1 Introduction
Globally, the continuous increase of freshwater demand led to more attention for nonconventional water resources as groundwater which play an essential role in water supply. In Egypt, the main constraint of future agriculture is the limited availability of renewable freshwater. Nile River is the main water resource; it is no longer sufficient for the increasing water requirements for the different developmental activities, so the role of groundwater is steadily increasing and is expected to cover about 20% of the total water supply in the upcoming decades especially in the newly reclaimed areas along the desert fringes of the Nile Delta and Valley [1]. There are six groundwater aquifers in Egypt, and the Nile aquifer represents 87% of the total groundwater pumping in Egypt. However, the groundwater aquifer of the Nile Delta is not considered as an additional or separate water resource from the Nile because it is directly connected to its river channels [2]. Aquifers generally are refilled by effective rainfall, lakes and rivers. This water may reach aquifers rapidly via macro-pores or fissures or slowly through soil infiltration and permeable rocks [3]. The direct seepage of Nile water from drainage and irrigation systems and, moreover, irrigated and cultivated lands are the main sources of Nile Delta aquifer recharge [4–6].
There is a hydraulic connection between the groundwater aquifer and the Nile Delta branches especially in the Middle and Southern parts of the Nile Delta due to the thinner and irregular clay layer [7]. The results of SF6 tracer study of groundwater in reclamation areas in South-west of the Nile Delta indicated that the wells that are located nearest to the artificial canals of irrigation (El-Rayah El-Behery and El-Rayah El-Nasiry) have a young age of between 0.5 and 12 years, which means that the wells close to the surface water are directly recharged by water derived from the river. With increasing the space from the surface water, the age of groundwater increases. This may be clarified by mixing between old water that is present in the aquifers and the infiltrated young water from the surface resources [8]. Drainage practices, soil type and irrigation method have a direct influence on the amount of water recharging the aquifer [9]. The Nile Delta aquifer thickness decreases from more than 900 m at the Mediterranean Sea to about 200 m in the South, near Cairo. The base is a clay aquiclude with an average slope of about 4 m/km [10].
2 Contribution of Groundwater in Agriculture
Groundwater is an essential source of freshwater. It could significantly contribute to agriculture through direct abstraction to use in irrigation mainly or supplemental irrigation. Also, groundwater could be used in irrigation via indirect way through control drainage system, so water table depth will decrease and, thus, contribute to crop water requirement.
2.1 Abstraction of Groundwater to Irrigate Newly Reclaimed Land
In arid and semiarid regions, coastal aquifers represent an essential source of freshwater. In these regions, groundwater resources are overexploited to meet the development and urbanization of coastal regions [6]. In Egypt, to face the challenge of high population density and constant freshwater resource from Nile River, Egypt adopted many policies to establish new agricultural communities outside the overpopulated Nile Delta and Nile Valley. These newly reclaimed areas depend almost exclusively on groundwater as water resource [8]. The growth of irrigation activities, urbanization and industrialization lead to rapid growth of groundwater abstraction. In most of the newly reclaimed areas in the Western Nile Delta region, groundwater is the main source for agriculture, industrial and domestic uses, which affect the quality of groundwater [1, 11].
Characteristics of the groundwater aquifers in the Nile Delta and its fringes [12]
Aquifer | Age | Rock type | Extent | Location | Transmissivity (m2/day) |
|---|---|---|---|---|---|
Mediterranean coastal aquifer | Pleistocene | Granular | Local | West of Alexandria | 500–1,500 |
Nile Delta aquifer | Pleistocene | Granular | Extensive | Nile Delta | 500–5,000 |
Northwest Sinai aquifer | Pleistocene | Granular | Extensive | Western Sinai | <500 |
Wadi El-Natrun aquifer | Pliocene | Granular/clay | Local | Western Delta fringe | <500 |
Moghra aquifer | Miocene | Granular | Extensive | Northwest Desert | 500–5,000 |
Cairo-Suez aquifer | Plio-Pleistocene/Miocene/Oligocene | Granular | Extensive | Cairo-Suez district | <500 |
Abu Zaabal volcanics | Oligocene | Fissured/volcanic | Local | South portion | <500 |
Upper carbonate aquifer | Middle Miocene | Fissured and karstified | Extensive | Almost the entire area | Largely unexplored |
Lower carbonate aquifer | Upper Cretaceous | Fissured and karstified | Extensive | Almost the entire area | |
Nubian Sandstone aquifer | Cretaceous to Paleozoic | Mixed granular and fissured | Extensive | Almost the entire area | Largely unexplored |
Basement | Precambrian | Fissured/igneous | Extensive | Almost the entire area | Unexplored |
2.2 Controlling Groundwater to Contribute Some Crop Water Requirements
A controlled subsurface drainage system allows farmers to adjust groundwater levels in their fields, simply by adjusting the overflow level in a control structure at the end of the lateral line. Conventional drainage is designed solely to remove excess water from the fields. The goal of controlled drainage is to reduce total drainage flow, reduce contaminant load, improve irrigation efficiency or some combination of these outcomes. In controlled drainage system, the groundwater table is maintained at shallower depth by a control structure which reduces deep percolation below the root zone by reducing hydraulic gradients and increases potential capillary upflow through careful water management at depth acceptable for the purpose for plant water [14].
Conceptual flow paths in controlled and uncontrolled subsurface drainage systems
Numerous studies indicated that controlled drainage significantly contributes to reducing drainage volumes from agricultural land and enhancing water use efficiency [17]. The drainage volume reduction under controlled drainage was 11% [18]. Also, the volume reduction ranges from 17 to 85% [19]. Water use efficiency on controlled drained plots with subirrigation was greater compared to controlled drained plots without subirrigation over a period of 6 years [20]. Application of controlled drainage has the potential to maintain and even increase yields per unit land whilst increasing the irrigation water use efficiency by 15–20% [21]. Water management techniques may be used to reduce drainage outflow during the growing season of rice. The use of controlled drainage and other water management practices plays an important role in reducing the amounts of irrigation water. Irrigation water requirements can be reduced to 80% of the total crop ET without reducing crop yield and increasing soil salinization in areas where water tables are shallow [22]. Introduced Modified drainage for rice areas in the Nile Delta by closing the main collector line during the rice season lead to water savings of up to 50% depending on local conditions. Thus, Farmers saved considerable irrigation time, and direct pumping costs were reduced by as much as 43% of total seasonal pumping costs [23]. Water requirements of rice under controlled drainage was 25% less than under conventional drainage system in Northern Western Nile Delta of Egypt [17].
Controlled drainage increases yields per unit land and increases water use efficiency by 15–20% [21]. Using controlled drainage systems (and combined subirrigation and controlled drainage systems) has positive results on crop yields of soybean and corn cropping systems in Canada and the United States [24].
3 Problems of Shallow Groundwater on Agriculture
3.1 Secondary Salinization of the Soils in Nile Delta
“Salinity management in the crop root zone is essential for the sustainability of irrigated agriculture and is a major consideration when proposing controlled drainage practices” [25]. Research has demonstrated that water and salt will move upwards from shallow groundwater and may result in the salinized soil in a short time. However, there have also been many studies over the years demonstrating that salinity in the root zone can be effectively managed from year to year by irrigation. Most investigators believed that shallow groundwater plays a major role in the soil salinization [26]. In shallow water table areas, due to applying controlled drainage, water and salt will move upwards from shallow groundwater and may result in the salinized soil in a short time, and it is highly dependent on water table position and salinity [25]. “High water tables are also a common problem in intensively irrigated areas and can lead to the development of salinity problems” [27]. “Results also show a positive correlation (r2 = 0.84) between the concentration of salt on the surface and water table levels of 70 cm or less” [27]. In the area of study, salt accumulation, groundwater and inadequate drainage conditions are the major causes of salinization [27].
Excessive irrigation and lack of adequate drainage cause rises in the ground table and consequently irrigation salinity [28–30]. The depth of groundwater, a distance of irrigation canals, subsoil and groundwater salinity and land use are the mean reasons of salinity risk [31].
Salinity levels in the soil and groundwater are increasing due to continuous irrigated crop production. Over time, crop production will be negatively affected due to shallow saline water table which causes salt accumulation in the soil surface through the capillary rise and/or directly as a result of waterlogging [32]. The salinity in wetlands of arid/semiarid areas will vary naturally due to sporadic rainfall, high evaporative conditions, groundwater inflows and freshening after floods or rains. However, wetlands are often at particular risk of secondary salinization which may be due to lower elevation in the landscape that causes an increase in saline groundwater inflows by rising water tables [33].
Soil processes lead to transient salinity accumulation in the root zone layers [35] (Reproduced by kind permission of CSIRO Publishing)
3.2 Waterlogging and Its Solution
Irrigation in arid and semiarid zones inevitably leads to water table variations and, thus, to salinization and waterlogging problems. About one-third of the world’s irrigated lands have decreased productivity as a consequence of poorly managed irrigation that has caused salinization and waterlogging [36]. In the irrigated lands such as Delta and Nile Valley, Egypt, groundwater levels have risen to produce waterlogging. This process has caused excessive salinity build-up in crop root zones and created yield reductions or caused land abandonment in severe cases [37]. Shallow groundwater table may have negative effects on crops; if water table is too shallow, crop yield could decrease due to waterlogging and root anoxia [38, 39]. “When the water table is very shallow, soil waterlogging limited the root growth of winter wheat due to the reduced oxygen concentration of the soil” [40].
Over the past 25 years, the role of drainage has improved from only purpose measure for controlling salinity and/or waterlogging to an important element of integrated water management under multiple land use [41]. In arid and humid zones, subsurface drainage is used to prevent waterlogging, increase the trafficability of soil and enhance aeration for better crop growth, thus allowing soil preparation for planting and harvest on time. In addition, in arid areas drainage contributes significantly to leaching capability to control salinity build-up in crop root zone [25]. Subsurface drainage systems have proved to be a technically feasible and cost-effective practice to combat the problems of root zone salinity and waterlogging in irrigated land and thus increase crop yields and rural income [42–47]. Drainage is one of the most effective solutions to reduce waterlogging and salinity problems [48].
4 Salinization of Groundwater and Seawater Intrusion
The coastal aquifers are essential sources of freshwater in coastal zones, but salinity intrusion can be the main problem in these areas. Saltwater intrusion is the process that saltwater moves from the ocean into coastal aquifers as a result of the overpumping of groundwater, and it is a dynamic equilibrium of groundwater movement and subsequently may cause polluting of the groundwater [3, 49]. Saltwater intrusion is a natural phenomenon that occurs almost in all coastal aquifers which are connected to the sea as a result of the higher density of the seawater compared to the groundwater in the aquifer [50]. Due to the extraction of groundwater from aquifers which are in hydraulic connection with the sea, the gradients that are set up may cause a flow of seawater from the sea towards the well [51]. Increasing water abstraction from coastal aquifers lead to decreasing the movement of freshwater to the sea and accordingly, increasing seawater intrusion inland [6, 52]. The intrusion of seawater in coastal aquifers is the main problem and is encountered, with different degrees, in almost all coastal aquifers. It is considered one of the main factors that decrease water quality through raising salinity. It may happen due to human activities as overpumping and/or by natural events like sea level rise or decreasing recharge from surface water which quickens saltwater intrusion [3, 10, 52]. Saltwater intrusion has a direct influence on the salinity of the soil, resources of groundwater, coastal zone quality and agricultural productivity [53].
In Egypt, Nile Delta aquifer is one of these aquifers which are subjected to face intrusion of seawater from the Mediterranean Sea [52]. The sea level rise and fluctuation of Nile River flow may have a significant impact on groundwater quality in the Nile Delta, which lead to an increase in the salinity levels of groundwater [54]. Moreover, the current and future human activities, mainly unplanned and extensive groundwater abstraction, are resulting in degradation of the available groundwater resources [55]. Previous studies indicated that seawater intrusion in the Nile Delta aquifer had extended inland more than 100 km from the Mediterranean coast [56, 57]. Sea level rising might have significant long-term impacts on the Nile Delta, which include the distribution of groundwater salinity and erosion of the narrow and low-lying barriers of the Manzala and Burullus lagoons [58]. Rising the Mediterranean Sea level by 0.5 m, it will cause additional intrusion of 9.0 km in the Nile Delta aquifer [3].
Groundwater salinity in the Nile Delta is classified into two zones; the first one in the Southern Nile Delta that is a low salinity level (total dissolved solids <1,000 ppm) and the second zone in the central Nile Delta, in which the salinity increases sharply to a total dissolved solids >10,000 ppm [4]. The projected seawater rise would cause a reduction in a hydraulic gradient of the water table and/or piezometric head in the Nile Delta aquifer. So, under climate change condition and seawater rise, the deep coastal aquifers with mild hydraulic gradients would be more vulnerable. In Nile Delta aquifer, increasing Mediterranean Sea level by 0.5 m would cause movement of the equi-concentration lines 35, 5, and 1 g/L inland by distances of 1.5, 4.5 and 9 km, respectively [59]. Rising seawater as a result of climate change will have many impacts on groundwater. First, additional pressure heads will be forced at the seaside causing more seawater intrusion in land. Second, seawater will submerge the low lands along the shoreline, and the groundwater below these lands will become saline. Third, climate change may cause differences in rainfall which would affect the natural replenishment of groundwater. Fourth, as a result of the expected reduction in surface water resources and rainfall particularly in arid and semiarid regions, the dependence on, and exploration of, groundwater resources would increase to contribute for water needs of the different sectors [10, 60].
The overextraction of groundwater leads to an increase in salinity concentration and covers about large area [61]. The most extremely serious result indicated that overpumping of the groundwater especially in the Nile Delta aquifers, which is followed by salinization by seawater intrusion, thus decreases the obtainability of freshwater sources [62]. The combination of overpumping and sea level rise results in the movement of the transition zone further towards land. Rising the Mediterranean Sea water level by 100 cm will cause extra intrusion in the Nile Delta aquifer by 10 km. Overpumping will cause an extra intrusion by 8 km and reduction in piezometric head water at the land side. The combination of sea level rise and overpumping will cause an extra intrusion of 15 km [6].
5 Contamination of Groundwater
Pollution can be defined as the modification of chemical, physical and biological properties of water preventing or restricting its use in the various uses. Water is polluted naturally due to saltwater intrusion and others without human intervention or artificially by human activities [63]. There are many groundwater pollution sources which include oil and mining fields (petroleum exploration and development, abandoned oil wells and test wells, buried pipelines and storage tanks, disposal of oil field brine and mining activities), water wells (disposal, drainage and abandoned wells, overpumping, well construction, river infiltration and seawater encroachment), cultivated lands (animal wastes, dryland farming and evapotranspiration from vegetation, feedlots, fertilizers and pesticides, irrigation return flow and sewage treatment plant discharge), urban areas (urban and industrial landfills, surface impoundments, solid wastes, natural pollution and septic tanks and cesspools) and others (lakes and spills, natural leaching, water from fault zones and volcanic origin) [64]. Contamination of the Nile Delta aquifers has resulted from saltwater intrusion, the absence of sewage systems in most of villages and towns in Delta and oxidation and dissolution of applied pesticides, in addition, fertilizers into infiltrating irrigation water. These contaminations are characterized by high concentrations of total dissolved solids [65]. The degradation of groundwater quality in the Eastern part of the Nile Delta could be related to the leakage from sewage storage tanks, transportation sewage pipes and El Khadrawya drain which receives the Mobarak industrial area wastewater. Moreover, dump disposal sites and factories’ illegal sewage disposal [66]. Land use and related human activities have significant correlation on groundwater quality, type and extent of pollution. Use and reuse of water for agricultural, domestic and industrial purposes result in the discharge of liquid or solid wastes into the geologic environment [63].
Shallow aquifers are more sensitive to degradation due to temporal and spatial changes in recharge and discharge. Extensive urban, industrial and agricultural expansions on the western fringe of the Nile Delta have applied much load on the water needs and lead to groundwater quality deterioration. These deteriorations may be due to salinization and contamination from nutrients and trace metals [67]. When the water table is shallow and the aquifers have low buffering capacity and are highly permeable, the risk of pollution increased [68]. There is hydraulic connection between groundwater in the Nile Delta aquifer and surface water from Nile River branches, drainage system and irrigation canals. Therefore there are many factors contributing to the decline of groundwater quality in the Nile Delta aquifer including leaching of fertilizers, pesticides and chemicals from agriculture areas, saline groundwater upconing and seawater intrusion, disposal of industrial waste and overpumping of groundwater [9].
The main pollutant of groundwater is saline water. Saline water intrusion takes place where saline water relocates or mixes with freshwater. The groundwater in most areas close to the sea in the western fringe of the Nile Delta, as North-Tahrir, Abis and Marriott, was unsuitable to irrigate regular crops. Also, West of the Cairo-Alexandria road except for the track of land between Nubaria Canal and this road has access to reasonably good groundwater [67]. The increase of salinity in the Middle part of the Nile Delta is caused by vertical seepage of sanitary, agricultural wastewater and the dissolving of salts from the sediments of the aquifer itself [69]. The excessive extraction of groundwater may affect groundwater quality by upward seepage of saline water [65]. Mobilization of stored salts in the unsaturated zone, local pollution and evaporites, which resulted from ancient marine intrusion, wind-driven sea spry and marine aerosols deposited at the topsoil, may further contribute to groundwater salinization [70, 71]. The salinization resulted from seawater mixing, dissolution of evaporite minerals in the water-bearing formation and the sabkhas and saline soils [67].
Agriculture, industrial and domestic discharges are the main sources of pollution of the Nile Delta aquifer. The overuse of chemical fertilizers in agriculture causes higher concentrations of nitrate, potassium, sulphate and phosphate. Also, higher concentrations of iron, manganese and aluminium were found in the old lands of the Nile Delta aquifer due to the dumping of industrial effluents into the drainage system. In addition, due to the lack of sewage systems in most rural areas and popularity of septic tanks in the Nile Delta [9]. Contamination with nutrients and trace metals is usually from agricultural, urban and industrial wastes [67]. Manganese and iron concentrations were higher in the old lands due to the general natural characteristics of the Nile Delta aquifer and more pronounced in the areas having a clay cap. Local contamination from industrial areas led to high concentrations of lead in Eastern Nile Delta and Western Nile Delta. Also, high concentrations of cadmium were found in the Middle of the Nile Delta [72]. The agricultural activities as the excessive use of pesticides and fertilizers particularly in the old land and using wastewater in irrigation which cause degradation of the groundwater quality are mainly attributing to enrich groundwater with nitrate, pesticides, phosphate and bacteria. In addition, domestic and industrial activities increase heavy metals, industrial pollutant and bacteria in groundwater [63, 73]. The maximum concentration of nutrients was mainly recorded in the old cultivated lands, indicating the contamination from irrigation water [11]. The shallow brackish coastal aquifer in northern Nile Delta is partially polluted with phosphates (0.24 mg/L) and nitrates (7.35 mg/L). Furthermore, groundwater contains excess amount of some trace elements (Fe, Mn, Ni) that are higher than the standard levels for drinking water [62]. Herbicides which were used to control submerged weeds in irrigation canals and water hyacinths in drains have caused serious environmental hazards [74].
6 Integrated Management for Sustainable Use of Groundwater
Maintaining groundwater quality and quantity for sustainable use requires an integrated management approach for both groundwater and surface water. For sustainable use of groundwater in areas where groundwater is renewable, the abstraction rate should not exceed the recharge rate. If this is not the case, the unplanned exploitation will lead to short term of use [75]. Many wells were dug on the Nubian aquifer in the Oases area in the Western Desert were stopped to produce naturally due to overextraction [64, 76]. Increasing recharge of groundwater by 10% and decreasing pumping by 10% are predicted to cause a 3 and 6 m increase in groundwater head in the heavily exploited coastal aquifer [77]. Under the condition of 0.5 m seawater level rise, it would sustain the freshwater resources when reducing the groundwater pumping by 50% [10].
The Nile Delta aquifer is strongly exposed to seawater intrusion from the Mediterranean Sea producing serious environmental impacts. Groundwater resources should thus be developed and managed carefully to avoid any further water quality deterioration [6]. In Western Nile Delta, the integrated and sustainable management plans for groundwater are essential to avoid the deterioration of the aquifer system in this area. The inclusive database that includes the characteristics of the aquifer system and modelling tools to achieve the impacts of decision alternatives are very important factors for efficient integrated and sustainable management of water resources [1].
Sustainability of groundwater is a very important issue, and it is not possible without water managers, monitoring and characterizing groundwater resources; local communities, management and hydrogeologists should work together to devise measure and policies by backcasting and, moreover, adapt future measures in achieving the long-term sustainable targets [78]. Decreasing, controlling or forbidding well drilling, treatment of sewage and drainage water before using in irrigation, integrated management of applying fertilizers in agriculture areas to prevent groundwater pollution and monitoring of surface and groundwater are very important actions to improve groundwater quality, avoid saline groundwater supply and prevent groundwater pollution of the Eastern Nile Delta aquifer [73].
7 Conclusions and Recommendations
Groundwater is a very important source of freshwater in Egypt besides River Nile. It is expected to cover about 20% of the total water supply in the upcoming decades especially in the newly reclaimed areas along the desert fringes of the Nile Delta and Valley. In Egypt, there are six groundwater aquifers, and the Nile aquifer represents 87% of the total groundwater pumping in Egypt. The direct seepage of Nile water from irrigation and drainage systems and the irrigated and cultivated lands are the main source of Nile Delta aquifer recharge. This is due to a hydraulic connection between the groundwater aquifer and the Nile Delta branches especially in the Middle and Southern parts of the Nile Delta due to the thinner and irregular clay layer.
Groundwater could significantly contribute to agriculture through direct abstraction to use in irrigation mainly or supplemental irrigation in rainfed areas. Also, it could be used in irrigation in an indirect way through controlling drainage system, so water table will decrease and, thus, contribute to crop water requirement. But, there are some problems with shallow water table which negatively affect agriculture production as secondary soil salinization and waterlogging. These two problems could be overcome through using surface and subsurface drainage systems.
Nile Delta aquifer is exposed to many challenges which decrease the quantity and/or deteriorate its quality as seawater intrusion. It may happen due to expected rising of seawater level due to projected climate change and/or the current and future human activities, mainly unplanned and extensive groundwater abstraction which lead to an increase in the salinity levels of groundwater, and also the transport of pollutant from surface sources to groundwater. So, to maintain groundwater resources in Nile Delta for sustainable use, an integrated management plan should be prepared. The plan should include preparing digital maps characteristic for groundwater distribution and depth and define the safe extraction. Also, controlling or forbidding drilling wells especially in North Nile Delta close to the Mediterranean Sea, increasing ground recharge and/or reducing ground abstraction, treatment of sewage and drainage water before using in irrigation, integrated management of applying fertilizers in agriculture areas to prevent groundwater pollution and applying controlled drainage in agricultural areas in Nile Delta because it reduces the transport of agricultural pollutant as pesticides, herbicides, nutrients and some of heavy metals in addition enhancing water use efficiency.
Finally, monitoring of surface and groundwater is a very important action to improve groundwater quality, avoid saline groundwater supply and prevent groundwater pollution in Nile Delta region.