Since safe and reliable drinking water is essential for economic vitality and public health [1
], goal 6.1 of the United Nations Millennium Development Goals to “achieve universal and equitable access to safe and affordable drinking water for all by 2030” has been included in the United Nations Inter-Agency and Expert Group on Sustainable Development Goals (SDG) Indicators. Progress is being made but verification is cumbersome due to a lack of data.
Investigations in developing countries on safe drinking water are mainly focused on the incidence of acute infectious diarrhea [1
] because it is a cause of death among young children. In addition, chemical agents have been associated with adverse health effects [2
]. One of these chemicals of concern is nitrate [6
]. Risks to human health associated with high levels of nitrate in drinking water include thyroid gland dysfunction, gastric cancer, and decrease in the capacity of blood to transport oxygen (known as methemoglobinemia) in infants below six months old [8
]. In addition, it poses health problems for pregnant women [11
]. Finally, excessive nitrates can cause health problems in ruminant animals and once released into the environment, can cause dead zones in the oceans near major rivers [12
Groundwater is preferred as a source of potable water because it is available throughout the year and is less contaminated than surface water [18
]. However, according to studies in both developed and developing countries, nitrate levels in groundwater have been increasing [19
] and can present serious problems [6
]. In the northeast of Spain, 80% of the groundwater nitrate concentration exceeded 5.6 mg/L due to use of nitrogen-based fertilizers, and animal and human wastes [27
]. In the North China plain, 50% of sampled wells exceeded the limit of 10 mg N-NO3
/L concentration due to application of untreated wastewater and excessive fertilizer on agricultural fields [7
]. Studies in developed countries such as UK found high nitrate concentrations related to intensive agriculture and high-density cattle and pig farms. Nitrate levels ranged from 4.5 to 11.3 mg N-NO3
/L in groundwater and were more than 22 mg N-NO3
/L in surface water in the winter [28
In developing countries, little is known about the impact of agriculture on water quality. Studies in Ghana reported nitrate concentrations reaching 9–10 mg/L in irrigation wells located close to intensively cultivated irrigated fields [31
]. Similar studies in India and parts of Africa indicated that 20–50% of the groundwater wells exceed the 10 mg/L nitrate level limit [32
]. A rapid assessment of drinking water quality in Ethiopia showed that 32% of the wells were contaminated with nitrate [33
]. A study on the Kebean and Akakie rivers of Addis Ababa, Ethiopia found poor quality surface and groundwater due to improper waste management, agricultural activities, poor sanitation, and low levels of hygiene [34
]. Research in the Ginchi watershed in southern Ethiopia found that there is pollution of surface water due to human and animal feces, agricultural activities, and a lack of waste disposal systems [35
]. Finally, groundwater quality monitoring results in the Ethiopian highlands agricultural watersheds indicated that nitrate levels are rising during the rainy season [36
Greater amounts of fertilizer are applied on farms because of both the loss of organic matter in the degrading soil base and the need to produce more food for a rapidly increasing population [37
]. Despite the greater application of fertilizers, the quality of surficial groundwater—which is used as the source of drinking water in the Ethiopia highlands—has not been well documented. The objective of this study is, therefore, to assess nitrate concentrations in the surficial groundwater. The South Gondar Zone in northern Ethiopia was selected as our study area because of widespread use of groundwater as a water supply, intensifying agriculture with increasing fertilizer use, and plenty of rainfall to leach the nitrate to the groundwater.
Nitrate concentrations were assessed in groundwater and the distribution of these concentrations was mapped in four woredas of South Gondar Province in the north–central part of the Ethiopian highlands. The results show that out of the total 213 randomly selected water points, consisting of springs and wells, only three wells had nitrate concentrations that were above the WHO permissible health standard. Nitrate concentrations in well water was almost twice those in spring water. As expected, nitrate concentrations in agricultural land, located mainly at the midslope position of the landscape, had the greatest concentration of groundwater, averaging 3.6 mg N-NO3/L. Upslope forest and shrub land had the lowest concentrations.
The mapped spatial distribution of nitrate concentrations shows that the lowest nitrate concentrations were found both in the highly productive Fogera Plain near lake Tana due to denitrification, and in the mountainous and steep areas in the upper parts of the watersheds without agriculture. Intermediate and high nitrate concentrations were found in the central portions of the study area with intermediate slopes and where agriculture was practiced. Since overall fertilizer use is still modest for most crops, with average application rates varying from 22 to 35 kg of N/ha, and less than can be taken up by the crop, most nitrate concentrations in groundwater for the major crops are below the WHO standard of 10 mg N-NO3/L. Two of the three wells with high nitrate concentrations were observed near potato fields with high levels of fertilizer. We expect that nitrate concentrations in drinking water from wells and springs will rise when fertilizer use increases. This will likely be more severe than in temperate climates because of the high rainfall over a four-month period in the (semi)humid climates; this rainfall promotes fast leaching of the nitrate before it can be taken up by the crop.