Groundwater quality assessment along the West of New Damietta Coastal City of Egypt using an integrated geophysical and hydrochemical approaches

Recently, the groundwater became very important source for the Egyptian water balance. Therefore, assessing its quality and quantity is necessary before initiating any developmental plans using this resource. In this research, a new operated power plant, which is located in the coastal area of West New Damietta City, north-east of Egypt was designed to work using the coastal groundwater aquifer. Therefore, an integrated approaches of both geophysical methods and hydrochemical analysis were applied to identify the subsurface lithology, the water-bearing layers and determine the potential use of such coastal groundwater. Therefore, a total of seventeen vertical electrical soundings (VES) and five time-domain electromagnetic soundings (TEM) were performed. Moreover, groundwater samples were collected from seven existing wells to be analyzed for water quality assessment. The geophysical results identified five geoelectric layers. The first geoelectric layer has a resistivity of 1–7.9 Ω m and a thickness range of 7–9 m. The second layer has a resistivity of 0.9–4.4 Ω m and a thickness range of 7–8 m. The third layer is 10–21 m thick with a resistivity value of 0.5–2 Ω m. The fourth layer, which is thicker (29 to 42 m), has a resistivity of 0.7–3 Ω m, while the last layer has a resistivity of 1.7–9 Ω m. According to the hydrochemical analysis, the aquifer is dominated by Na–Cl water type. The brackish nature of the water is revealed by the TDS range of 7035 mg/l to 7735 mg/l. The results collected demonstrate the groundwater's quantity and quality availability for the power plant’s sustainable use.


Introduction
Coastal areas have become progressively developed around the world. They cover over 10% of the planet's surface and contain more than half of the world's population, as well as human activities (Chatton et al. 2016). The Egyptian coastal region is undergoing tremendous demographic, environmental, economic, and social developments with a high concentration of water-demanding human activities (De Filippis et al. 2016). Development projects, including agriculture, domestic water use, and electricity generation, require a sustainable supply of fresh water. In the absence of applicability of the nearby surface water resources, the demand for groundwater resources is the alternative (Eltarabily and Negm 2019).
West New Damietta is one of Egypt's coastal regions, located northeast of the Nile Delta, where the increase in human demands and the expansion of national projects have necessitated the need for long-term water supplies. Therefore, this research is to investigate the groundwater Many studies were carried out on groundwater quality evaluation and hydrochemical characterization (Brindha et al. 2014;Sajil Kumar et al. 2014;Wu et al. 2014Wu et al. , 2015Bouzourra et al. 2015;Vetrimurugan and Elango 2015;Li et al. 2016). In addition, many researchers in Egypt have focused on groundwater resources in terms of quality (Masoud 2014;Armanuos et al. 2016;Negm and Armanuos 2017;El-Rawy et al. 2019;Salem et al. 2019;Ding et al. 2020;Hegazy et al. 2020;Mansour 2020;El-Kholy et al. 2022).
The physical and chemical properties of groundwater are influenced by both natural and anthropogenic causes. Natural factors include lithology, groundwater velocity, water-rock interaction, evaporation/crystallization, oxidation or reduction reactions, salt solubility and recharge water quality; anthropogenic factors entail agriculture, industry, mining, urban development, etc. (Jeong 2001;Hussein 2004;Hosseinifard and Mirzaei Aminiyan 2015). Numerous investigations focused on evaluating the hydrochemical changes and showing their spatial distributions using geographic information systems (GIS) (Ahmed et al. 2020a, b) as well as statistical techniques (Ahmed et al. 2020a). Therefore, this study aims at identifying the groundwater quality close to the newly constructed power plant in New Damietta coastal area and evaluate the underneath coastal aquifer for sustainable water supply.

The study area
The study area is located in the northeastern part of the Nile Delta. It extends between latitudes 31° 26′ 27.5″ and 31° 26′ 43.5″ N, longitudes 31° 36′ 14″ and 31° 36′ 39.5″ E (Fig. 1). It covers an area of 52.4 Acres (220,080.19 m 2 ). Topographically, it is generally flat with elevation not more than 2-3 m above sea level with gently slope toward the North. The climate is largely influenced by the Mediterranean Sea, with hot, arid summer and warm winter with moderate rainfall; the average precipitation is 200 mm/year (RIGW 1992). The coastal sand dunes, beach ridges and sabkha are the main geomorphic units that cover the surface of the study region.
Geologically, it is mostly covered by Quaternary deposits. These deposits are gravel and sand with some clay lenses belonging to Bilqas, Mit Ghamr and Wastany formations from the Holocene and Pleistocene periods. These deposits underlain by fluviatile and fluvio-marine sediments representing the main groundwater unit followed by the thick clay of the old deltaic succession of Tertiary (Sallouma 1983) (Fig. 2).
Hydrogeologically, the aquifer is part of the Nile Delta Quaternary aquifer, which is a moderately productive aquifer (Abu-Bakr et al. 2016). This aquifer consists of Pleistocene graded sand and gravel, changing to fine and clayey facies in the north . The thickness of the Quaternary major water-bearing unit is about 1000 m in the study area . It is mainly recharged from Damietta Branch, flooding irrigation water as well as rainfall. Static water level was measured in the study area through 7 wells and indicated that groundwater can be found at shallow depths of 1.0 to 2 m. The groundwater flow is directed from south to north.
The groundwater flow is synchronized with the topographic level, flowing from high elevation to low elevation (from south to north) as indicated by the map and the depth to groundwater ranges from 1 to 3 m below ground level and increases toward the south (Fig. 3) (Abu-Bakr et al. 2016). The hydraulic heads range between 29.8 and 35.84 m above (m.s.l.).
Structurally, the area is homoclinal, with a low northward dip (Elewa et al. 2013). It is a part of the unstable shelf and Nile Delta's northern block, which is an active tectonic region characterized by growth faults (Zaghloul et al. 2001). The primary fault in the study area is the Rosetta fault, which runs NE-SW (Abd el- Aal et al. 1994).

Material and methods
In this research, the integration between geophysical techniques and hydrogeochemical analysis was performed to describe the shallow aquifer along the study area.

Geophysical methods
Several geophysical techniques, including VES and TEM, not only identify the water-bearing layers, but also can quickly identify zones of saline groundwater as highly conductive (low resistivity) volumes beneath the surface (Jansen 2011). Therefore, a total of seventeen VES and five TEM soundings were carried out in the study area to determine the subsurface lithology, water-bearing layers and groundwater salinity.

Resistivity method
Electrical resistivity techniques determine earth resistivity by passing a direct current (DC) signal through the earth and measuring the resulting potentials (voltages). Using the collected data, the electrical properties of the earth (the geoelectric section) can then be calculated (Sundararajan et al. 2012). Bhattacharya 1968, Parasnis 1973and Zohdy 1975, 1965 have detailed descriptions of the theory and use of this method to groundwater studies. Schlumberger and Wenner arrays are the most common electrode arrangements for measuring potential differences, and they are utilized for a variety of reasons in exploration (Keller and Frischknecht 1966).
In this investigation, a one-dimensional Schlumberger array with a maximum current electrode spacing (AB) of 600 m was used to conduct a geoelectric survey that reached relatively deep. As shown in Fig. 4, a total of 17 VES were conducted to cover the entire research region. The field measurements started at two meters between the present poles and continued up to 600 m to determine the subsurface sequences and identify the various layers of aquifers. The Terrameter SAS 1000 system was employed, which can record changes in ground resistance values with high sensitivity. Finally, the obtained data were processed using 1X1D postprocessing software, which provides the true resistivity and depth from the observed apparent resistivity data at each location.

Transient electromagnetic sounding (TEM)
The TEM technique can be used in a variety of situations; it does not require direct electrical contact with the ground, and it can be used at depths ranging from a few meters to hundreds of meters. It is a quick and cost-effective technology, although it does not operate well in highly resistant rocks and is subject to interference from surrounding metal pipes, cables, fences, automobiles and induced noise from power lines (whether buried or not) (Barsukov et al. 2015). A direct current is transmitted into the ground-based transmitter loop in the TEM method. A primary, stationary magnetic field is created by the current. When the direct current is turned off, an eddy current system is created in the ground. The current system will fade due to the subsurface's ohmic resistance, causing a secondary magnetic field that can be measured in an induction coil (the receiver coil). The electromagnetic field's decay rate is determined by the subsurface resistivity distribution.
Depending on the exploration goal, TEM soundings are typically spaced between 150 and 250 m. The equipment's magnetic moment (i.e., transmitter loop size and number of turns, as well as the transmitted current), the ground's resistivity and the quantity of electromagnetic background noise all influence penetration depth (Macnae et al. 1984;McCracken et al. 1986). The TEM method has the advantage of being sensitive to conductors at considerable depths and having simpler equipment than VES (Abdelfattah et al. 2021).
In this study, a total of five TEM soundings that corresponded to the VES site ( Fig. 4) were monitored using a SIROTEM MK-3 time-domain system. These soundings used a simple loop setup at appropriate locations in the study area while avoiding noise. In each location, the single loop of 50 × 50 m was performed, and the measurements were taken four or five times and for processing and analysis, the best signal-to-noise data sets were selected. These measurements were acquired and analyzed using ZondTEM software, which resulted in a one-dimensional model with depth at each sounding, finally by combining VES and TEM soundings to produce a two-dimensional cross section and define subsurface layers along the research area.

Hydrogeochemical analysis
Water samples were taken from seven production water wells in the study area, with a total depth of 120 m to 140 m and chemically analyzed (Fig. 5). These water samples were extracted after 10 min of pumping to attain steady-state chemical conditions, which reduces the impact of leftover water in the well on analysis results. The samples were collected in 500-mL sealed polyethylene bottles that were cleaned, labeled and organized before being filled directly (after filtration through 0.45-μm filter paper) without any treatments according to APHA, 2005. Portable multi-parameters were used to measure electrical conductivity (mS/cm), hydrogen ion (pH), temperature (°C) and total dissolved solids (TDS in mg/l) in situ. Analysis of the water samples was carried out in the laboratory of Faculty of Science, Port Said University, using the atomic absorption instrument. All concentration values were expressed in milligram per liter (mg/l), and the analysis results were used to evaluate and identify the water type in the study area using Piper plot.

Geophysical interpretation
The integration of VES and TEM curves indicates the presence of low geoelectrical resistivity with depth, which could be connected to the prevalence of clayey, silty and fine sandy materials. The small variations in resistivity values within the zones might be due to changes in deposit grain size, clay interference limit and water saturation ratio (Figs. 6, 7). In addition, being near the sea causing reduction in the resistivity levels in both the lateral and vertical distributions, seawater invasion is expedited because the lithological section is predominantly permeable, and Sabakha occurs at the ground surface, especially Fig. 6 The processed VES soundings: a VES 1, b VES 2, c VES 3, d VES 4 and e VES 5 at low topographic landscape. Such geological setting is extremely affecting the water chemistry and thus its quality.
Four geoelectrical cross sections were created to highlight lateral and vertical hydrolithological variations. Two of the geoelectric cross sections are parallel to the shore (Figs. 8,9), while the other two are N-S oriented perpendicular to the coast (Figs. 10, 11). A total of five significant geoelectric layers were identified in all soundings. In all sections, • the uppermost geoelectrical layer has a regular expansion. It is primarily made up of poorly sorted sand and gravels with a thickness range between 7 and 9 m and resistivity of 1-7.9 Ohm.m. • The underneath layer is mostly clayey sand with silt, with a resistivity of 0.9 to 4.4 Ω m and a thickness varying from 7 to 8 m. The thickness of this layer increases toward the west in section A-A′ and B-B′ (Figs. 8, 9). • The third geoelectrical layer is thicker than the previous two (10 to 21 m). This layer displays resistivity ranging from 0.5 to 2 Ω m across the entire area, indicating that strong connection conditions exist. This resistiv-ity means that the layer above it is of finer facies. This resistivity range corresponds to finer facies as clay sediments. • The fourth layer is made up of sandy clay with a resistance of 0.7 to 3 Ω m and is thicker (29 to 42 m). In cross section A-A′ (Fig. 8), it thickens toward the west, while in section D-D′ (Fig. 11), it thickens toward the north. • The lowermost geoelectrical layer represents the main water-bearing formation in the area. It attains resistivity values ranging from 1.7 to 9 Ω m reflecting clayey coarse sands with brackish water conditions.
Despite the fact that resistivity measurements are influenced by a number of different factors, some standard ranges can be used to estimate the salinity of groundwater. For example, resistivity ranges of 30 to 50 Ω m or higher suggest sandy zones filled with air or fresh water, resistivity ranges of 20 to 30 Ω m represent soils that are clay-rich, and typically 10 Ω m or less demonstrate sandy zones filled with brackish or saline water (Jansen 2011). This result reveals that the aquifer in the study area contains brackish water because all of the measured resistivity values are less than 10 Ω m.

Groundwater quality
The analytical results of physical and chemical parameters of groundwater samples are tabulated in Table 1. Results of chemical analyses show slight variations in chemical compositions and also indicate high salinity of groundwater samples.
The pH value of an aquatic system is a key indication of water quality and pollution levels in watershed areas (Yisa and Jimoh 2010). The pH of the water samples ranged from 7.82 to 8.1, revealing that the water is somewhat alkaline in nature and within the World Health Organization (WHO) and Egyptian Higher Committee for Water's acceptable level.
The EC is a critical measure for determining salinity dangers and suitability for a variety of applications. It is a method of determining the total salt content of water based on the passage of electricity through the sample. The higher the salt content, greater will be the flow of electrical current. The EC measured ranged from 10.05 to 11.05 mS/cm. Water in this area is overly mineralized, according to the relation between EC and water mineralization. Indeed, seawater intrusion has been linked to elevated EC values along the shore (Stamatis and Voudouris 2003). In Well 3, the highest value was recorded.

Total dissolved solid (TDS)
To determine the acceptability of groundwater for any use, it is necessary to classify it according to its hydrochemical qualities as measured by TDS (Freeze and Cherry 1979). According to Egyptian standards, the acceptable TDS amount for drinking water is 1000 ppm, whereas it is only 600 ppm according to WHO guidelines. TDS ranged from 7035 to 7735 mg/l. Accordingly, all the samples above this limit due to saline intrusion and other anthropogenic factors. The water in this study region falls into the brackish water group, according to the classification shown in Table 2. TDS profile (Fig. 12) shows that the maximum value recorded in the well 3 and TDS generally increase toward the east. These TDS values show strong correlation with the resistivity results improving the aquifer's low resistivity, which is composed of fluvial sands and gravels with minimal clay intercalations and the brackish nature of the groundwater.
Seawater intrusion is the main factor affecting groundwater quality and significantly raising TDS in this coastal aquifer within the study area (Abu-Bakr et al. 2016).

Major cations
The ion concentration in the investigated area does not vary significantly. The concentration of all ions increases toward the eastern part of study area except Mg 2+ and K + decreases. In terms of main cations, Na + is the most common, followed by Ca 2+ , Mg 2+ and K + . Chloride and bicarbonate are the most common main anions, followed by sulfate.
Sodium concentrations range between 1600 and 1710 mg/l. The high sodium concentration may be likely owing to clay and shale leaching processes in the aquifer materials or seawater encroachment. The concentration profile demonstrates that Na + levels have risen toward the east (Fig. 13). The western region of the area has low Na + concentration. Moreover, due to excessive Na + concentration (Na + > 200 mg/l), all of the groundwater wells are unsuitable for drinking (WHO 2011).
Ca 2+ levels range from 195.68 to 302.84 mg/l increasing from west to east (Fig. 14). The approved limit of calcium for drinking according to the World Health Organization is 200 ppm, which means that the calcium concentration in the area's groundwater is unacceptable. The leaching processes of sulfate minerals, gypsum and anhydrite, as well as  carbonate minerals, calcite and dolomite, from the dominant sabkhas in the field of research may be responsible for the high elevated calcium levels.
In the research region, K + and Mg 2+ show a different trend. K + and Mg 2+ decreased to the east (Figs. 15, 16). Generally, wells with high K + and Mg 2+ concentrations are found in the study area's central and western region. Potassium concentrations are ranged between 36 and 41 mg/l, whereas magnesium ranged from 75.33 to 77 mg/l. In all samples, the potassium concentration surpasses the WHO guideline value (30 mg/l), while Mg 2+ concentrations are within. Magnesium content is mostly due to weathering of magnesium minerals and dolomites leaching, while potassium content is owing to silicate weathering process and human activities.

Major anions
The chloride content ranges between 2419 and 2684 mg/l. Salt dissolving, irrigation drain reverse flow, disintegration of chloride-bearing deposits and clay minerals within the aquifer materials and seawater intrusion contamination by industrial waste or domestic sewage, could be the main sources of high elevated chloride. The distribution profile of Cl − (Fig. 17) strongly correlates with that of Na + and depicts an increase in the chloride content toward the east. All of the groundwater wells are unfit for drinking as it exceeds the WHO permissible limit (250 mg/l).
In general, HCO 3 − concentration increases to the east of the study area (Fig. 18). HCO 3 − values in groundwater range from 446 to 548 mg/l, indicating a contribution from water-rock interaction (Park et al. 2002).  The concentration profile of SO 4 2− is different from the other ions, as high concentration is found within well 3 (Fig. 19). Sulfate content ranges from 402 to 462 mg/l in the study area. The washing of highly soluble sulfate-bearing minerals may be considered as the main sulfate source in the study area. Piper (1944) trilinear diagram can be used to examine the geochemical genesis of groundwater by looking at the concentrations of key cations and anions. Because they graphically depict correlations between the most major dissolved constituents in a batch of groundwater samples, trilinear diagrams can be used to identify the hydrogeochemical facies.
The Piper diagram realization was created using Surfer software. The hydrochemical classification of the groundwater (Fig. 20) shows that all samples are of Na-Cl water type which generally indicates a strong seawater influence.

Conclusion
The groundwater quality in the west New Damietta power plant was assessed using an integrated approach of hydrochemical and geophysical methods. The main aquifer in the research region is the Quaternary aquifer, which is composed of fluvial sands and gravels with minimal clay intercalations. Groundwater recharging in the studied area is still mostly accomplished through direct infiltration from the rainfall system. According to the interpretation of hydrochemical analysis, the groundwater in the area is brackish since the minimum and maximum TDS values are 7035 and 7735 mg/l, respectively. The PH value ranges from 7.82 to 8.1, indicating alkalinity. High concentrations of major ions were recorded in the eastern part of the power plant, wells 5, 6 and 7, with the exception of potassium, sulfate and magnesium. Potassium and sulfate were found in high concentrations in the central region, well 3, whereas magnesium was found in the western region, wells 1 and 2. According to the Piper plot, all groundwater is NaCl dominant, implying dissolution from evaporates or seawater intrusion.  The geophysical surveying results revealed the presence of five distinct geoelectric layers, including deep, sand with gravel, clayey sand, clay, sandy clay and coarse sand. The bottom one is the main water-bearing layer. It is found at depths of 65 to 70 m below ground and is made up of coarse sand with clay intercalation. Its resistivity ranges from 1.7 to 9 ohms and correlates with the productive zone of the Quaternary aquifer and indicates brackish water. The overlying thick clay zone confines the groundwater in this aquifer.
In general, the current work shows the integration between the hydrochemical analyses with hydrogeophysical methodologies can give a valuable information for assessing the hydrologic setting and groundwater chemistry. Accordingly, there is a need of the power plant to specified treatment and desalination in order to obtain acceptable water quality for sustainable use. Furthermore, proper groundwater extraction and use is critical in order to avoid the sweeping incursion of seawater, which might dramatically degrade the water quality.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of interest The authors declare no competing interests.
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