This research investigated the spatial distribution of heavy metals, including mercury (Hg), cadmium (Cd), copper (Cu), arsenic (As), nickel (Ni), lead (Pb), chromium (Cr), and zinc (Zn), in surface sediments from the coastal area near to an industrial harbor (the Tangshan Harbor, China) with 161 sediment samples. The results showed that the distribution patterns of Cr, Cu, Zn, Ni, and Pb were similar to each other, high in the northwest, southeast, and southwest regions of the study area and low in the northeast region, which corresponded well with components of sedimental sizes. The uncontaminated specimen proportions for Hg, Cd, Cu, As, Ni, Pb, Cr, and Zn were 6.8%, 4.3%, 31.1%, 28.6%, 42.9%, 64.6%, 44.7%, and 50.3%, respectively. Based on principal component analysis (PCA) and positive matrix factorization (PMF), four distinct sources of pollution were quantitatively attributed, including agricultural activities (22.08%), fossil fuel consumption (24.14%), steel production (29.78%), and natural sources (24.00%). Hg (80.29%), Cd (82.31%) and As (65.33%) in the region’s coastal sediments were predominantly contributed by fossil fuel, steel production and agricultural sources, respectively. Cr (40.00%), Cu (43.63%), Ni (47.54%), and Zn (38.98%) were primarily of natural lithogenic origin, while Pb mainly came from the mixed sources of agricultural activities (36.63%), fossil fuel (36.86%), and steel production (34.35%). Multiple factors played important roles in the selective transportation of sedimentary heavy metals, especially sediment properties, and hydrodynamic sorting processes in the study area.

Study area and sampling sites

As a shallow semi-enclosed epicontinental sea, the Bohai Sea received large amounts of terrestrial matter from coastal rivers, especially the Yellow River, which predominates in sediment transportation (Wang et al., 2014; Liu et al., 2020). There are several coastal rivers flowing into the study area with large amounts of sand, such as the Haihe River, the Jiyunhe River, the Daqinghe River, the Luanhe River, the Liaohe River, and the Yellow River.

Heavy metals within sediments were redistributed by currents in the area. The relatively warm and saline Yellow Sea Warm Current (YSWC) flows into the Bohai Sea from the Yellow Sea in winter (Xu et al. 2009). The North Shandong Coastal Current (NSCC) originates from the west coast of Bohai Bay and flows along the northern coast of the Shandong Peninsula in winter and summer (Fig. 1; Yuan et al., 2020). The main tidal constituent in the Bohai Sea is the semi-diurnal tidal component M2 with an average amplitude of 2 m (Li et al., 2019).

A total of 161 surface sediment samples (0~2 cm depth) collected from the south coast of Tangshan city in September and October 2021 were analyzed in this study (Fig. 1). These samples were distributed evenly across the study area and obtained with a stainless steel grab sampler dropped from a sampling ship. After the sediments were retrieved, the samples were placed in a sterile bag with a resealable zipper and stored at 4 °C in the dark until further analyses in the laboratory.

Grain size analyses

The organic and calcareous matters in each sample were removed by treatment with 30% H2O2 and 10% HCl. Before analysis, 0.5 M (NaPO3)6 was used to ensure complete disaggregation under ultrasonic dispersion. A laser particle size analyzer (Anton Paar PSA1190, Austria) was used for measuring sediment grain sizes in duplicate, with differences between measurements at < 1%. The percentages of clay, silt, and sand were determined with the grain size groups of <4 mm, 4~63 mm, and >63 mm, respectively. And the grain size parameter calculations were referred to a previous literature (Folk and Ward, 1957).

Heavy metals analyses

1. Cu, Cr, Zn, Pb, Ni, Cd, and As 

The sample was dried at 110°C for 5 hours, ground and sieved through a 200-mesh nylon sieve. A suitable amount of the sample was weighed and placed in a polytetrafluoroethylene sealed extraction vessel, followed by adding 1 ml of nitric acid and 3 ml of hydrofluoric acid. The mixture was shaken well and sealed tightly. The sealed vessel was subjected to thermal decomposition at 160°C~180°C for 48 hours using an automatic temperature-controlled hotplate. After cooling, the vessel was opened and the sample was steamed until nearly dry before adding 1 ml of perchloric acid. The sample was further steamed until fumes of white smoke appeared. After cooling, 2 ml of hydrochloric acid was added and the mixture was heated on an automatic temperature-controlled hotplate until the salt dissolved. The sample was then steamed until nearly dry before adding 2 ml of nitric acid to remove chloride ions. Next, 1.5 ml of nitric acid was added, the vessel was sealed tightly and heated to dissolve the sample for 12 hours at 160°C ~180°C on an automatic temperature-controlled hotplate. After cooling, the vessel was opened, and an appropriate amount of internal standard solution was added. The mixture was shaken well and held at 80°C for 12 hours on an automatic temperature-controlled hotplate. After cooling to room temperature, the vessel was opened, and the sample was transferred to a 50 ml volumetric flask with nitric acid and diluted to the mark. The contents were mixed well, and the levels of Cu, Cr, Zn, Pb, Ni, Cd, and As were determined using inductively coupled plasma mass spectrometry.

During the testing process, parallel analysis was carried out on 10% of the samples and two blank samples were included. The pass rate of the parallel samples was 100%, and the blank samples were all below the detection limit of the method. The results were validated using National Standard Materials of China (GBW07314, GBW07315, GBW07316), and the test values were within the standard value range.

2. Hg

The sample was dried at 110°C for 5 hours, ground and passed through a 200-mesh nylon sieve. An accurate amount of the sample was weighed and placed in a 50 ml stoppered colorimetric tube. To the tube, 2ml of nitric acid and 6ml of hydrochloric acid were added. Approximately 10ml of deionized water was used to rinse the inner wall of the tube thoroughly, and the mixture was stirred well. The tube was heated in a boiling water bath for 1 hour (with adequate shaking once during this process). The tube was removed and allowed to cool to room temperature, to which 1ml of potassium permanganate solution was added. The solution was shaken well for 20 minutes and then diluted with oxalic acid solution to the calibration mark. After thorough agitation, the tube was left standing to settle for 30 minutes. The determination of Hg was carried out using atomic fluorescence spectrometry.

During the testing process, 10% parallel sample analyses and two sample blanks were conducted for each batch of samples. The qualification rate of parallel samples was 100%, and the sample blanks were less than the detection limit of the method. Validation was conducted using National Standard Substances of China (GBW07314 and GBW07316), and the test values were within the range of standard values.

3. aluminum (Al)

To accurately weigh the rock powder sample dried at 105°C, 4g of sample was weighed with precision to 0.01g. The sample was transferred to a press mold, and boric acid was added for edge wrapping (press pressure of 30 MPa, held for 3s). The resulting pressed sample had a flat surface, and residual powder on the sample surface was blown away with an ear syringe. The sample was marked with a sample number and prepared for machine testing. The ZSX Primus II X-ray fluorescence spectrometer from Japan was used for testing the Al2O3 content.

During the testing process, 10% parallel sample analyses were conducted for each batch of samples. The qualification rate of parallel samples was 100%. Validation was conducted using National Standard Substances of China (GSD-10, GSD-11, GSD-12), and the test values were within the range of standard values.

In addition, the measurement error of the replicate sediments was within 5%. The Method Limit of detection were 0.07, 1.0, 0.2, 2.0, 1.0, 0.02, 0.1, and 0.002 mg kg–1 for Cr, Ni, Cu, Zn, As, Cd, Pb, and Hg, respectively. The Limit of quantitation were 0.28, 4.0, 0.8, 8.0, 4.0, 0.08, 0.4, and 0.008 mg kg–1 for Cr, Ni, Cu, Zn, As, Cd, Pb, and Hg, respectively.

4. Statistical analyses

As a classical assessment model, the Geo-accumulation index (Igeo) was widely used to assess the contamination extent of heavy metals in surface sediments by comparing the current concentrations with pre-industrial levels (Müller, 1979). The Igeos of the heavy metals were determined for analyzing the sources of heavy metals and understanding the heavy metal enrichment in the sediments near the study harbor, which were defined by the following equation:

Igeo = log2 (Metalsample/1.5Metalbackground)

where 1.5 serves as a correction factor to balance the variation of background values caused by diagenesis and the Metalbackground is the background geochemical values of the heavy elements (Sharifuzzaman et al., 2016).

The enrichment factor (EF) was used to evaluate anthropogenic influences of heavy metals in sediments because it can distinguish the sources of the metals from natural weathering processes and anthropogenic activities and display the status of environmental contamination (Zhang et al., 2007). As a conserved element, Al (Fig. S1) was used in calculating the enrichment factor for its standardization and elimination of the grain size effect (Fan et al., 2022). Referring to the previous literature (Buat-Menard and Chesselet, 1979), the EF was calculated as follows:

EF =(Metal/Al)sample/(Metal/Al)background

where (Metal/Al)background was calculated according to a previous reference (Zhao and Yan, 1994). Non-crustal sources of the heavy metals were corresponding to EF of >1.5, which were also thought to be related to human activities (Zhang et al., 2007).

The ecological effects of heavy metals were assessed by Effects range-low (ERL) and Effects range-mean (ERM), which represented the occasional occurrence of adverse biological effects (Long et al., 1995).

As an effective method to analyze the sources of heavy metals, the PCA was used in an attempt to further clarify the contribution of heavy metals in the sediments near the harbor (Liu et al., 2016a; Ogunlaja et al., 2019). In this analysis, we used the eight heavy metals as variables for PCA. The Kaiser-Meyer-Olkin-Measure of Sampling Adequacy (KMO) was 0.885 (> 0.6) and the significance level was zero (< 0.05), meaning suitable data for PCA.

The PMF model decomposes the original matrix into a contribution matrix and a factor allocation matrix to conduct data analysis, while also quantitatively describing the contribution of each sample (Jiang et al., 2017; Jiang et al., 2020). The computation principles and formula pertaining to PMF were derived from a scholarly work (Huang et al., 2022a, 2022b).

A Gao-Collins grain-size trends analysis (GSTA) method to produce a grain size trend map of the study area based on the spatial distribution of the grain size distributions (Gao and Collins, 2001). The GSTA exported the vector characteristics (magnitude and length) to deduct the vector pattern for the complete grain-size distribution, just as in the previous study (Gao, 1996; Lv et al., 2021).