Groundwater salinity distribution
Total dissolved solids (TDS) comprise inorganic salts that are dissolved in water. According to the total dissolved solids (TDS) of the investigated groundwater samples, the groundwater salinity shows that the TDS values of El Qaa plain groundwater vary from 394 ppm (sample No. 12) to 4907 ppm (sample No.19) reflecting fresh to brackish category, while El Gebail groundwater has brackish water to highly saline as it varies from 2578 ppm (sample No. 11) to 13864 ppm (sample No. 5). It is clear that 42 % are brackish (Nos. 3, 7, 11, 22, and 24) and 42 % are saline (Nos. 1, 2, 6, 8, and 9), while 16 % of samples are highly saline (Nos. 5 & 10). The salinity distributions in the study area are shown in (Fig. 6). It was found that the groundwater salinity in El Qaa area increased from Northeast to Southwest and from North to South in El Gebail area. This variation of salinity indicates different direct flows (seepage) of surface and subsurface water, which may be attributed to the location of wells near or away from the rainfall recharge area. The groundwater salinity is influenced by climate factors, groundwater level, and seawater intrusion.
Chemical water types
According to chemical analyses of groundwater samples, the water chemical types are classified according to the dominant anions and cations into:
According to the ion dominance, Chloride–Sodium chemical water type is recognized in El Gebail area. It is the dominant water type characterizing the saline groundwater in El Gebail area. Presence of Chloride–Sodium type reflects the advanced stage of groundwater metasomatism. Such type reflects the leaching and dissolution of marine salts.
Assemblages of hypothetical salts
Hypothetically, the ions of the strong acids (Cl− and SO4
2−) form chemical combination with alkalis (Na+ and K+) and the rest of the acid radicals combine with the alkaline earths (Ca2+ and Mg2+). If the cations of alkalis and alkaline earths are surplus in water, they well combine with the weak acids (CO3
2− and HCO3
−). To clarify such combinations, the relationships between cations and anions in the investigated waters are illustrated in the form of bar graphs.
The combination of major anions and cations in the investigated groundwater of El Gebail reveals the formation of one group of hypothetical salts (Fig. 7a).
All groundwater at El Gebail area are characterized by assemblage (NaCl, MgCl2, CaCl2, CaSO4, and Ca (HCO3)2), which has three chloride salts (more advanced stage) of chemical development reflecting leaching and dissolution of terrestrial and marine salts. On the other hand, the combination of major anions and cations reveals the formation of three main groups of hypothetical salts in El Qaa plain groundwater (Fig. 7b). These salts are:
-
I.
NaCl, Na2SO4, MgSO4, CaSO4, and Ca (HCO3)2.
-
II.
NaCl, MgCl2, MgSO4, CaSO4, and Ca (HCO3)2.
-
III.
NaCl, MgCl2, CaCl2, CaSO4, and Ca (HCO3)2.
The results reveal that (60 %) of Groundwater samples in El Qaa are characterized by assemblages II and III, (more advanced stage of chemical development), and (40 %) are characterized by assemblage I (less advanced stage of chemical development), where the presence of Na2SO4 reflects the impact of terrestrial salts dissolution.
Scale prediction modeling
Referring to the evaluation of water characteristics in study area, water samples in El Gebail which have excess salts are estimated as feed water for solar thermal desalination. Two computer programs were applied to achieve the main target:
-
NETPATH for windows.
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PHREEQC.
Chemical equilibrium and saturation indices (SI)
The saturation indices (SI) of the major mineral phases in the investigated water samples were calculated using the software package (NETPATH-WIN). The obtained results, (Figs. 8, 9, 10, 11) reflect that
-
1.
The groundwater samples (Nos. 2, 3, 11, 22) have negative (calcite, aragonite, and dolomite) indices, while sample (No. 5) has negative dolomite, as well as the rest of groundwater samples are supersaturated with respect to the main carbonate minerals (calcite, aragonite, and dolomite).
-
2.
Most of the groundwater samples have negative gypsum indices, indicating that, (the most of groundwater samples are unsaturated with this mineral). The dissolution of gypsum can be expressed as follows:
$${\text{CaSO}}_{ 4} \leftrightarrow {\text{ Ca}}^{ 2+ } + {\text{ SO}}_{ 4}^{ 2- }$$
-
3.
Increasing of Ca2+ concentration due to gypsum dissolution causes calcite to precipitate. However, the dissolution of gypsum induces the transformation of dolomite to calcite in the sediments and produces water with high Mg2+, Ca2+, and SO4
2− concentrations (Appelo and Postma 2005).
-
4.
Most of the groundwater samples are saturated with respect to quartz, chalcedony, and talc.
-
5.
The groundwater is supersaturated with respect to iron mineral phases (hematite, goethite). Hematite and goethite reflect the sensitivity of iron to oxidation even in low concentrations.
Corrosivity and scale formation
According to the saturation indices of minerals in the investigated water samples as indicator of water corrosivity or scale forming, the following could be deduced:
-
1.
About 31 % of water samples are faint coating, (Fig. 12). Faint coating in the municipal wells may (by time) lead to clogging of the pipes, which transport the water to the inhabitants, so treatment is recommended.
-
2.
About 38 % of samples are mild scale forming.
-
3.
About 31 % of water samples are mild corrosion.
Using PHREEQC Model
PHREEQC was used for applying evaporation with continuous water feeding model to simulate perspectives of solar-powered desalination unit (Fig. 1). For applying the model, the data of water chemical analyses of wells, Nos. 1, 2, 3, 4, 5, 8, and 10, are used in this study. Model was applied at 90 °C using different water recovery percentages from feed water, the first is 30 %, the second is 60 %, and the third is 90 %; the results are shown in (Figs. 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23). From the results of PHREEQC, we can make a comparison between the saturation indices of minerals in the investigated water samples and the results as indicator of water corrosivity or scale forming, The obtained results, (Fig. 23) reflect that the scaling tendency of the desalination unit increases as water recovery percentage increases from feed water by increasing ions concentration responsible to form scale Ca2+, SO4
2−, and HCO3
−. This threat of scaling is more serious when the temperature increases; indeed, these two salts, CaCO3 and CaSO4, present, particularly, have an inverse solubility phenomenon. Also results indicate that water recovery percentages of 60 and 90 % with continuous water feeding are suitable for operation in desalination systems.
Influence of feed water salinity on Scale Morphology
Three different feed waters (wells Nos. 1 and 8) and sea waters (No. 4) were tested with different water recovery percentages at 90 °C. Figure 24 shows the morphology micrograph of the precipitate. It can be observed that, as the water salinity increased, the structure of the precipitate changed. This does not mean that the morphology changed, but it means that the structure of the composite precipitate changed. For well No. 1, the precipitate was found to be attached to the test tube wall, with many crystals. Precipitates for well No. 8 have a structure contained more small needle-shaped crystals, it is clear from the figure that precipitate was found to adhere to the test tube wall. Precipitates for sea water No. 4 had a structure very different from those of the previous precipitates and consisted mainly of very fine powdery crystals. This precipitate was found to adhere to the test tube wall in a fine even layer.