Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Phetchaburi Province, Thailand

Journal Pre-proofs Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Prachuap-Khiri-Khan Province, Thailand Jiraporn Sae-Ju, Srilert Chotpantarat, Thanop Thitimakorn PII: DOI: Reference:

S1367-9120(19)30517-6 https://doi.org/10.1016/j.jseaes.2019.104165 JAES 104165

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Journal of Asian Earth Sciences

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28 December 2018 19 November 2019 26 November 2019

Please cite this article as: Sae-Ju, J., Chotpantarat, S., Thitimakorn, T., Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Prachuap-Khiri-Khan Province, Thailand, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104165

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Hydrochemical, geophysical and multivariate statistical investigation of the seawater intrusion in the coastal aquifer at Prachuap-Khiri-Khan Province, Thailand Jiraporn Sae-Ju1, Srilert Chotpantarat1, 2, 3, 4, and Thanop Thitimakorn1,3 1 Department 2 Center

of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand.

3 Research

Program on Controls of Hazardous Contaminants in Raw Water Resources for Water

Scarcity Resilience, Center of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand. 4 Research

* Corresponding

Unit of Green Mining (GMM), Chulalongkorn University, Bangkok, Thailand.

author. Tel.: (662)2185442; Fax.: (662)2185464; Email: [email protected].

ABSTRACT Seawater intrusion in coastal regions is an imperative ecological issues, and results in an undesirable consequence on future groundwater resources. To assess and mitigate seawater intrusion, the affected aquifers need to be characterized. By integrating geophysical investigation and multivariate statistical analysis of hydrochemical data, seawater intrusion into coastal aquifers at Cha-Am in Prachaup-KhiriKhan Province (Thailand), as a model area that is adversely affected by this problem, was evaluated. The vertical electrical sounding (VES) survey study was conducted at 80 locations, and then 47 VES locations were used to create four pseudo cross-section lines in a west-east direction. The geophysical results were described together with the hydrochemical analysis of 57 groundwater samples revealed that seawater intrusion occurred in the Quarternary colluvial sediment aquifer at an average depth of 50–60 m. A resistivity value of <5 Ωm represented areas that were highly impacted by seawater intrusion, while a range between 5–10 Ωm represented moderately contaminated areas. The hydrochemical characteristics, divided the groundwater into three groups according to the level of impact of seawater intrusion: (i) Ca2+-Na+-HCO3- and Ca2+- HCO3--Cl-; (slightly impacted) and (iii) Na+-Cl- (highly impacted). The hydrochemical facies evolution diagram (HFED) revealed that most of the samples fell close to the mixing line, demonstrating mixing between seawater and fresh water and that some samples fell in the intrusion phase. Multivariate analysis was in agreement with the HFED. Furthermore, the northern part of the study area faced seawater intrusion with a relatively higher impact than other areas.

Keywords: coastal aquifer; hydrochemical facies evolution diagram; hydrogeological characteristics; seawater intrusion; Thailand; vertical electrical sounding method

1. INTRODUCTION Many coastal areas in the world contain dense populations that have food cultivation and other important economic activities such as urban development, trade, and tourist activities. These factors that have attracted people to settle in these areas and as a result, the demand for water for consumption, agriculture and industry has increased. Groundwater resource is an alternative water source. Compared with surface water groundwater is of high quality and is barely affected by seasonal effects (i.e., constant temperature), with large available quantities. For the reasons above-mentioned reasons, groundwater resources are an alternative water source. As a consequence, groundwater has long been pumped out, but this can result in seawater intrusion in many coastal areas worldwide (Mas-Pla et al., 2014; Shi and Jiao 2014; Morgan and Werner, 2015). In coastal aquifers, seawater lies under the fresh water since fresh water is less dense than seawater, and so the zone of contact between the fresh and seawater form brackish water (Bear, 1999). The fresh water on the top of the heavier seawater serves to push the seawater interface seawards and restricts its landward penetration. In contrast, when pumping relatively large volumes of fresh groundwater from coastal aquifers, the fresh water pressure is reduced and allows the seawater to migrate further landwards. This seawater intrusion is one of the most important environmental issues that negatively affects groundwater resources, since the increased groundwater salinity can lead to a reduction in fresh water availability and degradation of groundwater quality (Essink, 2001; Werner et al. 2013; Kang and Jackson 2016; Ros and Zuurbier 2017). Therefore, the study of seawater intrusion into coastal aquifers is needed to identify the affected zones in order to prevent problems or remediate such areas efficiently. The coastal study area is located at Amphoe Cha-am, Changwat Phetchaburi (Thailand), and was selected as it is a relatively densely populated area and one of the most famous tourist areas in Thailand (Konkul et al., 2014). Therefore, the groundwater resource may become a primary water resource in the near future and is at risk of being drawn out at a level that exceed the aquifer yield, which would lead to a change in the natural equilibrium of the seawater interface and the sea water laterally moving landwards. Geophysical and hydrochemical techniques have previously been integrated to investigate areas disturbed by seawater intrusion (McInnis et al., 2013; Agoubi et al.,

2013; Kouzana et al., 2010; Cimino et al., 2008; Song et al., 2006; Kazakis et al., 2016; Fadili et al., 2017; Najib et al., 2017). Geophysical techniques, such as the one-dimensional (1-D) electrical resistivity survey or vertical electrical sounding (VES) have been used since the electrical resistivity between fresh water and seawater saturated zones show large differences (Van Dam & Meulenkamp, 1967; Sabet, 1975). Therefore, VES is capable of identifying the contrast, in terms of resistivity values, between seawater and freshwater in coastal aquifers. In addition, the VES technique can be enabled in large areas as it is less time-consuming and has a lower economical cost when compared with drilling exploration methods. However, geophysical survey data are not capable of clearly identifying the lateral penetration of seawater in various lithologic units in a hydrogeological formation and groundwater facies or chemical constituents, as the low resistivity depends on various other factors such as the formation materials and groundwater chemistry (Zohdy et al., 1974). As a result, to deal with the limitations of geophysics for delineating seawater intrusion areas, a hydrogeological investigation (with the help of lithologic information from drill wells), chemical analysis of groundwater samples and multivariate statistical analysis were incorporated in this study. The objectives of this study were to integrate multivariate statistical, hydrochemical and geophysical approaches to delineate the impact of seawater intrusion in coastal aquifers in the study area, and to further explain the geo-hydrochemical process in the coastal aquifers.

2. STUDY AREA The study area is located in Amphoe Cha-am, Changwat Phetchaburi, which is located in the central part of Thailand. The area covers approximately 360 km2 and lies between the latitudes 12°37.6'–12°53.845' N and longitudes 99°50.827'–99°729' E (Figure 1). The area is bounded by the northern and western borders of Tha-Yang District, by the southern border of Prachuap-Khiri-Khan Province, and by the eastern border of the Gulf of Thailand (GOT). The area can be classified by topography into two major landforms consisting of a plain interleaved mountain covers (~20% of the area) and low-plain or coastal plain covers (~80% of the area). The plain interleaved mountain landform is located to the west of the area, covered mainly by forest areas, while the eastern side is a

low-plain or coastal plain to GOT , and consists of various types of land use, including agriculture, facilities, forests, and water bodies(Land Developement Department, 2011).

[Insert Figure 1]

2.1. Geological Setting The study area is located on the Shan-Thai subcontinent and consists of Carboniferous rocks to Quaternary sediments (Figure 1). The Permo-Carboniferous sedimentary rocks and Permian limestone are the basement rocks and are mainly distributed to the east of the study area. In addition, basement rocks intruded by Cretaceous granite appears as a mountain range and isolated hills in the west of the study area. The basement rocks are filled with Quaternary sediments that consist of marine sediments ( coastal tide-dominate deposits), and colluvial sediments that are accumulated around the hill foot (Department of Mineral Resources, 2014).

2.2. Hydrogeology The study area is underlain with unconsolidated and consolidated aquifers (Figure 2). More than 60% are underlain with the Quaternary unconsolidated aquifer, which consists of a beach sand aquifer (Qbs), Quaternary flood plain aquifer (Qfd) and colluvial sediments aquifer (Qcl). The consolidated aquifers comprised 40% of the study area and consisted of the Silurian-Devonian metamorphic (SDmm), Permian-Carboniferous metasedimentary (PCms), Permian limestone (Pc) and Cretaceous granite (Gr) aquifers (Table 1) (Department of Groundwater Resources, 2015). [Insert Table 1] [Insert Figure 2] 3. METHODOLOGY Seawater intrusion into coastal aquifers can be detected through several methods. In most case studies, the electrical method has been universally recognized (George et al., 2015; Cimino et al., 2008; Samouëlian et al., 2005), since the electrical conductivity (EC) of a saturated subsurface depends primarily on : the porosity, connectivity of pores and the specific conductivity of water in the

pores (Telford et al., 1990). The difference between the EC of seawater and freshwater saturated subsurfaces is significant and so an electrical resistivity survey is well suited for evaluating the relationship between freshwater and seawater in coastal areas (Sherif et al., 2006). The electrical method analyzed in conjunction with hydrogeological measurements can map the interfaces of fresh water and seawater in coastal aquifers more precisely (Zarroca et al., 2011). In this study, we used both the direct method, by sampling groundwater samples for hydrochemical analysis, and the indirect method, by VES. Both the sampling of groundwater samples and the VES investigation were performed in the dry season (April to May) since the occurrence of seawater intrusion be more evident. 3.1. VES analysis The VES method, a 1D resistivity survey, was applied in this research. The principle of VES is that an electric current is released into the ground through current electrodes A and B, and the electrical potential is then measured by potential electrodes M and N. The resistivity meter converts the voltage to the resistance value, which is then plotted on log-log paper in the field to check for anomalies. In this research, 80 VES points were measured using a resistivity meter (Syscal R1 Plus model, Iris) in the Schlumberger configuration (Figure 3). The location of the 80 survey station points (Figure 1) could bedivided into the four transects: A-A’, B-B’, C-C’ and D-D’, all in a west to east direction oriented perpendicular to the coast line. The current electrode spacing (AB) and potential electrode spacing (MN) was based on the relationship of 2(AB/2) > 5 MN at all depths (Telford et al., 1990). The half-current electrodes spacing (AB/2) were varied from 1–200 m and the potential electrodes spacing (P1 and P2) were varied from 0.25–20 m. Apparent resistivity data derived from the field was interpreted using the IPI2WIN software version 3.1.2c developed by Moscow State University (Bobachev, 2003), which compared the inversion of the apparent resistivity measurements and the calibration field curves with the theoretical curves of the program. The results of the VES survey method were the vertical resistivity of the soil and rock layers. The shape and slope of the VES data on the graph represents changes in the layer that had a

different resistivity (Telford et al., 1990). Therefore, the correct interpretation of VES required the use of information regarding the local geology and drilling information of the groundwater wells.

[Insert Figure 3]

3.2. Chemical Analysis of Groundwater A total of 57 groundwater samples were collected in the study area. Prior to sampling, the groundwater was pumped out for 5–10 min to ensure it represented the groundwater in the aquifer. Samples were measured for physical and chemical parameters, such as pH, EC, groundwater and total dissolved solids (TDS) in the field using portable meters. Samples for the analysis of cations and anions were collected in two 500- mL poly-ethylene (one bottle with added nitric acid (HNO3) to maintain the acidity of the water for cation analysis and the other without to be used for anion analysis). The HNO3 acidification of the sample was performed in order to avoid precipitation reactions inside the sampling bottle in order to keep it stable until the analysis. All samples were kept at 4 °C to reduce microorganism metabolism and reduce the speed of physical and chemical processes. Chemical analysis was performed for the cations Ca2+, Mg2+, Na+, K+, and Fe2+ and the anions Cl-, Br-, NO32-, and SO42-, CO32- and HCO3- ). The results were analyzed by piper diagram (Galloway & Kaiser, 1980) and hydrochemical facies evolution diagrams (HFED) (Giménez-Forcada, 2010). Seawater that seawater intrusion can be indicated by an increased content of TDS and the major ions of seawater. Furthermore, correlation analysis and principal component analysis (PCA) were used to clearly acquire the relationship among the hydrogeochemical parameters measured in the groundwater samples, which enabled the identification of the hydro-geochemical processes occurring in the groundwater system. In this study, multivariate statistical analyses were performed using the SPSS software, version 22. Ten hydrochemical parameters (K+, Na+, Fe2/3+, Ca2+, Mg2+, F-, Cl-, Br-, SO42-, and HCO3-) as well as the pH and EC in the groundwater samples were evaluated using correlation

analysis and PCA. These two techniques group and establish the relationship between a group of groundwater samples based on hydrochemical characteristics (Abderamane et al., 2012).

4. RESULTS AND DISCUSSION 4.1. VES analysis The interpretation of the results from the 80 VES samples posts revealed that there were three layers in the subsurface, which consisted of a resistivity of ρ1 > ρ2 < ρ3, where ρ1, ρ2 and ρ3 are the resistivity of upper, intermediate and bottom layers, respectively. They were consistent with the characteristics of the geology and the hydrogeology of study area where the top, middle and bottom layers represented the unsaturated sediments, saturated sediments, and bed rocks, respectively. Since soil or rock layers with a low resistivity can be caused by many reasons, including the clay content or salt water in hydrogeologic formations, this research compared the lithologic data with the VES data survey, which revealed that the low resistivity at the area near the coast line may have been caused by seawater. Figure 4 shows the resistivity values compared with the litho-log of transect lineD-D’. The left side shows the lithological data of well no. Q168 vs the VES of St47 that represents a position located far from the coast, and the right side shows the lithological data of well no. PW7962 vs the VES of St34 that represents a position located near the coast. The low resistivity (< 5 Ωm) at VES St34 corresponded to a gravelly clay layer, whereas at VES St47, the layer with components of clay or clayey gravel showed a resistivity of only 10–20 Ωm. From the comparison, it can be concluded that although the layer had a clay component, the resistivity value was not very low (< 5 Ωm), and so the area that showed a very low resistivity may have been influenced by seawater intrusion. This is consistent with previous studies (Ravindran, 2013; Kaya et al., 2015; Gopinath and Srinivasamoorthy, 2015) who concluded that the areas with low resistivity (<5 Ωm) were influenced by seawater intrusion. The pseudo cross-sections (A-A’, B-B’, C-C’, and D-D’) were generated from the 1D VES data derived from the 47 selected VES points from this study and nine VES points from the DGR database. All transects were created from the west to east direction, which was perpendicular to the coast line (Figure 1).

The pseudo cross-section A-A’ (Figure 5a) had 14 VES points (St1, St2, St3, 42–28, St4, F5, 42–29, St5, St6, F6, St7, St8, St9, and St10) in a line that was located at the lower part of the study area with a total length of approximately 6,750 m. The surface layer (0–10 m) had a resistivity range from 10 to 1000 Ωm and so was interpreted as an unsaturated zone of the Qfd aquifer. At the western side of the cross section, a very high resistivity (400–1000 Ωm) was observed, which was due to the granite batholith that intrudes in the bedrock, as a granite outcrop was found near St8. In the eastern side, at a depth of 130–200 m, the layer that had a resistivity of 50–100 Ωm was the bedrock layer. The top of this side showed resistivity values between 3–40 Ωm and represented sediment saturated with water, especially near the coast line where there was a very low resistivity zone (< 5 Ωm). This may represent the influence of seawater intrusion that intruded inland by 1 kilometer from the coast line. The pseudo cross-section B-B’ (Figure 5b) consisted of 16 VES points (St11, St12, St13-19, St71, St20-23, 42–126, and 42–125) in a line of approximately 8,450 m that from east to west and was close to the A-A’ cross section. The surface layer (0–10 m) had resistivity anomalies between 50–400 Ωm, and represented the unsaturated zone of the Qfd aquifer. The next layer showed resistivity values ranging from 2–20 Ωm, which was interpreted as a sediment layer saturated with water. Near the coast line, there was a low resistivity zone (< 5 Ωm) that extended approximately 2 km inland, which may represent the influence of seawater intrusion. At the west side of the cross section, there was a zone of resistivity ranging from 60–250 Ωm at a depth 20-200 m, which represented the bedrock layer. The pseudo cross-section C-C’ (Figure 5c) consisted of 12 VES points (St24-31, 41–205, St32, 41–191, and St33) from the east to west of approximately 7,975 m. between the B-B’ and D-D’ cross sections. The surface (0–10 m) layer had a resistivity range from 6 to 700 Ωm, which represented the unsaturated zone of the Qfd aquifer. The next layer showed resistivity values between 2–20 Ωm and represented the water-saturated sediment layer , and there was a low resistivity zone (< 5 Ωm) near the coast line that extended inland by approximately 3.2 km, representing the influence of seawater intrusion. Furthermore, the western side of the cross section showed resistivity values between 60–200 Ωm at a depth 40-200 m, representing the bedrock layer.

The pseudo cross-section D-D’ (Figure 5d) consisted of 14 VES points (St34-37, St39, St4144, 41–174, 41–173, and St45-47) in an east to west line of approximately 14.3 km in the northern part of the study area. The surface (0–10 m) layer had a resistivity range from 1.5 to 400 Ωm, representing the unsaturated zone of the Qfd aquifer. In this profile, there were low resistivity anomalies (< 5 Ωm) near the surface at St7, possibly caused by the waste water from community areas. The next layer showed resistivity values in the range from 1.5 to 20 Ωm, representing the water-saturated sediment layer. The low resistivity zone (< 5 Ωm) extended far from the coast inland (approximately 4 km), representing the influence of seawater intrusion. In the west side of the cross section, the bedrock was found with resistivity values of 40–100 Ωm at a depth of 90–200 m. [Insert Figure 4] [Insert Figure 5] 4.2. Geological Interpretation Since this area had a limited number of borehole log data, interpretation was required to create a cross-section in some areas, as shown by the dashed line in the cross-section. Comparison of of borehole log and electric log data analyses from the pseudo cross-sections of the four transects are shown in Figure 6. The four geo-electrical cross-sections clearly showed the boundaries of each aquifer. The top layer was the Qfd aquifer and consisted of well sorted sand with a high sphericity and was approximately 0–20 m thick. The next layer was the Qcl aquifer, which consisted of clayey gravel, poorly sorted and angular to sub-angular, interbedded with sand in some areas with an average thickness of approximately 50–60 m, but it gradually increased by 100 m in the area near the coast line. The next layer underneath the Qcl aquifer was the PCms aquifer that consists of greenish-grey sandstone and shale with an average depth of 50–200 m. In the A-A’ and B-B’ cross section, located in the lower part of the study area, the PCms aquifer was not found, whereas Gr aquifers could be found in all cross sections. In the D-D’ cross section, the Pr aquifer was located in the central part of the cross section. It lay on top of the PCms aquifer at an approximate depth of 50–60 m.

[Insert Figure 6]

4.3. Apparent Resistivity Map Figure 7 shows the apparent resistivity map established from distinction of the apparent resistivity values for a half-electrode spacing (AB/2) of 10, 30, 50, 70, 100 and 150 m. The dark blue color on the map represents a low apparent resistivity (< 5 Ωm) and the red color represents a high apparent resistivity (> 1,000 Ωm). In the east side, which is near the coast line, every map showed a low apparent resistivity of <5 to 20 Ωm. This low apparent resistivity disperses in a sporadic pattern in the maps of the (AB/2) spacing of 10 m and was wider when the depth increased for an AB/2 spacing of 30, 50 and 70 m. When the depth reached the bedrock layer, the apparent resistivity value increased in the maps for an (AB/2) spacing of 100 and 150 m. The highest apparent resistivity (> 1000 Ωm) was located on the western side of the study area, and corresponded to the bedrock in the area. The map for an AB/2 of 70 m (Figure 7D) had the widest < 5 Ωm (dark blue) area, and corresponded to a depth of 50 m from the ground surface when compared to the cross-section, so this depth was located in the Qcl aquifer. As a result, it can be concluded that the Qcl aquifer has been highly influenced by seawater intrusion.

[Insert Figure 7]

4.4. Groundwater Chemistry The hydrochemical analysis of 58 groundwater samples is shown in Table SI.1 (see Supplementary Information). The TDS ranged from 195 to 3580 mg/L, while the EC ranged from 292 to 5360 mS/cm. Figure 8 shows the classification of the hydrochemical facies of the 42 groundwater samples with the chemical results plotted on a piper diagram (Galloway & Kaiser, 1980) using the Groundwater Chart Program from United States Geological Survey. The reliability from the results of the hydrochemical analysis was determined from the charge balance of cations and anions (%Δ), which in this study was deemed to be acceptable at %Δ < 10 (ALS Environmental). From Table

SI.1, it was found that 42 samples were acceptable (%Δ < 10) and were used for discussion and interpretation, while the other unreliable 16 samples may have resulted from the process of groundwater collection and preservation, as well as dilution of the samples. The piper diagram (Figure 8) could be divided into five facies of the (i) Ca2+-Na+-HCO3-, (ii) Ca2+-HCO3-Cl-, (iii) Ca2+-Na+-HCO3-Cl-, (iv) Ca2+-Na+-Cl- and Na+-Cl-. The Ca2+-Na+-HCO3- facies represents the fresh water and was found in sample no. W5, which was opened as a well screen to the Gr aquifer. This groundwater facies was younger and weakly acidic, which is generally found in high terrain or recharge areas (Appelo and Postma, 2005). The Ca2+-HCO3--Cl- facies was found in sample no. W19 and W20, which were opened as a well screen to the Gr aquifer. This facies was in waterbearing permeable rocks where ion exchange took place as the groundwater moves through the rock formation, (Appelo & Postma, 2005). The quality of the water in this facies was fresh water. The Ca2+-Na+-HCO3--Cl- facies was found in 21 wells, which were in the well screens from theGr (eight wells), PCms (nine wells), Pr (one well, sample no. W52), and Qcl (three wells) aquifers. This facies presents a complex chemical pattern since the water was influenced by many factors, but may have resulted from the mixing of fresh water with seawater (Appelo and Postma, 2005). The Ca2+-Na+-Clfacies was found in sample no. W53, which was opened as a well screen to the Pr aquifer, while three wells (sample no. W24, W25 and W34) were opened as a well screen to the Qcl aquifer and two wells (sample no. W22 and W23) were opened as a well screen to both the Qcl and PCms aquifers. The groundwater facies may have been changed due to the influence of seawater intrusion or by the ion exchange process (Appelo and Postma, 2005). The Na+-Cl- facies was found in 10 wells that were opened as a well screen to the Qcl aquifer. The groundwater facies indicated that these wells were influenced by seawater intrusion, and the quality of the water was saline water. According to the piper diagram, the groundwater facie could be classified into five facies, that corresponded to the influence of seawater intrusion and depended not only on the distance from the coast, but also on the depth of the well or the screen level of the well. Most groundwater samples in the Qcl aquifer were influenced by seawater intrusion. In general, the average depth of the Qcl aquifer ranged from 20 to 50 m and the thickness was typically higher than 100 m approaching the coast line. The Na+-Cl- facies (approximately ~48%) and the Ca2+-Na+-Cl- facies (approximately

~19%) were mainly found in the Qcl aquifer. The remaining (approximate 20%) groundwater wells represented the Ca2+-Na+-HCO3--Cl- facies. According to the geochemical facies, the average composition of cations (in meq/L) in the Qcl aquifer was ranked Na+ >> Ca2+ > Mg2+ ~ K+. Furthermore, the composition of anions (in meq/L) in the aquifer was ranked Cl- >> HCO3- > CO32- > SO4-. Therefore, their hydrochemical composition supported as seawater intrusion in this aquifer (Ahmed et al., 2017). The groundwater facies in the PCms and Gr aquifers, which opened a well screen in the high weathering zone (underneath the Qcl aquifer) at a depth of 40–60 m was composed of the Ca2+-Na+-HCO3-Cl- facies, suggesting that the groundwater was moderately affected by seawater intrusion. Moreover, the PCms and Gr aquifers (where a well screen was opened at a depth of 60–80 m) had Ca2+-HCO3--Cl- and Ca2+-Na+-HCO3- facies, suggesting that the groundwater was slightly affected by seawater intrusion due to the mixing of recharge rainwater. The relationship between the depth variation with the Cl- and Na+ concentrations in each aquifer is shown in the Supplementary Information (Figure SI.1). The Cl-e concentration was mostly found in relatively high concentrations in the Qcl aquifer, especially in shallow to moderate depths (approx. 15 – 70 m depth). However, this relationship of salinity with depth was not well correlated in the Pcms and Gr aquifers, implying that the groundwater in these aquifers were not highly impacted by seawater intrusion. The red line on the piper diagram (Figure 8) is the line that shows the hydrochemical evolution of the groundwater facies by cation exchange reactions (Appelo & Postma, 2005). This line resulted from two points plotted between the composition of seawater (blue point) and fresh water (red point) on the piper diagram. The hydrochemical results of the groundwater samples that fall in the zone between the blue and red points represent mixed water from the fresh water and seawater. The Na+-Cl- facies and Ca2+-Na+-Cl- groundwater facies were close to the blue point , while the Ca2+HCO3--Cl- and Ca2+-Na+-HCO3- groundwater facies were close to the red point and The Ca2+-Na+HCO3--Cl- groundwater facies were between the blue and red points. As mentioned earlier, this was similar to the previous study of contamination in the Korba unconfined aquifer, which was influenced by seawater intrusion (Zghibi et al., 2014). They showed the chemical analysis of water on the piper

diagram and then interpreted the results by creating a Theoretical Mixing Line (TML) of seawater and fresh water, which revealed that the groundwater showed paths of hydrochemical evolution along the TML line. The groundwater facies could change from a Ca2+-SO42- type to a Ca2+-Cl- type to an Na+Cl- type, and vice versa, from a Ca2+-SO42- type directly to an Na+-Cl- type, indicating that the chemical composition of groundwater is changed by cation exchange.

[Insert Figure 8]

Figure 9 shows the relationship between the Na+ and Cl- ions. The dominant ions in seawater are Na+ and Cl-, while the dominant ions in fresh water are Ca2+ and HCO3- (Appelo & Postma, 2005). Therefore, the study of seawater intrusion has to focus on the dominant ions of seawater, which are Na+ and Cl- ions. Comparison of the Na+ and Cl- ions revealed a strong correlation, where the groundwater samples fell on a 1:1 line with an R2 of 0.941. This relationship suggests that both ions hadthe same origin from seawater and so the concentrations of Na+ and Cl- depend on the degree of seawater intrusion into the aquifer. From Figure 9, the groundwater could be divided into three groups. The first group (red encircled), mainly consisted of samples in the Qcl aquifer that were severely influenced by seawater intrusion. The groundwater types in this group showed Na+-Cl- and Ca2+-Na+-Cl- facies. The second group (green encircled) consisted of groundwater samples from the PCms and Gr aquifers, which were moderately influenced by seawater intrusion and showed Ca2+-Na+-HCO3-Cl- facies. The last group (blue encircled) consisted of groundwater samples from the deeper zone (when compared to the second group) of the PCms and Gr aquifers, which were slightly influenced by seawater intrusion and showed Ca2+-HCO3-Cl- facies and Ca2+-Na+-HCO3- facies. These results are consistent with the previous studies (Agoubi et al., 2013; Zghibi et al., 2014) that revealed the groundwater chemistry was influenced by seawater intrusion. They found that the groundwater was influenced by seawater intrusion had dominant ions of Na+ and Cl-. Therefore, when plotting Na+ and Cl- concentrations, both ions were strongly correlated with the groundwater samples falling on the 1:1 Na+:Cl- line, indicating that both ions come from the same source (seawater).

Similarly, according to the ionic ratio estimated from Cl-/(HCO3- + CO32-) and recommendation of Raghunath (1990)suggested to use a ratio of 2.8 as the threshold for indicating the saltwater intrusion, we found that more than 85% of the groundwater samples in the Qcl aquifer were moderately to highly contaminated from seawater intrusion (Ebrahimi et al., 2016).

[Insert Figure 9]

Furthermore, plotting the results of groundwater samples on the piper diagram allowed the freshening and intrusion phases to be interpreted, but it was difficult to recognize the sequence of facies in detail (Al-Agha and El-Nakhal, 2004). Therefore, this research used the HFED approach (Giménez-Forcada, 2010) to describe the dynamics of the seawater intrusion. The percentage composition of the major ions in the hydrochemical process associated with the dynamics of the seawater intrusion interface (Ca2+, Na+, HCO3-, SO42- and Cl-) was considered in the HFED. Figure 10 shows that the red block (no. 4) is the Na+-Cl- facies (seawater), while the blue block (no. 13) is the Ca2+-HCO3- facies (fresh water). Plotting the percentage composition of ions on the diagram generated the mixing line between the fresh water and seawater to divide the phases of seawater intrusion. Groundwater samples above the mixing line represented the freshening phase, while those below the mixing linewere the intrusion phase. From this analysis, most of the samples fell close to the mixing line (facies path 4–7–10–13), demonstrating likely mixing between seawater and fresh water. Some samples fell in the intrusion phase (below the mixing line). In this initial phase, the water gradually increased in salinity along the facies path 13-14-15-16, which caused a reverse exchange, showing a Ca2+-Cl- facies (no. 16), which was not found in the groundwater samples in this facies. Finally, in this phase, water evolves towards facies that are closer to seawater (Na+-Cl- facies) along facies path 16-12-8-4, and most samples in this study were found in these groundwater facies. However, in this research, the groundwater sample fell in Na+-mixCl- (no. 3), Na+-mixHCO3- (no. 2), Na+-HCO3- (no. 1), and MixNa+-HCO3- (no. 5), which are characteristic of the freshening process, and were not found in all groundwater samples.

Most samples that fell in the Na+-Cl- (no. 4) facies were groundwater samples collected from the Qcl aquifer, and corresponded to the Na+-Cl- facies, which appeared in the piper diagram. This indicated that the groundwater samples in the Qcl were severely influenced by seawater intrusion. Moreover, the groundwater samples from the weathering PCms and Gr aquifers fell in the MixNa+-Clfacie (no. 8) and MixCa2+-Cl- facies (no. 12), which corresponded to the Ca2+-Na+-HCO3—Cl- facies in the piper diagram and indicated a moderate influence of seawater intrusion. In addition, the groundwater samples from the PCms and Gr aquifers at deeper levels (60–80 m) showed that the Ca2+-HCO3-Cl- facies in the piper diagram and the HFED fell in the MixNa+-Cl- facie (no. 8) and MixCa2+-Cl- (no. 12) facies. Even though it fell in the facies of fresh water in the HFED, it was close to the mixing line and so it was deemed to be slightly influenced by seawater intrusion. These data were consistent with a previous study

that used HFED for depicting the

salinization processes in coastal aquifers in Italy and found that the HFED plots followed the succession of facies along the mixing line, indicating that seawater and fresh water were slightly mixed or that ionic exchange occurred (Ghiglieri et al, 2012). Moreover, they compared the HFED results with the EC contour lines and revealed that seawater had intruded quite far inland. Najib et al. (2017) highlighted the succession of different water facies that developed between the intrusion and freshening phases by analysis on the HFED. The formation of Na+-HCO3- facies, which characterizes the last facies of the freshening phase, followed the succession of Na+-Cl-, MixNa+-MixCl-, MixCa2+MixCl-, MixCa2+-MixHCO3-, and Na-HCO3-. Moreover, the obtained HFED results in this study allowed us to extend the intrusion process in the Holocene groundwater and accept the fresh water recharge, such as meteoric water and lateral recharge from rivers (Liu et al., 2017).

[Insert Figure 10] 4.5. Multivariate Statistical Analysis 4.5.1. Correlation matrix Pearson correlation was applied to assess the relationship between the various hydrochemical variables of the groundwater samples, which were measured in the field and analyzed in the laboratory (Table 2). Results highlighted in bold show a significant relationship at p < 0.05. The high

correlation of these parameters indicated that these cations/anions contributed to mineralization in the aquifer systems. Chloride (Cl-) was highly correlated with Na+ (r = 0.960), SO42-(r = 0.835) and Br(r = 0.857), implying that the groundwater system was influenced by seawater intrusion (Askri et al., 2016). In general, the dissolution of halite may be investigated as the linear relationship between Na+ and Cl- ions (Hem, 1985). The TDS was strongly positively correlated with the EC (r = 0.963), Na+(r = 0.934), Cl- (r = 0.923), Br-(r = 0.826) and SO42- (r = 0.775) and moderately correlated with Mg2+ (r = 0.669) and Ca2+ (r = 0.435). Similarly, the EC was positively correlated with Na+ (r = 0.910), Cl- (r = 0.914), Br- (r = 0.779), SO42- (r = 0.769) as well as moderately correlated with Mg2+ (r = 0.636) and Ca2+ (r = 0.425). These good correlations with the EC implied that the increase in salinity was caused from groundwater mineralization (Moussa et al., 2011; Zghibi et al., 2012; Tiankao and Chotpantarat, 2018; Wongsasuluk et al., 2018; Ratchawang and Chotpantarat, 2019; Sonthiphand et al., 2019). Calcium (Ca2+) had a moderately positive correlation with Mg2+ (r = 0.583) and was moderately correlated with HCO3- (r = 0.422), implying the dissolution of calcite and dolomite minerals from the geologic formation. Moreover, the good correlation between Mg2+ and Na+ (r = 0705) indicated that ion exchange was occurring during seawater intrusion. 4.5.2. Principal components analysis (PCA) The varimax method used to rotate the parameters in the PCA uses the Kaiser criterion rotating with the varimax method. Therefore, all of the parameters were classified into four components (see Supplementary Information, Table SI.2). Each component included the values that represent a good correlation (shown in bold). Table SI.3 shows the Eigen value of all the parameters, where the four groups had Eigen values more than 1 and a cumulative variance of more than 80.4%. The TDS, EC, Mg2+, Na+, Cl-, Br-, and SO42- concentrations were included in the first component that had the highest factor loading (6.13) and accounted for 47.2% of the total variance. This factor, with a high positive loading, ranged from 0.757 to 0.965, probably indicating the consequence of seawater intrusion in the study area. Therefore, factor 1 was defined as the “seawater intrusion factor”. Similarly, the study of Ahmed et al. (2017) found that factor 1 (seawater intrusion impact) accounted for 66% of the total variance and consisted of the EC, Cl-, Na+, SO42-, K+, Mg+, Br-, Sr, B, Cr, Co, Arsenic, and Selenium.

The second component had a factor loading of 1.60, accounting for 12.3% of the total variance and showed apositive relationship between Ca2+ (0.62), F- (0.63) and HCO3- (0.77). This factor can be expressed as the natural process when recharge water infiltrates into the groundwater system and water-rock interactions occur and eventually release Ca2+ and HCO3- to groundwater (Jiang et al., 2009). The values of HCO3- ranged widely from 19.52 to 189.5 mg/L, depending upon various geological formations in this area. Fluoride is naturally released from the dissolution of fluorapatite and fluorite, which occur in sedimentary and igneous rocks with fluorite being detected in granite, gneiss, and pegmatites. (Rama Rao, 1982; Heinrich, 1948)Moreover, due to the similarity in charge and radius, F- can substitute the OH- ions via water-rock interactions. Similar to Ca2+ and HCO3-, F- in groundwater can be inferred from the weathering of rocks in this area. The third component consisting of K+ and Fe2/3+ showed positive loadings in the range of 0.7-0.81 and a factor loading of 1.448 or 11.1% of the total variance. Through the weathering of igneous rocks, K+ can mainly be released from potassium feldspar into the groundwater (Kim et al., 2004) and Fe2/3+ represents the natural dissolution of rocks and minerals via water-rock interactions. The last component consisted of only the pH, with a low factor loading (1.27) or 9.78% of the total variance. These parameters might be the result of various dynamic hydro-geochemical processes in the area, such as seawater intrusion, recharge, water-rock interaction, etc. In addition, all variables were plotted in a rotated space (Figure 11) to clearly demonstrate the separation of the four components.

[Insert Table 2] [Insert Figure 11]

4.6. Integrated Interpretation of the VES and EC Results The VES data has a limitation in interpreting seawater intrusion as there are many factors that show a resistivity value of < 5 Ωm in coastal aquifers. Therefore, the results of the hydrochemical analysis and EC values need to be interpreted with the VES data to overcome this limitation and to

further elucidate the seawater intrusion. The VES sections were overlaid on the EC map of the Qcl aquifer, which was severely affected by seawater intrusion, to obtain the extent of intrusion (Figure 12). The zone with resistivity values of < 5 Ωm were considered to be the seawater intrusion area. Furthermore, the EC map showed the location of a high EC (> 1500 μs/cm), corresponding to the location with resistivity values (< 5 Ωm). From this relationship, it can be concluded that the Qcl aquifer was highly influenced by seawater intrusion with resistivity values in the range of 0–10 Ωm, especially in the upper part of the area. The boundary line is shown in Figure 12. Section A-A’ was influenced by seawater intrusion to about 3 km inland, where the first km from the coast line was highly influenced (resistivity values < 5 Ωm), while the last 2 km were moderately influenced and represented brackish water (resistivity values ranging from 5 to10 Ωm). Section B-B’ was influenced by seawater intrusion to about 5 km inland, where the first 2 km from the coast line were highly influenced, while the last 3 km were moderately influenced. Section C-C’ was influenced by seawater intrusion to about 4.7 km inland, where the first 3.2 km from the coast line was highly influenced and the following 1.5 km were moderately influenced. Section D-D’ was influenced by seawater intrusion to about 8 km inland, where the first 4 km from the coast line was highly influenced and the next 4 km was moderately influenced. As shown from the geophysical and hydrochemical results, the levels of seawater intrusion could be classified into three zones using the criteria shown in Table 3.

[Insert Figure 12] [Insert Table 3]

5. CONCLUSIONS In this research, 80 VES surveys were conducted using a Schlumberger configuration integrated with the hydrochemical analysis from 58 groundwater samples to indicate the level of seawater intrusion into coastal aquifers. Four pseudo cross-sections were generated from the 80 VES data points. These were in good agreement with those obtained from the hydrogeological data and lithologic data in the study area. The resistivity map at different depths, generated from the VES data,

successfully revealed the interaction of seawater and freshwater along the coast line. The geophysical results found that seawater mainly intruded in the Qcl aquifer. Resistivity values of < 5 Ωm were found at a depth of approximately 50 m. However, the VES approach was limited when evaluating the seawater intrusion in highly contaminated aquifers located close to the coast line. Therefore, evaluation of seawater intrusion in coastal areas with VES data needs the assistance of hydrochemical and hydrogeological data to describe the seawater intrusion more accurately. From the hydrochemical analysis of 58 groundwater samples, five types of groundwater facies (Ca2+-Na+-HCO3-, Ca2+-HCO3--Cl-, Ca2+-Na+-HCO3--Cl-, Ca2+-Na+-Cl- and Na+-Cl-) were noticed, which were dependent upon the aquifer types and depths. The Na+-Cl- facies were typically found in the Qcl aquifer, corresponding to resistivity values of < 5 Ωm. From the geophysical and hydrochemical results showed, the levels of seawater intrusion could be classified into three zones depending on the degree of seawater intrusion. As a suggestion from this research, groundwater samples should be periodically collected from at least two periods to analyze the dynamics and evolution of seawater intrusion. The VES investigation should be concerned with the distance between the VES survey point and the VES point near the coast line as the depth of survey cannot penetrate through high seawater contaminated groundwater. Moreover, lithologic data for aquifers that have a lot of clay components are required to interpret the results more accurately. In the future, when this area needs to be used as the groundwater resources, people should use groundwater in the rock aquifers (PCms and Gr aquifers) at depths of more than 70 m. The proper criteria for selecting the study area of seawater intrusion should consider the following: groundwater demand in the area, ecological and hydrogeological characteristics, and the amount of groundwater recharge needed to prevent problems that may occur from too much groundwater pumping in the future.

ACKNOWLEDGMENTS Authors thank the Geology Department, Faculty of Science, Chulalongkorn University, the Graduate School of Chulalongkorn University, the International Research Integration: Chula Research Scholar program, the Ratchadaphiseksomphot Endowment Fund (GCURS-59-06-79-01), the Ratchadaphiseksomphot Endowment Fund (2019), Chulalongkorn University (CUGR_62_21_23_06)the Office of Higher Education Commission (OHEC), and the S&T Postgraduate Education and Research Development Office (PERDO) for providing financial support for this research program. We are also grateful to the Department of Groundwater Resources (DGR) for providing some of the data. We also thank Prof.Dr.Mei-Fu Zhou, Editor-in Chief of Journal of Asian Earth Sciences, and anonymous reviewers for their suggestions and critical comments which greatly improved an earlier version of this manuscript.

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Figure captions Figure 1. Geological map of the study area including the VES points and location of the four crosssection lines; A-A’, B-B’, C-C’, and D-D’ (adapted from the Department Mineral Resources 2014). Figure 2. Hydrological map of the study area showing the sample collection wells (adapted from the Department of Groundwater Resources (DGR), 2015). Figure 3. Schematic diagram showing the Schlumberger configuration of the current electrode (A, B), delivered currents (C1, C2) and the potential electrodes and measured potentials (P1, P2) Figure 4. Resistivity values vs. lithologic-log values from the pseudo cross-section line D-D’ (well Q168 vs. VES St 47 and well PW 7962 vs. VES St 34). Figure 5. Resistivity values in the pseudo cross-section lines A-A’, B-B’, C-C’ and D-D’ showing a comparison of the borehole and electricity log data. Figure 6. Geological cross-section lines of (a) A-A’, (b) B-B’, (c) C-C’ and (d) D-D’. Figure 7. Apparent resistivity map for AB/2 values of (a) 10, (b) 30, (c) 50, (d) 70, (e) 100 and (f) 150 m. Figure 8. Hydrochemical analysis of the groundwater samples as a piper diagram. Figure 9. The relationship between Na+ and Cl- concentrations (in meq/L) in the groundwater samples collected from different aquifers. Figure 10. The HFED for depicting the salinization process in the study area. Figure 11. Component plot in rotated space. Figure 12. Boundary of seawater intrusion in the Qcl aquifer, based on the EC contour map superimposed on the resistivity map.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Table 1 Characteristics of aquifer in the study area

Aquifer

Location

Average depth of aquifer

Groundwater Level

Well Yield

Total Dissolve Solid (TDS)

m.

m.

m3/hr

mg/L

Quaternary beach sand aquifer (Qbs)

found in the east coastal plain and are distributed from north to south

5-8

1-2

<2

500-1,000

Quaternary flood plain aquifer (Qfd)

dispersed over in the upper part and is lined between Qbs and Gr

25-45

3-8

2-10, 10-20 (some area)

600-2,000

Quaternary colluvial sediments aquifer (Qcl)

deposited near the foothill of the granite mountain are in the north and south of the area

30-50

n/a

>50

300->1,200

Silurian-Devonian metamorphic aquifer (SDmm)

found in a small area in the lower west

30-35

3-5

<2

<800

Permian-Carboniferous metasedimentary aquifer (PCms)

found in the northern part of the study area

50-200

n/a

<2, 5-10 (fracture zone)

n/a

found in the upper eastern part of the study area

50-100

n/a

2-10, >20 (fracture zone)

n/a

Permian limestone aquifer (Pc)

dispersed a small monadnock in the north of the area

10-50

3-5

10-20

<500

Cretaceous granite aquifer (Gr)

dispersed along the large mountain ranges in a northsouth direction to the west of the area

50-150

6-9

<2, >20 (fracture zone and fault zone)

200-1,000

Table 2 Pearson’s correlation coefficient )R2 (between the physiochemical and hydro chemical parameter from 58 groundwater samples.

Variables

pH

pH

1.000

TDS

-0.145

1.000

EC

-0.177

0.963

1.000

K+

0.032

-0.035

-0.118

1.000

0.056

-0.156

-0.189

0.324

1.000

-0.249

0.435

0.425

-0.067

-0.050

1.000

-0.227

0.669

0.636

0.052

-0.093

0.583

1.000

Na+

-0.196

0.934

0.910

0.046

-0.097

0.362

0.705

1.000

F-

0.216

-0.187

-0.169

-0.295

-0.010

0.038

-0.164

-0.237

1.000

Cl-

-0.162

0.923

0.914

0.003

-0.094

0.420

0.696

0.960

-0.290

1.000

Fe Ca

2+

Mg

2+

Br-

TDS

EC

K+

Fe

Ca2+

Mg2+

Na+

F-

Cl-

Br-

SO42-

-0.072

0.826

0.779

0.115

-0.048

0.564

0.849

0.847

-0.223

0.857

1.000

SO4

2-

-0.165

0.775

0.769

0.141

0.058

0.472

0.728

0.863

-0.233

0.835

0.817

1.000

alkalinity

-0.104

0.378

0.342

-0.046

-0.015

0.422

0.346

0.282

0.222

0.206

0.399

0.229

Values in the highlighted bold indicate a relationship between two parameters with a significance level at 0.05 (P<0.05)

Table 3. The levels of seawater intrusion with resistivity and EC values Level of seawater intrusion

Resistivity (Ωm)

EC (µs/cm)

Extremely

<5

>1500

Moderately

5-10

1000-1500

Slightly

>10

<1000

Groundwater Facies Na-Cl Na-Cl, Ca-Na-Cl , Ca-Na-HCO3-Cl, Ca-Na-HCO3, Ca-HCO3-Cl Ca-Na-Cl ,Ca-Na-HCO3, Ca-Na-HCO3-Cl, Ca-HCO3- Cl

alkalinity

1.000

Highlights     

The geophysical method combined with hydrochemical data to identify affected areas. The resistivity value of <5 Ωm represented highly contaminated areas. The relationship of VES data and EC map of affected aquifer was developed. Seawater intrusion clearly impacts in the Qcl aquifer at approx. 8 km inland. HFE-Diagram and PCA use successfully to explain intrusion and freshening process.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Author contributions statements

Ms. Jiraporn Sae-Ju conducted the experiments and field survey and wrote the main manuscript. Dr.Srilert Chotpantarat wrote the main manuscript and mainly response all questions of reviewers, and added supplemental discussion. I determined the research methodology, research plan and data analysis, summarized the results of research, including preparation of publication as the corresponding author. Dr.Thanop Thitimakorn suggested for the geophysical survey and gave comments and suggestions to improve the manuscript.