3.1. Groundwater quality parameters and spatial distribution pattern
Table 5 presents the results of physicochemical and bacteriological analyses of groundwater in the city of Ebolowa. Table 5 shows that the temperature values oscillate between 24°C and 30°C, with an average value of 25.5°C. They are mostly above the WHO guideline value of 25°C. These differences can be explained by the fluctuations recorded by the ambient temperature during the sampling as well as the various activities that develop around the sampling points.
Analysis of the data in Table 5 shows that all the water samples taken are acidic, with a pH between 3.7 and 5.6. This result implies positive values of the redox potential, which indicates the presence of strong oxidants with redox potential values between 70.2 mV and 179.5 mV. In addition, salinity is generally low, with values between 0.1‰ and 0.9‰. These values are consistent with the natural and geological environment of the center-south zone of Cameroon, where the superficial aquifer, depleted of basic cations, is recharged by rainwater, itself already acidic (Defo et al., 2016; Ewodo et al., 2016).
Table 5 also shows that 87% of the water samples analyzed presented conductivity values between 114 µS/cm and 1,818 µS/cm with an average value of 629.2 µS/cm. These waters therefore have low- to medium-level mineralization. With regard to TDS, the concentrations of this parameter vary from 169 to 913 mg/l in modern wells, from 189 to 559 mg/l in traditional wells, and from 54 to 351 mg/l in springs and boreholes. In addition, 47% of the samples analyzed have TSS concentrations that do not comply with WHO recommendations. Modern water points (Boreholes and modern wells) seem to be less exposed to TSS pollution than other types of structures.
Nitrate (NO3−) concentrations are not high, with values between 0 and 19.8 mg/l. These values are below the threshold value recommended by the WHO (50 mg/l). Nitrates are the main forms of nitrogen in waters and can be used as indicators of chemical pollution of waters. Fagbayide and Abulude (2018) link the presence of nitrates in waters, in addition to the normal nitrogen cycle, to different sources, namely: organic and inorganic chemical fertilizers, pesticides and herbicides, animal breeding facilities, and domestic and industrial effluents.
The concentrations of ammonium ions (NH4+) are between 0.2 and 12.9 mg/l, with an average value of 1.5 mg/l. About 40% of the water samples showed NH4+ concentrations above the limit value recommended by the WHO (0.5 mg/l). Of these, 50% come from modern wells, 17% from traditional wells, and 33% from springs. These high NH4+ values show proximity to the source of contamination through urine and feces due to the proximity of the toilets.
The FC concentration is between 1 and 51 CFU/100 ml, with an average value of 11.3 CFU/100 ml. The concentration of FS is between 0 and 25 CFU/100 ml, with an average value of 6.1 CFU/100 ml. The concentration of TC is between 2.0 and 84.0 CFU/100 ml, with an average value of 24.1 CFU/100 ml. In addition, all the samples analyzed showed the presence of germs, indicating fecal pollution. Indeed, the level of sanitation in the study area is very precarious with regard to waste management, particularly wastewater and latrines. The vast majority of excreta is stored in bottomless latrines, the pit of which generally reaches the water table and constitutes a risk of microbiological contamination of groundwater through the process of diffusion of contaminants in the soil.
3.2. Spatial distribution pattern
The spatial distribution pattern of the contour maps of the groundwater quality parameters has been generated, as represented in Fig. 2. The temperature distribution diagram (Fig. 2a) shows that groundwater in the city is generally above 25°C in the northern part and below 25°C in the southern part of the city. The spatial distribution pattern of pH (Fig. 2b) indicates that the northern part of the city has strongly acidic groundwater (pH < 4.5), while in the southern part it is moderately acidic (4.5 < pH < 5.5), with some spots where the water is strongly acidic. The north-central part of the town of Ebolowa has a high redox potential (Fig. 2c). The electrical conductivity is mainly highest (> 1000 µS/cm) in the central part north of the municipal lake (Fig. 2e). Similarly, the salinity is mainly highest in the central part, north of the municipal lake (Fig. 2d). The central part of the city, especially north of the municipal lake, has the highest TDS (> 750 mg/l) in groundwater (Fig. 2f) due to low flow and highly weathered rock formations.
The diagram of the spatial distribution of TSS (Fig. 2g) indicates that the northern part and the southwestern part of the city have the highest concentration of TSS. With regard to nitrate (Fig. 2h), it is generally low, with, however, two areas of high concentration in the center-north part of the city. Another quality parameter is ammonium (Fig. 2i), which shows its presence beyond the authorized limit. The area of high ammonium concentration is observed upstream of the municipal lake in the north-central area of the city. Fecal coliforms (Fig. 2j) are generally present in all groundwater in the city, with a high concentration (> 40 CFU/100 ml) in the northern part of the city. The spatial distribution of FS (Fig. 2k) is similar to that of FC, with an average presence of FS in the southeastern part of the city. On the other hand, the spatial distribution of TC (Fig. 2l) shows the highest concentrations in the northern part and in the southern part. The central area in which the municipal lake is located has the lowest concentrations.
Figure 2. Spatial distribution of temperature, pH, redox potential, salinity, electrical conductivity, TDS, TSS, Nitrate, Ammonium, FC, FS and TC
3.3. Correlation matrix
Table 4 present the correlation matrix between water quality parameters.
Table 4
Pearson correlation matrix between contamination indicator parameters
| TSS | NO3− | NH4+ | FC | FS | TC |
TSS | 1 | | | | | |
NO3− | 0.537 | 1 | | | | |
NH4+ | -0.158 | 0.013 | 1 | | | |
FC | 0.346 | 0.087 | -0.184 | 1 | | |
FS | 0.645 | 0.370 | -0.215 | 0.875 | 1 | |
TC | 0.001 | 0.048 | -0.220 | 0.759 | 0.608 | 1 |
In bold, significant values (> 0.5) |
Table 4 reveals three outstanding patterns. Firstly, a positive and strong correlation exists between fecal coliforms, fecal streptococci (0.875), and total coliforms (0.759). Faecal coliforms are positively and strongly correlated with total coliforms (0.759), and fecal streptococci are positively and strongly correlated with total coliforms (0.608). This suggests that the bacteriological contamination of the groundwater is mainly fecal. Total coliforms may also indicate the presence of additional bacteria that can cause waterborne infections such as typhoid fever, hepatitis, gastroenteritis, and dysentery (Lawson, 2011). As a result, the presence of coliforms in contaminated water makes the water unsafe for human consumption unless chlorinated.
Secondly, the correlation matrix shows the existence of a positive, although less pronounced, correlation between TSS and NO3− and fecal streptococci. The correlation between TSS and NO3− is positive and strong (0.537). This suggests that the nitrates present in the water come from the decomposition of the organic matter contained in the TSS. NO3− is the result of denitrification of NH4+ present in the water, which is oxidized to nitrite by bacteria of the genus Nitrosomonas and then to nitrate by bacteria of the genus Nitrobacter. The correlation between TSS and fecal streptococci is positive and strong (0.645). The correlation between TSS and NO3− is positive and strong (0.537). This suggests that fertilizers are not only responsible for nitrate enrichment. Finally, a negative and weak correlation exists between bacteria content (faecal coliforms, fecal streptococci, and total coliforms) and nitrogen (respectively 0.087, 0.370, and 0.048 with NO3− and − 0.184, − 0.215, and − 0.220 with NH4+) concentrations, showing heterogeneity between bacterial and chemical contamination.
Table 5
Concentration of contamination indicators in water samples
Samples name | Sample source | T (°C) | pH | Eh (mV) | Sal (‰) | EC (µS/cm) | TDS (mg/l) | TSS (mg/l) | NO3− (mg/l) | NH4+ (mg/l) | FC (CFU/100ml) | FS (CFU/100ml) | TC (CFU/100ml) |
PA1 | Modern well | 26.3 | 4.8 | 115.6 | 0.2 | 380 | 182.9 | 0 | 0 | 0.3 | 3 | 1 | 2 |
PA2 | Modern well | 25.8 | 4.9 | 109.6 | 0.2 | 327 | 156.9 | 0 | 7 | 0.4 | 5 | 2 | 9 |
PA3 | Modern well | 25.7 | 5.0 | 103.9 | 0.2 | 352 | 169.2 | 0 | 6 | 0.5 | 7 | 9 | 14 |
PA4 | Modern well | 25.8 | 4.9 | 113.1 | 0.9 | 1,818 | 913 | 0 | 8.3 | 12.9 | 3 | 1 | 6 |
PA5 | Modern well | 25.7 | 5.4 | 81.7 | 0.3 | 523 | 253 | 24 | 19.8 | 0.4 | 29 | 25 | 28 |
PA6 | Modern well | 25.7 | 4.6 | 127.4 | 0.6 | 1,131 | 559 | 0 | 4.5 | 3.6 | 14 | 6 | 28 |
P1 | Traditional Well | 25.4 | 3.7 | 179.5 | 0.6 | 1,131 | 559 | 3 | 0 | 2.0 | 1 | 0 | 9 |
P2 | Traditional Well | 25.3 | 4.0 | 163.0 | 0.2 | 474 | 231 | 2 | 0 | 0.3 | 51 | 20 | 84 |
P3 | Traditional Well | 24.9 | 5.6 | 70.2 | 0.3 | 114 | 374 | 5 | 10.9 | 0.3 | 2 | 0 | 7 |
P4 | Traditional Well | 24.4 | 4.0 | 159.9 | 0.1 | 987 | 189.1 | 0 | 3.8 | 0.2 | 7 | 3 | 18 |
P5 | Traditional Well | 24.6 | 4.4 | 136.1 | 0.2 | 393 | 198.1 | 0 | 9.3 | 0.2 | 10 | 7 | 75 |
S1 | Spring | 24.4 | 4.7 | 123.8 | 0.3 | 531 | 257 | 3 | 0 | 0.4 | 5 | 2 | 9 |
S2 | Spring | 24.1 | 4.2 | 147.3 | 0.2 | 355 | 170.6 | 0 | 11.7 | 0.8 | 17 | 9 | 33 |
S3 | Spring | 24.0 | 4.3 | 142.9 | 0.4 | 717 | 351 | 1 | 16.5 | 0.5 | 9 | 4 | 24 |
F | Borehole | 29.9 | 4.0 | 160.8 | 0.1 | 114.3 | 54 | 4 | 5.2 | 0.2 | 7 | 3 | 16 |
Mean | | 25.5 | 4.6 | 129.0 | 0.3 | 629.2 | 307.9 | 2.8 | 6.9 | 1.5 | 11.3 | 6.1 | 24.1 |
Median | | 25.4 | 4.6 | 127.4 | 0.2 | 474.0 | 231.0 | 0.0 | 6.0 | 0.4 | 7.0 | 3.0 | 16.0 |
SD | | 1.4 | 0.5 | 30.9 | 0.2 | 462.0 | 219.9 | 6.1 | 6.1 | 3.3 | 13.1 | 7.3 | 24.3 |
Min | | 24.0 | 3.7 | 70.2 | 0.1 | 114.0 | 54.0 | 0.0 | 0.0 | 0.2 | 1.0 | 0.0 | 2.0 |
Max | | 29.9 | 5.6 | 179.5 | 0.9 | 1,818.0 | 913.0 | 24.0 | 19.8 | 12.9 | 51.0 | 25.0 | 84.0 |
WHO standards | | 25 | 6.5-9 | - | - | 100–1000 | ‹ 1500 | 0 | 50 | 0.5 | 0 | 0 | 0 |
CFU Colony forming units, WHO World Health Organization, SD standard deviation, Min minimum, Max maximum
3.4. Water quality index
Table 6 presents the summary of the results on the WQI of the city of Ebolowa, while Fig. 3 presents the spatial distribution of the WQI. Overall, Table 6 shows that ten samples out of fifteen have water unfit for consumption, three samples are very poor, and one sample is poor. Only a yardstick of good quality.
Table 6
Water quality index and its quality class
Sample name | Sample source | WQI Value | Class |
PA1 | Modern well | 89.0 | Good |
PA2 | Modern well | 211.8 | Very poor |
PA3 | Modern well | 311.4 | Unsuitable for drinking |
PA4 | Modern well | 498.3 | Unsuitable for drinking |
PA5 | Modern well | 805.0 | Unsuitable for drinking |
PA6 | Modern well | 678.7 | Unsuitable for drinking |
P1 | Traditional Well | 209.9 | Very poor |
P2 | Traditional Well | 1 812.8 | Unsuitable for drinking |
P3 | Traditional Well | 145.0 | Poor |
P4 | Traditional Well | 357.1 | Unsuitable for drinking |
P5 | Traditional Well | 1 145.0 | Unsuitable for drinking |
S1 | Spring | 213.4 | Very poor |
S2 | Spring | 701.2 | Unsuitable for drinking |
S3 | Spring | 473.7 | Unsuitable for drinking |
F | Borehole | 322.0 | Unsuitable for drinking |
Figure 3 shows that there is a gradual variation in groundwater quality from good to poor quality from the central part of the city (around the municipal lake) towards the south of the city and towards the north of the city.