Concentration Levels of Heavy Metals and Selected Ions in the Irrigation Water: The Case of Little Akaki River, Addis Ababa, Ethiopia

Abstract

Irrigable water resources have been challenged by the contamination of heavy metals and unwanted ions that impair plant growth and human health. It impaired the quality of edible fruit & vegetables. The objective of this study was to determine the concentration of heavy metals (Pb, Cd, Cr, and Cu) and selected ions (chloride, Na, Mg, Ca), and to evaluate its suitability for irrigation use. Water samples were collected from three various locations (the upstream, middle stream, and downstream) of the river. Temperature (T), EC, pH, and total dissolved solids (TDS) were tested onsite using handheld multiparameter testing equipment, while the heavy metals (Pb, Cd, Cr, and Cu) and selected ions (Chloride, Na, Mg, and Ca) of the water sample were analyzed using (ICP-OES). ANOVA test was conducted to examine the concentration variations of heavy metals and selected ions between the sampling locations along the river. The concentrations of (Pb and Cd) were found (2.5–3.9), (0.03–0.4) mg/L respectively, and (Cr and Cu) were below the detectible limit of the (ICP-OES) equipment. Cadmium (Cd) was found to be higher than the permissible limit of FAO (0.01mg/L) for irrigation water. There was no significant variation of heavy metals and selected ions between the sampling locations.

1. Introduction

Inappropriately managed urbanization and expansion of industrialization are the major causes of river water pollution in urban and pre-urban areas due to the introduction of undesirable materials into soils and irrigation water sources [1]. Contamination of heavy metals and other toxic ions in irrigation water sources is a worldwide problem and harmful for human health and the ecosystems. The excessive accumulation of heavy metals in irrigation water and soils resulted in contamination of human diets [2]. Heavy metals are entering into the river water and the environment primarily through anthropogenic activities. The main sources of heavy metals and other pollutants entering into the Little Akaki River basin can be industrial effluent, municipal solid waste, oily wastes from garages, and fuel stations. Industries like textile, dyeing, garment, pharmaceutical, ceramic, paint, packaging, etc. discharge their effluents into the rivers could be the causes of heavy metal contamination in the irrigation water sources [3]. Industry expansion has brought severe water pollution in Little Akaki catchment from domestic, commercial, and industrial effluents [4] and the waste management systems of industries and other commercial centers are very poor. According to Addis Ababa environmental pollution Authority 2007 report, 90% of all industries lack waste treatment facilities and subsequently dispose of their effluents into river streams. Lack of proper waste management system in the catchment areas, the irrigation water source in Little Akaki River is highly polluted with point and non-point waste sources.

The rapid urbanization and expansion of industries together with poor effluent management systems have a significant effect on the quality of irrigation water sources in the catchment areas. In the recent few decades, the social and economic structure of Addis Ababa city has changed radically. Rapid urbanization and industries expansion are observed and all other economic activities are also highly concentrated in Addis Ababa city, particularly in the Little Akaki River catchment. Besides the urbanization and industries expansion, the intensification of pre-urban and urban farming activities is also becoming one of the other social and economic features of the city. However, most of these rapid changes are brought without considering the negative environmental consequences. As a result, irrigation water pollution with heavy metals and other undesirable pollutants become an increasingly emphasized problem. Wastes generated from households, industries, fuel stations, hospitals, different business centers, and sewerages are getting into the river streams. Contamination of water bodies from various sources denies current and future generations of a birthright and puts at risk ecological integrity [5].

Little Akaki River is the primary irrigation water source for pre-urban and urban agriculture in the catchment area. The demand for irrigation water is markedly increasing in the study area for the production of fruits and vegetables. Many households are involved in urban farming activities to sustain their life. The use of industrial and municipal wastewater in urban agriculture is a common practice in many parts of the world including Ethiopia [6]. The shortages of safe irrigation water sources in the study area forced the farmers to look for to use contaminated river water for irrigated agriculture and access to quality irrigation water sources is becoming a serious concern these days in the study catchment.

The heavy metals and other pollutant elements are entering the soil because using severely contaminated irrigation water source for prolonged periods in the catchment area affect the physical and chemical characteristics of the soil. The contaminated soil of irrigated farm sites in the catchment area harms vegetables and fruit production. The heavy metals become highly concentrated in the edible parts of fruits and vegetables which alter human health. Heavy metal accumulation in soils, and subsequently, in vegetation by long-term wastewater irrigation has a potentially detrimental effect on humans via their transfer along the food chain [7]. In the existing situation, vegetables and other edible crop products produced in contaminated soil are distributed in the local market of Addis Ababa city. Residents are consuming the infected vegetables subsequently by purchasing from the local market and farmers also use the contaminated vegetables for their home consumption before going to market.

Few studies have been conducted so far in Addis Ababa city particularly in Little Akaki River to investigate the contamination levels of Little Akaki River irrigation water with heavy metals [8, 9, 10, 11, 12] but it was still inadequately researched. Because the heavy metal contamination and irrigation water pollution is a very dynamic problem and become progressively increasing. The intensification of industrialization in the Little Akaki River catchment aggravated the progression of river water contamination with heavy metals and toxic pollutants over time. Thus, the dynamism of the problem and the need for updated information about heavy metal contamination are the triggering points for the initiative of this research work. Determination of the existing heavy metals and selected ions is a relatively newer issue or insight to provide possible suggestions. Therefore, this research has focused to investigate the concentration level of heavy metals (Pb, Cd, Cr, and Cu), selected ions (Chloride, Na, Ca, and Mg), and other physic-chemical parameters of the Little Akaki River and to evaluate the suitability of the river water for irrigation uses.

2. Materials and methods

The area description, materials and methods, water sample collection and laboratory testing procedures, and method of data analysis are discussed in detail in this chapter.

2.1 Description of the study area

This study was conducted in Addis Ababa city, particularly in the Little Akaki River basin. The Little Akaki River basin is located in the western part of Addis Ababa and the river flows starting from northwest upstream of the city about 40 km before it reaches the downstream of the Aba Samuel reservoir which is indicated in Figure 1.

Figure 1.

Water sampling location map in little Akaki River.

Little Akaki River is highly contaminated with industry effluents and other different anthropogenic activities. According to the Addis Ababa city administration industry development commission report, more than 667 different sized industries are found in Addis Ababa city and the distributions of manufacturing industries are relatively higher in the Little Akaki River catchment area than in other parts of the city. Most of the streams/tributaries flow from the Northwestern side of the catchment area meets Little Akaki River at Gullele area where different industries are found. Gullele Soap and Marble factories, Awash Winery, National Alcohol and liquor factory are found in Little Akaki River catchment around, Lideta and Mekanisa areas [8]. He also explained that tributaries that come from the Northwestern direction also receive wastes from abattoirs. These different industries release their effluents into the river stream thereby adversely influencing the irrigation water quality.

2.2 Water samples collection and sample preparation

Three water samples were collected from three purposively selected sampling locations from the upstream, middle, and downstream of Little Akaki River on March 2021. All three water samples are collected on a similar day of local time (at morning 4:00 am). The sampling points are selected by considering the different variation factors along the river stream and collecting representative water samples. The collected river water samples were tested for the analysis of major heavy metals (Pb, Cd, Cr, and Cu), selected ions (Chloride, Na, Ca, and Mg), and other physic-chemical quality parameters of the irrigation water. Samples were collected with 500 mL plastic bottles from the representative flowing river water of medium velocity or free from any turbulence by dipping the bottles in the deeper mid-stream of the river flow to fill it to overflowing. The current weather conditions during sample collection were cleared the sky and sunny condition and the air temperature was ranges between 20 and 28°C.

2.3 Water sample laboratory testing procedures

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was used to analyze the concentration level of heavy metals (Pb, Cd, Cr, Cu) and selected ions (chloride, Na, Ca, and Mg). Temperature (T), EC, pH, and TDS were tested onsite using handheld multiparameter testing equipment. After sample collection, the water samples were acidified with 10 ml of concentrated nitric acid and preserved in the refrigerator. The acidified water samples were digested to dissolve the organic matter and then the digested wastewater samples were analyzed for concentration levels of heavy metals (Pb, Cd, Cr, and Cu) and selected ions (chloride, Na, Ca, and Mg) by ICP-OES with ES ISO 11885:2007 testing standard procedures. In ICP-OES the samples are exposed to a high energy source such as inductively coupled plasma (ICP) at a temperature of 5000 to 10,000 K [13] and the elements emit light of a spectrum being characteristics of each element. The emitted light is collected by a photomultiplier and the intensity of emitted light is directly proportional to the concentration of elements within a sample.

Chemical oxygen demand (COD) was tested with American Public Health Association (APHA) 5220-B open reflux testing methods. Chemical oxygen demand (COD) is defined as the amount of a specified oxidant that reacts with the samples under controlled conditions [14]. Organic and inorganic components of samples were subject to oxidation [15]. The dichromate ion (Cr₂O₇⁻²) is the specified oxidant in 5220-B testing methods. Wastewater samples were refluxed in a strongly acid solution for a minimum of two hours with a known excess of potassium dichromate (K₂Cr₂O₇). After digestion, the remaining un-reduced K₂Cr₂O₇ is titrated with ferrous ammonium sulfate (FAS) to determine the amount of K₂Cr₂O₇ consumed and the oxidized matter is calculated in terms of oxygen equivalence. The very important apparatus that has been used in COD testing was 150 ml Erlenmeyer flasks with ground-glass 24/40 neck and jacket Liebig or equivalent condenser with 24/40 ground glass joint and a hot plate having sufficient power to produce at least 1.4 W/cm² of heating surface or equivalence.

COD was can be calculated by Eq. (1)

$$\mathrm{C}O\mathrm{D}\mathrm{as}\mathrm{mg}{\mathrm{O}}_2/\mathrm{L}=\frac{\left(\mathrm{A}-\mathrm{B}\right)\ast \mathrm{M}\ast 8000}{\mathrm{mL}\text{sample}}$$E1

Where:

A = mL FAS used for blank (volume of blank titrant).

B = mL FAS used for sample (volume of sample titrant).

M = molarity of FAS (Molarity of titrant).

8000 = mill equivalent weight of oxygen * 1000 mL/L.

The alkalinity of the wastewater samples was tested with APHA 2320-B titration methods. The alkalinity of water is explained by its acid-neutralizing capacity [16]. Bicarbonate, carbonate and hydroxide ions are the primary contributors to the alkalinity of water, other constituents such as borate, phosphates, or silicates may also contribute to alkalinity [17]. The alkalinity of the wastewater samples was determined from the volume of standard acid required to titrate a portion to a designated pH value. It was titrated at room temperature with a properly calibrated pH meter or electrically operated titration. The most important apparatus used for the alkalinity test were pH meter and electrode, magnetic stirrer, stir bar, Beaker, titration vessel, Burette, standard sulfuric acid titrant, Pipets volumetric, flasks volumetric.

Procedures: standardized sulfuric acid titrant solutions were prepared as required. The clean burette was filled with the standard acid titrant. Samples temperature is equilibrated with the room’s temperature and transferred volumetrically enough samples to 100/150 mL beaker to provide the titrant good volumetric precision. The stir bar is placed in the beaker and the beaker is placed on the magnetic stirrer and inserts the pH electrode into the beaker. The sample pH is measured and the initial burette reading was recorded when sample pH is measured above 8.3. And standard acid titrant is added until the pH endpoint of 4.5 is reached. The endpoint pH value of 4.5 is used for routine measurement of alkalinity in most environmental water and wastewater samples. Then the final burette reading is recorded.

Calculation and reporting of alkalinity were done by Eq. (2).

$$\text{Alkalinity}\mathrm{as}\left({\mathrm{T}}_{\mathrm{Alk}}\right)\mathrm{mg}/\mathrm{L}{\text{CaCO}}_3=\frac{\mathrm{B}\ast \mathrm{N}\ast 50000}{\mathrm{mL}\text{sample}\left(\mathrm{S}\right)}$$E2

Where:

B = mL of sulfuric acid titrant used to reach end point pH.

N = normality of the standardized acid titrant.

S = mL of sample volume.

Finally, total alkalinity is reported as “total alkalinity to endpoint pH mg/L as CaCO₃”.

2.4 Data analyses

Data were analyzed by the statistical tools (SPSS software version 21) and Microsoft excels software. The analysis results of water samples were presented in descriptive texts, tables, and respective graphs for each heavy metal element and other irrigation water quality parameters. The relationship of the variables or heavy metals and other irrigation water quality parameters were tested with correlation analysis. Variations of heavy metals and selected ions between water sampling locations along the river stream were tested with ANOVA. SAR was computed to test the level of existing sodium hazard in the Little Akaki River for irrigation uses.

3. Result and discussion

3.1 Descriptive statistics of the irrigation water quality parameters

The descriptive statistics illustrated the analysis result of heavy metals, selected toxic ions, and other physic-chemical parameters of Little Akaki River irrigation water (Table 1).

	COD	Chloride	Alkalinity	Pb	Cd	pH	TDS	ECw	Ca	Na	Mg	SAR
Max	295.6	394	400	3.9	0.4	8.42	1036	1584	24.46	87.9	5.25	4.25
Min	168.9	284	366.2	2.5	0.03	7.8	198	288	16.27	66.32	4.95	3.68
Mean	252.37	334.67	382.4	3.23	0.16	8.01	519.3	791	21.54	78.52	5.11	3.94
Range	126.7	110	33.8	1.4	0.37	0.62	838	1296	8.19	21.58	0.3	0.57
S. D.	72.3	55.51	16.943	0.70	0.21	0.35	451.86	694.97	4.57	11.06	0.15	0.29

Table 1.

The descriptive statics.

3.2 Levels of heavy metals in little Akaki River water

As presented in Table 2, the concentration levels of heavy metals (Pb and Cd) were ranged between (2.5–3.9 mg/L) and (0.03–0.4 mg/L) respectively. But Cr and Cu were found below the detectible concentration limits of the laboratory instrument (ICP-OES) in all three sampling locations along Little Akaki River. The detectible concentration limits of ICP-OES for both heavy metals (Cr and Cu) are 0.005 mg/L [18]. The result has revealed that the contamination level of the river water with Chromium and Copper was very low. Woldetsadik et al. has reported Cr (0.02–0.029 mg/L) and Cu (0.028–0.039 mg/L) around Lekuanda and Mekanisa respectively and (Aschale, 2015) also reported Cr (0.0074 mg/L) and Cu (0.0056 mg/L) around Kera [9]. This shows that previous studies also confirmed that the level of chromium and copper in the Little Akaki River was very low this is because the possible reasons are it could be the presence of hydrological process of the river water. In the natural aquatic environments or surface water, chromium does not persist for long in the dissolved states and is precipitated as a suspension in the river water and the soluble species of chromium are readily adsorbed by Phyto and zooplankton.

• RML-Recommended maximum limit for irrigation water set by FAO (Ayers and Westcot 1985)

• <0.005 represents the detectible limit of Cr & Cu. These two heavy metals were found below the detectible limits in all of the three sampling locations.

Sample ID	Sampling location	Pb	Cd	Cr	Cu
01	Gelan	2.5	0.03	<0.005	<0.005
02	Gofa	3.9	0.04	<0.005	<0.005
03	Lekuanda	3.3	0.4	<0.005	<0.005
	MRL (mg/L)	5	0.01	0.1	0.2

Table 2.

Concentration of heavy metals in (mg/L).

Cadmium ion found in the ranges between (0.03 and 0.4 mg/L) with an average value of 0.1566 mg/L (Figure 2). The study shows that cadmium was found extremely higher than the maximum recommended permissible limit of FAO (0.01 mg/L) guidelines for irrigation uses in the catchment irrigated areas.

Figure 2.

Concentration levels of cadmium ions between the sampling points across little Akaki River streams.

In addition, the concentration of cadmium was very higher upstream around lekuanda (0.4 mg/L) and it has decreased downstream of the river around Gelan (Figure 3). This is because the possible reason is that the concentration of cadmium in the upper catchment is influenced by naturally occurring sources like weathering of parent materials, sources of soils, and rocks than the anthropogenic effects. Cadmium is also governed by the types and extent of land use in the catchment areas. However, cadmium is decreasing downstream of the river. This is due to the presence of complex physic-chemical interaction and hydrological processes of the river water. The river flow rate also can determine the concentration of heavy metals, in a lower rate of river flow intensity, heavy metals become deposited at the bottom of the rivers and can be adsorbed with different suspended particulate matter that could be deposited in the bottom of the river flow while the heavy metals are transported in the long-distance along with the river flow. The other point is Cadmium concentration is highly influenced by the pH and other physic-chemical parameters of the river water. Because heavy metals like Cadmium are strongly adsorbent with the organic and inorganic matter in alkaline conditions. So, the analysis result also supported this assumption that pH, selected ions (chloride, Na, Ca, and Mg), TDS, and ECw are higher downstream of the river and they can influence the dissolution rate of cadmium downstream. Woldetsadik et al. has reported a similar trend of Cadmium concentration along the river streams that cadmium was higher in Kera (0.00282 mg/L) and Lafto (0.00148) but the very lower value of cadmium in Akaki (0.00033 mg/L) and it was also extremely lower than the current study result (0.03–0.4 mg/L) which indicated that the problem of cadmium concentration is progressively increasing in Little Akaki River water [9].

Figure 3.

Concentration level of sodium ion in mg/L between sampling locations of little Akaki River stream.

Lead (Pb) was also found in ranges between (2.5–3.9 mg/L) with an average value of (3.23 mg/L). The concentration level of lead was higher (3.9 mg/L) in the middle catchment around Gofa followed by the upper catchment in the lekuanda sampling location and it was lower downstream of the river (Figure 4).

Figure 4.

Concentration levels of Lead ions between the sampling points across the little Akaki River streams.

This is because in the middle catchment the anthropogenic activities such as industries and other commercial activities are higher and lead is accumulated in the river water through various sources such as industrial emission; burning of lead-containing gasoline etc. the concentration of leads has been reduced to the down catchment it is because of different hydrological process and interaction of other physicochemical properties in the river water. The low-intensity water flow downstream can reduce its carrying energy causing lead has to be adsorbed on suspended particles and become deposited in the bottom of the river and river banks. The downstream river water is also diluted with different small tributaries which are joining to the main river stream at the down catchment. Factors such as pH, Alkalinity of water, TDS, and ECw affect the concentration of leads in the down catchment. In the downstream, the pH is relatively higher and has a slightly alkaline nature than the upper catchment. So, this slightly alkaline water content tends to the heavy metal ions converted to poorly soluble forms and to adsorb on the suspended matter in the river water. And heavy metals like leads have strongly adsorbent properties and can be retained at river banks, aquatic vegetation, hydro-engineering structures, suspended particles, and other solid bodies in the river water. pH above 7 in irrigation water sources inactivate the heavy metals and reduce their mobility and availability to crops (Office, F A O Regional and Cairo, 2003). However, under acidic conditions (pH < 7) heavy metals could be a problem. The correlation analysis (Table 3) also supported these assumptions because Pb is perfectly (100%) correlated with Alkalinity and has a negative association. Woldetsadik et al. have reported a similar trend along the river stream that the level of lead was higher in Lafto (0.0369 mg/L) and kera (0.0477 mg/L) whereas the value was lower in 0.0168 mg/L) in Akaki [9]. The result of previous studies has revealed similar concentration trends along the catchment of the Little Akaki River and their result was extremely lower than the current study result. This shows that the concentration of heavy metals especially Pb and Cd are progressively increasing in Little Akaki River irrigation water. And the current study shows that the existing concentration level of lead somehow seems to be hazardous for irrigation agriculture to produce vegetables because the value was found nearly lower than the maximum recommendation limits of FAO (5 mg/L) guidelines for irrigation water. So, however, currently, it was found below the permissible limits of FAO for irrigation water, the possible hazardous condition should not be overlooked because it is persistent and highly toxicant heavy metals to plants and human health even at lower concentrations.

		COD	Alkalinity	Pb	Cd	pH	TDS	ECw
COD	P. Correlation	1
	Sig.
Pb	P. correlation	0.895	−1.000**	1
	Sig.	0.294	0.007
Cd	P. Correlation	0.538	−0.095	0.106	1
	Sig.	0.638	0.939	0.933
pH	P. Correlation	−1.000**	0.887	−0.892	−0.544	1
	Sig.	0.005	0.306	0.299	0.633
TDS	P. Correlation	−0.993	0.83	−0.836	−0.634	0.994	1
	Sig.	0.076	0.377	0.37	0.563	0.071
ECw	P. Correlation	−0.991	0.822	−0.828	−0.645	0.992	1.000**	1
	Sig.	0.085	0.386	0.379	0.554	0.08	0.009
Ca	P. Correlation	−0.463	0.008	−0.019	−0.996	0.469	0.565	0.576
	Sig.	0.694	0.995	0.988	0.056	0.689	0.618	0.609
Na	P.	−0.748	0.364	−0.374	−0.962	0.753	0.822	0.83
	Sig.	0.462	0.763	0.756	0.177	0.457	0.386	0.377
Mg	P. Correlation	−0.128	−0.338	0.328	−0.905	0.135	0.245	0.258
	Sig.	0.918	0.781	0.787	0.28	0.914	0.843	0.834
SAR	P. Correlation	−0.945	0.693	−0.701	−0.784	0.948	0.977	0.98
	Sig.	0.212	0.513	0.506	0.427	0.207	0.136	0.127

Table 3.

Correlation coefficient.

*

Correlation is significant at the 0.05 level (2-tailed).

**

Correlation is significant at the 0.01 level (2-tailed).

3.3 Heavy metals variations between sampling locations

Variation of heavy metals concentration levels between the three water sampling locations along Little Akaki River streams were tested with ANOVA and the result is illustrated in Table 4.

Source of Variation	SS	df	MS	F	P-value	F crit
Sampling location	0.285	2	0.142	1.079	0.39777	5.14325
Heavy metals	22.767	3	7.589	57.567	8.1750E-05	4.75706
Error	0.791	6	0.132
Total	23.843	11

Table 4.

The analysis of variation (ANOVA) for heavy metals.

The research Hypothesis was that Ha: the concentration of heavy metals has a significant concentration variation between the sampling locations at 0.05. According to the ANOVA test, the alternative hypothesis is rejected because the F calculated value (1.079) is less than the F tabulated value (5.14325). So, it is confirmed that there is no statistically significant concentration variation of heavy metals (Pb and Cd) at 0.05 and 0.01 between the sampling locations along the river stream.

3.4 Selected ions of water sample

The concentration levels of selected ions such as chloride, calcium, Sodium, and Magnesium were presented in Table 5.

Sample ID	Sample location	Chloride	Ca	Na	Mg	SAR
01	Gelan	326	23.88	87.90	5.13	4.25
02	Gofa	394	24.46	81.34	5.25	3.88
03	Lekuanda	284	16.27	66.32	4.95	3.68
	MRL (FAO in (mg/l)	350		69		6

Table 5.

Selected ions in little Akaki River water (mg/L).

The concentration of chloride was varying between 284 to 394 mg/L with an average value of 334.66 mg/L in Little Akaki River irrigation water. The result revealed that the value of chloride was surpassed the maximum permissible limit of FAO for irrigation water (350 mg/L) in the middle catchment (Figure 5). The Excess amount of chloride in the middle catchment is due to domestic and industrial wastes have been discharged into the river streams. Chloride is originating from natural resources, sewage, and industrial effluents, excessive chloride concentrations increase rates of corrosion of metals in the irrigation structure system, The excessive chloride ions in irrigation water have great impacts on the accumulation of chloride ions in soil solution through long time irrigation uses and can affect the vegetable production since excessive chloride in soil solution is very toxic to plants.

Figure 5.

Levels of chloride concentrations between sampling points across the downstream of Little Akaki River.

3.5 SAR and Sodium hazard

The computed SAR value of the water samples ranged between 3.68 to 4.25 and it is found below the Maximum limits of FAO (6) for irrigation water (Figure 6). SAR of the irrigation water has explained the impact of sodium in the destruction of soil structure and water infiltration problems through the application of contaminated irrigation water for long periods. The computed value of the Na/Ca ratio from (Table 6) was 3.64. In this regard, however, the computed SAR value is lower than the maximum limit of FAO, sodium ions can cause toxicities to sodium-sensitive crops at a lower SAR value in sodium-dominated irrigation water (ratio of Na/Ca > 3:1). At a given SAR value, the potential effect of sodium toxicity and soil water infiltration problems increases in sodium-dominated irrigation water (Na/ca >3:1).

Figure 6.

Computed SAR between sampling locations across Little Akaki River stream.

Sample ID	Sample location	COD	Alkalinity	PH	TDS	ECw (μS/cm)
01	Gelan	168.9	400	8.42	1036	1584
02	Gofa	292.6	366.2	7.82	324	501
03	Lekuanda	295.6	381	7.80	198	288
	MRL (FAO) (mg/l)			6.5–8.4	1400–2000	2000–3000

Table 6.

Physic-chemical characteristics of Little Akaki River water (mg/L).

Sodium-ion concentration was found in ranges between 66.32 and 87.9 mg/L with an average of 78.52 mg/L which is surpassed the maximum limit of FAO (69 mg/L) in the middle and downstream of the river for irrigation and the concentration is increasing to the downstream of the river (Figure 3). Sodium-ion concentration is told us the extent of its toxicity for plants. Therefore, according to the analysis result, sodium ion concentration is reached at the middle to slight restriction level for vegetable production in the study area because it has a significant toxicity effect on plants at higher concentration levels. The concentration of Sodium ions in the aquatic system is mainly derived from atmospheric deposition and silicate weathering [19].

3.6 Variation of selected ions between sampling locations

Variation of selected ions (Chloride, Sodium, Calcium, and Magnesium) concentration levels between the three sampling locations along the river streams were tested with ANOVA and the result is illustrated in Table 7.

Source of variation	SS	df	MS	F	P-value	F crit
Sample location	1791.28	2	895.642	1.538194	0.272124	4.45897
selected ions	237858.70	4	59464.68	102.1259	6.7E-07	3.837853
Error	4658.14	8	582.2684
Total	244308.20	14

Table 7.

Analysis of variation (ANOVA) test for selected ions.

The research Hypothesis was that Ha: the selected ions have a significant concentration variation between the sampling locations at 0.05. According to the ANOVA test, the alternative hypothesis is rejected because the F calculated value (1.538) is less than the F tabulated value (4.45897). So, it is confirmed that there is no statistically significant concentration variation of selected ions (Chloride, Na, Ca, and Mg) at 0.05 and 0.01 between the sampling locations along the Little Akaki River stream.

3.7 The physic-chemical characteristics of Little Akaki River water

As indicated in Table 6, COD has been decreased downstream of the rivers from 295.6 mg/L at Lekuanda to 168.9 mg/L at Gelan. Whereas, the other parameters such as Alkalinity, pH, TDS, and EC were increased across the downstream (Table 3). This is because the COD content was diluted and attenuated across the path of the river course from upstream to downstream. The physical and chemical properties of the river water are characterized by several interdependent interactions and their relationships are extremely complex. The hydrological process is also the most determinant factor that influence the concentration of the physic-chemical parameters of the river water.

The pH value of water samples ranged between7.8 to 8.4 and it has a mean value of 8.01. The value of pH also increases downstream of the river this is because the alkalinity and ECw of river water also increase downstream of the river and have a significant contribution to raising the pH of the river water downstream because they have a positive correlation. Hence the result indicated that Little Akaki River irrigation water is slightly alkaline and it lies in the acceptable ranges of FAO guidelines (6.5–8.4). pH is the most determinant factor for the quality of irrigation water and it can greatly influence the toxicity of heavy metals and other impairing selected ions. Alkaline irrigation water prohibits the solubility and bioavailability of heavy metals.

Electrical conductivity (EC_w) of the river water was varied between (288–1584 μS/cm) with an average of 791 μS/cm and the value was increased to the downstream of the river from 288 to 1584 μs/cm and it is found below the permissible limits of FAO (3000 μS/cm) for irrigation uses (Figure 7).

Figure 7.

Level of ECw in (μS/cm) between sampling locations across the Little Akaki River streams.

The total dissolved solids (TDS) of water samples were varied between 198 and 1036 ppm with an average of 519 mg/L and it has a higher value in the downstream (1036 mg/L) and the concentration is decreased to the upstream of the river (Figure 8). TDS is found below the maximum recommended limit of FAO (1400–2000 mg/L) for irrigation water. But relatively the higher value is obtained downstream of the river and it indicated the presence of a higher amount of basic or alkaline compounds like bicarbonates, sulfates, chlorides, etc. In general, according to the result in (Figures 7 and 8), the value of EC and TDS illustrated that salinity is not a serious problem in the existing condition in Little Akaki River irrigation water.

Figure 8.

Levels of TDS in mg/L between sampling locations across Little Akaki River streams.

3.8 The variation of physic-chemical parameters

Variation of physic-chemical parameters (COD, Alkalinity, pH, TDS, and EC_w) between the sampling locations along the river streams were tested with ANOVA and the result is illustrated in Table 8.

Source of Variation	SS	Df	MS	F	P-value	F crit
Sample location	474733.3	2	237366.6	2.09	0.18668	4.45897
Physic-chemical parameters	1,027,322	4	256830.5	2.25	0.152122	3.837853
Error	910608.30	8	113,826
Total	2,412,664	14

Table 8.

Analysis of variation (ANOVA) for physic-chemical parameters.

The research Hypothesis was that Ha: the physic-chemical parameters have a significant concentration variation between the sampling locations at 0.05. According to the ANOVA test, the alternative hypothesis is rejected because the F calculated value (2.085346) is less than the F tabulated value (4.45897). So, it is confirmed that there are no statistically significant variations in the physic-chemical parameters (COD, Alkalinity, pH, TDS, and ECw) at 0.05 and 0.01 between the sampling locations along the Little Akaki River stream.

3.9 Correlation of heavy metals and other physic-chemical parameters

The correlation analysis is conducted to show the relationship and interaction of heavy metals and other physic-chemical properties of the river water. The interaction between the heavy metals and other properties are the major factor for their concentration variations in the river water. Therefore, the correlation result is presented in Table 3.

The concentration of heavy metals and other selected ions were correlated with some physic-chemical interactions and different hydrological processes of the river water. The scatter plot analysis also depicted that ECw and TDS have positive associations and are strongly correlated with each other (Figure 9).

Figure 9.

Correlations between TDS and ECw.

TDS concentration describes the presence of inorganic salts and organic matter in the irrigation water and EC is the measure of irrigation water capacity to conduct electric current. Both EC and TDS are very determinant irrigation water quality parameters, which are used to describe the salinity level of the irrigation water [20]. These two parameters are correlated and usually expressed by a simple equation: TDS (mg/L) = k*EC (μS/cm in 25°C. The value of k will increase along with the increase of ions in water. However, the relationship between conductivity and TDS is not always directly linear; it depends on the activity of specific dissolved ions in the liquids and ionic strength [21].

Accordingly, the TDS/ECw ratio of the water samples in the Little Akaki River was = 519/791 = 0.656 or it can be written as equation TDS = 0.656*EC. This indicates that the correlation of both parameters is strongly influenced by the EC values. Unlike freshwater, the correlation between TDS and EC in wastewater cannot be described well because the water is heavily influenced by many contaminants [21].

4. Conclusions

The study was aimed to determine the concentration levels of heavy metals (Pb, Cd, Cr, and Cu) and selected ions (chloride, Calcium, Sodium, and Magnesium) of Little Akaki River irrigation water and to test the concentration variation of heavy metals and selected ions between the sampling locations along the river stream. To this end, the study was brought out the following observations and conclusions. The concentration level of Cadmium (Cd) ranged between (0.03–0.4 mg/L) and it was extremely higher than the permissible limits of FAO (0.01 mg/L) for irrigation water and the values of Pb was also varying in (2.5–3.9 mg/L) and it was found in approaching the maximum permissible limit of irrigation water set by FAO (5 mg/L). Both heavy metals (Cd & Pb) have higher concentrations in the middle and upstream than the downstream of the river and their concentration level reached the hazardous condition for irrigation water in Little Akaki River water. The concentration level of Pb and Cd were extremely higher than the previous study findings and this indicated heavy metal contamination problem is progressively increasing in the river stream. Whereas, heavy metals such as Cr and Cu were found below the detectible limits of the laboratory equipment (ICP-OES). Selected ions such as chloride and sodium were reached at the maximum permissible limits of FAO in the Little Akaki River and this can inhibit the growth of vegetables in the irrigation sites of the study catchment area. Other physic-chemical parameters (TDS, pH, and ECw) were found in optimum conditions for irrigation water in all three-sampling locations. The other main point that has been observed is a variation of heavy metals and selected ions between the sampling locations were not statically significant at 0.5 and 0.1. In general, the most important water quality parameters such as heavy metals (Pb and Cd), selected ions like (chloride and sodium) were exceeded the maximum recommendation limits of FAO guidelines for irrigation water in the Little Akaki River. Therefore, the study revealed that irrigation water quality is reached at a great concern for vegetable production and it could be a potential risk for human health through the food chain of vegetable consumption.

Acknowledgments

We would like to acknowledge Ethiopian Civil Service University for financing this study.

Conflict of interest

“The authors declare no conflict of interest.”