Distribution of physico-chemical parameters
Values of physico-chemical parameters (pH, Conductivity, total dissolved salts/TDS and turbidity of water from each well).
The values of pH, conductivity, TDS and turbidity of the waters of each well are recorded in Table 2.
Table 2
pH, conductivity, TDS and turbidity of well water.
Samples | pH | Conductivity (µS/cm) | TDS | Turbidity (NTU) |
p1 | 6.88 | 581.00 | 1 199.26 | 1.70 |
P2 | 7,04 | 1 064.00 | 807.09 | 0.70 |
P3 | 6.92 | 1 139.00 | 863.98 | 1.60 |
P4 | 6.56 | 1 634.00 | 1 239.46 | 0.40 |
P5 | 6.82 | 1 321.00 | 1 002.04 | 3.40 |
P6 | 6.91 | 1 133.00 | 859.43 | 1.10 |
P7 | 6.98 | 1 291.00 | 735.79 | 0.90 |
P8 | 6.95 | 1 273.00 | 979.28 | 0.20 |
P9 | 6.98 | 761.00 | 544.82 | 0.40 |
P10 | 6.76 | 970.00 | 735.79 | 0.50 |
P11 | 6.84 | 6.66 | 9.09 | 4.70 |
P12 | 6.94 | 676.00 | 483.96 | 0.40 |
P13 | 6.77 | 840.00 | 637.18 | 0.6 |
P14 | 7.00 | 1 389.00 | 1 053.62 | 1.50 |
P15 | 6.71 | 1 213.00 | 920.11 | 0.50 |
P16 | 6.64 | 108.00 | 822.26 | 0.60 |
P17 | 6.76 | 1 036.00 | 785.85 | 0.40 |
P18 | 6.89 | 1 367.00 | 1 036.93 | 0.30 |
P19 | 7.03 | 684.00 | 489.69 | 0.50 |
P20 | 6.53 | 859.00 | 651.59 | 0.90 |
P21 | 6.53 | 1 423.00 | 1 079.41 | 0.20 |
P22 | 6.67 | 987.00 | 748.68 | 0.70 |
P23 | 6.43 | 1 438.00 | 1 090.79 | 0.40 |
P24 | 6.57 | 742 | 531.21 | 21.0 |
Averages | 6.80 | 1 037.98 | 804.475 | 1.82 |
Standards | 6.5 < ph < 9.5 | 250 | 189.64 | 1 |
Pollution factors | - | 4.15 | 4.24 | 1.82 |
Averages | 6.80 | 1 037.98 | 804.475 | 1.82 |
µS/cm: micro siemens per centimeter; NTU: nephelometric turbidity unit; pH: Hydrogen potential; EU: European Union |
The results in Table 2 show that the pH values vary from 6.49 to 7.04. Values all comply with the standard set by the WHO for drinking water (6.5_9.5).
The conductivity ranges from 6.66 to 1634; and all these samples have a conductivity value above the WHO standard for drinking water (250) except the water from well P11.
Total Dissolved Solids (TDS) values fluctuate between 1239.46 and 9.09.
Turbidity values range from 0.2 to 21.0; seven samples have turbidity values higher than the standard set by the EU for drinking water (1 NFU): P1; P3; P5; P6; P11; P14; P24. The turbidity values of the other samples are within the EU standard for drinking water.
The pH values vary from 6.49 to 7.04 are all comply with the standard set by the WHO for drinking water (6.5_9.5). The average value of all the pH values of the waters of all the wells is 6.80. This shows a slightly acid pH. This acidic pH can be explained by acidifying organic matter as demonstrated by the blackish nature of the soil (Fig. 2) and the city's domestic wastewater.
All samples have a conductivity value above the WHO standard for drinking water (250) except the water from well P11. Seven on 24 samples have turbidity values higher than the standard set by the European Union (EU) for drinking water (1 NFU). The conductivity and TDS averages are 1 037.98 and 804.475 respectively, which are high compared to their standards set by the WHO for drinking water (250 and 189.64). Each of the two parameters have a pollution factor 4 times their standard. This shows a strong mineralization of this aquifer. This strong mineralization is due to saline intrusion of seawater, the lagoon system water and soil leaching. Studies of marine waters have shown strong mineralization; Solitoke et al in 2018 recorded a conductivity of 62 700 µS/cm from TDS of 35 900 mg/l (Solitoke et al. 2018). This leaching naturally leads to the dissolution of a certain number of mineral salts. It can also be caused by human activity caused by domestic effluents (wastewater). The turbidity of well water is higher than the standard set by the WHO with the degree of groundwater pollution in wells 1.8 times the WHO standard. The turbidity of these waters comes from the sandy character of the soil and the very superficial level of the water table. Indeed, the soils of this part of the coast are sandy and therefore permeable to suspended matter (clay, silt, colloidal organic particles), which explains the high turbidity of these waters. The presence of these suspended solids could also occur via the wells, since these large-diameter wells are unprotected.
With regard to total dissolved salts (TDS) this turbidity in these waters would come from the presence of suspended matter (silts, colloidal organic particles) because these wells are not protected.
Distribution of ETMs in water
The values of the metallic trace elements of the waters of each well are recorded in Table 3.
Table 3
Lead (Pb), cadmium (Cd), iron (Fe), chromium (Cr) and arsenic (As) contents of wells.
Samples | Concentration in mg/l |
Cd | Pb | Fe | Cr | As |
P1 | 0.04 | 0.25 | - | 0.08 | 3.43 |
P2 | 0.06 | 0.37 | - | - | 0.22 |
P3 | 0.04 | 0.19 | - | 0.08 | 1.62 |
P4 | 0.06 | 0.26 | - | 0.03 | 2.42 |
P5 | 0.05 | 0.33 | - | - | 4.48 |
P6 | 0.05 | 0.36 | - | 0.07 | 2.58 |
P7 | 0.04 | 0.29 | - | - | 0.54 |
P8 | 0.05 | 0.42 | 0.04 | 0.05 | 1.43 |
P9 | 0.06 | 0.33 | - | 0.07 | 2.84 |
P10 | 0.06 | 0.34 | - | - | 1.72 |
P11 | 0.06 | 0.32 | 0.04 | 0.03 | 0.10 |
P12 | 0.06 | 0.32 | - | 0.05 | 4.06 |
P13 | 0.06 | 0.29 | - | 0.03 | 2.45 |
P14 | 0.06 | 0.24 | - | - | 4.17 |
P15 | 0.05 | 0.49 | - | 0.05 | 1.51 |
P16 | 0.04 | 0.32 | - | 0.04 | 2.50 |
P17 | 0.04 | 0.34 | - | 0.04 | 0.58 |
P18 | 0.06 | 0.44 | - | 0.02 | 0.52 |
P19 | 0.06 | 0.39 | - | 0.06 | 4.40 |
P20 | 0.05 | 0.35 | - | 0.03 | 0.29 |
P21 | 0.04 | 0.44 | - | 0.01 | 0.04 |
P22 | 0.07 | 0.32 | - | - | 4.44 |
P23 | 0.06 | 0.32 | - | 0.05 | 3.87 |
P24 | 0.05 | 0.42 | 0.02 | - | 2.60 |
Minimum value | -0.04 | 0.19 | 0.02 | 0.01 | 0.04 |
Maximum value | 0.07 | 0.49 | 0.04 | 0.08 | 4.48 |
Average Contents | 0.05 | 0.34 | 0.00 | 0.03 | 2.20 |
Standard (mg/l) | 0.03 | 0.01 | 0.20 | 0.05 | 0.01 |
Pollution factors | 1.76 | 33.92 | 0.02 | 0.65 | 220.04 |
-value below detection limit |
The results show a variation in cadmium levels ranging from 0.4 to 0.7 mg/l, all these values are higher than the value of the standard (0.03 mg/l) recommended by the WHO for cadmium in drinking water.
The range of lead levels is 0.19 to 0.49 mg/l, which is higher than the value recommended by the WHO (0.01 mg/l) for lead in drinking water.
All water samples have iron concentrations below the detection limit of the instrument; except P8 samples; P11; P24, which have respective concentrations of 0.04 mg/l; 0.04mg/l; 0.02 mg/l; all less than 0.2 mg/l set by the French standard for iron in drinking water.
The chromium contents range from 0.01 to 0.08 mg/l; only five (5) wells: P1, P3, P6, P9 and P19 have a chromium concentration above the standard (0.05 mg/l) set by the WHO; i.e. 14% of the wells. Three wells have chromium concentrations equal to the standard; the rest of the wells have chromium concentrations below the standard or below the detection limit of the instrument.
The arsenic levels vary from 0.04 to 4.48 mg/l, and all these samples have an arsenic concentration above the standard (0.05 mg/l) set by the WHO for drinking water.
The results of the MTE analyzes of Cd, Pb, Fe, Cr and As show variability in content between these different elements. On the one hand, some elements have their levels higher than the WHO standards set for water potability, and, on the other hand, other elements, have acceptable levels (WHO 2011). The cadmium concentration is higher than the concentration set by the WHO for drinking water with a pollution factor of 1.7 times. The waters of the Kodjoviakopé wells are therefore polluted with cadmium and therefore harmful to health. This pollution would be due to the washings of the electrolytic coatings, to the accumulators of the paintings of the anthropic activities of the inhabitants of the locality and its surroundings because it is located at a low altitude compared to the other districts. This cadmium pollution can also come from rain, car fumes because the district is located near national road number 2 (№ 2). The high concentration of cadmium in the lagoon as shown by the work of BUAKA (2009) shows that it also contributes to the pollution of this element (Buaka 2009). Similarly, the lead concentration in the waters of the Kodjoviakopé wells is much higher than the concentration set by the WHO for drinking water with a very high pollution factor (33.92). Lead therefore poses health risks to consumers. The infiltration of domestic effluents, water from household waste dumps, lead from the combustion of fossil fuels by automobile engines due to the location of the district near national road number 2 (№ 2), leaching, paints, batteries, household waste of all kinds due to the old age of the district and especially the lagoon of Nyékonakpoè located in the North contiguous to the district. Indeed, there is a slight increase in lead concentration values from the sea to the lagoon. These results are similar to those obtained by BUAKA (2009) a little further north and north-east of the sampling site of this study, particularly in the lagoon system of Bè and Nyékonakpoè (Buaka 2009) and those obtained by LABOE (2004) on the assessment of water pollution from boreholes and wells in the districts of Adidogomé and Kegué (LABOE 2OO4). The iron concentration in the waters of the Kodjoviakopé wells is lower than the concentration set by the EU for drinking water with a low pollution factor (0.02) therefore not polluted in this element because of the high value of the standard fixed by the EU for this element which is a trace element therefore useful for human health.
The chromium concentration is generally lower than the concentration set by the WHO for drinking water with a low degree of pollution (0.65). For this element, the waters of certain wells are an exception, in particular wells P1, P3, P6, P9, P19 with degrees of pollution 1.6 times the WHO standard for wells P1, P3; 1.4 times for wells P6, P9; 1.2 times for P19 wells. Consequently, this parameter constitutes a risk of pollution in the waters of the aquifer studied. The presence of chromium in these waters would be due to discharges of domestic effluents and infiltration from the dye leaching.
The waters have arsenic levels well above the concentration set by the WHO for drinking water; with a very high pollution factor (220). Well water is therefore polluted with arsenic and is therefore dangerous. Its presence in these waters would be due to the combustion of coal or waste and detergents.
The average level of trace elements Cd, Pb, Fe and As respectively 0.05; 0.34; 0.00 and 2.20 mg/l in the present study are all higher than the average level of the same elements found by Djade et al (2020) in groundwater in the Department of Zouan-Hounien (Western Côte d'Ivoire). Ivory) which are respectively 0.8.10-2; 3.10. 10 − 3; 2.23 and 1.1810-3 mg/l except Fe (Djade et al. 2020). The high levels found in this study are due to the influence of marine pollution and the lagoon system.
Hazard Quotients (HQ) for Threshold Effects for Adults and Children (IR)
The results for the risks concerning the threshold effects are presented in the following table:
Table 6
Health Risk Indices for Threshold Effects
ETM | As | Cr | Pb |
Adultes | 4.10− 2 | 1.10− 4 | 3.10− 05 |
Enfants | 7.10− 2 | 2.10− 4 | 6.10− 05 |
On the one hand, the results of the calculation of the excess individual risk show that the ERI is higher than 10 − 4 for As in adults than in children, very slightly higher than 10 − 4 for Cr in children; on the other hand, Pb gave ERIs less than 10− 4.
We can deduce that the ingestion of groundwater from wells in the Kodjoviakopé district represents a danger to the health of the inhabitants in terms of these trace elements (As, Cd and Pb), in particular for children because of the hazard quotient which is greater than 1.
The assessment of the excess individual risk (ERI) for the carcinogenic effects concerning the trace elements Pb, Cr and As shows that exposure to Pb is unlikely (IRI ˂ 10− 4). However, exposure to As and Cr presents the risk of the appearance of carcinogenic effects (ERI > 10− 4) in children as well as adults.