Radioactive Traces in Moroccan Crops Reveal How Semiarid Soils Control Radiation Risk

Every meal carries a faint radioactive signature. Potassium-40, a primordial isotope present since the Earth formed, hums quietly inside every banana, potato and tomato. Alongside it travel trace amounts of radium, polonium, lead and thorium, all drawn up from the soil by the same roots that gather water and nutrients. For most of the world’s farmland this background radiation is negligible, but regulators still need hard numbers to prove it, and those numbers are scarce for one of the planet’s most extensive climate zones: the semiarid belt that stretches across North Africa, the Middle East and beyond. A new study from Morocco now provides some of the most detailed measurements yet of how naturally occurring and artificial radionuclides move from soil into the food crops of a dry agricultural region.

The research, published in Environmental Monitoring and Assessment, was carried out by a team from Hassan II University, the Moroccan National Centre for Nuclear Energy, Science and Technology (CNESTEN) and the National Institute of Agricultural Research, working within an International Atomic Energy Agency coordinated research project on radionuclide transfer in arid and semiarid environments. Led by Zineb Elaboudi, the scientists sampled soil and nine commonly consumed crops, including coriander, red pepper, potato, onion, pomegranate, cardoon, beet, tomato and eggplant, from farmland in a semiarid Moroccan agricultural area. Their goal was to quantify activity concentrations, calculate soil-to-plant concentration ratios and estimate the potential annual effective dose that people could receive from eating locally grown food.

The analytical work relied on high-resolution alpha- and gamma-spectrometry using high-purity germanium detectors, instruments capable of distinguishing the characteristic energy signatures of individual isotopes even at very low concentrations. The team measured eight radionuclides in both soil and plants: polonium-210, thorium-234, radium-226, thorium-228, radium-228, lead-210, potassium-40 and cesium-137. Together these isotopes span the full spectrum of environmental radioactivity, from members of the uranium and thorium decay chains that have leached slowly through bedrock for millennia, to potassium-40 dispersed uniformly through all living tissue, to cesium-137, a fission product whose global fallout from mid-twentieth-century nuclear weapons testing still lingers in surface soils.

In the soil, activity concentrations painted a picture of typical natural background. Polonium-210 ranged from 16.37 to 39.59 becquerels per kilogram, thorium-234 from 6.21 to 29.15, radium-226 from 4.84 to 26.26, thorium-228 from 9.37 to 46.04, radium-228 from 10.10 to 51.04, and lead-210 from 5.92 to 31.39. Potassium-40, as expected, dominated with values between 107.21 and 434.20 becquerels per kilogram, while cesium-137, the sole anthropogenic contributor, registered only 1.15 to 2.98 becquerels per kilogram, a faint residue of atmospheric fallout rather than any local contamination source.

Inside the plants, the story changed dramatically. Activity concentrations were generally far lower than in the underlying soil, with polonium-210 between 0.12 and 4.76 becquerels per kilogram, radium-226 between 0.36 and 5.89, and thorium-228 between 0.22 and 1.61. Potassium-40, however, ran in the opposite direction, reaching 386.33 to 3265.53 becquerels per kilogram, sometimes exceeding its soil concentration. The reason is biochemical rather than radiological: potassium is an essential plant macronutrient, recruited in large quantities to regulate osmotic pressure and enzyme function, and potassium-40 constitutes a fixed fraction of all natural potassium. Because the isotope cannot be separated chemically from its stable siblings, plants that accumulate potassium necessarily accumulate its radioactivity. Meanwhile thorium-234, radium-228, lead-210 and cesium-137 mostly fell below detection limits in the crops, indicating that roots act as effective barriers against these species.

The concentration ratio, defined as the activity concentration in plant tissue divided by that in dry soil, is the single most important parameter in radiological dose models, and the Moroccan values revealed striking crop-specific differences. Ratios spanned 0.01 to 0.14 for polonium-210, 0.02 to 0.43 for radium-226, and 0.01 to 0.17 for thorium-228, but climbed as high as 20.03 for potassium-40. Two crops stood out as the strongest accumulators: cardoon, a thistle-like vegetable with deep roots and a place of honor in North African cuisine, and eggplant. Elevated transfer in these species suggests that rooting depth, plant family physiology and nutrient demand all shape which isotopes cross the root membrane, a finding consistent with observations from Syria, Saudi Arabia, Abu Dhabi and other dry regions where comparable studies have been conducted.

Perhaps the most scientifically valuable contribution lies in the correlation analysis linking radionuclide uptake to soil chemistry. The researchers found strong interrelationships among the measured isotopes and demonstrated that transfer processes are governed primarily by soil physicochemical properties, particularly organic matter content, cation exchange capacity and the suite of exchangeable cations held on clay and humus surfaces. These parameters control how tightly radionuclides are bound to soil particles and therefore how available they are for root uptake. In contrast, carbonate content and salinity tended to restrict radionuclide mobility, effectively locking the isotopes in place. This has a counterintuitive implication for semiarid agriculture: the very conditions that make soils challenging for crop productivity, including high carbonate levels and salt accumulation under intense evaporation, may also suppress the movement of radioactive elements into the food chain.

The study also estimated the potential annual effective dose from food ingestion, the standard metric used by the International Commission on Radiological Protection to translate environmental measurements into health-relevant terms. By combining crop activity concentrations with consumption rates and isotope-specific dose coefficients, the team could assess whether locally grown vegetables contribute meaningfully to the radiation burden that residents already receive from cosmic rays, radon and terrestrial gamma radiation. The dominance of potassium-40 in plant tissues means it inevitably drives much of the ingested dose, yet because the human body maintains potassium homeostasis, biologically regulated excretion keeps the internal burden from this isotope essentially constant regardless of dietary source, a nuance that distinguishes potassium-40 from radium or polonium, which follow bone and soft-tissue metabolism and are treated with greater regulatory caution.

For the wider scientific community, the Moroccan dataset fills a recognized gap. The IAEA has repeatedly noted that most soil-to-plant transfer parameters in its handbooks derive from temperate European and North American ecosystems, while arid and semiarid environments, home to a large share of the world’s irrigated agriculture, remain underrepresented. Studies from Turkey, Jordan, Nigeria, Vietnam, Australia and now Morocco are steadily building the evidence base needed to adapt radiological environmental impact assessment to dry climates. The finding that organic matter and cation exchange capacity promote transfer while carbonates and salinity suppress it offers modelers concrete, measurable soil descriptors that can be incorporated into transfer predictions without requiring site-specific experiments at every location.

The practical message for consumers is reassuring. Cesium-137 was barely detectable, the natural radionuclides were mostly well below levels of concern, and the estimated doses fall within the range regarded as normal background exposure from food. Yet the research carries lasting value beyond its immediate conclusions. As phosphate fertilizer use, water scarcity and climate change reshape agriculture across the semiarid world, understanding the soil chemistry that governs radionuclide mobility becomes essential for protecting both food safety and the environmental systems that sustain it. This Moroccan study, rooted in rigorous gamma and alpha spectrometry and framed by international collaboration, demonstrates that even the quiet radioactivity of an ordinary vegetable patch can be measured, modeled and managed with precision.

Subject of Research: Soil-to-plant transfer of natural and artificial radionuclides and associated radiological doses in semiarid Moroccan agriculture

Article Title: Radiological impact of radionuclides in Moroccan food crops under semiarid conditions

Article References: Radiological impact of radionuclides in Moroccan food crops under semiarid conditions. (n.d.). https://doi.org/10.1007/s10661-026-15992-w

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15992-w

Keywords: radionuclides, soil-to-plant transfer, concentration ratio, potassium-40, radium-226, polonium-210, cesium-137, semiarid agriculture, Morocco, gamma spectrometry, radiological dose, soil chemistry