Introduction

The rapid development of the atomic energy industry has continuously raised the demand for natural uranium since the mid-1950s [1,2,3]. The industries of uranium mining and smelting have accordingly developed. The mining of uranium mines would give rise to the increasing of a large number of tailings, waste dumps, and relic, which would change the local environment, and cause profound environmental pollution [4]. Of particular concern is the possible increase of radiological hazard on the local environment and public [3].

China’s uranium deposits are mainly distributed in southern and eastern regions, such as Guangxi, Guangdong, and Jiangxi Province. It was reported that 85% of proven uranium deposits are distributed in these regions [5], which are as well as densely populated, and are the most important rice production bases [6, 7]. In the process of uranium mining, radionuclides in the mining area would be transferred in the rice through various pathways (especially irrigation), and increase hazard to the local public [5, 7,8,9,10].

In the last 20 years, the early-developed uranium mines have gradually decommissioned [11]. The radiological impact was assessed effectively to meet the requirement of environmental restoration. One of the most important concerns during the assessment is the mine water [1,2,3,4, 11]. Years of mining would result in a large number of barren rock and disturbed residual mine ores or soils around uranium mine. Due to the heavy rainfall and high groundwater level, reservoirs would form locally for some open-pit uranium mine, and the pit-water was often used for irrigation of surrounding paddy fields. The use of the pit-water for agricultural purposes (irrigation) would result in the transfer of the natural radionuclides through the terrestrial food chain, and the subsequent internal exposure for the local public individuals due to ingestion of contaminated foodstuffs. The possible environmental impact and radioactive contamination on paddy rice have always attracted local public concern [9]. Therefore, the site release in reality for the uranium mine is far from over, local public complaints and government affairs about the assessment of the radiological hazard still happened occasionally in China [12].

In this paper, we take a decommissioned uranium mine in eastern China as an example to assess the radiological hazard from the paddy field, where the around paddy field was irrigated with the pooled mine pit-water. The spatial distribution of radionuclides concentration and the health risk indexes in the local environment was presented. The study will provide a reference for effective evaluating the radiological impact and the extent of contamination, and help to solve the public’s concerns about the impact of pit-water irrigation around the decommission uranium mine through the scientific assessment method.

Materials and methods

The research area

The uranium mine is located in a province in eastern China (Fig. 1). It had been mined for more than 50 years and was decommissioned in 2010. The area to which the uranium mine belongs is a subtropical greenhouse climate zone. The average annual temperature is 18.4 °C, the average relative humidity is 76%, the average annual rainfall is 1750 mm, and the frost-free period is 267 days.

Fig. 1
figure 1

Locations of the research area and the sampling sites. S1–S25, the sampling sites at the research area; C01–C05, the contrast sites; W1, the sampling site at the mine pit-water (note: the contrast site of a small-scale reservoir is situated on the 0.5 km west of the research area (W2), and is not marked on the figure)

Full size image

It was an open-pit mine. During the mining, the ore was just transported to another site for centralized smelting. A pit with an area of about 3 hectares was formed at the mine site with a maximum depth of 80 m through years of catchment after abandoned years ago. The nearest settlement is located about 200 meters north of the pit boundary, and has a population of about 400 people. Since the mining area is mainly surrounded by paddy fields, the residents use the pit-water for irrigation, mainly through gravity irrigation or pumping. The irrigation area is about 6 hectares in the north of the pit. In the rainy season, the water would overflow through a water gate located in the north of the mine pit.

Sampling

Surface water from the mine pit and the contrast site was sampled with a sampling pump for the analysis of total uranium and natural gamma radionuclides. 238U and 226Ra were mainly concerned radionuclides in this study. Before the sampling, the polyethylene containers with volumes of 25 L were washed with the sampling water twice. After filled with the water, 10 ml of concentrated nitric acid was added into each container to acidify the water to avoid the possible adsorption of the radionuclides on the interior.

According to the uniform distribution of paddy fields and shape of each farming block, 30 points were selected for surface soil sampling and analyzing (within 20 cm depths), including 5 contrast sites (C01–C05). The contrast sites are located across a creek to the research area that are unaffected from the mine pit-water. 5 points of the total sampling sites were selected for profile analysis (focus on the sites most likely to be affected, as well as one control site), i.e., S13, S16, S21, S24, and C05. The soils for profile analysis were sampled every 20 cm, and the total sampling depth was about 80 cm. The soils were sampled in the harvest season, and the field was soft without free water. The sampled soil was analyzed for natural gamma nuclides, including 238U, 232Th, 226Ra, 40K.

Sample processing and radioactivity determination

Mine water

  1. 1.

    Laser fluorimetry

Total uranium was analyzed on a laser fluorescence analyzer (WGJ-III Trace uranium analyzer, Hangzhou Daji Electric Instrument Co. Ltd). The analysis principle is based on the fluorescence of the uranium complex with high luminescent yield formed with an enhancer reagent and the uranyl species (UO2)2+ in water [13]. The detection limit for uranium is about 0.05 ng ml−1.

The analysis for each sample includes the following procedures. Take a 5.00 mL water sample in a quartz cuvette, determine the fluorescence intensity as N0, add 0.5 mL of fluorescent enhancing reagent, mix well, and determine the fluorescence intensity as N1. Then add 5μL of uranium standard solution to the sample, mix well, and record the fluorescence intensity as N2. Then total uranium (CU) can be calculated by the following equation:

$$ C_{\text{U}} = \frac{{(N_{1} - N_{0} )C_{0} V_{0} }}{{(N_{2} - N_{1} )V}} $$
(1)

where \( C_{\text{U}} \) is the total uranium concentration in the sample (μg L−1); N0, N1, and N2 are the fluorescence intensity recorded according to the procedures mentioned above; C0 is the uranium concentration in the standard (μg L−1); V0 and V are the volume of the uranium standard (0.05 mL) and sample (5 ml), respectively.

  1. 2.

    Gamma spectrometry

For the water sample subjected to a high-resolution gamma spectrometer, took a 40 L water sample, evaporated, and concentrated to 1 L with an electric heater. Then it was filled into a 1 L PVC Marinelli beaker, sealed for up to 20 days to ensure the radioactive secular equilibrium among the daughter nuclides of 226Ra, 220Rn, and their short-lived decay products, and then counted on a high-resolution gamma spectrometer (GMX5084-P4 & DSPEC-jr2.0-POSGE, Ortec inc.). It used a GMX (N-type) coaxial radiation detector with a carbon fiber window, and the relative efficiency is about 50%. The ultra-low-background lead shield system was used that features an overall thickness of 4 inches of lead materials and an inner liner of copper and thin layers. A liquid nitrogen dewar was used for cooling the detector.

The spectrometer was calibrated with radionuclides of 238U, 232Th, 226Ra, 40K, 60Co, and 137Cs in freshwater standards in a PVC Marinelli beaker with the same dimensions, which was obtained from the National Institute of Metrology, China (NIM), and were guaranteed within the validity period during the monitoring stage in this study

The activity concentration of 238U was determined from its daughter nuclide of 234Th(63.3 keV), and the activity concentration of 226Ra was determined from its daughter nuclide of 214Pb (351.9 keV). To determine the activity concentration of 232Th, the gamma-ray from its daughter nuclide of 212Pb (238.6 keV) was used. To determine the activity concentrations of 40K, the principal gamma-ray of 1460 keV was used. The total counting time for each sample was 80,000 s, which could make one sample could be measured every 24 h. The activity concentrations of 238U, 232Th, 226Ra, and 40K were calculated using the following equation:

$$ C_{w} = N/\varepsilon \eta V $$
(2)

where Cw is the activity concentration of 238U, 232Th, 226Ra, and 40K in water (Bq L−1); N is the net gamma counting rate (cps); ε is the detection efficiency at the corresponding energy; \( \eta \) is the absolute transition probability of gamma-ray; V is the volume of the sample (40 L).

The a priori minimum detection concentrations (MDC) for 238U, 232Th, 226Ra, and 40K in water were about 0.18 Bq L−1, 0.006 Bq L−1, 0.004 Bq L−1, and 0.04 Bq L−1.

Soil

The surface paddy soil was sampled within 20 cm depth with a shovel for all the sites except for the sites of profile analyzing. As the paddy field was cultivated in blocks, a mixture with separate samples of a random number in each block was collected. Firstly, debris such as grassroots was removed for each sample, and then it was mixed thoroughly. For profile sampling, a dynamic sampler was used (core diameter 4 cm), and take profile samples every 20 cm depth down to 80 cm. Then, the samples were dried in an oven for 48 h (110 °C), weighed, and crushed into fine powders. After sieving for 60 mesh-size, each sample was sealed into an airtight PVC container and kept for up to 20 days to allow radioactive secular equilibrium among the decay products of 226Ra and then measured on a high-resolution gamma spectrometer. Simulated soil sample standard with known activity concentrations was used for sample calibration, which was obtained from the National Institute of Metrology, China (NIM), and the spectrometer and gamma-ray energy of each nuclide were the same as that of the water samples. An average 300 g of weight for each sample was used, and the counting time was about 80000 s, which could make one sample could be measured every 24 h. The activity concentrations of 238U, 232Th, 226Ra, and 40K were calculated using the following equation:

$$ C_{s} = N/\varepsilon \eta m $$
(3)

where Cs is the specific activity of 238U, 232Th, 226Ra, and 40K in soil (Bq kg−1); m is the mass of the sample (kg); N, ε, and \( \eta \) have the same meaning as in the previous equation.

The a priori minimum detection concentrations (MDC) for 238U, 232Th, 226Ra, and 40K in soil were about 21 Bq kg−1, 1.3 Bq kg −1, 1.9 Bq kg −1, and 13 Bq kg −1.

Radiation risk index

  1. 1.

    Gamma absorbed dose rate (Dr)

The environmental surface gamma absorbed dose rate (Dr) reflects the environmental background gamma radiation level and the changes caused by human practice, and is also used to assess the radiation dose received by the public individuals [10]. In this study, through the specific activity of natural radionuclides in the shallow surface soil, the Beck’s formula was used to estimate [14,15,16]:

$$ D_{\text{r}} = 0.462C_{\text{Ra}} + 0.604C_{\text{Th}} + 0.0417C_{\text{K}} $$
(4)

where CRa, CTh and CK are the specific activities (Bq kg−1) of 226Ra, 232Th, and 40K, respectively; the coefficients are the conversion factor corresponding to the nuclides.

Although the Beck’s formula was established based on the assumption of uniformly distributed of the natural radionuclides within 5 cm depth and the secular equilibrant of U-series and Th-series(Beck et al., 1964), it was used in this study as an approximate method.

  1. 2.

    Annual effective dose equivalent

The annual average effective dose equivalent (Aex) received by an adult individual was calculated using the following equation [14]:

$$ A_{\text{ex}} = D_{\text{r}} \times t \times 0.7 \times 10^{ - 6} $$
(5)

where 0.7 is the conversion factor from absorbed dose in the air to the effective dose received by an adult individual (Sv Gy−1); t is the working time in the paddy field. According to the characteristics of local farming, it is assumed that the annual arable time is 50 h, planting time is 50 h, weeding and fertilization time is 50 h, and harvest time is 50 h. So, the total working time in the paddy field was estimated as 200 h a−1 conservatively.

  1. 3.

    Radium equivalent concentration

The radium equivalent concentration (Raeq) is widely used to evaluate the radiation hazard of the material [9, 14, 16, 17]. It is assumed that 370 Bq kg−1 of 226Ra, 259 Bq kg−1 of 232Th, and 4810 Bq kg−1 of 40K produce the same gamma dose rate, which gives the annual dose of 1.5 Gy (1 mSv) at a height of 1 m above the ground. It can be calculated by the following equation [14, 16]:

$$ Ra_{\text{eq}} = C_{\text{Ra}} + 1.43C_{\text{Th}} + 0.077C_{\text{K}} $$
(6)

where CRa, CTh, and CK have the same meaning mentioned in Eq. (4).

  1. 4.

    Exposure index

Beretka and Matthew defined two indexes (internal/external exposure index) to limit the radiation dose to 1 mSv a−1 [18]. The external exposure index (Hex) is calculated using equation [14, 18]:

$$ H_{\text{ex}} = C_{\text{Ra}} /370 + C_{\text{Th}} /259 + C_{\text{K}} /4810 $$
(7)

If the calculated value is less than the unity, the radiation hazards caused by radioactive 226Ra, 232Th, and 40K in this area to the human body can be ignored [16, 18].

The internal exposure index (Hin) is calculated using equation [14, 18]:

$$ H_{\text{in}} = C_{\text{Ra}} /185 + C_{\text{Th}} /259 + C_{\text{K}} /4810 $$
(8)

If the value of Hin is less than unity, the hazard from radon and its short-lived progeny to the respiratory organs is negligible [16, 18].

Results and discussions

Results

Radioactivity in mine water

The analysis result of the radioactivity in water is shown in Table 1. It is indicated that total uranium in mine pit-water is 10.4 μg L−1, which is higher than the result from the contrast site, i.e., 0.090 μg L−1. It is shown that there is obviously enhancing of uranium content in the mine pit-water, with almost 116 times the result from the contrast site. The result is within the range of uranium concentrations in the freshwater of China (0.02–42.35 μg L−1), but higher than the average (1.66 μg L−1), which was investigated for natural radiation in China during the 1980s [19]. Estimated by the natural abundance of 238U, 1 μg U is equivalent to 12.24 mBq 238U [20], and the total uranium measurement results in the mine pit-water could be converted to the activity concentration of 238U as 127.3 mBq L−1.

Table 1 The radioactivity concentrations in mine pit-water and contrast reservoir
Full size table

For the gamma spectrometric analysis, it was shown that the activity concentration of 238U in mine pit-water is about 140 ± 41 mBq L−1, which is in accordance with the transfer values from total uranium (127.3 mBq L−1). The results of 232Th and 40K are lower than that of 238U, which indicates the principal impact from 238U and its progenies should be concerned. The related results of 238U and 226Ra in the contrast reservoir are lower, while those of 232Th in both water samples are below the MDC. For the activity concentration of 40K, the higher value in contrast reservoir is related possibly to the lower depth and capacity, as well as its usage for aquaculture.

Radioactivity in paddy soil

  1. 1.

    Surface paddy soil

Statistics of radionuclides activity concentrations in surface paddy soil are shown in Table 2, the detailed result is displayed in Fig. 2. It is shown that the activity concentrations of 238U in the research area vary from 77.0 to 5600.0 Bq kg−1 with an average of 1212.9 Bq kg−1 and a standard deviation of 1262.5 Bq kg−1. All the values of activity concentrations of 238U is higher than the national average activity concentration that investigated during the 1980s (with an average of 39.5 Bq kg−1 and a standard deviation of 34.4 Bq kg−1) [19]. 40% of the results are higher than 1000 Bq kg−1 (ten sites), which indicated further intervention should be carried out according to the IAEA recommendation of 1000 Bq kg−1 for radionuclides excluding 40K was derived using the exclusion concept for the natural region [21]. Particularly, higher values occurred in the north shore side of the mine pit and the south-east region of the research area. Sampling site S23 had the largest value of 5600.0 Bq kg−1, which was affected possibly by the residual soil during the mining around the region. The deposition of the activity is also expected as a factor for the high values, which need further identification through detailed retrospective examinations. Meanwhile, it could be observed that higher values occurred near the mine pit, and have a decreasing trend in the distance.

Table 2 A statistics of the activity concentrations in surface paddy soil (Bq kg−1)
Full size table
Fig. 2
figure 2

The radar plots of activity concentrations 238U, 232Th, 226Ra, and 40K in the sample of surface paddy soil (Bq kg−1)

Full size image

A similar trendency of 226Ra could be observed as a daughter nuclide of 238U. It could be seen that the activity concentrations of 226Ra in the research area vary from 577.6 to 1073.4 Bq kg−1, with an average of 577.6 Bq kg−1 and a standard deviation o 1073.4 Bq kg−1, which is higher than the result in the national soil that investigated during the 1980s (with an average of 49.1 Bq kg−1 and a standard deviation of 27.6 Bq kg−1) [19]. There are only 3 soil samples with the analysis results that are higher than the derived value of 1000 Bq kg−1 from the exclusion concept for the natural region [21], which indicated an influence of the different geochemical characteristics of 238U and 226Ra in the natural environment [22].

The activity concentrations of 232Th are not notable, which are in the range of 48.2–81.8 Bq kg−1 with an average of 62.6 Bq kg−1 and a standard deviation of 7.6 Bq kg−1. They are lower and more homogenous than that of 238U and 226Ra. The corresponding activity concentrations were in the range of the national investigated values (ranged in 1–437.8 Bq kg−1 with an average of 49.1 Bq kg−1 and a standard deviation of 27.6 Bq kg−1) [19].

The activity concentrations of 40K are in the range of national activity concentrations and are comparable to the national average (ranged in 11.5–2185.2 Bq kg−1 with an average of 580 Bq kg−1 and a standard deviation of 202.0 Bq kg−1) [19].

As shown in Table 2 and Fig. 2, the results of 238U and 226Ra in the research area are higher than those in the contrast site, while those of 232Th and 40K in the research area are closed to those in the contrast site, which indicated that the paddy field in the research area was affected from the uranium mining significantly.

To explore further the activity concentrations of 238U, 232Th, 226Ra, and 40K in the surface paddy field of the research area, we plot the contour distribution of the activity concentrations as displayed in Fig. 3. It could be found that the activity concentrations of 238U and 226Ra in the south-east region are higher, and have a decreasing trend with the distance to the mine pit. Especially in the north region, the corresponding results are closed to those in the contrast sites. Also, the paddy field area about 3 hectares for 238U and 226Ra above 1000 Bq kg−1 could be identified, mainly in the north and south-east regions.

Fig. 3
figure 3

The contour plots of the distribution of the activity concentrations (Bq kg−1)

Full size image

To explore the correlations between the activity concentrations of 238U, 232Th, 226Ra, and 40K, the Pearson’s correlations coefficients are shown in Fig. 4. It could be observed for the strong correlation (r between 0.6 and 0.8) between 238U and 226Ra (r = 0.76) and that between 226Ra and 40K (r = 0.61), while weak correlation (r between 0.2 and 0.4) between 232Th and 40K(r = 0.47) and between 238U and 40K (r = 0.31). None correlation between the activity concentrations between 238U and 232Th, 226Ra and 232Th could be observed as the Pearson’s coefficients are both 0.03 respectively. The correlation analysis may provide some meanings of the geochemical of the radionuclides in the local environment. The strong correlation between the activity concentrations of 238U and 226Ra indicates a similar source and the transfer characteristics [10].

Fig. 4
figure 4

The matrix illustration of the Pearson’s coefficients between the activity concentrations of 238U, 232Th, 226Ra, and 40K

Full size image
  1. 2.

    Profile distribution

The profile distribution of radioactivity is important to understand the behavior along with the history and give some information such as weathering, erosion, biological function, and cultivation influence [23, 24]. Five paddy soil sampling sites were selected for profile investigation, including a contrast site (C05). The corresponding results of the activity concentrations of 238U, 232Th, 226Ra, and 40K are displayed in Fig. 5.

Fig. 5
figure 5

The profile distribution of the activity concentrations in paddy soils

Full size image

238U and 226Ra profile from the research area showed a similar pattern, obviously higher values occurred for the surface soil, getting mainly lower values as deeper positions. At the depth of about 60–80 cm, the activity concentrations in the research area decrease to similar levels of that in the contrast site. The phenomenon indicates a deposition process of the anthropogenically introduced activity. The profile distribution of the 238U for site S24 is slightly different. It was beginning with a lower value at the surface (0–20 cm), then getting a higher value at 20–40 cm, and a lower value at a deeper position. The special profile is related to the fact that it is remediated soil with a coating of other soil materials. The profile distributions of 226Ra are relatively weak for the others sites of the research area, which was ascribed to the possible disequilibrium in the U-series and the difference of the geochemical characteristics, as the ratios of 238U/226Ra are mainly in the range of 1.1–6.7 for the surface paddy soil (excluding S24) with average about 3.9, and the ratio of 238U/226Ra are mainly in the range of 0.5–7.5 for profile soil with an average about 2.5. Since the profile distribution is highly associated with the geochemical characteristics of the radionuclides and the soil, and further investigations should be carried out to give more scientific meaning.

Note the comparison of the profile distributions of activity concentration between the research area and the contrast site (C08), it could be seen that profile variations of the 238U, 226Ra are inconspicuous, which is shown the possible impact from the mining process and the irrigation impact with the mine pit-water.

For the activity concentrations of 232Th and 40K, it presents quite homogeneous profile distributions, and could not give any meaningful conclusions. Although 40K activity always revealed a high variability due to the cultivating and the fertilization, there is none uniformed profile characteristic in our study.

Radiological risk assessment

The corresponding statistics of the radiological risk indexes calculated with the activity concentrations of surface paddy soil are shown in Table 3, and the distribution in the research area are displayed in Fig. 6.

Table 3 A statistic of the radiological hazard indexes for the research area and the contrast area
Full size table
Fig. 6
figure 6

The contour plots of the radiological hazard indexes

Full size image

Dr is observed with values range from 82.0 nGy h−1 (S02) to 2289.1 nGy h−1 (S23) with an average of 328.1nGy h−1 and a standard deviation of 498.9nGy h−1. The results are higher than the national average of 62.8nGy h−1 [19]. Note Dr in the south-east area is higher, the higher risk in the region should be concerned, which was attributed mainly to the radionuclides of U-series. Take site S23 as an example, 96.7% of the values of Dr was contributed from 226Ra, while only 1.9% and 1.4% of Dr were contributed from 232Th and 40K respectively.

The results for the Aex were calculated with values varied from 0.01 mSv a−1 to 0.32 mSv a−1 with an average of 0.05 mSv a−1 and a standard deviation of 0.07 mSv a−1. The corresponding average is lower than the world average annual effective dose from outdoor terrestrial gamma radiation (0.460 mSv a−1) [14]. It indicates that the radiation hazard is insignificant for the local residual public concluded from the assessment of Aex, as only the 200 h of farming time per year were assumed in this study.

The radium equivalent activity (Raeq) provides a basis for comparing the activity concentrations of 226Ra, 232Th, and 40K in soil to obtain the total radioactivity. The calculated values could be found varying from 177.6 Bq kg−1 to 4953.1 Bq kg−1 with an average of 710.4 Bq kg−1 and a standard deviation of 1079.0 Bq kg−1. 60% of the values in the research area (15 sites) are higher than the maximum permissible values of 370 Bq kg−1, while all the values in the contrast area are lower than the corresponding limit. As the annual effective dose fro Raeq value of 370 Bq kg−1 corresponds to an effective dose of 1.0 mSv for public individuals, the observed maximum value of Raeq of 4953 Bq kg−1 (S23) corresponds to an annual effective dose of about 13.4 mSv a−1, and the soils should be restricted for building materials to avoid radiological health hazards.

The external hazard index (Hex)could be observed varying from 0.5 to 1.8 with an average of 0.8 and a standard deviation of 0.4. The acceptable value of 1.0 corresponds to 1.5 mSv a−1 for the radiological health hazards for its usage as building raw material. It could be observed that 40% of the values (10 sites) from the research area are higher than 1.0. The internal hazard index (Hin) could be observed varying from 0.6 to 3.2 with an average of 1.2 and a standard deviation of 0.8, and 60% of the values (15 sites) could be identified to be higher than the limit of 1.0. It could be observed that all the external exposure and internal exposure indexes in the contrast area are lower than the limit of 1.0, and the corresponding hazard could be concluded as acceptable.

Overall, the higher radiological hazard in the research area could be identified compared to the contrast, especially in the south-east region of the research area. The area for Raeq values above 370 Bq kg−1, for Hex and Hin values above unity, is all about 3 hectares.

The estimation of the impact of irrigation

According to the previous description, the results of 238U and total uranium in the mine pit-water are in good agreement. A conservative model with the activity concentrations of 238U was used to estimate the cumulative impact of irrigation water on the paddy fields assuming that all the radionuclides in the irrigation water were deposited in the surface soil. Estimating the deposition flux of the radionuclide 238U is based on irrigation water consumption:

$$ M = C_{\text{U}} \cdot V/666.7 $$
(9)

where M is the deposition flux from the irrigation water (Bq m−2·a−1); CU is the activity concentration of 238U in the pit-water (Bq L−1); V is the irrigation water consumption (m3); 666.7 is the conversion factor between Chinese area unit of mu and square meters. The activity concentration of 238U in the mine pit-water (CU) is 140 mBq L−1 (i.e. 140 Bq m−3). The water consumption of 500 m3 mu−1 in the research area was assumed [25].

According to estimates, the irrigation deposition flux through the pit-water is about 105 Bq m−2·a−1. Considering that the typical tillage depth of the paddy soil is within 20 cm, the 238U radioactivity deposited in the soil due to the irrigation can be estimated by the following equation:

$$ C_{\text{s}} = M/\rho d $$
(10)

where Cs is the annual radioactivity level deposited in the soil (Bq kg−1·a−1); ρ is the density of paddy soil (kg m−3) with a typical value of about 2500 kg m−3; d is the tillage depth, assuming about 0.02 m.

It can be estimated that if the mine pit-water was used for irrigation every year, the annual deposited 238U amount in the paddy soil is about 2 Bq kg−1. This level is very small compared to the level of 238U activity concentration monitored in paddy soil. So, the higher values of 238U and 226Ra in the surface paddy soil of the research area maybe are correlated with other significant issues such as the disturbance of the land surface during mining, the temporary storage of ores on-site, dewatering of mine workings, and the other reclamation activities that all have potential to significantly affect the concentrations and loads of dissolved and suspended materials in surface water off-site [4]. So, further investigation for the source of the radioactivity in the research area should be carried out based on retrospective examinations.

Conclusion

In this paper, the natural radionuclides concentrations and the radiological hazards, as well as the distribution characteristics around a decommissioned uranium mine in eastern China were investigated. It was shown that the total uranium concentration in the mine pit-water is about 116 times higher than that in the contrast reservoir, and the activity concentrations of 238U and 226Ra are higher levels in the south-east region of the research area and north shore side of the mine pit. The maximum activity concentrations of 238U and 226Ra were observed as 5600 Bq kg−1 and 4791 Bq kg−1, respectively. It was estimated that a paddy field area about 3 hectares for 238U and 226Ra above 1000 Bq kg−1 or an area with high radiological hazard in the research area could be identified.

The profile distributions analysis of the selected sites showed that the activity concentration of 238U and 226Ra presents the characteristics of higher at surface layer and lower at the bottom layer, which may be related to the geochemical conditions in the region and the deposition of radioactive materials during uranium mining. Based on conservative estimates, the cumulative radioactive effect of mine water irrigation on the paddy field is considered relatively limited.

The study would provide a scientific reference and decision basis for the concerns about the radiological impact on the local public individuals and local government. Further research on the source of the elevated concentration of 238U and 226Ra in the paddy soil should be carried in the future.