Anthropogenic nitrate contamination of water resources in Ethiopia: an overview Open Access

Nitrate is a chemical compound composed of one nitrogen and three oxygen atoms in combination. It occurs naturally in moderate amounts in soil and water (surface water and groundwater) (Wakida & Lerner 2005). The other types of nitrogen found in the environment include nitrite and ammonia. On the other hand, the organic nitrogen fertilizers used in agriculture include animal manure, human wastes, composts, sewage sludge, legume crops and green manure crops (Zhou et al. 2015).

Nitrate is an essential component of the nitrogen cycle. Other nitrate sources include ammonia chemical oxidation, microbial nitrate synthesis in soil, and fossil fuel burning (Zhou et al. 2015). Nitrate is frequently employed in chemical fertilizers and as an oxidizing agent in explosive manufacturing. Purified potassium nitrate, in particular, is used to manufacture glass. In cured meats, sodium nitrite is used as a preservative. Furthermore, nitrate is occasionally employed as a nitrite reservoir (Monarca et al. 2004). Such diverse applications of nitrate are the causes of nitrate release into the environment, and it has become one of the major water contaminants in the world.

The anthropogenic activities responsible for increasing nitrate concentration in water include animal and human waste, open septic or sewage systems failure, fertilizer application in agricultural activities, concentrated animal farming, industrial waste and leachates from solid waste landfills (Manassaram et al. 2006). These activities are classified as the point or non-point sources of nitrate contamination of water. Generally, nitrate contamination of water resources involves the mass transfer of nitrate from the anthropogenic point to water. There are two mechanisms of nitrate transport from the soil into the water resources. These are leaching and surface runoff (Wang et al. 2015). Nitrate reaches the unsaturated zone in the deeper layers, joining both aquifers (shallow and deep groundwater) (Wang & Li 2019). Surface runoff is another mechanism by which nitrate from the top-soil is carried into surface water resources by water flow. These two mechanisms increase the rate of nitrate pollution of water. Nitrate contamination of water resources varies with place and season. In highly populated areas with high agricultural activities, the nitrate contamination problem is expected to be more concerning. On the other hand, in seasons with high rainfall and surface run-off, fertilizer applied to the land can be washed away and enter into the water. As a result, nitrate contamination becomes evident.

Water contamination with nitrate is a growing problem because of the high population growth rate (Mekonnen et al. 2014). This problem is becoming a substantial environmental burden. As a result, water quality is being degraded with time. In many industrialized countries, nitrate could be one of the factors for a gradual decline in drinking water quality during the last three or four decades. Since the mid-1950s, nitrate in groundwater and surface water has increased in many countries (Monarca et al. 2004). Thus, the problem is currently well recognized, and attention has been given globally. In particular, most developed countries, such as the USA, Spain, Canada, Britain, and France, have adopted strict control strategies to remove and even prevent the water from being polluted by nitrate. Water treatment methods such as ion exchange, reverse osmosis, and biological denitrification are technologies employed in developed countries to remove nitrate from water. On top of that, blending nitrate polluted water with unpolluted water is a conventional low-cost approach to minimize the nitrate contamination level through dilution (Zhou et al. 2015).

No or little attention is given to nitrate pollution by developing countries. Nitrate contamination of ground and surface water is a problem commonly occurring in Africa due to the poor management of domestic and industrial wastewater (Ouedraogo & Vanclooster 2016). Some studies in Africa and India have reported that 20–50% of the groundwater exceeds the maximum acceptable limit of nitrate level set by the World Health Organization (WHO) (50 mg NO₃/L) (Rao 2006; Reddy et al. 2009; Dar et al. 2010; Dan-Hassan et al. 2012; Pantaleo et al. 2018; Eblin et al. 2019; Egbi et al. 2020). Nitrate contamination of ground and surface water can be considered the prevalence of drinking water pollution. In most cases, groundwater is used as the source of drinking water with little or no treatment depending upon the economic condition of the countries. The high cost of nitrate removal from the water makes the problem more complicated for the developing world. Therefore, nitrate remediation measures such as prevention and source treatment need to be considered for the safety of humans and other organisms. In Ethiopia, several studies (Abreha 2014; Mekonnen et al. 2014; Misganaw 2015; Woldemariyam & Ayenew 2016; Teklu et al. 2018) conducted in different regions indicated that nitrate in the water exceeded the maximum acceptable WHO standard with varying intensities.

Nitrate level in water that exceeds the WHO maximum allowed limit has a negative impact on humans and the environment (Monarca et al. 2004; Ahada & Suthar 2018; Qasemi et al. 2018). The health impact of nitrate ‘methemoglobinemia’, commonly called a blue-baby syndrome, is the most widely reported health problem in infants. This is a well-known health effect of nitrate on infants. Moreover, it affects adults’ health by causing gastric cancer, respiratory trouble, headache, fatigue, thyroid gland hypertrophy, and multiple sclerosis (Ahada & Suthar 2018). Besides, the high nitrate level in water also causes eutrophication. Eutrophication affects the life in the water body due to oxygen shortage. Also, animals' health conditions will be affected by ingesting plants or taking water with high nitrate content (Bhatnagar & Sillanpää 2011).

This overview work is aimed to investigate the current status of nitrate contamination of water resources in Ethiopia. Moreover, it also aims to assess the sources of nitrate and its transport mechanism in the environment to communicate with water experts, decision-makers, water scientists, and the general public. An overview of the current state of nitrate pollution of water resources in Ethiopia is provided by reviewing and analyzing the available data from published sources and responsible authorities. This work presents foundational information on the current status of nitrate pollution of water in Ethiopia for further remedial actions. In the meantime, it creates public and institutional awareness regarding the effectiveness of nitrogen management policies. Above all, the information generated in this work would be valuable for national water resource management.

Nitrate is very soluble, colourless, odourless, and tasteless in water. Due to these properties, it is difficult to visually identify whether the water contains nitrate or not. Nitrate in the aquatic environment results from various activities, including natural (soil nitrification and atmospheric deposition) and anthropogenic activities (Vystavna et al. 2017).

Anthropogenic activities are the leading causes of nitrate rise in surface and groundwater. These include agricultural activities (inorganic fertilizers and manure), wastewater treatment plants, human nitrogenous waste products, and discharges from industrial processes. Agriculture is the main economic source in Ethiopia which may lead to the contamination of water resources. On the other hand, the existing sanitation practices in Ethiopia (open defecation and poor or no urban wastewater treatment facilities) are another possible reason for contaminating water resources with nitrate. These man-made activities easily disseminate nitrate to the water environment. As a result, lakes, rivers, streams, and groundwater are being contaminated due to these human activities. Naturally, groundwater contains nitrate up to 3 ppm. However, for nitrate levels exceeding 3 ppm, anthropogenic activities would be responsible for the increase (Mekonnen et al. 2014).

Natural land degradation phenomena such as soil erosion and mineral depletion, first induced by anthropogenic activities of resource exploitation, result in hydrological impacts such as surface run-off and low infiltration. On the other hand, the natural nitrification process (decaying plant or animal material) carried out by microorganisms (WHO 2003) are another source where nitrate evolves. Some percentage comes from the plant metabolism process of fixation. Also, nitrate can be released from geologic materials containing soluble nitrogenous compounds (Peechattukudy & Dhoble 2017). Consequently, the surface water becomes loaded with such nutrients and gets contaminated (Zinabu et al. 2018). Mengistu et al. (2019) reported high microbiological and nitrate concentrations in shallow unconfined aquifers around major metropolitan cities in Ethiopia. Moreover, some geological formations also contain nitrate ions that increase nitrate in groundwater in such locations. The nitrate ions in water vary with location, with some exceeding the WHO drinking water standards (Peechattukudy & Dhoble 2017).

The Chemical Abstracts Service numbers for nitrate and nitrite are 14797-55-8 and 14797-68-0, and their molecular weights are 62.00 and 46.01, respectively. Other properties are shown in Table 1 (WHO 2003).

The excess amounts of nitrate and nitrite compounds in the nitrogen cycle (beyond plant uptake capacity) would cause nitrate pollution in water which then causes health problems in humans and eutrophication of water bodies (Wakida & Lerner 2005). Thus, a proper understanding of the nitrogen cycle and nitrate transport route is crucial for the accounting of these contaminants. Moreover, the nitrogen cycle is of particular interest to ecologists as nitrogen availability can affect the rate of key ecosystem processes such as primary production and decomposition. Anthropogenic activities have dramatically altered the natural global nitrogen cycle. These activities have changed the global nitrogen cycle, which can disturb the natural ecological balance (Galloway et al. 2004; Reis et al. 2016; Kuypers et al. 2018). Reactive nitrogen can be naturally formed through primary processes such as nitrogen fixation, ammonification, denitrification and nitrification (Zhou et al. 2015). The various forms of nitrogen in the soil and the corresponding conversion processes are shown in Table 2.

Generally, nitrate has point and non-point sources. Point sources of nitrate are sources that can be identified (traced). These include accidental spills of nitrogen-rich compounds, absence of slurry storage facilities, manure tanks in rural areas, concentrated livestock confinement, leaky septic or sewer systems, industrial and municipal wastewater effluents (Rahm et al. 2016). Water sources such as uncovered wells, boreholes, poorly constructed wells and sinkholes can be exposed to the point source contamination of nitrate. The nitrate level in groundwater rises due to the introduction of nitrate from these point sources to the surface and groundwater. Moreover, inadequate domestic wastewater management systems may allow nitrate formation through oxidation under aerobic conditions and nitrate leaching into the groundwater (Zhou et al. 2015).

The non-point source is termed a diffuse source of nitrate; that is difficult to exactly identify the point of pollution. The pollution extent of the non-point source is greater than that of the point source contamination of water with nitrate. The extensive application of synthetic and organic fertilizers in agricultural activities is one of the most common anthropogenic non-point sources of nitrate (Mcnickle 2019). Plants cannot use the excess fertilizer's entire nitrate; consequently, nitrate accumulates in the soil and becomes an available source for leaching into groundwater (Zhou et al. 2015; Ahada & Suthar 2018). Therefore, the magnitude of groundwater contamination depends on the amount of fertilizer applied (Zhou et al. 2015). Using an irrigation system can exacerbate water pollution due to nitrate leaching phenomena (Burkart & Stoner 2007).

Nitrate has a high solubility in water and low retention in soil. Thus, it is prone to be leached; because the negatively-charged nitrate anion is repelled by negatively charged surfaces of clay minerals and soil organic matter. This keeps nitrate dissolved in the soil solution and moves freely in the soil through percolating rainfall or irrigation (Padilla et al. 2018). An excess amount of nitrate above plant utilization capacity will readily be denitrified to N₂O and N₂, and it will then easily leach to the sub-soil layer and then finally to the groundwater (Ahada & Suthar 2018). The two nitrate transport mechanisms from the soil into the water resources (groundwater and surface water) are leaching and surface runoff. Nitrate moves through the unsaturated zone into the deeper horizons, and ultimately reaches both aquifers (shallow and deep groundwater). Surface runoff is another way in which nitrate from the top-soil is washed by the water flow to surface water. These two processes reduce water quality by increasing the risk of water contamination with nitrates (Wang & Li 2019). Moreover, microorganisms and plants are the main factors in the nitrogen cycle, contributing to the overall nitrogen transport in the environment in different forms.

Consequently, nitrogen forms such as ammonia, nitrate, nitrite and nitrogen gas are compounds of nitrogen present with spatial variations in the ecosystem (Wakida & Lerner 2005). The nitrate concentration spatial variation may be due to the difference in groundwater gradient (Buvaneshwari et al. 2017) and the difference in human activities from place to place. Nitrate transport is controlled by the physical, chemical, and biological processes in groundwater with various interacting processes, advection, dispersion, and chemical reactions that influence the contaminants' movement and fate (Abate 2010).

The health impact of nitrate on humans is well-known. As a result, many countries and organizations have set the maximum permissible concentration limit for nitrate in water to safeguard humans and the environment. Ethiopia has adopted WHO standards for monitoring nitrate levels in drinking water. Nitrate regulation standards are presented in Table 3 below.

Groundwater is a widely used drinking water source due to its higher quality. Groundwater is preferred as a drinking water source because it is available throughout the year and is less vulnerable to contamination than surface water. Thus, it is a vital resource with high economic value and social importance. Groundwater supplies almost half of all drinking water globally. It plays a crucial role in food production, accounting for over 40% of global agricultural irrigation (Zhou et al. 2015). Moreover, it is the primary water source for domestic, agricultural, and industrial purposes in many countries of the world (Chen et al. 2016). Naturally, groundwater contains a nitrate level of less than 3 mg N/L (nitrate-N). An extensive study by Madison & Brunett (1984) concluded that nitrate concentration exceeding 3 mg N/L indicates human inputs (anthropogenic activities). However, a recent analysis suggested that 2.0 mg N/L is a probable threshold for background concentration of NO₃⁻ (Burkart & Stoner 2007). However, the background concentration varies with region and soil characteristics. The higher concentration of nitrate in groundwater might be associated with animal and human waste, open septic or sewage systems, fertilizer application, concentrated animal farming, industrial waste and leaches from solid waste landfills (Manassaram et al. 2006; Asrat 2014; Romanelli et al. 2020).

Nitrate is a readily water-soluble ion that cannot bind to the soil. Thus, it can easily reach the groundwater and become a widespread groundwater contaminant imposing a major threat to drinking water supplies and promoting eutrophication (Bhatnagar & Sillanpää 2011). Globally, nitrate levels in the groundwater of the world have been annually increasing from 1 to 3 N-NO₃ mg/L. Several studies in different parts of the world showed that groundwater contamination with nitrate is a serious problem (Strebel et al. 1989; Zhang et al. 1996; Kumazawa 2002; Nakagawa et al. 2016). Previous studies have also shown that nitrate level in groundwater increases with time, both in developing and developed countries. For example, the excessive use of nitrogen-based fertilizer, animal and human waste in Spain has resulted in high nitrate concentration, with 80% of the groundwater exceeding 5.6 mg/L. Due to a similar reason, 50% of sampled wells exceed the limit of 10 mg N-NO₃ in North China. In the UK, nitrate ranging from 4.5 to 11.3 mg N-NO₃/L in groundwater had been associated with intensive agriculture and pig farms. Similar studies in India and Africa have reported that 20–50% of the groundwater wells exceed the 10 mg NO₃/L nitrate level limit. A rapid assessment of drinking water quality in Ethiopia has shown that 32% of the wells were contaminated with nitrate. Notably, the Ethiopian highlands agricultural watersheds' water quality monitoring has revealed that nitrate level is rising during the rainy season (Akale et al. 2018). However, little is known about the relationship between water quality and agricultural activities in developing countries.

Surface water and groundwater are the principal components of the Earth's water bodies where life began and the issues of quality and quantity have become more sensitive in a global context (Bawoke & Anteneh 2020). Surface waters such as lakes and rivers can be contaminated with nitrate. The surface waters are predominantly polluted by nitrate from surface run-off, sub-surface flow (Monarca et al. 2004), and nitrate leaching from agricultural farms using chemical fertilizers and animal manure (Zinabu et al. 2018). Intensive farming activities using excess fertilizer beyond the uptake capacity of plants can lead to the pollution of surface water with nitrate. The rate and direction of nitrate movement in soils is roughly related to the nitrate concentration in soil solution (Keeney & Olson 1986). The nitrate concentration in the surface water is naturally low (up to ∼4 mg/L as nitrate-nitrogen) (Abera et al. 2018) but can reach high levels due to anthropogenic activities. Higher nitrate concentration of surface water is reported in densely populated cities in sub-Saharan countries (Mengistu et al. 2019). A nitrate concentration of 9–10 mg NO₃/L was reported in the intensively irrigated fields in Ghana.

The concentration often fluctuates with the season and may increase when nitrate-rich aquifers feed the river. In winter, the nitrate level becomes 22 mg N-NO₃/L in surface water (Akale et al. 2018). Nitrate concentration in surface water has gradually increased in many European countries in the last few decades and, in some cases, doubled over the past 20 years. For example, in the United Kingdom, an average annual increase in nitrate concentration of 0.7 mg/L has been observed in some rivers (WHO 2003). In Ethiopia, water resources such as the Legedadi water supply, Geffersa water supply, Dire water supply and Sibilu water reservoirs are highly contaminated with nitrate, phosphate and ammonia due to anthropogenic activities. In the past 10 to 15 years, fertilizer use per hectare for agricultural irrigation in the Central Rift Valley (CRV) of Ethiopia has significantly increased and caused a nitrate water contamination problem (Abera et al. 2018). Previous studies in Ethiopia have reported nitrate levels in surface waters (river and lake), ranging from 0.06 to 296 mg/L (Misganaw 2015; Yasin et al. 2015). The nitrate concentration in surface water varies with the season and may increase when the river is fed by nitrate-rich aquifers (WHO 2003).

Groundwater and surface water are important sources of drinking water. These water sources can be used with or without treatment depending upon their quality and economic conditions for treatment. Primarily, groundwater can be used directly due to its good quality in most cases. Drinking water resource is the most valuable resource for the health of human beings. The occurrence of high concentrations of NO₃⁻ in drinking water sources has become a serious concern worldwide over recent decades (Bhatnagar & Sillanpää 2011). Nitrate contamination of groundwater and surface water in Ethiopia is mainly due to improper waste management, agricultural activities and poor sanitation (Akale et al. 2018). The maximum permissible nitrate level in public drinking water in the United States is 10 mg/L as nitrate-nitrogen (Nitrate-N). This standard is approximately equivalent to the WHO guideline of 50 mg/L as NO₃ or 11.3 mg/L NO₃-N (multiply NO₃ mg/L by 0.2258). Drinking water contamination by nitrate is a major health concern because nitrate is reduced to nitrite and nitrosation of nitrites can form N-nitroso compounds which are potent carcinogens (Tadesse et al. 1981).

Ethiopia is located in the Horn of Africa between 3°N and 15°N of the equator and 33° E and 48° E longitudes. Eritrea surrounds it to the north and northeast, Djibouti and Somalia to the east, Sudan and South Sudan to the west and Kenya to the south. Ethiopia's high central plateau varies from 1,290 to 3,000 m above sea level, with the highest mountain reaching 4,533 m (Ras Dashen). The total area of the country is about 1.104 million km². Currently, there are 10 administrative regions and two administrative states (Addis Ababa and Dire Dawa city). These regions are Afar, Sidama, Amhara, Benishangul-Gumuz, Gambela, Harari, Oromiya, Somali, Tigray and the Southern Nations Nationalities and Peoples Region (SNNPR) and two administrative councils (Addis Ababa and Dire Dawa).

In this study, published data was gathered using scientific databases (Google Scholar, PubMed and Science Direct) for most recent publications (2015–2022). Keywords and phrases used during the review include nitrate, nitrogen cycle, health impact, groundwater, surface water, nitrate pollution, nitrate forms, physicochemical properties of nitrate, nitrate contamination in Ethiopia, and standards for nitrate in drinking water to access publications on nitrate contamination of water. Moreover, we also searched for internal publications from the WHO and US. EPA using the organizations' official websites. Based on the literature review, 39 publications with nitrate reports for water resources of Ethiopia were found and primary data from the Addis Ababa Water and Sewerage Authority (AAWSA) were also used. Moreover, nitrate data for 108 wells in Addis Ababa was obtained from AAWSA. All the data gathered from the literature and AAWSA were combined, analyzed and compared against WHO standard.

About 80% of the total national water supply in Ethiopia is covered by water drawn from the aquifer (Asfaw & Mengistu 2020). The spring and well water are the major water supply sources used in urban and rural areas (Mekonnen et al. 2014). Several studies have been conducted on the general physico-chemical analysis of water resources (lakes, rivers, groundwater, wastewater and drinking water) in Ethiopia. However, no or few works have focused on nitrate contamination of water resources in Ethiopia based on our search on the scientific databases such as Google Scholar, PubMed, and Science Direct to the best of our knowledge. However, Mekonnen et al. (2014) conducted a retrospective study on the spatial distribution of nitrate in drinking water sources based on the data obtained from the Ethiopian Public Health Institute (EPHI). They reported nitrate concentrations ranging from 0.09 mg/L to 409 mg/L (as a maximum value) for drinking water sources such as wells, springs and tap water. Based on their findings, 15.3% (n = 186), 10% (n = 33) and 12.4% (n = 21) for well, springs and tap water respectively exceeds the concentration of 20 mg/L at a national level. Also, 5.7% (n = 70) and 2.7% (n = 9) of spring water samples had nitrate concentrations higher than the WHO standard for drinking water (50 mg/L) (Mekonnen et al. 2014). These authors found the highest average nitrate concentration value (104.8 mg/L) exceeding the WHO standard in Dire Dawa well-water. On the other hand, nitrate levels in well-water exceeding 20 mg NO₃/L were found in the Ethiopian Somali regional state (37.0 mg NO₃/L), Afar (34.9 mg NO₃/L), Harari (26.3 mg NO₃/L) and Addis Ababa Regions (20.5 mg NO₃/L). However, the nitrate level in all the spring water was below 20 mg NO₃/L except Dire Dawa and Harari regions (Mekonnen et al. 2014).

Abera et al. (2018) indicated nitrate leaching phenomena from onion farm activities by considering different chemical fertilizer loads per hectare for different irrigation water quantities. The previous reports found a higher nitrate level in water bodies in rift valley areas and cities like Addis Ababa and Dire Dawa (Sahele et al. 2018). Nitrate concentration ranging from 43.78 to 407.85 mg NO₃/L was found in Mekelle (Tigray) (Abreha 2014). Despite the clear indication of nitrate contamination of water resources in Ethiopia, organized measures have not been implemented yet. Moreover, Abera et al. (2018) indicated nitrate leaching from the furrow irrigation on the onion farm by farmers practicing higher fertilizer applications per hectare (368 kg/hectare). However, urban non-point source nitrogen transported into rivers is becoming a new concern due to its critical role in eutrophication and water quality deterioration in highly urbanized regions (Guo et al. 2021). In areas where a higher concentration of nitrate is found, proper management action should be taken to protect the water from further contamination.

The gathered nitrate data from all sources varied from below the detection limit to the maximum of 407.85 mg NO₃/L throughout the country. In this study, at least one nitrate data has been found for all Ethiopia regions, including Addis Ababa, Dire Dawa, and the central rift valley of Ethiopia. The discussion is based on the available data for each region of Ethiopia. As reported in the literature, 20 mg NO₃/L of nitrate is considered the threshold value of nitrate in water resources (Mekonnen et al. 2014). However, recent studies have reported groundwater's natural nitrate content as less than 3 mg NO₃/L nitrate (Burkart & Stoner 2007). Based on this report, nitrate above 3 mg NO₃/L indicates water contamination by anthropogenic activities.

Data obtained from the published articles and unpublished materials are assigned under groundwater, surface water, and drinking water. The total number of nitrate data gathered is 147 (published data (39) plus unpublished data (108)). Among the 147 reported nitrate data, 29 data exceed the WHO standard, as presented in Table 4 below. The number of data obtained from AAWSA and published papers exceeding the WHO for drinking water is 7 and 22, respectively. At this concentration, nitrate is known to cause several health impacts on humans and the environment. According to this overview work, groundwater is more contaminated than other water resources, as shown in Table 4. The reason might be nitrate leaching from the point and non-point sources. Addis Ababa and Dire Dawa are the areas where a higher number of nitrate data is reported. The number of previous publications containing nitrate reports in decreasing order is: Oromia (9) > Amhara (6) > Southern region (5) > Tigray (4) > Dire Dawa (4) > Addis Ababa (3) > Afar (3) > Benishangul-Gumuz (1) > Harari (1)> Somali (1) > Gambela (1) as indicated in Table 4. Based on the published reports, Addis Ababa and Dire Dawa have a higher number of data (3) on nitrate levels in water exceeding the WHO standard. The southern regions, Amhara and Oromia, has an equal number of data (2) exceeding the WHO standard, whereas Tigray, Benishangul-Gumuz, CRV, Afar, Somali and Harari have an equal number of data higher than the maximum acceptable level, as presented in Table 4. However, no data (published reports) has been found for Gambela with nitrate values above the maximum permissible limit (50 mg NO₃/L). On the other hand, the maximum value of nitrate found in the surface water is 296 mg NO₃/L (Lake Ziway). Drinking water containing a nitrate concentration of 294.5 mg NO₃/L is found in the Afar region (spring water). The highest reported nitrate level in groundwater is 409 mg/L in the Benishangul-Gumuz region (well-water). Groundwater has been found to have the maximum nitrate concentration (409 mg NO₃/L) and a higher number of reported data, as shown in Table 4. The number of reports containing nitrate data exceeding the WHO standards for groundwater, surface water and drinking water is 12, 6 and 4, respectively.

The total number of previous studies containing values exceeding the WHO standard is 15, of which 22 data of nitrate level is higher than the WHO standard (50 mg/L). All the regions or areas studied (except Gambela) contain at least one nitrate datum that exceed the WHO standard for drinking water. These reports indicate the occurrence of nitrate contamination in water resources in Ethiopia. Currently, the intensity of nitrate contamination of water resources in Ethiopia may not be severe. However, more data are required for a comprehensive analysis of nitrate contamination of water in Ethiopia. On the other hand, only a few studies focusing on nitrate contamination of water resources are available. Among the studied areas, Addis Ababa and Dire Dawa city have relatively higher intensities of nitrate contamination. In both areas, higher numbers of reports have indicated nitrate values exceeding the WHO standard. This may be due to the higher urbanization rate, poor septic systems and age of cities that resulted in the leaching of nitrate from human activities to the water resource.

Figure 2 depicted the distribution of nitrate in wells (n = 108) in Addis Ababa based on the data obtained from the Addis Ababa Water and Sewerage Authority (AAWSA). In Addis Ababa, areas between Arada, Kirkos, Lideta and Addis Ketema, have higher nitrate (≥50 mg/L) in the wells. Among these data, seven exceeded the WHO and the national standards for nitrate in drinking water, as presented in Table 5. Therefore, well-water contaminated with nitrate should not be used and further nitrate analysis should be conducted to identify the root cause for remediation.

Dire Dawa is one of the fast-growing cities in Ethiopia. Despite its fast growth rate as an industrial and commercial center, the lack of proper sewers and other waste disposal facilities favours geological, morphological, climatological conditions and facilitates the hydrogeological system (Alemayehu 1999). According to the world population review, the metro area population of Dire Dawa in 2020 was 408,096, a growth rate of 4.39%. However, in 2019, it was 390,924 with a growth rate of 4.37%. In 2018, it was 374,553, with a population growth rate of 4.36%. This data shows an increase in the population growth rate yearly. This fast population growth has resulted in more significant groundwater pollution due to increased anthropogenic activities (Sahele et al. 2018). In semi-arid parts of Ethiopia, 6% of the samples (n = 6) showed nitrate concentrations higher than the acceptable limit (50 mg NO₃/L) (Woldemariyam & Ayenew 2016). Water supply that conforms to the increasing demand is a challenge for the city.

Another prominent challenge is the contamination of groundwater with nitrate. In this regard, previous studies on assessing groundwater vulnerability to nitrate contamination by Tilahun & Merkel (2010) indicated vulnerable zones in the Dire Dawa groundwater basin. Their work showed that some boreholes in the Sabian well field (Dire Dawa area) had nitrate levels above the WHO standards. In contrast, boreholes in the western part had no nitrate in the water. Based on this finding, it is possible to conclude that nitrate contamination showed a spatial difference. This might be due to the differences in anthropogenic activities from location to location. Urban areas generate a large amount of wastewater daily, and some industrial areas release industrial effluents that take a large share of nitrate sources. Moreover, intensive agricultural area uses more nitrogen fertiliser, which contributes to higher nitrate in the nearby water resources. Generally, a high nitrate level in the water resources of Dire Dawa has been reported by many researchers compared to other cities in Ethiopia, with Addis Ababa being an exception (Mekonnen et al. 2014; Sahele et al. 2018). A maximum nitrate level of 104.8 mg/L was reported in the well-water of Dire Dawa city. This is far beyond the WHO standards of 50 mg NO₃/L. Moreover, Woldemariyam & Ayenew (2016) have found a higher nitrate concentration than the minimum permissible health safety level of 50 mg NO₃/L in the Dawa River, southern Ethiopia. In the Dawa River basin, nitrate concentrations vary below the detection limit (BDL) to 433 mg/L of NO₃⁻.

Nowadays, nitrate contamination in groundwater has become a concerning environmental problem due to its growing trend and negative health consequence. Nitrate (NO₃⁻) pollution of well-water is a significant problem in Dire Dawa city. Many water-wells showed more than a two-fold increase in two decades (Sahele et al. 2018). The nitrate level is not expected to decrease with time unless some preventive strategies are implemented. Instead, it increases with time due to the increasing anthropogenic activities with population growth. However, in some areas, no nitrate is reported in the water, as in the report of Tilahun & Merkel (2010), from their spatial studies on nitrate contamination. This suggests the presence of lower human activities in those areas. However, this should not be the reason for not considering control or remediation strategies for nitrate contamination of water resources.

This overview has assessed Ethiopia's anthropogenic nitrate contamination of water resources based on the available published data and data from the responsible authorities (AAWSA). Nitrate data exceeding the WHO standard limit have been found in water resources (groundwater and surface water) of almost all Ethiopian regions with varying intensities. Among the studied areas, relatively higher nitrate contamination intensities were found in Addis Ababa and Dire Dawa. Water resources contaminated with nitrate could be potentially dangerous for humans and animals. Nitrate contamination of water is a growing problem and increasing from time to time due to the increasing human activity with population growth. The information generated in this overview might be an early warning for the responsible authorities, scientists and the community to take judicious measures to stop further stress on water resources. Besides the health impacts of nitrate on humans, nitrate treatment is very expensive due to a series of physical, chemical and biological treatment processes required to reduce nitrate concentration in water.

Nationally, Ethiopia has no organized practice for investigating and monitoring nitrate. Centralized nitrate monitoring approaches need to be implemented as this problem is very challenging if not solved at its initial stage. Different methods, such as preventive and treatment approaches, can halt this problem. Private wells should be checked twice a year for nitrate contamination of the water and a health risk assessment should be done for nitrate in the drinking water. The responsible authorities need to conduct organized nitrate analysis for water resource management. Despite the low current intensity of nitrate in some areas of Ethiopia, the nitrate contamination problem will increase in the future with anthropogenic activities. Future research works need to consider the data of populations, implementation of sewage treatment plants and amount of fertilizer applied in each region for a comprehensive analysis of nitrate contamination of water in Ethiopia. Moreover, further research on low-cost nitrate removal options is required.

The authors appreciate the Africa Center of Excellence for Water Management, Addis Ababa University, Ethiopia, for its financial support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.