Aniekan Edet
3. Results and Analysis
A statistical summary of the data for the three countries are presented in Table 2. The Table also includes the WHO (1993) limits for drink-ing and domestic purposes and the percent of each parameter higher than the WHO limits.
Table 1. Characteristics of the study area Country
Temp
°C
Precipitation (mm)
Relative humidity
(%) Geology
Main lithology
SWL (m) Benin 34–43 2,000–4,000 60–96 Coastal
Sedimentary Basin
Fluviatile and Alluvial Sands,
Clays Nigeria 34–43 2,000–4,000 60–96 Coastal
Plain Sands
Continental Sands, Silts,
Clays
10.00–40.00
Senegal 28–29 600–00 Continental
Terminal
Sandstones, Sands, Sandy
Clay, Silt and Clay
2.87–38.25
Notes
SWL = Static water level.
Benin data: after Boukari et al., 1996; Senegal data: after Faye et al., 2003.
SENEGAL
BENIN 0°00
8°00 NIGERIA
Figure 1. Map of West Coast of Africa showing Benin, Nigeria and Senegal
What do we know about transboundary aquifers in Africa?109
Session 1
Table 2. Descriptive statistics of physicochemical parameters of groundwater in the study area (Units in mg/l except Temp [oC] and SEC [μS/cm], pH [no units])
Benin (n = 212)
Max 1,174.00 898.00 8.30 101.00 38.70 162.00 136.00 0.28 89.40 387.00 3.47 144.00 132.00 33.40
SD 293.50 193.28 0.62 21.04 7.54 31.55 29.93 0.05 25.00 86.85 0.67 38.63 45.27 7.78
WHO 1,400 1,000 6.5-8.5 75 100 200 12 0.1 400 0.3 250 10
No. > WHO 1 15 24
% > WHO 3.3 50.0 80.0
Nigeria (n = 48)
Mean 28.08 1,223.59 575.20 6.16 40.71 28.72 112.84 16.78 0.24 35.25 66.69 273.65 0.57 1.90 52.63
Med 28.17 120.50 174.49 6.17 14.15 7.55 22.50 2.49 0.06 8.35 18.70 71.50 0.46 0.25 1.92
Min 24.40 16.28 17.64 4.08 0.50 0.09 0.06 0.01 0.00 0.04 0.20 9.92 0.08 0.00 0.00
Max 32.85 21,538.00 5,017.04 8.32 520.30 230.70 159.60 1.72 968.90 787.20 2,670.00 3.50 14.75 575.00
SD 1.82 4,133.22 1,136.98 0.81 98.57 50.40 262.28 35.22 0.42 139.36 149.91 552.87 0.63 3.17 133.26
WHO 1,400 1,000 6.5-8.5 75 100 200 12 0.1 400 250 10 0.3 0.1
No. > WHO 6 5 32 3 4 6 11 19 1 10 23 30
% > WHO 12.5 10.4 66.7 6.3 8.3 12.5 22.9 39.6 2.1 20.8 47.9 62.5
Senegal (n = 108)
Mean 33.37 886.55 449.85 6.10 56.89 11.81 119.38 15.69 26.29 49.12 257.25 67.07
Med 33.60 213.00 99.00 6.10 26.96 1.82 18.00 2.00 2.45 23.00 30.78 20.87
Min 24.10 37.80 17.00 4.30 1.84 0.19 4.00 0.40 0.00 0.00 3.88 0.00
Max 43.90 11,180.00 6,190.00 7.50 420.00 88.04 1,980.00 160.00 459.03 309.00 3,666.47 834.70
SD 4.24 1,845.32 997.27 0.64 88.97 21.81 314.01 30.45 77.45 65.86 687.74 137.38
WHO 1,400 1,000 6.5-8.5 75 100 200 12 400 250 10
No. > WHO 9 8 4 8 11 1 7 40
% > WHO 14.8 13.1 6.6 13.1 18.0 1.6 11.5 65.6
Benin data: after Boukari et al., 1996; Senegal data: after Faye et al., 2003.
4. Discussion
Data for Benin showed that 50% and 80% for K and NO3 were higher than the WHO (1993) maximum value of 12 and 10 mg/l respectively for drinking water purposes. The data from Nigeria showed that most of the parameters considered except for NO3were higher than the WHO (1993) maximum admissible values (Table 1). For Senegal, the EC, TDS, Ca, Na, K, SO4, Cl and NO3 were higher than the WHO values by certain level of percentages. The highest of 65.6% was obtained for NO3.
4.1 Hydrogeochemical facies and processes
Six types of hydrogeochemical facies identified in the study area and are presented in Table 3.
The table also includes the different processes
controlling the groundwater chemistry in the different countries.
Besides the above processes, groundwaters affected by wastewater and sewages from human activities including pit latrines and sep-tic tanks are generally high in nitrate as shown in the present data (Table 1).
4.2 Classification
The water samples were classified into four groups based on the concentrations of Cl-and NO3-, which represents the influences of sea-water and human activities (Kim et al., 2004). In this classification, the regional threshold values were obtained from the inflection point of the S-shaped cumulative frequency distribution plots based on the method of Sinclair (1976).
For this work, the threshold values obtained are 88 mg/l for chloride and 70 mg/l for nitrate. Four water types have been identified on the basis of this classification (Table 4).
Facies Countries Process
Na–Cl Benin, Nigeria, Senegal Seawater Intrusion
Na–HCO3 Benin, Nigeria, Senegal Ion exchange
Ca–HCO3 Benin, Senegal Carbonate dissolution
Ca–Cl Benin, Nigeria, Senegal Ion exchange
Ca–SO4 Senegal Gypsum dissolution
Mg–Cl Nigeria Ion exchange/Seawater Intrusion
Table 3. Hydrochemical facies and processes controlling water chemistry in Benin, Nigeria and Senegal
Table 4. Classification of groundwater in the study area based on a threshold value of 88 mg/l (Cl) and 70 mg/l (NO3) in Benin, Nigeria and Senegal
Groups
Class
Benin Nigeria Senegal Total % Total Remarks Cl NO3
1 < 88 < 70 18 26 43 87 62,6 Freshwater
2 < 88 > 70 5 3 8 5,8 Anthropogenic influence
3 > 88 < 70 1 22 9 32 23 Seawater intrusion
4 > 88 > 70 6 6 12 8,6 Seawater intrusion
and anthropogenic influence
4.3 The Nigerian situation
The development of hydrocarbon reserves over the years has contributed significantly in the crease in the activities oil and gas related in-dustries in the Niger Delta Region of southern Nigeria. These activities have lead to an in-crease in the demand of potable water whose main source in the area is groundwater. The groundwater resources provide more than 85% of the water used in the area and its con-tamination through oil spillage, leakage from pipelines and storage facilities, natural and anthropogenic sea water contamination is one of the major problems facing the region.
The contaminated groundwater may not only reduce the available water for use but also pose a threat to the human health.
Figure 2 shows a plot of mean concentrations of chloride and total hydrocarbon content in groundwater. Group A: Groundwater samples were collected near petroleum processing facilities located inland thus the groundwater has no problem with seawater but affected by hydrocarbon; Group B: The groundwater samples were collected near petroleum pro-cessing facility located along the coast. The water were affected by seawater and hydro-carbon; Group C: The groundwater samples were not collected near petroleum processing facility and far away from the coast. The water has no problem with seawater and hydro-carbon; Group D: The groundwater samples were not collected near petroleum processing facility but near the coastline. The water has no problem with hydrocarbon but affected by sea-water intrusion.
5. Conclusions
1. The quality of groundwaters in parts west coast of Africa have been evaluated using the WHO limits for drinking and domestic purposes.
2. The groundwaters in the studied areas are predominantly as follows:
3. The main geochemical processes which contribute to groundwater chemistry:
• rainwater originated freshwater (Benin, Nigeria, Senegal) very dilute Na+-HCO3 -dominated),
• seawater (tidal flushing) related water (Nigeria, Senegal) (Na+-Cl-dominated),
• human waste-related contaminated water (Benin, Senegal) (NO3-dominated),
• cation exchange in Benin, Nigeria and Senegal (resulting in relative loss of Ca2+),
• calcite dissolution (Senegal) (Ca2+-HCO3 -dominated),
• gypsum dissolution (Senegal) (Ca2+ -SO42- dominated),
• hydrocarbon contamination in Nigeria (High THC).
4. For management of transboundary aquifers, the source of pollution should be identified.
References
Boukari M., Gaye, C.B., Faye, A. and Faye S., 1996. The impact of urban development on coastal aquifers near Cotonou. Benin, Jour African Earth Sciences22 (4): 403-408.
Faye S., Faye S.C., Ndoye S. and Faye A, 2003.
Figure 2. Plot showing mean concentrations of chloride (Cl–) and total hydrocarbon content (THC) in groundwater for different
scenarios in the coastal parts of Nigeria
Hydrochemistry of the Saloum (Senegal) superficial coastal aquifer. Environmental Geology,44:127-136.
Kim, K., Rajmohan, N., Kim, H.J., Hwang, G.S., Cho and M.J., 2004. Assessment of ground-water chemistry in a coastal region (Kunsan, Korea) having a complex contaminant sources: a stoicometric approach. Environ-mental Geology,46, 763-774.
Sinclair, A. J., 1976. Application of Probability Graphs in Mineral Exploration. Association of Exploration Geochemists, Rexdale, Ont.
p 95.
World Health Organisation (WHO), 1993. Guide-lines for drinking water quality, Vol. 1, Rec-ommendations.Geneva, Switzerland.
1. Introduction
Tunisia and Libya are considered among countries which suffer from limited surface water resources because most parts of these countries are either semi-arid or arid. Con-sequently, groundwater constitutes the main water resource in southern Tunisia and Libya.
The North West aquifer system (NWSAS) is one the most important aquifers in the world. It extends over much of Algeria (700,000 km2), Libya (250,000 km2) and Tunisia (80,000 km2).
In 2000, the NWSAS supplied an estimated
volume of 2.2 billion m3fresh water for domes-tic water supply, agriculture and other indus-trial purposes. Groundwater withdrawal from the NWSAS increased from about 14 m3/s in 1950 to 82 m3/s in 2000, resulting in decrease in the natural water flows (OSS, 2003). The NWSAS is constituted of three important aquifers: The continental Intercalaire overlain by the Complexe Terminal and the Djeffara aquifer which covers the coastal lowlands straddling Tunisia and Libya over an area of 34,000 km2. The presented study is focused on Djeffara aquifer system which suffers from