Hidden Radioactivity and Rare Earths Mapped in Vietnamese Lake Sediments

Deep in the pine-covered hills of Vietnam’s Central Highlands, TuyenLam Lake has long been prized as a scenic reservoir above the city of Da Lat. A new study shows that its lakebed is quietly recording a chemical story about geology, land use, and human pressure on the watershed. Researchers from the Da Lat Nuclear Research Institute analyzed 35 surface sediment samples for rare earth elements and natural radionuclides, and their results reveal a clear spatial fingerprint: sediments near urbanized and farmed shores carry measurably more lanthanum, cerium, neodymium, samarium, and terbium, along with elevated activities of radium-226 and thorium-232, than sediments fed by forested catchments.

The team divided the lake into three hydrologically distinct groups based on dominant inflow sources and catchment characteristics. Group 1, influenced by urban development and agricultural runoff, showed significantly higher concentrations of most of the studied rare earth elements together with elevated radionuclide activities. Group 2, representing predominantly forested catchments, exhibited comparatively lower values across the board. Group 3, located near the dam outlet, displayed intermediate characteristics that the authors attribute to sediment mixing processes, where material from different parts of the catchment converges before settling.

Rare earth elements have become strategically critical materials for magnets, batteries, and electronics, so understanding where they accumulate in ordinary environments matters both for resource geology and for ecotoxicology. At the same time, radium-226, thorium-232, and potassium-40 are the principal natural contributors to gamma radiation in soils and sediments, and their concentrations determine whether an area poses any radiological concern. Because rare earth elements often travel geochemically with thorium and uranium decay chains in minerals like monazite, the co-occurrence of elevated REEs and radionuclides is not coincidental but reflects shared mineral hosts.

To quantify contamination, the researchers applied two widely used indices. The geoaccumulation index, or Igeo, compares measured concentrations against background levels and classifies enrichment from unpolluted to heavily polluted. The pollution load index, or PLI, aggregates multiple elements into a single value for each site. Both indices indicated generally low to moderate relative enrichment across the lake, with localized enrichment at several sites, particularly within Group 1. Enrichment factor calculations, however, suggested that the REE distributions are predominantly controlled by natural geological sources rather than by pollution, meaning the elevated values near developed shores likely reflect weathering of local bedrock and soils transported by runoff rather than industrial discharge.

The radiological assessment told a more nuanced story. Radium equivalent activity and the external hazard index, two standard measures used to gauge gamma dose potential from sediment or building materials, remained below recommended reference values at most sampling sites. However, several radiological indices exceeded commonly adopted global reference levels at some locations. The authors identify these spots as areas where continued environmental monitoring is warranted, not as immediate hazards but as signals that parts of the lakebed warrant periodic rechecking as land use evolves.

Multivariate statistical analyses added a key mechanistic insight: strong associations emerged between the rare earth elements and the radionuclides radium-226 and thorium-232, whereas potassium-40 behaved distinctly in space. This pattern is geochemically coherent. Radium-226 sits in the uranium decay chain and thorium-232 heads its own chain, and both tend to reside in the same refractory mineral phases that concentrate rare earths. Potassium-40, by contrast, tracks potassium-bearing minerals such as feldspars and micas, which weather and transport differently, explaining its decoupled spatial behavior.

The study’s spatial variability, the authors conclude, is consistent with the combined influence of the regional geological setting, sediment provenance, catchment-scale sediment transport processes, and recent land use disturbances. In practical terms, the lake is acting as an integrator: every inflow carries the chemical signature of its sub-catchment, and the lakebed preserves a mosaic of those signatures. Urbanized and agricultural areas deliver finer, more weathered, and chemically enriched particles, while forested catchments contribute comparatively pristine material.

For Da Lat, a city whose economy depends heavily on tourism, agriculture, and a clean mountain environment, the findings provide a valuable geochemical baseline for sustainable catchment management. Establishing reference concentrations now allows future monitoring to detect departures from natural variability, whether from expanding agriculture, construction, or changes in hydrology. The work also fits into a broader national effort, as Vietnamese researchers have recently published similar radiological assessments for the Red River, Ha Long Bay, and central coastal lagoons, building a country-wide picture of natural radioactivity in sediments.

Methodologically, the study demonstrates the value of combining nuclear analytical techniques with multivariate statistics and geographic information systems. By classifying sampling sites according to hydrological connectivity rather than arbitrary grid positions, the researchers could link chemical patterns to catchment processes, a design that other lake studies could replicate. The approach turns a routine monitoring exercise into a diagnostic tool for identifying which parts of a watershed contribute the most chemically distinctive sediment.

As demand for rare earth elements grows worldwide, and as awareness of naturally occurring radioactive materials in everyday environments increases, studies like this one fill an important gap. They show that even a seemingly pristine mountain lake carries measurable geochemical gradients shaped by both bedrock and human activity, and that careful baseline mapping is the first step toward protecting both environmental and public health in rapidly developing regions.

Subject of Research: Distribution of rare earth elements and natural radionuclides in lake sediments

Article Title: Assessment and spatial distribution of natural radionuclides and some rare earth elements in the surface sediments of TuyenLam Lake, Central Highlands, Vietnam

Article References: Assessment and spatial distribution of natural radionuclides and some rare earth elements in the surface sediments of TuyenLam Lake, Central Highlands, Vietnam. (n.d.). https://doi.org/10.1007/s10661-026-15854-5

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15854-5

Keywords: rare earth elements, natural radionuclides, lake sediments, radiological risk, Vietnam, TuyenLam Lake, sediment contamination, geochemistry, environmental monitoring, land use, radium-226, thorium-232