Introduction

Most Ethiopians, 80–85%, work in the agricultural sector. However, the sector only accounts for as low as 40% of the GDP because of ongoing droughts and food shortages (World Bank 2011). In the sub-Saharan region, Ethiopia is home to more than 80 million people, of which 44% are poor. Over the past few decades, the population has increased by 2.5% annually, but crop yields have only increased by 1.4%. Even in a good year, these farming techniques are unable to meet the demand for food, and their deficiencies are clear. This is mostly the result of a scarcity of water brought on by unpredictable and poor infrastructure (Marx 2011). Since the agricultural sector in Ethiopia is mainly dependent on rain-fed farming, the sector is under significant pressure to cut its water use because of the rise in demand, climate change, and recurring droughts (Misra 2014).

To reduce demand, the Ethiopian government focuses on the irrigation sub-sector by encouraging farmers to enhance irrigation management methods and promote contemporary irrigation technologies. Both surface and groundwater resources are abundant in Ethiopia. The Ministry of Water, Irrigation, and Energy (MOWIE) projected that the country’s annual groundwater flow was 40 billion cubic meters (BCM), and its annual surface water flow was 122 BCM, with an estimated irrigation potential of 3.7 million hectares (Awulachew et al. 2007; Gebul 2021). However, a country’s total irrigated land is just 197,000 ha, or 5% of its potential, and more than half of this is deemed small-scale (Eriksson 2012).

The Abbay basin in Ethiopia has an irrigation potential of about 1 million hectares. The completion of the Koga scheme, found in the Abbay basin, offers crucial lessons for developing and managing massive irrigation projects. The Koga irrigation and watershed management project is the first largest irrigation project in Ethiopia (Abeba 2018) that has drawn interest not only in Ethiopia but also internationally (Marx 2011). This irrigation and watershed management project was created in response to the ongoing droughts, which made it difficult for small-scale farming to meet the growing population’s food demands (Ministry of Water Resources, 2008).

While the irrigation system expands, ongoing evaluation of irrigation water quality becomes one of the anticipated difficulties. According to Molla and Fitsume (2017), the challenges of agricultural intensification, climate changes, and excessive use of ground aquifers in arid and semi-arid settings are causing irrigation water quality to become a major concern worldwide.

In the Koga irrigation scheme, irrigation is carried out using rainwater as well as surface water gathered in the Koga dam from rivers, lakes, and streams. Due to the pollution loads from both natural and man-made sources, including animal and human waste, chemical fertilizer leaching, and pesticides and weedkillers, the quality of this irrigation water may deteriorate with time. Thus, a well-timed assessment of the quality of this irrigation water is very important to produce healthy crops and maintain human health. Although earlier studies have been conducted on the quality of Koga irrigation water (Eriksson 2012; Densaw et al. 2016), updating the data set and examining the trend on the quality status of Koga’s irrigation water, and how the agriculture and ecosystems in the area are affected is very important. The objective of this study was, therefore, to assess the quality of irrigation water in Koga Ethiopia using physiochemical and biological measures, and compare it to quality standards of irrigation water. The study also examined the trend of the water quality based on reported results and determined whether the water quality is suitable for the production of food crops and vegetables.

Materials and methods

The research area description

The investigation was carried out in Northwestern Ethiopia at the Koga irrigation dam located in the Mecha District of the West Gojam Zone of the Amhara Regional state (Fig. 1). The irrigation system extends as high as 3200 m across a 266 km2 area of rugged mountains. The northern and southern halves of the irrigation area are 1860, and 2000 m above sea level, respectively (Gebrehiwot et al. 2010).

Fig. 1
figure 1

Map of the study site’s location: a. Ethiopia and Amhara Region; b. Amhara Region, West Gojjam Zone, and North Mecha District; c. North Mecha district, d. Kebeles in the North Mecha district where the sampling points are located

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The region is impacted by the Inter-tropical Convergence Zone (ITCZ), northern trade winds, and southern monsoon (UNESCO 2004). It consequently goes through the Bega, a dry season that lasts from the start of December through the end of May. The rainy season in Kiremt lasts from June/July through September/October. Meher in Ethiopia would only allow for one growing season of rain-fed crops (Marx 2011). Ethiopia’s most important climate variable at the moment for crop and feed output is rainfall (UNESCO 2004). Peak flows occur during the rainy season (June–October), coinciding with increased delivery of fine sediments tosurface waters. The annual precipitation ranges from 800 to 2200 mm (mean 1420 mm) (MoWR 2006).

Water sample collection

The standard method outlined by APHA (1998) was used to collect water samples from the Koga irrigation dam. Water samples were collected in 250 mL pre-cleaned plastic containers.

Water samples were collected from 13 sites during March 2022 (Table 1). In the study site, people’s activities vary with time and the degree of solid and liquid trash they produce also differ. In the dry seasons, the animals enter the irrigation area for feeding and more excretes of animals and humans enter the irrigation water canals; high wind in the dry season causes a large amount of dust and soil particles to enter the irrigation water; high amounts of chemical fertilizers and pesticides used for the irrigation activities enter to the irrigation water. The application of chemicals and fertilizers and most of the people’s activities which may cause pollution to the irrigation water reaches a climax around the end of March. Taking this into consideration, the last week in March 2022 was selected as a sampling time.

Table 1 Description of the sampling sites
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The sampling sites were selected purposively. The main dam is open to animals and humans, pollutants from all sources enter the water in the dam in all directions. Due to this, the main/primary canal head was selected to be the first sampling site. At night, water from the main dam flows and collects in the night storage reservoirs, which are open and exposed to pollution. Because of this, the four sampling sites were at night storage reservoirs. From the night reservoirs, water moves a long distance to reach the farmers’ cultivated land. The distribution canals through which the water flows are exposed to different pollutants, including animal and human waste, dust particles, fertilizer, and chemicals, and hence pollutants entering the canals are carried by the irrigation water. Therefore, taking samples at the end of the canals helps to know the water quality used for crop production. Taking this into consideration, eight sampling sites were at the canals through which the irrigation water from the night reservoirs moves to the farmers’ cultivated land. The description of the sampling locations along with their GPS coordinates is depicted in Table 1. The canal network and site-to-site relationships of the sampling sites are shown in Fig. 2.

Fig. 2
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A map of the canal network and site-to-site relationships of the sampling sites

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The water samples were collected using the protocols stated by Wondimagegne and Tarekegn (2016). All sampling materials were washed with detergent, rinsed with distilled water, soaked with 10% HNO3 for 24 h, re-rinsed with deionized water, and air-dried. Three separate locations at each sampling site were used to collect water samples using pre-rinsed 250 mL polyethylene bottles. Water samples from the three locations of each site were combined into a one-liter polyethylene container with 2 mL of nitric acid to reduce the adsorption of metals onto the walls of the plastic bottles.

Temperature, EC, pH, turbidity, and TDS were measured on-site using portable EC, pH, turbidity, and TDS meters (Gebremedihin and Berhanu 2015). For bacterial analysis, water samples were taken using very clean, sterilized, tightly closed containers, and transported using an ice box, and the microbial analysis was done within 24 h of collection. To avoid contamination, the researchers’ hands were cleaned before sample collection. For the remaining water quality parameters, water samples were shipped in an ice box and kept at 4 °C until analyzed in the laboratory.

Apparatuses, chemicals, and methods of water quality analysis

The instruments, materials, and procedures for analyzing the water quality in this investigation were the same as those utilized in Lewoyehu’s research (2021). Equipment includes pH meter (Hana pH meter, Germany), EC meter (DDB-11A portable conductivity meter), hotplate, volumetric flasks, hand lens vacuum pump, Palintest test tube, and Palintest photometer (photometer 8000, England) and inductively coupled plasma optical emission spectrophotometer (ICP-OES) (Perkin Elmer optima 8000).

Buffer solutions (pH: 4.0 and 7.0), laurel sulfate broth, hydrogen peroxide (30%), hydrochloric acid (37%), lanthanium nitrate, calcium chloride (anhydrous), standard tablets of phosphate, nitrate, sulfate, ammonia, alkalinity, and hardness were utilized in the study. Distilled water was used throughout the research.

Total suspended solids (TSS), total dissolved solids (TDS), turbidity, pH, electrical conductivity (EC), total alkalinity (TA), total hardness (TH), nitrate (NO3−1), phosphate (PO4−3), sulfate (SO4−2), and ammonia (NH3)) were analyzed using the standard analytical methodologies stated by the American public health association (APHA) (APHA, 1995, 1998, and 2012). Microbial analysis was carried out following the methods stated by WHO (2004). The membrane filter technique was employed since the method is more efficient, less labor-intensive, requires less culture medium and glassware, and provides precise results directly from colony counts (UNEP/WHO, 1996). The concentration of metal cations (Ca2+, Mg2+, and Na+) was measured using ICP-OES. Using the data obtained, the sodium absorption ratio (SAR), Kelley’s ratio (KR), and magnesium absorption ratio (MAR) were computed.

The following equations were used to calculate: SAR (USDA, 1954), KR (Kelley 1940), and Magnesium hazard (MAR) (Paliwal 1972).

$${\text{SAR}} = \;\frac{{{\text{Na}}^{ + } }}{{\sqrt {\frac{1}{2}\left( {{\text{Ca}}^{2 + } + {\text{Mg}}^{2 + } } \right)} }}$$
$${\text{KR}} = \;\frac{{{\text{Na}}^{ + } }}{{{\text{Ca}}^{2 + } + {\text{Mg}}^{2 + } }}$$
$${\text{MAR}} = \;\frac{{{\text{Mg}}^{2 + } }}{{{\text{Ca}}^{2 + } + {\text{Mg}}^{2 + } }} \times 100$$

where the cation concentrations are all represented in meq/L.

Data analysis

Each water quality parameter was measured three times from the same water sample and expressed as a mean and standard deviation. Differences among sites were tested using a one-way ANOVA using SPSS version 22, followed by Tukey’s post hoc multiple comparisons test at a confidence level of 95%. Correlation values were calculated for the quality parameters.

Results and discussion

Physicochemical characterization

Turbidity

All sites had turbidity levels greater than 170 NTU (Table 2) which exceeds the turbidity standard of 35 NTU (US FAO, 1999) (Fig. 3). The ANOVA results showed significant differences among the various sampling sites. Eriksson (2012) reported turbidity in the range of 84.1–147.0 NTU for the irrigation water used at Koga, with an average turbidity of 100 NTU. In his report, water in the main dam showed higher turbidity compared to the turbidity of water in the canals and night reservoirs. In this study, however, the turbidity of water in the canals and night reservoirs was higher than the turbidity of water in the main dam. This shows that the irrigation water is exposed to various pollutants, including suspended particles that can block light from entering the water, as it goes from the storage via the canals into the reservoirs; as a result, turbidity increased. According to Densaw et al. (2016), the irrigation water in the Koga irrigation canal had a turbidity of 228.5 NTU. The study’s higher turbidity results revealed that the water source in the study location needs to be better maintained. The increased turbidity of the irrigation water at the research location may have been caused by human activity near the irrigation project. Among these, changes in agricultural production practices may have disrupted the land’s surface, possibly introducing soil particles and nutrients to the irrigation water canal. Chemicals used for weed and pest management as well as fertilizer frequently leak into the water canal.

Table 2 Summary of the physicochemical parameter levels of the examined water samples (n = 13, sampling time: March 2022, statistical test: one-way ANOVA followed by Tukey’s post hoc multiple comparisons test at P = 0.05)
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Fig. 3
figure 3

Comparison between the physicochemical characteristics of Koga irrigation water and the corresponding FAO recommended threshold limits

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Total suspended solids (TSS)

The TSS results of this study were greater than the 45 mg/L permissible standard (FAO, 1999) (Fig. 3), and the 25.5 mg/L TSS reported in 2016 (Densaw et al. 2016). Although the site 11, 12, and 13 values are high (> 100 mg/L), it is important to note that the rest of the samples are just below the severe standard, the higher end of the moderate range (50–100 mg/L) (FAO, 1985). This is important because if TSS were to continue to trend upward, it will likely be severe shortly if management action is not implemented relatively quickly.

Total dissolved solids (TDS)

TDS was below 168 mg/L, well below the 450 mg/L FAO standard (FAO, 1985) (Table 2). These observed TDS values classify as excellent water quality (TDS = 160 mg/L) except the sample at S1 which could be categorized as good for irrigation (TDS < 500 mg/L). Based on the TDS level, none of the samples were found in the medium (TDS = 500 mg/L), bad (TDS = 1500 mg/L), or extremely bad (TDS > 2500 mg/L) category. If the TDSlevel is between 450 and 2000 mg/L, there may be a small to considerable restriction on use while TDS greater than 2000 mg/L is harmful to plants. The TDS levels of the tested samples exceeded the TDS ranges reported by Densaw et al. (2016) (75–148.2 mg/L).

Electrical conductivity (EC)

EC levels in the irrigation system were less than the 750 S/cm FAO allowed limits, indicating that EC is not limiting crop production. Water samples from sites 11, 12, and 13 were rated as excellent (< 250 S/cm), and the rest were rated as acceptable (250–750 S/cm) based on Bauder et al. (2011). EC values recorded in this study showed an increment compared to reported EC values for the study site: 277 S/cm (Eriksson 2012) and 124.9 S/cm (Densaw et al. 2016). However, there was no upward trend over time based on these data.

pH

Hydrogen ion content determines the pH scale measuring acidic to basic. For example, high pH in irrigation water could lead to the precipitation of metal cations and the absorption of nutrients by plants could be affected.

All pH levels were within the range of 6.5–8.4 (Table 2, Fig. 3) set by the FAO as acceptable for irrigation water, indicating that they could not pose a risk to crops. According to FAO guidelines, water with a pH of 7–8 is mildly moderate for irrigation activities and may be severe if it has a pH of > 8. Therefore, the water samples from S12 and S13 may be severe for irrigation while the water samples from the remaining locations were deemed to be slightly moderate for irrigation. Water quality sampling indicated an increasing trend of pH values ranging from 6.95–7.6 (Eriksson 2012) to 7.97–8.17 (Densaw et al. 2016) and 7.33–8.31 in this study.

Total alkalinity (TA)

The TA measured at all sites exceeded the optimal range (100 mg/L) (Table 2, Fig. 3), and exceeded 150 mg/L at eight sites (2, 3, 4, 8, 10, 11, 12, 13), indicating the TA at these sites can raise the pH of the growth environment, and may cause a variety of nutritional issues such as iron and manganese deficiency, and calcium and magnesium imbalance. Any of the tested water samples didn’t have TA below 30 mg/L, and hence have buffering power against pH variations if acid fertilizers are employed. The TA values in this study were higher than the values in previous studies, 47–92 mg/L (Eriksson 2012) and 67.5 mg/L (Densaw et al. 2016).

The main grains and vegetables farmed in the Koga irrigation project include wheat, barley, maize, potatoes, cabbage, garlic, onions, tomatoes, and green peppers. For these main crops and vegetables, the TA tolerance level is 40 mg/L. Therefore, the irrigation water samples from the investigated areas had TA levels unsuitable for the region’s main crops and vegetables.

Total hardness (TH)

Hardness is a measure of the Ca and Mg concentration. All hardness values in this study exceeded the optimal values for plant growth (100–150 mg/L). The TH level of Koga irrigation water didn’t show an upward trend compared to previous studies though TH levels at S10, S12, and S13 were similar to the TH reported by Densaw et al. (2016) (222 mg/L).

Sulfate (SO4 2−)

All examined samples had sulfate concentrations over the suggested threshold limit (20 mg/L) of the FAO regulations for Ethiopia (FAO, 1996) for irrigation water. The research site’s irrigation water’s sulfate content showed an upward trend compared to the reports by Densaw et al. (2016) (3–10.5 mg/L). This sulfate load may be the result of agricultural activities where the farmers employed a lot of inorganic fertilizer and other chemicals on the irrigation field, which allowed sulfate to enter the irrigation water.

Phosphate (PO4 3−)

The suggested allowable limit for phosphate in irrigation water is 3 mg/L. All samples greatly exceeded this limit (Fig. 3). Phosphorus from fertilizers and chemicals used on irrigated land can enter the irrigation delivery canal from spraying and associated wind transport and field run-off during irrigation and/or precipitation events. In this manner, the continued application of phosphorus-containing products can continually accumulate. Previous studies measured much lower phosphorous concentrations < 1 mg/L (Eriksson et al. 2012) and < 1.5 mg/L (Densaw et al. 2016). Therefore, the results of this study revealed that there is a substantially higher application and retention of phosphate in the study area. The phosphate levels of all water samples exceeded the 2 mg/L tolerance limit of the crops and vegetables grown in the area and hence were unsuitable for the production of the primary crops and vegetables in the area.

Nitrate (NO3 1−)

All samples had nitrate below the 45 mg/L suggested allowable level of nitrate for irrigation water (FAO 1996). However, the nitrate level of the study site showed an increment compared to the values reported by Densaw et al. (2016) (0.66–1.7 mg/L). The nitrate levels of the study site surpassed the 10 mg/L tolerance limit of the primary crops and vegetables and were unsuitable for the cultivation of the area’s common crops and vegetables.

Ammonia (NH3)

The ammonia level of the studied water samples was above 0.1 mg/L. The ammonia level of Koga’s irrigation water showed an upward trend compared to the reported values. In Eriksson’s (2012) study, out of the 15 examined irrigation water samples, only one sample at a quaternary canal had a 0.04 mg/L of ammonia. Densaw et al. (2016) reported ammonia levels between 0.09 and 0.16 mg/L.

Above all, the physicochemical characteristics results of this study’s water samples are shown in Table 3 along with data from earlier sampling and analysis conducted by the Ministry of Water Institute of Ethiopia (MoWIE, 1995), the Tana Sub-basin Office (2011–2012), Eriksson (2012), and Densaw et al. (2016) as well as a comparison of each parameter observation to the FAO’s recommended standards for irrigation water quality.

Table 3 Comparison of Koga irrigation water quality with early reported values and FAO irrigation water quality standards (for the present study: n = 13, sampling time: March 2022)
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Microbial and biochemical oxygen demand characterization

Total coliform and fecal coliform count (TCC and FCC)

In this study, every water sample showed coliform contamination (Table 4). Although all samples’ fecal coliform counts were less than the FAO criterion of 800 cfu/100 mL, the coliform levels in every sampling point were higher than the zero/100 mL limits of the EU (1998) and WHO (2006) set for irrigation waters. Eriksson (2012) reported higher FCC ranging from 100 to 400 cfu/100 mL, indicating the FC contamination of the study site didn’t show an upward trend.

Table 4 The bacteriological quality and biochemical oxygen demand level of the Koga irrigation water (n = 13, sampling time: March 2022, statistical test: one-way ANOVA followed by Tukey’s post hoc multiple comparisons test at P = 0.05)
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For the primary crops (wheat, barley, maize, bean) and vegetables (garlic, onion) grown in the study site, the tolerance level for coliforms is 1000 cfu/100 mL. Therefore, the production of the aforementioned crops and vegetables could not be impacted by Koga irrigation water. However, the tolerance limit for coliforms in potato, cabbage, green pepper, and tomato is 10 cfu/100 mL. Hence, Koga irrigation water could be unsuitable for the growth of potatoes, cabbage, green peppers, and tomatoes.

Biochemical oxygen demand (BOD5)

All investigated water samples showed BOD5 levels higher than the irrigation water’s 30 mg/L standard (FAO, 1999) (Table 4). When there is typically organic matter and bacteria break down this waste, water with a BOD5 level of more than 5 mg/L is regarded as slightly contaminated. Bacteria that use oxygen have access to more organic matter or food when the BOD5 value is higher. When nutrient loading and the accumulation of plant decomposing matter in sampling points rise, BOD5 readings rise as well.

The acceptable limit of BOD5 for the use of greywater in Ethiopia to irrigate vegetables that are likely to be consumed raw is 20 mg/L. Given this, even though the water was being utilized to grow vegetables, the BOD5 level at all monitoring sites exceeded the allowable limit.

Additionally, the BOD5 results of the examined samples exceeded the BOD5 standards set forth by various nations: 10 mg/L for all irrigation methods (EU directive); 8 mg/L for food crops (South Korea); 20 mg/L for all crops except those consumed raw (Italy); 60 mg/L for all crops except those consumed raw (France); and 10 mg/L for crops (Israel); 8 mg/L for food crops (South Korea) (European Commission 2018; National Green Tribunal Order, 2019).

Metal cation characterization

High levels of dissolved ions like sodium, calcium, and magnesium in irrigation water may have physical and chemical effects on crops, and agricultural land, and reduce production. These ions can build up and lower the osmotic pressure in the structural cells of the plant, which prevents water from getting to the branches and leaves. Taking this into account, the irrigation water samples of the current study were also analyzed for important metal cations (Na+, Ca2+, and Mg 2+), in addition to the physicochemical and biological water quality characteristics.

Sodium (Na+)

The hydraulic conductivity (permeability) of soil is impacted by a high sodium ion content in irrigation water, which leads to issues with water infiltration. This is because soil particles become dispersed when sodium, which is present in the soil in an exchangeable form, replaces calcium and magnesium and becomes adsorbed on soil clays; if calcium and magnesium are the predominant cations adsorbed on the soil exchange complex, the soil tends to be easily cultivated and has a permeable and granular structure. The sodium content in all water samples was below the 40 mg/L FAO threshold limit (Ayers & Westcot 1985), and there was no predicted sodium salinity and sodicity risk for crop and vegetable production. Except for the samples at S11, S12, and S13, the Na+ concentration of the examined samples was higher than the reported values (3.5–3.7 mg/L) (Densaw et al. 2016).

Sodium absorption ratio (SAR)

SAR is used to predict how much sodium will be absorbed by the soil. It accurately forecasts how much irrigation water will typically contribute to a cation-exchange reaction in the soil. A risk of sodium replacing adsorbed calcium and magnesium ions is implied by high levels of SAR, which could harm the soil’s structure and plant roots. High SAR irrigation water can cause high soil Na levels to accumulate over time, which can negatively impact soil permeability.

In the present study, SAR values fell between 0.996 and 14.32 (Table 5). SAR values at the 12 sites (1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13) were in the lower range (< 10), indicating excellent quality. SAR at S10 was found in the medium range (10–18) (USDA, 1954; Ayers and Westcot 1985), indicating fair quality with a modest alkali hazard and little risk of exchangeable salt levels rising to unhealthy levels when used as irrigation, especially for maize production. None of the samples had SAR in the high (18–26) or very high (> 26) range. In 1995, the SAR of Koga irrigation water was found to be 1.95 (MoWIE, 1995) while in Densaw et al. (2016) an SRA of 0.12–0.18 was reported.

Table 5 Koga irrigation water suitability analysis results based on SAR for the primary crops and vegetables cultivated in the study area (n = 13, sampling time: March 2022). The sodicity hazard (SAR) classification is adapted from USDA (1954) and Ayers and Westcot (1985)
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Kelley’s ratio (KR)

Kelley’s ratio greater than one indicates an over-abundance of sodium, which exceeds suitability for irrigation (Kelley 1940). All water samples of this study, except S2 and S10, had Kelley ratios < 1 (Table 6). Water samples at sites 2 and 10 had Kelley’s ratios of 2.0 and 4.2, respectively, indicating sodium concentrations are not appropriate for irrigating crops.

Table 6 The concentration of metal cations, sodium-calcium, and magnesium-calcium proportions, Kelley’s ratio (KR), and magnesium absorption ratio (MAR) of the analyzed Koga irrigation water samples (n = 13, sampling time: March 2022, statistical test: one-way ANOVA followed by Tukey’s post hoc multiple comparisons test at P = 0.05)
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Calcium (Ca2+) and Magnesium (Mg2+)

Water hardness is mostly caused by the concentrations of Ca and Mg, and the ideal ranges for each are 75–100 mg/L and 30–100 mg/L, respectively. The irrigation water’s calcium level was below the 20 mg/L tolerance level of the region’s main crops and vegetables.

The lowest desirable level of magnesium in irrigation water is 25.0 mg/L (FAO, 1985). In this study, magnesium was the dominating cation and a magnesium hazard was foreseen. Ratios of Mg2+:Ca2+greater than 4 will cause soil structural instability and impermeability (FAO, 1985). Results indicate that Koga water may affect soil structure and may impair the soil’s ability to absorb water since the Mg2+:Ca2+ ratios of all samples were greater than 28.74 (Table 6).

Ratios of Na:Ca greater than 3 will cause infiltration issues (FAO, 1985). Results indicate that Koga water may result in soil dispersion, crusting, plugging, and sealing of the surface pores since the Na:Ca ratios of all samples were higher than the threshold limit (Table 6). Utilization of the irrigation water at site 10 may cause the soil impermeable and non-granular due to the highest content of Na+ at this site which in turn results in the highest Na:Ca proportion (123.66:1). The Ca ions were very small quantities to counterbalance for the Na ion concentration, and this may have had linked implications, such as permeability issues. Eriksson (2012) reported calcium concentrations ranged from 0 to 110 mg/L while Densaw et al. (2016) reported a calcium level of 12–23 mg/L and magnesium level of 11–42.5 mg/L, indicating calcium showed a downward trend while magnesium rose upward. The same trend was observed in this study also since the calcium level ranged from nil to 3.99 mg/L while magnesium ranged from 55 to 81 mg/L.

Magnesium hazard (MH)

To determine whether there was a magnesium hazard or not, the magnesium absorption ratio (MAR) was determined. Accordingly, the studied water samples’ MAR ranged from 23.05% (S10) to 365.28% (S12) (Table 6). If the MAR is higher than 50%, a magnesium hazard will manifest and the water is hazardous and unfit for irrigation (Ayers & Westcost, 1985). When MAR is greater than 50%, plant growth can be negatively affected and lower yields occur (Paliwal 1972).

In this study, the MAR levels of the water samples were found to be extremely high (100.81–365.28%) and a considerable magnesium danger was anticipated for these samples. Consequently, the waters were deemed inappropriate for irrigation except the samples at S2 (MAR = 47.79%) and S10 (MAR = 23.05%) (Table 6).

According to the suitability indices (EC, SAR, KR, and MAR) for irrigation, Table 7 summarizes the appropriateness of the analyzed water samples in general.

Table 7 Classification of irrigation water based on EC, SAR, KR, and MAR indices of appropriateness for irrigation
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Diagrammatic illustration of Koga irrigation water suitability

The irrigation water in the research region is classified using the Wilcox diagram (Wilcox 1955), which is based on electrical conductivity (EC) and sodium adsorption ratio (SAR) (Fig. 4). The sodium hazard is represented by the SAR, while the salinity hazard is indicated by electrical conductivity. Two key factors in the classification of irrigation water are the sodium and salinity concerns. The soil will become saline and unproductive if irrigation water contains a lot of salt. Table 7 lists the various water classifications according to Wilcox’s classification. Accordingly, Koga’s irrigation water samples are grouped into three quality classes: C1S1 (low salinity and low sodium hazards), C2S1 (medium salinity and low sodium hazards), and C2S2 (medium salinity and medium sodium hazards). These classes correspond to “low saline suitable for irrigation” and “slightly saline suitable for irrigation” quality classes.

Fig. 4
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Wilcox’s diagram illustration to categorize the water samples under study based on SAR and EC to determine whether they were suitable for irrigation or not

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Correlation among parameters

The correlation result displayed in Table 8 is used to determine the relationships between the water quality metrics for the tested water samples.

Table 8 The physicochemical properties of the Koga irrigation water samples’ Pearson’s correlation matrix
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The correlation analysis’s results (Table 8) showed that turbidity and TSS, TA, TH, SO42− and PO43− had a strong and direct association. The correlation between pH and TA, TH, SO42−, and PO43− was very strong, whereas pH and EC had a strong and inverse correlation. As EC and TDS of water are correlated, in this study, samples with greater EC had higher TDS values than samples with lower EC. There is a weak and inverse correlation between Ca2+ and Mg2+. The correlation matrix shows that because Ca2+ concentration was found to be low, the effect of Ca2+ on water’s TH was minimal due to precipitation and irrigation effects; however, Mg2+ concentration was very high and shows a strong and positive association with TH and TA.

Conclusions and recommendations

Conclusions

  • The study showed that turbidity, TSS, TA, and TH of the Koga irrigation water exceeded the FAO’s recommended limit and did not meet international quality requirements.

  • The SO4−2, PO4−3, and BOD5 levels of 100% of the samples were higher than the FAO’s threshold limit and did not support the use of greywater for irrigation.

  • The levels of TDS, EC, pH, and NO3− were acceptable.

  • The coliform and fecal coliform counts exceeded the zero/100 mL recommended limits of WHO and EU set for irrigation waters.

  • The examined water samples were found to be unsuitable for the crops and vegetables grown in the area since TA, PO4−3, and NO3− levels surpassed the tolerance limit of the crops and vegetables cultivated in the site, and the coliform and BOD5 levels crossed their tolerable limits in vegetables grown in that area. Therefore, it is not recommended to eat the crops and vegetables cultivated in the study site without first properly preparing them.

  • All samples had Mg2+:Ca2+ ratios greater than 4; so, soil structure instability may occur, and the soil’s ability to absorb water may be hampered.

  • Koga water may result in soil dispersion, crusting, plugging, and sealing of the surface pores since the Na:Ca ratios of all samples were greater than the FAO’s permissible limit.

  • The greater sodium concentration may have an impact on maize productivity concerning the important irrigated crops in the research area.

  • Turbidity, TSS, TDS, pH, TA, SO42−, PO43−,NO3−, NH3,Na+, Mg 2+ showed trending upward compared to the results early reported.

  • Ca2+ showed a downward trend while EC, TH, and FCC didn’t show an upward or a downward trend over time.

  • In general, the majority of the water quality parameters of the studied water samples were not found within the recommended limits of FAO, WHO, EU, and irrigation water quality standards of different nations, even though additional samples evaluating seasonal fluctuations are needed to fully assess the water quality.

Recommendations

  • To come to firm findings, studies with seasonal fluctuations and additional samples are required.

  • The Koga irrigation project office should create and implement an appropriate legal framework with the assistance of the relevant organizations to control and avoid the pollution load on Koga irrigation water.

  • Human or animal excretes may be a source of FC. So, there should be educational outreach to residents to implement practices, reduce fecal pollution, and properly manage fertilizers, pesticides, and herbicides.

  • The increment in turbidity, TSS, and TDS may be due to the entrance of soil or dust particles and other wastes emanating from the agricultural activities of the local people. So, the local people and the Koga irrigation project office should conduct soil and water conservation activities so that the entrance of contaminants to the irrigation water can diminish.