1. Introduction
Many challenges that are currently faced by humanity are due to environmental pollution, for example, disease outbreaks, climate change, the scarcity of safe drinking water, biodiversity, forest and wetland losses [
1,
2,
3,
4,
5]. These are fueled by the ever-increasing human population which has in turn caused great pressure on the pristine environment with their need for habitats, resources and waste assimilation [
6]. Consequently, the direct and indirect introduction of contaminants such as heavy metals (HMs), plastics, agrochemicals (such as fertilizers and pesticides), preservatives, endocrine-disrupting compounds, personal care products and pharmaceuticals into the environment has raised concerns [
7]. This is arguably because they pose threats to the pursuit and realization of some Sustainable Development Goals and thus, need to be continuously monitored [
8]. Of immediate concern is the pollution of water, which is an indispensable necessity for life on earth.
HMs are chemical elements with high molecular weights and a specific gravity (that is at least five times greater than that of water) and are toxic at concentrations that exceed their threshold values [
9]. Their high densities and toxicities are believed to be inter-related, and these HMs include metals such as cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), mercury (Hg), molybdenum (Mo), nickel (Ni), strontium (Sr), titanium (Ti), vanadium (V) and zinc (Zn), and metalloids such as lead (Pb), arsenic (As) and tin (Sn) [
10]. HMs are used in domestic, industrial, agricultural, medical and technological applications which have led to their uncontrollable distribution in the environment. The physicochemical nature of HMs makes them persistent, toxic and bio-accumulative. Owing to their high degree of toxicity, As, Cd, Cr, Pb and Hg are listed as priority HMs that are of public health significance [
11]. HMs can react with biological systems by losing one or more electrons and forming cations that can ably bind with the nucleophilic sites of the vital macromolecules. Their toxicity is caused through the disruption of cellular activities such as growth, differentiation, damage-repairing processes and apoptosis. These may be mediated through the generation of reactive oxygen species (thus, causing oxidative stress), weakening the organism’s antioxidant defense system, complexation or ligand-formation with organic compounds and the active sites of enzymes [
9,
11]. Their toxicity is contingent on the exposure route, dose, chemical form and the age, gender and nutritional status of the individual that is in question.
Upon entry into aquatic ecosystems, HMs equilibrate between aqueous and solid phases, and can bioaccumulate depending on their solubility and the toxicokinetics in the organism. Due to the ability of HMs to exist in two forms i.e., dissolved and accumulated, a large proportion of HMs tend to accumulate in sediments [
7,
12,
13]. Thus, sediments and biota form a useful portion of the passive samples that can be used to reflect the exposures and developments of the detection of compounds that are otherwise undetectable in the aqueous phase [
14,
15,
16,
17]. Human exposure to HMs is routinely
in utero, through inhalation or by contact with contaminated matrices (occupational exposure), or by the ingestion of contaminated foods or water [
18,
19]. HMs exert various health effects in humans once their permissible levels are exceeded. For example, Pb is a toxic, non-essential metal that is known to result in kidney failure, anaemia, weakness and brain damage upon exposure to it in high doses [
20,
21]. Long-term exposure may result in Pb poisoning, an increased risk of hypertension and the toxicity of the hematopoietic and nervous systems [
22,
23]. In addition, inorganic Pb compounds are cited as probable carcinogens to humans (Group 2A) according to the International Agency for Research on Cancer [
24].
Whereas HM pollution is systematically monitored in developed countries, this is not the case in developing countries. On the African continent, one of the regions with marked pollution challenges is the East African community (EAC), which is constituted by countries: Burundi, Democratic Republic of Congo, Uganda, Kenya, Tanzania, Rwanda and South Sudan. It is a region that is rich in water resources, such as Lake Turkana, Lake Tanganyika, Lake Kivu, Lake Malawi, the Western Indian Ocean and L. Victoria. Of great interest is L. Victoria, the world’s largest tropical lake and Africa’s largest freshwater lake which is shared among Tanzania (49%), Uganda (45%) and Kenya (6%) [
25]. It is an exoreic African Great Lake that is primarily drained by the longest river in the continent (the river Nile) into the Mediterranean Sea [
26]. The size of L. Victoria (surface area: 68,800 km
2) [
27], its complex shorelines with iconic island clusters and it having a rich fish species diversity has positioned it as the largest freshwater inland fishery in the world, which is largely based on Nile perch and Nile Tilapia [
28,
29]. Thus, it is a source of food (in the form of fish and other edible freshwater animals and plants), employment (livelihood), foreign exchange, water and other ecosystem services to at least 42 million people in the riparian EAC countries [
30].
The high levels of urbanization, industrialization and the large number of human settlements on the shores of L. Victoria has led to its inevitable pollution by both legacy and contaminants of emerging concern [
30,
31]. With a long retention time (23 years) and a flushing time of 123 years [
29], various studies have found contaminants such as microplastics [
30,
32,
33,
34,
35], polycyclic aromatic hydrocarbons [
36,
37,
38], per- and poly-fluoroalkyl substances [
39,
40,
41], active pharmaceutical ingredients and personal care products [
42], agrochemicals [
43], HMs, polybrominated diphenyl ethers, alternative flame retardants [
44,
45] and cyanotoxins [
46,
47,
48] in the water and fish from L. Victoria. Despite the foregoing studies, there is paucity of reports on the HM contamination of sediments from fish landing sites and ports on the Ugandan portion of L. Victoria. Some of the fish landing sites on the Ugandan Portion of L. Victoria include Ripon, Wairaka and Masese in Jinja, Katosi in Mukono, and Port Bell in Kampala. From an industrial perspective, Port Bell was considered for this study because it has one of the first instant tea factories in Uganda. Currently, the major industries in its vicinity are the Uganda Breweries (a subsidiary of the East African Breweries and maker of Uganda Waragi and Bell beer), Afroplastics Enterprises Limited (manufacturers of plastic items), and Cipla Quality Chemical Industries Limited which manufactures antiretroviral drugs [
49]. These factories are industrially located in the Luzira Industrial and Business Park. In addition to these, L. Victoria is endowed with alluvial depositions that contain sand, which is mined at Port Bell and sold to construction industries by the indigenous communities. Though previous reports have raised concerns that the mining activities might disfigure the lake shores, bed and fish breeding grounds [
50], no study has assessed the health risks that are associated with such dredging works. The main objective of the present study was, therefore, to assess the levels of HMs in the sediments from Port Bell, Northern L. Victoria, Uganda and the associated health risks that they could pose to humans and the lake’s ecosystem.
2. Materials and Methods
2.1. Description of Study Area
Port Bell is a 0.79-km-long port and fish landing site (
Figure 1) that is situated in Luzira, Nakawa-East, in the greater metropolitan Kampala area, Central Uganda (0°17′20.0″ N, 32°39′13.0″ E). The port takes its name from the British commissioner (Henry Hesketh Bell) who was an administrator who executed the interest of Britain in Uganda from 1906 [
51]. The port is positioned at the end of a narrow inlet of L. Victoria (shores of Murchison Bay), southeast of the central business district and the capital city of Uganda, Kampala (
Figure 2). Ferries operating from Port Bell provide a linkage between Kampala and other ports on L. Victoria, e.g., Mwanza and Musoma in Tanzania, Jinja in Uganda and Kisumu in Kenya [
49].
2.2. Collection and Preparation of Samples
The sediments were selected to determine the concentration of HMs that were being retained in the solid phase. The samples were obtained on Thursday 24 February 2022 in triplicate (n = 9; 3 for each site) using a grab sampler at 0–5 cm. Within each sampling station, the samples were collected at distances of at least 500 m from one another. The three sampling points (SP1, SP2 and SP3) were chosen as follows:
- (i)
SP1 is at the end of the terminal where Nakivubo channel pours its water into Port Bell.
- (ii)
SP2 is situated near the shores of Nakivubo channel.
- (iii)
SP3 is at the extreme end of the port towards the mainland.
The samples that were obtained were transferred into sterilized plastic polypropylene bottles, tightly sealed, labelled and submitted within 4 h to the Chemistry Laboratory, Uganda Industrial Research Institute (UIRI), plot 42A, Mukabya road, Nakawa industrial area, Kampala, Uganda. The samples were oven-dried at 80 °C to 95 °C for 16 h and then homogenized. They were crushed in a stone mortar and passed through a 0.63 μm nylon mesh sieve. The powdered sediment samples were preserved at 4 °C in an ice block.
2.3. Analysis of Physicochemical Parameters
The pH of the samples was determined at a sediment-to-water ratio of 1:2.5 using a precalibrated Hanna 211 digital microprocessor-based bench top pH/mV/°C meter (Hanna instruments, Italy) [
7]. A measurement of 20 g of the sample was transferred into a 100 mL beaker and 50 mL of distilled water was added to it. The mixture was shaken using a magnetic stirrer for 15 min. After 30 min, the suspension was shaken another 2 min and the pH of the suspension was directly recorded. The probe of the pH meter was rinsed with distilled water in between measurements. All of the measurements were performed in triplicate.
The moisture content of the samples was measured using the oven method at 60 °C. Briefly, 1.0 g of the samples were weighed into moisture dishes and transferred to the oven for 2 h, followed by their cooling in a desiccator [
52]. Thereafter, their weights were determined, and the moisture content was determined as a percentage of the differences between the wet and dry weights divided by the wet weight. The moisture analyses were done in triplicate.
2.4. Heavy Metal Analysis
Measurements of 0.25 g of the samples were digested on a hot plate in 20 mL of aqua regia (3 HNO3:1 HCl v/v) until the solution became colourless. The digestates were thereafter heated to near dryness and then they were cooled. A measurement of 20 mL of 1% nitric acid was added, mixed, and heated. After, filtration of the sample was done and then transferred into 50 mL sample vials. The solutions were then analysed for the presence of Cu, Pb, Cr and Cd (at wavelengths of 324.8 nm, 283.3 nm, 357.9 nm and 228.8 nm, respectively) using an Atomic Absorption Spectrometer (Perkin Elmer 3030) with a graphite furnace. All of the instrumental analyses were replicated twice.
The results that were obtained from the instrument were converted to mg/kg dry weight. A quality control was performed through an analysis of the blank and spiked samples according to the same procedure. Recoveries that were obtained ranged from 94% to 101%. Analytical precision (expressed as Relative Standard Deviation) varied between 3% and 5%. The method detection limit (LOD) of each metal was computed as Blank + 3 × Standard deviations of four samples analyzed in triplicate.
2.5. Human Health Risk Assessment
The United States Environment Protection Agency (US EPA) suggested that the human health risk assessment model should estimate the potential health risks of contaminants based on exposure and toxicity assessments [
53]. This study appreciated that lake sand mining occurs at Port Bell and inside L. Victoria, and miners come into contact with dredged HMs-contaminated sediments. Accordingly, the average daily dose (ADD
therm) in mg/kg/day was calculated (Equation (1)) to discern if there is any potential HM intoxication through skin contact [
54,
55,
56,
57].
where
Cm = heavy metal concentration (mg/kg),
SA is the exposed surface area = 4350 cm
2 for adults [
56],
DAF is the dermal absorption factor = 0.001,
AF is the skin adherence factor in mg/cm
2/day = 0.7 for adults [
12],
Ef = exposure frequency (365 days/year),
Ed = exposure duration, the average lifetime (58.65 years for an adult Ugandan) [
14,
58],
Wab = average body weight (considered to be 60 kg for adults) and
Taet is the average exposure time for non-carcinogens =
Ef × Ed [
59].
The hazard quotient (HQ) was calculated using Equation (2). On the whole, a HQ ≤ 1 implies that the exposure is very unlikely to have adverse effects while a HQ > 1 represents a possibility of non-carcinogenic effects, with its probability increasing as the value of the HQ increases [
12].
The dermal reference doses (
RfD) for Cu, Pb, Cr and Cd through dermal contact are 4.0 × 10
−4, 5.4 × 10
−4, 3.0 × 10
−3 and 1.0 × 10
−3 mg/kg/day, respectively [
12]. The reference dose is the maximum daily dose of a metal from a specific exposure pathway that is believed not to lead to an appreciable risk of deleterious effects to sensitive individuals during their lifetime [
60]. For this study, the HQ was computed through a single pathway (dermal contact) with the assertion that such contact of lake sand miners with periodically dredged sediments are inevitable [
12,
14].
Since exposure to multiple HMs can increase the non-carcinogenic health risks due to instances of dermal contact with contaminated sediments, the cumulative risk (hazard index, HI) was estimated using Equation (3) [
61].
2.6. Sediment Quality Assessment
To evaluate the level of contamination of the sediments from Port Bell, four pollution indices were computed namely; contamination factor (CF), geo-accumulation index (I
geo), pollution load index (PLI) and the potential ecological risk index (PERI) [
62,
63,
64]. The CF was calculated using Equation (4), which was suggested by Hakanson [
62].
where
Cm is the metal concentration in the sediment sample and C
Bn is the geochemical background/preindustrial concentration of the same metal.
Geo-accumulation index (I
geo) for the sediments was obtained from computations utilizing Equation (5), which was suggested by Müller [
65].
where
Cm and C
Bn follow from Equation (4), whereas, in this equation, 1.5 is the background matrix correction factor which was introduced to cater for lithological variability [
66,
67].
The pollution load index (PLI) was calculated using Equation (6) as follows:
where CF is the contamination factor (Equation (4)) and n = 4 is the number of HMs that were studied.
Lastly, the potential ecological risk (
) and potential ecological risk index (PERI) were computed employing Equations (7) and (8) in tandem:
where
is the biological toxic factor for the HMs = 5, 5, 2 and 30 for Cu, Pb, Cr and Cd, respectively [
68,
69]. The degrees of ecological risk for single elements and PERI for all factors combined are outlined in
Table 1.
2.7. Statistical Analysis
All experiments were performed in triplicate, and the data that were obtained were checked for normality and averaged prior to statistical evaluation. One-way analysis of variance was used to examine significant differences between the means, followed by Tukey post hoc test. Pearson’s bivariate correlation and principal component analysis (PCA) were used to explore the inter-relationships between metal concentrations and aquatic environmental parameters (pH and moisture content). All statistical analyses were executed at 95% confidence interval using GraphPad Prism for Windows (v9.3.1, GraphPad Software, San Diego, CA, USA).