Kumba generally has a hot and humid equatorial climate with two seasons: a short dry season of about 4 months (December to March) and a long rainy season (April to November). Annual rainfall ranges from 2298 mm to 3400 mm. The average annual temperature is approximately 27˚C.

Kumba is located in the tropical rainforest with vegetation that varies from savannah to forest (around Lake Barombi Mbo). The evergreen and semi-deciduous forests contain economically important tree species (iroko, mahogany, obeche, ebony, padouk, tiama, framire, sapelline, makore and bobinga, etc.). The herbaceous layer is dominated by Pennisetum, purpureum and Imperata cylindrica with a ligneous cover that is heavily affected by human activity. The river valleys are covered with Indian bamboo (Bambousa species) whose stems are used for handicraft activities. Deforestation is the main cause of environmental degradation in Kumba. It arises from human activities, especially inappropriate farming practices (shifting cultivation), over grazing, bushfires, poaching and illegal logging.

Kumba is located in the Kumba Plain, a graben intercalated between the strato- volcanoes of Mt Cameroon and Mts Rumpi, at the northwestern edge of the Douala Basin as in Figure 2. The Cameroon Line (CL) is an alignment of Tertiary-to-Recent alkaline volcanoes, plutons and grabens extending over more than 1600 km stretching from the Atlantic oceanic island of Annobon through the Gulf of Guinea and within the African continent. The Douala basin probably formed from a Precambrian cratonisation, granitisation and sedimentation phase followed by the Pan-African orogenesis, the Afro-Brazilian depression (the site of the future Cameroon Atlantic basin) with epi-continental sedimentation which may have begun during the lower Cretaceous, discordant Cretaceous to Pliocene sediments on the Precambrian Pan-African basement and covered in

some areas by Miocene sedimentation and volcanism. The geology of the Kumba Plain (Kumba Volcanic Field) is controlled by three main volcanic activities (which probably occurred between the Eocene and 1 Ma ago). These include: old basaltic lavas covering the entire plain; cinder cones and phreatomagmatic units; and short vesicular basaltic lava flow.

There are 4 maars in the Kumba Plain. These include: the Barombi Mbo, Barombi Koto, Mbwadong and Dissoni Maars, with the first two occupied by Lakes Barombi Mbo and Lake Barombi Koto. Based on the composite fragments contained in the Barombi Mbo Maar (BMM) pyroclastic deposits, it is likely that the maar cuts through a geological succession composed by granite gneissic formation, sandstones, and basaltic lava flows; the same formations that make up the Kumba volcanic field.

Volcanic formations of the plain have been emplaced over Pan African metamorphic formations intruded by granitoids and locally covered by Cretaceous continental sandstones. They commonly enclose mantle peridotite xenoliths [4].

Weathered basement (regolith), fractured-gneiss, basalts, pyroclastics and recent alluvium are the aquiferous formations in the area. Saturated hydraulic conductivities of the aquiferous formations range from 2.88E−08 to 1.60E−06 m/d [5], groundwater velocities from 1.96E+01 to 6.34E+02 m/d [4], first estimates of well yields from 4.6E−01 to 2.28E+01 m/d and the hydraulic conductivities of the vadose zone from 7.96E+02 to 3.27E+04 m/d. and stream discharges 1.79E+05 to 9.13E+05 m³/d [6].

A

Twenty one samples were collected from 21 pre-selected wells, boreholes and springs. Site selection was based on spatial distribution of the wells, boreholes, springs and population. At each site, groundwater temperature, electrical conductivity, total dissolve solids and pH value were measured in situ, using portable field pH, EC and TDS meters as shown in Table 1.

It involves the identification of the chemical of concern and documenting its toxic effects on human beings after field mapping. It also involves the characterization of potential contaminants and their relative mobilities [7] as shown in Table 2.

This is the process of measuring or estimating the intensity, frequency and

duration of human exposures to an environmental agent [7]. The main exposure pathway taken into consideration in this study was intake of the metals through water consumption. The daily environmental exposures to metals were assessed for carcinogenic and non-carcinogenic elements.

The intake of metals through ingestion of groundwater were calculated using Equation (1) [8].

ADD = C ⋅ IR ⋅ ED ⋅ EF BW ⋅ AT ⋅ 360 (1)

where:

ADDs is Exposure duration (mg/kg-day)―The Average Daily Dose (ADD) of the contaminant through water pathway indicates the quantity of chemical substance ingested per kilogram of body weight per day.

C is Concentration of contaminant in the environmental media (e.g., µg/L, mg/L).

IR is Ingestion rate per unit time (e.g., mg/day or L/day).

EF is Exposure frequency (day/year).

ED is Exposure duration (years).

BW is Body weight of receptor (kg).

AT is Averaging time = life expectancy (years) 365 is the conversion factor from years to days.

For non-carcinogenic effects, AT = ED in days; carcinogenic effect, AT = 70 years or 25,550 days [8].

This is the quantitative relationship that indicates the contaminants degree of toxicity to exposed species. It also involves the identification of the toxicity criteria used to evaluate human health risk associated with the chemical of concern in the study area. The amount of chemical that can affected human health is estimated. The Reference Dose RfD is used for non-carcinogen risk.

This is the final phase of the risk assessment process. In this phase, cumulative exposure and dose-response assessments are integrated to yield probabilities of effects occurring in human beings under specific exposure conditions and time scales. Also incorporated is information from hazard identification, exposure assessment, toxicity assessment and risk estimation to evaluate the potential risk to residents.

Carcinogenic risks was determined by calculating the potential of an individual to develop cancer as a result of cumulative exposure to each potential carcinogen over a lifetime. For carcinogen, identified by a weight-of-evidence classification of the chemical [9].

The estimated daily dose and the cancer slope factor are multiplied together to find the lifetime cancer risk posed by the chemical. Cancer slope factors are estimates of carcinogenic potency and were used to relate estimated daily dose of the trace metal over a lifetime exposure to the lifetime probability of excess tumors Equation (2) [9] [10] [11].

CR = ADD ⋅ SF (2)

CR is the excess probability of developing cancer over a lifetime as a result of exposure to a contaminant or carcinogenic risk. It is unit less; SF is the slope factor of the contaminant [mg/kg/d]⁻¹.

Non-carcinogenic hazards are characterized by the hazard quotient (HQ). HQ is a unitless number that is expressed as the probability of an individual suffering an adverse effect. To estimate noncarcinogenic risk, the hazard quotient (HQ) was calculated using Equation (3) [12].

HQ = ADD RfD , (3)

RfD is the reference dose mg/kg/d. It represents a toxicity index of a daily exposure to the population in comparison to a safe level of exposure orally over a lifetime [13].

It is the toxic risks due to all the potentially hazardous substances present in the same media simultaneously [14]. Since more than one toxicant is evaluated, the interactions of all the toxicants were considered and assumed to be cumulative. Thus, the HI was calculated by summing all the HQ for all toxicants, Equation (4) [12].

HI = ∑ i = 1 n HQ i (4)

Generally, pollution indices are estimated for a specific use of the water under consideration. The trace metal degree of contamination (DC), enrichment factor (EF), ecological risk index (Er), potential ecological risk index (RI), pollution load index (PLI) and geo-accumulation index (Igeo) were used to evaluate the pollution potential of the study area (Table 3).

The physicochemical parameters groundwater in the study area: temperature, pH, EC and TDS were evaluated as shown in Table 4.

Depth-to static water level (m) of groundwater ranged from: 0.2 - 6.2 m as in

Figure 3. Areas with low depths to static water levels such as; Kumba, Ediki, Bombe and Kwakwa are susceptible to pollution if the wells are not appropriately constructed and protected.

Groundwater flows towards the Southwestern and Northwestern parts of the study area which could probably be a recharge zone as in Figure 4.

Temperature values of groundwater ranged from: 26.7˚C - 29.1˚C as seen in Figure 5. The temperature variation is similar in the different areas, suggesting a single aquifer since groundwater in the same aquifers have similar parameter values and temperature is one of them.

The pH value of most of the groundwater samples in the study area ranged from 2.3 - 6.4 as in Figure 6. The value of pH of a water sample is recognized as an index of classifying groundwater as acidic < 5.5, slightly acidic 5.5 - 6.5, neutral 6.5 - 7.5, slightly alkaline 7.5 - 8, moderately alkaline 8 - 9 and alkaline > 9. This clearly shows that the groundwater in the study area is acidic to slightly acidic.

The EC ranged from 1.8 - 50.1 mS/m as in Figure 7. The low electrical conductivity

is due to less solute concentration in groundwater.

The total dissolved solids ranged from 1.2 - 33.6 mg/L as in Figure 8. These TDS values are < 500 mg/L thus indicating that the water is fresh.

The results for twenty one samples of trace metal analysis ICP-MS are presented in Table 5. The concentrations of fourteen trace metals; Mn, Fe, Ba, Sr, Zn, Ni, Cu, Co, Pb, Li, Cr, V, As, and Cd were evaluated since they were of significance. All concentrations of these trace metals are below the [20] allowable limits Mn and Ba which had concentrations above permissible limits. However, the cumulative effects of long term consumption of these trace metals in the groundwater necessitated a health risk assessment.

Average concentrations of these trace metals were in the decreasing order as follows in µg/L: Mn (193.11), Fe (151.71), Ba (142.32), Sr (65.78), Zn (60.29), Ni (14.15), Cu (10.12), Co (7.47), Pb (2.31), Li (1.62), Cr (1.53), V (0.72), As (0.21) and Cd (0.16). The trace metal with the highest concentration is Mn with a maximum value of 687.20 µg/L detected at Bombe.

Correlation between trace metals in groundwater within the study area was carried out using Pearson’s correlation analysis (PCA) as shown in Table 6 to establish the relationships that exist between the variables; trace metals and the physico-chemical parameters as in Table 2. r values > 0.5 or < −0.5 are significant, 0.72 strong (moderate) and 0.80 Very strong correlation.

In Table 6, a very strong positive correlation exists between the following; Pb and Cu (r = 0.82), Cd and Zn (r = 0.86), EC and As (r = 0.98), TDS and As (r = 0.98), Ba and Sr (r = 0.86). Moderately positive correlation exists between the following:

Ni and Li (r = 0.79), Co and Mn (r = 0.75), Ni and Co (r = 0.72), Pb and Zn (r = 0.79), Pb and Cd (r = 0.74). A moderately negative correlation exist between V and Li (r = −0.04), Sr and Li (r = −0.03), Cu and Co (r = −0.08). A high correlation coefficient (nearly 1 or −1) means a good relationship between two variables, and a correlation coefficient around zero means no relationship. Positive values indicate a positive relationship while negative values of r indicate an inverse relationship [13].

The R-mode Cluster Analysis; hierarchical cluster analysis HCA performed on the groundwater of Kumba and environs area shows two clusters based on spatial similarities and dissimilarities. The trace metals fall in two clusters: Cluster one; (01) element Ba; soluble. Cluster two (13) non soluble elements divided into three classes; class one (06) As, Cd, V, Li, Pb, and Cr; less enriched. Class two (03) Co, Cu, Ni and Zn; enriched; Class two (04) Zn, Sr, Fe and Mn; more enriched (Figure 9).

Human health risk assessment was done to estimate the intensity, frequency, and duration of human exposures to an environmental contaminant. Exposure assessment was carried out by measuring the average daily dose ADD of the trace

metals selected in Table 7. Carcinogenic and non carcinogenic risk were calculated from the ADD.

ADD values ranged as follows; (Li) 8 × 10⁻⁶ to 1 × 10⁻⁴, (V) 5.5 × 10⁻⁶ to 5.1 × 10⁻⁵, (Cr) 7 × 10⁻⁶ to 3.2 × 10⁻⁴, (Mn) 5 × 10⁻⁴ to 0.022, (Fe) 2 × 10⁻³ to 0.013, (Co) 4 × 10⁻⁵ to 9 × 10⁻⁴, (Ni) 7 × 10⁻⁵ to 1.2 × 10⁻³, (Cu) 1 × 10⁻⁴ to 7 × 10⁻⁴, (Zn) 1.4 × 10⁻⁴ to 5 × 10⁻³, (As) 1 × 10⁻⁶ to 2 × 10⁻⁵, (Sr) 2 × 10⁻⁴ to 0.011, (Cd) 1 × 10⁻⁶ to 1 × 10⁻⁵, (Ba) 1 × 10⁻⁴ to 0.052, (Pb) 3 × 10⁻⁵ to 2 × 10⁻⁴ as in Figure 10.

HQ values ranged as follow; (V) 2.7 × 10⁻³ to 2.5 × 10⁻², (Cu) 2.4 × 10⁻³ to 1.8 × 10⁻², (Pb) 7.8 × 10⁻³ to 5.7 × 10⁻², (Zn) 3.3 × 10⁻⁵ to 1.7 × 10⁻², (Fe) 2.2 × 10⁻³ to 1.8 × 10⁻², (Mn) 7.6 × 10⁻² to 1.59 as in Figure 11.

CR for the carcinogenic elements; Cr, Cd, Ni and As are; (As) 2.2 × 10⁻⁴ to 4.4 × 10⁻⁴, (Cd) 8.5 × 10⁻⁶ to 8.8 × 10⁻⁵, (Cr) 3.9 × 10⁻⁴ to 1.8 × 10⁻², (Ni) 8.6 × 10⁻⁵ to 1.4 × 10⁻³ as in Figure 12.

HI is the cumulative sum of HQ. The values ranged between 0.07 and 1.59 for each of contaminant indicating no toxicity as in Figure 13.

All the groundwater hazard risk indices; ADD, CR, HQ and HI were less than 1 but for sample 20 which had HI value greater than 1, thus the water is of significant health risk as shown in Table 8.

Degree of contamination (DC)

The degree of contamination (DC) is used as reference of estimating the extent of metal pollution. The DC values in the groundwater ranged from −13.53 to 12.1 According to classification [15], 95.2% of the samples have degree of contamination factor < 10 which is indicative of low contamination whereas 4.8% of the samples have degree of contamination factor between 10 - 20 thus indicating moderate contamination.

Enrichment factor

Iron (Fe) was chosen as a stationary reference element to perform this calculation [22]. EF values < 2 indicate that the metal is entirely from crustal materials or natural processes; whereas EF values > 2 reveal that the sources are more likely to be anthropogenic [23]. The enrichment factors of trace metals in Kumba and environs were as shown in Figure 14, Figure 15 and Table 9. The sequence of EF in the sediments was Ba > Co > Mn > Ni > Pb > Cd > Cr > As > Sr > Zn> Li > Cu > V. EF values in the study area are between 1.15 to 874.13 which is indicative of significant enrichment and that the source of these metals is from natural and anthropogenic processes. Barium and Cobalt are the most enriched elements in the study area; this could be attributed to agricultural wastes.

Ecological Risk Factor (Er) and Ecological Risk Index (RI)

Er and RI of the heavy metals in the investigated area are given in Table 9 and Figure 16 and Figure 17. All analyzed trace metals showed low potential ecological risk, it varied from −29.68 to 9.45 (Er < 40). RI of the studied trace metals ranged from −61.06 to −43.03. All the samples show low ecological risk index, this indicates low polluted according to [17].

Pollution Load Index (PLI)

This index is a quick tool in order to compare the pollution status of different places [24]. The values of Pollution Load Index are <1 which is indicative that there is no pollution. These results attributed principally to natural sources.

Geo-Accumulation Index Igeo

The geo-accumulation index Igeo is a quantitative measure of the degree of pollution in groundwater [25] as shown in Table 9 presents the indices for the

quantification of trace metal accumulation in the Akwa-Mundemba. The values range as in Table 4. Groundwater is Unpolluted to moderately polluted by Pb, V, Cr, Co, As, Cd, Pb; while it is Extremely polluted by Mn, Fe, Ni, Cu, Sr, Zn, Ba (Figure 18).

The results of pollution evaluation indices are presented in Table 10.