Delineation of seawater intrusion in the Yanbu industrial area, northwest Saudi Arabia, using geoelectric resistivity sounding survey

1 Introduction

The water demand is being further increased by continual improvement in living conditions in addition to a rise in population (Neumann et al., 2015). Groundwater, the principal freshwater source in coastal areas, is utilized indiscriminately to meet expanding residential, agricultural, and industrial water demands (Hamed et al., 2018). Excess groundwater withdrawal disturbs the hydrodynamic equilibrium between freshwater and seawater in the aquifer, pushing seawater upward (van Camp et al., 2014). As a result, fresh groundwater resources are being depleted in coastal locations (Alfarrah and Walraevens, 2018; Werner et al., 2013). The “Ghyben-Herzberg relationship” is a well-known mathematical relationship that controls whether seawater moves uphill or downward into the coastal aquifer (Narayan et al., 2007). According to the relationship, the depth of seawater decreases by 40 m for every meter that the water table rises. When groundwater levels drop below the mean sea level, coastal aquifer seawater flows inland, reversing the hydraulic gradient (Lee and Cheng, 1974; Nair et al., 2013). The inland migration of seawater into the coastal aquifer is the principal cause of coastal groundwater decrease. Seawater intrusion not only hinders the development of the local agricultural and industrial sectors but also lowers people's quality of life (Demirel, 2004).

Seawater intrusion has been studied extensively in coastal locations worldwide (Abdalla, 2016; Allow, 2011; Barlow and Reichard, 2010; Felisa et al., 2013; Garing et al., 2013; Manivannan and Elango, 2019; Rajaveni et al., 2016; Shammas and Jacks, 2007; Shi and Jiao, 2014; Suhartono et al., 2015; Werner and Gallagher, 2006; Zghibi et al., 2013). However, there have been few attempts to link these separate researches to gain a better knowledge of the seawater intrusion process and associated corrective actions. The goal of the current effort is to describe the numerous aspects that affect seawater intrusion and mitigation tactics. Regional and global phenomena including sea level rise, storm surges, climate changes, shoreline erosion, coastal floods, etc. affect coastal aquifers (Barlow, 2003). Salinization in coastal locations is accelerated by human activity (Rapti-Caputo, 2010). Surface water sources are also impacted by their interaction with seawater, in addition to the coastal aquifers. Due to seawater backwater, rivers and estuaries naturally bring seawater and salt to surface water (Oude Essink, 2001; Vijay et al., 2011).

Lithology, geomorphology, structural features, and other geological factors regulate coastal aquifer seawater intrusion. The aquifer lithology, or the makeup of the geological formations, determines how much water flows inland from the coastal aquifer (Michael et al., 2013). The magnitude of seawater intrusion is directly influenced by the geological history of water-bearing formation, hydraulic gradient, pace of groundwater extraction, and replenishment (Ammar et al., 2016; Barlow, 2003; Choudhury et al., 2001). Various porosity and permeability levels in geologic formations have various water-holding capacities. Paleo-saltwater may arise when seawater that is flooding inland regions becomes stuck in the under-saturated pore spaces (Werner and Gallagher, 2006). Clayey layers are typically found in small patches along the coast, where they function as barriers to protect the light-colored saltwater (Barrett et al., 2002; Cary et al., 2015). In locations that do not experience seawater inundation, however, these clay patches may grow perched aquifers and assemble freshwater (Yousif and Bubenzer, 2012). In such cases, the distribution of clay patches in coastal aquifers determines the distribution of freshwater and seawater. The coastal groundwater, however, may have a significantly higher salinity due to the interaction of freshwater with old trapped seawater and saltpans (Ayolabi et al., 2013; Nair et al., 2016).

Climate change and sea level rise are the main factors regulating coastal seawater incursion. The main source of groundwater recharge is atmospheric precipitation, but its volume varies across time and space. Furthermore, there is significant inter-annual variability in atmospheric precipitation. During the rainy season, groundwater levels rise, but in summer, evapotranspiration increases multifold, and precipitation decreases (Rapti-Caputo, 2010; Alarifi et al., 2022a, 2022b). As a result, even relatively minor groundwater removal during non-rainy seasons may cause seawater intrusion. Thus, dry seasons increase seawater intrusion risk compared to rainy seasons. Other aspects of climate change that are directly related to seawater intrusion include meteorological drought, storm surges, and coastal erosion (Terry and Falkland, 2010). Storm surges pose a threat to coastal aquifers by saline coastal groundwater and salt-depositing soil by flooding low-lying coastal areas with seawater (Rezaie et al., 2019). Because of this, the soil and groundwater cannot be used for farming or other purposes.

Tidal action affects coastal groundwater quality, and many scholars have used simulation, field, and experimental methods to study it. Kim et al. (2005) discovered that groundwater quality in Kimje, Korea's coastal districts correlates with periodic changes in tidal activities using time series analysis. A different study found that tides on Jeju Island, Korea, affect the coastal groundwater system up to 3 km inland (Kim et al., 2006). According to studies (Carr and van der Kamp, 1969; Nielsen, 1990), the oscillation in tide movement modulates the groundwater head in coastal zones, which results in periodic variations in the groundwater table. The freshwater-seawater mixing zone is directly impacted by such changes in groundwater head (Bear, 1972; Strack, 1976; Wang and Tsay, 2001). According to Shalev et al. (2009), this variation in the freshwater-seawater interface results in a seawater inflow into the pumping wells during the high tide period.

Seawater intrusion is considered an increasing threat to coastal communities causing enormous risks to the coastal environment and making the groundwater unsuitable for consumption. Several researchers have studied the environmental risks of seawater intrusion (Alfaifi et al., 2019, Hussain et al., 2019, Khadra and Stuyfzand, 2018; Chang et al., 2018, and Alharbi et al., 2023). The problem of the salinization of shallow groundwater aquifers has been recorded in many areas along the Saudi Red Sea coast, where the excessive pumping of unconfined coastal aquifers by water wells leads to the intrusion of seawater (e.g. Mogren, 2015).

The area of study (Fig. 1) witnesses high-speed urban growth and high-speed industrialization, where the city of Yanbu has been developed to become the third largest oil refinery center in the world. This area has few deep boreholes to study the shallow groundwater aquifer, so a vertical electrical sounding (VES) survey using the Schlumberger array was conducted on 11 localities (Fig. 1) to determine how seawater intrusion affected the shallow coastal aquifer. Many countries have employed resistivity-sounding surveys to detect seawater incursion and freshwater/saline water interface (Mogren, 2015, Batayneh, 2006). The high resistivity contrast between the layers saturated with seawater and layers saturated with freshwater; makes vertical resistivity sounding (VES) effective for coastal seawater intrusion detection. The VES survey in this work was conducted primarily to gather data on the underlying lithology, groundwater salinity, and hydrogeological behavior of the aquifers.

Close

Fig. 1

Location map of the study area including the measured VES stations and boreholes.

Export to PPT

2 The study area

The study area (Fig. 1) lies along the eastern Red Sea coast west of Al Madinah and NW of Jeddah. It has one of Saudi Arabia's largest and oldest marine ports located in this area, which is increasing in population and tourism. Recently, the area attracted several development and industrial projects; where the area plays a major role in the kingdom’s gross national income. This area is occupied by the Yanbu Industrial City which represents the last station for the raw oil and liquid natural gas pipelines.

The area lies on the “Tihamah” littoral plain (Fig. 2) at the foot of the highly dissected Precambrian granitic mountains of Alhijaz. The coastal plain is covered by the Quaternary sediments (Fig. 2); mainly gravel, sand, silt, and clay. Flash floods from close-lying coastal escarpments have deposited terrigenous material in many old wadis, forming most of the sediments along the coastline. Parts of the area are covered by Sabkha soil and the coastal morphology of the area is characterized by an extension of various open bays on its shores referred to as “Sharm”. The coastal zone of Yanbu is a dynamic and morphologically unstable area (Nofal and Abboud, 2019). The area is bounded from the eastern side by basement rocks of Alhijaz mountains and basalt ridge forming part of a set of so-called Harrats (lava flows today in an inversion of relief), coming from mouths located at a hundred kilometers inside, and going back to the old Pleistocene (Monnier and Guilcher, 1993).

Close

Fig. 2

Geologic map of the study area (modified after Monnier and Guilcher, 1993).

Export to PPT

3 Data acquisition and processing

Among all surface geophysical methods, the electrical resistivity method has been applied most widely for groundwater investigations. A Schlumberger array with 600 m current electrode spacing was used for the VES survey in this investigation. In this respect, a total of 11 VES stations were conducted along four NNW-SSE profiles, using the Iris Syscal-R2 resistivity meter. Fig. 1 depicts the positioning and distribution of the VES spreads. Using IX1D-Interpex software (ver. 3.52, 2013), the 1-D inversion approach was used to process the VES field data in terms of the layer parameters. With the least root mean square (RMS) error between the generated model and the field data, this 1-D inversion generates a resistivity model that fits the VES field data. In this respect, the initial forward model for each VES was proposed based on the available geological and borehole information in the current study. After the first forward model was calculated, multiple iterations were applied until the RMS fitting error between the observed and calculated data for the assumed model. In addition, smooth model inversion was applied using a model with the same number of layers as the data points and fixed thicknesses related to the electrode spacing. The output is subsurface geoelectric resistivity layers with their true resistivities, thickness, and depths. The interpreted resistivity layers with their true resistivities and thicknesses are listed in Table 1.

4 Results and interpretation

The interpretation shows that the study area's subterranean rock units' resistivity varies greatly. Depending on the VES curve type, the amount of geoelectrical layers, and their connection to resistivity; geoelectrically, the study region has two zones. The western zone is characterized by descending Q-type VES curves (ρ1 > ρ2 > ρ3) that show decreasing resistivity with depth. H-type VES curves (ρ1 > ρ2 < ρ3) in the eastern zone indicate decreasing resistivity with depth till reaching high resistivity bedrock at the bottom. Fig. 3 displays examples of the two types of sounding curves along with respective 1-D inversion models and data fit.

Close

Fig. 3

Inversion modeling in comparison with field data for Q-type VESes (VES-1, VES-3, and VES-9) that characterize the western zone of the study area and H-type VESes (VES-2, VES-4, and VES-8) that characterize the eastern zone of the study area.

Export to PPT

A correlation between the interpreted VES-sounding layers and the available borehole data was conducted (Fig. 4). The correlation shows good agreement within the available lithologic information. The first top layer is correlated with the Quaternary sediments (gravel, coarse sand, silt, and clay) that attain a resistivity of 100 Ohm.m; followed by the second layer of sand and clay contaminated by seawater with a resistivity of 0.6 Ohm∙m (Fig. 4). This shows that lithology, water, clay, and salinity determine subsurface rock strata electrical resistance.

Close

Fig. 4

Correlation of VES-1 with the nearest borehole lithologic log.

Export to PPT

VES data show three resistivity layers. The resistivity of the interpreted three geoelectric layers (listed in Table 1) is plotted in iso-resistivity contour maps to show resistivity dispersion horizontally and vertically over the research area (Figs. 5–7). The first layer (Fig. 5) attains a thickness, that ranges from 1.5 to 9 m and a wide range of resistivities (ranges from 52 to 1650 Ohm.m). The wide range of resistivity indicates inhomogeneous Quaternary sediment varied from gravel, and coarse sand to silty and clayey soil with different moisture content. This layer has significant resistivities toward the east of the study region; indicating dry gravel and coarse sand sediments, while the western side is dominated by low resistivities; which indicates clayey moist soil. The second layer (Fig. 6) shows relatively low resistivities (2.4–28 Ohm.m), except for a local rise in the northwestern part where the resistivity reaches 240 Ohm.m; indicating a change in the lithology to gravel and coarse sand. This layer is composed of silty sand clay-graded gravel and coarse sand in some places’ upper parts. The sediments of this layer are saturated with saltwater. The third layer (Fig. 7) is geoelctrically divided into two zones; the western zone is dominated by its low resistivity (1.2–11 Ohm.m) and the eastern zone is characterized by relatively very high resistivities that reach 4463 Ohm.m. These findings show a fault, which causes a basement uplift, is present in the studied area. In the eastern half of the region, the fractured basement uplift may function as an impermeable or sealing wall within the shallow aquifer, preventing the infiltration of seawater. This explains the very obvious difference in the resistivity values between the western and eastern zones; suggesting a possible seawater intrusion that may be controlled by a subsurface faulting that divides the area into two zones (Fig. 8).

Close

Fig. 5

A map shows the distribution of the true resistivity of the first geoelectric resistivity layer along the study area.

Export to PPT

Close

Fig. 6

A map shows the distribution of the true resistivity of the second geoelectric resistivity layer along the study area.

Export to PPT

Close

Fig. 7

A map shows the distribution of the true resistivity of the third geoelectric resistivity layer along the study area.

Export to PPT

Close

Fig. 8

A possible extension of the seawater intrusion into the coastal area around the industrial city of Yanbu.

Export to PPT

5 Geoelectrical cross-sections

Three geoelectrical cross-sections have been constructed throughout the study area (Fig. 9). All cross-sections are oriented NE-SW perpendicular to the Red Sea’s main direction. Three geoelectrical layers make up the subsurface section, according to their cross-sections. Surface dry sand, gravel, and silt form the first layer. Wet sand, silt, and clay form the second geoelectrical layer while the third layer is represented by clayey sand soaked with saline water. Basement rocks create the fourth geoelectrical layer. The resistivity of the second unit ranges between 9 and 29 Ω.m, 1.7–13.2 Ω.m in geoelectrical cross-sections a and b respectively while not recorded in cross-section c. The third geologic unit has resistivities in the range of 4.3–11.9 Ω.m, 1.2 Ω.m, and 0.6–3.2 Ω.m for cross-sections a, b, and c respectively. The fourth geoelectric unit has resistivity values of 958 Ω.m, 803–4464 Ω.m, and 843 Ω.m for cross-sections a, b, and c respectively. Structurally, these cross-sections reveal a fault with a downthrown side toward the western direction.

Close

Fig. 9

Geoelectrical cross-sections in the area of study. (a) Between VES-1 and VES-2, (b) between VES-6, VES-7 and VES-8, (c) between VES-9, VES-10 and VES-11.

Export to PPT

6 Discussion and conclusions

The Schlumberger array was used to conduct a vertical electrical sounding (VES) survey at 11 locations in the Yanbu industrial area to collect data on subsurface lithology and groundwater salinity to help identify seawater intrusion in the shallow aquifer along the coastal zone. The interpretation of the VES data reveal the presence of three distinct resistivity layers. The first layer is an inhomogeneous layer with a wide large range of resistivities, which is correlated to Quaternary sediments of variable lithology ranging from gravel, and sand to silt and clay; besides variable water content. The variation of resistivities of the second layer indicates abundant clayey soil as indicated by its low resistivity values; except a local high resistivity in the northern part; indicating a local change in the lithology to gravel and coarse sand. The resistivity values of the third layer show a sharp and sudden change in the resistivity values; indicating a possible structural fault affected the coastal plain that may result in a basement uplift. The fractured basement uplift in the eastern investigated area may seal seawater from entering the shallow groundwater aquifer. In general, structural faulting and lateral lithologic changes have a major role in controlling seawater incursion in the research area. A large resistivity contrast between seawater that greatly lowers the contaminated zone's resistivity values compared to the uncontaminated zone in current data suggest that the VES survey is beneficial for assessing coastal seawater intrusion. To safeguard the freshwater in the eastern side of the area, it is recommended to reduce the pumping from the shallow wells because the primary cause of saltwater intrusion is the over-pumping of fresh groundwater from coastal aquifers, which reduces the hydrostatic pressure and drawing saltwater into the aquifer.