Ecological and Health Risks from Trace Elements Contamination in Soils at the Rutile Bearing Area of Akonolinga, Cameroon

Abstract

This study evaluates the concentrations of trace elements (TEs) in soils from the rutile deposit area of Akonolinga, Cameroon, and analyzes the associated health risks. A total of 25 samples were analyzed using flame atomic absorption spectrometry (FAAS). The results show that TE concentrations follow the decreasing order Fe, Ti, Zr, Mn, Cr, V, Ba, Zn, Nb, Ni, Pb, Ga, Cu, Co, Y, Br, and Sn. Pollution indices and the Pearson correlation matrix reveal moderate correlations between Fe and several other TEs, indicating a common origin. Enrichment Factors show significant enrichment in Zr, Nb, and Ti, with notable enrichment in Cr and Co at certain sites. Although the Geo-Accumulation Index indicates no direct contamination and the overall ecological risk is low, the Contamination Factor reveals high levels for Cr, Nb, Ti, and Zr. The carcinogenic risk is moderate, while non-carcinogenic risks are high for children and considerable for adults. These research highlight the potential public health impacts in this mining region and provide essential baseline data for future environmental risk management.

1. Introduction

Environmental pollution from trace elements (TEs) is an increasing concern due to the potential impacts of these toxic substances on ecosystems and human health [1,2]. Mining activities are a major source of TE emission from the soils which contaminate water resources and affect the quality of agricultural crops [3,4,5]. Both essential elements (iron and copper) and non-essential elements (lead and nickel) can accumulate in food chains, exposing local populations to significant health risks. Various scientific studies worldwide have addressed the impact of TE on the environment, highlighting the risks associated with the accumulation of heavy metals in agricultural soils and their transfer to crops and animals [6,7].

In Cameroon, similar studies have recently evaluated the effects of TE in mining areas: Gondji et al. [8] examined trace elements pollution and their potential health risks in the cobalt–nickel-bearing areas of Lomié, and Njayou et al. [9] studied contamination levels in the abandoned gold mining area of Bindiba. Likewise, Blanchard et al. [8] assessed the risks associated with TE in the gold-mining region of the country’s east [3]. However, despite the economic importance of the rutile deposits in Akonolinga [10], little research has focused on the current contamination levels and the environmental and health risks related to TE in this region, which is potentially vulnerable to significant pollution effects. It is important to note that rutile (TiO₂) found in the aquatic environment can interact with organisms and induce toxic effects [11]. Barmo et al. [12] showed that TiO₂ affected the immune system and digestive gland function of mussels. Hence, the importance and necessity of assessing the level of environmental pollution in Akonolinga, where fishing and farming are the main activities of the inhabitants

This study investigates the ecological and health issues related to trace elements (TEs) in Akonolinga, Cameroon, for the first time before the planned rutile mining activities in the coming years. Although rutile exploitation has not yet begun in the study area, ongoing exploration activities are contributing to environmental pollution. Thus, because exploration implies the excavation of soil and the release of dust into the aquatic and terrestrial environments, it becomes imperatively essential to assess the current level of contamination in order to provide baseline data for comparison with assessments conducted during and after exploitation. The aim of this study is to determine the current level of TE contamination in soils and assess the potential risks to human health and the environment. In addition, this research will help public health policymakers and international researchers to better understand the effects of TE and also increase awareness among local stakeholders about the importance of sustainable management of mineral resources.

2. Materials and Methods

2.1. Study Area

Akonolinga is located in the Administrative Region of the Centre of Cameroon. It is the capital of Nyong-et-Mfoumou’s administrative division. The locality is located at the latitude 3°45′ north and longitude 12°15′ east with an altitude of 643 m (Figure 1). It has a surface area of 1420.9 km² and 80,361 inhabitants. The climate type is tropical, and the landscape is made up of a sizable plateau covered with secondary vegetation. The soils are lateritic on the mainland, and silty–sandy in low-lying places (swamps) [13]. Agriculture is the main activity in the area. Fishing is the second activity [13]. Akonolinga’s soil is rich in minerals, particularly rutile. Rutile was discovered in Cameroon at the beginning of the 20th century [14].

2.2. Sampling and Sample Preparation

The soil samples were collected using reusable stainless-steel trowels and shovels, cleaned with distilled water, sun-dried and sterilized with alcohol, after each collection to avoid cross-contamination. The sampling followed a protocol described in a previous study [16]. Twenty-five samples were collected to ensure adequate coverage of each targeted area, allowing for a statistically robust and representative analysis of contamination levels based on proximity to rutile deposits. The sampling points were strategically distributed among the rutile discovery site, surrounding villages, and urban peripheries to provide a comprehensive overview of areas potentially affected by trace element contamination. Samples from inhabited areas were taken in close proximity to houses. This approach helps to better understand soil contamination in relation to the proximity of rutile deposits and the potential risks to residents. A distance of approximately 150–250 m was considered between the different sampled sites to ensure characteristic evaluation.

Each sample, consisting of 1 kg of soil, was collected from a depth of 5 to 15 cm and then stored in hermetically sealed polymer bags. The samples were subsequently placed in biodegradable containers and transported to the Nuclear Science and Technology Research Center (CRSTN) of the Geological and Mining Research Institute (IRGM) in Yaoundé for conditioning. During sample conditioning, foreign plant debris, stones, gravel, and roots were removed. Afterward, the samples were dried for 24 h at 105 °C in an oven and then ground and sieved with appropriate machinery. Finally, the samples were placed in marked polyethylene bags, carefully sealed to prevent contamination, and transported for analysis by flame atomic absorption spectrometry (FAAS) at the Research Center for Nuclear Safety, Metrology, and Technology, National Agency for Research and Innovation of Indonesia (BRIN).

2.3. Identification of Trace Elements

For trace element (TE) identification, each sample was heated to a high temperature of 400 °C for 2 h in a furnace to fully carbonize organic materials. Metal quantification in each sample was carried out using the aqua regia extraction technique, involving partial digestion with an HNO₃-HCl mixture on 1 g of each sample at 90 °C for 2 h. After digestion, each sample was diluted with 100 mL of distilled water and filtered after 3 h [17,18]. The acids and standards used in this study were provided by Merck (Darmstadt, Germany). TE concentration analysis was performed using a flame atomic absorption spectrometer (ContrAA 300, Analytic Jena, Jena, Germany), referred to as FAAS. Before the analysis of the samples, the FAAS was calibrated using standards from the same company. The data mean was determined using three different measures; to ensure accurate and precise measurements, we also analyzed certified reference material (CRM) OREAS 465 from Mantra Resources Nyota prospect in Tanzania, with results aligning with the CRM values. The trace elements studied include Ba, Br, Co, Cr, Cu, Fe, Ga, Mn, Nb, Ni, Pb, Sn, Ti, V, Y, Zn, and Zr. This approach is almost identical to the one used by Golia et al. [19] in their study on Cu and Zn contamination in urban soils, where they emphasize the importance of a rigorous methodology to assess the health risks associated with soil contamination. The quantification limits for the analyzed elements are as follows: 0.5 ppm for Br, Cu, Ga, Nb, Ni, Sn, Y, Zn, and Zr; 0.6 ppm for Pb; 1 ppm for Cr, Fe, Mn, and V; 1.5 ppm for Ba; 2 ppm for Ti; and 3 ppm for Co.

2.4. Assessment of Environmental Contamination by Trace Elements

To assess TE contamination in the study area and to estimate the health risk of human exposure, the following parameters were used: Contamination Factor (CF), Enrichment Factor (EF), Pollution Index (PI), Potential Ecological Risk Index (RI), Hazard Quotient (HQ), Hazard Index (HI) and Geo-Accumulation Index (I_geo). These parameters are calculated from TE concentrations.

2.4.1. Geo-Accumulation Index (I_geo) and Enrichment Factor (EF)

I_geo and EF are used to determine the presence and severity of anthropogenic contaminant deposition on surface soils [20]. The reference elements mostly used to calculate the EF are Al and Fe [21,22]. In this work, due to its geochemical properties similar to several TE properties, we applied Fe as a reference element [23,24,25]. Equation (1) expressed the EF formula.

$$EFi={\displaystyle \frac{\left(\frac{Ci}{Cr}\right) sample}{\left(\frac{Bi}{Br}\right) crust}},$$

(1)

where EFi is the EF of element i measured; Ci is the concentration of element i in the sampling environment; Cr is the concentration of the reference element in the sampling environment; B_i is the concentration of element i in the reference environment; and B_r is the concentration of the reference element. The reference values used in this study are those for the Earth’s crust and soil [25] and are presented in

Table 1. Reference values of elements in the Earth’s crust.

TE	Ba	Cr	Mn	Fe	Cu	Ni	Cu	Zn	Ga	Pb	Zr	Nb	Y	Ti	V	Br	Sn
Geochemical Background Values(ppm)
	580	90	850	47,200	19	68	45	95	19	20	160	11	26	4600	130	4	6

. EF values of 0.5 to 1.5 indicate a natural origin of the element, while values above 1.5 indicate human contributions [26].

The I_geo proposed by Muller in 1979 is a quantitative indicator for measuring the extent of TE’s soil contamination [27]. Equation (2) expressed the I_geo formula.

$$I_{geo}={Log}_2\left({\displaystyle \frac{Ci}{1.5\ast Bi}}\right)$$

(2)

where Ci and B_i are as defined in Equation (1), and 1.5 is the background matrix correction factor. Müller defined seven classes of I_geo [28] (

Table 2. I_geo and EF classification.

Classes	Igeo Values	Soil Quality	EF Values	Soil Quality
0	Igeo ≤ 0	Not contaminated	EF < 2	Deficient to minimal enrichment
1	0 < Igeo < 1	Not contaminated to moderately contaminated	2 < EF < 5	Moderate enrichment
2	1 < Igeo < 2	Moderately contaminated	5 < EF < 20	High enrichment
3	2 < Igeo < 3	Moderately to heavily contaminated	20 < EF < 40	Very high enrichment
4	3 < Igeo < 4	Heavily contaminated	EF > 40	Extremely high enrichment
5	4 < Igeo < 5	Heavily to extremely Contaminated
6	Igeo ≥ 5	Extremely contaminated

), ranging from class 0 (I_geo = 0) to class 6 (I_geo > 5). The categorization of the Enrichment Factor indicates the soil quality, ranging from deficient enrichment to extremely high enrichment, as shown in Table 2 [29].

2.4.2. Contamination Factor CF

CF was introduced by Hakanson [30] as a tool for monitoring soil contamination by TE and indicates the level of contamination. CF is obtained by comparing the TE concentrations (C_i) in samples to the reference TE concentrations (C_n); its formula is expressed by Equation (3) [4,31]:

$$CF={\displaystyle \frac{Ci}{Cn}}$$

(3)

CF is classified into 4 categories as follows:

• CF < 1: Low contamination;
• 1 < CF < 3: Moderate contamination;
• 3 < CF < 6: Considerable contamination;
• CF ≥ 6: Very high contamination.

The contamination degree (C_deg) is used as a reference to assess the level of TE contamination in soils [32]. It does not take into account the level of soil pollution and is calculated using Equation (4).

$$C_{deg}=\sum _{i=1}^{n}CFi$$

(4)

For C_deg, four classifications have been established. A C_deg < 8 indicates a low degree of contamination; a C_deg from 8 to 16 indicates a moderate degree of contamination, from 16 to 32 indicates a considerable degree of contamination, and over 32 means a very high level of contamination [33].

2.4.3. Individual Ecological Risk Index (Ei) and Potential Ecological Risk Index (RI)

The ecological risk index is used to determine the ecological risk of contaminants such as trace elements in soils or sediments [34]. RI (see Equation (5)) is the summation of Ei for all indices of trace elements (Ei) and represents the biological sensitivity to toxic substances [30]. Ei and RI classifications are displayed in

Table 3. Ei and RI classification.

Ei Value	Interpretation	RI Value	Interpretation
Ei < 40	Low	RI < 95	Low
40 ≤ Ei < 80	Moderate	95 ≤ RI < 190	Moderate
80 ≤ Ei < 160	Considerable	190 ≤ RI < 380	Considerable
160 ≤ Ei < 320	High	380 ≥ RI	Very High
320 ≥ Ei	Very High

[35].

$$RI={\sum }_{i=1}^{n}Ei, \mathrm{where} Ei=Ti\times CFi$$

(5)

and RI is the total potential ecological risk of all the elements likely to be toxic for a site; Ei stands for the ecological risk for element i; T_i is the toxic reaction factor for element i; CF is the Contamination Factor for element i. The toxic reaction factors (T_i) for the trace elements studied (Cr, Mn, Ni, Cu, Zn, Pb) according to Hakanson are 2, 1, 5, 5, 1, and 5, respectively [30].

2.5. Pearson Correlation

In this study, we used the Pearson correlation analysis to evaluate the linear associations between the measured concentrations of the trace elements in soil. This method allowed us to determine the strength and direction of relationships between the variables, shedding light on how variations in the concentration of one element can influence those of another. A significance level of p < 0.05 was set to identify significant relationships, facilitating the interpretation of the results.

2.6. Health Risk Assessment of Trace Elements in Soil Using RSL of USEPA

USEPA has introduced health risk parameters to determine the probabilistic non-carcinogenic and carcinogenic risks to the public from long-term exposure to heavy metals. The aim of utilizing the RSL calculator to analyze a site is to pinpoint the regions, pollutants, and circumstances that need special attention at a certain place [36]. The different routes considered here are inhalation, ingestion, and the cutaneous route. An in-depth assessment of the potential risks must be carried out when the screening level (SL) is exceeded at a site. The SLR, target Hazard Quotient (HQ), or target risk can be calculated using Equations (6) and (7) [37].

$$\mathrm{Non}-\mathrm{carcinogenic} \mathrm{risk}: {\displaystyle \frac{THQ}{RSL}}={\displaystyle \frac{HQ}{C}}$$

(6)

$$\mathrm{Carcinogenic} \mathrm{risk}: {\displaystyle \frac{TR}{RSL}}={\displaystyle \frac{Risk}{C}}$$

(7)

where C is the input concentration in site-specific mode [mg/kg; μg/m³; μg/L]; TR is the target risk provided by the user in site-specific mode; the Total Hazard Quotient (THQ) is the target HQ; HQ represents the potential non-carcinogenic risk for each TE and RSL is the regional screening level (mg/kg; μg/m³; μg/L). The risk associated with exposure to metals is evaluated using HI, which is the summation of the HQs. There are non-carcinogenic effects if HI is greater than 1 [38]. Using Equations (6) and (7), the RSL, the target risk or target quotient used to calculate the RSL, and a concentration entered by the user are sufficient to calculate the risk [37]. The New York State Department of Health (NYSDOH) has described carcinogenic risks as follows:

• ≤10⁻⁶: Low carcinogenic risk;
• 10⁻⁴ à 10⁻³: Moderate carcinogenic risk;
• 10⁻³ à 10⁻¹: High carcinogenic risk;
• ≥10⁻¹: Very high carcinogenic risk.

3. Results and Discussion

3.1. Trace Elements Concentrations

The TE identified in the present study area are Ba, Br, Co, Cr, Cu, Fe, Ga, Mn, Nb, Ni, Pb, Sn, Ti, V, Y, Zn, and Zr. The soil samples from our study area showed a broad range of average concentrations of these TE.

Table 4. Descriptive statistics for TE concentrations (ppm) in soil.

TE	Min.	Max.	Mean	SD	Detection Limit	Global Average ⸭	Max. Permissible Con. in Soil
Ba	16	207.2	89	61	1.5	550	NA
Br	2	13	7	3	0.5	2.8	NA
Co	11	22	17	3	3	10	30a
Cr	73	267.3	110	36	1	35	150a
Cu	1	68	22	14	0.5	25	100a, 30b
Fe	6500	84,700	42,006	20,227	1	35,000	50,000a
Ga	11	40	23	7	0.5	17	NA
Mn	57.5	696	293	178	1	600	2000a
Nb	33	98	57	15	0.5	25	NA
Ni	21	84	52	18	0.5	20	50a, 40b
Pb	15	111	31	22	0.6	20	100b
Sn	3	8	5	1	0.5	5.5	NA
Ti	8900	19,300	13,004	2537	2	3000	NA
V	12	142	96	27	1	60	NA
Y	10	23	17	4	0.5	22	NA
Zn	33	148	62	30	0.5	71	300a
Zr	411	1080	652	176	0.5	190	NA

NA = not available, ⸭ global average of the higher continental crust, a and b [39,40,41].

presents these different concentrations, comparing them with the world average and maximum permissible values in the soil. They are ranked in ascending order as follows: Fe > Ti >Zr > Mn > Cr >V > Ba > Zn > Nb > Ni > Pb > Ga> Cu > Co > Y > Br >Sn. Although TE is necessary for human health, and can also have harmful effects when present in inappropriate concentrations. Optimal concentrations of TE vary from one individual to another and can be influenced by factors such as age, sex, health status, and lifestyle.

The results presented in Table 4 summarize the concentrations of various elements analyzed in the samples. Among the essential elements identified, iron (Fe), copper (Cu), and zinc (Zn) show significant concentrations. Iron concentrations, particularly concerning, range from 6500 to 84,700 ppm, with an average of 42,006 ppm, exceeding the global average of 35,000 ppm. Copper concentrations vary between 1 and 68 ppm, and zinc between 33 and 148 ppm, with averages of 22 ppm and 62 ppm, respectively. The analysis of the essential elements indicates that metals like iron, copper, and zinc, though necessary for human health, are present in soil at concentrations above recommended levels. This raises concerns about potential toxicity from prolonged exposure. For instance, excess iron is linked to liver diseases and metabolic disorders, while copper excess can lead to neurological and psychological issues [42].

As for non-essential elements, lead (Pb), nickel (Ni), and chromium (Cr) present specific risks due to their toxic effects. Nickel concentrations range from 21 to 84 ppm, exceeding WHO recommended limits [43], and lead ranges from 15 to 111 ppm, which is especially concerning for its neurotoxic effects. Lead is known to affect children’s cognitive development and cause neurological disorders [43,44]. Chronic nickel accumulation in the body can result in health issues like pulmonary fibrosis, kidney and cardiovascular diseases, and respiratory cancers [45,46], as well as an increased risk of nasal and oral cancer in workers [47,48]. Chromium levels between 11 and 22 ppm are associated with increased cancer risks and respiratory issues for exposed workers [49,50,51].

Trace elements like gallium (Ga), titanium (Ti), vanadium (V), manganese (Mn), and barium (Ba) also show notable concentrations. Titanium concentrations, varying from 8900 to 19,300 ppm, are four times the global average, raising concerns about long-term respiratory health risks [52,53,54]. Inhalation of fine particles of this mineral is linked to pleural diseases [52,53]. Vanadium, with levels between 12 and 142 ppm, and manganese, varying from 57.5 to 696 ppm, are both required in small amounts but can become toxic at high concentrations. Although barium is generally considered less concerning, prolonged exposure can also lead to adverse effects.

Finally, non-trace elements such as zirconium (Zr), niobium (Nb), Tin (Sn), and bromine (Br) were also analyzed. Zirconium concentrations range from 411 to 1080 ppm, niobium from 33 to 98 ppm, strontium from 3 to 8 ppm, and bromine from 2 to 13 ppm. All these values exceed the global average. Although often considered non-essential, these elements can pose health and environmental risks when present at elevated levels. Research indicates that exposure to these substances can have adverse effects on human health, particularly in the context of intensive mining activities.

TE concentrations in Akonolinga were compared with those found in other regions of Cameroon and the world in

Table 5. Average TE concentrations in this study compared to other reported studies.

	TE Concentrations (ppm)
TE	This Study	Lomié	Meiganga	Dschang	Douala	Nigeria	China	Burkina	Niger
Ba	89	175	-	147	-	-	-	-	-
Cr	110	903	122	186	89	25	22	5	16
Mn	292	421	-	-	551	146	-	-	-
Fe	42,005	126,343	-	-	7	784	-	-	-
Co	17	-	-	23	5	4	-	-	-
Ni	52	189	169	64	43	20	10	1	8
Cu	22	36	192	8	96	39	19	10	17
Zn	61	35	-	67	421	10	82	40	-
Ga	22	18	-	59	-	-	-	-	-
Pb	31	16	6	18	302	71	41	1	1
Zr	651	264	-	2209	-	-	-	-	-
Nb	57	-	-	-	-	-	-	-	-
Y	17	9	-	33	-	-	-	-	-
Ti	13,004	-	-	-	-	-	-	-	-
V	96	-	-	-	-	-	-	-	-
Br	7	10	-	-	-	-	-	-	-
References	-	[8]	[9]	[57]	[60]	[41]	[7]	[55]	[56]

. According to the results, the average concentrations of Cr, Cu, Ni, Pb, and Zn in the present study are significantly higher than those found in Burkina Faso [55] and Niger [56]. However, the concentrations of Ba, Br, Co, Fe, and Mn in Lomié [8] are higher than those found in this study. Furthermore, the average values of Pb and Co in Niger [56] are, respectively, higher and lower than those found in this study; meanwhile, the opposite is true for the town of Dschang [57]. The comparative results show that TE concentrations vary from place to place. These results are coherent with those of many authors who have shown that TE concentrations in soil vary according to each trace element, soil type, sampling site, and anthropogenic activities [58,59]. As for Ni and Cu, the concentrations in Douala [7,60] are insignificant compared to those found in our work.

3.2. Assessment of Contamination Parameters

Because of its impact on ecosystems and human health, TE pollution of the environment is a serious concern. To assess this contamination, the contamination parameters CF, I_geo, EF, and CF were estimated. The results are illustrated in

Table 6. EF in the study area.

Pollution Indices	Descrip Tive Statistic	TE
		Ba	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Pb	Zr	Nb	Y	Ti	V	Br	Sn
EF	Mean	0.18	1.81	0.43	1.00	1.50	0.96	0.59	0.86	1.61	2.05 *	7.05 *	7.77 *	1.01	4.97 *	0.64	2.06 *	1.15
	Min	0.08	0.80	0.17	1.00	0.35	0.31	0.21	0.41	0.59	1.06	1.80	2.36	0.26	1.36	0.45	0.14	0.44
	Max	0.34	7.16 *	1.01	1.00	6.96 *	2.39	1.27	2.49	5.20	5.85 *	34.68 *	36.51 *	4.14	25.42 *	0.96	4.38	4.59
	SD	0.06	1.81	0.20	0.00	1.80	0.52	0.19	0.59	1.26	1.42	9.26	8.39	1.15	6.71	0.11	1.17	0.96

* significant enrichment.

(EF),

Table 7. Potential Ecological Risk Index (RI).

Sample ID	Ei						RI
	Cr	Mn	Ni	Cu	Zn	Pb
A1	3.16	0.24	6.01	3.51	0.64	8.43	21.99
A2	2.49	0.45	6.15	5.91	0.75	9.15	24.91
A3	2.34	0.62	3.74	2.60	1.51	7.55	18.36
A4	2.01	0.44	5.32	3.72	0.60	8.13	20.21
A5	2.55	0.49	4.11	3.60	0.57	7.90	19.22
A6	3.43	0.63	2.91	7.58	0.77	6.35	21.67
A7	2.04	0.19	1.56	1.38	0.49	3.68	9.34
A8	2.13	0.82	2.97	2.54	0.51	5.48	14.45
A9	2.46	0.60	4.18	2.88	0.47	5.80	16.38
A10	2.67	0.31	4.90	3.59	0.65	8.23	20.34
A11	2.28	0.26	4.89	4.27	0.71	7.78	20.18
A12	2.45	0.26	4.51	2.77	0.51	7.90	18.39
A13	2.52	0.09	2.10	0.26	0.36	4.70	10.03
A14	2.48	0.10	2.35	0.68	0.39	4.90	10.90
A15	2.88	0.40	5.19	3.57	1.21	13.63	26.88
A16	5.94	0.50	2.76	4.31	0.90	27.80	42.22
A17	2.53	0.31	4.55	3.52	0.83	8.10	19.85
A18	2.37	0.26	5.73	4.28	0.59	8.43	21.65
A19	2.16	0.51	4.72	3.53	0.53	7.98	19.43
A20	1.72	0.07	1.51	0.14	0.34	4.03	7.82
A21	2.29	0.80	4.14	3.42	1.56	22.85	35.06
A22	2.26	0.41	5.02	2.96	0.73	7.75	19.12
A23	1.62	0.45	4.14	2.99	0.56	5.85	15.61
A24	2.16	0.72	5.18	2.84	0.70	7.90	19.51
A25	2.51	0.33	4.12	2.58	0.65	8.75	18.94
Mean	2.54	0.41	4.11	3.18	0.70	8.76	19.70

(Ei and RI), and Figure 2 (CF and I_geo).

The CF average values are below 1 for the elements Ba, Mn, Zn, and Y indicating low soil contamination from anthropogenic sources for these elements. Moderate contamination is observed for the elements Fe, Co, Cu, Ga, Ni, V, and Pb, with CF mean values of 1 ≤ CF ≤ 3. For the elements Cr, Nb, Ti, and Zr, the CF mean values are over 3, indicating that our study area is heavily contaminated by these elements. The works carried out by Kabata-Pendias [61] and Sello et al. [62] show that high CF values indicate an anthropogenic origin of the pollutants and relatively low average values could be attributed to a geogenic origin of the elements.

The results presented in Table 6 show that the average Enrichment Factor values for the elements considered in this study are <2, with the exception of the average value for Zr (=7.05), Pb (=2.05), Br (=2.06), Ti (=4.97) and Nb (=7.77) indicating a significant enrichment of these elements. Values <2 indicate minimal soil enrichment in this area. Maximum EF values for Cr and Co are 7.16 and 6.96, respectively, i.e., 5 < EF < 20, indicating a significant enrichment of some Cr and Co measurement sites. These results show that Zr, Pb, Ti, Nb, Cr, and Co are the main elements of the anthropogenic load in the soil sampled at these sites [63]. The study area bears rare earth deposits that have been explored but not yet mined; future mining activity may increase the enrichment of the trace elements in topsoil.

I_geo values of the trace elements taken into account in our study are all below 0 except for those of Pb, Zr, Nb, and Ti, which are between 0 and 1. The results indicate there is no soil contamination by TE in the study area and found that indirect human activities are linked to the buildup of oligo-elements in soil, sediments, and water. The box plots in Figure 2 show the results of the Igeo and CF evaluations for the trace elements examined in the soil.

The values of Ei and RI for Cr, Mn, Ni, Cu, Zn, and Pb elements are shown in Table 7. The results of the ecological risk index of the soil contamination with trace elements show the mean values of the individual environmental risk index (Ei) ranging from the lowest value (0.41) for the Mn to the highest value (8.76) for the Pb. Individual risk indices imply low potential environmental risk (RI < 40) for this study area [30] except for measurement site A16 (3°47066′ north latitude and 12°15424′ east longitude) where the RI = 42.22.

3.3. Assessment of Pearson Correlation

The Pearson correlation matrix (

Table 8. Pearson correlation matrix of trace elements.

TE	Ba	Cr	Mn	Fe	Co	Ni	Cu	Zn	Ga	Pb	Zr	Y	Sn	Br	Ti	V	Nb
Ba	1.00
Cr	0.27	1.00
Mn	0.46 *	0.09	1.00
Fe	0.77 *	0.49 *	0.41 *	1.00
Co	−0.60 *	−0.40	0.02	−0.60 *	1.00
Ni	0.58 *	−0.08	0.18	0.74 *	−0.40 *	1.00
Cu	0.64 *	0.37	0.44 *	0.79 *	−0.32	0.52 *	1.00
Zn	0.72 *	0.21	0.51 *	0.41 *	−0.31	0.24	0.33	1.00
Ga	0.42 *	0.03	−0.09	0.66 *	−0.49 *	0.89 *	0.43 *	0.05	1.00
Pb	0.53 *	0.70 *	0.35	0.55 *	−0.31	0.13	0.31	0.61 *	0.11	1.00
Zr	−0.42 *	−0.19	−0.39	−0.30	0.14	−0.01	−0.41 *	−0.25	0.06	−0.23	1.00
Y	−0.16	−0.15	−0.32	0.06	−0.11	0.45 *	−0.17	−0.20	0.55 *	−0.17	0.77 *	1.00
Sn	−0.03	−0.09	0.48	−0.02	0.11	−0.10	0.00	−0.03	−0.13	−0.07	−0.16	−0.13	1.00
Br	0.16	−0.06	0.04	0.23	−0.14	0.47 *	−0.04	−0.07	0.35	−0.12	0.12	0.34	−0.01	1.00
Ti	−0.74 *	−0.25	−0.43	−0.67 *	0.40 *	−0.40 *	−0.75 *	−0.57 *	−0.24	−0.40 *	0.64 *	0.43 *	0.12	0.05	1.00
V	0.52 *	0.27	0.01	0.85 *	−0.53 *	0.70 *	0.44 *	0.17	0.64 *	0.30	−0.53 *	−0.09	−0.15	0.37	−0.60 *	1.00
Nb	−0.11	−0.12	−0.05	0.10	0.04	0.22	−0.13	−0.09	0.20	0.05	0.58 *	0.43 *	−0.05	−0.06	0.42 *	−0.30	1.00

* p ≤ 0.05; bold values: important values.

) is used to assess whether trace elements may originate from common pollution sources. However, this correlation simply indicates simultaneous contamination, which could possibly result from a common source [64]. The matrix shows an average correlation between Fe and several elements Ni (0.74), Cu (0.79), Ga (0.66), Pb (0.55), and Ti (0.67) and has a strong correlation with V (0.85). The positive average correlation between these elements means that they originate from the same anthropogenic activities, such as agriculture, and are probably governed by the same physicochemical processes [65,66]. Similarly, Ba is moderately correlated with the elements Fe (0.77), Ni (0.58), Cu (0.64), Zn (0.72), and Pb (0.53), and has a negative moderate correlation with Ti (−0.74).

3.4. Assessments of Carcinogenic and Non-Carcinogenic Risks

To safeguard human health and the safety of the public, it is essential to assess the potential risk for TE exposure. TE is necessary for many biological functions, but it may also be harmful to people’s health, especially if the TE concentrations in the surrounding living environment exceed certain thresholds. USEPA which based its recommendations on rigorous scientific criteria with the Regional Screening Level (RSL) [37], plays a significant role in establishing guidelines for assessing TE threats. This study assesses the carcinogenic and non-carcinogenic risks associated with exposure to TE in the study area.

To assess the risk of cancer to human health associated with exposure to TE, two approaches were used and three specific TEs were considered such as chromium, cobalt, and nickel. The ways of exposure used were inhalation and ingestion. No dermal exposure was observed in this study. According to the USEPA guidelines [37], the inhalation risk for residents in the study area is low and is 7.01 × 10⁻⁶, while the ingestion risk is moderate (3.73 × 10⁻⁴). The cancer risk for these residents is also moderate (3.8 × 10⁻⁴).

Table 9. Carcinogenic risk from exposure to TE.

TE	Ingestion Risk	Inhalation Risk	Carcinogenic Risk
Chromium	3.73 × 10−4	6.96 × 10−6	3.8 × 10−4
Cobalt	-	4.18 × 10−8	4.18 × 10−8
Nickel	-	3.81 × 10−9	3.18 × 10−9
Total Risk/HI	3.73 × 10−4	7.01 × 10−6	3.8 × 10−4

shows the various carcinogenic risks associated with exposure to TE. The carcinogenic risk for cobalt and nickel in this study is lower than that estimated by Gondji et al. in the cobalt–nickel region of Lomié [8]. This difference could be attributed to the fact that the mining area in Lomié has already been exploited, leading to increased contamination of the soil and environment.

The non-carcinogenic risk to children and adults from the ingestion and inhalation of trace elements, including Ba, Cr, Co, Cu, Fe, Rb, Sr, Zn, and Zr, is presented in

Table 10. Non-carcinogenic risk from exposure to trace elements.

TE	Ingestion Child HQ	Inhalation Child HQ	Non-Carcinogenic Child HI	Ingestion Adult HQ	Inhalation Adult HQ	Non-Carcinogenic Adult HI
Barium	5.72 × 10−3	1.26 × 10−5	5.73 × 10−3	5.36 × 10−4	1.26 × 10−5	5.48 × 10−4
Chromium	2.82 × 10−1	2.60 × 10−4	2.83 × 10−1	2.65 × 10−2	2.60 × 10−4	2.67 × 10−2
Cobalt	7.42 × 10−2	6.14 × 10−4	7.48 × 10−2	6.95 × 10−3	6.14 × 10−4	7.57 × 10−3
Copper	7.16 × 10−3	-	7.16 × 10−3	6.71 × 10−4	-	6.71 × 10−4
Iron	7.67 × 10−1	-	7.67 × 10−1	7.19 × 10−2	-	7.19 × 10−2
Rubidium	1.32 × 10−1	-	1.32 × 10−1	1.23 × 10−2	-	1.23 × 10−2
Strontium	1.05 × 10−4	-	1.05 × 10−4	9.89 × 10−6	-	9.89 × 10−6
Zinc	1.67 × 10−1	-	1.67 × 10−1	1.56 × 10−2	-	1.56 × 10−2
Zirconium	104	-	104	9.76	-	9.76
Total Risk/HI	106	8.86 × 10−4	106	9.90	8.86 × 10−4	9.90

. The average daily dose for each method is calculated, with ingestion typically resulting in a higher dose for children (106) than inhalation (8.86 × 10⁻⁴). The non-carcinogenic risk value exceeding 106 indicates trace element pollution, posing a health risk to children [67]. According to Kamunda et al. [31], this high level highlights the non-carcinogenic risk in this study area. Adults also show a non-carcinogenic risk value above 1 (9.9), with ingestion being riskier than inhalation. These findings indicate that exposure to trace elements like Ba, Cr, Co, Cu, Fe, Rb, Sn, Zn, and Zr can cause various health effects, particularly gastrointestinal, respiratory, and cardiac issues, especially with chronic exposure [42,49,50,51]. The particularly high risk associated with zirconium in this study could suggest a specific industrial or mining pollution source in the area, requiring further assessment to identify and manage this source to limit human exposure. Children are especially vulnerable to environmental hazards due to their biological development, limiting their ability to metabolize, detoxify, and eliminate contaminants.

4. Conclusions

This study carried out an extensive assessment of the trace elements present in soil samples at the Akonolinga rutile deposits zone and estimated the health risk of human exposure. The average concentrations of the various trace elements in the soils were higher than the global average values except Ba, Mn, Sn, Y, Cu, and Zn. The elements Ni and Fe in particular have an average that exceeds the maximum limits assimilable by the soil authorized by the WHO. The EF results indicate high enrichment in Zr, Pb, Ti, and Nb and significant enrichment for some sites in Cr and Co. The potential environmental risk (RI) in this study is low and the Geo-Accumulation Index (I_geo) indicates indirect non-contamination. As for CF, it indicates high contamination in Cr, Nb, Ti, and Zr, moderate contamination in Fe, Co, Cu, Ga, Ni, V, and Pb, and low contamination in Ba, Mn, Zn, and Y. The carcinogenic risk is moderate (3.8 × 10⁻⁴), and the non-cancerogenic risk is significant for children (106) and considerable for adults (9.9). The Pearson correlation matrix revealed a moderate correlation between Fe and the elements, Cu, Ga, Ni, Pb, and Rb, indicating the same source of these trace elements. The assessment of TE concentrations in this area enables us to anticipate the potential challenges associated with the incoming mining activities of rutile ore. This first assessment can help to develop safety measures for future miners. However, certain limitations of this study should be noted; in particular, the limited sample size and the lack of studies to assess long-term effects. To overcome these shortcomings, future research could focus on a more thorough assessment of exposure ways, as well as an in-depth analysis of the effects on human health and surrounding ecosystems.