Distribution and Relationship of Radionuclides and Heavy Metal Concentrations in Marine Sediments from the Areas Surrounding the Daya Bay Power Plant, Southeast China

Abstract

Radionuclides and heavy metals pose potential risks to marine ecosystems and human health. Daya Bay, the site of China’s first commercial nuclear power plant, has experienced significant anthropogenic impacts, yet the extent of radionuclide and heavy metal contamination remains unclear. Nineteen surface sediment samples were collected in January 2024 and analyzed for natural (²¹⁰Pb, ²²⁸Th, ²²⁶Ra, ²²⁸Ra, and ⁴⁰K) and anthropogenic (¹³⁷Cs) radionuclides, heavy metals (Cu, Pb, Zn, Cd, Cr, Mn, Hg, and As), grain size, and total organic carbon (TOC). The surface sediments of Daya Bay were predominantly fine-grained, with TOC levels ranging from 0.41% to 1.83%, influenced significantly by riverine input from the Dan’ao River. Natural radionuclides exhibited distinct spatial patterns: ²¹⁰Pb and ²²⁸Th activity levels were higher in fine-grained sediments, and correlated with TOC, indicating adsorption and sedimentation controls. In contrast, anthropogenic ¹³⁷Cs activity was low and showed no significant impact from the nuclear power plant. Notably, the absence in the samples of key anthropogenic radionuclides typically associated with nuclear power plant operations further confirmed the negligible impact of the power plant on local sediment contamination. The results indicated that the baseline levels of both natural and anthropogenic radionuclides and heavy metals were predominantly influenced by natural processes and local anthropogenic activities rather than the operation of the nuclear power plant. This study establishes critical baselines for radioactivity and heavy metals in Daya Bay, underscoring effective pollution control measures and the resilience of local ecosystems despite anthropogenic pressures.

1. Introduction

Radionuclides and heavy metal elements are widely present in marine environments, accumulating through marine food chains and posing hazards to human health [1]. Based on their origins, radionuclides can be categorized into natural and anthropogenic types. “Natural radionuclides” refers to those produced during Earth’s formation and evolution and existing in the natural environment (e.g., ²¹⁰Pb, ²²⁶Ra, and ⁴⁰K), whereas anthropogenic radionuclides are generated by human activities. For instance, ¹³⁷Cs primarily originates from atmospheric nuclear weapons tests, with additional contributions from nuclear accidents (such as the Chernobyl and Fukushima accidents) and discharge from nuclear waste processing facilities [2,3]. Depending on their sources and properties (e.g., particle-reactivity, half-life), different radionuclides can serve as indicators of various geological processes [4]. Examples include calculating sedimentation rates using vertical decay profiles of excess ²¹⁰Pb in sediment cores [5]; tracing terrestrial sediment transport pathways across continental shelves using ¹³⁷Cs [6,7]; and employing ⁷Be to identify newly deposited estuarine sediments while analyzing riverine versus marine influence ratios [8]. Notably, naturally occurring gamma radiation from radionuclides accounts for 85% of global annual ionizing radiation exposure [9,10], with prolonged contact potentially leading to health risks including various cancers. Similarly, due to their physiological toxicity, heavy metals constitute another critical marine pollutant threatening human health. The primary sources of heavy metals include natural rock weathering and anthropogenic activities (e.g., industrial/mining discharges), with subsequent adsorption onto particulate matter for transportation and sedimentation [11,12]. Upon entering marine systems, radionuclides and heavy metals from different sources can accumulate in marine organisms through sediment and water mediums and can eventually threaten human health through trophic transfer and biomagnification. The determination of radionuclide activity in sediments is a crucial component in assessing human exposure to background radiation, providing important information regarding health risks associated with natural radioactivity. Meanwhile, heavy metal concentrations serve as key indicators in the evaluation of the degree of marine environmental pollution. Therefore, investigating the activity levels of radionuclides and concentration characteristics of heavy metals in marine sediments not only offers critical insights for sedimentological process but also holds significant implications for the protection of marine life and, subsequently, human health.

The Daya Bay Nuclear Power Plant was China’s first commercial nuclear power plant, and has been in operation for over thirty years, since 1994. During the operation of the power plant, it has exerted impacts on the surrounding marine environment, especially the water environment, including changes in the local flow field, increases in the temperature at the discharge outlet, alterations in the geochemical characteristics of the water in the discharge area, and changes in the structure of the plankton community [13,14,15]. Moreover, the operation of the nuclear power plant also has certain impacts on the sediment characteristics of the surrounding sea areas; however, there is currently no systematic and comprehensive conclusion regarding the extent of these impacts. At present, the focus of pollution research on the sea areas surrounding the Daya Bay Nuclear Power Plant is mainly concentrated on the organic carbon and heavy metal pollution [16,17,18]. In comparison, the levels of radionuclides (both natural and anthropogenic) in the surrounding sediments and their relationship with heavy metals have not yet been fully clarified.

Therefore, this study focuses on the radioactivity level and heavy metal pollution in the marine sediments of Daya Bay, especially those near the power plant, aiming to clarify the spatial distribution of natural (²¹⁰Pb, ²²⁸Th, ²²⁶Ra, ²²⁸Ra, and ⁴⁰K) and anthropogenic (¹³⁷Cs) radionuclides and heavy metals (Cu, Pb, Zn, Cd, Cr, Mn, Hg, and As) in surface marine sediments near the Daya Bay Power Plant, and relate these findings to the determinations of lithology, gran-size composition, and total organic carbon content. Finally, clarification of the relationship between radionuclides and heavy metals, as well as the extent to which they are affected by the operation of nuclear power plants, will be made. This study provides important references for pollution assessment and the formulation of comprehensive management policies in the marine areas surrounding nuclear power plants.

2. Materials and Methods

2.1. Study Area and Sample Collection

Daya Bay is situated on the eastern coast of Guangdong Province, Southeastern China (Figure 1a), covering an area of approximately 650 km². The water depth in most parts is <30 m, with an average depth of around 10 m. This semi-enclosed bay with limited water exchange has a water residence time of about 90 days [18,19]. The region has a typical subtropical monsoon climate, with an annual rainfall of 1827 mm, 80% of which occurs between May and August [20]. Daya Bay is characterized by an irregular semidiurnal tide with a narrow tidal range of 1.0–1.5 m. Flood tide in Daya Bay basically flows from the mouth to the top of the bay (south to north). There is only a small river, named the Dan’ao River, flowing into Daya Bay from the northwest of the bay (Figure 1b). Since the 1980s, the rapid development of marine aquaculture and the petrochemical industry in Daya Bay, along with the successive construction of two nuclear power plants (Daya Bay Power Plant and Ling’ao Power Plant, which began operation in 1994 and 2002, respectively), along with the accelerated process of urbanization, has exerted a substantial impact on the marine environment of the bay [12].

Nineteen surface sediment samples (uppermost 5 cm) were collected using a grab sampler near the Daya Bay power plants during 25−26 January 2024 (the stations are shown in Figure 1b). The samples were separately preserved to be used for grain size, total organic carbon (TOC), radionuclides, and heavy metals analysis.

2.2. Laboratory Sample Analyses

2.2.1. Grain Size and Total Organic Carbon (TOC) Analysis

The grain size of sediments was determined by a laser particle-size analyzer (Mastersizer 2000, Malvern Instruments, Ltd., Worcestershire, UK). About 0.5 g of fresh sediment was pretreated using 10 mL of a 30% H₂O₂ (AR, Xilong Scientific Co. Ltd., Shantou, China) solution to oxidize the organic matter, and then dispersed and homogenized using an ultrasonicator for 30 s. The measurement error of the instrument was within 3%. The grain size fractions were < 4 μm for clay, 4–63 μm for silt, and >63 μm for sand.

Before TOC measurement, the sediment was pretreated with 1.2 mol L⁻¹ HCl (AR, Xilong Scientific Co. Ltd., Shantou, China) to remove inorganic carbon. The samples were dried at 60 °C after the excess acid had been removed. About 25–30 mg of carbonate-free samples were wrapped in a tin cup for TOC measurement using an Elemental Analyzer (EA, Integra 2, SerCon, Crewe, UK). The instruments were first run with three blank samples, and two standard samples were also run between every twelve field samples. The analytical accuracy was ±0.02% for TOC.

2.2.2. Radionuclides Measurement

High-resolution gamma-ray spectrometry was used to measure the radionuclides in the surface sediment. After being dried in an oven at 80 °C for 12 h, sediment samples were ground and sieved (100–150 mesh), and then packed into containers of identical height and sealed for 30 days, prior to radionuclide measurement using a high-purity germanium (HPGe) γ-spectrometer (Canberra-GR4021). By the time of measurement, the radionuclides in the samples had reached a relative secular equilibrium. A certified reference material for environmental radioactivity in river sediment (GBW08034a), prepared by the National Institute of Metrology of China, was used as the standard reference source to determine the full-energy peak efficiency. During sample measurement, decay correction and background subtraction were performed for all samples. Each sample was measured for 24 h in the HPGe γ-spectrometer. The specific activity of ²¹⁰Pb was determined from the peak area at 46.5 keV. The specific activity of ²²⁶Ra was calculated using the peak areas of its progenies ²¹⁴Bi (609.3 keV) and ²¹⁴Pb (295.2 keV and 352.0 keV). The specific activity of excess ²¹⁰Pb (²¹⁰Pb_ex) was obtained by subtracting the activity of ²²⁶Ra from that of total ²¹⁰Pb. The specific activity of ²²⁸Ra was calculated based on the peak areas of ²²⁸Ac (338.5 keV and 911.1 keV), while that of ²²⁸Th was determined using the average specific activities of ²⁰⁸Tl (583.2 keV) and ²¹²Pb (238.6 keV). The specific activity of ⁷Be was determined using the peak area at 477 keV, with a detection limit of 5 Bq kg⁻¹. The specific activity of ⁴⁰K was calculated from the peak area at 1460.8 keV. The specific activity of ¹³⁷Cs in the sediment samples was determined based on the peak at 661.5 keV.

The counting error was <10% for all radionuclides. The minimum detectable activity levels were 0.58 Bq kg⁻¹ for ²¹⁰Pb, 1.30 Bq kg⁻¹ for ²²⁶Ra, 0.27 Bq kg⁻¹ for ²²⁸Ra, 0.19 Bq kg⁻¹ for ²²⁸Th, 5.0 Bq kg⁻¹ for ⁷Be, 1.91 Bq kg⁻¹ for ⁴⁰K, and 0.41 Bq kg⁻¹ for ¹³⁷Cs, respectively.

2.2.3. Heavy Metals Measurement

Closed-container dissolution was used to digest the sediments prior to heavy metal measurement. Approximately 40 mg of dried and ground fine powders underwent digestion using 1 mL HF (GR/Merck, 40%, Boston, MA, USA) and 3 mL HNO₃ (GR/Merck, 65%, Boston, MA, USA) in Teflon containers for 12 h at 180 °C. The samples were then removed and placed on an electrothermal board to evaporate the HF. The residue was solubilized by 1 mL HNO₃ and 1 mL deionized water for 12 h at 150 °C. The Cu, Pb, Zn, Cd, Cr, Mn, and As contents were measured by ICP-MS (iCAPQ, Thermo, Waltham, MA, USA) and the Hg was measured using an atomic fluorescence spectrophotometer. The parallel samples were set up with an interval of five samples, and two blank samples and two standard samples (GBW07316, marine sediment) were set up for all samples to calibrate the results. The error for the parallel sample was less than 3%, and the test result of the standard sample fell within the standard range (the concentration of the GBW07316).

2.3. Heavy Metals Pollution Assessment Method

The single-factor index method was employed to assess the heavy metal pollution levels in the sediments of Daya Bay. The calculation formula is as follows:

$$Q_i={\displaystyle \frac{C_{\mathrm{i}}}{C_0}},$$

(1)

where $Q_i$ is the pollution index of the i-th sampling station, $C_{\mathrm{i}}$ represents the measured concentration, and $C_0$ denotes the reference standard value (in this study, the Marine Sediment Quality Standard of China [21] was adopted). A value of $Q_i$ ≤ 1 indicates that the corresponding heavy metal is not present at polluting levels, whereas $Q_i$ > 1 indicates the presence of pollution.

3. Results

3.1. Deposition Background: Grain Size and TOC

The results for sedimentary grain size composition and TOC contents in the study area are presented in

Table 1. Lithology; size distribution (in wt%) of sand (>63 μm), silt (4–63 μm), and clay (<4 μm) fractions; mean grain size (Mz, μm); and TOC contents (in wt%) in the sediments of Daya Bay.

Station	Lithology	Particle-Size Distribution, %			Mz, μm	TOC, %
		>63 μm	4–63 μm	<4 μm
D01	Sandy silt	25.04	65.72	9.26	22.51	1.26
D02	Silt	6.47	79.52	14.01	11.70	1.23
D03	Sandy silt	28.60	51.18	20.23	16.28	0.69
D04	Clayey silt	8.09	71.01	20.90	10.81	0.71
D05	Sandy silt	25.97	61.35	12.68	20.27	0.41
D06	silt	0.06	76.70	23.24	7.07	1.48
D07	Clayey silt	16.67	62.11	21.22	12.43	1.29
D09	Sandy silt	33.31	50.89	15.80	18.89	1.03
D10	Silt	1.23	78.35	20.42	8.39	1.26
D11	Sandy silt	35.37	53.08	11.55	22.22	0.48
D14	Silt	9.12	76.42	14.47	13.12	0.62
D15	Silt	2.88	85.91	11.21	12.00	1.23
D16	Sandy silt	17.23	67.07	15.70	14.61	0.78
D17	Clayey silt	10.60	73.99	15.41	12.74	0.99
D19	Silt	3.29	79.32	17.39	8.86	1.51
D20	Sandy silt	28.63	51.73	19.65	16.27	1.55
D21	Silt	3.08	82.83	14.10	10.51	1.83
D22	Silt	7.86	78.15	13.99	11.68	1.22
D25	Sandy silt	27.47	60.67	11.86	18.81	0.62
Average		14.57	69.04	16.39	13.60	1.06

. The surface sediments of Daya Bay are generally fine-grained, with the lithology dominated by silt and sandy silt (Table 1). The mean grain size ranges from 7.07 to 22.51 μm, with an average of 13.60 μm. Silt is the most abundant component, accounting for 50.89–85.91% (an average of 69.04%), followed by clay, which ranges from 9.26% to 23.24%, with an average of 16.39%. The sand content exhibits considerable variability, ranging from 0 to 35.37%, with an average of 14.57%. Coarser sediments are primarily distributed in the eastern and western parts of the study area, whereas two centers of fine-grained sediments can be observed in the central and northern regions (Figure 2). The northern fine-grained center is mainly composed of silt, while the central one is jointly dominated by silt and clay. The TOC content of sediments in the study area ranges from 0.41% to 1.83% (average: 1.06%). A tongue-shaped high-TOC zone extends from the estuary of the Dan’ao River in the northwest toward the southern part of the study area (Figure 3a), indicating the significant influence of riverine input on the spatial distribution of organic carbon. Moreover, areas with high TOC content strongly coincide with regions of fine-grained sediment accumulation (Figure 2 and Figure 3), suggesting that sedimentary organic carbon in the study area is predominantly associated with fine particles.

3.2. Baselines of Natural and Anthropogenic Radionuclides

The activities of the natural and anthropogenic radionuclides in surface sediments at different stations in the study area are presented in

Table 2. Activity concentrations (in Bq kg⁻¹) of ⁷Be, ²¹⁰Pb, ²²⁸Th, ²²⁶Ra, ²²⁸Ra, ⁴⁰K, ¹³⁷Cs, ²¹⁰Pb_ex, and ²²⁸Th_ex radionuclides in the sediments of Daya Bay.

Station	7Be	210Pb	228Th	226Ra	228Ra	40K	137Cs	210Pbex	228Thex
D01	–	125.9	53.3	35.1	49.4	455.9	1.3	90.9	3.9
D02	–	166.9	56.8	32.7	56.7	476.9	0.5	134.1	0.1
D03	–	118.2	44.8	29.5	43.3	428.8	1.2	88.8	1.5
D04	–	162.7	54.9	29.4	51.2	356.8	1.2	133.3	3.7
D05	–	139.7	54.2	34.1	43.1	421.4	0.4	105.6	11.2
D06	–	117.9	72.2	30.5	59.7	484.9	1.1	87.5	12.5
D07	–	129.8	56.7	30.1	43.5	409.5	0.8	99.7	13.2
D09	–	105.0	53.8	31.6	46.9	422.1	0.5	73.3	6.9
D10	14.5	104.7	65.3	29.8	53.4	436.7	0.4	74.9	11.9
D11	11.4	116.6	47.5	32.9	44.7	449.6	0.6	83.7	2.8
D14	–	123.2	54.2	33.6	52.3	437.2	0.7	89.6	1.9
D15	–	132.8	66.6	35.1	55.8	462.1	1.0	97.7	10.8
D16	–	119.0	51.2	35.4	45.5	433.2	0.4	83.6	5.6
D17	17.3	137.8	61.9	35.4	55.7	456.4	0.9	102.4	6.2
D19	10.9	140.2	58.2	30.7	54.9	462.8	1.8	109.5	3.3
D20	8.0	150.0	59.4	29.8	55.1	359.3	0.9	120.2	4.3
D21	–	139.9	46.6	28.8	50.2	418.5	1.2	111.1	−3.6
D22	13.7	128.5	54.4	29.2	55.7	448.5	0.5	99.3	−1.3
D25	–	136.2	61.8	30.6	48.0	431.5	0.3	105.6	13.8
Average	12.6	131.3	56.5	31.8	50.8	434.3	0.8	99.5	5.7
Global average [10]	–	–	–	35.0	–	420.0	–	–	–
Chinese average [22]	–	–	–	37.6	–	584.0	–	–	–
Sacca Di Goro [23]	–	–	–	25.8	–	550.6	11.8	–	–
Mediterranean Sea [1]	–	–	–	20.1	–	467.3	–	–	–

“–” represent no data.

, and their spatial distributions are shown in Figure 3. Due to its short half-life (only 53.3 days), elevated activity of ⁷Be can only be detected in recently deposited sediments. In the study area, ⁷Be was detected only at certain stations in the southern and northeastern regions, with the highest activity reaching 17.21 Bq kg⁻¹ in the south (Figure 3b). In contrast, the ⁷Be activities in the western part of the study area near the nuclear power plant were all below the detection limit (<5 Bq kg⁻¹). The specific activity of total ²¹⁰Pb in the sediments ranges from 104.7 to 166.9 Bq kg⁻¹, with an average of 131.3 Bq kg⁻¹. High ²¹⁰Pb activities occurred near the shore in the western part and near the estuary in the northwestern part of the study area, while the central and northeastern regions showed distinctly lower levels of activity (Figure 3c). The specific activity of ²²⁸Th ranges from 44.8 to 72.2 Bq kg⁻¹, with an average of 56.5 Bq kg⁻¹. High ²²⁸Th values are concentrated in the central region and extend both northward and southward, while the eastern and western parts exhibit notably low values (Figure 3d). The specific activity of ²²⁶Ra varies from 28.8 to 35.4 Bq kg⁻¹, with an average of 31.8 Bq kg⁻¹, slightly lower than the global average of 35 Bq kg⁻¹ [10] and the Chinese average of 37.6 Bq kg⁻¹ [22]. Elevated ²²⁶Ra activity is mainly observed in the southwestern part of the study area, forming a tongue-shape pattern extending from the bay mouth toward the vicinity of the Daya Bay Nuclear Power Plant (Figure 3e), suggesting a potential influence from the external input of the open sea. The specific activity of ²²⁸Ra ranges from 43.05 to 59.7 Bq kg⁻¹, with an average of 50.8 Bq kg⁻¹. Its spatial distribution is similar to that of ²²⁸Th, with its high values mainly concentrated in the central region (Figure 3f). The specific activity of ⁴⁰K ranges from 356.8 to 484.9 Bq kg⁻¹, with an average of 434.3 Bq kg⁻¹, slightly higher than the global average of 420 Bq kg⁻¹ [10], but lower than the Chinese average of 584 Bq kg⁻¹ [22]. The spatial pattern of ⁴⁰K in the study area is characterized by higher values in the central region and lower values in both the eastern and western parts, closely resembling the distribution of ²²⁸Ra (Figure 3g).

¹³⁷Cs is an anthropogenic radionuclide primarily originating from atmospheric nuclear weapons testing and nuclear accidents. The ¹³⁷Cs activity in the study area is generally low, ranging from 0.4 to 1.8 Bq kg⁻¹, with an average of 0.8 Bq kg⁻¹. High ¹³⁷Cs activity zones are found at the northern and southern ends of the study area. The northern high-value region is significantly influenced by riverine input, while the southern one may be attributed to contributions from Pearl River-derived materials. In contrast, the area surrounding the nuclear power plant shows distinctly low ¹³⁷Cs activity (Figure 3h).

The discrepancies in the geochemical background give rise to variations in the radionuclide fingerprints of sediments across different regions (Table 2). This implies that the accumulation of baseline data is of paramount significance.

The results of Pearson correlation analysis (

Table 3. Pearson correlation coefficient matrix of radionuclides with grain size composition and TOC contents, for Daya Bay (n = 19). Significant values (p < 0.05) are shown in bold.

	Sand	Silt	Clay	Mz	TOC	210Pb	228Th	226Ra	228Ra	40K	137Cs
Sand	1
Silt	−0.946 **	1
Clay	−0.249	−0.079	1
Mz	0.917 **	−0.754 **	−0.566 **	1
TOC	−0.513 *	0.443	0.253	−0.544 *	1
210Pb	−0.214	0.239	−0.050	−0.161	0.127	1
228Th	−0.482 *	0.407	0.266	−0.475 *	0.302	0.010	1
226Ra	0.128	0.073	−0.592 **	0.382	−0.355	−0.071	0.062	1
228Ra	−0.700 **	0.673 **	0.150	−0.648 **	0.566 *	0.287	0.671 **	−0.060	1
40K	−0.323	0.447	−0.324	−0.149	0.108	−0.279	0.287	0.405	0.338	1
137Cs	−0.314	0.257	0.196	−0.303	0.479 *	0.222	−0.029	−0.173	0.265	0.045	1

* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed).

) indicate that in this study, only ²²⁸Th, ²²⁶Ra, and ²²⁸Ra show significant correlations with sediment grain size characteristics, suggesting that grain size exerts a controlling influence on their specific activities. Specifically, ²²⁸Th and ²²⁸Ra exhibit significant negative correlations with the sand fraction, while showing relatively strong positive correlations with the silt fraction. This is because ²²⁸Th and ²²⁸Ra in the sediment are mainly bound to the fine-grained aluminosilicate grain lattice [24]. In contrast, ²²⁶Ra demonstrates a significant negative correlation with the clay fraction, but no apparent correlation with either the sand or silt fractions. In addition, both ²²⁶Ra and ¹³⁷Cs display significant positive correlations with TOC. Aside from these relationships, most radionuclides do not exhibit significant inter-correlations, with the exception of the significant positive correlation observed between ²²⁸Th and ²²⁸Ra.

3.3. Concentration and Pollution Assessment for Heavy Metals

The concentrations of heavy metals in surface sediments at different stations in the study area are presented in

Table 4. Concentration of heavy metals in the sediments of the Daya Bay. The unit is ng g⁻¹ for Hg and μg g⁻¹ for others.

Station	Cu	Pb	Zn	Cd	Cr	Mn	Hg	As
D01	12.90	29.80	76.10	0.19	27.60	667.00	61.00	7.73
D02	76.30	180.00	187.00	0.37	41.30	639.00	52.00	6.23
D03	8.01	19.60	49.10	0.16	14.90	412.00	38.00	5.62
D04	26.00	38.50	72.00	0.19	33.30	383.00	99.00	7.53
D05	77.50	245.00	161.00	0.46	27.90	378.00	78.00	5.31
D06	15.00	32.20	80.80	0.18	33.30	643.00	67.00	6.23
D07	14.80	33.10	80.90	0.18	30.60	822.00	33.00	7.01
D09	12.50	27.20	64.40	0.18	27.70	489.00	62.00	6.38
D10	14.10	33.20	78.80	0.18	31.40	666.00	44.00	6.83
D11	6.26	16.50	41.70	0.15	12.00	334.00	34.00	4.97
D14	7.74	20.50	45.70	0.17	14.90	405.00	71.00	6.08
D15	12.60	26.70	66.40	0.18	27.60	540.00	79.00	6.17
D16	7.46	21.50	47.70	0.16	14.80	425.00	47.00	6.17
D17	10.20	25.30	60.80	0.17	22.20	565.00	58.00	6.86
D19	16.00	28.50	75.60	0.19	39.10	590.00	88.00	5.92
D20	17.90	33.90	66.70	0.19	34.30	467.00	78.00	6.74
D21	16.20	28.70	78.00	0.23	38.70	701.00	145.00	6.90
D22	11.80	23.70	67.30	0.18	39.40	528.00	51.00	6.29
D25	5.54	16.10	39.00	0.15	11.60	391.00	51.00	5.77
Average	19.41	46.32	75.74	0.20	27.51	528.68	65.05	6.35

, revealing three distinct spatial distribution patterns. Cu, Pb, Zn, and Cd follow the first distribution pattern, with high-concentration zones all being located in the western part of the study area, and particularly exhibiting a peak near the power plants (Figure 4a–d). Cr, Mn, and As represent the second distribution pattern, characterized by a saddle-shaped pattern with higher concentrations in the central region and lower concentrations toward the east and west. Notable high-value centers are observed near the estuary in the northern part and the bay mouth in the southern part of the study area (Figure 4e–g). Hg exhibits a unique distribution pattern, showing a gradient of high concentrations in the northwest and lower values toward the southeast (Figure 4h).

The calculated results of the single-factor index method are shown in

Table 5. Evaluation results for sediment quality, single-factor index. The values > 1 are shown in bold.

Station	Cu	Pb	Zn	Cd	Cr	Hg	As
D1	0.37	0.50	0.51	0.39	0.35	0.31	0.39
D2	2.18	3.00	1.25	0.73	0.52	0.26	0.31
D3	0.23	0.33	0.33	0.33	0.19	0.19	0.28
D4	0.74	0.64	0.48	0.37	0.42	0.50	0.38
D5	2.21	4.08	1.07	0.93	0.35	0.39	0.27
D6	0.43	0.54	0.54	0.37	0.42	0.34	0.31
D7	0.42	0.55	0.54	0.36	0.38	0.17	0.35
D9	0.36	0.45	0.43	0.36	0.35	0.31	0.32
D10	0.40	0.55	0.53	0.36	0.39	0.22	0.34
D11	0.18	0.28	0.28	0.31	0.15	0.17	0.25
D14	0.22	0.34	0.30	0.33	0.19	0.36	0.30
D15	0.36	0.45	0.44	0.36	0.35	0.40	0.31
D16	0.21	0.36	0.32	0.32	0.19	0.24	0.31
D17	0.29	0.42	0.41	0.34	0.28	0.29	0.34
D19	0.46	0.48	0.50	0.37	0.49	0.44	0.30
D20	0.51	0.57	0.44	0.38	0.43	0.39	0.34
D21	0.46	0.48	0.52	0.45	0.48	0.73	0.35
D22	0.34	0.40	0.45	0.36	0.49	0.26	0.31
D25	0.16	0.27	0.26	0.31	0.15	0.26	0.29

. In this study, at stations D2 and D5 the $Q_i$ values for Cu, Pb, and Zn exceeded 1, indicating relatively severe pollution by these metals at these two locations. Apart from this, the concentrations of heavy metals in surface sediments across Daya Bay remain at relatively low levels, suggesting a generally low degree of pollution.

4. Discussion

4.1. The Sedimentary Process Indicated by Radionuclides in Daya Bay

Excess ²¹⁰Pb (²¹⁰Pb_ex) in sediments can be used as an indicator of deposition [4]. In Daya Bay, the specific activity of excess ²¹⁰Pb_ex ranges from 73.3 to 134.1 Bq kg⁻¹, with an average of 99.5 Bq kg⁻¹ (Table 2), suggesting that Daya Bay is in a continuous accumulation state. High ²¹⁰Pb_ex activity zones are mainly distributed in the southwestern near-shore area with weak hydrodynamic conditions and the northwestern estuarine area of the study region, indicating that these locations represent the major depositional and accumulation zones of sediments in Daya Bay (Figure 5a). In addition, excess ²²⁸Th (²²⁸Th_ex) can also be used to indicate sedimentation processes on a time scale shorter than ²¹⁰Pb_ex due to the differences in their half-lives (1.9 years for ²²⁸Th and 22.2 years for ²¹⁰Pb). This also leads to significant differences in their spatial distribution characteristics. In this study, the specific activity of ²²⁸Th_ex is obviously lower than ²¹⁰Pb_ex, ranging from −3.6 Bq kg⁻¹ to 13.8 Bq kg⁻¹, with an average of 5.7 Bq kg⁻¹. This result indicates that the ²²⁸Th in the sediments of Daya Bay is primarily derived from the decay of ²²⁸Ra, and the two are in a state of approximate secular equilibrium. Notably, the ²²⁸Th_ex values at Station D21 in the northwestern part of the study area near the Dan’ao River estuary and at Station D22 in the northeastern region are negative (−3.6 and −1.3 Bq kg⁻¹, respectively), indicating a deficit of ²²⁸Th relative to its parent ²²⁸Ra, and that secular equilibrium has not yet been established at these stations, a condition that may be influenced by the underground water discharge in these regions [25]. The high values of ²²⁸Th_ex are primarily concentrated in the central and eastern parts of the study area, whereas the northern and southern parts exhibit distinctly low values (Figure 5b). Despite the differences in spatial distribution, the subtle numerical variation indicates that ²²⁸Th cannot be effectively used to indicate marine sedimentary processes in the study area.

The distribution characteristics of ⁷Be also show significant differences from those of ²¹⁰Pb and ²²⁸Th. Compared with ²¹⁰Pb and ²²⁸Th, which have half-lives measured in years, the 53.3-day half-life of ⁷Be means that it is primarily used to indicate seasonal or event-related sedimentation processes [8,26]. The ⁷Be activities in the western part of the study area near the nuclear power plant were all below the detection limit (<5 Bq kg⁻¹), indicating that sediments in this region had not received recent riverine sediment input [8]. The results of ⁷Be indicate that the sedimentation process in the study area during winter (the season when samples were collected) is relatively weak and mainly occurs in the bay mouth area. It is worth noting that, although ²¹⁰Pb, due to its high particulate reactivity, can also be used to indicate episodic sedimentation processes in some cases [4], its long half-life tends to dilute short-term non-episodic sedimentation signals given its long-term background, resulting in differences from the results of ⁷Be. This indicates that the sedimentation process of sediments in Daya Bay during winter differs from the long-term accumulation process, because the sedimentation primarily occurs during summer in Daya Bay. In winter, on one hand, the stronger dynamics are not conducive to sedimentation, and on the other hand, the limited material input restricts the occurrence of the sedimentation process.

In summary, although different radionuclide indicators exhibit varying spatial distribution characteristics, none of them show a clear impact effect from the nuclear power plant (Figure 3 and Figure 5). Therefore, whether considering a short time scale (winter) or a long-time scale (several years to several decades), the sedimentation processes in the waters surrounding the Daya Bay Nuclear Power Plant are primarily controlled by the bay’s own sedimentary dynamic processes. The activities related to the nuclear power plant (such as the intake and discharge of cooling water) do not have a significant influence on the sedimentation processes in the bay. Admittedly, the present study is confined to sampling and analysis conducted solely in the winter season. The absence of data corresponding to the strong summer monsoon period may introduce a degree of temporal bias into the findings. To address this limitation, our future work will encompass sampling and analysis across multiple seasons and extended time frames, thereby enhancing the comprehensiveness and accuracy of the insights derived.

4.2. The Sources of Heavy Metals and Their Relationship with Radionuclides

According to the spatial distribution characteristics for the different heavy metals and the results of correlation analysis among heavy metals with respect to grain size and TOC, these eight heavy metals in the study area can be classified into three groups, as influenced by different sources and geochemical behavior processes. Cu, Pb, Zn, and Cd form the first group. They are significantly correlated with one another but show no correlation with grain size or TOC (

Table 6. Pearson correlation coefficient matrix of heavy metals with grain size composition and TOC contents in the Daya Bay (n = 19). Significant values (p < 0.05) are shown in bold.

	Sand	Silt	Clay	TOC	Cu	Pb	Zn	Cd	Cr	Mn	Hg	As
Sand	1
Silt	−0.943 **	1
Clay	−0.267	−0.069	1
TOC	−0.513 *	0.443	0.253	1
Cu	−0.061	0.102	−0.113	−0.103	1
Pb	0.034	0.027	−0.179	−0.213	0.975 **	1
Zn	−0.200	0.229	−0.062	0.115	0.962 **	0.916 **	1
Cd	−0.006	0.080	−0.214	−0.115	0.965 **	0.987 **	0.919 **	1
Cr	−0.552 *	0.484 *	0.251	0.749 **	0.414	0.281	0.564 *	0.351	1
Mn	−0.506 *	0.444	0.229	0.784 **	0.015	−0.057	0.271	0.006	0.599 **	1
Hg	−0.385	0.416	−0.051	0.422	0.131	0.068	0.118	0.192	0.462 *	0.088	1
As	−0.317	0.251	0.223	0.502 *	−0.144	−0.253	−0.019	−0.205	0.436	0.557 *	0.289	1

* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed).

), suggesting that these metals mainly result from nearby anthropogenic inputs and have not undergone long-distance transport [11]. Cr, Mn, and As exhibit significant inter-element correlations, showing significant negative correlations with coarse-grained components and significant positive correlations with TOC (Table 6). This indicates that these metals are predominantly associated with fine-grained sediments and organic matter and likely originate from riverine input [11,17]. Next, Cr, Mn, and As form the second group, metals which are likely influenced by both the Dan’ao River discharge and external inputs from the open sea. Hg belongs to the third category, and exhibits a unique distribution pattern while displaying a positive, though not statistically significant, correlation with fine-grained sediments and TOC (Table 6); this suggests that it primarily originates within the bay, including a partial influence from river input (but not as the dominant source), and offshore contributions are likely negligible. Previous studies have identified the discharge of large volumes of industrial wastewater along the coast and atmospheric pollutants released from fossil fuel combustion in industrial activities as the major sources of Hg in sediments of the Daya Bay region [12,27].

The activity of radionuclides and the content of heavy metals are both important components of the chemical characteristics of marine sediments [1,4,11,12]. In addition to being influenced by their sources, their geochemical behaviors (such as adsorption, sedimentation, and redox regulation) also show certain similarities [16,24,25,28]. To further explore the relationship between heavy metals and radionuclides in the sediments of the sea area surrounding the Daya Bay Nuclear Power Plant, this study conducted a correlation analysis between the two, with the correlation coefficients shown in

Table 7. Pearson correlation coefficient matrix of heavy metals with radionuclides in Daya Bay (n = 19). Significant values (p < 0.05) are shown in bold.

	210Pb	228Th	226Ra	228Ra	40K	137Cs
Cu	0.557 *	−0.005	0.136	0.011	0.035	−0.220
Pb	0.431	−0.023	0.221	−0.104	0.064	−0.302
Zn	0.503 *	0.090	0.097	0.132	0.169	−0.142
Cd	0.442	−0.063	0.189	−0.089	0.059	−0.226
Cr	0.443	0.240	−0.400	0.545 *	−0.015	0.333
Mn	−0.003	0.300	−0.175	0.304	0.280	0.288
Hg	0.428	−0.061	−0.199	0.261	−0.271	0.499 *
As	0.218	0.158	−0.120	0.273	−0.320	0.310

* Correlation is significant at the 0.05 level (2-tailed).

. The results reveal a significant spatial distribution correlation between some heavy metals and radionuclides in the sediments (Figure 3 and Figure 4), with the correlation patterns mainly controlled by the adsorption mechanisms of pollutant transport carriers, chemical precipitation behavior, and the homogeneity of external inputs. Cu and Zn show significant positive correlations with ²¹⁰Pb (correlation coefficients of 0.557 and 0.503, respectively), which may be related to the high homogeneity of Cu, Zn, and Pb in the study area (Figure 4 and Table 6). Although Cu and Zn, as heavy metals, have high particle activity similar to ²¹⁰Pb, their weak correlation with fine-grained sediments indicates that in the study area, the adsorption and sedimentation processes are not the dominant factors controlling their spatial distribution characteristics. The significant positive correlation between Cr and ²²⁸Ra (correlation coefficient of 0.545) indicates a connection between the two in the sediments, which may be realized through the enrichment of organic matter and the reductive environment generated by its degradation. In high-organic-matter sediment environments with strong reducing conditions, Cr can be fixed in the sediments in the form of hydroxides or sulfides [29], while ²²⁸Ra can co-precipitate with Ba²⁺ in sulfate form in high-organic-matter environments and become fixed in the sediments due to the similar chemical behavior between Ra and Ba [30]. The high correlation between both elements and TOC also supports this inference (Table 3 and Table 6). In addition, the significant correlation between Hg and ¹³⁷Cs (correlation coefficient of 0.499) also reveals same source input, that is, they are derived from river inputs at the head of the bay and atmospheric deposition inputs. ¹³⁷Cs, as a residual from nuclear testing, and mercury, which is highly volatile, may be transported over long distances through the atmosphere and then co-precipitate or enter the ocean through river-transported terrestrial particles [2,6,12]. They can then be synchronously retained in organic-rich sediments, showing a strong positive correlation with TOC (Table 3 and Table 6). The general negative correlation between most heavy metals (such as Pb and Cd) and ¹³⁷Cs may be due to differences in mobility: ¹³⁷Cs is strongly fixed in the lattice by clay minerals [31], while heavy metals adsorbed on the surface of particles are more easily activated and transported [28,32], causing their differentiation in sediments. From a geological perspective, these correlations not only provide key clues for pollution source tracing (for example, the combination of ²¹⁰Pb and Cu (Zn) indicates industrial input, while the Hg-¹³⁷Cs association reflects atmospheric deposition), but also provide a new perspective for reconstructing the evolution of sedimentary environments. For example, the co-precipitation of Cr and ²²⁸Ra marks the development of reducing environments in recent years. This has potential value for reconstructing sedimentary environments (such as anoxic events). Moreover, the negative correlation between ¹³⁷Cs and heavy metals suggests that heavy metals adsorbed on the surface of particles may undergo further activation and migration, posing a greater ecological risk, and thus targeted remediation strategies need to be developed. Therefore, the geochemical behavior of pollutants in the sedimentary system is jointly regulated by the adsorption-desorption equilibrium of carriers, redox oscillations, and input pathways. The correlation network provides important geochemical evidence for regional environmental quality assessment and the reconstruction of pollution history.

It is worth noting that, although there is a certain correlation between radionuclides and heavy metals, the sources and distribution characteristics of the two in Daya Bay are not significantly affected by the operation of the nuclear power plant. In addition, during the detection processes associated with the samples, no effective spectral peaks of key anthropogenic radionuclides commonly produced by the operation of nuclear power plants (such as ⁶⁰Co, with a minimum detectable activity of 0.44 Bq kg⁻¹ and ^110mAg, with a minimum detectable activity of 0.35 Bq kg⁻¹) were found. This further indicates that the operation of the Daya Bay Nuclear Power Plant has a negligible impact on the radionuclide contamination of the sediments in the surrounding marine areas.

In summary, monitoring radionuclides and metal pollutants in the marine environment is crucial for the environmental protection of coastal areas significantly affected by human activities and for the study of relevant environmental processes. For example, Pappa et al. (2018, 2019) successfully established the historical pollution situation of a mining area and identified the environmental impacts of different mining periods by monitoring radionuclides and metal pollutants in the sediments of the different mining area [33,34]. Their research not only effectively assessed the pollution levels of the area but also provided important scientific references for the formulation of environmental management and remediation measures in those regions. Although the results of this study show that the operation of the Daya Bay Nuclear Power Plant has not significantly affected the levels of heavy metals and radionuclides in the surface sediments of the surrounding sea areas, and radionuclides and heavy metals are not necessarily closely related, the joint study of radionuclides and heavy metals is still of great significance. In addition to using radionuclides for dating, the indicator radionuclides of material transport such as ⁷Be and ²¹⁰Pb can further help to understand the sources, migration and transformation processes of modern pollutants (such as heavy metals), so as to better protect the marine ecosystem and promote sustainable development.

5. Conclusions

This study comprehensively investigated the distribution and relationship of radionuclides and heavy metals in marine sediments surrounding the Daya Bay Nuclear Power Plant. The results indicated that the baseline levels of both natural and anthropogenic radionuclides and heavy metals were predominantly influenced by natural processes and local anthropogenic activities rather than the operation of the nuclear power plant. Fine-grained sediments dominated the surface sediment composition, with radionuclide activities and heavy metal concentrations exhibiting distinct spatial patterns. Notably, the absence of key anthropogenic radionuclides typically associated with nuclear power plant operations (such as ⁹⁰Sr, ⁶⁰Co, and ^110mAg) in the samples further confirmed the negligible impact of the power plant on local sediment contamination. Heavy metals exhibited distinct spatial patterns: Cu, Pb, Zn, and Cd peaked near industrial zones in the western region, likely from local anthropogenic sources; Cr, Mn, and As correlated with riverine and marine inputs; Hg showed localized enrichment in the northwest, possibly linked to industrial emissions. However, the single-factor pollution indices for heavy metals also suggested low pollution levels in the sediments. This research provides critical baseline data for assessing environmental quality and pollution control in Daya Bay, highlighting the resilience of the local ecosystem despite anthropogenic pressures. Future studies should focus on long-term monitoring and further exploration of the interaction mechanisms between radionuclides and heavy metals in marine sedimentary environments.