Introduction

Clean water is very important for all forms of life to survive. Although water covers three-fourth part of the earth’s surface, getting of freshwater has become a serious problem in the world. The quality of water deterioration is due to different factors that result from human development. The major ones are the expansion of industrialization and the rapid growth of the world population. The amount of heavy rain that results in high runoff from the agricultural land, lack of soil, and water conservation system management also affect the quality of water bodies. As a result, many perpetual rivers become short-lived and even dried out (Thillai Arasu et al. 2007). Massive physical, chemical, and biological interactions result in water quality characteristics of aquatic ecosystems. The water bodies such as rivers and lakes are continuously experiencing a dynamic change in their geological age and geochemical characteristics (Sahoo 2014). This complex equilibrium in the aquatic ecosystem is disrupted by human activities resulting in pollution, which in turn manifests dramatically as fish kill, the bad taste of drinking water, and unregulated aquatic weed growth (Fan et al. 2012).

Quality of water is now a great concern for people of all parts of the world. Different sources of pollutants could damage the quality of water resources. Similarly, there is no such environmental protection practice in Ethiopia, particularly in Bahir Dar City, as there are several polluting sources that have continuously deteriorated the quality of surface water.

The following hazard centers were considered as major categories of pollutant sources in the study area based on information obtained, observations made during site visits, and analytical results. These include industrial establishment, agriculture, municipal waste, fuel stations, garages, car washing, hotels, homes, and health centers (Wosnie and Wondie 2014).

These bodies of surface water are the source for disposal of industrial effluent and household waste. As a result, the natural dynamic equilibrium between the environmental segments is disturbed leading to the condition of polluted rivers (Mahananda et al. 2010). Food for consumers should be free of pathogenic organisms and harmful substances (Thillai Arasu et al. 2007), according to the decision of the World Health Organization (WHO). Ethiopia faces persistent problems of floods and droughts and high contamination of aquatic resources, despite large water resources in lakes and rivers (Tamiru et al. 2004). River Abay is also one of the water resources that have been affected by point sources effluent generated by textile (Mehari et al. 2015a, b), tannery (Wosnie and Wondie 2014), and municipal (Mehari et al. 2016) wastewater sources. As a result, there will be a high concentration of nutrients (Wagh et al. 2017) that enhance the growth of floating plants such as hyacinth (Stålnacke et al. 2003). The higher growth of such plants reduces the amount of dissolved oxygen inside the river. This reduction of the amount of dissolved oxygen will affect the life of aquatic life such as fish (Miller 2005; Dudgeon et al. 2006) that cannot survive below 4 mg/L amount of oxygen (Mehari et al. 2016). The present study analyzed various parameters (water temperature, total suspended solids (TSS), total dissolved solids (TDS), total dissolved solids (TS), turbidity, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), pH, electrical conductivity (EC), salinity and chloride content) of water samples from 12 sites which are very important in assessing water quality.

Materials and methods

Description of the study area

The study area is found in Northern Ethiopia Bahir Dar town which is located 578 km from Ethiopian capital city Addis Ababa. It also found at latitude 11° 8′ N and longitudes 39° 38′ E (Fig. 1). Abay river originates from Gish Abay, Sekela woreda, west Gojam zone, Ethiopia. The average elevation in the Bahir Dar town is about 1795 m.a.s.l with the “Woina Dega” type of agroecological zone. The town covers an area of about 16,000 hectares. The study area experiences average annual rainfall that ranges from 1200 to 1600 mm and it has a mean annual temperature of 26 ℃ (Mehari et al. 2015a, b). Bahir Dar town is a beautiful, rapidly growing and main tourism center for the country. Although the water from this river is used by the downstream communities for various purposes such as drinking, livestock watering, hygiene, and irrigation, different sources of contamination such as textile factory, tannery, municipal ditch located at the edge of the head of Blue Nile River (Abay River), which directly discharges its effluents into the river is mainly the polluting factor.

Fig. 1
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Location map of study area

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Data collection

Accurate data collection is very critical to maintaining the integrity of research. Both the choice of acceptable data collection methods and delineated guidelines for their proper use decrease the risk of errors occurring. The collection of relevant information and data for the study area is, therefore, essential. Laboratory and on-site measurements were used to collect the data. Water samples from the mouth, middle, and mouth of the river were taken to determine the water quality status of the river. Twelve sampling stations were usually chosen by taking into account the sources of wastewater effluent contributing to the river. These 12 sampling stations consisted of 4 effluent mixing stations from point sources such as textiles, municipal ditches, and tanneries, 4 upstream parts of the mixing stations, and 4 upstream parts of the mixing stations after mixing (Table 1). The geographical location of sampling points as shown in Table 2.

Table 1 Water quality sampling station of the Abay River
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Table 2 Sampling points with its coordinate system
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Sampling sites

Table 1.

Sampling and laboratory analysis

Water samples from 12 sampling stations were collected in March and May 2017. During sample collection, acid-washed polyethylene bottles were used for physicochemical analysis. The collected samples were transported to the Bahir Dar institute of technology water quality lab by keeping them in a cooled icebox and later stored in a refrigerator below 4 ℃ until use. Parameters such as total dissolved solids (TDS), water temperature, dissolved oxygen (DO), pH, electrical conductivity (EC), and salinity were measured on-site with the multimeter prob by dipping the probe about 20 cm below the water surface. Whereas the other parameters like biochemical oxygen demand (BOD), ammonia, turbidity, and chemical oxygen demand (COD) were analyzed in the lab.

Result and discussion

During dry seasons, water samples have been collected from the Abay River to examine physico-chemical parameters. The essential parameters of water quality were temperature, pH, TDS, BOD, COD, DO, turbidity, EC, and salinity. Assessment of contaminated water samples is achieved by comparing the measured values of all physico-chemical parameters with the relevant criteria recommended by the World health organization for drinking water (Fig. 2; Tables 3 and 4).

Fig. 2
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Temperature value at sampling points

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Table 3 Water quality parameters versus sampling station
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Table 4 Water quality parameters versus sampling station
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Temperature and pH

As it is shown in Fig. 3 the temperature was 23.2–26 ( ℃) which is within WHO and EPA limits of 25–40 ℃ and values of pH concentration at all sampling points was (5.5–7.96) was within WHO and Environmental Protection Authority (EPA) limit of 9.0.

Fig. 3
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DO, BOD, and COD values at different sampling points

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DO, BOD and COD

In the case of dissolved oxygen (DO), the maximum tolerance is 3 mg/L for surface waters used as raw water and swimming, and 4 mg/L for aquatic sustainability. In comparison, it is 6 mg/L for drinking purposes. The Abay River DO level ranges from 2.5 to 7.8 mg/L. The DO value at all sampling points except 1, 3, 4, 6 and 12 is lower than the recommended WHO value of 4 mg/L. both the concentration of BOD and COD values in all sampling sites were above the standard for drinking purposes is 0.2 mg/L and 4 mg/L, which is not acceptable in terms of the Abay river water sample analyzed.

Total dissolved solids and electrical conductivity

The TDS content of water samples obtained at all stations was well below the 500 mg/L WHO limit value. [8]. But electrical conductivity value at sampling points 10 and 11 were above the permissible limit of WHO of 300 mg/L (Figs. 4, 5). Increasing conductance indicates that water is unsafe to drink.

Fig. 4
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Total dissolved solids and electrical conductivity

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Fig. 5
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Salinity, ammonia, and turbidity values at sampling points

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Salinity, ammonia, and turbidity

The mean salinity of the Abay River was found with a range from 0.11 to 1.5 mg/L which is within the EPA standard for fish < 100 mg/L. The value of turbidity at all sampling points except at 2, 7, 8, 9 all sampling points were within the WHO standard of 30 mg/L. NH3–N values were less when compared with the WHO standard of 30 mg/L at all the sampling stations.

Conclusion

The majority of Abay rivers water quality parameters such as DO, COD, BOD, electrical conductivity, turbidity at sampling points 2, 5, 7, 8, 9, 10, and 11 were set for drinking purposes and aquatic life beyond the WHO maximum permissible amount. In particular, the extent of contamination of the quality of river water near the point sources is further impaired due to the uncontrolled discharge of effluent. From this finding, it can be concluded that the water quality has been affected by the point sources and unfit for drinking purposes. Hence, there are many immediate downstream users of the water river for drinking, fishing, bathing, and irrigation; the effluent demands regular control and proper treatment before being discharged to the environment.