1 Introduction

Potentially toxic metals (PTMs) migrate in sediments through natural or anthropogenic sources and affect various environmental components in aquatic ecosystems. Mining is a major cause of water, tailings, sediment and soil pollution worldwide [1]. The primary source of metals that can leak into the environment and build up in sediments at high amounts is mining [2]. Gold mining activities are considered to be one of the major sources of trace metals emissions into the environment [1, 2]. Mining activities are considered a major source of metals that can be released into the surrounding environment, pollute rivers and accumulate in high concentrations in sediments [3]. The consequences of mining include habitat destruction, loss of biodiversity and environmental pollution [3]. Because of their environmental toxicity and accumulation, hazardous metals (toxic metals) cause a significant problem in ecosystems [4]. Potentially toxic metals can come from both natural and man-made sources, and they are absorbed into sediments as contaminants from industrial activities [5]. Pathways of hazardous metals into the environment include inheritance from source rocks, wastewater application and marine deposition.

Numerous studies have indicated that sediments are responsible for approximately 90% of toxic metal contamination in riverine systems [6]. Sediments act as environmental sinks for trace metals in water and are of environmental concern. Stream sediment survey is a tool to monitor environmental quality [7]; it is commonly known that metals are present in sediments, and that biological factors influence their distribution [8, 9]. Mining operations, urbanization, industrialization, agriculture and household practices are to blame for the contamination of dangerous metals. Several authors reported metals contamination in stream sediments, mining, tailings, sediment and soil pollution in Tunisia [10,11,12] and worldwide. The combination of geochemical indices and multivariate analyses was successfully used to assess contamination status and source apportionment of trace elements in stream sediments [10]. Potentially toxic metals like Mn, Fe, V, Cr Cd, Pb, Hg, Cu, Ni, Zn and Co can have harmful effects on humans [13]. They can enter the human body mainly through direct ingestion and dermal contact during bathing and balneology. The Mbal area is rich in gold and mining is carried out by local communities using rudimentary tools and small firms with heavy machinery. Intensive agriculture using agrochemicals also prevails in Mbal area. Mining and agricultural practices are one of the anthropogenic activities responsible for contamination in many aquatic ecosystems in the world. Mining has a significant impact on the contents of hazardous metals which are recognized as probable pollutants following the computation of pollution indices [14,15,16]. There are increasing concerns about toxic metals discharged by artisanal and semi-mechanized mining contaminating the ecosystem. In this regard, the significant content of metals in water is impacted by mining activities [17]. Further investigations have revealed that mine tailings, sediments and soils have been contaminated with variety of toxic metals [14].

The Mbal area is one of the areas in Cameroon where gold is being mined. It is found between longitude 14°04'E to 14°21'E and latitude 5°36’N to 6°60'N and underlined by the Pan-African fold Belt. The area is characterized by gentle topography with many vast alluvial plains and hills with a drainage system well developed within the highly undulating topography. The flat lying topography provides the area with dendritic drainage pattern characterized by many tributaries that empty their content into the Lom basin. The study area is characterized by a tropical monsoon climate and experience high temperatures of 24 degrees Celsius on average. The vegetation of the area is the guinea savanna type dominated by wooded savanna and the trees are sparsely distributed. Soils in the study area are predominantly acrisols, alisols, ferralitic, plinthosols, acid soil with clay-enriched lower horizon and low saturation of bases.

The available literature on the Mbal area focus on the trace metal concentration and spatial distribution without the use of integrated single and complex pollution indicators to quantify the level of sediments contamination and metallic pollution. Few or no studies have looked at the integrated evaluation of metal contamination using single and complex pollution indices, despite the fact that some have been done in this area based on the potential environmental risk of mining and agricultural practices [14]. Considering the factors governing sediments accumulation such as geology, climate and anthropogenic activities as well as the correlation with commonly used environmental background, the data from this study can be considered as a baseline of pollution to plan major mining operations in the area. The study underlines the importance of continuous monitoring and adaptive management for the preservation of sediments ecosystem. The findings from this study provide new and valuable information, and a suitable foundation for maintaining ecosystem, safeguarding public health and managing pollution problems in mining districts of Cameroon. Furthermore, data from these studies would enable the local authorities and the government to better anticipate and plan for potential environmental impact of mining operations and are useful for setting realistic targets for monitoring and remediation programs. This study aims to assess the PTMs including Cd, Cu, Co, Cr, Fe, V, Ni, Cu, Zn, Hg and Pb using multivariate statistics through correlation and factor analyses, and integrated single and complex pollution indices.

2 Geologic setting

2.1 Regional geology

Mbal (Fig. 1) is found within the Pan-African fold belt [18,19,20] which extends to NE Brazil [21]. In Cameroon this fold belt is structured into three domains [22]:

  1. (1)

    The Yaoundé domain (YD) corresponds to the north boundary of the Congo Craton (Fig. 1). This domain comprises 616 Ma metasedimentary rocks [23, 24] and an alkaline magmatism [23] that has undergone a high-grade metamorphism.

  2. (2)

    Mbal is located within the Adamawa–Yade domain (AYD; Fig. 1) which continuous to the east of the Tchollire–Banyo fault [25]. This is an Archean microcontinent domain that separated from the Archean crust in early Neoproterozoic rifting [26]. The Lom Serie (700 Ma) comprises of metasediments, volcanoclastic rocks, amphibolite and granitoids. These rocks represent a Paleoproterozoic basement that was fractured during the Neoproterozoic orogeny. The basement formations have undergone four stages of deformation [27].

  3. (3)

    The West Cameroon domain (WCD) is situated west of the Tcholliré–Banyo fault (Fig. 1). It is composed of Neoproterozoic (800 Ma) metasediments, metavolcanic rocks of tholeiitic and alkaline affinities [28], 660 and 580 Ma granitoids [19], sedimentary and volcanic rocks.

Fig. 1
figure 1

Geologic map of Cameroon [22] showing the location of the Mbal area and the lithotectonic domains. YD: Yaoundé domain; AYD: Adamawa-Yade domain; WCD: Western Cameroon domain; BOSZ: Betare-Oya Shear Zone; SSZ: Sanaga Shear Zone; Pe: Permambuco Shear Zone; Pa: Patos Shear Zone TBSZ: Tibati-Banyo Shear Zone; SL: Suture Line; KF: Kandi Fault; KCSZ: Kribi-Campo Shear Zone; NT: Ntem complex; DS: Dja Series; NS: Nyong Complex

Full size image

2.2 Local geology

The Mbal area is found within 14° 04' E to 14° 21' E and 5° 36’ N to 6° 60' N (Fig. 1) in AYD. This study area forms the north–south extension of the Lom Basin [29, 30]. This Basin is regarded as a syn-depositional Neoproterozoic pull-apart basin [31, 32], composed of metasedimentary rocks that are divided into the monocyclic and polycyclic units, which are the two primary structural and metamorphic units. The monocyclic unit is related with grabens and consist of rocks that underwent metamorphism under the greenschist facies [29]. The polycyclic unit comprises of gneisses and mylonites, all associated with horst structures [31]. Granitoids and quartz veins typically crosscut both units and exhibit sinistral deformation. The Mbal area includes metatuff, metaconglomerate, volcaniclastics and sedimentary rocks [33]. The gold mineralization which is associated with granitoids intrusions [33, 35] is found in quartz veins [31]. Small-scale and semi-mechanized gold exploitation are intensively developed in the Mbal area [35]. The quartz veins hosted gold minerals are formed during transpressional and transtensional event [36].

3 Materials and methods

A total of twenty (20) stream sediment samples were recovered from the riverbank deposits using a stainless-steel shovel and auger in February 2021. The duration between sample collection, dispatch to the laboratory, and preparation is last for three months. It was not clear how the geochemical anomalies behaved at depth, but the representative sampling sites were selected according to the anomalies reported in the area, the mining operations and possible pollution sources. Therefore, the 20 samples were collected from the existing artisanal and semi mechanized gold mining exploitation along the Mbal and Bedere rivers, the tributaries of the River Lom over the anomaly, and from the sites of possible pollution sources between 200 and 250 m. Pits were dug along the riverbank at depths that vary from 50 cm to 1 m and samples collected (~ 3 kg) using a spade. The samples were first put into a 10 litres rubber bucket and further pan using and aluminium basin. At each sampling point, geographic coordinates were thought with the help of the global positioning system (GPS), the recovered sediment of a fraction size of < 150 µm was collected, labeled and placed in the plastic sample bags. Twelve (12) representative samples were selected for geochemical analysis based on site and area with high intensity of mining operations, agricultural practices, and proximity to pollutants due to mining discharge into streams. In the laboratory, they were dried in an oven at 40 degrees Celsius for 48 h and then sieved through a nylon sieve with an aperture of 125 µm. The importance of the drying and sieving process in the analytical procedure is to determine the particle size. Also, sieve analysis is the standard for production and quality control of powders and granules. The specific fraction size of 125 µm was preferred for the analysis because this is the optimum fraction for chemical analysis. A 0.5 g of each representative sample was digested in a microprocessor-controlled digestion block and was further diluted and analysed using inductively coupled plasma mass spectrometer (ICP-MS) at activation laboratories (ACTLABS) in Canada. Sample preparation was conducted in the same laboratory. Statistical standards such as the minimum, maximum, mean, standard deviation and range have been calculated. The potentially toxic elements (PTEs) interrelationships were ascertained through the application of multivariate studies, such as factor analysis and Pearson’s correlation matrix. Quality assurance and quality control (QA/QC) measures have undertaken in this study. Indeed, analytical grade solvents and reagents were utilized in the sediment sample examination. During the metal analyses, calibration solutions, blanks, and standards were run after every 10 samples. The mean measured values for the blanks were below the reported detection limits. The analysis of blanks, randomization of sample numbers and use of in-house reference materials and sample triplicates, provided a measure of the accuracy and precision of the analytical results and precision. Laboratory controls of known concentrations and spiked samples of known concentrations were employed for quality control. The certified values indicated an acceptable degree of uncertainty in the results. Throughout the trial, deionized water was utilized to adequately rinse and wash glassware using liquid soap. The assessment of contamination and pollution degree of sediments was done using integrated pollution indicators (Table 1).

Table 1 Contamination and integrated pollution indicators
Full size table

4 Results and discussion

4.1 Trace metal concentration

The analysed stream sediment samples (Table 2) collected from the Mbal area showed high enrichment in Hg (540–> 10000 ppb), Mn (825–1950 ppm; average = 1445.66 ppm), Zn (50.80–97 ppm; average = 72.30 ppm) and Pb (9.80–87.20 ppm; average = 31.25 ppm) relative to the standards [42, 43]. The concentrations of V (7–16 ppm), Cr (7–47; average = 15.91 ppm), Fe (1.80–3.39 ppm), Co (1.30–16.20 ppm), Ni (0.90–8.70 ppm) and Cu (4.60–8.70 ppm) are low than the standard. The high content of Hg are remarkable in samples S01, S14, S06 and S15 respectively. Cd (< 0.01–0.09 ppm) content are very low in the samples. The mean content of metals is ranged in the order of Hg > Mn > Zn > Pb > Cr > V > Cu > Co > Fe > Ni > Cd (Table 2). Anthropogenic sources related to human activities such mining operation, agricultural practices and domestic activities are primarily responsible for pollution in the Mbal area, comforting by the elevated concentrations of several PTMs (Hg, Zn, Pb and Mn). Weathering and leaching of basement rocks (granitoids, gneiss) may be the cause of the low levels of Ni, Cu V, Cr, Fe, Co and Cd. The content of Mn, Zn, Pb and Hg is higher than those reported in Bambui and Nkwen area respectively [44, 45], lacustrine ecosystems [46], Mari [15] and Pouma [47] areas. Elevated content of Mn, Zn, Pb and Hg could be linked to mining, agriculture, oxidation and hydrolysis of the main ore mineralization, and dust and vehicle emission deposited in the atmosphere [14, 15, 17]. Potentially toxic elements (Mn, Zn, Pb, Hg) intake by humans through food chain results in chronic health problems such as lung damage, brain damage, nervous system disorder, liver damage, kidney disorder, cancer and gastrointestinal disorder [45]. The standard deviation and range values for Mn, Zn, Pb and Hg are high indicating significant variable heterogeneity. Previous research has demonstrated that acidic sediments provide conditions for metal mobility, solubility, sensitivity and bioavailability in the sample medium and thus, increase the availability of PTMs [14].

Table 2 Potential toxic metal concentrations in the stream sediments of the Mbal area
Full size table

4.2 Source apportionment

4.2.1 Correlations between trace metals

The correlation matrix is a tool showing correlation coefficient between variables. Correlation is regarded as the degree to which a pair of variables are linearly associated. The value of a correlation coefficient varies between -1 and + 1. The limitation of correlation coefficients is that it assumes the linear association, meaning that the correlation coefficient is not affect by linear and scale transformation. The coefficients of correlation of PTMs of Mbal area are given in Table 3. Except for Ni and Zn (r = − 0.52), the correlations matrix exhibited positive correlations (r ≥ 0.5). The positive correlations between some metals are assigned to the lithology and mining activities controlled by physico-chemical processes [48]. Correlation coefficients of V/Cr, V/Pb, Cr/Pb, Mn/Fe, Cu, Co/Ni and Ni/Zn indicate their transition metal relationships suggesting evidence for similar input sources due to the occurrence of basement rocks and common geochemical characteristics. Therefore, these trace metals have geogenic origin. V and Pb (r = 0.58) show moderate correlations because they are relatively immobile.

Table 3 Inter-element correlation matrix of potentially toxic metals in the stream sediments of the Mbal area
Full size table

4.2.2 Factor analysis

The factor analysis helps to identify variable elements that are mutually associated into principal associations according to their correlation coefficient. This method assumes linear relationships among variables and requires a large sample size for reliable results. The maximum variance from all variables are extracts and puts into factors. The eigenvalue, percentage of variance, and cumulative percentage have been calculated in accordance with the factors and the larger quantities are extracted and rotated. The limitation of this methods is that the quality of results depends on the quality of the data. Therefore, the interpretation of tactors is subjective and can vary. For this study, four factors explaining 81.89% of variance is generated (Table 4). Factor 1 (F1) accounts for 25.65% and shows high positive loadings for transition metals such as Mn (0.97), Fe (0.89) and Cu (0.82). The F1 association shows low factor score relating to the ferromagnesian mineral from the metamorphic basement, and mobile in the environment. The chalcophile element (Cu) in this factor indicates the occurrence of sulphide mineral (sulphide phase), and therefore, Cu is associated to the sulphidation event. Sulphidation is relevant to the formation of sulphide minerals. The sulphidation event describes the chemical reaction of a chalcophile metal with bisulphide (HS-) forming a metal with sulphide as the final reaction product [49]. The second factor is formed with two siderophiles (Co, Ni) representing 21.24% and reflecting mantle origin. The third factor accounts for 23.43%, it comprises V, Cr and Pb consistent with mafic minerals. Factor 4 (Hg) accounts for 11.59% and also reflects their influence from anthropogenic source, since Hg is used for gold separation.

Table 4 Factor analysis of toxic metals in the sediments of the Mbal area
Full size table

4.3 Indicators of pollution

The computed contamination factor (CF) results indicate that Cd, Ni, Co, Cr, Cu, Fe, V were low (CF < 1), suggesting that sediments enriched with these metals are not contaminated (Table 5, Fig. 2). Cd-rich sediments (sample S11), Mn, Pb and Zn are moderately contaminated. Mn-rich sediments (samples S08-S14, S02, S04) and Pb-rich sediments (samples S05, S15) were significantly contaminated. The sediments enriched in Pb (sample S06) are significantly contaminated while sediments with high Hg contents are classified as very highly contaminated (CF > 6). Human activities (agriculture, mining) have contaminated Mbal area [14, 17]. The use of Mn-based fungicides and fertilizers, to boost crop production raises Mn abundance. Abrasion of tyres, fuel, Pb containing exhaust equipment, agricultural practices, and sulphide minerals can all contribute to high Zn and Pb contents. Hg contamination is caused by the use of Hg in gold amalgamation [15]. Contamination of Mbal sediments is generally attributed to alteration of parent rocks, large scale gold mining operations, agricultural practices and runoff, domestic activities and atmospheric deposition. This result is similar to other studies carrier out in Betare-Oya gold district [14, 15, 17].

Table 5 CF, Cdeg, mCd and PLI of potentially toxic metals in the stream sediments of the Mbal area
Full size table
Fig. 2
figure 2

Contamination factor and pollution load index of toxic metals in sediments of Mbal area

Full size image

The degree of contamination (Cdeg; Table 5) ranges from 200005 (sample S01) to 10809.33 (sample S06). It follows the order: S01 > S02 > S11 > S05 > S014 > S08 > S13 > S07 > S03 > S15 > S04 > S06. The samples show a high contamination degree (Cdeg > 24), which indicates anthropogenic and geogenic inputs. High degree of contamination in the Mbal area is influenced by a range of factors. These include meteorological characteristics such as chemical and physical weathering as well as the erosion of surrounding rocks in the area, without neglecting the contribution of other factors such as large mining operations, atmospheric conditions, productivity, pH, soil texture, redox potential and cation exchange capacity, among others. Internal geochemical processes can lead to remobilization of heavy metals and this phenomenon should be taken into account when assessing contamination.

The modified degree of contamination (mCd) ranges from 18182.33 (sample S01) and 982.67 (sample S06; Table 5) and follows the order: S01 > S02 > S11 > S05 > S014 > S08 > S13 > S07 > S03 > S15 > S04 > S06. This illustrates that Mbal area is ultra-highly contaminated in terms of sampling sites, and shows significant anthropogenic input (mining operation, agricultural practices and domestic activities). Therefore, adequate investigation should be placed on the monitoring the origin of various PTMs in the streams from geogenic sources (alteration of parent rocks and atmospheric deposition), as well as mining and agricultural activities [45]. Therefore, the study area is at risk of growing contamination; this required the remediation and a good policy of mining operations as well as agricultural practices and domestic activities.

The pollution load index (PLI) values range as follows: S02 > S15 > S05 > S11 > S01 > S14 > S06 > S08 > S04 > S13 > S03 > S07 with values of 1.18, 1.15, 1.04, 1.04, 1.03, 0.94, 0.92, 0.91, 0.91, 086, 0.67 and 0.59 respectively. PLI yields 1.18 (sample S02) to 0.59 (sample S07), indicating significant pollution (Table 5, Fig. 2). Samples S01, S15, S11, S05 and S01 with PLI > 1 were polluted and represent 41.66% while the majority of the sampling sites, representing 58.33% exhibited low pollution (PLI < 1). This could be the results of agricultural and mining operations [44, 47]. Therefore, Mbal is at risk of growing pollution; this required the remediation, the good policy of mining operations and the indiscriminate use of Hg and agrochemicals. This is strongly supported by the CF, Cdeg and mCd values, but also by those that confirm the natural contribution of the mineral assemblage and weathering processes (physical and chemical). These results mean that the PTMs present are pollutants in this area since calculated indices suggest their hazardous nature in their present state and quantity.

The geo-accumulation index (Igeo) ranges from -0.03 (Fe) in samples S14 to 18.19 (Hg) in sample S01. (Table 6, Fig. 3). It indicates that sediments are essentially unpolluted for Cu, Fe, Ni, V, Cd, Co, Cr, Cu, Fe, Ni (Igeo < 0). The sediment enriched in Zn, Mn and Pb are minimally polluted (Igeo < 1); samples with elevated content of Pb (sample S06) are moderately polluted (1 < Igeo < 2) and those showing elevated values of Hg are extremely polluted (Igeo > 5). Polluted sediments in Zn, Mn, Hg, Pb are attributed to anthropogenic (mining operation, agricultural practices and domestic activities) and geogenic sources (alteration of parent rocks and atmospheric deposition). Human exposure to Hg and Pb results in poor immunological response, cardiovascular illness, cancer, vital organ damage, and nervous system and reproduction problems [45, 50]. Finally, more than 95% of the sediments would be unpolluted to moderately polluted according to the geo-accumulation index calculated with the geochemical background of [42]. This result underscores the presence of moderate pollution and the risk of toxicity that the Mbal sediment ecosystem would face. Therefore, the necessary preventive measures must be taken to avoid any deterioration of the environment.

Table 6 Geo-accumulation index of toxic metals in the sediments of the Mbal area
Full size table
Fig. 3
figure 3

Geo-accumulation index of toxic metals in sediments of Mbal area

Full size image

The enrichment factors (EF) are shown in (Table 7; Figs. 4 and 5). The sediments rich in Hg have  the highest EF (404,819.28) in sample S01 followed by Pb (13.90) in sample S06 and Mn (4.15) for (sample S15). Sediments do not show any enrichment in Cd, Co, Cr, Cu, Ni or V (EF < 1). The sediments rich in Mn, Zn, Pb in samples S5 and S15 are considerably enriched (EF = 2–5). The sediment are significantly enriched in Pb (sample S06) and severely enriched in Hg (EF = 20– > 40). This clearly shows that the processing of metal ores was the predominant anthropogenic activity that resulted in build-up of Pb, Zn, Mn, Hg. Pb and Zn compounds are exposed to the air and can be transported to multiple sites to induce accumulation through sedimentation processes [51]. These elements could be associated to the alteration of minerals [14, 17]. As aforementioned, Hg enrichment (Fig. 5) is attributed to its use for gold separation. The enrichment of Mn could also be attributed to alteration and leaching from the rocks, mine tailings bile into streams and agricultural practices [44]. This enrichment of PTMs could also be justified by the alteration of parent rocks, oxidation and dissolution of sulphide minerals and atmospheric deposition. This results concord with other research works carrier out in the study area [14, 15, 20].

Table 7 Enrichment factors of toxic metals in the sediments of the Mbal area
Full size table
Fig. 4
figure 4

Enrichment factor of toxic metals in sediments of Mbal area

Full size image
Fig. 5
figure 5

Enrichment factor of Hg in sediments of Mbal area

Full size image

The ecological risk factor (Er) ranges from 0.10 (Ni) to 8000000 (Hg) (Table 8, Figs. 6 and 7). Samples exhibits low Er (Er < 40) for Mn, Ni, Zn, Cd, Co, Cu, Cr, Fe, Pb, V, Pb. This suggests a reduced potential ecological influence in aquatic ecosystem (Fig. 6). Meanwhile, sediments show extreme Er for Hg (Er ≥ 320; Fig. 7). Thus, the constant use of Hg should be restricted and prohibited within the mining communities. This results concord with other research works in the study area [14, 15].

Table 8 Ecological risk factor and potential ecological risk index of toxic metals in the sediments of the Mbal area
Full size table
Fig. 6
figure 6

Ecological risk factor of toxic metals in sediments of Mbal area

Full size image
Fig. 7
figure 7

Ecological risk factor of Hg in sediments of Mbal area

Full size image

The potential ecological risk indices (PERI) are shown in Table 8 and Fig. 8. The PERI is in the order of S01 > S02 > S11 > S05 > S14 > S08 > S13 > S07 > S03 > S15 > S04 > S06. The Mbal displayed an extreme ecological risk index probably due to mining exploitations, agriculture, domestic activities and atmospheric deposition, which are deleterious to humans and the entire environmental components [52,53,54,55]. Consistent with the ecological risk factor results, the study area is in the class of extreme risk environment with respect to various pollutants and sites.

Fig. 8
figure 8

Potential ecological risk index of toxic metals in sediments of Mbal area

Full size image

The results of anthropogenic input are listed in Table 9. Anthropogenic input in the Mbal area refer to Cd (1.56 -14.06) and Hg (0.60–11.11). Cd is sourced from oxidation and dissolution of sulphide minerals associated with ore mineralization and by the use of Cd- batteries by local miners and agricultural runoff (phosphate fertilizers, pesticides and other agrochemicals). Hg is sourced from the use of Hg by companies and local miners in gold separation from sediment concentrate and from agrochemicals use in crop production. This call for concern as Cd and Hg are very toxic at all concentration to humans and biota.

Table 9 Anthropogenic input of potential toxic metals in the sediments of Mbal area
Full size table

4.4 Spatial variability

The coefficient of variation (CV) is used to compare the variability for similar values of variance and different means. The low CV values infer to a spatially homogeneous distribution, while high CV values denote a non-homogenous surface distribution [56]. The studied PTMs show low CV values that range from 17.79 to 96.34% (smaller than 100%; Table 2), indicating a low spatial variability. The spatial variability maps for the studied PTMs are generated and cluster along River Mbal and River Bedere, the tributaries of River Lom where mining and anthropogenic activities are more developed (Figs. 9 and 10). High concentration of Mn, Hg, Pb and Zn cluster western and eastern part of the study area. The concentrations of Hg, Pb, Zn, Cu and Cd indicate sulphide mineralization related to vein gold deposits found in the study area. Zn and Pb can form stable complexes with Fe and Mn oxides through adsorption or co-precipitation processes [57]. In the study area, Hg is used for gold separation, this is therefore the main source of Hg in the studied sediments. The natural occurrence of Hg is low in the crust, his value is not up to 0.05 µg/g [42]. It can be discharged into water, sediments and atmosphere through its use in mining. This toxic metal could affect the reproductive and nervous system; therefore, it is important to monitor his use in mining within the catchment. The spatial distribution of Ni, Co, Cr, Fe, V (Figs. 9 and 10) are low; these are the important constituent elements in ferromagnesian silicates and oxides in the metamorphic rocks of the study area. Fe could also originate from the sulphide minerals associated with the primary gold mineralization in the Mbal area. Therefore, the chemical weathering in this area is related to the enrichment of Fe and Mn in sediments. Hence Fe and Mn are environmental scavengers [58]. The geochemical distribution of Cd is very low and irregular. The potential sources of Cd in the environment (air, land, water) include natural processes and those associated to human activities [59]. In the study area, Cd originated from oxidation and dissolution of sulphide minerals, Cd- batteries and/or tobacco smoke over time. High levels of Cd damage human’lungs causing death. Exposure to low levels of Cd may cause kidney disease and fragile bones. Also, Cd is considered a cancer-causing agent.

Fig. 9
figure 9

Spatial variation maps of V, Pb, Ni, Mn, Hg and Cu

Full size image
Fig. 10
figure 10

Geochemical distribution maps Fe, Cr, Co, Cd and Zn

Full size image

5 Conclusions

This work was aimed at assessing the contamination of potentially toxic metals (Cd, Cu, Co, Cr, Fe, V, Ni, Cu, Zn, Hg and Pb) in stream sediments of Mbal area. The most important findings are followed:

  1. 1

    Hg and Mn were strongly enriched compared to their standard values in the Upper Continental Crust. Multivariate statistical analysis reveals evidence for similar input sources due to occurrence of granitic and metamorphic rocks in the study area and common geochemical characteristics of these trace metals. However, low content of V, Fe, Co, Ni, Cu, Cd are attributed to the weathering and leaching of granitic and metamorphic rocks. The Mbal area is facing a high risk of trace metals contamination due to the high concentration of potential toxic metals in stream sediments associated with the extensive development of artisanal gold mining and anthropogenic activities.

  2. 2

    The PTMs exhibit a low variability and their spatial distribution is broadly controlled by source geology. The CF, Cdeg, mCd, PLI, EF and RI of Cd, Cu, Co, Cr, Fe, V, Ni, Cu, Zn, Hg and Pb indicates metal pollution of sediments in the Mbal area. Also, the EF values show a significant influence of anthropogenic activities on the supply of Hg, Pb, Mn and Zn in stream sediments. Natural and anthropogenic metal input assessment reveal that the dominantly mafic lithologies in the area, mining and domestic activities as well as agricultural activities are the main source of toxic metal contamination. The illegal and uncontrolled use of Hg in artisanal mining during gold separation constitutes a major source of pollution as Hg was the most enriched toxic metal in the entire study area.

  3. 3

    There is increasing potential for rising pollution levels in virtually all the study sites, this called for remediation and control of illegal mining and indiscriminate use of Hg and agrochemicals in all the study area. Thus, there is a need to educate the companies and local population by providing them with more training on the safe method of mining, control use of mercury, application of agrochemicals and fertilizers input to reduce the risk of PTMs contamination in Mbal sediments.

  4. 4

    Constant monitoring and implementation of environmental laws and regulation in mining districts of Cameroon are of great important to protect the environment, biota and humans for sustainable development. These data can enable the local councils and the government to better anticipate and plan for potential environmental impacts and are useful for setting realistic targets for monitoring and remediation programs.