Hydrochemistry and Isotope Hydrology for Groundwater Sustainability of the Coastal Multilayered Aquifer System (Zhanjiang, China)
Groundwater sustainability has become a critical issue for Zhanjiang (China) because of serious groundwater level drawdown induced by overexploitation of its coastal multilayered aquifer system. It is necessary to understand the origins, material sources, hydrochemical processes, and dynamics of the coastal groundwater in Zhanjiang to support its sustainable management. To this end, an integrated analysis of hydrochemical and isotopic data of 95 groundwater samples was conducted. Hydrochemical analysis shows that coastal groundwater is fresh; however, relatively high levels of Cl−, Mg2+, and total dissolved solid (TDS) imply slight seawater mixing with coastal unconfined groundwater. Stable isotopes (δ18O and δ2H) values reveal the recharge sources of groundwater in the multilayered aquifer system. The unconfined groundwater originates from local modern precipitation; the confined groundwater in mainland originates from modern precipitation in northwestern mountain area, and the confined groundwater in Donghai and Leizhou is sourced from rainfall recharge during an older period with a colder climate. Ionic relations demonstrate that silicate weathering, carbonate dissolutions, and cation exchange are the primary processes controlling the groundwater chemical composition. Declining trends of groundwater level and increasing trends of TDS of the confined groundwater in islands reveal the landward extending tendency of the freshwater-seawater mixing zone.
Increases of both population and water demand in coastal areas have made groundwater an important water resource for coastal regions; however, coastal groundwater is vulnerable to overexploitation and contamination [1, 2]. Therefore, sustainable management of coastal groundwater has become a critical issue . Understanding the hydrochemical characteristics of coastal groundwater could provide guidance for sustainable groundwater management [4–6].
The characteristics of groundwater chemistry are primarily influenced by recharge water chemistry, water-rock interactions, solute transport, and chemical processes occurring along the flow paths [6–10]. By analyzing the hydrochemical and isotopic data together with considering the hydrogeological conditions, the origins, chemical compositions, and dominating hydrochemical processes (e.g., water-rock interactions, evaporation, and mixing between different water) of groundwater in aquifers can be assessed comprehensively [11–14].
In hydrogeological studies about the coastal groundwater management, analysis of the hydrochemistry and hydrogen-oxygen isotopes data has been used widely to determine the hydrogeological conditions, such as groundwater recharge sources, recharge rates, and flow patterns [15–20]. The application of chemistry and hydrogen-oxygen isotopes can be used also to identify processes of groundwater salinization induced by seawater intrusion [21–25]. In addition, many other isotopes (e.g., radium, carbon, chlorine, boron, and strontium) have been used as tracers for characterizing the hydrogeological conditions and hydrochemical processes in coastal aquifers, specifically identifying submarine groundwater discharge and describing seawater intrusion [10, 13, 26, 27].
This study focused on the coastal multilayered aquifer system (including three layers of aquifer and two layers of aquitard) of Zhanjiang, which is located in the southwest of Guangdong Province, China (Figure 1). The groundwater in the middle and deep confined aquifers (Figure 2) has been the sole source of drinking water for the population of the city of Zhanjiang since the 1960s. According to the water resources bulletin of Zhanjiang, groundwater pumping amount has been about 2.2 × 108 m3/a for the resident population and local industry in recent years. Because of this intense exploitation of groundwater, the confined groundwater level has dropped to about 20 m below sea level since the 1990s [28, 29] (Figures 3(b) and 3(c)). Recent investigations have shown that the groundwater in this multilayered aquifer system remains fresh, but parts of the unconfined groundwater in island areas (e.g., Donghai and Naozhou) and small parts of the confined groundwater in Naozhou island have suffered seawater intrusion [30–33]. It is a concern that the confined groundwater in Zhanjiang city will be risky in suffering from seawater intrusion in the future. Therefore, it is necessary and urgent to conduct a research to identify the origins, mineralization processes, and hydrochemical dynamics of the coastal groundwater to assess the risk of seawater intrusion.
The main objective of this study is to identify the origins, material sources, and hydrochemical processes of the groundwater in the coastal multilayered aquifer system of Zhanjiang through integrated analysis of hydrochemical and isotopic data. In addition, the risk of seawater intrusion into the confined groundwater is assessed by analysis of the dynamic data of groundwater level and hydrochemistry. The results will contribute to generate scientific information for the local coastal hydrogeology and be supportive for the sustainable management of the groundwater in this multilayered aquifer system.
2. Study Area and Its Hydrogeology Condition
Zhanjiang city with a land area of 1491 km2 is located in southwestern Guangdong, China (Figure 1). The topography is high in the northwest and low in the south. The average annual precipitation and evaporation are 1347 and 1774 mm, respectively [29, 32].
The geology of the study area mainly consists of continental and marine sediments of upper Tertiary-Quaternary age overlying a basement of muddy sandstone of Cretaceous age (K2). According to earlier geological investigation [29, 35, 36], the sedimentary formations are characterized by five stratigraphic units, which include Holocene stratum (sand and clay), Beihai Group of middle Pleistocene age (Q2b, sand with gravel in the lower portion and clayey sand in the upper portion), Zhanjiang Group of lower Pleistocene age (Q1z, coarse sand with gravel and scattered lenses of clay), Xiayang Group of Pliocene age (N2x, medium to coarse sand with gravel and thin layers or scattered lenses of clay), and Weizhou Group of Miocene age (N1w, silty sand and fine sand with clay). These geological formations are intercalated with basalt and pyroclastic rock. The sediments mentioned above constitute the multilayered aquifer system that includes three aquifers (the unconfined aquifer, the middle confined aquifer, and the deep confined aquifer) separated by clay layers (aquitards) (Figure 2).
The unconfined aquifer is about 30-m thick and is composed of deposits of Holocene age, Beihai Group of middle Pleistocene age, and upper portion of Zhanjiang Group of lower Pleistocene age. This aquifer overlies a thick layer of clay that extends laterally under the seabed. The hydraulic conductivity () of this unconfined aquifer is 5–25 m/d. Because exploitation of the unconfined groundwater is scattered and intermittent, the groundwater flow field remains an approximately natural flow regime with the water table above the mean sea level (Figure 3(a)). The groundwater, which is recharged mainly by rainfall infiltration and discharged through evaporation and runoff to the ocean, flows radially from the watershed to the ocean (Figures 2 and 3(a)).
The middle confined aquifer is composed of Zhanjiang Group deposits of lower Pleistocene age (Q1z), with thickness of about 120 m and hydraulic conductivity () of 20–60 m/d. Induced by overexploitation, the groundwater level of this confined aquifer has dropped to −24 to 16 m (Figure 3(b)). The deep confined aquifer is composed of Xiayang Group deposits of Pliocene age (N2x), with of 20–50 m/d. The groundwater level of the deep confined aquifer has dropped to −22 to −4 m (Figure 3(c)). These two confined aquifers are recharged mainly via lateral runoff and they are discharged by pumping.
3. Sampling and Analysis Method
To investigate the hydrochemistry of the groundwater in the multilayered aquifer system of Zhanjiang, 3 times of groundwater sampling activities were conducted from March 2009 to March 2011. As shown in Figure 1 and Table 1, a total of 95 groundwater samples were collected from public supply wells. These comprised 22 samples from the unconfined aquifer (depth < 30 m, sample numbers starting with Q), 35 samples from the middle confined aquifer (50 < depth < 140 m, sample numbers starting with Z), and 38 samples from the deep confined aquifer (depth > 200 m, sample numbers starting with S). All samples were filtered through membranes (0.45-μm pore size) and stored in high-density polyethylene bottles, which were pretreated using deionized water and rinsed using sampled water. Then, the samples were preserved and acidified with HNO3 for cation analysis. All bottles were sealed with wax to ensure a watertight seal.
The total dissolved solid (TDS), temperature, and pH were measured in situ using a portable multiparameter water analyzer (Hach, Sension156). The concentration of was also determined in the field via titration on the day of sampling. The major cations (K+, Ca2+, Na+, and Mg2+) were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, pHPerkin-Elmer Sciex Elan DRC-e) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The anions (Cl−, , and ) were measured by Ion Chromatography System (Dionex, ICS-1500) at IGGCAS. Dissolved silica (SiO2) was analyzed by spectrophotometry using the molybdate blue method. The charge balance (, where is the milligram equivalent of the cations and is the milligram equivalent of the anions) varied from −4.97% to 4.95% (within ±5%), with an average of −1.84% (within ±5%). This balance number can indicate the accuracy of the data.
The analyses of stable oxygen (18O), hydrogen (2H), and sulfur (34S) isotopes were conducted using mass spectrometers (Finigan MAT 253 for 18O-2H and Delta S for 34S) at the Stable Isotope Laboratory, IGGCAS. The isotope ratios (δ18O, δ2H, and δ34S) were given in the usual δ-units calculated with respect to standard sample: , in which and represent the ratio of heavy to light isotopes of the sample and standard, respectively. The results of the stable isotope are shown in Table 2.
In this study, first, according to the groundwater chemical and isotopic data (Tables 1 and 2), statistical analyses (including general statistics and Pearson correlation analysis) and Piper diagram were used to illustrate the general hydrochemical characteristics (e.g., groundwater composition, dominating ions, and groundwater type) (Table 3 and Figure 4) and to assess the correlation between the hydrochemical compositions (Table 4) in the groundwater of this multilayered aquifer system. Second, isotope analyses, Gibbs plots, and bivariate analyses of the compositions were conducted to determine the origins, controlling physical/chemical processes and material sources of the groundwater. Third, based on the understanding of the groundwater origins and the controlling processes, the groundwater dynamics were analyzed to assess the risk of seawater intrusion into this coastal aquifer system.
4.1. Groundwater Hydrochemistry
Understanding the characteristics of groundwater chemistry is the base to identify the groundwater origins and the hydrochemical processes occurring in the aquifer. The hydrochemical data of groundwater in the multilayered aquifer system of Zhanjiang are presented in Table 1, and the statistical results of those data are presented in Table 3. As shown, the TDS of the unconfined groundwater varies from 149 mg/L to 823.39 mg/L with a mean value of 360.28 mg/L; that of the middle confined groundwater ranges from 64.31 mg/L to 252.94 mg/L with an average value of 124.47 mg/L; and that of the deep confined groundwater changes from 99.52 mg/L to 325.98 mg/L with a mean value of 158.01 mg/L. These low values of TDS indicate that the groundwater in this aquifer system is mainly fresh (TDS < 1000 mg/L). According to the hydrogeological conditions of this aquifer system, it can be concluded that the approximately natural flow regime of the water table above the mean sea level (Figure 3(a)) is the primary reason why most of the unconfined groundwater has not become salinized. Furthermore, the confined aquifer’s roof with extremely low permeability prevents seawater intrusion into the confined groundwater. The pH value of the unconfined groundwater varies from 4 to 8.25 with an average value of 6.04; that of the middle confined groundwater changes from 4.15 to 7.34 with an average value of 6.25; and that of the deep confined groundwater ranges from 5.34 to 7.81 with an average value of 6.72. Therefore, the groundwater is generally acidity. The average concentrations of major cations in the unconfined and the middle confined groundwater follow the order of Na+ > Ca2+ > K+ > Mg2+, and those of the major anions follow the order of > Cl− > > (Table 3). The average concentrations of major cations in the deep confined groundwater follow the order of Na+ > K+ > Ca2+ > Mg2+, and those of the major anions follow the order of > > Cl− > (Table 3).
The relations between the major ions and TDS are useful for interpreting the major hydrogeochemical evolution processes occurring in the aquifer and for deducing the material sources of the ions in the groundwater [37–39]. In this study, correlation coefficients were calculated to represent the relations between TDS and the major ions (Na+, K+, Ca2+, Mg2+, , , Cl−, and ). As shown in Table 4, the correlations between TDS and the major ions are not strong (correlation coefficients < 0.900), which indicates no single ion can take dominant role in groundwater mineralization. This result also implies that the dissolution of various minerals together constitutes the groundwater composition.
4.2. Groundwater Types
Piper diagram can help to understand the groundwater type and the potential hydrochemical processes controlling groundwater chemistry . According to the concentrations of major ions shown in Table 1, the Piper plot was made (Figure 4). As shown, the unconfined and middle confined groundwater show a relatively large range in the rhombus area (areas I and II). The unconfined groundwater is characterized by HCO3-Ca·Na, HCO3·Cl-Ca·Na, Cl·SO4-Na·Ca, and Cl-Na·Ca·Mg hydrochemical types. With comprehensive consideration of the complicated hydrochemical types and relatively high levels of Cl−, Mg2+, and TDS (i.e., samples Q2, Q4, and Q6, Figure 1 and Tables 1 and 3) of the unconfined groundwater, it can be concluded that slight seawater mixing occurs in the unconfined groundwater near the coastline but that seawater intrusion is still in the initial phase . The middle confined groundwater is characterized by HCO3-Ca·Mg·Na, Cl·HCO3-Na·Ca, and Cl·SO4-Na·Ca·Mg hydrochemical types. The low TDS values and complicated hydrochemical types of this confined groundwater suggest that water-rock interactions (e.g., mineral dissolution or cation exchange) might occur in the middle confined aquifer.
Figure 4 also shows that the deep confined groundwater samples, which are distributed mainly in the bottom-left corner of rhombus area (II), are mainly represented by HCO3-Na·Ca(Mg) hydrochemical type, indicating that deep confined groundwater naturally evolves without any intensive hydrogeochemical process or anthropogenic impact.
4.3. δ18O and δ2H Compositions
The stable oxygen (18O) and hydrogen (2H) isotopes of groundwater samples are related to the recharge sources, flow paths, and residence times of groundwater. The method of isotope analysis has been used widely in many hydrogeological studies [4, 5, 41, 42]. The analysis results of δ18O and δ2H are shown in Table 2 and Figure 5. According to the monthly rainwater data obtained from the GNIP (Global Network of Isotopes in Precipitation) of the IAEA (International Atomic Energy Agency), the local meteoric water lines (LMWLs) of Hong Kong and Haikou weather stations were calculated as and , respectively. Then, these two LMWLs were used as the LMWL of Zhanjiang. As shown in Figure 5 and Table 2, the isotopic compositions of δ2H and δ18O of the unconfined groundwater vary from −38.40 to −46.50 (average: −42.07) and from −5.56 to −7.05 (average: −6.40), respectively. The isotopic compositions of δ2H and δ18O of the middle confined groundwater vary from −43.60 to −52.90 (average: −47.58) and from −6.42 to −7.42 (average: −6.91), respectively. The isotopic compositions of δ2H and δ18O of the deep confined groundwater vary from −46.50 to −54.62 (average: −50.79) and from −6.90 to −7.45 (average: −7.16), respectively.
5.1. Origin of the Groundwater
For the unconfined groundwater, as shown in Figure 5, all samples (except Q2, Figure 1) plot along the global meteoric water line (GMWL)  and LMWL, indicating that the unconfined groundwater is mainly of meteoric origin. In comparison with the other groundwater samples, sample Q2 is relatively enriched in stable isotopes, it deviates slightly from the LMWL (part I in Figure 5), and it is characterized by comparatively high levels of TDS (563.74 mg/L) and a low water table (2.55 m). This might imply that this sample is influenced either by relatively intense evaporation or by slight mixing with seawater in the area near the coastline.
For the middle and deep confined groundwater, as shown in Figure 5, the distribution of the confined groundwater samples presents a pattern: the deeper the aquifer depth is, the more depleted the isotopic data are. This pattern may imply that the hydraulic connection between this two confined aquifers is relatively weak. The samples collected in the mainland area (i.e., Zhanjiang city area) are mainly located along the GMWL and LMWL, indicating that the confined groundwater in the mainland area is of meteoric origin. However, the relatively more depleted isotopic data compared to those of the unconfined groundwater implies that meteoric recharge to the confined groundwater is sourced from the mountain area of the north and northwest area, where the precipitation’s isotopic data are more depleted. Meanwhile, the confined groundwater flow fields (Figures 3(b) and 3(c)) certify the occurrence of recharge from the northern and northwestern areas. In addition, the confined groundwater samples collected in the southern and southwestern areas (samples Z13, Z14, and S14 in Donghai and samples Z12 and S13 in Leizhou, Figure 1) are characterized by more depleted isotopic data than samples of the mainland area. These samples deviate significantly from the LMWL and they are distributed to the bottom-left of the LMWL (part II in Figure 5). This indicates that the confined groundwater of the Donghai and Leizhou areas is sourced from rainfall recharge during an older period with a colder climate. From the sulfur isotopes (δ34S) of the groundwater in Donghai (Table 2), it can be concluded that the δ34S values in groundwater become more enriched with increasing depth. This trend of enrichment of δ34S values in the confined fresh groundwater of Donghai also demonstrates that the confined groundwater of island is palaeowater. According to the recharge pattern of the confined groundwater in the southern and southwestern areas (Figures 3(b) and 3(c)), we consider that the palaeowater stored in the confined aquifers of Donghai and Leizhou will flow toward Zhanjiang through lateral flow because of the intensive groundwater pumping of recent years.
In conclusion, the unconfined groundwater is recharged by local modern precipitation. However, the confined aquifers are recharged by precipitation in northern and northwestern mountain areas and by palaeowater sourced originally from rainfall infiltration during an older time with a colder climate.
5.2. Controlling Processes and Material Sources of Groundwater Chemistry
To quantitatively study the controlling processes and material sources of the groundwater in Zhanjiang, Gibbs plots and bivariate analyses of the ionic relations were discussed in this section.
5.2.1. The Dominating Hydrochemical Process
Gibbs plots (i.e., a TDS versus graph and a TDS versus graph) can be used to determine the primary hydrochemical processes (e.g., atmospheric precipitation, rock weathering, and evaporation) controlling groundwater chemistry . According to the hydrochemical data (Table 1), Gibbs plots were made as Figure 6. Those plots indicate that rock weathering is the major mechanism controlling the groundwater chemistry of the multilayered aquifer system of Zhanjiang. This conclusion is coincident with the results deduced from the analysis of groundwater hydrochemistry and groundwater types.
5.2.2. Dissolution Material and Dissolution Process
To identify the dominant mineral in the rock weathering process in this aquifer system, molar ratio bivariate plots of Na-normalized Ca, Mg, and HCO3 were made [45, 46]. As shown in Figure 7, the groundwater of the multilayered aquifer system is mainly influenced by silicate weathering and carbonate dissolution, especially for the confined groundwater.
The milligram equivalent ratio of (Na+ + K+)/C1− can be an indicator of the sources of cations and of the occurrence of silicate weathering, where a ratio greater than 1 implies Na+ released from silicate weathering and a ratio of 1 indicates halite dissolution . As shown in Table 3, the (Na+ + K+)/C1− ratio values of the unconfined groundwater vary from 0.38 to 4.44 with an average value of 1.58; those values of middle confined groundwater range from 0.57 to 6.28 with an average value of 2.22; those values of deep confined groundwater change from 0.91 to 25.03 with average value of 6.95. These averages (Na+ + K+)/C1− ratio > 1 indicate the derivation of Na+ and K+ from silicate weathering. Moreover, the increase of the (Na+ + K+)/Cl− ratio with groundwater depth reveals that the silicate weathering in the confined aquifer is more remarkable than in the unconfined aquifer. Furthermore, the relatively higher concentration of SiO2 (Table 3) in confined groundwater verifies evident silicate weathering in confined aquifers. The scatter plot of C1− versus Na+ + K+ (Figure 8(a)) shows that the unconfined groundwater samples are distributed along the 1 : 1 line (or on either side of this line), which implies that ions (Na+ and K+) are mainly resultant from the silicate weathering and halite dissolution. Conversely, most samples of the confined groundwater fall below the 1 : 1 line, indicating that silicate weathering is the primary hydrochemical process in the confined aquifers. In addition, as shown in Figure 8(a), the excess of (Na+ + K+) over C1− also implies that cation exchange may occur in the confined aquifers.
The plot of ( + ) versus (Ca2+ + Mg2+) (Figure 8(b)) shows that most samples of fresh unconfined groundwater fall along the 1 : 1 line and that some samples fall below the 1 : 1 line, which indicates that the combined dissolutions of carbonate and silicate are the main sources of Ca2+ and Mg2+ in the unconfined groundwater . Most samples of the confined groundwater fall above the 1 : 1 line, which demonstrates that silicate weathering is the main source of Ca2+ and Mg2+ in the confined groundwater [9, 49–51]. The deficiency of Ca2+ + Mg2+ (Figure 8(b)) and the excess of Na+ (Figure 8(a)) indicate the occurrence of cation exchange in the confined aquifers.
The plot of versus (Cl− + ) (Figure 8(c)) shows that the groundwater samples of the unconfined and middle confined aquifers are distributed on both sides of the 1 : 1 line, which implies that carbonate and evaporite dissolutions are also the main material sources of the chemical compositions of the unconfined and middle confined groundwater. The groundwater samples of the deep confined aquifer mainly plot above the 1 : 1 line, indicating that carbonate dissolution is another material source of the chemical composition of the deep confined groundwater.
In conclusion, silicate weathering is the dominant process influencing the material source of ions (Na+, K+, Ca2+, and Mg2+) in the coastal aquifer system of Zhanjiang. Carbonate and evaporite dissolutions also contribute to the groundwater compositions. With the dissolution by carbonic acid (H2CO3), the general reaction of silicate weathering is See .
5.2.3. Ion Exchange
As described in Section 5.2.2, most of the confined groundwater samples show an excess of Na+ over Cl+ and a deficiency of Ca2+ + Mg2+ over + , which may indicate the contribution of cation exchange to the groundwater composition [39, 52]. The chloroalkaline index (CAI) of the groundwater samples can be an indicator of the type and the intensity of the ion exchange reactions between the groundwater and the aquifer matrix . The CAI is calculated using the following formulae: . Positive and negative values of the CAI indicate reverse cation exchange () and cation exchange (), respectively. As shown in Figure 9, the CAI values of the confined groundwater, especially that of the deep confined groundwater, are mainly negative, supporting the assumption of the occurrence of cation exchange in the confined aquifers. Meanwhile, the absolute value of the CAI can reflect the intensity of the cation exchange reactions. Figure 9 shows that the absolute CAI values of the deep confined groundwater are greater than the values of the middle confined groundwater, which means that the cation exchange reaction in the deep confined aquifer is more intense than in the middle confined aquifer.
In addition, to further investigate the occurrence of cation exchange in the confined aquifers, a bivariate plot of (Ca2+ + Mg2+ − − ) versus (Na+ − Cl−) can be used . If cation exchange is an important process controlling the groundwater chemistry, the groundwater samples will fall in the lower-right quadrant of this diagram and along a line with a slope of −1. According to the chemistry data, plot of (Ca2+ + Mg2+ − − ) versus (Na+ − Cl−) was made (Figure 10). Figure 10 shows that most of the deep confined groundwater samples showed an excess of Na+ over Cl− and a deficiency of Ca2+ + Mg2+ over + and that they mainly lie along the line with a slope of −1. Thus, it can be concluded that cation exchange occurs in the confined aquifer.
5.2.4. Anthropogenic Input
In an area with considerable demand for groundwater, anthropogenic activity is always an important factor regarding groundwater quality. The concentrations of can reflect the influence of anthropogenic activity on groundwater chemistry. In this study, the concentrations in the samples of the unconfined groundwater range from 0.5 to 120.0 mg/L with an average value of 41.36 mg/L, exceeding the quality standard for groundwater in China. This implies that the unconfined groundwater has been influenced by anthropogenic activities. The concentrations in the samples of the middle and deep confined groundwater are <15 mg/L, which meet the groundwater quality standard. This indicates that anthropogenic influence on the confined groundwater is minor.
5.3. Salinity Indications of the Seawater Intrusion Risk
As introduced in Section 2, the confined groundwater level has dropped to about 20 m below the mean sea level since the 1990s [28, 29]. Although the confined aquifers have not experienced seawater intrusion, it is of concern that the confined groundwater will be at risk from seawater intrusion because of continuous groundwater demand, low confined groundwater level, and landward recharge flow pattern. In this section, based on the analysis of the flow regime, groundwater level dynamics, and salinity dynamics of the confined aquifers, the seawater intrusion risk for confined aquifer is discussed.
First, as shown in Figures 3(b) and 3(c), the confined groundwater level has dropped to approximately 20 m below the mean sea level. Two groundwater depression cones have formed in the confined aquifers and the confined aquifers are partially recharged by lateral groundwater runoff from the south (i.e., the direction of the ocean). This recharge characteristic of confined aquifers constitutes a potential risk of seawater intrusion.
Second, the long-term monthly monitoring data in boreholes (Figure 3) of the multilayered aquifers are analyzed to assess the dynamics of the groundwater level of Zhanjiang. Plots of observed groundwater levels of the unconfined aquifer, middle confined aquifer, and deep confined aquifer are shown in Figure 11. The groundwater level dynamics can be concluded as follows: (i) the unconfined groundwater level is in a relatively steady state with fluctuations induced by rainfall dynamics (Figure 11(a)) and (ii) the confined groundwater levels fluctuate with declining trends (Figures 11(b) and 11(c)). The declining trends of the confined groundwater level are obvious even for the groundwater in recharge-runoff areas such as Donghai island (boreholes L38-1(B) and L40-1(B)) and Nanshan island (L39-1(B)). The declining trends of the confined groundwater levels reveal that groundwater exploitation in Zhanjiang city is excessive and unsustainable. Meanwhile, from the δ18O-δ2H isotopic analysis result of the recharge sources of the confined groundwater, it has been established that the confined groundwater in Donghai is palaeowater which is unrenewable in short time. This means that the unsustainable groundwater exploitation is consuming the groundwater storage resources in the confined aquifers. Therefore, this unsustainable groundwater exploitation will increase the risk of seawater intrusion.
Third, the TDS values during 2009–2011 were compared to analyze the salinity dynamics. Figure 12 shows the salinity dynamics in some of the monitoring boreholes of the three aquifers. For the unconfined groundwater, the average TDS value presents a slightly decreasing trend (Figure 12(a)). However, for the middle confined aquifer, the average TDS value shows a slightly increasing trend (from 113.27 to 140.53 mg/L), while the trend of increase of the groundwater in Donghai (borehole Z10, increasing from 144.74 to 252.94 mg/L) is comparatively obvious (Figure 12(b)). Similarly, the average TDS value of the deep confined groundwater also shows a slightly increasing trend (from 151.52 to 162.33 mg/L), while the trend of increase of the groundwater in Donghai (borehole S10, increasing from 159.68 to 196.66 mg/L) is relatively notable (Figure 12(c)).
According to the increasing trends of TDS values in the confined groundwater (especially the groundwater in Donghai island), it can be deduced that the freshwater-seawater mixing zones in the coastal confined aquifers have begun extending landward as the inland groundwater levels decline. From the groundwater flow fields (Figure 3), it can be seen that Naozhou island which is located in the offshore area of Donghai island will be the first to suffer from seawater intrusion. To address the possibility of seawater intrusion in Naozhou island, we referred to the hydrochemistry data of Naozhou island investigated by Zhang et al.  in March 2011 (Table 5). These hydrochemical data showed that the confined groundwater in the southern and eastern coastal area (e.g., samples 26, 22, 20, and 14 in Figure 1(d)) has been saline with TDS value of 1.39–10.28 g/L. This means that freshwater-seawater mixing zones in the confined aquifers of Naozhou island have extended landward. This landward extension constitutes the reason for the increase in TDS of the confined groundwater in Donghai island. Thus, the confined groundwater in Donghai island and Zhanjiang city will be risky in suffering from saltwater intrusion if the current unsustainable groundwater exploitation is not optimized.
Based on the analysis of the hydrochemistry and isotope of groundwater, this study revealed the recharge sources, hydrochemical processes, and seawater intrusion risk of the coastal groundwater in the multilayered aquifer system of Zhanjiang, China. The stable isotope values of unconfined and confined groundwater indicate that the recharge sources of groundwater in those aquifers are different. The unconfined groundwater is recharged from local modern precipitation, the confined groundwater in the city area (mainland area) is sourced from modern rainfall from the mountain area in the northwest of Zhanjiang, and the confined groundwater in Donghai island and Leizhou areas is recharged from palaeowater originated from precipitation during an older time under a colder climate. Natural hydrochemical processes such as silicate weathering, carbonate dissolutions, and cation exchange reaction are the dominant processes controlling the material sources of ions in groundwater of this multilayered aquifer system. However, anthropogenic activities also have affected the quality of the unconfined groundwater leading to higher concentrations of nitrate. In addition, increase trend of TDS in the confined groundwater reveals the occurrence of seawater intrusion induced by groundwater exploitation. The confined groundwater exploitation is unsustainable and is consuming the palaeowater storage.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was supported by the National Natural Science Foundation of China (Grant no. 41502255). The authors thank the laboratory technicians of Institute of Geology and Geophysics for their great help in testing groundwater samples.
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