Introduction

Nuclear radiation has both natural and artificial sources1. The nuclides with the longest half-lives contribute to natural radioactivity that has persisted since the Earth’s birth, and cosmic rays also form many. Radionuclides, e.g., radon and uranium, could be taken into the human body by soil, consumption of plants, fish, meat, vegetables, drinking water, and inhalation of air2. Nature and man-made (artificial) activities are sources of the alpha-emitting radionuclides that enter soil, accumulate in it. Some alpha-emitting radionuclides, such as 238U, 235U, 232Th, and their decay products (including 226Ra and 222Rn) can naturally be found in the Earth’s crust3. With the passage of geological time, rock weathering, erosion, and leaching lead to the migration of these radionuclides into surface soils. As the parent isotopes decay, their daughter products (many of which are also alpha emitters) persist in restoring soil radioactivity3. Apart from the natural ones, several activities by man have brought more alpha-emitters to the environment. Facts: Uranium-bearing ore mining and milling, phosphate fertilizer production, industrial waste disposal, mishandling of radioactive materials, and nuclear test fallout are all sources of increased levels of alpha-emitters in the soil1. Upon release, these radionuclides can be associated with soil minerals, sedimented or migrated into the groundwater, which may result in long-term radiological and environmental hazards. To estimate environmental radioactivity, human exposure, and to develop strategies for remediation and monitoring, it is crucial to clarify the way of alpha emitters get into soil4. Soil borne alpha-emitting radionuclides may also exert substantial environmental and health effects owing to their high ionizing potency2. While alpha particles are not very penetrating and cannot travel long distances in the air or ground, they present a significant hazard if any radionuclides become airborne or contaminants are ingested4. In soil, alpha emitters (mainly uranium, thorium, and radium and their daughters) may perturb soil chemistry by inducing pH shifts, mineral stability changes, or trace element mobility. These radionuclides are taken up by plant roots and then transferred to crops and thus the food chain, thereby enhancing internal radiation exposure in humans and animals. High levels of alpha emitters could also have an impact on soil microorganisms, resulting in a loss of biodiversity and changes to key biological processes, including nutrient cycling and decomposition of organic matter5. Moreover, soil pores serve as a migratory pathway for radon gas produced by radioactive decay of radium, which may collect in buildings and greatly increase lung cancer risk. In general, alpha emitters in soil are a radiological hazard in the long run and therefore should be monitored on an ongoing basis with good environmental management5. Uranium and its decay products are present in soil worldwide as a result of geology, mineral composition, soil texture and porosity, and climatic factors3. Granitic and phosphate-bearing rocks have higher concentrations of uranium, as does radium (giving its chemical and dimensional similarities with uranium), which also tends to be enriched in sites where there is uranium mineralization. Radon exhalation from the soil is also influenced by parameters like soil moisture, grain size distribution of the soil, and pressure gradients. Thus, understanding these radionuclides in different soil depths is critical for radiometric study, natural background establishment, as well as predicting migration to water and air systems. The majority of previous studies around the world were carried out on surface soils (0–30 cm), since it is closely related to human activities and vegetation6,7,8. However, deeper parts of the soil profile, including values at 50 cm depth, are especially important in terms of radionuclide migration, root uptake, and transfer to waters due to “groundwater replenishment” and long-term retention by radioactive substances9. Radon investigations performed in a few residential and agricultural regions of NAJAF revealed that remarkable heterogeneities can be observed in the soil-gas radon exhalation rates, which indicate that the distribution of alpha radiating is greatly influenced by bedrock structure and properties of soils10. Nevertheless, it has been difficult to find data for profiling concentrations of alpha-emitting radionuclides at deeper depths (up to 50 cm). This type of information is needed to support the interpretation of surface measurements that have measured high radon and provide critical information about deeper material reservoirs and potential mechanisms driving uranium and radium transport in groundwater. The analysis of 222Rn, 226Ra, and 238U in soil samples from KUFA city of Iraq is very important for several reasons. First, it allows for a better assessment of the vertical distribution of alpha emitters and their retardation processes. Secondly, this allows the local database to be compared with normalized international safety standards like those of the International Commission on Radiological Protection (ICRP) as well as those of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Third, it provides the opportunity to evaluate the radiological hazards in terms of the absorbed dose rates, annual effective doses, and radon emanation risks. Ultimately, it also contributes to environmental radiation-wise planning and to capacity building in a growing governorate such as Al-NAJAF facing population growth, agriculture development, and religious tourism increase, where land and soil management must take into account potential long-term exposure.

Area of study

The KUFA region is part of Al-NAJAF Governorate in central southern Iraq that it is situated on the eastern bank of the Euphrates River at 10 km northeast of NAJAF city. Geographically, KUFA is situated in the Mesopotamian alluvial plain with average dimensions of 32.02 latitude and 44.40 longitude E, hence lying in Iraq’s semi-arid climate zone with hot summers and mild winters11. The topography is characterized by residential areas, agricultural areas, and antiquities sites, all of which are served by fertile alluvial soil from date palms to vegetables and seasonal crops. KUFA is also home to some of the most identified and respected centers of Arab culture, including The House of Poetry, and one example could have been formed by the contemporary Nexus that connects it directly with NAJAF, creating together a huge metropolitan area. The choice of the KUFA district has been determined by demographic and certain socio-economic features, its location as a regional meeting/active spot, and also for the type of study requested, concerning alpha-emitting radionuclides in the environment. Hence, 30 sampling sites were determined through GIS (Geographic Information System) methods and were mapped by an expert in the field of GIS; the locations are shown on a map (Fig. 1).

Fig. 1
figure 1

Area of study.

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Materials and methods

Collection samples

In the present study, thirty soil samples were collected from KUFA district, NAJAF province, in Iraq. The samples were collected throughout March 2025. Soil samples were collected at a 50 cm distance layer from the KUFA area homogenously following the standard method to determine the recommended procedures for environmental radioactivity pollution as provided by the International Atomic Energy Agency12. Thus, each sample point used to clean a certain area from leafage and waste material on the ground and then drilled to a depth of 50 cm using an appropriate and clean corrosion-free auger. Following the drilling of the hole, about a similar amount of soil from neat layers was placed in a retrieved auger and transferred into pre-labeled clean polyethylene containers. The containers were then appropriately sealed and used control conditions during transfer to the laboratory where it was conditioned for further analysis. This method of obtaining samples from subsurface soil lay provides the trustworthiness and uniformity of the alpha-emitting radionuclide estimates.

Preparation of the samples

Soil samples were put into covered plastic bags and carried out to the laboratory of the Nuclear Physics Department/Faculty of Science/University of KUFA. Samples were dried for 1 h at 100 °C in an electric oven. Dried samples were then powdered using an electric mill, with the powder being passed through a sieve by hand. The processed soil was then filled into a 0.13-L plastic cup (container (and left for 30 days to allow secular equilibrium between 222Rn and 226Ra13.

Experimental of detection

Radon soil concentrations were measured in the current study by a CR-39 (TASTRACK: 1 mm thickness, 0.32 g/cm3) detector developed by ASTRAK Analysis System, Ltd., UK, with dimensions (2.5 cm × 2.5 cm). The detectors were set at the top of a plastic cup (average radius is 5.5 cm and height is 7 cm, as shown in Fig. 2), while the soil samples were on the bottom. After which, the container was closed properly with a lid for an exposure duration of 90 days. After the exposure, the detectors (along with their holders) were taken out of the container and placed in an electrical water bath of 98 °C for one hour in a NaOH solution (6.25 N)14. The detectors were then taken out of the bath, rinsed with distilled water, and finally dried. Finally, the tracks of alpha particles on the detector area were counted using the TASLIMAGE technique.

Fig. 2
figure 2

Container used in the present study.

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Theoretical calculations

Many quantities of alpha emitters in soil samples, such as 222Rn concentrations in the air space of the container (C), 222Rn concentration in soil samples (CRn), activity concentration of 222Rn (ARn), activity concentration of 226Ra (CRa), and 238U concentrations (CU) were calculated as the following equations8,15,16,17:

$$C\left( {\frac{Bq}{{m^{3} }}} \right) = \frac{\rho }{K T}$$
(1)
$$C_{Rn} \left( {\frac{Bq}{{m^{3} }}} \right) = \frac{{C \lambda_{Rn} h T}}{l}$$
(2)
$$A_{Rn} \left( {\frac{Bq}{{kg}}} \right) = \frac{C l A}{M}$$
(3)
$$C_{Ra} \left( {\frac{Bq}{{kg}}} \right) = \frac{C h A}{M}$$
(4)
$$C_{U} \left( {ppm} \right) = \frac{{W_{U} }}{{W_{S} }}$$
(5)

where ρ, T, K, λRn, h, \(l\), A, W, WS are referred to track density, time of exposure, calibration factor, decay constant of 222Rn, distance between sample and CR-39 in container, thickness of sample in container, area of surface container sample, weight of sample, and weight of uranium in sample, respectively. A track is a latent etch-pit track left on CR-39 detectors. The CR-39 detector was calibrated with a 226Ra source (222Rn source) that it is activity 6.6 kBq. The detector samples were irradiated for a different duration (0.5, 1, 1.5, 2, 2.5 and 3 days) to find the relation between track density (track/cm2) and radon exposure (Bq/m3. day). Considering the linear relationship between measured track density and corresponding radon concentration, the calibration factor was derived as K = (0.28 ± 0.043) track/cm2 per Bq/m3. day. This factor was then applied to convert the apparent track density (track/cm2) of all measured samples. Also, the following important radiological parameters due to 222Rn in air space of container: EM (exhalation rate in term of mass) and EA (exhalation rate in term area), as given below18,19.

$$E_{M} = \frac{C V \lambda }{{M T_{eff} }}$$
(6)
$$E_{S} = \frac{C V \lambda }{{A T_{eff} }}$$
(7)

where Teff is the time of actual exposure which can be determined by following20:

$$T_{eff} = \left[ { T - \lambda_{Rn }^{ - 1 } \left( {1 - e^{{ - \lambda_{Rn } T}} } \right)} \right]$$
(8)

Results and discussion

Table 1 shows the results of 222Rn, 226 Ra, and 238 U concentrations for 30 locations of KUFA city in Al-Najag governorate, Iraq. From Table 1, the results of C ranged from 220.73 Bq/m3 to 613.59 Bq/m3, with an average value of 398.08 ± 94.75 Bq/m3, whereas the results of CRn and ARn varied from 10,811.06 Bq/m3 to 30,052.34 Bq/m3, with an average value of 19,496.95 ± 4640.56 Bq/m3 and from 4.99 Bq/kg to 13.872 Bq/kg, with an average value of 9.00 ± 2.14 Bq/kg, respectively. The results of activity concentrations of 226Ra (CRa) ranged from 0.326 Bq/kg to 0.905 Bq/kg, with an average value of 0.587 ± 0.140 Bq/kg. While the results of 238U (CU) in unit’s ppm and Bq/kg were from 0.403 to 1.119, with an average value of 0.726 ± 0.173, and from 4.932 to 13.710, with an average value of 8.895 ± 2.117, respectively.

Table 1 Results of 222R, 226Ra, and 238U in soil samples of the present work.
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In the first step, the ratios are compared to worldwide reference (safe limit) levels for alpha emitters. 222Rn concentration in the air space of a cup is radon that you trap inside a closed container during your lab analysis, and it does not tell anything about health. Radon in soil gas monitors radon in soil pores, for which no direct safety standard exists, either as it dissipates outdoors21. Indoor radon is the only kind that poses a health risk, because people breathe in the accumulated gas. It is known that the safe limits of 222Rn concentrations indoors were 100 Bq/m3, 148 Bq/m3, and 300 Bq/m3, according to the world organizations such as WHO22, EPA23, and ICRP24, respectively. Typical soil gas radon levels range from 4000 to 40,000 Bq/m3 varying in soil permeability and geological structure 25,26,27. Therefore, soil radon of > 40 kBq/m3 is recognized as a good predictor for high indoor radon. Comparison of CRn in soil samples from KUFA city by the current study is shown in Fig. 3 about the CRn safe limit interval. So, all these values are well under 4000 Bq/m3 so although it doesn’t feel like they should be, all the samples you’ve measured so far are safe; in fact, significantly lower than that threshold. Nevertheless, although the majority of the study area is within control limits prescribed for safety to humans at a global scale, areas with higher concentration levels are recommended for monitoring, especially in cases where these may likely be used for residence by humans or as agricultural land. Typically, activity concentrations of 226Ra in natural soils are 30 Bq/kg, and those for uranium (238U) concentrations in natural soils are 11 ppm according to UNSCEAR 200021. Therefore, the values of 226Ra and 238U in all soil samples of KUFA city were less than the acceptable safe limit according to UNSCEAR 2000, as shown in Figs. 4 and 5, respectively.

Fig. 3
figure 3

Comparing the results of CRn in the present study with the range of safe limits.

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Fig. 4
figure 4

Comparing the results of CRa in the present study with the range of safe limits.

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Fig. 5
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Comparing the results of CU in the present study with the range of safe limits.

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From Table 1, the activities of α-emitting 222Rn, 226Ra, and 238U have significant variability in KUFA soil. These differences are a result of the parent lithology, the soil texture, and mineral composition that governs in nature the distribution of uranium series radionuclides. Soils with higher content in clay keep 238U and 226Ra in a weaker way, since the transport of radon is improved and so its exhalation by sandy to very porous layers28. Soil moisture, temperature, and inhomogeneity of soil porosity are additional factors that affect radon release. Moreover, land use practices, irrigation, and local geochemical conditions may mobilize or dilute radionuclides, leading to spatial variability in this regard as well29. The variation in Table 1 is consequently an expression of the integrated influence among geological, environmental, and soil-physical factors.

The concentration of 222Rn (C) is also characterized by the various degree found over investigated sites (Fig. 6). The median is around 390–400 Bq/m3, which suggests that the central portion of the data has moderate concentration levels. The majority of the measurements concentrate around 330–460 Bq/m3 (see interquartile range). This means that half of the samples have similar concentrations, with a fair degree of deviations around central value. Nevertheless, there is different diffusion in the full data range and the minimum and maximum value are about 220.73 Bq/m3, and 613.59 Bq/m3 respectively. Many high measurements with values > 500 Bq/m3 increase the right whisker of the boxplot, showing a distribution elongated to the right. Those enhanced levels could result from site-specific release of airborne radionuclides, transient increases in emissions or environmental factors that increase the suspension of radionuclides in air. The fact that there are no significant statistical outliers indicates that, while not the norm, the highest measurements fall within the sampled population’s expected range. In general, the distribution pattern reveals a system of meridionally oriented lines depicting more or less constant air radioactivity and some points of known exceptionally high values which should be paid further attention to. These results demonstrated the necessity of continued monitoring to detect spatial or temporal trends as well as to assess any potential health or environmental effects.

Fig. 6
figure 6

Boxplot of C in the present study with the range of safe limits.

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Looking at Fig. 7, most of the C values are between 300 and 500 Bq/m3 showing that this range groups most peaks in data. It is right-skewed, and only a few measurements reach higher values above 500 Bq/m3. This skewing means that high concentrations are only occasional and not the usual for most of these samples. Few occurrences of relatively lower values which are less than 300 Bq/m3, indicate that very low airborne radioactivity condition is rare in the sampling area. In general, however, the histogram expression shows a moderate centrality effect with some higher values possibly due to local or temporal accumulation of airborne radionuclides.

Fig. 7
figure 7

Histogram of C in the present study with the range of safe limits.

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The EM and EA of the soil samples varied obviously as can be seen from Table 2. The minimum EM was 2.03 mBq/kg h obtained from the sample 20, and the highest value was 5.63 mBq/kg. h in sample 29, with the average values in all samples was 3.65 ± 0.87 mBq/kg h. Also, from Table 2, the minimum value of EA was 213.27 mBq/m2. h in sample 20, and the maximum value was 592.84 mBq/m2. h in sample 29, with an average value of 384.61 ± 91.54 mBq/m2. h.

Table 2 Results of EM and EA due to 222R in soil samples of the present work.
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In the second step, the ratios are compared to worldwide reference levels for potential radiological risk. In the second step, the results of EA are compared to worldwide reference levels for potential radiological risk. Where the safe limit of EA due to inhalation of 222Rn concentrations in natural soils is 57.6 Bq/m2 h (57,600 mBq/m2 h) according to UNSCEAR 1988 30. Therefore, the values of EA in all soil samples of KUFA city in the present study were smaller than the safe limit, as shown in Fig. 8.

Fig. 8
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Comparing the results of EA in the present study with the range of safe limits.

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Figures 9, 10, 11, 12, and 13 were obtained by distributing the results of C, CRa, CU, EM, and EA in the entire sample of the study area, which was drawn using the GIS technique. Different colors were used to distinguish between high, medium, and low quantities.

Fig. 9
figure 9

Distribution the results of C in soil samples of KUFA city.

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Fig. 10
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Distribution the results of CRa in soil samples of KUFA city.

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Fig. 11
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Distribution the results of CU in soil samples of KUFA city.

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Fig. 12
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Distribution the results of EM in soil samples of KUFA city.

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Fig. 13
figure 13

Distribution the results of EA in soil samples of KUFA city.

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Comparison of measured 222Rn gas concentration for soil samples at a depth of 50 cm with tabulated values. The content of Table 3 is the comparison for (KUFA city, Iraq) and also compiles the reported data for different countries.

Table 3 Comparison of 222Rn concentrations in area of study with previous research at depth 50 cm.
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Comparison of 222Rn concentrations in soil samples at a depth of 50 cm. The differences in concentration of radon at the 50 cm depth between India, Cameroon, Iraq (Baghdad), Iraq (KUFA) are evident indicating the influence of geology and soil type from one place to another as shown in Table 3. It has generally higher concentrations in India reflecting the high content of uranium and its decay products in its soils; The Cameroon ratios, on the other hand, were generally lower reflecting a low background radioelement content. Baghdad falls within the medium zone of alluvium types. The current investigation for KUFA city indicates that the radon activity concentration is quite high, and this is an indication of a higher emanation rate from local geological deposits. In summary, these differences illustrate the manner in which local fine-scale soil texture, mineralogy, and surficial geology dominate radon migration in the subsoil.

Over the long term, consumption and inhalation of elevated levels of radon and uranium can have negative health impacts, leading to an increased risk of lung cancer or other radiological effects. As such, the hotspots can be used to help manage and mitigate something more general, like land remediation or radon-venting systems in buildings, for example. Overall, the results of this study indicate that the soil under investigation has moderate natural radioactivity, with some high activity spots. These findings could be a significant baseline to be used for environmental observation, risk evaluation, and land utilization planning in the future.

Conclusion

An investigation of alpha radioactivity (222Rn, 226Ra, and 238U) in soils in the depth of KUFA district at 50 cm revealed a moderate natural radioactivity; most values were within UNSCEAR world levels. The exhalation rates of radon (EM and EA) showed different potentials of emission, which depended on both the composition of the soil and its content in radium. Although most samples were below the dangerous value, local hot spots (sample 29) had higher values, which need consideration for environmentally sensitive areas, health risk assessment, etc. These results could be used for future surveillance, land-use planning, and radiation protection programmes in the Al-NAJAF governorate.