Spatiotemporal Dynamics and Health Risk Assessment of Gross Beta Radioactivity in a Tropical Reservoir: Insights from Phase Partitioning and Long-Term Monitoring

Every drop of water we drink carries a silent history written in the atoms it contains. Some of these atoms are naturally radioactive, remnants of the Earth's formation that have been slowly decaying for billions of years. Scientists call these naturally occurring radioactive materials, or NORM. While the word "radioactivity" often triggers thoughts of industrial accidents or nuclear weapons, these natural traces are everywhere, found in the soil, the rocks, and the water that flows over them. For the people who manage our drinking water, the challenge is not to eliminate these natural traces entirely—an impossible task—but to understand their levels and ensure they remain far below the threshold where they could harm human health. This requires a deep understanding of how water moves through the landscape, how it picks up these invisible particles, and whether the treatment plants that clean our water can actually remove them.

In the tropical south of Taiwan, where the landscape is carved by mountains and drenched by intense seasonal rains, a team of researchers set out to map this invisible world. They focused on the Mudan Reservoir, a vital source of drinking water for fifteen townships in the region. The scientists wanted to answer a simple but critical question: how does the heavy rain of the summer monsoon change the amount of natural radioactivity in the reservoir? For decades, a common assumption in water science has been that pollutants become more concentrated when water levels drop during dry seasons, much like salt left behind as a puddle evaporates. However, in a place defined by violent storms and massive runoff, the researchers suspected the story might be different. They spent nearly four years, from 2022 to 2025, collecting hundreds of water samples to see if the heavy rains actually flush more radioactive material into the water, or if the water simply dilutes it.

The team established a network of ten sampling stations, ranging from the deep center of the reservoir to the very top of the mountain streams that feed it. They collected water every few months, tracking how the levels of "gross beta activity"—a measure of the total radiation from all beta-emitting particles—changed over time. What they found overturned the traditional idea of dry-season concentration. Instead of finding the highest levels when the water was low and still, the researchers discovered that the radioactivity peaked during the summer monsoon. In July, when the southwest monsoon brings torrential rains and typhoons, the average radioactivity in the water rose to its highest point. The most intense readings came from a specific upstream tributary, where the rain washed soil and rock from the mountains directly into the reservoir. The data showed that the heavy rains did not dilute the radioactivity; instead, they acted as a powerful flush, sweeping naturally radioactive materials from the land into the water supply.

Once the water reached the reservoir, the scientists needed to understand what form these radioactive materials took. This distinction is crucial because the machines used to clean drinking water are designed to catch solid particles, like dirt or clay, but they often let dissolved chemicals pass right through. To test this, the researchers took samples and filtered them through a very fine membrane that traps tiny solid particles. They found that nearly all of the radioactive material, about 99 percent, passed right through the filter. This meant the radioactivity was not stuck to dirt or sand; it was dissolved in the water itself, likely as tiny ions or attached to microscopic colloids that are too small to be caught by standard filters. This discovery explained a puzzling observation at the local water treatment plant: the water coming out of the plant was almost identical in radioactivity to the water going in. The plant's standard processes, which include mixing chemicals to clump particles together and then filtering them, were simply not designed to remove dissolved substances.

Despite the fact that the treatment plant could not remove these dissolved particles, the final assessment of the water's safety was reassuring. The researchers calculated the potential health risk to an adult drinking this water every day for a year. They used a conservative approach, assuming that all the radiation came from a single, highly toxic isotope known as strontium-90, which is much more dangerous than the natural mix actually found in the reservoir. Even with this worst-case assumption, the maximum dose a person would receive in a year was only 2.01 microsieverts. To put this in perspective, the international safety guideline for drinking water allows for up to 100 microsieverts per year. The actual risk from the Mudan Reservoir is roughly two orders of magnitude lower than the limit considered safe by the World Health Organization. The water is so clean, in terms of radiation, that even the most extreme monsoon rains only pushed the levels to about 5.5 percent of the national legal limit.

The study concludes that while the monsoon rains do temporarily increase the amount of natural radioactivity in the water, the reservoir remains a safe source for municipal supply. The natural background levels are simply too low to pose a threat, even when they are at their highest. This finding has a practical implication for water managers: there is no need to invest in expensive, advanced technologies like reverse osmosis to remove these specific natural particles, as the current system is already sufficient to protect public health. The water is safe to drink, and the invisible dance of natural radioactivity, driven by the seasons and the mountains, continues without posing a danger to the people who rely on it.