Contamination and Ecological Risk Assessment of Metal(loid)s in Sediments of Two Major Seaports along Bay of Bengal Coast

Abstract

Pollution from shipping, industrial, and municipal wastewater discharges is a major source of heavy-metal contamination at seaports located near estuaries or along the coast. In this study, for the first time, nine metal(loid)s (Pb, Cd, Cr, Cu, Mn, Zn, Ni, Fe, and As) were analyzed from the surface sediment of two major seaports on the Bay of Bengal coast to evaluate the degree of pollution and ecological risk. The average concentrations of metal(loid)s followed the decreasing order of Fe (53,800 ± 4002 mg/kg) > Mn (590 ± 116.8 mg/kg) > Zn (67.59 ± 13.5 mg/kg) > Ni (62.8 ± 22.5 mg/kg) > Cr (36.59 ± 7.22 mg/kg) > Cu (32.63 ± 6.78 mg/kg) > Pb (16.78 ± 3.93 mg/kg) > As (6.33 ± 1.9 mg/kg) > Cd (0.71 ± 0.16 mg/kg). Both sites had much greater Fe concentrations (compared to other metals) than the levels that had been previously recorded at nearby localities. Furthermore, elements such as Fe and Ni surpassed the recommended NOAA and EPA limits for maximum samples from both ports. However, with the exception of one sampling point in Chattogram Port, the pollution-load-index (PLI) values were <1, indicating no heavy-metal contamination. For all metals except Cd, the enrichment factor (EF) values were also <1, indicating that the metals came from natural sources. Besides, the contamination factor (CF) was 1 < CF < 3 for Cd and <1 for other metals, therefore, the study area was under moderate risk for Cd contamination. The geo-accumulation index (I_geo) values indicated that the study area is moderately polluted with Cd (I_geo > 0). In addition, the potential ecological-risk index (PERI) revealed that the both areas are under considerable (PERI > 80) to moderate (PERI > 40) risk due to Cd pollution. Correlation and Principal Component Analyses (PCA), demonstrated the anthropogenic sources of some metals, especially Fe, Ni, and Cd. As a result, it is suggested that the study area should be followed up on, to track changes and design a pollution-control strategy to reduce future pollution hazards.

1. Introduction

Seaports are the primary route for business and commerce around the world, and they are also heavily used for recreation and tourism [1,2]. The activities of seaports have a significant impact on the global economy, accounting for roughly 80% of global trade volume and 70% of global trade value [1,3]. Metal contamination, oil leakage, rubbish, greenhouse gas emissions, and a variety of other toxins put stress on the ecosystem, as a result of severe monetary and sports workouts [1,4]. Due to their abundance, high toxicity, inherent persistence, non-biodegradable existence, pervasiveness, biogeochemical cycle, environmental risk, and bioaccumulation, heavy metals are among the most important harmful pollutants in seaport areas and the aquatic ecosystem [5,6,7]. Under favourable water-driven conditions, metals released into the waterbody from seaports are often dissipated by binding with suspended matter and eventually sink to sediment [1,7,8]. As a result, aquatic sediments act as a metal deposit and also provide strong evidence of anthropogenic pressure [9]. Toxicity and heavy-metal mobility in sediment, on the other hand, are determined by a number of criteria, including metal-binding state, chemical type, total accumulation, and metal characteristics [10]. High salinity, for example, enhances the solubility of weakly bound metals due to cation exchange, whereas pH dictates metal partitioning due to Ca²⁺ and H+ ionic characteristics [7,11]. Moreover, metals entering the marine systems generally settle down and assimilate into silt, along with organic matter, Fe/Mn oxides, sulfides, and clay [1].

Generally, seaports around the world are highly polluted with heavy metals, as they accommodate industry, trade, tourism, and recreation [1]. Different port activities, e.g., transportation, loading and unloading goods, hazardous material storage, industrial installation, fishing, excavation and dredging, ballast-water discharges, ship maintenance, and drainage of highly populated catchment areas, contribute to the high level of heavy metals in seaports. Like other ports in the world, the Chattogram and Mongla ports are located on the bank of two estuaries (the Karnaphuli and Passur rivers), draining two highly urbanised and industrialised catchment areas. Untreated sewage disposal and effluents from industries such as spinning mills, dying, textiles, steel mills, ship scrapings, oil refineries, and continuous dredging contribute to metal pollution in both ports [2,5,6]. High levels of heavy metals/metal(loid)s, such as Fe, As, Cr, Cd, and Pb, were reported from these areas in earlier studies [5,6]. These contaminants are deposited in the aquatic environment and eventually sink to the bottom of the water table [12]. As a result, aquatic sediments are being investigated as a possible indicator of aquatic-ecosystem monitoring [13]. Furthermore, sediments have an advantage over other indicators in that they record and aggregate environmental events over time within the aquatic system (such as shipbreaking enterprises), a phenomenon called “sediment memory”. Sediments also enable the prediction of future environmental change and status, by combining environmental events throughout time [14,15] in addition to providing useful geographical and temporal information [2,14]. Due to the potential harm to human health, the buildup of heavy metals in sediments and their bioaccumulation and biomagnification through plants and other species is a growing concern. When excessive concentrations of metals become available to marine organisms, they may be absorbed and accumulate in their tissues, causing biological reactions and eventually inhibiting growth and development [16]. Furthermore, metal transfer in the food chain is one of the most important avenues for harmful compounds to enter the human body. The accumulation of these heavy metals in the human body and other animals over time can lead to serious illness [17]. For example, lung cancer, kidney dysfunction, skin lesions, central nervous system damage, and hypertension are caused by chronic exposure to Pb, Cd, and As [13].

Due to its massive impact on the biological health of the marine environment and humans, heavy metal(loid)s have raised the concerns of researchers all over the world. Heavy-metal contamination in aquatic sediments and its possible effects on the environment and human health have been widely investigated in most parts of the world [18,19,20,21,22,23,24,25]. In Bangladesh, several investigations on heavy metals in coastal surface sediments [26,27,28,29,30] and in ship-breaking sediments [31] have been conducted. Many studies have evaluated heavy metals from port areas in various regions of the world [3,32,33], but no particular information regarding heavy metals in sediments and their risks from seaport areas of Bangladesh is available so far.

Therefore, this study intended to assess the metal(loid)s’ levels, their risks, the sediment quality, and the ecological status of the two major seaports of Bangladesh, using a number of indices such as the geo-accumulation index, pollution-load index, enrichment factor, and potential ecological-risk index. Key stakeholders, such as marine and estuary managers, government officials, and policymakers, will be able to use this tool to conserve aquatic biota and the environment.

2. Materials and Methods

2.1. Study Area

Two major seaports (Chattogram and Mongla) in Bangladesh were chosen for sampling (Figure 1). Chattogram Port (22°20′0″ N, 91°50′0″ E), located on the bank of the Karnaphuli River near Chattogram city, is one of Bangladesh’s major seaports. This port handles over 90% of the country’s export and import business. For example, in 2018, it dealt around 2.80 million TEUs (twenty-foot equivalent units) of import, export, and empty containers. The port is engaged in the shipping of large containers, crude oil, chemicals, and fertiliser and steel products. Mongla Port (22.49′98″ N, 89.58′83″ E) is the second largest and busiest seaport in Bangladesh, located along the coast of Bay of Bengal near the Passur River in Bagerhat District. In 2018, it handled 29,330 TEUs (twenty-foot equivalent units) of import and export containers. In total, 33 ships can load and unload the import and export items at a time. It is surrounded and protected by mangrove forest. Semidiurnal tides with a 2–4 m range predominate in both estuaries. The bottom is made up of tertiary rocks that are covered with alluvial deposits, which are composed of successive layers of mud and sand. The climate of Bangladesh’s riverine or estuary varies from season to season, and is heavily impacted by the Bay of Bengal. The southern section of the country has a tropical climate, with a warm and sunny winter from November to February, a hot spring from March to May, and a long rainy season from June to October due to the summer monsoon. The annual rainfall is 17.8 inches, with temperatures ranging from 58 °F to 90 °F.

2.2. Sample Collection

A total of 30 sediment samples were collected from 10 sampling stations (5 stations from each port) of both port areas from 8–13 December 2020. Three replicate sediment samples were collected from each station within a 1 m radius. In each case, top soil (from the 0–10 cm depth) was collected using the Peterson grab. Sample locations were fixed using a Global Positioning System (GPS) device (Germin, Olathe, KS, USA). The sediment samples were then placed in pre-labelled plastic Ziploc bags, and transported to the Analytical Chemistry Laboratory (ISO/IEC 17025 accredited) of the Atomic Energy Centre Dhaka (AECD) for further analysis.

2.3. Sample Pre-Treatment and Analysis

In the laboratory, sediment samples were oven dried at 90 °C, then ground into powder form using a mortar and pestle. The samples were sieved through 0.63 μm mesh to obtain a homogeneous mass following the work of Tasrina [34]. In this study, nine heavy metals, namely Pb, Cd, Cr, Cu, Mn, Zn, As, Fe, and Ni, were detected due to their common occurrence in port areas. Elemental analyses were determined following the US EPA method [35]. Briefly, for digestion, 15 mL of aqua regia (3HCI + HNO₃) was mixed with 0.5 gm of dry sediment sample in a beaker. The mixture was then placed on a heated plate and covered with a watch glass in a digesting chamber, where the temperature was kept between 50 °C and 100 °C for 10 h to 12 h. After filtering the mixture with a filter paper (Whatman no. 41), the extracted solution was poured into a 50 mL volumetric flask and diluted with deionised water to make a volume of 25 mL. Finally, the vial’s mouth was sealed with a cork and stored at 4 °C until further analysis. After cooling, the solution was placed into Flame Atomic Absorption Spectroscopy (FAAS) for the selected elemental analyses, but for As a Hydride Generation Atomic Absorption Spectroscopy (HGAAS; VGA 77, United States Patent No. 4559808) by Varian Analytical Instruments (Models AA DUO 240 FS and AA 280 Z) was used [36,37].

Figure 1. Study area and location of two major seaports, Chattogram and Mongla, in Bangladesh.

Figure 1. Study area and location of two major seaports, Chattogram and Mongla, in Bangladesh.

2.4. Quality Assurance and Control

The chemicals and reagents used for digestions and elemental analyses were AR grade (E. Merck, Germany). Standard solutions of the individual elements used were with the highest purity level (99.98%). In each case, 100 mL of different concentrations of working standard solutions of the studied metals were prepared from stock standard solutions (1000 mg/L) by diluting with Milli-Q water (water resistivity > 18.2 MΩ-cm, 25 °C; Millipore, MA, USA). To avoid contamination and improve precision, analytical quality control also included the analyses of a blank, a spike, a sample duplicate, and the reference materials of a similar matrix.

The accuracy of the procedures was evaluated using the certified reference material IAEA-433 (estuarine sediment). The detection limit, precision, and recovery values obtained are presented in the supplementary tables (Tables S1 and S2). The recovery rate of 90%–97% was achieved and was deemed satisfactory for the purposes of the analysis.

2.5. Assessment of Pollution Levels and Potential Ecological Risk

Metal pollution in sediments and its ecological implications have been measured using a variety of ways. To quantify varying levels of pollution, the “geo-accumulation index, enrichment factor, pollution load index, contamination factor, and possible ecological risk index” have been estimated.

2.5.1. Contamination Factor (CF) and Pollution Load Index (PLI)

The contamination factor and pollution load index were calculated to determine the extent of heavy-metal contamination and to provide a measure of overall contamination. The Contamination Factor (CF) was calculated as follows [38]:

$$\mathrm{CF}={\mathrm{C}}_{\mathrm{metal}}/{ \mathrm{C}}_{\mathrm{background}}$$

(1)

where CF = contamination factor, ${\mathrm{C}}_{\mathrm{metal}}$ = pollutant levels in sediment, and ${\mathrm{C}}_{\mathrm{background}}$ = background value of the metal. The background was based on average metal concentrations in shale. The geochemical background value of a given metal was taken from Turekian and Wedapohl [31] because they provided background values for every kind of sediment. The CF values were classified into four groups to reflect the metal enrichment in the sediment. The CF values < 1 denote low contamination, 1 ≤ CF < 3 indicates moderate contamination, 3 ≤ CF ≤ 6 designates considerable contamination, and CF > 6 specifies very high contamination in the sediment [38].

Each site was evaluated for the extent of metal pollution by employing the method based on the pollution-load index (PLI) developed by [39], as follows:

PLI = (CF₁ × CF₂ × CF₃ × …… CFn)^1/n

(2)

where n = number of metals studied and CF = contamination factor. The PLI provides a simple but comparative means for assessing site quality. When PLI is <1, it represents no pollution, and PLI > 1 shows a deterioration of site quality [27].

2.5.2. Enrichment Factor (EF)

The enrichment factor (EF) was used to determine the extent of metal pollution and the possible natural and anthropogenic sources. EF is defined mathematically, as follows [40]:

$$\mathrm{EF}=\frac{\left[\frac{\mathrm{Cx}}{\mathrm{Cref}}\right]\mathrm{Sample}}{\left[\frac{\mathrm{Bx}}{\mathrm{Bref}}\right]\mathrm{Background}}$$

(3)

where (Cx/Cref) _sample = ratio of concentration of a target metal and the reference metal in the sediment samples and (Bx/Bref) _background denotes the ratio of concentration of a target metal and the reference metal in the background material. The element concentrations in the sediment served as a background, while Al was employed as a normaliser. Furthermore, EF aids in the assessment of metal-contamination levels. On the basis of the enrichment factor, the following five contamination categories are recognised: deficiency to minimal enrichment (EF < 2), moderate enrichment (2 ≤ EF < 5), significant enrichment (5 ≤ EF < 10), very high enrichment (20 ≤ EF < 40), and extremely high enrichment (EF ≥ 40) [41].

2.5.3. Geoacumulation Index (I_geo)

The index of geoaccumulation (I_geo) is widely used in the assessment of contamination, by comparing the levels of the heavy metals obtained to the background levels originally used with bottom sediments [42]. It is estimated by the following equation:

$${\mathrm{I}}_{\mathrm{geo}}={\log }_2\left(\raisebox{1ex}{$\mathrm{Cn}$}\!\left/ \!\raisebox{-1ex}{$1.5\mathrm{Bn}$}\right.\right)$$

(4)

where Cn denotes the heavy-metal concentration measured, and Bn denotes the heavy-metal concentration in the geochemical background. Due to lithogenic effects, a constant factor of 1.5 is used to adjust the background matrix. The following seven categories of geo-accumulation index were used to determine the categorisation contamination level: [42]: practically unpolluted (I_geo < 0), unpolluted to moderately polluted (0 < I_geo < 1), moderately polluted (1 ≤ I_geo < 2), moderately to strongly polluted (2 ≤ I_geo < 3), strongly polluted (3 ≤ I_geo < 4), strongly to extremely polluted (4 ≤ I_geo < 5), and extremely polluted (I_geo ≥ 5).

2.5.4. Assessment of Potential Ecological Risk

Hakanson [38] developed a methodology to assess the ecological risks potential for heavy-metal pollution. The potential ecological-risk index (R_I) was introduced to assess the degree of heavy-metal pollution in sediments, according to the toxicity of heavy metals and the response of the environment as follows:

$${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}={\mathrm{T}}_{\mathrm{r}}^{\mathrm{i} }{\mathrm{C}}_{\mathrm{f}}^{\mathrm{i}}$$

(5)

$${\mathrm{C}}_{\mathrm{f}}^{\mathrm{i}}={ \mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}/{\mathrm{C}}_{\mathrm{o}}^{\mathrm{i}}$$

(6)

$${\mathrm{R}}_{\mathrm{I}}={{\displaystyle \sum }}^{ }{\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$$

(7)

where R_I is considered as the sum of all risk factors for heavy metals, ${\mathrm{E}}_r^i$ is the monomial potential ecological-risk factor, ${\mathrm{T}}_{\mathrm{r}}^{\mathrm{i} }$ is the toxic-response factor for a given substance, which accounts for the toxic and sensitivity requirements, and the values for Cr, Mn, Cu, Zn, As, Pb, and Cd are 2, 1, 5, 1, 10, 5, and 30, respectively [31,38]. The potential ecological risk of coastal sediments posed by heavy metals was classified into five categories, per Hakanson [38]: low risk (E_rⁱ < 30 and R_I < 100), moderate risk (30 ≤ E_rⁱ < 50 and 100 ≤ R_I < 150), considerable risk (50 ≤ E_rⁱ < 100 and 150 ≤ R_I < 200), very high risk (100 ≤ E_rⁱ < 150 and 200 ≤ R_I < 300), and disastrous risk (E_rⁱ ≥ 100 and R_I ≥ 300). Turekian and Wedapohl [43], who gave background values for all types of sediment, were used to calculate the geochemical-background value of a given metal.

2.5.5. Statistical Analysis

The concentration of heavy metals was calculated as mg/kg (wet weight). The normality of the dataset was tested by a Shapiro–Wilk test, and then ANOVA was conducted to compare the variation in the mean values metals. The Pearson correlation coefficient and principal component analysis (PCA) were conducted through PAST (version 3.0) to identify the relationship and association of the heavy metals. The graphical representation of the study was conducted through GraphPad Prism (version 7.0).

3. Results and Discussion

3.1. Metal(Loid)s in Sediments of Chattogram and Mongla Port

The mean concentration of the nine studied metal(loid)s (Pb, Cd, Cr, Cu, Mn, Zn, As, Fe, Ni) in Chattogram Port were 19.18 ± 2.74 mg/kg, 0.81 ± 0.15 mg/kg, 42.02 ± 4.95 mg/kg, 37.15 ± 3.68 mg/kg, 682.82 ± 75.5 mg/kg, 78.22 ± 7.83 mg/kg, 6.5 ± 0.99 mg/kg, 56,007 ± 3125, and 75.75 ± 22.70 mg/kg, respectively, whereas in Mongla Port their levels were 14.37 ± 3.58 mg/kg, 0.60 ± 0.08 mg/kg, 31.15 ± 4.37 mg/kg, 28.1 ± 6.21 mg/kg, 497.86 ± 60.50 mg/kg, 56.96 ± 8.10 mg/kg, 6.16 ± 2.80 mg/kg, 51,495 ± 3680 mg/kg, and 49.85 ± 14.3 mg/kg, respectively (

Table 1. Comparison of detected values of heavy metals/metal(loid)s with recommended values of unpolluted sediments (mg/kg); n = 30 for this study.

Metals	Present Study Mean (Range)	EPA Guideline for Sediments (EPA, 1977)			(NOAA, 1999)	Turekian and Wedepohl (1961) [47]
		NP	MP	HP
Pb	16.78 ± 3.93 (9.69–22.18)	40	40–60	60	46.7	20
Cd	0.71 ± 0.16 (0.47–0.93)	*	*	>6	1.2	0.3
Cr	36.59 ± 7.22 (23.56–47.44)	25	25–75	75	81	90
Cu	32.63 ± 6.78 (20.22–42.78)	25	25–50	50	34	45
Mn	590 ± 116.8 (427.18–787.56)	300	300–500	500		850
Zn	67.59 ± 13.5 (44.88–86.48)	90	90–200	200	150	95
As	6.33 ± 1.9 (3.38–10.76)	3	3–8	8	8.2	13
Fe	53,800 ± 4002 (46,702.86–58,719.55)	17,000	17,000–25,000	25,000		47,200
Ni	62.8 ± 22.5 (33–106.92)	20	20–50	50	20.9	68

Note: NP = not polluted, MP = moderately polluted, HP = heavily polluted. * Not detected.

). In both port areas, the metal(loid)s followed the decreasing order of Fe > Mn > Zn > Ni > Cr > Cu > Pb > As > Cd. Among the studied metals, the concentrations of Fe and Ni in all samples exceeded the guidelines adopted by the NOAA and the EPA [43,44]. However, among the two studied ports, Chattogram Port has the higher metal concentration than Mongla Port. It might be due to shipping, industrial activities, and other human activities. Chattogram Port is the oldest port in the country, which possesses an annual container volume of 3.097 million TEUs [45], whereas Mongla Port is relatively new, possessing an annual container volume of 100,000 TEUs [46]. Therefore, it is obvious that the Chattogram Port area is enduring severe anthropogenic pressure.

Among the metals, the concentrations of Fe were highest in both ports. In Chattogram Port’s sediment, the concentration of Fe ranged from 50,716.35 mg/kg to 58,719.55 mg/kg (mean 560,069.90 mg/kg), whereas in Mongla Port’s sediment the Fe concentration ranged from 46,702.90 mg/kg to 56,702.48 mg/kg (mean 51,495.15 mg/kg). These concentrations of Fe in both sediments exceeded the sediment-standard guidelines by the EPA [43] (Table 1). Besides, they also exceeded the average shale value provided by Turekian and Wedepohl [47]. Moreover, our findings of Fe exceeded the reported values of similar national and international studies (

Table 2. Comparison of metals/metal(loid)s in sediments (mg/kg) with different other studies in the world. N = 30 for this study.

Region	Pb	Cd	Cr	Cu	Mn	Zn	As	Fe	Ni	References
Chattogram Port (Karnaphuli River)	19.18	0.81	42.02	37.15	682.82	78.22	6.5	56,006.90	75.75	Present study
Karnaphuli River	15.49		28.17	20.05	-	-	-	24,075	29.8	[53]
Karnaphuli River		92.11	70.06		-		16.79		-	[5]
Mongla Port (Passur River)	14.37	0.6	31.15	28.1	497.86	56.96	6.16	51,495.15	49.85	Present study
Passur River	6.92		19.37	15.83				21,306.03	20.62	[48]
Passur River (Mongla Port)	-	-	-	-	649	69.8	6.73	41,600	-	[54]
Port Jackson	18		4	6 ± 1		85 ± 7	6	7300 ± 142	3 ± 1	[51]
Port Newcastle	24		4	4	185 ± 50	78 ± 3		5300 ± 566	3	[51]
ASV (average shale value)	20	0.3	90	45	850	95	13	47,200	68	[47]

). In Bangladesh, previously the highest concentration of Fe was recorded from Passur River (21,306.03 mg/kg) [48], but our study found an extensively higher concentration than those findings. However, the high concentrations of Fe in the sediment of Chattogram Port and Mongla Port might be associated with excessive port activities.

Increased Fe in the port region could be due to terrestrial loads/sources. Again, this area is generally within the turbidity-maximum zone, which is defined as the region with higher suspended particulate matter concentration than the adjacent up and down streams of one estuary, which influences the biogeochemical cycle of both inorganic and organic material, resulting in an increase in the Fe level.

Mn concentration varied from 427.18 mg/kg to 787.56 mg/kg in both seaports in the study. Chattogram Port had a higher concentration of Mn than Mongla Port (Table 1). However, in terms of Mn concentration in the sediments, both of the study areas were moderately polluted, based on the scale provided by the EPA [43]. However, the average concentration of Zn in Chattogram Port was 78.22 mg/kg, whereas in Mongla Port it was 56.96 mg/kg. According to the NOAA [44], the threshold level of Mn in sediment is 150 mg/kg, whereas the EPA confined it to 90 mg/kg (to be not polluted by Mn), and our findings were within both limits (Table 1).

Ni concentration in both studied ports ranged from 33 mg/kg to 106.92 mg/kg. According to the NOAA [44], the maximum limit of Ni in the sediment is 20.9 mg/kg. Therefore, the findings of the study exceeded the guideline value (Table 1). Besides, according to the EPA [43] standard, both ports were moderate to highly polluted (Table 1). Environmental pollution from nickel may be due to industry, the use of liquid and solid fuels, and municipal and industrial waste [49]. Industrial waste must be responsible for the increased concentration of Ni in those stations.

In the present study, the average concentrations of Zn in Chattogram Port was 78.22 mg/kg, whereas in Mongla Port it was 56.96 mg/kg. The concentrations of Zn were within the threshold value of the sediment-quality guideline provided by the NOAA [44]. Besides, the study areas were not polluted by Zn, according to the EPA [43]. In addition, the average Cr concentrations in Chattogram Port was 42.02 mg/kg, whereas in Mongla Port it was 31.15 mg/kg. These concentrations were within the threshold level of the NOAA [44]. However, sediments of the both studied ports were moderately polluted, based on the EPA guideline (Table 1).

Concentration of Cu is higher in Chattogram Port and was moderately polluted by Cu, according to the EPA sediment-quality guideline (Table 1). Besides, the Cu concentration in Chattogram Port sediment exceeded the NOAA standard. Copper is present in all aquatic environments. It is an essential element for all living organisms, but can be toxic when taken in high doses. Copper is a common ingredient in anti-biofouling paints, which are applied on the surfaces of ships and in offshore engineering (Pan and Wang, 2012). The painting of ships is one of the reasons for the increase in Cu. The activities of nearby industries and their sewage are other reasons, since copper pollution in the marine environment generally occurs due to sewage and industrial activities (Hichey, 1992).

The highest concentration of Pb in Chattogram Port has been found to be highly influenced by industrial processes. Though in both ports, Cr in sediment was within the limit, so they were moderately polluted, according to the EPA. However, according to Islam et al. [50], oil can be contaminated with Pb through atmospheric deposition from various point sources, such as smelting and industrial processes. Besides, the adjacent industrial wastewater can be a factor that increases Pb in these stations. However, the average concentrations of As were almost similar in both of the study areas (Table 2). Though most of the As concentrations were within the threshold, the study areas were moderate to highly polluted. However, the highest concentration of Pb and As in Mongla Port has been found at a sampling point adjacent to the jetty. Therefore, loading and unloading activities in the jetty might be responsible for higher Pb and As concentrations.

Among the metal(loid)s studied, Cd had the lowest concentration in both Chattogram and Mongla ports. The concentrations of Cd varied from 0.47 mg/kg to 0.93 mg/kg throughout the study; the average concentration in Chattogram Port was 0.81 mg/kg, while in Mongla Port it was 0.6 mg/kg. According to the NOAA, the concentration of Cd in the sediment should not exceed 1.2 mg/kg, and our findings were within this limit. However, the findings of Cd concentration are almost similar to the findings in Karnafuli River [51] and Naples Harbor [33] but are lower than Quseir Harbor [52].

The levels and variations of metals in this study could be explained by the prevailing environmental conditions. Many geochemical and hydrodynamic parameters, such as pH, particle size, organic matter, and dissolved oxygen, influence the mobility, distribution, and toxicity of metals in aquatic sediment. In general, the higher the pH is, the lesser the mobility. At pH 7, for example, Zn and Cd ions begin to break away from their complexes, whereas Cd, Co, and Cr ions are less mobile. At pH 5, these metals will start to dissolve [25]. Oxidised Fe-, Mn hydroxide, nitrate, and sulphate compounds can be found in sediments with high levels of dissolved oxygen [25]. As heavy metals are adsorbed to organic particles, organic matter reduces their potential bioavailability by redistributing them into less bioavailable forms. Furthermore, when sediment grain size decreases, metal concentration rises, indicating that smaller sediments contain more heavy metals than coarser sediments. The fundamental reason for this is that smaller grain-size particles have a higher surface-to-volume ratio than bigger grain-size particles. Heavy metals can be absorbed on the negatively charged surfaces of clay minerals, organic materials, or iron- and manganese-hydrous oxides and then eliminated through ion exchange with hydrogen ions and other cation species [23,25].

3.2. Ecological Risk Assessment of Heavy Metals

3.2.1. Pollution-Load Index (PLI) and Contamination Factor (CF)

The PLI can provide some perception for the occupants about the quality of the environment. Moreover, it also gives useful information to the decision-makers about the status of the pollution rate of the area [55]. In this study, PLI values in each station were found < 1, which indicates the perfection of the sediment, except only one sampling station of Chattogram Port, which represents a progressive pollution. The order of the PLI is Chattogram Port > Mongla Port. However, the metals’ total-contamination factor (CF) followed the order of Cd > Fe > Ni > Pb > Cu > Zn > Mn > As > Cr throughout the study (Figure 2), which revealed that the study areas were polluted by Cd. Especially in Chattogram Port, the CF values in most of the sampling points were >1, which depicted moderate contamination by heavy metals in this area.

3.2.2. Geo-Accumulation Index (I_geo)

I_geo values have been used to explain the sediment quality and metal-accumulation status [56]. The values of I_geo indicate that the study area is moderately polluted with Cd, since I_geo values of all the metals (Except Cd) remained below 0, which means unpolluted. The I_geo values of Cd was >0 (0.62), which revealed that it was moderately polluted by Cd. The I_geo values of all metals in the present study showed the decreasing order of Cd > Fe > Ni > Pb > Cu > Zn > Mn > As > Cr (Figure 2).

3.2.3. Enrichment Factors

To distinguish the metal sources that originated from anthropogenic and natural means, a normalised enrichment factor is implemented [57]. Enrichment factor (EF) < 1 indicates natural sources of the heavy metals, while enrichment factor (EF) > 1 indicates anthropogenic sources, [58]. The sediments were normalised with respect to the reference elements, including Fe [47]. In the current study, EF values for Pb, Cr, Cu, Mn, Zn, As, and Ni are less than 1 in both ports (Figure 2). The EF value of Cd is greater than 1 in both ports, which indicates Cd is coming from both anthropogenic sources and natural sources.

3.2.4. Potential Ecological-Risk Index (PERI)

The ecological-risk-assessment index reflects not only the single ecological risk of individual contaminants but also indicates the integrated ecological and toxicological effects of the increased pollution, by driving the ecological-risk levels of soil contamination [59]. The average monomial risk factor E_rⁱ of metals in sediment samples were ranked in the following order: Cd > As > Ni > Pb > Cu > Cr > Zn. The value of monomial ecological risk for As, Ni, Pb, Cu, Cr, and Zn metals were found to be below 30, which indicates that these metals pose a low risk in the surrounding ecosystem. This was true except for Cd, which was above 80 in most of the stations in Chattogram Port, indicating considerable risk, and above 40 in the other stations, indicating moderate risk. The RI was also found to be below 100, which indicates the surrounding environment is exposed to a low-risk condition, except for some stations in Chattogram Port. Metal (viz. Fe and Mn) with Er value < 1 is not considered for the calculations of RI (

Table 3. Heavy-metal potential-ecological-risk indexes in sediment (n = 30).

$\mathbf{Ecological} \mathbf{Risk} \mathbf{for} \mathbf{Sin}\mathbf{gle} \mathbf{Metal} {\mathit{E}}_{\mathit{r}}^{\mathit{i}}$
	Pb	Cd	Cr	Cu	Zn	As	Ni	RI
C1	5.55	93.00	1.05	4.75	0.88	4.82	6.22	116.28
C2	4.30	92.00	0.86	3.77	0.72	4.38	5.88	111.91
C3	5.46	90.00	0.97	3.76	0.91	6.29	7.86	115.25
C4	4.66	71.00	1.00	4.23	0.85	4.98	4.11	90.84
C5	4.02	60.00	0.79	4.14	0.75	4.50	3.78	77.96
M1	2.42	47.00	0.52	2.25	0.47	2.60	5.32	60.59
M2	4.91	58.00	0.74	4.13	0.69	8.28	3.59	80.34
M3	3.47	65.00	0.70	2.83	0.60	3.85	3.73	80.18
M4	3.38	65.00	0.74	3.09	0.57	4.23	2.43	79.44
M5	3.78	67.00	0.76	3.31	0.66	4.74	3.26	83.51
Mean	4.19	70.80	0.81	3.63	0.71	4.87	4.62

C1–C5: five sampling points from Chattogram Port; M1–M5: five sampling points from Mongla Port.

).

3.2.5. Source Identification

Pearson correlation analysis and PCA were analyzed to find the interrelations and associations of metals in the ports’ sediments. The interrelations and associations of metals in aquatic sediments provide factual information of their origin and pathways. Strong positive correlations between metals may depict their release from the same origin of pollution and identical behaviour [19,60]. However, most of the metals are strongly positively correlated to each other, except As and Ni. Fe, Zn, Mn, Cu, Cr, and Cd had a strong positive correlation with most of the metals (

Table 4. Correlation matrix of heavy metals obtained from the sediment of Chattogram and Mongla ports (p < 0.05 or significant correlations are given in bold) (n = 30).

	Pb	Cd	Cr	Cu	Mn	Zn	As	Fe	Ni
Pb
Cd	0.73
Cr	0.88	0.82
Cu	0.88	0.56	0.84
Mn	0.86	0.70	0.85	0.73
Zn	0.93	0.75	0.95	0.85	0.91
As	0.71	0.19	0.37	0.58	0.43	0.49
Fe	0.93	0.57	0.78	0.80	0.71	0.79	0.85
Ni	0.50	0.65	0.45	0.22	0.67	0.55	0.07	0.25

Note: Bold numbers indicate strongly correlated.

), which revealed that these metals may be derived from similar sources, either natural or anthropogenic. However, As had positive correlations with Pb only (p < 0.05), and Ni had a positive correlation with Cd and Mn (p < 0.05).

PCA also shows a similar association of metals, compared to the correlation matrix. In PCA, the first two components depict 87.28% of the total variance, where component 1 describes 71.86% and component 2 describes 15.41% of the total variance. Component 1 was dominated by most of the studied metals, such as, Pb, Cd, Cr, Cu, Mn, Zn, and Fe. However, component 2 was highly loaded with As and Ni but shows a negative association with the others (Figure 3). A similar result in PCA was observed in the previous study of comparative metal concentration in the seaports [1]. Therefore, based on the correlation and PCA, we speculate that most of the heavy metals in Chattogram and Mongla port’s sediment may be aroused from different ship- and transportation-related anthropogenic activities, such as ship loading and unloading, ship oil, painting, hauling, anchoring, and other related activities. Besides, seaports are the centre of development throughout the world. Many cement factories, oil refineries, and urbanisation projects are located around these studied seaport areas, which might also be important sources of heavy-metal intensification in seaport sediments.

However, there are many factors that regulate the transportation and distribution of heavy metals in aquatic ecosystems. Terrestrial transports, hydrodynamic conditions, and the type of sediment are major factors regarding the distribution of heavy metals [61]. Fine sediment possesses larger concentration of metals, which allows them to travel a longer distance along the river basins [62]. Besides, despite various environmental features, such as pH, organic matters, and carbon content, influence these processes, which are not included in our study due to it having a different focus. Therefore, it is further recommended to study the transport and distribution of heavy metals considering sediment types, particle size, environmental conditions, and organic matter.

In the cluster analysis, the metal-concentration data were normalised and followed Ward’s method using Euclidean distance. In terms of Chittagong Port, there were three clusters (C1–C4, C2–C5, and C1–C4–C3) observed based on dendrograms (Figure 4a) where a station wise difference is justified. On the other hand, two clearly observed clusters (M2–M5, and M3–M4) were identified for the different research stations of Mongla Port (Figure 4b). However, the metal concentration data for the different two sampling spots, Chittagong and Mongla ports, were identical or almost similar, meaning the levels of pollution and sources of contamination are more or less similar (Figure 4c).

4. Conclusions

This study was designed to provide the first baseline details on the concentrations of nine selected metalloids in sediments of two major seaports in Bangladesh. The results demonstrated that the levels of the metalloids followed the decreasing order of Fe > Mn > Zn > Ni > Cr > Cu > Pb > As > Cd for both seaports. In both ports, Cd concentration outstripped the average shale value on Earth, while the accumulation of Cd in the sediment poses a considerable ecological risk. Due to the high concentrations of Cd, the potential ecological-risks index (PERI) showed that the study areas are considerable-risk zones. The study area is moderately polluted with Cd (I_geo < 0). Moreover, the PLI values in each station were found to be below 1, which indicates that the sediment was practically unpolluted, though the order of the PLI is higher in Chattogram Port than Mongla Port. Besides, the EF value of Cd is greater than 1 for both ports, which indicated that the Cd was from both anthropogenic sources and natural sources.