3.1 Source of 210Pb and 210Po in the Dongshan Bay coastal zone groundwater
210Po, 210Pb, 222Rn, and 226Ra have been detected in all the coastal zone groundwater samples and the specific results are shown in Table 1 and Fig. 2. The groundwaters in the Dongshan Bay coastal zone have 210Po, 210Pb, 222Rn, and 226Ra activities ranging from 0.09 ± 0.01 to 1.09 ± 0.14 Bq/m3 (Fig. 2a) with a mean value (Mean ± SD) of 0.35 ± 0.29 Bq/m3, ranging from 0.41 ± 0.03 to 6.28 ± 0.25 Bq/m3 (Fig. 2b) with a mean value of 2.09 ± 1.84 Bq/m3, ranging from 1180 ± 530 to 177000 ± 2200 Bq/m3 (Fig. 2c) with a mean value of 44720 ± 54387 Bq/m3, and ranging from 2.43 ± 0.28 to 93.7 ± 0.9 Bq/m3 (Fig. 2d) with a mean value of 31.2 ± 27.0 Bq/m3, respectively. Obviously, from Fig. 3, we ca see that the radioactivity levels of these four radionuclides in groundwater meet the following order of size: 210Po < 210Pb < 226Ra < 222Rn. The activity concentrations of 210Po and 210Pb in the coastal zone groundwater samples of Dongshan Bay were not exceeding the US EPA safety thresholds for 210Po (26 Bq/m3) and for 210Pb (37 Bq/m3). A similar work carried out in Guangxi province (China) coastal regions by Zhong et al. (2020) reported that the 210Po, 210Pb, and 222Rn activities in groundwater samples of the Beibu Gulf-Guangxi coastal zone ranged from 0.24 to 6.96 (Mean ± SD: 2.23 ± 1.97) Bq/m3, from 2.17 to 13.08 (Mean ± SD: 6.29 ± 3.29) Bq/m3, and from 1500 to 31800 (Mean ± SD: 10700 ± 9000) Bq/m3, respectively. Comparing the results from Beibu Gulf and Dongshan Bay, we can see that activity concentrations of 210Po and 210Pb in the groundwaters of Beibu Gulf-Guangxi coastal zone are 6.4 and 3.0 times that of the Dongshan Bay coastal zone, respectively, however, the activity concentration of 222Rn in the groundwater of Dongshan Bay is 4.2 times that of the Beibu Gulf-Guangxi coastal zone. These results indicated that the groundwater in Dongshan Bay is characterized by a background of low 210Pb and high 222Rn activity concentration. The reason for the low 210Pb feature in groundwater environment of Dongshan Bay might be related to the strong scavenging of particle-reactive materials by aquifer solids.
Figure 4 shows the relationships between 210Pb and 210Po and between 222Rn and 210Pb in the Dongshan Bay coastal zone groundwaters. There is a strong positive correlation between 210Pb and 210Po activities in groundwaters (R2 = 0.51, Fig. 4a), which is consistent with the finding in the coastal zone groundwaters of the Beibu Gulf (210Po vs. 210Pb, R2 = 0.67, Zhong et al., 2020). This positive correlation indicates that 210Po is supported by 210Pb and the decay of 210Pb in the water from these wells is sufficient to account for the amount of 210Po present. From Fig. 4b, we can see that 222Rn showed a very strong correlation with 210Pb (R2 = 0.89). Similarly, we also found that a good relationship (R2 = 0.66) between 226Ra and 222Rn occurred in the coastal zone groundwater (see Fig. 5), but 222Rn activity was significantly higher than that of 226Ra (Fig. 2c and d), hence this relationship indicated that accumulation of 222Rn in groundwater was partly related to 226Ra in the Dongshan Bay coastal zone groundwater. If we fitted the relationship between 222Rn and 226Ra, we can found that the slope of the fitted curve could be much much greater than unity (Fig. 5), which indicated a great excess of 222Rn relative to 226Ra (see Table 1, 222Rn/226Ra=1887 ± 2923). However, the slope (0.11) of the fitting line between 210Pb and 210Po was much lower than unity, indicating that the occurrence of a strong 210Po-210Pb disequilibrium (exactly, deficiency of 210Po relative to 210Pb) in the Dongshan Bay coastal zone groundwaters. The occurrence of 210Po deficiency in the coastal zone groundwater should be caused by the preferential removal of 210Po, because the residence times of groundwater are generally up to several years or decades, if there is no scavenging of 210Po, a secular equilibrium between 210Po and 210Pb would be created. Interestingly, the slope (0.11) of the fitting line of the Dongshan Bay is much lower than that of the Beibu Gulf (0.35), which indicated that deficiency of 210Po in the Dongshan Bay coastal zone groundwater is more serious than that of the Beibu Gulf coastal zone. Another evidence is the 210Po/210Pb activity ratio. From Table 1 and Fig. 4a, we can see that 210Po/210Pb activity ratio ranged largely from 0.09 to 0.93 and the mean value of 210Po/210Pb activity ratio is 0.24 ± 0.23 in the Dongshan Bay, which is lower than the mean value of 210Po/210Pb activity ratio (mean ± SD: 0.348 ± 0.210, Zhong et al., 2020) in the Beibu Gulf. This phenomenon confirms again the occurrence of 210Po deficiency in the China coastal zone groundwater, and besides, there is spatial difference of 210Po deficit in China coastal zone. However, the reason for this difference is still unknown, and a further investigation is needed.
As we mentioned above, there existed a strong relationship between 222Rn and 210Pb (Fig. 4b) and this strong positive correlation between 222Rn and 210Pb indicates that the origin of 210Pb in groundwater is strongly controlled by the decay of parent 222Rn. In some cases, researchers also found that 210Pb correlated positively with the concentration of 222Rn in groundwaters (Seiler et al., 2011; Dickson et al., 1992). However, some other researchers also observed that 210Pb did not correlate with the concentration of 222Rn in groundwater (Burnett et al., 1987; Seiler, 2011; Bonotto and Caprioglio, 2002; Bonotto et al., 2009), and these studies highlighted that elevated 222Rn activities are not necessary for there to be elevated 210Pb activities. It seems that correlation between 210Pb and 222Rn are greatly variable depending upon the aquifer property. The 210Pb/222Rn activity ratios in groundwater samples are < < 1 (from 3.0×10− 5 to 86×10− 5, mean: (9.2 ± 2.4)×10− 5, see Table 1), which shows a strong disequilibrium between 210Pb and 222Rn in the Dongshan Bay coastal zone groundwater. The large deficiencies of 210Pb relative to 222Rn in Guangxi coastal zone groundwaters indicated that 210Pb was also strongly scavenged by particles in the groundwater-aquifer systems due to the particle affinity of 210Pb.
Table 1
Concentrations of 210Po, 210Pb, 222Rn and 226Ra in the groundwaters of Dongshan Bay coastal zone.
Station | Longitude | Latitude | Salinity | 226Ra (Bq/m3) | 222Rn (Bq/m3) | 210Pb (Bq/m3) | 210Po (Bq/m3) | 210Po/210Pb | 210Pb/222Rn | 222Rn/226Ra |
G-1 | 117.4134 | 23.9238 | 0.2 | 33.2 ± 0.7 | 40600 ± 1100 | 1.25 ± 0.09 | 0.11 ± 0.01 | 0.09 ± 0.01 | (3.1 ± 0.2)×10− 5 | 1223 ± 42 |
G-2 | 117.4631 | 23.8130 | 0.6 | 47.6 ± 2.4 | 34200 ± 900 | 2.38 ± 0.11 | 0.38 ± 0.04 | 0.16 ± 0.02 | (7.0 ± 0.4)×10− 5 | 718 ± 41 |
G-3 | 117.4134 | 23.9238 | 0.2 | 68.7 ± 0.7 | 58200 ± 1200 | 2.61 ± 0.19 | 0.27 ± 0.03 | 0.10 ± 0.01 | (4.5 ± 0.3)×10− 5 | 847 ± 19 |
G-4 | 117.5330 | 23.9260 | 0.3 | 15.6 ± 0.8 | 177000 ± 2200 | 6.28 ± 0.25 | 0.66 ± 0.07 | 0.10 ± 0.01 | (3.5 ± 0.2)×10− 5 | 11346 ± 599 |
G-5 | 117.5340 | 23.9260 | 0.5 | 19.4 ± 0.7 | 1180 ± 530 | 1.02 ± 0.06 | 0.26 ± 0.02 | 0.25 ± 0.03 | (86 ± 39)×10− 5 | 61 ± 27 |
G-6 | 117.6100 | 23.8823 | 0.5 | 11.8 ± 0.5 | 3390 ± 290 | 0.45 ± 0.04 | 0.17 ± 0.02 | 0.38 ± 0.06 | (13.4 ± 1.7)×10− 5 | 287 ± 27 |
G-7 | 117.6108 | 23.8926 | 0.3 | 2.43 ± 0.28 | 2310 ± 240 | 0.41 ± 0.03 | 0.12 ± 0.02 | 0.30 ± 0.06 | (17.7 ± 2.2)×10− 5 | 951 ± 147 |
G-8 | 117.5526 | 23.9108 | 29.9 | 32.6 ± 0.7 | 82300 ± 1500 | 4.61 ± 0.23 | 1.09 ± 0.14 | 0.24 ± 0.03 | (5.6 ± 0.3)×10− 5 | 2525 ± 71 |
G-9 | 117.5120 | 23.7450 | 0.3 | 93.7 ± 0.9 | 109000 ± 1700 | 3.23 ± 0.19 | 0.37 ± 0.04 | 0.11 ± 0.01 | (3.0 ± 0.2)×10− 5 | 1163 ± 21 |
G-10 | 117.5361 | 23.9269 | 0.5 | 5.05 ± 0.34 | 7550 ± 390 | 0.61 ± 0.07 | 0.57 ± 0.06 | 0.93 ± 0.14 | (8.1 ± 1.0)×10− 5 | 1495 ± 127 |
G-11 | 117.4117 | 23.9253 | 0.6 | 40.9 ± 0.5 | 15630 ± 790 | 1.65 ± 0.10 | 0.14 ± 0.01 | 0.09 ± 0.01 | (10.6 ± 0.8)×10− 5 | 382 ± 20 |
G-12 | 117.5360 | 23.9265 | 0.5 | 3.21 ± 0.18 | 5280 ± 300 | 0.63 ± 0.06 | 0.09 ± 0.01 | 0.14 ± 0.02 | (11.9 ± 1.3)×10− 5 | 1645 ± 131 |
Min | | | | 2.43 | 1180 | 0.41 | 0.09 | 0.09 | 3.0×10− 5 | 61 |
Max | | | | 93.7 | 177000 | 6.28 | 1.09 | 0.93 | 86×10− 5 | 11346 |
Mean | | | | 31.2 | 44720 | 2.09 | 0.35 | 0.24 | 9.2×10− 5 | 1887 |
SD | | | | 27.0 | 54387 | 1.84 | 0.29 | 0.23 | 2.4×10− 4 | 2923 |
3.2 Evaluation of potential radiological hazard for groundwater digestion by local residents
The surrounding area of Dongshan Bay is a mixed coastal zone of agriculture and industry, the demand of fresh groundwater is still extensive in local villages. As mentioned before, although compared with the activity levels of 210Pb and 210Po in the groundwater of the coastal zone of Beibu Gulf-Guangxi, the activity levels of Rn-daughters in the coastal zone of Dongshan Bay in Fujian are relatively low. However, there are still potential radiation risks to local residents after drinking local groundwater. Considering the volatility of radon, the dissolved 222Rn is very easy to escape from the groundwater, so the radiation dose of 222Rn caused by drinking groundwater is not considered in this work. Based on the results of 226Ra, 210Pb and 210Po activity concentrations in different fresh groundwater samples (except for G-8) displayed in Table 1, the total annual effective ingestion dose of these three radionuclides to the local residents can be estimated as follow:
$${E}_{i}={A}_{i}\times q\times {e\left(g\right)}_{i, ingestion}$$
1
$${D}_{total}=\sum \left({E}_{i}\right)$$
2
where Ei is the annual effective ingestion dose for radionuclide i in µSv, Ai is radionuclide i’s activity concentration in the water samples in Bq/m3, q is the annual consumption amount of groundwater (approximately 500 L for adults, UNSCEAR, 2000), e(g)i,ingestion is the dose conversion coefficient of radionuclide i for adults in µSv/Bq, and Dtotal is the total annual effective ingestion dose for all mentioned radionuclides in µSv.
The annual effective ingestion doses for 210Po (EPo), 210Pb (EPb), and 226Ra (ERa) for adults of local residents are calculated and displayed in Fig. 6. The EPo ranged from 0.05 to 0.40 µSv and EPb ranged from 0.14 to 2.17 µSv (Fig. 6a). Compared with 210Po and 210Pb, annual effective ingestion dose for 226Ra (ERa) was highest, varying between 0.34 and 13.12 µSv (Fig. 6a). Similarly, the total annual effective doses (Dtotal) for all three radionuclides (210Po, 210Pb and 226Ra) ranged from 0.55 to 14.45 µSv for adults (Fig. 6b). Hence, the potential radiation doses from groundwater ingestion for local adult residents are much lower than the recommended reference dose level (RDL) of 200–800 µSv (UNSCEAR, 2000) by radionuclide ingestion for one year. In order word, our results indicate that the background radiation doses by drinking groundwater with natural occurring 226Ra and 222Rn daughters is safe from radiological point of view. Therefore, our results can provide background radioactivity and potential radiation dose of groundwater in Dongshan Bay before the formal construction and operation of Zhangzhou Nuclear Power Plant.
3.3 210Pb sources in Dongshan Bay
In addition to the consumption of coastal groundwater by agricultural cultivation and daily use by local residents, another naturally occurring process defined as submarine groundwater discharge (SGD) is also an important pathway of coastal groundwater disappearance or transport (Burnett et al., 2003; Moore, 2010; Yu et al., 2022; Zhong et al., 2022). However, this SGD process is very hidden and has been ignored for a long time. And little attention has been paid to the potential environmental impact of radioactive materials transport to the coastal seas with SGD process. One way to understand the importance of SGD in coastal bay is to quantitatively assess its relative proportion to a certain well-evaluated source such as riverine input or atmospheric input of a certain radionuclide.
Zhong et al. (2023) systematically studied the 210Po and 210Pb activities and behaviors from the upper Zhangjiang River to the outer Dongshan Bay for the same study area. Dissolved 210Po and dissolved 210Pb in the river water of the Zhangjiang River were 0.36 ± 0.04 and 0.56 ± 0.05 Bq/m3, respectively and similarly, dissolved 210Po and dissolved 210Pb in sea waters of the Dongshan Bay varied from 0.13 to 0.59 Bq/m3 (mean value: 0.33 ± 0.11 Bq/m3) and from 0.18 to 0.82 Bq/m3 (mean value: 0.46 ± 0.16 Bq/m3), respectively (Zhong et al., 2023). However, highest 210Po and 210Pb activities was found in rainwaters of the Dongshan Bay watershed, which were 122 ± 68 Bq/m3 and 430 ± 590 Bq/m3, respectively (Zhong et al., 2023). Therefore, we can compare the activity levels of 210Po and 210Pb in all available water bodies in the Zhangjiang River-Dongshan Bay region (see Fig. 7). Interestingly, 210Pb showed a significantly different activity in different waters and the order of 210Pb concentrations in these water samples is sea water < river water < groundwater < rain water, which is very similar to the finding reported by Zhong et al., (2022), however, 210Po had almost the same activity concentration in seawater, river water and groundwater apart from rain water (Fig. 7). This phenomenon implied that groundwater discharge is likely to be a potentially important source of 210Pb in the Dongshan Bay.
To confirm whether this low 210Pb background groundwater discharge is important for Dongshan Bay, we need to quantitatively assess all possible sources of 210Pb in Dongshan Bay. Possible important sources of 210Pb to the Dongshan Bay include 1) river discharge; 2) atmospheric deposition; 3) in-situ production from 226Ra; and 4) submarine groundwater discharge (SGD).
1) Riverine input: 210Pb from river input in the Dongshan Bay is estimated by multiplying the Zhangjiang River freshwater discharge with the dissolved 210Pb activity in the river fresh water endmember. In this study, dissolved 210Pb in the river water of the Zhangjiang River was 0.56 ± 0.05 Bq/m3 (Zhong et al., 2023). The freshwater discharge of the Zhangjiang River was ~ 4 m3/s (Sun et al., 2023) during our sampling period (December, 2020). Consequently, the dissolved 210Pb fluxes from Zhangjiang River input is calculated to be (1.94 ± 0.17)×105 Bq/d.
2) Atmospheric input: Zhong et al. (2023) measured the atmospheric deposition of 210Pb at an observation station in the Dongshan Bay and the atmospheric 210Pb flux was 0.07 ± 0.01 Bq/m2/d in December, 2020. Multiplying the atmospheric 210Pb deposition flux (0.07 ± 0.01 Bq/m2/d) by the total area of Dongshan Bay (240 km2) yields a total atmospheric input of (1.68 ± 0.24)×107 Bq/d.
3)226Ra production: Yu et al. (2022) reported a mean 226Ra concentration of 6.22 ± 0.30 Bq/m3 in 27 surface seawater samples from Zhangjiang Estuary to Dongshan Bay in September, 2019. Sun et al., (2023) investigated the radium isotopes activities in surface and bottom seawaters of Dongshan Bay and found that 226Ra activity concentrations varied from 2.83 to 6.50 Bq/m3 (with a mean value of 4.38 ± 1.23 Bq/m3) in May, 2020. By averaging the 226Ra results of these two study cases in Dongshan Bay, a value of 5.30 ± 0.92 Bq/m3 was used and the averaged 210Pb production rate from this quantity of 226Ra is calculated by multiplying the Dongshan Bay volume (7.69×108 m3, Sun et al., 2023), the decay constant of 210Pb (8.52×10− 5 d− 1) and the averaged 226Ra activity concentration (5.30 ± 0.92 Bq/m3, n = 49). Therefore, the in-situ production of 210Pb from mother nuclide 226Ra is only (3.47 ± 0.60)×105 Bq/d.
4) SGD flux of210Pb: Submarine groundwater discharge (SGD) is now being defined as all the water flow on the continental margin from the seabed to the coastal ocean, regardless of fluid composition or driving force, which includes fresh groundwater (FSGD) and recirculated seawater (RSGD) (Burnett et al., 2003; Moore, 2010). Based on a mass balance model of Ra isotopes (223Ra, 224Ra, 226Ra and 228Ra), Yu et al. (2022) estimated that the average Ra-derived SGD flux in December 2020 in Dongshan Bay was (4.1 ± 1.4)×107 m3/d, and in this bay, FSGD contributed approximately 13% of the total SGD, which was estimated to be approximately (5 ± 2)×106 m3/d and correspondingly, the RSGD flux was estimated to be (3.5 ± 1.2)×107 m3/d (Yu et al., 2022). Our previous work pointed that 210Pb concentrations were considerably different in fresh groundwater and recirculated groundwater (pore water) (Zhong et al., 2022), hence, SFGD-210Pb flux and RSGD-210Pb flux should be estimated separately. Luckily, we obtained the 210Pb activity concentration (4.61 ± 0.23 Bq/m3) in a saltwater well (station G8) with salinity up to 29.9, thus, this value minus the averaged seawater dissolved 210Pb concentration (0.46 ± 0.16 Bq/m3) can be used as the endmember of RSGD (4.15 ± 0.28 Bq/m3). This would result in an RSGD-derived 210Pb flux of (1.45 ± 0.51)×108 Bq/d. For FSGD-210Pb flux, the averaged 210Pb activity concentration of (1.87 ± 1.67) Bq/m3 in fresh groundwaters of the Dongshan Bay was used as the 210Pb concentration in the FSGD endmember. Then, the FSGD-derived 210Pb flux was calculated to be (9.35 ± 9.15)×106 Bq/d.
Figure 8 showed the fluxes of 210Pb sources based on the above calculation. Apparently, RSGD-210Pb flux was the largest one comparing with other four 210Pb sources. Due to the extremely low Zhangjiang River water discharge during the dry season in our observation period, the riverine input flux of 210Pb was the smallest in this study. Specifically, the Zhangjiang River contributed 210Pb flux was approximately 87 times lower than the 210Pb flux from atmospheric deposition (Fig. 8). In order words, the 210Pb contribution of Zhangjiang River is even negligible compared with other sources. As we pointed, there might exist a large uncertainty in the calculation of RSGD-210Pb, because we select the only observed salt well as the endmember in this study. However, even if we do not consider RSGD-210Pb and only focus on FSGD-210Pb, from Fig. 8, we can see that FSGD-210Pb flux was one order of magnitude higher than riverine input flux of 210Pb and 226Ra production flux of 210Pb. On the other hand, FSGD-210Pb accounted for approximately 56% of atmospheric deposition flux of 210Pb. Consequently, we concluded that although Dongshan Bay has groundwater with low 210Pb activity concentration background, groundwater discharge is still a vital source of 210Pb that cannot be ignored.
SGD has been recognized as an important source of dissolved chemicals to the coastal seas in recent years (Moore, 2010; Santos et al., 2021). Most studies focused on nutrients, carbons and metals. However, gradually, some researchers start to pay attention to the role of SGD as a conveyor of anthropogenic radionuclides and naturally-occurring radionuclides to the coastal seas (Garcia-Orellana et al., 2013, 2016; Sanial et al., 2017; Otosaka et al., 2020; Kambayashi et al., 2021; Zhong et al., 2022). The most intuitive environmental problems caused by radioactive materials entering the coastal seas through SGD could be the increase of the radioactivity level in seawater, bottom sediment and coastal plants and fishes. For example, after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, the 137Cs transported by SGD was estimated to be 0.6 ×1012 Bq/yr, which was comparable to the flux of dissolved 137Cs discharged from the local rivers into the nearshore waters of the FDNPP (Sanial et al., 2017), and this process undoubtedly increased the activity concentration of 137Cs in the nearshore seawaters of the Japan coast. As for SGD derived natural-occurring radionuclides, Garcia-Orellana et al. (2013) found that the SGD released 210Pb with a flux of 6×105 Bq/d, which increased the concentration of 210Pb in marsh waters of Peníscola in the Mediterranean Sea, besides, this SGD should be responsible for the observed increase of 210Po and 210Pb activities in both plants and fishes in this salt marsh wetland (Garcia-Orellana et al., 2016). In addition to the above mentioned environmental problems, what other impacts of the radioactive materials discharged from submarine groundwater on the offshore ecological environment still need more continuous attentions and works in the future.