Study areas
Every year, the Hong–Thai Binh river system discharges ~ 120×109 m3 of water and deposits 114×106 tons of silt in coastal areas. The annual flow of water is mainly concentrated in the rainy season (300 m3.s− 1 and 2200 m3.s− 1, from June to September), and just 50–300 m3.s− 1 during the remaining months are typically low (Table S4) (Vietnam Government Portal, 2010).
The coastal estuaries of the Thai Binh Province have a relatively uniform tidal regime, with the diurnal tide gradually decreasing from the north to the south. The maximum amplitude of tides ranges from 3.5 to 4.0 m, with an average of 1.7–1.9 m and a minimum of 0.3–0.5 m. The highest tide in a year can reach 4.0 m, and the lowest tide can be 0.08 m. Because of the high tide amplitude, salinity ranges from 5‰ to 20‰, and tides can result in considerably deep penetration of river mouths: 14 km for the Red River and 20 km for Tra Ly River. As shown in Table S4, the flow of rivers is lower in April than in the rainy season. During this time, the low water level facilitates seawater penetration into the land, especially saline intrusion.
Coastal flow in Thai Binh is a combination of tides, wind, waves, river flow, and the characteristics of the Gulf of Tonkin. Flows in estuarine areas are often strongly influenced by the river, whereas flow is predominantly caused by wind in the outermost sea surface. In the northeast wind field, the longshore surface current flow, influenced by the north wind, varies in a direction depending on the intensity of the river flow and the coastal terrain. The south wind is prevalent in the southeast wind field, and the surface flows along with the shore. The tidal flow and tidal composition play a decisive role in the integrated flow in the coastal area of Thai Binh.
Sampling
Sediment samples for the study were collected from 27 intertidal sites in northeast Vietnam during the dry season (i.e., April 1–10). Samples were collected from the estuaries of the Thai Binh, Diem Ho, Tra Ly, Lan, and Hong River mouths (Ba Lat estuary) and adjacent sites (Fig. 1). For each cross-section, three sampling sites, namely low, middle, and high tide marks, were obtained from inland. Specific sampling locations are shown in Fig. 1. All sediment samples were collected and carefully preserved to avoid contamination of samples. At each sampling site, five large samples were collected from a square area (each edge 20-m long), one sample from each corner, and the fifth from the intersection of the two diagonals. Each large sample consisted of three smaller samples that were collected 5-m apart. Samples were taken from a depth of 0–10 cm and stored in polyethylene bags at 4°C until later analysis. A handheld GPS device was used to fix the sampling location. All steps were accorded to the National Technical Regulation on the use of certified reference materials (CRM) in the analysis of geology and minerals (QCVN43:2012/BTNMT 2017).
Analytical procedures
The grain-size distribution of sediments was calculated by wet sieving of sand and gravel and by using the pipetting technique for silt and clay fractions (Robert L. 1980). Sediment samples were placed with weighting paper and dried out at 60°C in the oven until the weight was unchanged. Then, the dried sediments were milled to have a size < 2 mm before further measurement. Total organic carbon (TOC) of samples was calculated following the ISO 14235:1998, which is by reaction of samples with excess K2Cr2O7-H2SO4 solution and the excess being titrated using Mohr’s salt [(NH4)2Fe(SO4)2].6H2O (ISO 1998). Heavy metals were extracted from accurately weighed 0.25 g sample in polyfluorocarbon tubes by wetting with 2 mL of HNO3, 6 mL of concentrated HCl, and 2 mL of HF (El-Sorogy et al. 2016) and allowing to stand at room temperature overnight. The next day, samples were digested under pressure using a microwave system, increasing the temperature to 180°C in 15 minutes and maintaining that temperature for 30 min (QCVN43:2012/BTNMT 2017). Solutions were allowed to cool to room temperature, then transferred to plastic volumetric cylinders and diluted to 50 mL with deionized water. According to the manufacturer’s recommendations, the concentrations of As, Cd, Cr, Cu, Hg, Ni, and Zn were determined by inductive coupled plasma mass spectrometry (ICP-MS, model 7900 Agilent, USA). All digests were repeated at least twice, i.e., the values are means of at least three independent measurements. The standard deviation of independent replicate analyses was less than ± 10%. The ICP-MS was calibrated using purchased solutions of certified concentration. Accuracy was assessed using the European Commission’s standard reference material BCR-277R, and the recovery of spikes of As, Cd, Cr, Cu, Hg, Ni, and Zn, and of elements in BCR-277R ranged from 90–110%. In all the analyses blanks were performed and modifications were given if required. All the measurements were taken in triplicate and average results are provided.
Pollution assessment criteria
This study used the standards on sediment quality established by Ontario Canada’s Ministry of the Environment (OCME 2008) and the Environment Protection Agency (EPA), USA (Long et al. 1995). The ratings for these parameters are shown in Table 1.
Table 1
Standards for assessment of metal pollution in sediments
Elements
|
Guidelines of EPA USA
(mg.kg− 1, dry weight)
|
Ontario Canada’s Ministry of the Environment
(mg.kg− 1, dry weight)
|
|
ERL
|
ERM
|
No effect
|
lowest effect
|
Severe effect
|
As
|
8.20
|
70.0
|
N. A
|
6.00
|
33.0
|
Cd
|
1.20
|
9.60
|
N. A
|
0.60
|
10.0
|
Cr
|
81.0
|
370
|
N. A
|
26.0
|
110
|
Cu
|
34.0
|
270
|
N. A
|
16.0
|
110
|
Hg
|
0.15
|
0.71
|
N. A
|
0.20
|
2.00
|
Ni
|
20.9
|
51.6
|
N. A
|
16.0
|
75.0
|
Zn
|
150
|
410
|
N. A
|
120
|
820
|
ERL: effects range low; ERM: effects range medium. |
Assessment of heavy metal contamination in sediment by using indicators.
The Igeo, potential ecological risk of a single element (Eri), and potential toxicity response index for various heavy metals in the sediments (RI) were used to assess the cumulative origin of heavy metals in the intertidal sediments.
The geoaccumulation index (Igeo)
Igeo is a quantitative measurement of the level of contamination in sediments and freshwater environments. Igeo is calculated by comparing the total metal content in a sample with the base value of the metal (in this study, heavy metal content in the earth’s crust was used as the base value); thus, an increase in the present level is predicted to be due to human activity affecting the natural environment. This index is calculated after Muller (1969), and after that, it has been successfully applied by many researchers (Muller 1969; El-Sorogy et al. 2016; Beata et al. 2018; Dash et al. 2021). The index was calculated as Eq. (1):
(1)
Where Cn is the total measured concentration of the metal n, and Bn is the geochemical background of the metal n obtained from Turekian Karl’s measurements (Turekian et al. 1961). Factor 1.5 is a background matrix correction factor proposed to minimize the impact of possible changes on the baseline because of lithological changes in sedimentary rocks. According to (Loska et al. 1997), the classification of the sediment pollution level is as follows: Igeo ≤ 0 (unpolluted); 0 < Igeo < 1 (unpolluted to moderately polluted); 1 < Igeo < 2 (moderately polluted); 2 < Igeo < 3 (moderately to strongly polluted); 3 < Igeo < 4 (strongly polluted); 4 < Igeo < 5 (strongly to extremely polluted); and 5 < Igeo (extremely polluted).
The contamination factor (CF) and the potential ecological risks (Eri and RI)
CF is used to evaluate the contamination of heavy metals in sediments and is calculated by the ratio of the concentration of each metal in sediment to its base value. The formula (2) is as follows (Hakanson 1980):
(2)
is the heavy metal content of the studied sediment, whereas CB is the heavy metal content in the substrate, after accounting for the average content in the shale (Turekian et al. 1961). Pollutants are classified into four levels (Liu et al., 2005): CF < 1 (low CF ); 1 ≤ CF < 3 (moderate CF); 3 ≤ CF < 6 (considerable CF); and 6 ≤ CF (very high CF).
Eri is the potential ecological risk factor of single metal i. The calculation of Eri is deduced as follows:
where Tri is the response coefficient for the toxicity of the ith heavy metal. The corresponding Tri listed values are as follows: Hg = 40, Cd = 30, As = 10, Cu = Ni = 5, Cr = 2, and Zn = 1 (El Nemr et al. 2016); CFi is the contamination factor which calculated in Eq. (2). The levels of impact of the element on the sediment environment were as follows: Eri < 40 (low potential ecological risk), 40 ≤ Eri < 80 (moderate potential ecological risk), 80 ≤ Eri < 160 (considerable potential ecological risk), 160 ≤ Eri <320 (high potential ecological risk), and Eri ≥ 320 (serious ecological risk).
Finally, RI was obtained using Eq. (4).
(4)
The impact of potential ecological risk factors on the study elements was classified into four levels: RI < 150 (potentially low ecological risk), 150 ≤ RI < 300 (moderately potential ecological risks), 300 ≤ RI < 600 (considerably potential ecological risk), and 600 ≤ RI (seriously potential ecological risk) (Guo et al. 2010).
Statistical analysis
Multivariate statistical techniques, including Pearson Correlation Matrix (PCM) and Principal Component Analysis (PCA), were performed using OriginPro 2021 learning edition (OriginLab Corporation, Northampton, MA, USA) software packages. PCA is a strong pattern recognition approach that seeks to explain the variation of a large set of inter-correlated variables (Ranjbar Jafarabadi et al. 2018). It denotes the connection of variables, therefore decreasing the dimensionality of the data collection. PCA was conducted on the raw data set of heavy metals, TOC content, and grain components of the sediments in this work. The loading from PCA analysis provides information on the correlation between variables. Besides, their contribution to modeling and their effect on sample cluster separation also can be obtained. We are grateful to Originlab for providing a free student edition.