Ecological Risk Assessment of Heavy Metals Pollution in the Loskop Dam of the Olifants River System, South Africa

Abstract

The Loskop Dam is a major reservoir on the upper Olifants River in South Africa. Many human activities in the upper river catchment are causing contamination in the river including heavy metals. Although several studies have investigated water pollution in the river system, limited information exists regarding the spatial distribution and ecological risks of heavy metals in the Loskop Dam and their ecological implications. Seasonal heavy metal concentrations and ecological risks associated with heavy metal contamination in the dam were assessed. Though most of the heavy metal concentrations were below detection levels in the water, the concentrations were substantially higher in the sediments, with higher concentrations mainly recorded during winter than summer. Chromium and nickel concentrations in the sediments exceeded the permissible guideline values. Furthermore, contamination factor, enrichment factor and geoaccumulation index were used to determine the extent of chemical pollution, and ecological risk index was used to assess the potential ecological risks. The contamination indices found the sediments to be moderately to highly contaminated by Cr, Pb and Zn. However, the ecological risk values were low, indicating a low ecological risk of contamination posed by heavy metals in the dam. During winter, Cd had the highest ecological risk and during summer, the ecological risk was dominated by Pb, but the values indicated a low contamination (ER <40) and the potential ecological risk index values were also low (RI < 150). Nonetheless, effective conservation strategies are needed to prevent further degradation of the river system. Furthermore, the study reinforces the importance of addressing metal pollution and conservation of freshwater ecosystems, which aligns with the United Nations Sustainable Development Goal (SDG) 6, particularly in enhancing water accessibility and responsible sanitation management.

1. Introduction

Globally, many freshwater ecosystems are facing increasing degradation, resulting in a decline in ecosystem services and biodiversity. Freshwater pollution is mainly caused by discharges from activities such as agriculture, mining, industrialization, urbanization and household [1,2,3,4]. Although heavy metals in freshwater ecosystems may originate naturally, their concentrations are usually low and may have little impact on the environment [5,6], but discharges from human activities contain, among other substances, heavy metals that can cause considerable ecological effects, such as impairing ecosystem functions, disrupting food webs and reducing biodiversity.

Environmental pollution, particularly by heavy metals, is of serious concern due to their ability to bioaccumulate, be non-biodegradable, and have potential toxic effects [7,8]. Heavy metals are usually trapped in the water column and accumulate in sediments and aquatic organisms. In sediments, changes in physicochemical conditions such as pH, redox, temperature and sediment disturbance can cause the release of heavy metals back into the water; thus, sediments may serve as a secondary source of heavy metal pollution [9,10,11]. Hence, the presence of heavy metals in both water and sediment poses serious threats to the aquatic ecosystem and humans. Assessing ecological risks resulting from heavy metals in sediments such as the Loskop Dam cannot be overemphasized. When heavy metals occur in solutions, they enter the food chain, bioaccumulate and biomagnify in various trophic levels [12]. South Africa is a water-scarce country, and freshwater resources are under continuous threat from rapid population growth, economic expansion and climate change. Similar to many developing countries, demand for water is increasing, coupled with aging water and sanitation infrastructure and poor management, which are worsening the water situation [3,13]. It is estimated that water demand would soon exceed supply as economic activities in the country continue to grow [14].

The Olifants River is considered among the most polluted rivers in South Africa [15]. The catchment is experiencing a rapid increase in agricultural, mining and industrial activities, including coal-fired power plants to generate electricity. Currently, 11 of the 15 coal-fired power plants (CFPPs) owned by the national power utility, Eskom, are in Mpumalanga Province [16]. The Loskop Dam is the first major reservoir on the Olifants River in the Mpumalanga Province, and its water quality is deteriorating due to runoff from agricultural fields, acid mine discharge and return flows from sewage treatment plants [17]. Though, studies have been done previously, they did not focus on a comprehensive assessment of heavy metals and their ecological implications. This study aims to determine (i) seasonal variation in heavy metal concentrations, arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) in the inflow (S1), middle (S2) and dam wall (S3) of the Loskop; and (ii) to determine if the contamination levels pose significant ecological risks to the aquatic environment. The findings of this study intend to fill the existing knowledge gap by assessing the concentrations, spatial distribution, and ecological risks of selected heavy metals in the Loskop Dam, thereby providing critical information for environmental monitoring, pollution control, and sustainable catchment management. It would also contribute to Sustainable Development Goal (SDG) 6, which focuses on advancing efforts toward universal access to clean water and effective sanitation practices.

2. Materials and Methods

2.1. Study Area

The Loskop Dam is one of the largest reservoirs on the Olifants River. It is in Mpumalanga Province, South Africa (25°25′01″ S; 29°21′01″ E). It has a full capacity of 362 million m³ and is 43 m high. It is about 30 km and covers a surface area of about 148 km². The dam was built to accommodate a design flood of 2886 m³/s [18]. The dam supplies water to two municipalities, Groblersdal and Marble Hall, and is used for irrigation. The area has an average annual rainfall of 649 mm. Vegetation in the surrounding area consists mainly of broad-leaved woodland, riparian grasses, reeds, and bushveld species. The catchment is heavily influenced by human activities and the dam experiences increasing environmental stress associated with eutrophication, sedimentation, mining-derived pollution, agricultural runoff, elevated nutrient loading, and fluctuating dissolved oxygen conditions. During warmer months, the dam experiences thermal stratification, resulting in oxygen-poor bottom waters. These stratified conditions may promote nutrient cycling and the mobilization of heavy metals from sediments into the water column. Three sites were selected: the inflow region where the Olifants River enters the reservoir (S1), the middle section (S2), and the dam wall (S3), to ascertain the level of pollution at different areas of the dam (Figure 1).

2.2. Sampling of Water and Sediment

Water samples were collected seasonally, during summer (March) and winter (July), to account for high flow and low flow respectively. A handheld multiparameter instrument (YSI) was used to measure temperature, salinity, dissolved oxygen, electrical conductivity, pH, and total dissolved solids. Water samples were collected at each site by immersing an acid-pretreated plastic bottle into the water column. The samples were labeled and put in a container with ice and sent to the laboratory. The samples were kept at 4 °C in a refrigerator until subsequent analysis.

Seasonal sediment samples were taken at the same sites where the water samples were collected, using a Friedlinger mud grab. The samples were transferred to 1 L acid-pretreated polyethylene sampling bottles and labeled. They were put in a container with ice and sent to the laboratory. In the laboratory, the samples were refrigerated prior to chemical analysis. The water and sediment samples were analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for concentrations of As, Cd, Cr, Cu, Hg, Pb, Mn, Ni, Fe and Zn in an accredited chemical laboratory (WaterLab (Pty) Ltd.) in Pretoria, South Africa. The samples were sub-divided into glass vials and heated at 60 °C for 24 h. Nitric acid (HNO₃) and hydrochloric acid (HCl) mixture at 3:1 ratio was used to mineralize the samples. Heavy metal determinations of the samples were conducted in parallel, in three batches. Approved guidelines were used to obtain analytical accuracy, and recoveries were within 10% of standard values. To ensure data accuracy, quality control/quality assurance (QC/QA) measures such as certified reference materials (CRMs) were used to ensure the validation of data and the accuracy and precision of the analytical method. Analytical precision and accuracy were verified through replicate analyses and comparison with certified standards from De Bruyn Spectroscopic Solutions 500 MUL20-50STD2 (Johannesburg, South Africa), and recoveries were within 10% of the certified values. All the reagents used were of superior quality and of analytical grade. All the solutions were prepared using ultra-pure water. All plastic, quartz and glassware were soaked in HNO₃ (10%) for at least 24 h and rinsed repeatedly with ultra-pure water. Total heavy metal concentrations were expressed in mg/kg dry sediments.

2.3. Data Analysis

Mean and standard deviation of the physicochemical parameters and metals were calculated using descriptive analysis. Data of the physicochemical parameters and metals were assessed for normality and homogeneity of variance and were found to conform to parametric assumptions. Differences in metal concentrations among sites were, therefore, analyzed. An independent-samples t-test was used to determine seasonal (winter and summer) differences (p < 0.05). Spearman’s correlation analysis was performed to establish the associations between the physicochemical parameters and heavy metals. Principal component analysis (PCA) was used to obtain the main components of the data accountable for explaining metal variations and to deduce those originating from common sources. All the analyses were done using the Statistical Package for Social Sciences (SPSS) v. 26.0.

2.3.1. Enrichment Factor (EF)

The presence and degree of sediment contamination were evaluated using the enrichment factor (EF). To calculate EF values, usually Al or Fe concentration is used to normalize heavy metal concentrations.

$$EF=\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{x}}}{Fe}}\right)/\left({\displaystyle \frac{Baseline {\mathrm{C}}_{\mathrm{x}}}{Baseline Fe}}\right),$$

where C_x is the element concentration, and Fe is a reference value to adjust for natural metal concentration. Iron (Fe) was used as a normalization element in the calculation of the enrichment factor (EF) because it is considered a stable and naturally abundant element in the Earth’s crust [19].

2.3.2. Geoaccumulation Index (I_geo)

The level of contamination was assessed using the geoaccumulation index (I_geo)

$$Igeo=log2 (Cx/1.5Bn),$$

where C_x is the concentration of heavy metal in sediment and Bn is the geochemical background (average shale value) for a given metal, and 1.5 is applied to adjust for possible variations in the background values. The classes of EF and I_geo are presented in

Table 1. Sediments classification using enrichment factor and geoaccumulation index.

EF Classes	Enrichment Level	Igeo Value	Igeo Class	Contamination Level
EF < 1	No enrichment	Igeo ≤ 0	0	Uncontaminated
EF = 1–3	Minor enrichment	Igeo = 0–1	1	Uncontaminated/moderately contaminated
EF = 3–5	Moderate enrichment	Igeo = 1–2	2	Moderately contaminated
EF = 5–10	Moderately severe enrichment	Igeo = 2–3	3	Moderately/strongly contaminated
EF = 10–25	Severe enrichment	Igeo = 3–4	4	Strongly contaminated
EF = 25–50	Very severe enrichment	Igeo = 4–5	5	Strongly/extremely contaminated
EF > 50	Extremely severe enrichment	Igeo > 5	6	Extremely contaminated

.

2.4. Pollution and Ecological Risk Assessment

To assess sediment pollution and ecotoxicological risk at Loskop Dam, the following indices, contamination factor (CF) and ecological risk indices (ERI), were used to assess the degree of contamination and potential risk [20].

2.4.1. Contamination Factor (CF)

Contamination factor determines the degree of contamination by an element compared to pre-industrial reference concentrations [21]. Many studies have measured metal baseline values from an area with geologically similar material or average crustal composition [22]. The formula for calculating this index is given by Hakanson [21] as follows:

$$CF={\displaystyle \frac{ C\left(metal\right)}{ C\left(background\right)}}$$

where C(metal) is the concentration of the metal of interest measured at a particular site, and C(background) is the average shale values [23]. The average shale values were used because of the absence of local or regional reference values for heavy metals [24,25]. There are four levels of contamination [26], as shown in

Table 2. Classification of contamination indices for sediments.

CF		Ecological Risk Indices
CF Range	Contamination Categories *	ER **	RI ***	Classification
CF < 1	Low contamination	<40	<150	Low risk
1 < CF < 3	Moderate contamination	40 ≤ Er < 80	150 ≤ RI < 300	Moderate risk
3 < CF < 6	Considerable contamination	80 ≤ Er < 160	300 ≤ RI < 600	Considerable risk
CF > 6	Very high contamination	160 ≤ Er < 320	>600	High risk
		>320		Very High risk

* Hakanson [21], Wang et al. [27]; ** Rostami et al. [28], Mahabadi et al. [29]; *** Kang et al. [30], Hu et al. [31].

.

2.4.2. Ecological Risk Assessment of Sediment

Ecological risk index [21] is the potential ecological risk caused by the contamination of all the selected heavy metals in view of their combined ecological impact [26]. The following equations were used to assess the ecological risk potential [20,21,32].

$$ER=TR\times CF$$

(1)

$$RI=\sum ER$$

(2)

where CF is the pollution factor, ER indicates the ecological risk of each metal, RI is the ecological risk index which represents the sum of the metals, and TR is the toxicity response factor of the elements [21]. The TR values are as follows: As = 10, Cd = 30, Cr = 2, Cu = 5, Hg = 40, Mn = 1, Ni = 5, Pb = 5, and Zn = 1. The classification of ER and RI is shown in Table 2 [20,21].

3. Results

3.1. Physicochemical Parameters and Trace Metal Concentrations in Water

Seasonal readings of the physicochemical parameters of the water column are presented in Table 3. The mean temperature ranged from 15.32 °C in winter to 23.54 °C in summer. The temperatures at the three sites were within the permissible guideline values. Mean dissolved oxygen (DO) concentrations decreased from 9.05 ± 3.00 mg/L in winter to 6.34 ± 1.13 mg/L in summer. The pH ranged from 7.56 to 9.43 in winter and 7.63 to 9.41 in summer. These values were within the SANS guideline of 6–9. The mean electrical conductivity increased from 407.98 μS/cm in winter to 453.58 μS/cm in summer. The total dissolved solids (TDS) decreased slightly from 326.57 ± 2.92 mg/L in winter to 299.33 ± 23.06 mg/L in summer. Mean salinity ranged from 0.24 ppt in winter to 0.22 ppt in summer. There were no significant differences in all the parameters between the two seasons (p > 0.05), except temperature (p < 0.05).

The concentrations of Cd, Cr, Hg and Pb in the water were below detection limits across sites and seasons. However, Cu, Ni and Zn were only detected during summer, and As, Fe and Mn were only detected during winter. The concentrations of the metals detected were within the permissible guideline values, except for Mn during the summer season (Table 3).

Table 3. Seasonal physicochemical parameters and heavy metal concentrations in the water from Loskop Dam.

Parameters	Winter (Mean ± SD)	Summer (Mean ± SD)	Guideline Value
Temperature (°C)	15.32 ± 0.08	23.54 ± 0.26
pH	7.56–9.43	7.63–9.41	6.5–9.0 2
DO	9.05 ± 3.00	6.34 ± 1.13	-
Conductivity (µS/cm)	407.98 ± 2.10	453.58 ± 49.64	-
TDS	326.57 ± 2.92	299.33 ± 23.06	-
Salinity (‰)	0.24 ± 0.0	0.22 ± 0.02	<0.5‰ 1
As	-	0.001	0.01 3
Cu	0.001	-	2.00 3
Fe	-	0.23	0.30 3
Mn	-	0.08	0.01 3
Ni	0.002	-	0.07 3
Zn	0.003	-	5.00 3

¹ DWAF [17], ² BC-MECCS [33], ³ WHO [34].

Table 3. Seasonal physicochemical parameters and heavy metal concentrations in the water from Loskop Dam.

Parameters	Winter (Mean ± SD)	Summer (Mean ± SD)	Guideline Value
Temperature (°C)	15.32 ± 0.08	23.54 ± 0.26
pH	7.56–9.43	7.63–9.41	6.5–9.0 2
DO	9.05 ± 3.00	6.34 ± 1.13	-
Conductivity (µS/cm)	407.98 ± 2.10	453.58 ± 49.64	-
TDS	326.57 ± 2.92	299.33 ± 23.06	-
Salinity (‰)	0.24 ± 0.0	0.22 ± 0.02	<0.5‰ 1
As	-	0.001	0.01 3
Cu	0.001	-	2.00 3
Fe	-	0.23	0.30 3
Mn	-	0.08	0.01 3
Ni	0.002	-	0.07 3
Zn	0.003	-	5.00 3

¹ DWAF [17], ² BC-MECCS [33], ³ WHO [34].

3.2. Concentrations of Heavy Metals in the Sediments

Heavy metal concentrations were substantially higher in the sediment compared to the water column. The concentrations showed a distinct seasonal variation, with higher concentrations in winter than in summer, except for As (Table 4). The range of metal concentrations were as follows: As (3.46–3.73 mg/kg), Cd (0.00–0.15 mg/kg), Cr (154.09–182.6 mg/kg), Cu (7.89–23.05 mg/kg), Fe (20,193.67–21,719.97 mg/kg), Mn (225.58–480.95 mg/kg), Ni (14.52–23.20 mg/kg), Pb (15.86–17.14 mg/kg) and Zn (25.33–97.33 mg/kg) (Table 4). During winter, heavy metal concentrations decreased in the following order: Fe > Mn > Cr > Zn > Ni > Cu > Pb > As > Cd, and during summer, contamination levels decreased in the following order: Fe > Mn > Cr > Zn > Pb > Ni > Cu > As > Cd. During winter, higher levels of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn were recorded, while As had a higher concentration in summer. The mean Cr concentrations exceeded the permissible standard value during both seasons and Ni exceeded the permissible standard value in winter.

Table 4. Mean metal concentrations (mg/kg) in the sediments during summer and winter in the Loskop Dam.

Elements	Winter	Summer	SQG	Average Shale Value
	Mean ± SD	Mean ± SD
As	3.46 ± 1.69	3.73 ± 3.70	5.9	13
Cd	0.15 ± 0.16	0.00 ± 0.00	0.6	19
Cr	182.6 ± 35.97	154.09 ± 69.37	37.3	90
Cu	23.05 ± 21.32	7.86 ± 4.28	35.7	45
Fe	21,720 ± 1734	20,194 ± 1999	-	47,200
Mn	480.95 ± 49.13	225.58 ± 81.57	-	850
Ni	23.20 ± 13.64	14.52 ± 5.05	18	68
Pb	17.14 ± 4.81	15.86 ± 4.41	35	20
Zn	97.33 ± 18.41	25.33 ± 3.17	123	95

SD = Standard deviation; SQG = Sediment Quality Guideline (CCME) [35].

Table 4. Mean metal concentrations (mg/kg) in the sediments during summer and winter in the Loskop Dam.

Elements	Winter	Summer	SQG	Average Shale Value
	Mean ± SD	Mean ± SD
As	3.46 ± 1.69	3.73 ± 3.70	5.9	13
Cd	0.15 ± 0.16	0.00 ± 0.00	0.6	19
Cr	182.6 ± 35.97	154.09 ± 69.37	37.3	90
Cu	23.05 ± 21.32	7.86 ± 4.28	35.7	45
Fe	21,720 ± 1734	20,194 ± 1999	-	47,200
Mn	480.95 ± 49.13	225.58 ± 81.57	-	850
Ni	23.20 ± 13.64	14.52 ± 5.05	18	68
Pb	17.14 ± 4.81	15.86 ± 4.41	35	20
Zn	97.33 ± 18.41	25.33 ± 3.17	123	95

SD = Standard deviation; SQG = Sediment Quality Guideline (CCME) [35].

3.2.1. Spatial Distribution of Heavy Metals in the Sediments

There were variations in the distribution of heavy metals, with S2 having the highest contamination during winter (Figure 2). Arsenic showed a substantial increase from 1.6 mg/kg at S1 and S2 in summer to 8.00 mg/kg at S3 in winter. The highest Cd and Cr concentrations were recorded at S2 during winter. Similarly, Cu showed uneven spatial distribution, with winter concentrations of 54.02 mg/kg at the middle site compared to 2.38 mg/kg at the dam wall and 12.8 mg/kg at the inflow. Iron exhibited the highest concentrations (7,549.4 to 42,254.4 mg/kg), with the highest concentration at S1, while the highest concentration of Mn (913.01 mg/kg) was recorded at S2 and the lowest concentration (99.5 mg/kg) at S3. Nickel concentrations ranged from below detection at S3 to 18.4 mg/kg at S1, Pb concentrations ranged from 6.0 at S1 to 31.7 mg/kg at S2, and Zn concentrations ranged from 2.82 mg/kg at S3 to 230.15 mg/kg at S2.

3.2.2. Correlation Analysis

The correlation analysis revealed strong relationships among metals in the sediments (Figure 3). Arsenic showed a strong positive relationship with Fe and Pb (r = 0.94, p < 0.01; r = 0.95, p < 0.01 respectively); Cd had a positive association with Cu, Mn, Ni and Zn (r = 0.94, p < 0.01; r = 0.90, p < 0.01 and r = 0.86, p < 0.01; r = 0.92, p < 0.01 respectively); Cu had a strong association with Mn and Zn (r = 0.97, p < 0.01; r = 0.99, p < 0.01 respectively); Fe had a strong positive correlation with Pb (r = 1.00, p < 0.01); Mn had a strong correlation with Zn (r = 0.97, p < 0.01); and Ni correlated strongly with Zn (r = 0.94, p < 0.01). Strong correlations were also observed among the water parameters and with the heavy metals. A strong positive relationship existed between TDS and salinity (r = 0.97), while EC was strongly correlated with temperature (r = 0.77). Metals such as As, Fe, Pb, Mn, and Zn exhibited positive correlations with TDS and salinity. In contrast, Cr correlated negatively with EC (r = −0.71), and Cd and temperature correlated negatively (r = −0.64).

3.2.3. Principal Component Analysis (PCA)

The PCA was used to show the seasonal clustering behaviour of the metals across sites (Figure 4). Two components of the PCA explained 96.3% of the total variance. The first and second components explained 70.7% and 25.6% of the data variation respectively. All the metals except As are clustered together during winter, showing strong positive loadings on PC1. Besides seasonal effects, both trends indicate spatial influence in contamination characteristics, as most of the metals had higher concentrations in the middle of the dam. This could be attributed to localized discharge from human activities in the catchment.

Furthermore, Figure 5 describes two distinct seasonal contamination trends. The blue represents the overall contamination during winter, with higher heavy metal concentrations, while the red represents a lower contamination gradient of heavy metals during summer.

3.3. Sediment Contamination Indices

Enrichment Factor (EF) and Geoaccumulation Index (I_geo)

Heavy metal contamination in the sediments from the Loskop Dam was evaluated using the EF and I_geo. In accordance with the EF classifications, As, Cd, Cu, Mn, Ni, Pb and Zn showed deficiency to minimal enrichment (EF < 3) and Cr showed moderately to severe enrichment. The highest EF values were at S3 during winter and S1 during summer (Figure 6). The I_geo values of As, Cd, Cu, Mn and Ni were below zero during both seasons, but Cr, Pb and Zn were between zero and 1; thus, they fall under the non-contaminated to moderately contaminated class (Figure 7).

4. Ecological Risk Assessment

4.1. Contamination Factor (CF)

The calculated CF values of the majority of metals were higher in winter than in summer; however, for the sites, the highest CF values were collected at S2 and S3 in winter and summer respectively (

Table 5. Mean spatio-temporal contamination factors (CFs) of heavy metals in sediment of the Loskop Dam (2024–2025).

Season	Elements	S1	S2	S3
Winter	As	0.21	0.42	0.18
	Cd	0.21	1.11	0.22
	Cr	1.59	2.38	2.11
	Cu	0.29	1.20	0.05
	Fe	0.35	0.87	0.16
	Mn	0.51	1.07	0.12
	Ni	0.18	0.85	0.01
	Pb	0.60	1.60	2.24
	Zn	0.62	2.24	0.03
Summer	As	0.12	0.12	0.62
	Cd	0.00	0.00	0.00
	Cr	1.96	2.33	0.84
	Cu	0.12	0.12	0.29
	Fe	0.15	0.23	0.90
	Mn	0.16	0.30	0.34
	Ni	0.26	0.13	0.24
	Pb	0.03	0.46	1.60
	Zn	0.09	0.05	0.65

). Most of the elements showed low seasonal contamination; however, in winter, the following elements showed moderate contamination: Cr at all the sites; Cd, Cu, Mn and Zn at S2; and Pb at S2 and S3. During summer, Cr showed moderate contamination at S1 and S2, and Pb at S3. The CF value of Cd decreased from low to moderate in winter and to zero in summer.

4.2. Ecological Risk (ER) and Risk Index (RI)

The results of the potential risk factor and potential ecological risk index for the metals in the sediments are shown in

Table 6. Ecological risk (ER) and risk index (RI) for heavy metals in the sediments from the Loskop Dam of the Olifants River.

Elements	As	Cd	Cr	Cu	Mn	Ni	Pb	Zn	RI
Winter
S1	2.1	6.3	3.18	1.45	0.51	0.18	3.0	0.62	17.34
S2	4.2	33.3	4.76	6.0	1.07	0.85	8.0	2.24	60.42
S3	1.8	6.6	4.22	0.25	0.12	0.01	11.2	0.03	24.23
RI	8.1	46.2	12.16	7.7	1.7	1.04	22.2	2.89	102.0
Summer
S1	1.2	0.00	3.92	0.6	0.16	1.3	0.15	0.09	7.42
S2	1.2	0.00	4.66	0.6	0.30	0.65	2.3	0.05	9.76
S3	6.2	0.00	1.68	1.45	0.34	1.2	8.0	0.65	19.52
RI	8.6	0.00	10.26	2.65	0.8	3.15	10.45	0.79	36.70

. The ER values were higher in winter than in summer. During winter, Cd had the highest ecological risk of 33.3 at S2, followed by Pb (11.2) and Cr (4.76) at S3. During summer, the ecological risk was dominated by Pb (ER = 8.00) at S3, followed by As (6.2) also at S3 and then Cr (4.66) at S2. All the seasonal ER values showed low ecological risk (ER < 40) at all the sites, indicating low contamination of the heavy metals. Cadmium and Pb exhibited the highest contamination during winter and summer respectively; though, the RI values were low (RI < 150). Generally, RI for heavy metals in the surface sediment of the Loskop Dam was classified as less contaminated.

5. Discussion

5.1. Physicochemical Parameters

The mean temperatures at the selected sites fell within the guideline values, including permissible limits by the World Health Organization [36], which are between 25 and 30 °C. Temperature usually affects the state of different inorganic components and chemical pollutants in water. The mean dissolved oxygen was higher in winter than in summer, as the solubility of gases increases with a decrease in temperature. The lower DO in summer could also be attributed to the higher rate of oxygen consumption by aquatic organisms and the high rate of dead organic matter decomposition. The pH of the water was within the acceptable and permissible limit (6.5–8.5) for domestic use [34]. The pH indicated an alkaline nature of the river and could be due to the soil and rock–water interactions as the river flows [37] and industrial and sewage discharges into the river [38]. Electrical conductivity measures how water conducts electricity and is usually related to the TDS in water. Conductivity of the water did not exceed the recommended guideline value of 1000 μS/cm [35]. Electrical conductivity was higher in summer than in winter, indicating elevated ionic strength and higher concentrations of dissolved inorganic matter during warmer months [39]. Similar pH, TDS and EC values were recorded in the Vaal River in South Africa [40].

The higher concentrations of Fe and Mn during summer could be linked to increased surface runoff from the catchment. In contrast, the lower levels during winter could be due to the settling of metals in the sediment. This can lower the concentration of dissolved metals in the water column. Furthermore, factors such as human activity cycles (e.g., agriculture) and chemical conditions that control the movement and persistence of metals in rivers could cause seasonal detection of specific metals in the water [40]. Recent studies in the Vaal River also recorded high concentrations of Fe and Mn [40].

5.2. Heavy Metals in the Sediment

Heavy metals in freshwater ecosystems are distributed between the water column and sediments, with concentrations in sediments usually higher than in the water [41]. Chemicals in the water are normally bound to organic and inorganic particles that eventually settle in sediments. These chemicals then become a potential source of secondary contamination in the water [9]. Thus, sediments serve as both active and passive components at the bottom of water bodies and control the movement, storage, and release of substances within Earth’s biogeochemical systems. During both seasons, the mean Cr concentration exceeded both the Sediment Quality Guideline (SQG) and the average shale value [23]. The mean concentrations of Ni and Zn exceeded the SQG values and the average shale values, respectively, during winter. Higher concentrations of Cr, Cu, Pb and Zn than their average shale values have also been recorded in other rivers in South Africa such as the Molopo River [42,43]. Studies in the Nyl River in South Africa also recorded higher concentrations of Ni and Zn in sediments than the SQG values [44,45]. High concentrations of heavy metals including Ni and Zn have also been reported in the Olifants River basin [46]. The high concentration of these elements found in the sediments might be associated with human activities in the catchment.

Correlations among heavy metals may indicate their origin and migration; however, if there is no correlation among the metals, then they are not controlled by a single factor [47]. The strong relationship observed between some heavy metals suggests common sources [8]. However, the weak correlation of Cr with other heavy metals indicates that Cr in the sediments may be coming from different sources. The correlation analysis and PCA results exhibited strong positive inter-relationships among As, Fe and Pb, and strong relationships among Cd, Cu, Mn, Ni and Zn, which further affirm that they originate from a common source or have similar geochemical characteristics except for Cr, which showed a weak correlation with other metals and even inverse relationships with some metals. In the study area, the possible pollution is from agricultural and coal mining discharges [15,48]. When PCA was done to evaluate the most common pollution sources, the cumulative variance exceeded the 70% threshold that is commonly accepted, indicating good dimensional representation. The high communalities suggest that most variables were well represented by the retained components, supporting the suitability and validity of the PCA model. Furthermore, it revealed site-specific contamination profiles (Figure 5). The middle site, S2, was characterized by relatively high levels of contamination, signifying the existence of anthropogenic pollution. In contrast, S1 and S3 showed low concentrations of heavy metal pollution, reflecting localized anthropogenic pressure. The concentration of most of the metals increased during the dry winter compared to the wet summer. This observation is in accordance with studies that demonstrated that the seasonal differences in concentrations could be attributed to evaporation and dilution effects during winter and summer respectively [24].

5.3. Sediment Risk Evaluation Indicators

The high EF values for Cr could be linked to coal mining, coal power stations and agricultural activities [14]. Low EF values were obtained for the rest of the metals, indicating minor enrichment. Geoaccumulation is mainly used to quantify chemical element accumulation in sediments. Chromium and Zn were the only metals that showed moderate contamination in the sediments. The EF and I_geo values suggest that Cr contamination in the river is from both geochemical and human activities, while the other trace elements are mainly from geological and natural activities in the area. The heavy metals except Cr, Pb and Zn had I_geo values lower than 0, indicating possible non-anthropogenic pollution.

The CF values for As, Fe and Ni were below 1 at the three sites, suggesting minimum contamination conditions for these metals; however, Cd, Cr, Cu, Mn, Pb and Zn showed moderate contamination, and therefore are environmental concerns. The highest potential ecological risk index was posed by Cd during winter, but the value was still in the low-risk range. The following metals, As, Cr, Cu, Mn, Ni, Pb and Zn had very low RI and do not pose an immediate risk to the aquatic environment. The whole dam can be categorized as having a low ecological risk level (RI < 150). The low ecological risk is consistent with findings from a study conducted in the Molopo River in South Africa [42]. The ecological risk index focuses on the possible risk heavy metals pose to the environment [21,49,50], and living organisms [28,51,52].

From the results, the Loskop Dam is facing a low ecological risk; however, As, Cd, Cr, Ni and Pb levels in the sediments need constant monitoring, as the CF, EF, I_geo and CF are showing potential risk should contamination continue in the dam. In addition to the risk, heavy metals may pose individually, simultaneous exposure to two or more metals may have synergistic effects [53], due to their shared tendency for binding, and may present greater toxicity than a single metal [54]. The current study is similar to other studies that recorded high heavy metal concentrations that could pose a risk to aquatic organisms, though the potential ecological risk was low [42,55]. In the present study area, agricultural products such as pesticides and fertilizers could be major contributors [56]. Studies have shown that pesticides and fertilizers contribute to the accumulation of heavy metals in sediments [57,58]. Furthermore, metal accumulation in the sediments could have come from wastewater from mining and emissions of many large coal-fired thermal power stations in the region. Industrial wastewater from mining activities has been reported to contribute significantly to the pollution of freshwater ecosystems [59,60]. In coal mining areas and coal power stations, during the rainy season and at high temperatures, large amounts of heavy metals are released into the environment from coal gangue piles due to spontaneous combustion and may pose an ecological risk [61].

6. Limitations

This study used the global average shale values as background values for calculating the enrichment factor, contamination factor, and geoaccumulation index, rather than using local or regional geological background values for South Africa, which are not available. The sample size for principal component analysis and correlation analysis was small and could have affected statistical reliability. The distribution of heavy metals in sediments is strongly influenced by other factors such as organic matter content, iron-manganese oxides, and grain size composition. In the absence of such measurements, it is difficult to determine whether the high correlations between Fe, Mn, and certain metals stem from natural geochemical processes. Long-term monitoring is recommended for future studies and should include organic matter content, iron-manganese oxides, and grain size composition, along with biological indicators such as macroinvertebrates and fish.

7. Conclusions

The study investigated the ecological risk of As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn concentrations in the Loskop Dam of the upper Olifants River. The results demonstrated that the Cr concentration in the sediments was considerably higher than the SQG and the average shale values. The findings further show that the factors causing metal accumulation varied across the sites and some of the metals originated from common sources. The highest contamination was in the middle of the dam (S2), indicating localized contamination sources. Seasonal variation in contamination was observed, with higher metal concentrations in winter than in summer. The assessment of EF and I_geo indicated that the sediments were severely enriched and moderately contaminated with Cr respectively. Based on sediment chemical analysis and the ecological risk index, current heavy metal pollution levels do not indicate significant short-term ecological risk, and further studies are recommended that may include bioindicators to validate this assessment.