Four water distribution centers and their catchment areas were included in our study as shown in Table 1. The distribution and their catchment areas are the main sources of drinking water to Addis Ababa city and they also represent river dam and ground water sources of drinking water that supply the city.
Table 1. Distribution of water samples based on catchment of actual main treatment plant.
|
Main treatment plant
|
Number of samples
|
%
|
|
Akaki deep-well system
|
4
|
33.33
|
|
Legedadi SWTP
|
4
|
33.33
|
|
Gefersa SWTP
|
4
|
33.33
|
|
Total
|
12
|
100
|
*SWTP= surface water treatment plant
As shown in the table, equal numbers of samples were taken from the distribution centers that supply water to the city. The Gefersa and Legedadi river dams are sources of surface water but the Akaki deep-wells are alternate ground water sources that were dug to overcome water shortage of the city.
The mean lead concentration of the total water samples was 0.028 mg/l, which was higher than the maximum admissible limit (MAL) of lead by the WHO at 0.01mg/l [11], but lower than the USEPA MCL of 0.1mg/l [10]. Even though it is not statistically significant (p>0.39), the mean level of lead in surface water was higher than that of ground water. There was a statistically significant difference (p<0.01) in the mean level of lead among water samples taken from different treatment plants and their respective catchment areas (Table 2).
Though not statistically significant (p>0.5), the mean lead level of tap water samples (0.03 mg/l) is higher than that of water samples of major treatment plants (0.02 mg/l).
Table 2. Comparison of heavy metal levels in ground and surface water samples.
|
Parameter
|
Ground water samples
|
Surface water samples
|
p-value
|
|
Pb (mg/l)
|
0.02±0.01
|
0.03±0.02
|
0.39
|
|
Cd (mg/l)
|
BDL*
|
0.002±0.004
|
0.31
|
|
Cr (mg/l)
|
0.005 ±0.006
|
0.002±0.005
|
0.43
|
|
Mn (mg/l)
|
0.005 ±0.01
|
0.007 ±0.008
|
0.67
|
All values are expressed as mean ± Standard deviation. BDL*(Below detectable limit).
The mean level of cadmium in surface water samples of Addis Ababa town was 0.002 mg/l which is lower than the maximum admissible limit of Cadmium set by WHO at 0.003 mg/l [11] and the USEPA maximum contaminant level of Cadmium at 0.005 mg/l [10]. The mean level of Cadmium in water samples of the Gefersa surface water treatment plant and its catchment was higher than the maximum admissible limit of Cadmium by WHO at 0.003 mg/l [11] but aligns with the USEPA maximum contaminant level of cadmium in drinking water at 0.005 mg/l [10].The possible justification for high levels of cadmium may be due to the disposal of waste and industrial effluents containing Cadmium and Cadmium related compounds [12] and due to industrial activities such as electroplating, dye production, as well as plastic, stabilizer and battery industries [13].
Long-term exposure to cadmium is associated with renal dysfunction, obstructive lung disease and lung cancer. Cadmium may also be implicated in bone abnormalities such as osteomalacia and osteoporosis. Soon after ingestion Cadmium is first transported to the liver where it binds with many proteins to form complexes. These complexes then are transported to the kidneys, ultimately affecting the filtering mechanism ( ATSDR,1999).
Table 3. Comparison of concentration of heavy metals in water samples taken from different treatment plants.
|
Parameter
|
Akaki deep-well& catchment area
|
Legedadi SWTP& catchment area
|
Gefersa SWTP& catchment area
|
ANOVA
P- value
|
|
Pb (in mg/l)
|
0.02±0.02
|
0.01±0.01
|
0.05±0.01
|
0.01
|
|
Cd (in mg/l)
|
BDL
|
BDL
|
0.005±0.005
|
0.1
|
|
Cr (in mg/l)
|
0.005±0.005
|
BDL
|
0.005±0.003
|
0.27
|
|
Mn (in mg/l)
|
0.005±0.01
|
0.002±0.005
|
0.01±0.01
|
0.27
|
Values are expressed as mean ±Standard deviation. SWTP= Surface water treatment plant. BDL (Below detectable limit)
The mean level of Chromium in water samples of Addis Ababa town was 0.003 mg/l, which is below the maximum admissible limit of Chromium by WHO at 0.05mg/l [11] and the USEPA maximum contaminant level of Chromium at 0.1mg/l [10].The possible explanation for lower levels of Chromium might be due to limited availability of ophiolite rocks capable of generating Chromium and derivative compounds [14]. Low-level exposure to Cr can irritate the skin and cause ulceration. Long-term exposure can cause kidney and liver damage, and also circulatory and nerve defects (ATSDR, 1999).
The mean level of Manganese in water samples of Addis Ababa town was 0.007mg/l, which is below the maximum admissible limit of Manganese by WHO at 0.4 mg/l [11] and the USEPA maximum contaminant level of Manganese at 0.05 mg/l [10].The possible justification for lower levels of manganese may be due to dilution of dissolved manganese in surface and ground water sources [15], Oxidation of Manganese by Chlorine during the process of water treatment in treatment plants and other distribution systems [16] and the influence of water waves in surface waters and rainfall resulting in extensive dissolution of manganese ions [15]. Prolonged exposure of high levels of manganese negatively affects the central nervous system, visual reaction time, hand steadiness and eye-hand coordination (ATSDR, 1999)
Table 4. Comparison of concentration of lead between water samples of major treatment plant and tap water samples.
|
Parameter
|
Samples of treatment plants
|
Tap water samples
|
p- value
|
|
Pb (mg/l)
|
0.02±0.03
|
0.03±0.02
|
0.4
|
|
Cd (mg/l)
|
BDL
|
0.002±0.004
|
0.4
|
|
Cr (mg/l)
|
0.003±0.006
|
0.003±0.005
|
0.9
|
|
Mn (mg/l)
|
0.006±0.01
|
0.007±0.01
|
0.9
|
Values are expressed as mean ±Standard deviation.
Water is one of the natural resources that support the existence of human beings and other living organisms on earth(Musa et al, 2013).Water pollution is the leading worldwide cause of deaths and disease and it accounts for the deaths of more than 14,000 people daily (Larry,2006). Accumulation of heavy metals in living organism can be toxic and carcinogenic due to their non-biodegradable nature. So, water quality management and assessment with regard to levels of heavy metal is of prime importance. The overall water quality status and identification of source of origin of heavy metals are required for water quality management (Bodaghpour etal., 2012). Heavy metals exhibit toxic effect by entering into food chains and the ecosystem where they pose adverse impact on both the living and nonliving components of the ecosystem.
The possible justification for high levels of lead may be due to the practice of mining and other anthropogenic activities in the area [8], composed manure deposited on the farms around the study area [2], junk auto spare parts which may contain lead residues [7] and mobilization of lead from sediments and leaching of lead from waste rocks and dumps in the area [9]. Lead is the most significant of all the heavy metals because it is toxic and very common and harmful even in small amounts. Lead is a potent toxic metal that is dangerous to the environment and can be a serious threat to human health. Acute lead poisoning is associated with seizure, coma and death, while long-term and low-level exposure leads to chronic poisoning which is commonly associated with diseases such as anemia, neurotoxicity, hemo-toxicity, nephrotoxicity and toxic metabolic encephalopathy. Furthermore, lead toxicity causes brain damage and mental retardation especially in children (DEAT & DME 2003).
