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Vadose Zone Wells and Surface Spreading Basins Modeling to Augment the Groundwater Resources in a Semi Arid Area in Egypt

Peter H. Riad 1, 2, Max H. Billib 1, Peter-W. Boochs 1, Ahmed A. Hassan 2, Mohamed N. Eldin2, Maha A. Omar 3

1Institute of Water Resources Management, Hydrology and Agricultural Hydraulic Engineering, Leibniz University of Hannover, Germany; (riad@iww.uni-hannover.de), (boochs@wbbau.uni-hannover.de), (billib@iww.uni-hannover.de), Tel.: +49 511 7622498; Fax: +49 511 7623731 2Irrigation and Hydraulics Department, Faculty of Engineering, Ain Shams University, Cairo/Egypt; (ahmad9657@yahoo.co.uk) 3Research Institute for Groundwater, El Kanater-Cairo/Egypt; (birdblue_2008@yahoo.com)

ABSTRACT In arid and semi-arid regions, where water scarcity is almost endemic, artificial recharge of groundwater by treated wastewater is one of the most effective techniques for the augmentation of groundwater resources. In this study two artificial recharge techniques, surface spreading basin and shallow vadose zone wells, have been suggested to an industrial city in the western desert of the Nile Delta fringes in Egypt called Sadat City which is suffering of groundwater depletion due to the unmanaged over exploitation. Non reactive transport model MT3DMS has been applied along with the flow model MODFLOW 2000 to study the most effective techniques for the groundwater artificial recharge. Total Dissolved Solids (TDS) was taken as an indicator parameter for the water mixing and the quality improvement, considering that the recovered water will be used for the non-potable uses. From the field studies it was found that the TDS of the native groundwater in the city area is about 1200 mg/1 while the TDS of the recharged water from the treatment plants is about 800 mg/1. After the model calibration and validation processes it was found that recharging by wells gives almost the same TDS concentrations as in basins; however the basins cover wider area just only after half a year. Also the wells diameters did not show so much influence on the TDS concentrations, however it is recommended by the authors to use more wells with smaller diameters (e.g. 1 m) to reduce the clogging effect which increases with high continuous flow rates and for maintenance purposes. Another reactive transport model has been applied using PHT3D to study the cation exchange effect on the sodium adsorption ratio (SAR) by the mixed water. It was found that higher SAR is expected in case of recharging by basins or wells with high flow rates; hence the study recommends to apply artificial recharge by using 1 surface spreading basin and 5 vadose zone wells (1 m diameter).

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INTRODUCTION

Since the early eighties, the private sectors in Egypt have started to reclaim the western fringes of the Nile Delta region depending mainly on the groundwater resources. The reclamation process was accelerated during the eighties and up till now (Wassef, 2011). The total groundwater abstraction was 870 million m3/year by the year 2000 and increased to more than one billion m3/year in 2007 (The World Bank, 2007). The groundwater table started to decline due to the unplanned groundwater abstraction, and over pumping from production wells while the aquifers receive little or no replenishment particularly during the last decades because of the scarcity in the rainfalls, (Ghaly, 2001). Accordingly, the ministry of water resources and irrigation in Egypt initiated the main objective of finding out some other non conventional fresh water resources; hence special attention has been paid to artificial groundwater recharge in the arid and semi-arid regions. Artificial recharge is an effective technique for the augmentation of groundwater resources. A variety of methods have been developed to recharge groundwater, and most use variations or combinations of direct-surface, direct sub-surface, or indirect recharge techniques (Aish, 2004). The advantage of the direct-surface techniques (surface spreading basins and vadose zone wells) lies in the ability to replenish underground water supplies in the vicinity of metropolitan and agricultural areas, where the groundwater overdraft is severe; and there is an added benefit from the filtering effect of soils and the transmission of water through the aquifer (Asano, 1985) and (Bouwer, 1978). Direct surface techniques are suggested here in this study, as the soil of the study area, El Sadat City, has a good permeability and a wide area for implementation. The source of the recharging water is the treated waste water effluents (45,000 m3/d) from the existed treatment plants in the north east of the city, where the oxidation ponds are. It is expected that these effluents will be increased to 150,000 m3/d in 2025, so a new advanced treatment plant, under construction, has been designed for this purpose and after separating the domestic waste water from the industrial waste water. Quality parameters of concern in the operation of surface infiltration systems (basins or vadose wells) are the suspended solids (SS) and total dissolved solids (TDS) content as well as the concentrations of nutrients that stimulate biological growth and of major cations such as calcium, magnesium, and sodium, which determine the sodium adsorption ratio (SAR). The studied water quality parameters in this study are the TDS and SAR only. The TDS was studied by a non reactive transport model MT3DMS, while the SAR was studied by a reactive transport model which is designed mainly for studying the cations exchange by using the numerical models PHT3D and PHREEQC. OBJECTIVE

The main objective of this study is to predict the short and long-term impacts of recharging a groundwater aquifer, on the surrounding environment and to get water with a quality compatible with national and international standards for irrigation requirements. The regional numerical model is designed mainly to answer these questions:

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1. After how long the recharged water can be recovered and what are the best

pumping locations? 2. Is the quality of the extracted water will be suitable to be used

immediately for irrigation or does it need further treatment? 3. What are the most effective recharging techniques in this case study?

THE STUDY AREA

The selected study area is El Sadat City which is located between 30° 21´ E and 30° 41´ E longitude and 30° 19´ N and 30° 34´ N Latitude and it lies at the kilometer 93 on Cairo – Alexandria highway north-west of Cairo, its total area is 523.5 km2, Figure 1. The total inhabitant area is 18 km2, this area is divided into 12 parts with about 250,000 persons and it includes 5 industrial areas. The new city, as mentioned before, included several activities within it, figure 2 shows the land use and the groundwater flow directions from the north east to the south west. All these activities are mainly depending on the groundwater; most of the supplying wells were constructed near to the residential parts of the city to cover the city needs for water. The average recharge for the groundwater in the green areas is about 0.5 mm/day (RIGW, 2002).

Figure 1: The study area location

Figure 2: Sadat City Land use and groundwater flow directions

N

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METHODOLOGY

After carrying out a calibration and validation on a regional model by using a groundwater piezometric map in 2007, as shown in figure (3), several scenarios were done by studying the two artificial recharge techniques (surface spreading basins and vadose zone wells) and their influences on the water quality changes (total dissolved solids, TDS and sodium adsorption ratio, SAR). Table 1 summarizes the different applied techniques on the model and the corresponding studied parameters and models used. For the practical and effective operational purposes the flooding and drying periods were taken 9 & 12 days respectively to accommodate the large effluents of the treatment plant and to allow the formation of the aerobic conditions in the soil to remove the Nitrogen by denitrification up to within 65 % (Bouwer, 1995).

Figure 3: The model calibration on a groundwater piezometric map 2007

Table 1: The studied parameters in each artificial recharge technique

The studied parameters TDS SAR

Model used Technique MT3DMS PHREEQC

& PHT3D

1. Vadose zone wells

2 m (diam.)

1 m (diam.) 0.3 m

(diam.) X 2. Surface spreading

basins (2 basins)

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TOTAL DISSOLVED CHANGES

The two applied techniques in this study are the vadose zone wells and the surface spreading basins. Figure 4 shows the vadose zone wells and the TDS concentration observation wells distribution. Table 2 presents the distances of the TDS concentration observation wells from the recharging wells. The vadose wells are located in the direction of the groundwater flow and on the center line of two columns in the model (250 m apart) and in several rows (around 1 km apart), this order makes all the wells work together as a group source for the recharged water. Table 2: TDS observation wells

Figure 4: Vadose zone wells and TDS concentration observation wells

Design of vadose-zone wells Vadose zone recharge wells inject reclaimed water into the unsaturated zone between the ground surface and the water table of the saturated aquifer or it can be punched through impermeable geologic layers to introduce the recharge water to more transmissive geology. They also can be used to take advantage of the subsurface geology's natural treatment capacity before introducing the water to the aquifer (American Water Works, 2008). The following equation can be used to calculate the recharge flow rate in a typical vadose-zone well with groundwater levels significantly below the bottom of the well and a water depth in the well of at least five well diameters, (Bouwer, 2004).

𝑄 = 2𝜋𝑘𝐿𝑤2

𝑙𝑛�2𝐿𝑤 𝑟𝑤� �−1 (1)

TDS Observation

wells

Horizontal distances from

wells (7 &8) center (m)

1W, 2W, 3W 985.44 2(1)W 338.86

1E, 2E, 3E 919.08 2(1)E 319.09

TDS Observation

wells

Vertical distances from

the end of wells (11 & 12) in (m)

1M 440.15 2M 1079.92 3M 1688.00 4M 2201.10 5M 2657.27

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Where Q is the recharge flow rate, K is the horizontal hydraulic conductivity of the upper soil material, Lw is the water depth in the well, and rw is the radius of the well. The upper layer soil actual hydraulic conductivity K = 20 m/d to 45 m/d. In this equation (8.1) K was taken 20 m/d to take the clogging effect into consideration. Lw was taken 10 m. Two radii were taken for the proposed vadose wells (0.5m and 1m). In case of rw =0.5 m, the recharge flow rate Q ≅ 4600 m3/d, hence the number of the required wells = 45,000/4600 ≅ 10 wells, while in case of rw = 1 m, Q = 6300 m3/d and number of wells = 45000/6300 ≅ 7 wells. Design of the basins Since that the upper soil layer is mainly fine sand then the average infiltration rate could be assumed 1 m/d (Kopec, 1995), (Bouwer, 2004). Assuming that the artificial ponds will work only in three–fourth of the year for the clogging and the maintenance considerations (Bouwer, 1978) the annual infiltration rate can be calculated as 1m/d * 365 d/year * 0.75 = 273.75 m/year. Notice that during the drying period (after stopping providing water for the basins) the basins are still working in draining the existed water into the aquifer. The treatment plants produce about 45,000 m3/d or 16.425*106 m3/yr of the treated waste water, hence the total needed area for the ponds (1 m depth for the soil stability) = Total water volume/ Infiltration rate = 60,000 m2 considering that the basin depth is 1 m for soil stability and not to provide larger water head on the soil which can increase the clogging (Bouwer, 2004). This area can be achieved by two alternatives; either one pond (245 * 245 m2) or two ponds (173 * 173 m2 for each), while a separate study showed that the second alternative is much better than the first one for some reasons related to the maximum groundwater mound height below the basins. The two surface spreading basins and the TDS observation wells are shown in figure 5. The distances of the TDS observation wells from the surface spreading basins are shown in table 3. Table 3: TDS observation wells in

case of 2 basins

Figure 5: The 2 recharging basins and the TDS observation wells

According to previous studies by Ahmed, M.A. et al. (2011) the TDS of the native groundwater within the recharging area, North East of El Sadat City, in the model was assigned to 1200 mg/l (brackish water) while and the TDS of the treated

TDS Observation

wells

distances from basins center (m)

B1 548.84 B2 173.09 B3 380.81 B4 1092.31 1E 1037.54 2E 428.47 3E 1128.82 1M 1392.08

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waste water used in the recharging process was assigned to 800 mg/l (fresh water). Table 4 shows the TDS problems for irrigation (U.S.EPA, 2004).

Table 4: Total dissolved solids TDS problems for irrigation (U.S.EPA, 2004)

TDS (ppm) Effect on Plants

Less than 500 No detrimental effects usually noticed

500-1000 Detrimental effects on sensitive crops

1000-2000 Adverse effects on many crops requiring careful management practices

2000-5000 Only for tolerant crops on permeable soils with careful management practice

MT3DMS Limitations The TDS was studied by a non reactive transport model MT3DMS, the following points are the MT3DMS limitations: (1) The non reactive transport model MT3DMS considers that the flow of the

infiltrated water through the vadose zone is fully saturated. (2) MT3DMS does not take into consideration the natural purification which can

be done by the soil in the vadose zone, so it is expected that the model results would give quality results less than that can be actually obtained from the nature. However, it gives a good indication for the mixing process with the native groundwater and its propagation in the aquifer.

Results

The TDS breakthrough curves of the all observation wells in case of 2 m vadose well are shown in figure (6) as an example. The values of this graph and the other recharging cases are summarized in tables (5 to 7). The lowest TDS values in this case (2 m wells) are detected in the nearest observation wells, like in 2(1)E the TDS = 839.6 mg/l. however these observations wells have the shortest periods (≈ 5,000 days, 14 years) to restore the full TDS value of its native groundwater (1200 mg/l) after stopping recharge, this is due to the advection and dispersion effects which make the recharged water plume moves far away from the recharging wells and replaced by the native groundwater after stopping recharge.

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Figure 6: TDS breakthrough curves, 2 m diameter wells (for 2 years recharging) Figures (7 a, b, c) show the TDS plumes in case of recharging by 2 m vadose wells (7 wells), case of recharging by 1 m diameter vadose wells (10 wells) and by 2 basins, respectively. The figures show that The TDS plume < 1000 mg/l in case of 2 m diameter wells is wider than the 1 m diameter wells, while in case of 2 basins it is wider in the horizontal direction and tends to propagate in the eastern direction with the groundwater flow to cover an area around 2250 (Horizontal) * 1650 m2 (Vertical), however for getting water with TDS < 1000 mg/l it can be withdrawn after at least half a year of recharging and within an area of 1700 m (horizontal) * 1000 m (vertical) around the recharging field.

a

b

c

Figure 7: TDS plume (after 2 years recharging): a. case of 2 m diameter vadose wells, b. 1 m diameter vadose wells, c. 2 spreading basins

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Tables 5 to 7 summarize the TDS concentrations of the treated waste water and the native groundwater before recharging and the TDS concentration of the new mixture after 1 and 2 years of recharging at the observation wells in cases of recharging in 2, 1 m diameter vadose zone wells and 2 surface spreading basins, respectively. Also these tables are including the number of days which are needed to restore the native groundwater TDS concentration (1200 mg/l) at each observation well, in addition to the recharge water fraction (F) which was calculated after 1 and 2 years of recharging according to equation 2. The recharge water fraction (F) is defined as the degree to which the recharge water has mixed with ambient ground water at a site from a recharge facility (wells or basins). It can be estimated from water quality parameters like TDS or some conservative substances.

F = Cobs−CGWCrech−CGW

∗ 100 (2)

Where Cobs is the concentration at the observation well, CGW is the concentration at the ambient groundwater before recharging and Crech is the concentration of the recharged water.

Table 5: TDS breakthrough curves, 2 m diameter wells

Table 6: TDS breakthrough curves, 1 m diameter wells

From tables (5 and 6) it is obvious that the observation well 2(1)E has the lowest values in TDS as it is one of the nearest observation wells (319 m) to the recharging field and in the eastern side at which the groundwater flows. Consequently the recharge water fraction in this well 2(1)E is the highest value (90 % in the first case and 86 % in the second one), nevertheless it is one of the first observation wells which restore their original TDS (1200 mg/l) especially in the higher flow rate (Q = 6300 m3/d), as the mixing plumes start leaving the nearest observation wells after stopping recharge and be replaced by the ambient

Time (days) 5M 4M 3M 2M 1M 3E 2E 2(1)E 1E 3W 2W 2(1)W 1W Remarks

0 800 800 800 800 800 800 800 800 800 800 800 800 800TDS of treated waste

water

0 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200TDS of native

groundwater at time 0 of recharging

365 1200 1200 1200 1196.1 1068.6 1171.9 1165.5 888.75 1200 1191.2 1185.3 1058.9 1200 1 Year

recharge water fraction (F) % 0 0 0 0.98 32.85 7.02 8.63 77.81 0 2.20 3.68 35.28 0

F= (Cobs-CGW)/(Crech-CGW)*100

730 1200 1200 1199.6 1174.9 980.25 1079.9 1085.3 839.56 1200 1165.4 1162.6 992.01 1200 Recharge stopsDays to restore

1200 mg/l NA NA NA 19000 17000 17000 14000 14000 NA 15500 NA 14000 NA

recharge water fraction (F) %

0 0 0.1 6.275 54.9375 30.025 28.675 90.11 0 8.65 9.35 51.9975 0F= (Cobs-CGW)/(Crech-

CGW)*100

Time (days) 5M 4M 3M 2M 1M 3E 2E 2(1)E 1E 3W 2W 2(1)W 1W Remarks

0 800 800 800 800 800 800 800 800 800 800 800 800 800 TDS of treated waste water

0 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200TDS of native

groundwater at time 0 of recharging

365 1200 1199.8 1197.9 1181.5 1113.5 1184.4 1173 926.98 1190.7 1198.1 1186.5 1084.9 1193 1 Year

recharge water fraction (F) %

0 0.05 0.52 4.63 21.63 3.90 6.75 68.26 2.32 0.48 3.38 28.78 1.75F= (Cobs-CGW)/(Crech-

CGW)*100730 1199.8 1197.6 1184.7 1132.5 1035.8 1113.4 1093.2 857.11 1164 1187.3 1154.3 999.91 1179.8 Recharge stops

Days to restore 1200 mg/l

NA NA 19000 19000 17500 18000 15000 15700 12600 15500 13400 14800 11800

recharge water fraction (F) %

0.05 0.60 3.82 16.88 41.05 21.65 26.70 85.72 9.00 3.18 11.43 50.02 5.05F= (Cobs-CGW)/(Crech-

CGW)*101

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native groundwater. Also it is clear that there is no big difference between the TDS values after 2 years in both used vadose zone wells. Table 7 gives a short summary about the TDS values in the observation wells near to the two surface spreading basins. From the table it is clear that the lowest values of TDS are found in the nearest observation wells (B2 and B3). Moreover it is obvious that recharging by the two basins is giving faster and wider coverage of the recharged water than in cases of recharging by the vadose zone wells, as the recharge water fraction in case of the two basins just after one year from recharging ≈ 93 %, while in the vadose zone wells (Q = 6300 m3/d) after 2 years ≈ 90 %.

Table 7: TDS concentrations changes in the observation wells (case of 2 basins)

SODIUM ADSORPTION RATIO CHANGES The increase of Sodium Adsorption Ratio (SAR) in the water leads to a rapid reduction in the infiltration rates and affect negatively the sensitive crops if irrigated with this water. SAR changes were studied by using PHT3D v.1.46, which is a multi-component transport model for three-dimensional reactive transport in saturated porous media. It couples the two existing and widely used computer programs, the solute transport model MT3DMS v5.1 (Zheng and Wang, 1999) and the USGS geochemical code PHREEQC-2 (v2.9), (Parkhurst and Appelo, 1999). The coupled model forms a powerful and comprehensive three-dimensional reactive multicomponent transport model (Visual MODFLOW v.4.2 Manual, 2006). PHREEQC-2 is a computer program written in the C programming language for simulating chemical reactions and transport processes in natural or polluted water. PHREEQC is based on an ion-association aqueous model and has capabilities for calculating the speciation and saturation-index and doing batch-reaction and one dimensional (1D) transport calculations. The used models here PHT3D v.1.46 and PHREEQC which are compatible with MODFLOW v.4.2 were used to study the cations (Na+, Ca+2, Mg+2) exchange between the recharged water and the native groundwater during and after the artificial recharge process. These cations concentrations could be used later to calculate the Sodium Adsorption Ratio (SAR) from equation 1. SAR is used to determine whether water will disperse or flocculate clay as Sodium increase in the soil has severe hazards on plants, soil and recharging process.

Time (days) B4 B3 B2 B1 3E 2E 1E 1M

0 800 800 800 800 800 800 800 800 TDS of treated waste water

0 1200 1200 1200 1200 1200 1200 1200 1200TDS of native groundwater

at time 0 of recharging

365 1049.9 830.29 829.17 863.92 1128.2 913.37 1190.9 1198 1 Year

recharge water fraction (F) % 37.53 92.43 92.71 84.02 17.95 71.66 2.27 0.50

F= (Cobs-CGW)/(Crech-CGW)*100

730 931.09 814.64 813.27 827.4 996.72 854.91 1160.4 1172.9 Recharge stopsDays to restore

1200 mg/l 16500 14000 14800 16000 17200 14600 12200 15700

recharge water fraction (F) % 67.23 96.34 96.68 93.15 50.82 86.27 9.90 6.77

F= (Cobs-CGW)/(Crech-CGW)*100

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𝑆𝐴𝑅 = �𝑁𝑎1+�

�12([𝐶𝑎2+]+[𝑀𝑔2+]) (1)

The used PHT3D model has several PHREEQC databases for different reactions of different case studies which can be used for other similar cases. The used database here was designed for studying the cation exchange during artificial recharge based on the work which was done by Valocchi et al. (1981), this work has served as a benchmark problem for several numerical models (Appelo and Postma, 1993; Walter et al., 1994; Zysset, 1993 and Prommer et al., 2003). The database which has been used here is explained in details in PHT3D Manual, Example 5 (Prommer, 2002). The SAR of the treated waste water was calculated from equation (1) to give SAR = 6.8 which is quite higher than the SAR value (6.0) at the water quality standards of the recharged water (Who, 2006). However the existed treated waste water SAR = 6.8 was used here in the modeling, assuming that no improvements in the treatment process. Table 8 represents a summary for the most important compounds and the compounds of the existed treated waste water in the oxidation ponds which would be used as a source for water recharge and the native groundwater by the model, these values were extracted from Ahmed et al. (2011). This table was used as input data for the PHREEQC model. All the extracted minerals are having units of mg/l. However they have been converted into mol/l to agree with the PHREEQC default units. Table 8: The physiochemical data of the treated waste water and the native groundwater

Data from Table 4.4

For PHT3D by MODFLOW

Units Parameter Treated water (mg/l)

Native GW

(mg/l)

Treated water

(Mol/l)

Native GW (Mol/l)

oC Temp. 21.3 23.7 0.0213 0.0237

-- pH 7.56 7.86 7.56 7.86

(Mas

s/V

ol.)

DO 1.04 2.56 0.000065 0.00016 TDS 800 1,200 800 1,200 Na 247 266 0.0107391 0.0115652 Ca 53.4 141 0.001335 0.003525 Mg 27.6 64 0.0011358 0.0026337

Limitations and Assumptions

1. The cations exchange was studied by recharging through 2 basins or only one vadose zone well with different flow rates from Q = 6300 to 3000 m3/d to save the time of simulation.

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2. The mixing is continuous and saturated. 3. It is assumed that there is no cation adsorption by the soil sediment (CEC,

Cation Exchange Capacity of the soil is negligible) because the upper layer of the study area is mostly sandy and no clay or organic matters (TerraGIS, 2007).

4. There is no oxidation or redox reaction (which depends on Iron or Sulfates).

5. The recharge simulation extended from 2 years and more without drying periods and only one observation well within 500 m from the recharging field.

Results The following figures (8 to 11) are showing the cations concentrations in the native groundwater before the recharging starts and the cations concentrations changes along the time after recharging for 5.5 years or 20 years. All the models showed that the cations reach to their minimum values within 5.5 years and maintain constant until stopping the recharge process. The figures show clearly that the cations concentration reductions, especially in Ca2+ and Mg2+, in the groundwater in case of recharging by basins are higher than the case of recharging by 2 m diameter well, which is also higher than the 1 m diameter well, which is higher than 0.3 m well, which means that SARBasin> SAR2m > SAR1m > SAR0.3m.

Figure 8: Cations changes in case of recharging by 2 Basins

Figure 9: Cations changes in case of recharging by 2 m diameter well

Figure 10: Cations changes in case of recharging by 1 m diameter well

Figure 11: Cations changes in case of recharging by 0.3 m diameter well

The following graph, figure (12), represents the calculated SAR values in the different recharging techniques used in this study. It is clear that recharging by the

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surface spreading basins are giving the highest value of SAR which is equal to the SAR of the recharged water, then the SAR of the highest flow rates in the vadose zone wells is also higher than the others which have lower flow rates, which means that operational practice and the applied technique play an important role in determining the SAR of the new mixture. Accordingly it was found that the best design diameter in case of recharging by the vados zone wells is 0.3 to 1.0 m which is corresponding to Q = 3000 and 4600 m3/d respectively to guarantee the lowest values for SAR.

Figure 12: SAR in different cases according to the operational practices

CONCLUSIONS

• The changes in the vadose zone wells diameters have no significant influence on the TDS concentrations in the subsurface, while in case of recharging by surface spreading basins the TDS are decreasing more rapidly and covering wider area.

• It takes about 20 years (after 2 years recharging only) to restore the native groundwater TDS concentration within the recharging field.

• The plume of the recharged water tends to move towards the eastern side of El Sadat City, quite far from the drinking wells in the south of the city, especially in case of recharging by spreading basins.

• The treated waste water has a relatively high sodium adsorption ratio (SAR) = 6.8, which can increase the clogging rate in the surface spreading basins or vadose wells.

• Operational practices play an important role in determining the SAR, as it increases when the given flow rates in the vadose zone wells increase or in case of recharging by 2 basins; consequently the recovered water can be harmful to the sensitive plants and the infiltration rates.

RECOMMENDATIONS

• For getting water with TDS < 1000 mg/l in case of vadose wells it is recommended to withdraw it after at least half a year and from the closest eastern well (not further than 500 m) like 2(1) E, or after 2 years from the closest eastern or western wells or downstream the recharging field up to 500 m, while in case of recharging by basins it is recommended to

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withdraw it after at least half a year and within an area of 1700 m (Hz) * 1000 m2 around the recharging field.

• In case of recharging by shallow (vadose zone) wells it is recommended to use more wells with smaller diameter (0.3 to 1 m) to reduce the flow rates and the SAR.

• In case of recharging by surface spreading basins it is recommended to reduce the sodium adsorption ratio (SAR) of the treated waste water to be less than 6.0, otherwise the recovered water should not be used with sensitive crops or precautions should be taken before irrigation.

• It is recommended to use the combined techniques, 1 basin and 5 vadose zone (1 m diam.) wells, as the best alternative in this area in the case that the existed treated waste water SAR has not been reduced. However, in case of the combined technique it is recommended to still have 2 basins, as one of them can be set as a standby or to work alternately with the another one in its drying period or to be used in the emergency cases.

• Cation exchange process between the recharged water and the soil particles should be studied more in details by getting the actual cation exchange capacity and redox state (Pe) of the soil in the project area.

• A water quality monitoring network near to the project after implementation to follow up the recharged water progress, the quality changes and the influences on the surroundings.

REFERENCES

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