SNPP is located in Sanmen County, Taizhou City, Zhejiang Province, China, which is 171 km to the north of Hangzhou City, 83 km to the east of Ningbo City, 51 km to the west of Taizhou City, and 150 km to the south of Wenzhou City. During 2011–2020, all samples were collected within 30 km of the SNPP.

Sampling sites of surface water, factory water and tap water were selected in the distance of 5–10, 10–20, 20–30 km from SNPP based on the water source distribution and water supply characteristics of Sanmen County. The reservoir located 10.2 km away from SNPP is one of the main water sources for local residents, which was set as surface water sampling site. Sanmen County has a complete water supply system, and the tap water comes from the municipal water supply company. Factory water sampling site was set at the water plant 6.0 km away from SNPP, and tap water sampling site was set at the Sanmen County Center for Disease Control and Prevention, which is 22.7 km away from SNPP. In order to reflect the time trend and seasonal changes, water samples were collected in the wet season (May) and dry season (October) in each year.

According to the dietary habits of local residents, four typical types of food were selected in this work: rice, cabbage, crucian carp and mullet, which were collected in the harvest season every year. The sampling information of water and food samples in this work are summarized in Figure 1 and Table 1.

The prevailing wind direction of SNPP region was north and north-northwest (16). In this work, all sampling sites were located in the south or southwest of SNPP (except for tap water sampling site, located in the west of SNPP), which were located in the leeward direction of the prevailing wind direction. In addition, tap water was transported through pipelines, the influence of wind direction was considered negligible. Therefore, we believe the sampling sites in this work were representative.

Anhydrous ethanol (CH₃CH₂OH) (Analytical grade, Anhui Antell Food Co., LTD.), acetic acid (CH₃COOH) (Analytical grade, Tianjin Damao Chemical Reagent factory), sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium sulfide (Na₂S), nitric acid (HNO₃), ammonium phosphomolybdate [(NH₄)₃PMo₁₂O₄₀], hydrogen peroxide (H₂O₂) (Analytical grade, Sinopharm Chemical Reagent Co., LTD.), ammonia (NH₃·H₂O) [Analytical grade, Aladdin Reagent (Shanghai) Co., LTD], 2- (2-ethylhexyl) phosphoric acid (Analytical grade, 60~80 mesh, Beijing Research Institute of chemical Engineering Metallurgy). ⁹⁰Sr-⁹⁰Y standard solution (9.78 Bq/g of ⁹⁰Sr in 0.1 mol/L nitric acid) and ¹³⁷Cs standard solution (1.47 Bq/g of ¹³⁷Cs in 0.1 mol/L nitric acid) were purchased from National Institute of Metrology, China. ⁹⁰Sr-⁹⁰Y reference source was purchased from Beijing Research Institute of Chemical Engineering Metallurgy, with source intensity (surface particle number/2π·min) of 1.20 × 10³.

After cleaning (washing the sample with water), each fresh food sample was weighted and loaded into a microwave furnace for drying, carbonization and ashing under programmed gradient heating conditions (17). The detailed microwave heating conditions for different types of samples are shown in Table 2. The schematic radioanalytical procedures used in this work for ⁹⁰Sr and ¹³⁷Cs determination are demonstrated in Figures 2, 3, respectively.

To analyze ⁹⁰Sr concentrations in water, 30 L of water sample was taken and adjusted to pH = 1.0 with HNO₃. After adding 2.0 mL of 50 mg/mL strontium carrier and 1.0 mL of 20 mg/mL yttrium carrier, the pH was adjusted to 8.0 with addition of NH₃·H₂O. 240 g of (NH₄)₂CO₃ were added to form Ca(Sr)CO₃ precipitate, which was separated by centrifugation (twice at 4000 r/min for 5 min). The precipitate was washed with 1% (NH₄)₂CO₃ solution and dissolved in 6 mol/L of HNO₃. After adjusting the pH to 1.5 with NH₃·H₂O, the solution was loaded onto a chromatographic column (1.0 cm inner diameter × 14 cm length) packed with di-(2-ethylhexyl) phosphate (HEDHP) at a flow rate of 2 ml/min. The column was sequentially washed with 10 ml of 1.5 mol/L HNO₃, 30 ml of 1.0 mol/L HCl and 30 ml of 1.5 mol/L HNO₃ at 2 ml/min. Yttrium (Y) was eluted with 30 ml of 6 mol/L HNO₃ at 1 ml/min, the eluate was collected. The starting and ending times of the column separation were recorded for decay correction of ⁹⁰Y. 5 mL of saturated H₂C₂O₄ solution was added into the eluate. The sample solution was adjusted to pH = 2.0 with ammonia, and heated in a water bath at 80 °C for 30 min to precipitate Y as yttrium oxalate Y₂(C₂O₄)₃, then cooled to room temperature. Yttrium oxalate precipitate was filtered through filter paper and washed successively with 10 mL of 1% H₂C₂O₄, 10 mL of water and 10 mL of CH₃CH₂OH, then dried at 45–50°C until the weight was constant. The chemical recovery of Y was calculated using the weight of obtained yttrium oxalate according to the molecular formula of Y₂(C₂O₄)₃·9H₂O. The yttrium oxalate precipitate was detected by low-background α-β counter (BH1217II, China National Nuclear Corporation, China; LB790, Berthold Technologies, Germany), with each sample counted for 10 cycles and 100 min. for each cycle (18, 19).

To analyze ⁹⁰Sr concentrations in food, 10g of ash sample was spiked with 2.0 mL of 50 mg/mL strontium carrier and 1.0 mL of 20 mg/mL yttrium carrier. After adding 10 mL of concentrated HNO₃ and 6 mL of H₂O₂, the sample was evaporated to dryness and ashed at 600 °C until a complete decomposition of organic substance. The sample was digested twice with 40 mL of 2 mol/L HCl solution by heating on a hotplate. The leachate was filtered and combined to proceed with Ca(Sr)C₂O₄ precipitation for Sr pre-concentration. The precipitate was ashed in a muffle furnace at 800°C for 1h, dissolved with a few milliliter of 6 mol/L HNO₃. 35 mL of 1 mol/L HNO₃ was added to completely dissolve the precipitate. After the addition of 0.5 mL of bismuth carrier (20 mg/mL), 0.4 mL of 0.3mol/L Na₂S solution was dropped to generate black Bi₂S₃ precipitate to removal of interfering ions such as ²¹⁰Bi and rare earth elements. The solution was filtered with filter paper and the filtrate (about 60 mL) was collected for further chromatographic purification using HDEHP as described above (19, 20).

To analyze ¹³⁷Cs concentrations in water, 30 L of water sample was adjusted to pH < 3.0 with HNO₃. 1.0 mL of 20 mg/mL cesium carrier and 6 g of (NH₄)₃PMo₁₂O₄₀ were added to the sample, which was stirred by electric mixer, then kept still for the settling of the precipitate. The supernatant was discarded by siphon, and the remaining precipitate was centrifuged at 3500 r/min for 5 min to remove the liquid phase. The precipitate was dissolved in 60 mL of 2.0 mol/L NaOH, 10 g of solid citric acid and 10 ml of concentrated HNO₃ were added. After adding 0.8 g of (NH₄)₃PMo₁₂O₄₀, the sample was filtrated to separate the (NH₄)₃PMo₁₂O₄₀ on which cesium was adsorbed. 10 mL of 2.0 mol/L NaOH were added to dissolve (NH₄)₃PMo₁₂O₄₀ and the solution was filtered again. 5 ml of sodium citrate solution (mass ratio 30%) was added, followed by addition of 2 ml of glacial CH₃COOH and 2.5 ml of bismuth sodium iodide solution (mass ratio 42%). The sample was cooled in an ice water bath to finally precipitate cesium as Cs₃Bi₂I₉. Cs₃Bi₂I₉ precipitate was dried and weighed to obtain the chemical yield of Cs, and then detected by low-background α-β counter (BH1217II, China National Nuclear Corporation, China; LB790, Berthold Technologies, Germany). Each sample was counted for 10 cycles with 100 min. for each cycle (21, 22).

To analyze ¹³⁷Cs concentrations in food, samples were stored for 1 month to allow ¹³¹I to decay completely. Each dried food sample was screened by gamma spectrometry to eliminate interferences to the ¹³⁷Cs measurements from other short-lived radiocesium (i.e., ¹³⁴Cs, ¹³⁶Cs, and ¹³⁸Cs). To 10 g of ash sample, 1.0 mL of 20 mg/mL cesium carrier, 10 mL of concentrated HNO₃ and 3 mL of H₂O₂ were added. After evaporating to dryness, the sample was burned in a muffle furnace at 450°C, then repeatedly leached with 1.5 mol/L HNO₃. 0.8 g of (NH₄)₃PMo₁₂O₄₀ were added to the leachate and the sample was thereafter processed following the same procedure as described above (22, 23).

The main factors affecting the uncertainty of the measurement results of ⁹⁰Sr and ¹³⁷Cs in water and food samples were the uncertainties of β radioactivity measurement (counting statistics), instrument detection efficiency, chemical yield and sample quality. The typical relatively uncertainties for ⁹⁰Sr and ¹³⁷Cs activity concentrations obtained in this work were at the level of 5–27%.

In order to carry out dose assessment for residents around SNPP, Zhejiang Center for Disease Control and Prevention conducted a food consumption survey of residents in Taizhou, Zhejiang Province from 2015 to 2017. Three counties in Taizhou city, Zhejiang province were chosen for this survey, using multi-stage stratified proportional to population cluster random sampling method, i.e., each site from three villages and towns (streets), each street draw two villages (neighborhood committees), every village (neighborhood committees) extract 50 families, the permanent members of each selected family (resident for at least 6 months, resident aged 3 years and above) were identified as the respondents after signing an “informed consent”.

The recorded data of the consumption frequency and average consumption amount of rice, cabbage, crucian carp, mullet and related products were collected by face-to-face survey (using the method of 24 h retrospective for 3 consecutive days). The questionnaire was based on the recommendations from China National Center for Food Safety Risk Assessment. The dietary information of all respondents under the age of 18 was provided by their guardians. According to the survey results, the average consumption of four types of food in Taizhou city, Zhejiang Province was calculated for people of different ages (grouping criteria: 3–12 years old, 12–18 years old and more than 18 years old).

The annual effective dose (AED) due to the ingestion of ¹³⁷Cs and ⁹⁰Sr in food was estimated using the following equation:

Where, I_Sr or I_Cs is the annual intake of ⁹⁰Sr or ¹³⁷Cs (Bq/year) from food. e(g)_Sr or e(g)_Cs is the age-dependent dose conversion coefficient for ingestion of ⁹⁰Sr or ¹³⁷Cs. The corresponding e(g)_Sr and e(g)_Cs values used in this study were according to Chinese national standards (24), namely, 6.0 × 10⁻⁸ Sv/Bq and 1.0 × 10⁻⁸ Sv/Bq, 8.0 × 10⁻⁸ Sv/Bq and 1.3 × 10⁻⁸ Sv/Bq, 2.8 × 10⁻⁸ Sv/Bq and 1.3 × 10⁻⁸ Sv/Bq, for people with age of 3~12 years, 12~18 years and more than 18 years, respectively.

SPSS 25.0 software was used for statistical analysis, and Mann–Whitney U-test was used to compare the radioactive activity concentration levels of water. p < 0.05 was considered as statistically significant.

The time series of ⁹⁰Sr and ¹³⁷Cs activity concentrations in water samples collected around SNPP from 2011 to 2020 are shown in Figure 4. Statistical analysis indicated that, ⁹⁰Sr and ¹³⁷Cs activity concentrations in all types of water had no significant difference between the wet season and dry season in each year (p > 0.05). The monitoring results of ⁹⁰Sr and ¹³⁷Cs vary within ranges of (2.8 ± 0.2)–(9.9 ± 0.8) mBq/L and (0.19 ± 0.05)–(7.6 ± 0.4) mBq/L, (2.7 ± 0.3)–(8.6 ± 0.6) mBq/L and (0.30 ± 0.02)–(7.0 ± 0.3) mBq/L, (1.2 ± 0.2)–(8.3 ± 0.5) mBq/L and (0.10 ± 0.02)–(5.1 ± 0.2) mBq/L, for surface water, factory water and tap water, respectively. Compared to the concentration limits recommended by WHO (10 Bq/L for ⁹⁰Sr and 10 Bq/L for ¹³⁷Cs), the activity concentrations of ⁹⁰Sr and ¹³⁷Cs in water samples around SNPP from 2011 to 2020 were 3-4 orders of magnitude lower (25).

The annual mean activity concentrations of ⁹⁰Sr in all three types of waters showed similar temporal changes, namely, notable increases are observed during 2016–2020 from relatively low and stable levels in 2011–2015 (Figure 4A). Different from ⁹⁰Sr, activity concentrations of ¹³⁷Cs in the three types of waters were more fluctuated during 2011–2020 (Figure 4B). For example, the difference between the maximum and minimum annual mean activity concentrations of ¹³⁷Cs reached 3.4 times in factory water, and more than 9 times in tap water and surface water. In the case for ⁹⁰Sr, the difference was <2.9 times. This might be related to the higher solubility and mobility of Sr in water, whereas Cs is more particle reactive in the natural environment (26). The highest ¹³⁷Cs annual mean activity concentrations were seen in 2016 for both surface water and tap water.

Compared with tap water, the annual mean activity concentrations of ⁹⁰Sr in surface water were generally higher (Figure 4A). Statistical analysis also confirmed this difference (p < 0.05). This phenomena was consistent with our previous findings (27), which should be related to additional water treatment processes (e.g. coagulation, filtration and precipitation) from surface water to tap water.

Table 3 showed the activity concentrations of ⁹⁰Sr and ¹³⁷Cs in waters from different regions of the world. The values of waters around SNPP were at background levels and at the same order of magnitude as radioactivity levels in different waters around the globe (11, 27–37).

Figure 5 shows the activity concentrations of ⁹⁰Sr and ¹³⁷Cs in different types of food samples around SNPP from 2011 to 2020. The variations in activity concentrations of ⁹⁰Sr and ¹³⁷Cs in all four types of food showed no significant inter-annual trend. The activity concentrations of ⁹⁰Sr and ¹³⁷Cs in rice, cabbage, crucian carp and mullet were (0.037 ± 0.004)–(0.075 ± 0.004) Bq/kg fresh weight (f. w.) and (0.020 ± 0.003)–(0.081 ± 0.008) Bq/kg f. w., (0.065 ± 0.005) – (0.37 ± 0.02) Bq/kg f. w. and (0.011 ± 0.002)–(0.14 ± 0.01) Bq/kg f. w., (0.34 ± 0.04)–(1.3 ± 0.2) Bq/kg f. w. and (0.13 ± 0.02)–(0.45 ± 0.02) Bq/kg f. w., (0.21 ± 0.03)–(0.88 ± 0.08) Bq/kg f. w. and (0.14 ± 0.02)–(0.31± 0.04) Bq/kg f. w., respectively (Table 4). Comparing to the limits of ⁹⁰Sr and ¹³⁷Cs activity concentrations in grain (96 and 260 Bq/kg), vegetables (77 and 210 Bq/kg) and fish (290 and 800 Bq/kg) recommended by Chinese national standard (38), the activity concentrations of ⁹⁰Sr and ¹³⁷Cs in food samples around SNPP were significantly lower.

The activity concentrations of ⁹⁰Sr varied greatly with food species (Figure 5A), with mean activity concentrations over the 10 years generally decreasing following the order of rice (0.055 ± 0.012 Bq/kg f. w.) < cabbage (0.19 ± 0.01 Bq/kg f. w.) < mullet (0.50 ± 0.22 Bq/kg f. w.) < crucian carp (0.73 ± 0.32 Bq/kg f. w.) (Table 4). In contrast to rice, other three types of food showed significant inter-annual variations in activity concentrations of ⁹⁰Sr during 2011–2020. The difference between maximum and minimum of ⁹⁰Sr activity concentrations was 5.7, 3.8, and 4.2 times in cabbage, crucian carp and mullet, respectively. The highest ⁹⁰Sr values were both found in 2016 for cabbage and crucian carp, and in 2011 for mullet.

Different from ⁹⁰Sr, the mean activity concentrations of ¹³⁷Cs were comparable between rice (0.050 ± 0.018 Bq/kg f. w.) and cabbage (0.041 ± 0.039 Bq/kg f. w.), and between crucian carp (0.21 ± 0.10 Bq/Kg f. w.) and mullet (0.21 ± 0.07 Bq/Kg f. w.), with much lower levels in the former pair than the latter. These characteristics might be related to the different environmental conditions for the growth of these biota and different physiological metabolisms for ⁹⁰Sr and ¹³⁷Cs uptake and excretion among different biological species (4, 5).

Table 5 lists the activity concentrations of ⁹⁰Sr and ¹³⁷Cs in foods from different regions in the world. The values obtained in this study were at the same order of magnitude as ¹³⁷Cs and ⁹⁰Sr activity concentrations in food samples from other areas reported earlier (27, 29, 36, 39–49).

Figure 6 shows the time series diagram of ¹³⁷Cs/⁹⁰Sr activity ratios in the water and food samples investigated from 2011 to 2020. The activity ratios of ¹³⁷Cs/⁹⁰Sr in surface water, factory water and tap water were in the ranges of (0.020 ± 0.004)–(1.55 ± 0.12), (0.035 ± 0.001)–(2.05 ± 0.13) and (0.014 ± 0.002)–(1.57 ± 0.04), respectively. The activity ratios of ¹³⁷Cs/⁹⁰Sr in rice, cabbage, crucian carp and mullet ranged within (0.28 ± 0.03)–(1.94 ± 0.03), (0.031 ± 0.003)–(2.13 ± 0.13), (0.12 ± 0.006)–(0.71 ± 0.03) and (0.23 ± 0.02)–(0.92 ± 0.11), respectively (Table 4).

The main pollution source of ⁹⁰Sr and ¹³⁷Cs in surface environment would be the global fallout from the atmospheric nuclear weapon tests conducted in 1950–1980s (50, 51). In the study region, in addition to the contamination caused by global fallout, there was also the possibility of radioactive fallout from the operation of local and regional nuclear power plants, e.g., SNPP and Qinshan NPP. Other pollution source, e.g., Chernobyl nuclear accident, was deemed to have contributed little to the quantity of ⁹⁰Sr and ¹³⁷Cs in the western North Pacific (52, 53). In addition, there were no evidences showing that Fukushima nuclear accident has significant impact on China. So far, no significant release of radioactivity has been reported from SNPP and Qinshan NPP. Therefore, ⁹⁰Sr and ¹³⁷Cs detected in water and food around SNPP should reflect the global radioactive fallout levels.

¹³⁷Cs/⁹⁰Sr activity ratio has been used in order to obtain evidence to clarify the origin of these radionuclides (54, 55). The activity ratio of ¹³⁷Cs/⁹⁰Sr from the global fallout deposition was estimated to be about 1.6 (51). Much higher ¹³⁷Cs/⁹⁰Sr activity ratios have been reported for the local fallout from the Chernobyl (up to 250) (56) and Fukushima (up to 1000) (2, 57) nuclear accidents. Our results showed that the ¹³⁷Cs/⁹⁰Sr activity ratios in water and food around SNPP were lower than the global fallout signal, except for individual samples. This may be related to the fact that ⁹⁰Sr deposited in the soil is easier to migrate to crops and water, because of higher solubility and mobility of ⁹⁰Sr in the surface environment (58). In our previous studies,the ¹³⁷Cs/⁹⁰Sr activity ratios in food and water samples form Qinshan NPP (China) (water: <1.2, food: <1.2) (27) and Hangzhou [water: <1.5(except for individual samples), food: <0.72] (44) were similar to this study, both lower than the global fallout signal.

In this study, the highest activity ratio in surface water occurred in wet season of 2011 (1.55 ± 0.12), which was 5 times higher than that in dry season (0.31 ± 0.02) in the same year, and 2.7 times higher than that in wet season of 2012 (0.58 ± 0.02). This might indicate additional radioactive input in the study area during the wet season of 2011. In addition, the maximum activity ratio of cabbage and rice appeared in 2011 and 2012 respectively, which further proved the possibility of our speculation (Figure 4). However, more in-depth investigation would be needed to clarify this potential source input.

In addition, after the operation of SNPP in 2018, activity ratios of ¹³⁷Cs/⁹⁰Sr in either food or water samples during 2018–2020 were not increased compared to earlier year. This indicates that radioactive substances released during the operation of SNPP in 2018–2020 were negligible.

According to the annual average food intake of Sanmen County residents (Located in Taizhou City), as shown in Table 6, the mean AED values from 2011 to 2020 were calculated to be 7.2 × 10⁻⁷, 1.3 × 10⁻⁶ and 6.8 × 10⁻⁷ Sv/y, respectively, for people of 3-12 years old, 12-18 years old and more than 18 years old. They were significantly lower than the reference value recommended by Chinese national standards (7.84 × 10⁻⁴ Sv/y) (38), indicating that radioactivity concentrations of ⁹⁰Sr and ¹³⁷Cs in food from Sanmen County were at safe levels from 2011 to 2020.