Materials, Equipment and their uses for this study are listed in Table 1.

153 hand-dug wells including some rivers were located around the city of Douala. At each of the sites, a GPS was used to get readings of the coordinates of the locations. 10 groundwater samples were collected from 10 pre-selected wells.

Prior to sampling, the bottles were rinsed with the water to be sampled so as to avoid contamination. The well water was withdrawn with a small bucket on which a rope was tied on to reach water in the well.

The groundwater samples collected in 50 ml were acidified and sent to Activation laboratory Canada for the determination of the concentrations of trace elements using ICP-OES analysis.

Involved identification of the chemical of concern and documenting its toxic effects on humans after contact and the characterization of potential contaminants and their relative mobility [19] as in Table 2.

This is the process of measuring or estimating the intensity, frequency and duration of human exposures to an environmental agent [19]. 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 [20].

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

where;

ADDs are 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: Concentration of contaminant in the environmental media (e.g, μg/L, mg/L).

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

EF: Exposure frequency (day/year).

ED: Exposure duration (years).

BW: Body weight of receptor (kg).

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

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

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 affect 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 were determined by calculating the potential of an individual to develop cancer as a result of cumulative exposure to each potential carcinogen over a lifetime. Carcinogenic properties were identified by a weight-of-evidence classification of the trace element concentration [21]. 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) [22] [23].

CR = ADD ⋅ SF (2)

where; CR is the excess probability of developing cancer over a lifetime as a result of exposure to a contaminator 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 non carcinogenic risk, the hazard quotient (HQ) was calculated using Equation (3) [24].

HQ = ADD RfD (3)

where; 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 [25]

It is the toxic risks due to all the potentially hazardous substances present in the same media simultaneously [26]. 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 the entire hazard quotient (HQ) for all toxicants, Equation (4) [22] [27].

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), Contamination factor (CF), 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 Bafoussam as in Table 3.

Determination of Indices for contamination level assessment

The determination of indices for trace metal assessment includes the human health risk assessment indices and the pollution indices.

Human Health Risk Assessment Indices

A human health risk assessment estimates the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media, now or in the future. Human health risk assessment includes 4 basic steps: EPA begins the process of a human health risk assessment with planning and research; hazard identification, dose-response assessment, exposure assessment, risk characterization

Hazard identification

This examines whether a stressor has the potential to cause harm to humans and /or ecological systems, and if so, under what circumstances.

Exposure assessment

The main exposure pathway taken into consideration in this study was intake of water through water consumption. The daily environmental exposure to metals was assessed for carcinogenic and non-carcinogenic elements. This is the process of measuring or estimating the intensity, frequency and duration of human exposures to an environmental agent [24]. 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 procedures used to extrapolate from high to low doses are different for assessing carcinogenic effects and non-carcinogenic effects:

・ Carcinogenic effects in general are not considered to have a threshold and mathematical models are generally used to provide estimates of carcinogenic risk at very low dose levels.

・ Non-carcinogenic effects are considered to have dose thresholds below which the effect does not occur. The lowest dose with an effect in animal or human studies is divided by the safety factors to provide a margin of safety.

The primary health risk assessment classifications schemes are those of the Environmental Protection Agency [28]. The intake of metals through ingestion of groundwater was calculated (Equation (1)) [29].

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

where;

・ ADDs: Exposure duration (mg/kg-day)

・ The Average Daily Dose (ADD) is the amount of aqueous contaminant ingested per kilogram of body weight per day

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

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

・ EF: Exposure frequency (day/year)

・ ED: Exposure duration (years)

・ BW: Body weight of receptor (kg)

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

・ For non-carcinogenic effects, AT = ED in days. For carcinogenic effect, AT = 70 years or 25,550 days.

The geo-accumulation index (I_geo) is a quantitative measure of the degree of pollution in sediments. It renders seven degrees of contamination ranging from unpolluted to very extremely polluted conditions. To describe the level of pollution in the water, the formula of the I_geo is as follows:

I g e o = log 2 [ C i / ( 1.5 C r i ) ] [17].

where C_n is the metal concentration, and B_n is the background geochemical value of a particular substance in the samples. Factor 1.5 is attributed to lessen the provable difference in the background values that may influence lithogenic effects. The I_geo values were categorized into 7 classes as in Table 3.

The field measured physicochemical parameters of groundwater in Douala compared to the WHO guidelines for drinking water were as presented in Table 4. The Groundwater temperatures ranged from 22.9˚C - 36.7˚C in the wet season and 24.1˚C - 37.8˚C in the dry season; this is indicative that the wells are good for drinking. The pH ranges from 3.77 - 7.7 in the wet season and 3.64 - 7.4 in the dry season; this is indicative that some of the well water in both seasons are not suitable for drinking. Some EC and TDS values of groundwater were above WHO permissible limit.

The results of chemical analysis of the major and minor elements of groundwater in Douala compared to WHO 2017 drinking water guidelines are as shown in Table 5.

Comparing average major cation and anion concentrations the groundwater in Douala is potable.

The results of the relevant trace elements in groundwater in Douala compared to the WHO guidelines and presented in Table 6.

The values of Ba, Al, As and Sb were above the WHO guidelines as presented in Table 6. With the rapid industrialization and economic development, trace elements are being introduced to groundwater via several pathways viz; fertilization, irrigation, groundwater inflow and runoffs. The amount of trace elements in groundwater in Douala is generally low compared to the WHO guidelines but for Ba, AL, and Sb which are slightly higher. This is similar to the results of [26] in Ejisu-Juaben Municipality, Ghana.

This was done to assess the potentially harmful effects of long term human exposures to the low intensity and high frequency of the trace element concentration in groundwater used as drinking water in Douala so as to discriminate the health effects that might result from exposure to carcinogenic and non-carcinogenic chemicals. These health risk indices include ADD, CR, HQ and HI.

1) For the average daily dose (ADD), values range from 0 - 60 mg/kg/day, amongst all the investigated trace elements, Mn and Ag have the highest or the greatest daily intake. All the other trace elements have acceptable to low average daily dose values as seen in Figure 3.

2) All the carcinogenic risk (CR). All values are >10⁻⁴ and fall under the intolerable class; intollerable to human health. This implies that there is a tendency/potential for the inhabitants in the Douala drinking groundwater over a long period to suffer from cancer as in Figure 4.

3) The non-carcinogenic hazard quotient (HQ) values were all found to be >1 using the USEPA 2012 [31] classification. This is indicative of no non-carcinogenic adverse effects in the groundwater in Douala as in Figure 5.

4) The carcinogenic hazard index (HI) values were found to be >1 as in Figure 6, according to USEPA, 2012 [31] classification. This indicates that the trace element concentrations in the groundwater in Douala is unsafe for human

consumption and may cause adverse effects to the human population with time.

The health risk assessment indices in the study area are below acceptable limits as presented in Table 7.

These results contrast those of [32] in Buea in the Piedmont regions of Mount Cameroon.

1) Geo-accumulation index (Igeo): The values for the trace elements in the groundwater in Douala are in the range of 0 - 1 of Class II, indicating unpolluted to moderately polluted groundwater. The maximum value of 0.15 moderately polluted groundwater geo-accumulation indexes is at Akwa Nord, New Bell and Ndogbati as in Figure 7.

2) Degree of contamination (DC) values shows low degree of contamination in the groundwater in Douala as in Figure 8. The low degree of contamination by these trace elements could be due to the absence of external discrete sources like agricultural runoff, other anthropogenic inputs or the occurrence of large volumes of groundwater flow through the Wouri delta (a discharge zone) on which Douala City is found which dilutes the effects of pollution.

3) Enrichment factor (EF) sequence in the sediments was Pb > Sr > Ba > Mn > Bi > Co > Fe > Li > V (Figure 8). Significant enrichment ranged from 0 - 1.2 indicating that the sources of these elements are not from rock weathering processes (geogenic). Pb and Sr are the most abundant in all the groundwater in Douala; this could be attributed to agricultural and industrial wastes. High values of Sb occurred at Akwa, Akwa Nord, New Bell and Ndogbati; Al at Bepanda and As Akwa, Makepe Deido as in Figure 9.

4) Pollution Load Index (PLI) values of indicated that 80% of the trace elements in the groundwater in Douala were <1 therefore considered unpolluted, and 20% were = 0, which is indicative of perfection, meaning there is no pollution in the groundwater as in Figure 10. Pollution load index measured the pollution severity and its variation in the groundwater in the various localities in Douala. This index is a quick tool used to compare the pollution status of different locations in a study area [33].

5) Ecological Risk assessment Er of the trace elements in the groundwater of Douala is given in Figure 11. All values showed low potential ecological risk, (Er < 40) indicating low pollution according to [34]. Steps should be taken to minimize contamination.

6) Ecological risk index (RI) of the trace elements in groundwater ranged from −17.58 to −17.64 as in Figure 12 classifying groundwater in Douala as being of low ecological risk, [34]. RI varies with time depending on activities in the various ecological niches in the area.

A classification of all the pollution indices for groundwater in Douala is presented in Table 8.

1) Pearson’s Correlation Analysis (PCA). The Pearson’s correlation matrix (Table 9) exhibited very strong positive correlation exists between the following; Fe and Ba (r = 0.84), Cu and Si (r = 0.80) and TDS and EC (r = 1). Strong positive correlation existed between Mn and Ba (r = 0.74), Sb and Si (r = 0.70). Good positive correlation existed between Cu and Fe (r = 0.68), and an average correlation existed between Sb and Cu (r = 0.50). Very strong negative correlation existed between pH and temperature (r = −0.80) as in Table 9.

2) Hierarchical cluster Analysis

(HCA) was conducted based on the normalized data along with Ward’s linkage method. The R-mode cluster analysis performed on the groundwater analysis produced a Dendrogram Figure 13 has two clusters based on similarities and dissimilarities. Cluster1 comprised of Si and cluster 2 comprised of two sub clusters; Sub-clusters1; Sb and Sub-clusters which is divided into two classes: Class1; Sb and Class2; TI, V, Ti, Sn, Pb, Mo, Ni, Ag, Cd, Co, Cu, U, Y, Ba, Mn, Cr, Se, Bi, As, Fe Li, Zn, Sr, Al. as seen in Figure 13.

The elements in Cluster2 Class2 associate with one another because they are from the same parent rocks, which could possibly be that they resulted from the same, melt during crystallization. Sb and Al have different mobility Si also clustered

0.8 - 1.00 Very strong correlation; 0.7 Strong correlation.

together but in different subclasses which indicates that they also emanate from same parent rock. The presence of Si in Cluster1 reveals lithogenic contribution from environmental processes and thus of a different provenance. This is due to the marked difference in their thermal stability and chemical activity.

The concentration of trace elements in water could be influenced by natural and anthropogenic factors. Naturally, trace elements are associated with different rock types in varying degrees with Cr, V, Ni, Co, Cu, Fe, commonly associated with low silica content rocks [34]. The most likely mode of occurrence of trace elements in groundwater within the study area is geogenic with slightly elevated concentrations of Sr, Ba, and Li.