The Pearl River, formed by the convergence of the Xi River, Bei River, Dong River, and numerous tributaries within the Pearl River Delta, is the most extensive river system in southern China. The third longest in the country, spanning 2,320 km with a drainage area of 450,000 square kilometers, of which 440,000 square kilometers are within Chinese territory (Chen et al., 2008). The Pearl River’s annual runoff exceeds 330 billion cubic meters, second only to the Yangtze River, and it produces seven times the annual runoff of the Yellow River (Xu et al., 2010). This study focuses on the surface sediments of the Pearl River Estuary, extending from Duntouji in Guangzhou (113.51°E, 23.05°N) in the north to near the Jitimen Bridge in Zhuhai (113.26°E, 22.09°N), including the Lingding Yang region, which is characterized as a tide-dominated estuarine bay. Sampling was conducted in January 2022, with 17 sampling sites established across the study area, as depicted in Figure 1.

Due to the mild and consistent climate conditions in the Pearl River Estuary region throughout the year, coupled with the continuous and stable industrial activities along its banks, the levels of heavy metal pollution in this area are generally consistent year-round (Ye et al., 2020). Sediment samples were collected using a grab sampler to obtain undisturbed surface sediments (0–2 cm) and stored in polyethylene bags at 0°C–4°C. The samples were air-dried, and visible gravel, plants, and animal residues were removed. Subsequently, the sediments were oven-dried at 105°C to a constant weight, ground using an agate mortar, and sieved through a 200-mesh screen. The processed samples were stored in a desiccator for further analysis. According to the GB 15618–2008 Environmental Quality Standards for Soils, Cu, Zn, Cd, and Pb were selected as target heavy metals of concern (Ministry of Environmental Protection of China, 2008). The total concentrations of these metals in the sediment samples were determined after digestion using a mixture of HCl, HNO₃, HF, and HClO₄, following the four-acid digestion method (GB/T17140) (Standardization Administration of China, 1997). Cd and Pb concentrations were measured using a graphite furnace atomic absorption spectrophotometer, while Cu and Zn were quantified using a flame atomic absorption spectrophotometer. These different analytical techniques are chosen based on the concentration sensitivity and detection requirements specific to each metal. The graphite furnace atomic absorption spectrophotometer (GFAAS) offers a significantly higher sensitivity compared to FAAS, making it ideal for detecting trace levels of metals like Cd and Pb, which are often present in lower concentrations. In contrast, flame atomic absorption spectrophotometry (FAAS) is suitable for metals such as Cu and Zn, which are generally found in higher concentrations in environmental samples. FAAS is also efficient for rapid analysis when ultra-trace sensitivity is not required. Utilizing these methods in tandem ensures accurate measurement across different concentration ranges, enhancing the precision and reliability of the heavy metal quantification (Garnier et al., 2006; Medeiros et al., 2020).

The chemical forms of the heavy metals were extracted using the modified BCR sequential extraction procedure (Rauret et al., 1999; Sungur et al., 2014). The specific extraction protocol consisted of (1) Acid-extractable fraction (F1), including exchangeable and carbonate-bound metals, extracted using 0.11 mol·L⁻¹ acetic acid; (2) Reducible fraction (F2), representing metals bound to iron-manganese oxides, extracted using 0.5 mol·L⁻¹ hydroxylamine hydrochloride; (3) Oxidizable fraction (F3), representing metals bound to organic matter and sulfides, extracted by digesting the F2 residue with 30% (8.8 mol·L⁻¹) hydrogen peroxide followed by 1 mol·L⁻¹ ammonium acetate; and (4) Residual fraction (F4), representing metals bound to silicates or within the mineral lattice, extracted by digesting the F3 residue using a mixture of three acids.

Three parallel samples and an environmental standard reference material (ESS-4) were used for quality control to ensure analytical precision and accuracy. The relative error of heavy metal concentrations in parallel samples was maintained below 10%, and the recovery rates of standard reference materials ranged from 95% to 120%. The detection limits for Cu, Zn, Cd, and Pb were 0.5 mg·kg⁻¹, 7.0 mg·kg⁻¹, 0.07 mg·kg⁻¹, and 2.0 mg·kg⁻¹, respectively.

While the BCR sequential extraction procedure is widely used for assessing heavy metal speciation, potential limitations exist. Interferences from other sediment components, such as organic matter or carbonate content, may affect the extraction accuracy, particularly for acid-extractable and reducible fractions. Additionally, the four-acid digestion method used for total concentration measurement could lead to variations in recovery rates for certain metals due to complex matrix effects (Ryan et al., 2008).

A comprehensive assessment of the total and speciated pollution risks for Cu, Zn, Cd, and Pb is presented in Table 6, using the I g e o , ERI, and RAC indicators. Results indicate that Cd poses the highest risk levels across all three indicators. Cu is categorized as moderately to mildly polluted (MMP) according to the I g e o , with both its single-element ecological risk and bioavailable fraction risk being classified as low risk. Zn is also classified as MMP by I g e o and as low risk in terms of its single-element ecological risk; however, its bioavailable fraction presents a medium risk, indicating a higher potential ecological risk than Cu. Pb is considered clean according to I g e o , with its single-element ecological risk and bioavailable fraction risk classified as low risk.

This assessment highlights that Cd poses the most significant risk across all indicators, necessitating prioritized mitigation efforts, whereas Cu and Pb present minimal risks. Despite Zn’s lower total concentration, its higher bioavailable fraction contributes to a medium risk, warranting attention to potential ecological impacts.

These ecological risks are closely linked to the chemical forms of the metals. For Cd, the dominance of its acid-extractable fraction indicates high bioavailability, which increases its mobility and potential for uptake by aquatic organisms, posing severe ecological threats. In contrast, Zn, although lower in total concentration, shows a substantial bioavailable fraction, highlighting its potential for mobility and ecological impact under variable environmental conditions. On the other hand, Cu and Pb have lower proportions of bioavailable forms and higher residual fractions, suggesting they are less mobile and present a reduced ecological risk (Xu et al., 2016; Liu et al., 2021).

Cu, Zn, Cd, and Pb concentrations in the Pearl River Estuary are substantially higher than other major river systems in China, such as the Yangtze River and Bortala River (Xinjiang), as shown in Table 7. While the sediment Cd concentrations in the Yangtze and Liao Rivers slightly surpass those in the Pearl River Estuary, Cu, Zn, and Pb concentrations are significantly lower. This finding underscores that the Pearl River Estuary remains one of the most heavily contaminated estuaries in China concerning heavy metal pollution in sediments, likely due to the highly industrialized nature of the lower Pearl River region. Furthermore, all heavy metal concentrations in the sediments exceed the background values for Guangdong Province soils (Table 7), indicating substantial anthropogenic contributions to heavy metal contamination, particularly for Cd, which is elevated by 39 times compared to the background level. Numerous studies have consistently identified Cd as the most severe contaminant in this estuary over the years (Xie et al., 2012; Liu et al., 2017; Jia et al., 2021). Despite the implementation of stringent control measures in recent years, the persistence and accumulation of these metals imply that heavy metal pollution remains a critical environmental challenge in the region, especially for Cd, which necessitates sustained remediation efforts.

Regarding potential ecological risk, the single-element ecological risk index (ERI) for Cu, Zn, and Pb at all sampling points indicates low risk (Table 5). In contrast, Cd poses a high ecological risk across the 17 sites. The contribution of each metal’s ERI to the comprehensive potential ecological risk index (RI) is ranked as follows: Cd (96.94%) > Cu (1.94%) > Pb (0.84%) > Zn (0.28%), clearly indicating Cd’s dominant role in the overall ecological risk. This result aligns with a study by Zhang et al. (2016), which assessed the potential ecological risk of heavy metals in the Bortala River. Their findings similarly showed that Cd accounted for 97% of the total ecological risk, followed by Pb, Cu, and Zn. The congruence between these studies further confirms that Cd exhibits the highest single-element ecological risk and makes the most significant contribution to the comprehensive ecological risk in the Pearl River Estuary.

Heavy metals in sediments often exhibit complex interrelationships, influenced by factors such as the original composition of heavy metals in the parent rock, soil formation processes, and anthropogenic activities. High and significant correlations between heavy metals can imply a common source of contamination (Zhang et al., 2016; Ke et al., 2017; Fu et al., 2022). This study conducted Pearson correlation analysis to examine the interrelationships between Cu, Zn, Cd, and Pb in the sediments from 17 sampling sites. As shown in Table 8, all four metals exhibited significant correlations (P < 0.01), suggesting that they share a common source, potentially linked to the intense industrial development in the lower reaches of the Pearl River. Many heavy metals are discharged into the river through industrial effluents, leading to severe sediment contamination in the estuary. These findings are consistent with research by Zhang et al. (2016), who studied the distribution of heavy metals in sediments from the Lingding Yang area of the Pearl River Estuary over the past century. Their results indicated that the strong correlations between heavy metals suggest a common source or that the environmental conditions and biogeochemical processes have remained highly consistent and stable. Other studies have attributed the primary sources of heavy metals in the Pearl River Estuary to industrial activities, shipping, and agricultural production, with Cd contamination particularly associated with the lead-zinc mining and smelting industries in the upper reaches of the Beijiang and Dongjiang Rivers in Guangdong Province (Gu et al., 2014; Du et al., 2019; Lu et al., 2020; Niu et al., 2021b). Pollutants from these industries are transported downstream, exacerbating heavy metal contamination in the estuarine sediments.

The RAC results indicate that Cd poses the highest pollution and ecological risk among the four metals. The acid-extractable fraction of Cd, representing its most bioavailable form, constitutes more than one-third of its total concentration, the highest proportion among the studied metals. Furthermore, the combined content of the acid-extractable, reducible, and oxidizable forms of Cd exceeds 90% of its total concentration, with the residual fraction contributing less than 10%. This distribution pattern suggests that anthropogenic activities heavily influence Cd in the surface sediments of the Pearl River Estuary, which are present predominantly in bioavailable forms. Consequently, Cd is highly susceptible to environmental changes, such as variations in water chemistry, and can be quickly released back into the water, where it may be assimilated by aquatic organisms, adversely impacting their growth and survival.

In contrast, Pb exhibits the lowest RAC value, with the acid-extractable fraction accounting for only 2.12% of its total concentration and a relatively high residual fraction (28.45%), indicating low ecological risk. Cu also has a low proportion of acid-extractable content but shows the highest percentage of oxidizable forms, exceeding 50% of its total concentration. This suggests Cu may migrate from sediments to the water column under oxidative conditions. On the other hand, Zn has a relatively high proportion of acid-extractable content and a substantial residual fraction (33.07%), the highest among the four metals. The other chemical forms of Zn (acid-extractable, reducible, and oxidizable) are present in relatively similar proportions, ranging from 19.28% to 28.08%, indicating that Zn can quickly shift between its bioavailable forms under both oxidative and reductive conditions.

Although research on heavy metals in river sediments across China began relatively late, most major river basins have been comprehensively studied. The distribution of heavy metal chemical forms shows significant variation among different river systems, likely due to the diverse anthropogenic activities and environmental conditions that influence heavy metal partitioning in sediments (Jiang et al., 2020; Deng et al., 2023).

The estuarine environmental conditions, such as pH, salinity, and organic matter content, are considered to interpret the observed speciation patterns further. For instance, the high bioavailability of cadmium (Cd) in the Pearl River Estuary may be attributed to the estuary’s slightly acidic conditions and moderate salinity levels, which enhance the solubility and mobility of Cd. Similarly, the presence of oxidizable forms of Cu is likely influenced by the organic-rich sediments found in this region, which provide binding sites for metal-organic complexes. These environmental factors contribute to the differential bioavailability and mobility observed for each metal.