Numerical Investigation for Riverbank Filtration Sustainability Considering Climatic Changes in Arid and Semi-Arid Regions; Case Study of RBF Site at Embaba, Nile Delta, Egypt

: Changes in riverine hydrography and reduced aquifer recharge due to projected climate changes in arid and semi-arid regions are the main issues of water supply, especially in the Nile Delta, Egypt. Continuous degradation results from reduced Nile water ﬂow, poor management of groundwater extraction, and human activities throughout the Nile’s course and drainage channels. Contamination of this water with heavy metals and dissolved organic solids reduces the quality of this water, which increases the price of treatment. River Bank Filtration (RBF) is a water treatment technology used for improving the quality of drinking water taken from polluted rivers where abstraction wells are installed on the banks. This study was applied to the RBF site at Embaba, Nile Delta, Egypt using the numerical code MT3D. The study was simulated and calibrated for the current situation and number of scenarios to investigate the effect of climatic changes on RBF sustainability. Four scenarios were simulated to identify and estimate the RBF portion and the total water travel time from the river to the wells. The ﬁrst scenario involves a reduction in river stages, the second a decrease in aquifer recharge, the third a combination of the ﬁrst two scenarios, and the fourth scenario combines scenarios 1, 2, and 3. The results indicate that the RBF portion decreased from 67.42% in the base case to 35.46% and 64.99% with a reduction in river stage by 75% from the base case and a decrease in aquifer recharge from 182.50 (base case) to 50 mm per year, respectively. Moreover, the RBF portion increased to reach 87.75% with a reduction in the General Head Boundary of 75% from the base case, while the combination of the three scenarios decreased the RBF portion to 67.24%. Finally, the water supply systems in arid and semi-arid regions should be extended by installing and operating RBF facilities to manage the negative effects of climatic change through reduction in river stages and aquifer recharge, and increasing abstraction due to overpopulation.


Introduction
Overpopulation and drought cause great pressure on water resources. This requires developing new resource approaches to water planning and management to prevent escalating conflicts and reverse environmental degradation. The Middle East and north Africa region can be considered one of the world's most water-scarce regions [1], as the use of its water resources with increasing intensity and increasingly low precipitation is leading to national water crises with low water levels and the emptying of reservoirs [2,3]. Evaluating water quality indicators is a crucial issue for integrated water resource management, since potable water sustainability is an essential resource for the world's health [4]  Numerical modelling is useful for studying groundwater systems, as it is a relatively low-cost method with the ability to experiment with multiple scenarios, and the results can be taken further into account, for example, when implementing an RBF system. Numerical simulations using MODFLOW software [18] have been applied concerning the effects of temperature on water contamination during soil passage transport, and to examine the influence of potential hydrological changes on the efficiency of bank filtration and artificial feeding [19,20]. Potential climate change has been assessed as discussed in applicable climate scenarios, including its impact on BF as a sustainable, cost-effective method of drinking water supply. In particular, increased biological activity at higher temperatures in the groundwater layer appears to improve the cleaning efficiency of the soil path, and the soil path functions as a temporary barrier to pollution. In the event of an increase in extreme situations such as floods and low flow periods, application strategies are provided to protect the drinking water supply [21].
This study aims to investigate the effects of projected climate change on the sustainable operation of RBF systems in arid and semi-arid regions using numerical modelling. Moreover, the study is intended to identify and estimate the RBF portion and the total water travel time from the river to the RBF wells by solute transport model MT3D, considering the effects of changes in river stages, precipitation, the combination of river stages with precipitation, and finally the effects of the General Head Boundary (GHB) from the groundwater side on the portion of the RBF site at Embaba, Nile Delta, Egypt.

Study Area
The study area is located in Giza Governorate, Egypt between latitudes 30° 06\ to 30° 07\ N and longitudes 31° 12\ to 31° 13\ E, occupying about 4 km 2 , as presented in Figure 2a. This is the location of the drinking water and sewage plant at Embaba, which operates for a population of more than 8 million [22] .

Geomorphology, Land Use, and Metrological Conditions
The geomorphologic features of the study area consist of Al-Warraq island to the east and the surrounding western branch of the river Nile. The average land levels range between 17 to 20 m above mean sea level (masl) reaching 23 m at the western boundary, while the eastern boundary is the river Nile branch with average bed level 8.8 (masl) and width 250 m. The groundwater reservoir in the study area depends on the river Nile at the source of the delta barrages, and its flow is generally in a northerly direction. However, due to the presence of two side depressions, at the Giza pyramids to the west and the Marg to the east, and due to the high levels of groundwater extraction at the-Remayyah Golf Club and the Giza Pyramids site [23], the flow conditions in the southern part of the map are different on the left from those on the right side of the river.
The climate of the Nile Delta region is moderate, with a relative rise in temperature during the summer and average mean temperatures between 12° C and 31° C in January Numerical modelling is useful for studying groundwater systems, as it is a relatively low-cost method with the ability to experiment with multiple scenarios, and the results can be taken further into account, for example, when implementing an RBF system. Numerical simulations using MODFLOW software [18] have been applied concerning the effects of temperature on water contamination during soil passage transport, and to examine the influence of potential hydrological changes on the efficiency of bank filtration and artificial feeding [19,20]. Potential climate change has been assessed as discussed in applicable climate scenarios, including its impact on BF as a sustainable, cost-effective method of drinking water supply. In particular, increased biological activity at higher temperatures in the groundwater layer appears to improve the cleaning efficiency of the soil path, and the soil path functions as a temporary barrier to pollution. In the event of an increase in extreme situations such as floods and low flow periods, application strategies are provided to protect the drinking water supply [21].
This study aims to investigate the effects of projected climate change on the sustainable operation of RBF systems in arid and semi-arid regions using numerical modelling. Moreover, the study is intended to identify and estimate the RBF portion and the total water travel time from the river to the RBF wells by solute transport model MT3D, considering the effects of changes in river stages, precipitation, the combination of river stages with precipitation, and finally the effects of the General Head Boundary (GHB) from the groundwater side on the portion of the RBF site at Embaba, Nile Delta, Egypt.

Study Area
The study area is located in Giza Governorate, Egypt between latitudes 30 • 06\ to 30 • 07\ N and longitudes 31 • 12\ to 31 • 13\ E, occupying about 4 km 2 , as presented in Figure 2a. This is the location of the drinking water and sewage plant at Embaba, which operates for a population of more than 8 million [22].

Geomorphology, Land Use, and Metrological Conditions
The geomorphologic features of the study area consist of Al-Warraq island to the east and the surrounding western branch of the river Nile. The average land levels range between 17 to 20 m above mean sea level (masl) reaching 23 m at the western boundary, while the eastern boundary is the river Nile branch with average bed level 8.8 (masl) and width 250 m. The groundwater reservoir in the study area depends on the river Nile at the source of the delta barrages, and its flow is generally in a northerly direction. However, due to the presence of two side depressions, at the Giza pyramids to the west and the Marg to the east, and due to the high levels of groundwater extraction at the-Remayyah Golf Club and the Giza Pyramids site [23], the flow conditions in the southern part of the map are different on the left from those on the right side of the river.
The climate of the Nile Delta region is moderate, with a relative rise in temperature during the summer and average mean temperatures between 12 • C and 31 • C in January and July, respectively. The rainfall is less than 40 mm per year; the humidity ranges from 45% to 84%, and the daily evaporation reaches 15 mm [24]. and July, respectively. The rainfall is less than 40 mm per year; the humidity ranges from 45% to 84%, and the daily evaporation reaches 15 mm [24].

Geology Setting
The geology of this study area is formed from two layers: the upper layer known as the Bilqas formation was laid down in the Holocene age and is made up of alternating fine to medium-grained sand, silt, and clay interbeds, and the lower layer which functions as a semi-confining layer for the aquifer is the Mit-Ghamr formation, which includes organic matter and peat beds [26]. The Nile Delta Quaternary aquifer is classified as a semi-confined aquifer [27,28]. Figure 3 presents a vertical section through the formation of aquifer systems in the Nile Delta area, with gradient thickness ranging between more than 800 m in the north and 200 m in Cairo, where the RBF site is located [29]. The depth of groundwater is 2 m in the north, gradually increasing to 5 m in the south [30].

Hydrogeological Systems
Based on the relevant hydrogeological data, the current study area is located on the western fringes of the Greater Cairo area (Giza Governorate) overlying a regional aquifer, the quaternary aquifer of the Nile alluvium, which consists of graded sand and gravel

Geology Setting
The geology of this study area is formed from two layers: the upper layer known as the Bilqas formation was laid down in the Holocene age and is made up of alternating fine to medium-grained sand, silt, and clay interbeds, and the lower layer which functions as a semi-confining layer for the aquifer is the Mit-Ghamr formation, which includes organic matter and peat beds [26]. The Nile Delta Quaternary aquifer is classified as a semi-confined aquifer [27,28]. Figure 3 presents a vertical section through the formation of aquifer systems in the Nile Delta area, with gradient thickness ranging between more than 800 m in the north and 200 m in Cairo, where the RBF site is located [29]. The depth of groundwater is 2 m in the north, gradually increasing to 5 m in the south [30]. and July, respectively. The rainfall is less than 40 mm per year; the humidity ranges from 45% to 84%, and the daily evaporation reaches 15 mm [24].

Geology Setting
The geology of this study area is formed from two layers: the upper layer known as the Bilqas formation was laid down in the Holocene age and is made up of alternating fine to medium-grained sand, silt, and clay interbeds, and the lower layer which functions as a semi-confining layer for the aquifer is the Mit-Ghamr formation, which includes organic matter and peat beds [26]. The Nile Delta Quaternary aquifer is classified as a semi-confined aquifer [27,28]. Figure 3 presents a vertical section through the formation of aquifer systems in the Nile Delta area, with gradient thickness ranging between more than 800 m in the north and 200 m in Cairo, where the RBF site is located [29]. The depth of groundwater is 2 m in the north, gradually increasing to 5 m in the south [30].

Hydrogeological Systems
Based on the relevant hydrogeological data, the current study area is located on the western fringes of the Greater Cairo area (Giza Governorate) overlying a regional aquifer, the quaternary aquifer of the Nile alluvium, which consists of graded sand and gravel

Hydrogeological Systems
Based on the relevant hydrogeological data, the current study area is located on the western fringes of the Greater Cairo area (Giza Governorate) overlying a regional aquifer, the quaternary aquifer of the Nile alluvium, which consists of graded sand and gravel with intercalations of clay lenses at various depths. The aquifer changes from phreatic (where the top silty layer vanishes) to semi-confined (where the silty clay layer appears on the surface). The hydraulic conductivity of the aquifer decreases towards the south and west, where the vertical conductivity of the Holocene clay ranges from 0.001-0.01 m/d [32], that of the Pleistocene sand ranges from 3.6-24.0 m/d [33], and the common value of effective porosity ranges from 0.05-0.45 [34] for the reservoir. Surface water is represented by the Nile to the east, with a water hydrograph showing from 16.00 to 16.40 m 3 ·sec −1 according to Embaba water plant measurements [25]. The groundwater reservoir is over 60 m in depth, with the groundwater flow direction and the piezometric heads presented in Figure 4, which also shows the effects of abstraction wells in the south-west by the Remayyah Golf Club, which cause a reduction in piezometric water head in a westerly direction. The water quality of this aquifer shows variations in salinity ranging from less than 100 up to 1000 ppm, while the river Nile total dissolved soilds (TDS) values are close to 260 ppm [35]. with intercalations of clay lenses at various depths. The aquifer changes from phreatic (where the top silty layer vanishes) to semi-confined (where the silty clay layer appears on the surface). The hydraulic conductivity of the aquifer decreases towards the south and west, where the vertical conductivity of the Holocene clay ranges from 0.001-0.01 m/d [32], that of the Pleistocene sand ranges from 3.6-24.0 m/d [33], and the common value of effective porosity ranges from 0.05-0.45 [34] for the reservoir. Surface water is represented by the Nile to the east, with a water hydrograph showing from 16.00 to 16.40 m 3 .sec −1 according to Embaba water plant measurements [25]. The groundwater reservoir is over 60 m in depth, with the groundwater flow direction and the piezometric heads presented in Figure 4, which also shows the effects of abstraction wells in the south-west by the Remayyah Golf Club, which cause a reduction in piezometric water head in a westerly direction. The water quality of this aquifer shows variations in salinity ranging from less than 100 up to 1000 ppm, while the river Nile total dissolved soilds (TDS) values are close to 260 ppm [35].

Embaba River Bank Filtration Site
In 2015, the Holding Company for Water and Wastewater [36] drilled six pumping wells (PW) at the current RBF case study site at Embaba, Nile Delta, Egypt, close to the river Nile. The schematic PW depth ≈ 54 m, 450 mm in diameter with a discharge of 150 m 3 /h for each well, and total capacity ranging from 14 000 to 21 000 m 3 /day, as shown in Figure 2b. The distance between the wells and the river bank ranged between 10 and 15 m, and, moreover, four test wells each with a depth of 64 m and a maximum diameter of 216 mm were drilled to identify the lithology. Two further test wells, each with a depth of 30 m, were sunk, one beside the river and the other further inland, each with a diameter of 100 mm. Multiple core samples from all wells were taken to the HCWW laboratory [25,37]. The location was valid for RBF as the Nile naturally recharges the aquifer. The results show good values of total dissolved solids (TDS) and NH4, compatible with the Egyptian drinking water standards.

Contaminant Transport Model
The numerical model was used to investigate the potential of this system by simulating the RBF water quality for the current situation and different scenarios. The MT3D code was used to simulate the solute transport model and the partial differential equation [38]:

Embaba River Bank Filtration Site
In 2015, the Holding Company for Water and Wastewater [36] drilled six pumping wells (PW) at the current RBF case study site at Embaba, Nile Delta, Egypt, close to the river Nile. The schematic PW depth ≈ 54 m, 450 mm in diameter with a discharge of 150 m 3 /h for each well, and total capacity ranging from 14,000 to 21,000 m 3 /day, as shown in Figure 2b. The distance between the wells and the river bank ranged between 10 and 15 m, and, moreover, four test wells each with a depth of 64 m and a maximum diameter of 216 mm were drilled to identify the lithology. Two further test wells, each with a depth of 30 m, were sunk, one beside the river and the other further inland, each with a diameter of 100 mm. Multiple core samples from all wells were taken to the HCWW laboratory [25,37]. The location was valid for RBF as the Nile naturally recharges the aquifer. The results show good values of total dissolved solids (TDS) and NH 4 , compatible with the Egyptian drinking water standards.

Contaminant Transport Model
The numerical model was used to investigate the potential of this system by simulating the RBF water quality for the current situation and different scenarios. The MT3D code was used to simulate the solute transport model and the partial differential equation [38]:

Boundary Conditions and Hydraulic Parameters
The boundary conditions for the NDA were assigned using a river package where the river Nile was assigned at the east side with stage starts from 16.17 to 16.15 (masl) at the south side. The northern and southern boundaries had no-flow boundaries. However, along the model's western boundary a general head boundary (GHB, Cauchy BC) representing the unaffected groundwater head was set, starting from 16.18 to 16.20 (masl) at a distance of 150 m from the river Nile, where the natural groundwater slope was assigned 0.083‰ [25]; these values were based on the groundwater map published by [23]. Figure 5a,b show the study area boundary conditions. The aquifer hydraulic parameters including hydraulic conductivity [K] ranged from 0.25 to 5 m per day [m/day] for the clay cap and the fine sand top layer, while the course of coarse Quaternary sand ranged from 10 to 60 m/day. The storage coefficient [S] for the upper layers was 10 −3 -5 × 10 −3 and 2.5 × 10 −3 for the lower layers, the specific storage [Ss] ranged from 0.1 to 0.2, and effective porosity from 40% to 25% [ Table 1]. The total abstraction from the study area reached 50,000 cubic meters per day (m 3 /day), while the average net recharge was 0.5 mm/day [40]. A constant concentration of 260 ppm was assigned along the river boundary, and initial concentration was set at 1000 ppm [41]. These parameters are reported by [42,43].

Model Calibration
The model calibration started with a comparison of the simulated head with the measured data from the observation wells using the piezometric contour map of Greater Cairo developed by [23] (see Figure 4). The flow model was calibrated for steady-state by changing the aquifer hydraulic conductivity using a trial-and-error approach to match the model results and piezometric head. Figure 6 shows the difference between the calculated and observed heads, where the root mean square [RMS] and normalization root mean

Model Calibration
The model calibration started with a comparison of the simulated head with the measured data from the observation wells using the piezometric contour map of Greater Cairo developed by [23] (see Figure 4). The flow model was calibrated for steady-state by changing the aquifer hydraulic conductivity using a trial-and-error approach to match the model results and piezometric head. Figure 6 shows the difference between the calculated and observed heads, where the root mean square [RMS] and normalization root mean square reached 0.366 m and 8.135%, respectively; the correlation coefficient also reached 94%.  The groundwater head ranged from 13 to 15 (masl) as shown in Figure 7a, and TDS in the study area ranged from 250 to 350 ppm at layer #2 as presented in Figure 7b,c. The travel time for river particles was calculated for the transverse flow paths to redirect the trace and reverse trace particles, which were simulated using the Visual Modflow modpath, assigning particles in the middle of the simulated river at a distance of 75m from the shore line and 90 m from the pumping wells along the river. Using forward tracking, the travel time reached 200 days, as presented in Table 2.  The groundwater head ranged from 13 to 15 (masl) as shown in Figure 7a, and TDS in the study area ranged from 250 to 350 ppm at layer #2 as presented in Figure 7b,c. The travel time for river particles was calculated for the transverse flow paths to redirect the trace and reverse trace particles, which were simulated using the Visual Modflow mod-path, assigning particles in the middle of the simulated river at a distance of 75 m from the shore line and 90 m from the pumping wells along the river. Using forward tracking, the travel time reached 200 days, as presented in Table 2. The solute transport model was calibrated by changing the hydraulic conductivity of the clogging bed layer (kclogging) on the RBF portion by 0.01 × 10 −5 , 0.1 × 10 −5 , 0.2 × 10 −5 , 0.5 × 10 −5 , 1 × 10 −5 and 1.16 × 10 −5 m/s. Figure 8 shows that the relationship between kclogging and RBF share reached 46.54%, 62.07%, 65%, 66.59%, 66.71% and 66.88%. The model results give a good match with the field measurements and the numerical investigation by [20,25,37], where the RBF portion at Embaba, Nile Delta, Egypt is around 60%, which corresponds to kclogging of 0.5 × 10 −5 m/s (0.43 m/day).  The solute transport model was calibrated by changing the hydraulic conductivity of the clogging bed layer (k clogging ) on the RBF portion by 0.01 × 10 −5 , 0.1 × 10 −5 , 0.2 × 10 −5 , 0.5 × 10 −5 , 1 × 10 −5 and 1.16 × 10 −5 m/s. Figure 8 shows that the relationship between k clogging and RBF share reached 46.54%, 62.07%, 65%, 66.59%, 66.71% and 66.88%. The model results give a good match with the field measurements and the numerical investigation by [20,25,37], where the RBF portion at Embaba, Nile Delta, Egypt is around 60%, which corresponds to k clogging of 0.5 × 10 −5 m/s (0.43 m/day).

Results and Discussion
The influence of climate change on the availability and quality of both surface-and ground-water resources is well recognized nowadays [44][45][46][47]. The impact of climate change on the share of the bank filtration technique is calculated using the mean value of conservative species of TDS. This selection of the TDS parameter in the evaluation of RBF systems was based on the published field investigation maps for groundwater salinity for this study area, and this parameter is also a very important indicator for groundwater

Results and Discussion
The influence of climate change on the availability and quality of both surface-and ground-water resources is well recognized nowadays [44][45][46][47]. The impact of climate change on the share of the bank filtration technique is calculated using the mean value of conservative species of TDS. This selection of the TDS parameter in the evaluation of RBF systems was based on the published field investigation maps for groundwater salinity for this study area, and this parameter is also a very important indicator for groundwater quality in arid and semi-arid regions.
The percentage of bank filtrate in the pumped water was calculated with the following equation: where (TDS) GW is the concentration of TDS in the aquifer around the pumping well, (TDS) RBF is the concentration of TDS in pumped water after using a filter, and (TDS) River is the concentration of TDS in the surface water or river. Particle tracking was used to calculate the travel time of river particles to the top of the RBF well screens by adding a line of particles in the centre of the river in layer #2 of the model at a distance of 90 m from the RBF wells using forward tracking (Figure 7a).

Impact of River Boundary Stages on RBF Portion
Four scenarios were enacted to estimate the RBF efficiency for decreasing the river stage to reach 75%, 50%, 25%, and 12.5% from the base case, where the river boundary was changed to start from 14.70, 13.23, 11.17, 9.77 m and end at 14.68, 13.21, 11.15 and 9.75 m, respectively. The RBF portion reached 61.05%, 49.20%, 35.46%, and 32.11%, respectively, compared with 67.42% for the base case (see Table 2). Figure 9 shows the relationship between the different river stages with the percentage of RBF portion, whereby the results indicate that this system has high sensitivity to the river stage. Moreover, Figure 13a presents the vertical distribution of TDS in the study area for a 50% reduction in river stage. The expected impacts of climate change on the RBF system in this scenario are shown in Table 2, where the travel time reached 220, 280, 800 and 1000 days, respectively. The results show a corresponding decrease in the RBF portion and increase in travel time as the river stage drops due to climatic changes, so the RBF system requires high heads for increasing the system portion and sustainability.   Table 2). Moreover, the TDS distribution for a reduction in aquifer recharge to value 100 mm per year is presented in Figure 13b. The results also show that low values of rainfall in arid and semi-arid regions have an effect on RBF portion, but no effect on particle travel time.   Table 2). Moreover, the TDS distribution for a reduction in aquifer recharge to value 100 mm per year is presented in Figure 13b. The results also show that low values of rainfall in arid and semi-arid regions have an effect on RBF portion, but no effect on particle travel time.

Impact of Aquifer Recharge Reduction on RBF Portion
based on the expected scenarios of climate change for the charge by 182.5, 150, 100, 50, and 10 mm per year. The results of the RBF portion reached 67.42%, 66.50%, 65.55%, 64.99%, and 64.47%, respectively, while the travel time reached 200 days for all scenarios (see Table 2). Moreover, the TDS distribution for a reduction in aquifer recharge to value 100 mm per year is presented in Figure 13b. The results also show that low values of rainfall in arid and semi-arid regions have an effect on RBF portion, but no effect on particle travel time.

Effect of General Head Reduction on RBF Portion
The General Head Boundary [GHB, Cauchy BC] was studied as a product of decreasing groundwater levels due to increased groundwater abstraction as a result of overpopulation. The current scenarios involved reduction of groundwater levels to reach 80%, 60%, 40%, and 20% from the base case. The GHB changed, starting at 12.90, 9.68, 6.45, and 3.22 (masl) and ending at 12.15, 8.10, 4.05, and 2.03 (masl). The simulation results show that decreasing the GHB led to increased RBF portion due to changing groundwater flow direction from the high level at the riverside to the low level at GHB. Moreover, Figure 11 and Table 2 show the RBF portion reaching 78.94%, 84.43%, 87.75%, and 89%, while the travel time reached 140, 95, 80, and 70 days. Moreover, the results of TDS distribution at 50% reduction in GHB are shown in Figure 13c. These results confirm that decreasing groundwater heads led to an increase in RBF portion and a decrease in travel time due to an increased slope of groundwater heads from the river to land-side.

Effect of General Head Reduction on RBF Portion
The General Head Boundary [GHB, Cauchy BC] was studied as a product of decreasing groundwater levels due to increased groundwater abstraction as a result of overpopulation. The current scenarios involved reduction of groundwater levels to reach 80%, 60%, 40%, and 20% from the base case. The GHB changed, starting at 12.90, 9.68, 6.45, and 3.22 (masl) and ending at 12.15, 8.10, 4.05, and 2.03 (masl). The simulation results show that decreasing the GHB led to increased RBF portion due to changing groundwater flow direction from the high level at the riverside to the low level at GHB. Moreover, Figure 11 and Table 2 show the RBF portion reaching 78.94%, 84.43%, 87.75%, and 89%, while the travel time reached 140, 95, 80, and 70 days. Moreover, the results of TDS distribution at 50% reduction in GHB are shown in Figure 13c. These results confirm that decreasing groundwater heads led to an increase in RBF portion and a decrease in travel time due to an increased slope of groundwater heads from the river to land-side.

Combined Effects of River Stage, General Head and Groundwater Recharge on RBF Portion
This part of the study was carried out using a combination of a reduction in river stage with groundwater recharge and with general head as a result of climatic changes in arid and semi-arid regions. The river stage and the GHB boundary were reduced to 75%, 50%, 25%, and 12.50% from the base case, while the aquifer recharge was changed by 150, 100, 50, and 10 mm per year. The RBF portion and the travel time are presented in Figure  12 and Table 2. The results show that reduction in river stage and GHB by 75% and 50% with aquifer recharge of 150, 100 mm/yr increased the RBF portion to 67.80% and 69.79% with constant particle travel time at 220 days, whereas the RBF portion decreased to 67.24% and 63.10% and the travel time increased to 220 and 240 days with further reduction in river stage and GHB by 25% and 12.50%, and aquifer recharge by 50 and 10 mm/yr, respectively. Moreover, Figure 13d shows the distribution of TDS at 50% reduction in river stage and GHB with aquifer recharge at 100 mm/yr. These scenarios indicate that surface water and groundwater boundary conditions are more critical for using RBF in arid and semi-arid regions to decrease the negative impacts of climate change on river water qual- Figure 11. Relationship between RBF portion % and general head.

Combined Effects of River Stage, General Head and Groundwater Recharge on RBF Portion
This part of the study was carried out using a combination of a reduction in river stage with groundwater recharge and with general head as a result of climatic changes in arid and semi-arid regions. The river stage and the GHB boundary were reduced to 75%, 50%, 25%, and 12.50% from the base case, while the aquifer recharge was changed by 150, 100, 50, and 10 mm per year. The RBF portion and the travel time are presented in Figure 12 and Table 2. The results show that reduction in river stage and GHB by 75% and 50% with aquifer recharge of 150, 100 mm/yr increased the RBF portion to 67.80% and 69.79% with constant particle travel time at 220 days, whereas the RBF portion decreased to 67.24% and 63.10% and the travel time increased to 220 and 240 days with further reduction in river stage and GHB by 25% and 12.50%, and aquifer recharge by 50 and 10 mm/yr, respectively. Moreover, Figure 13d shows the distribution of TDS at 50% reduction in river stage and GHB with aquifer recharge at 100 mm/yr. These scenarios indicate that surface water and groundwater boundary conditions are more critical for using RBF in arid and semi-arid regions to decrease the negative impacts of climate change on river water quality and groundwater recharge, whereby this system is the key to improving water quality management and sustainability in these regions.
arid and semi-arid regions. The river stage and the GHB boundary were reduced to 75%, 50%, 25%, and 12.50% from the base case, while the aquifer recharge was changed by 150, 100, 50, and 10 mm per year. The RBF portion and the travel time are presented in Figure  12 and Table 2. The results show that reduction in river stage and GHB by 75% and 50% with aquifer recharge of 150, 100 mm/yr increased the RBF portion to 67.80% and 69.79% with constant particle travel time at 220 days, whereas the RBF portion decreased to 67.24% and 63.10% and the travel time increased to 220 and 240 days with further reduction in river stage and GHB by 25% and 12.50%, and aquifer recharge by 50 and 10 mm/yr, respectively. Moreover, Figure 13d shows the distribution of TDS at 50% reduction in river stage and GHB with aquifer recharge at 100 mm/yr. These scenarios indicate that surface water and groundwater boundary conditions are more critical for using RBF in arid and semi-arid regions to decrease the negative impacts of climate change on river water quality and groundwater recharge, whereby this system is the key to improving water quality management and sustainability in these regions. Figure 12. Relationship between RBF portion % and combination of river stage, groundwater recharge and general head. Figure 12. Relationship between RBF portion % and combination of river stage, groundwater recharge and general head.

Conclusions
RBF sustainability takes into account current and future climate changes. As this technology actually provides surface water from rivers or streams through bank-side aquifers, which depend on the stage of the river and the head of groundwater, the quality of

Conclusions
RBF sustainability takes into account current and future climate changes. As this technology actually provides surface water from rivers or streams through bank-side aquifers, which depend on the stage of the river and the head of groundwater, the quality of the water in rivers and aquifers is an important factor. MT3D code was used to run the simulations in this study at the RBF site at Embaba, Giza Governorate, Egypt. The RBF portion and the travel time of river particles were investigated for the base case in the Nile Delta aquifer, and four scenarios involving changes in river stages, aquifer recharge, and General Head Boundary [GHB] were applied, followed by combinations of these scenarios. The simulation results indicate that the RBF portion reached 67.42% and 200 days of river particles travel time for the current situation. Reduction in river stage decreased the RBF portion and increased the travel time of river particles to reach 61.05%, 49.20%, 35.46%, and 32.11%, and 220, 280, 800, and 875 days, respectively. Different reductions in river stage by 75%, 50%, 25%, and12.50% from the base case, as well as changes in aquifer recharge by 150, 100, 50, and 10 mm, caused the RBF portion to reach 66.50%, 65.55%, 64.99%, and 64.47%, respectively, while the travel time remained at 200 days for all scenarios. The GHB was also a factor, decreasing the particle travel time to 140, 95, 80, and 70 days while the RBF portion increased to 78.94%, 84.43%, 87.75%, and 89% with reduction in GHB by 75%, 50%, 25%, and12.50% from the base case, whereby the flow increased in the direction from the river to the aquifer. A combination of the three scenarios produced an increase in the RBF portion to 67.80% and 69.79% with constant particle travel time at 220 days and reduction in river stage and GHB by 75% and 50% with aquifer recharges of 150 and 100 mm/yr, respectively. Further reduction in river stage and GHB by 25% and 12.50% with aquifer recharges of 50 and 10 mm/yr led to decreases in the RBF portion to 67.24% and 63.10%, while the travel time increased from 220 to 240 days, respectively. The conclusion of this study is that the effects of climate change are inevitable, and that resorting to unconventional counter-measures will be necessary, such as natural water treatment systems including RBF, taking into account the hydraulic factors which affect it, such as decreases in river level and aquifer recharge, which impact the efficiency of the system. However, RBF is considered to be an effective treatment or pre-treatment method, while intensive withdrawal from the groundwater systems showed a good influence on the effectiveness of the system due to increased flow velocity from the river to these wells.