Content of Radionuclides in Coastal Seawater and Saline Groundwater in Jeju, Korea
This 2024 study of Jeju Island's coastal seawater and saline groundwater found that while 137Cs was detected at low levels in seawater, other artificial radionuclides including 60Co, 131I, 90Sr, and 3H were generally below detection limits in the sampled environments.
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The ocean is not a static pool of water; it is a moving, breathing system that constantly exchanges with the land beneath it. In places where the sea meets the shore, saltwater can push inland, seeping into the porous rocks and soil to create a hidden layer of salty groundwater. This mixing zone is a critical part of the Earth's water cycle, acting as a bridge between the vast ocean and the fresh water we drink. However, the ocean also carries a faint, invisible history of human activity. Decades ago, nuclear weapons testing and more recent industrial accidents released tiny amounts of radioactive atoms into the atmosphere and the sea. These atoms, known as radionuclides, do not vanish quickly. Some of them, like the one called cesium-137, can linger in the water for decades, traveling with the currents and settling into the environment. Because these substances are so long-lasting, scientists can use them as markers to trace how water moves, how long it stays in one place, and whether it is mixing with other water sources. Understanding where these markers end up is essential for knowing the health of our coastal ecosystems and for establishing a clear picture of what is normal, so that any future changes can be spotted immediately.
In 2024, a team of researchers from Jeju National University set out to map this invisible history in the waters surrounding Jeju Island, a volcanic island off the southern coast of South Korea. They were interested in two specific environments: the open coastal seawater that laps against the island's shores, and the saline groundwater that exists just beneath the surface in the island's volcanic rock. The scientists collected water samples from ten different spots along the coast and from two locations where salty groundwater is drawn for industrial use. Their goal was to measure the presence of three specific artificial radioactive elements: cesium-137, strontium-90, and tritium. These elements are of particular interest because they behave differently in water. Cesium tends to stay dissolved and move with the water flow, strontium acts chemically like calcium and can be taken up by living things, and tritium is a form of hydrogen that becomes part of the water molecule itself, making it a perfect tracer for tracking the movement of water.
The researchers found that the coastal seawater around Jeju Island still contains measurable traces of cesium-137. In the ten samples they tested, the concentration of this element ranged from 1.04 to 2.04 millibecquerels per kilogram, with an average of 1.37 millibecquerels per kilogram. This level is consistent with what has been observed in other parts of the Korean coast and the wider North Pacific Ocean, suggesting that the water around Jeju is part of the same broad, low-level background of radiation that exists globally. The team also looked for cobalt-60 and iodine-131, two other radioactive elements, but found none of them in the seawater. This absence makes sense because iodine-131 decays very quickly, within days, so its presence would only indicate a very recent, strong release of radiation, which did not happen during their sampling period. The presence of cesium, by contrast, is a sign of the long-term accumulation of radiation from past events, such as the atmospheric nuclear tests of the mid-20th century and the 2011 accident at the Fukushima Daiichi Nuclear Power Plant, rather than a new, immediate danger.
The story changes when the researchers turned their attention to the saline groundwater. Despite the fact that this groundwater is formed by seawater mixing with the island's volcanic aquifers, the results were strikingly different. In every sample of saline groundwater they tested, the levels of cesium-137, strontium-90, and tritium were so low that they fell below the minimum amount the instruments could detect. This does not mean these elements are completely absent, but rather that their concentration is too faint to be measured with the methods used. The scientists calculated that if cesium-137 were present, its amount would be less than 0.78 millibecquerels per kilogram, and for strontium-90, it would be less than 0.39 millibecquerels per kilogram. For tritium, the limit was even lower, below 2.27 becquerels per liter. This finding is significant because it shows that the radioactive signature found in the open ocean does not simply copy itself into the groundwater. The process of water moving through the volcanic rock, mixing with older freshwater, and spending time underground appears to dilute or filter out these radioactive markers, or perhaps the water in these specific aquifers has a different history that does not include the same exposure to recent oceanic radiation.
The study provides a crucial baseline for the future. By establishing exactly how much cesium is in the seawater and confirming that the groundwater currently shows no detectable levels of these specific radioactive elements, the researchers have created a reference point. If a future event were to release new radiation into the area, scientists would have a clear, pre-existing record to compare against. The work highlights that while the ocean and the groundwater are physically connected, they can hold very different chemical and radioactive histories. The volcanic rocks of Jeju Island act as a complex filter, creating a distinct environment where the water behaves differently than it does in the open sea. This detailed look at the invisible components of the water helps us understand the hidden pathways of our planet's water systems and ensures that we are ready to recognize any changes that might occur in the years to come.
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