Elucidating the Pollution Sources and Groundwater Evolution in Typical Seawater Intrusion Areas Using Hydrochemical and Environmental Stable Isotope Technique: A Case Study for Shandong Province, China

Seawater intrusion has a serious impact on industry, agriculture, and people ’ s daily life. Thus, the present study was designed to elucidate the pollution sources and groundwater evolution in typical intrusion areas of Shandong Province by hydrochemistry and environmental isotope techniques. The water samples were collected to analyze the groundwater evolution under di ﬀ erent intrusion, and groundwater evolution in the south of Laizhou Bay from 2005 to 2019. The ﬁ ndings indicated that the groundwater level dropping funnel caused by overexploitation was the direct causation of seawater intrusion in the three typical intruded areas. The groundwater evolution paths demonstrated that the groundwater in the south of Laizhou Bay had the fastest evolution rate and the highest degree of evolution, followed by the Dagu River Basin. The groundwater evolution extent and ﬁ tting of mixing lines indicated that the groundwater in the south of Laizhou Bay, Longkou, and Dagu River Basin was dominated by palaeosaltwater intrusion, modern seawater intrusion, and sea-saltwater mixed intrusion, respectively. Palaeosaltwater mixing produces a more severe salinization e ﬀ ect compared to seawater mixing. Meanwhile, the isotopes are gradually enriched with the deepening of intrusion, while the decrease of isotopes is delayed compared with the saltwater retreat. This is caused by that the stable isotopes enriched in the aquiclude due to the chemical permeation e ﬀ ect will be released into the aquifer after the salinity attenuates in the aquifer. The palaeosaltwater intrusion caused by anthropogenic activities has promoted serious ﬂ uorine pollution in the south of Laizhou Bay, while the groundwater nitrate pollution in Longkou was the most serious, followed by the Dagu River Basin due to high-density agricultural and domestic activity.


Introduction
About 40% population of the world live within 100 km in coastal zones [1], and freshwater stored in coastal aquifers has become a vital resource for sustaining the economy of coastal communities around the world [2]. However, with the increase in water demands, groundwater pollution caused by seawater intrusion [3][4][5][6][7], waste disposal of urban residents, and industrial and agricultural production [8][9][10] have caused severe threats to freshwater resource. Seawater intrusion is widely regarded as a common environmental problem affecting groundwater in coastal areas [11][12][13]. With 1% mixing of seawater would render the groundwater nonpotable and further induce soil salinization, ecological degradation, and decline in the quality of agricultural and industrial products [13,14]. In addition, seawater intrusion changes the pore medium by changing the cohesion between particles and the microbial community in the aquifer, which affects the flow of groundwater in different natural conditions [15][16][17][18]. Consequently, it is important to identify the types and evolution of seawater intrusion for proper management actions.
Normally, generalized seawater intrusion includes modern seawater intrusion and palaeosaltwater intrusion [3,19]. Modern seawater intrusion is mainly caused by the reversal of the hydrodynamic direction between seawater and freshwater in the coastal aquifer due to the changes of natural or human factors. Naturally, seawater intrudes into the freshwater aquifer under the effect of hydraulic gradient and density difference. This type of intrusion mainly occurs in the groundwater system where the seawater is coexisting [13], while palaeosaltwater originates ancient seawater intrusion, which is stored in aquifer after evaporation. In contrast, saltwater intrusion is mainly due to groundwater overexploitation and more susceptible to ecological degradation [20,21].
Laizhou Bay, Longkou, and Dagu River Basin are the typical areas prone to seawater intrusion in Shandong Province [7,22,23]. The types of intrusion include modern seawater, palaeosaltwater, and sea-saltwater mixed intrusions. The environmental problems in the study areas are mainly groundwater problems and a series of problems that it induced. The groundwater in the coastal area was salinized under the influence of the saltwater intrusion, thereby resulting in the deterioration of groundwater quality and the reduction of fresh groundwater resources. This phenomenon has led to environmental problems (such as soil salinization, vegetation degradation, and ecological deterioration) and social problems (such as the disposal of wells, the decline in the quality of industrial and agricultural products, and poor health of residents) [7,[24][25][26][27].
Previous studies on seawater/saltwater intrusion in Shandong Province mainly focused on the hydrochemical characteristics of groundwater, influencing factors of seawater intrusion, and characteristics and distribution of seawater intrusion [5,7,19,[28][29][30]. However, comprehensive approaches to type determination of seawater intrusion and comparative analysis to its influence on the ecological environment such as fluorine enrichment are few in coastal aquifer research. Based on previous studies, this work com-prehensively studied the current status of seawater intrusion in typical intruded areas in Shandong. The research objectives mainly focus on the following aspects: (1) identification of types of seawater/saltwater intrusion on the south of Laizhou Bay, Longkou, and Dagu River Basins based on the physicochemical indicators and isotopic composition of groundwater; (2) carry out a comparative analysis of characteristics of groundwater evolution under different intrusion conditions and its implication on the environment. For these purposes, data from 1,031 groundwater observation wells were collected from 2005 to 2019 to establish the evolution of groundwater in the seawater/saltwater intrusion, construct a determination system for different intrusion types, and comprehensively analyze saltwater intrusion and its assessment on the ecological environment in Shandong in recent years. The results are expected to provide deep insights into the invasion mechanism under different types and have significant implications for the management of groundwater exploitation in coastal areas.

Location and Climatic
Conditions. Shandong Province is located on the coast of eastern China, with a land area of approximately 150,000 km 2 and a coastline of length 3,024 km ( Figure 1). The south of Laizhou Bay is located on the northern coastal area of the Shandong Peninsula, with a land area of 2,600 km 2 and a coastline of 43 km. The study area is bounded between 36°38′-37°20′N and 118°40′-119°50′E. The slope of the terrain is gentle and gradually flattens from the south (land) to the north (sea). Longkou is located in the northeast of Shandong Province and the northwest of the Jiaodong Peninsula. The topography of the region is high in the southeast and low in the northwest, thereby showing a stepwise decline. The study area is bounded between 37°31′-37°47′N and 120°17 ′ -120°40 ′ E. The Dagu River Basin is located on the west of Jiaodong Peninsula in Shandong Province, which is bounded between 36°10 ′ -37°12 ′ N and 120°03 ′ -120°25 ′ E, with a drainage area of 4,631.3 km 2 and a total river length of 179.9 km. The northern part of the basin is mountainous, and the southern portion is plain. The terrain is high in the north and low in the south, and the slope of the terrain gradually decreases from north to south. The study area ranges from 36°12 ′ N to 36°26 ′ N and 120°05 ′ E to 120°18 ′ E. These areas belong to semihumid monsoon/warm monsoon climate with the average annual temperature of 11.9−12.5°C. The average temperature is the highest in July with 25.3−40.4°C, and the minimum temperature is -19.5−-4°C in January. The average annual precipitation is 600-656 mm, which is concentrated from June to September [7,31].

Hydrogeological
Conditions. The south of Laizhou Bay gradually changes from alluvial-diluvial deposits to marine sediments from south to north in the coastal plain. The bedrock is distributed in the mountainous area in the south of the study area. The aquifer is composed of Quaternary sediments [19,32]. The upper alluvial fan in the south is mainly 2 Lithosphere composed of gravel and coarse sandstone and gradually evolved into fine sand, silty sand, sandy clay, and silty clay toward the northern seashore. The aquifers in Longkou are alluvial and diluvial layers formed in Quaternary, which is composed of medium-fine sand-coarse sand and pebbles. The marine deposits near the coastline overlying alluvial deposits and lagoon sediments are composed of fine-coarse sand and fine gravel with good permeability. The marine surface sediments are also mainly silty sand and fine-coarse sand. Accordingly, a good hydraulic connection exists between the seawater and the groundwater in the coastal zone. Ancient river channels exist on land, and they are constructed by coarse constituent materials. Therefore, high permeability makes the ancient river channels a favorable way for seawater intrusion, and it also extends the distance of seawater intrusion [33].
In Dagu River Basin, the Quaternary sediments are dominated by late Pleistocene and Holocene alluvial-alluvial deposits, with a little of marine or continental alternating silt deposits on the southeast edge of the study area. The ancient valley of the Dagu River is an alluvial-diluvial double-layer structure. The upper layer is mainly composed of silty sand, mixed with silty clay and clay. The lower layer is composed of sand and gravel with a large thickness. The grain size of sand and gravel gradually becomes thinner from north to south and becomes thicker from shallow to deep. And the sand layer in the Quaternary is the main aquifer in the study area.

Sample Collection.
The data of water samples are shown in Table 1. All groundwater samples were taken from local domestic supply and monitoring wells. The brine of Laizhou Bay was taken from wells of chemical and aquaculture plants. These wells were continuously pumped. The surface water was from a local river and reservoir, and seawater was local coastal seawater or estuarine seawater. The detailed data are shown in Figure 1 and Table S1 to S8 in Supplementary Data. 3 Lithosphere was sealed with tape to avoid air contact. Thereafter, the collected samples were stored in refrigerators with a temperature below 4°C until analyzed.
The total dissolved solids (TDS) were analyzed on-site with a portable multiparameter water quality analyzer (YSI ProPlus, American YSI). HCO − 3 was tested by titration method with phenolphthalein and standard HCl solutions. The major anion concentrations (F − , Cl − , Br − , SO 2− 4 , and NO − 3 ) were analyzed with an ion chromatograph (ICS-3000, American DIONEX). The cation concentrations (K + , Ca 2+ , Na + , and Mg 2+ ) were measured with an inductively coupled plasma-atomic emission spectrometer (ICP-MS, American Thermo Fisher). Before the cation was tested, the samples were acidified to pH ≈ 2 with HNO 3 . The charge balance error in all sample analyses was less than 8%.
The stable isotopic compositions ( 18 O and 2 H) were analyzed with an LGR liquid water isotope laser spectroscope in Nanjing Hydraulic Research Institute. The values of δ 18 O and δ 2 H were calculated based on the Vienna Standard Mean Ocean Water (V-SMOW). The analytical precision values of the long-term standard measurement of δ 18 O and δ 2 H were ±0.2‰ and ±0.6‰, respectively.
where δ 18  All groundwater samples were classified into four types based on Cl − concentration: freshwater (<250 mg/L), brackish water (250-1,000 mg/L), saline water (1,000-19,400 mg/L), and brine (>19,400 mg/L). (1) Hydrochemical Compositions. The dominant hydrochemical type of groundwater in the study area can be found in Table 2. The water samples are linearly scattered on both sides of the mixing line (Figures 2 and 3). And the mixing lines are distributed closely parallel to the seawater Br − /Cl − ratio line ( Figure 3). The properties of Br − are stable, and most Br − in nature is stored in the ocean; thus, Br − can be used to infer the source of salt in coastal groundwater [23].

Results and Discussions
The Gibbs diagram usually is used to indicate the dominant geochemical effects in groundwater evolution [14]. The TDS in water samples gradually increases with an increase of evaporation degree ( Figure 4). The degree of evaporation of the brackish water in the study areas is as follows: Laizhou Bay > Dagu River Basin > Longkou. The evaporation level of brine in Laizhou Bay is higher than that of seawater. The mixing line of fresh groundwater and brine coincides with that of fresh groundwater and seawater. The results indicated that brine originated from seawater. The comparison between Figures 4(a) and 4(b) indicated that the increase of Ca 2+ concentration in brackish water in Longkou is greater than that of Na + with increasing salinity, while the increase of Ca 2+ concentration in brackish water is more suppressed compared with that of Na + with increasing salinity in the Laizhou Bay and Dagu River Basin, especially for Laizhou Bay. This phenomenon in Longkou is caused by cation exchange, while these phenomena in Laizhou Bay and Dagu River Basin were caused by the palaeosaltwater intrusion. Ca 2+ gradually precipitates out of the water with the evaporation and concentration, and the Ca 2+ /Na + ratio continuously decreases.  Table 3 after statistical analysis.
The δ 2 H and δ 18 O of the groundwater and surface water indicated that the δ 2 H and δ 18 Figure 5(b)). Therefore, it can be concluded that mod-ern precipitation promotes the renewal of groundwater, and rivers are the source of the lateral recharge of groundwater.
The mixing lines are distributed closely parallel to the seawater Br − /Cl − ratio line, and the water samples are linearly scattered on both sides of the mixing line ( Figure 3), which indicates that the salt in groundwater comes from the saltwater intrusion. However, the deviation of the water sample from the mixing line in Laizhou Bay, Longkou, and Dagu River Basin is different (the deviation of Longkou > that of Dagu River Basin > that of Laizhou Bay), which indicated that the groundwater mixing sources (brine, seawater, or saltwater), the mixing level, and geochemical evolution were different. The Gibbs diagram indicates that rock dissolution contributed to the salt in fresh groundwater, and saline water and brine in Laizhou Bay are dominated by evaporation [36], which is different from that of Longkou and Dagu River Basin.

Evolutionary II: Mixing Process
(1) δ 18 O and Cl − . These water samples are not strictly distributed along the mixing line of two different water bodies due to the diversity of mixed water and geochemical effects. The properties of 18 O and Cl − are stable, and 18 O in groundwater is closely related to the mixing line of originated waters. Therefore, the deviation of water samples from the mixing lines can be used to determine the dominant source in the mixing process.
Most groundwater samples are scattered on the freshwater member ( Figure 5(b)). The brackish water samples in 2005 and 2018-2019 are closely scattered to the mixing line of freshwater and seawater, with the seawater mixing proportion ranges of 0%-20% and 0%-5%, respectively. The saline water samples in 2005 and 2018-2019 are distributed near the mixing line of freshwater and brine, with the brine mixing proportion ranges of 5%-15% and 10%-40%, respectively. Brine (palaeosaltwater) mixing produces a more severe salinization effect compared with seawater mixing; thus, it can be used to determine the type of intrusion. Most brackish water fits a freshwater-seawater mixing trajectory, thereby suggesting classic seawater intrusion. The saline water fits a freshwater-brine mixing trajectory, thereby suggesting brine (palaeosaltwater) intrusion. The δ 2 H and δ 18 O values of brine in Laizhou Bay (2018-2019) are greater than those of Laizhou Bay (2005). In combination with long-term fresh groundwater and brine mining history, the results indicated that brine was diluted with seawater intrusion.
The brackish water samples are close to the mixing line of freshwater-local seawater, and the mixed level is less than 5%. The results indicate that the brackish water in Longkou is salinized by modern seawater intrusion. Some water samples in Dagu River Basin are scattered near the mixing line of freshwater-local saltwater, which is shown in the red hollow circle. These samples are collected from the east bank of the Dagu River Basin and the north side of the cutoff wall, where residual saltwater exists [23]. The results indicate that the brackish water in these two areas is salinized by residual saltwater intrusion. The other groundwater collected from the south of the Dagu River Basin is closely distributed to the mixing line of freshwater-seawater, which is affected by seawater intrusion [23]. The mixed level of brackish water is in the range of 0%-15%, and that of saline water is 30%-50%. The results further verified that groundwater in the south of the Dagu River Basin is salinized by seawater intrusion. The river samples are distributed between two mixed lines (freshwater-local saltwater and freshwater-seawater), thereby indicating that rivers play an important role in groundwater salinization.
(2) Hydrochemical Facies Evolution. It can be found from Table 2 that the hydrochemical type of freshwater is Ca-Na-HCO 3 -Cl or Na-HCO 3 -Cl in Laizhou Bay in 2005. The brackish is Na-Cl-HCO 3 . The saline water and brine are Na-Cl. The complete groundwater evolution path is Ca-  6 Lithosphere The hydrochemical type of freshwater in Laizhou Bay in 2018-2019 is Ca(Na)-HCO 3 or Na-Mg-HCO 3 -Cl. The brackish is Mg-Na-Cl-HCO 3 or Na-Cl-SO 4 . The saline water and brine are Na-Cl. The complete groundwater evolution path is Ca- The evolution path in 2018-2019 is shorter, and the evolution speed is faster than that in 2005. The Na + enrichment rate is higher compared with Cl − . The hydrochemical type of fresh groundwater in Longkou is Ca-HCO 3 -Cl, and that of brackish is Ca-Cl-HCO 3 or Ca-Na-Cl-HCO 3 . The complete groundwater evolution path is Ca-HCO 3 ⟶ Ca-HCO 3 -Cl ⟶ Ca-Cl-HCO 3 ⟶ Ca-Na-Cl-HCO 3 ⟶ (Ca-Na-Cl) ⟶ (Na-Cl). The current terminal evolution stage is Ca-Na-Cl-HCO 3 , and the Cl − enrichment rate is higher compared with that of Na + .
The hydrochemical type of fresh groundwater in the Dagu River Basin is Ca(Na)-HCO 3 or Na-Ca-HCO 3 -Cl. The brackish and saline water types are Na-Cl-HCO 3 (SO 4 ) and Na-Cl, respectively. The groundwater evolution path is Ca-Na-HCO 3 ⟶ Na-Ca-HCO 3 -Cl ⟶ Na-HCO 3 -Cl ⟶ Na-Cl-HCO 3 ⟶ Na-Cl. During salinization, the increased rate of Na + and Cl − in groundwater determines the evolution speed and path, while the type of mixed water and the degree of mixing determine the enrichment rate of Na + and Cl − . The salinization efficiency of brine (palaeosaltwater) mixing in Laizhou Bay is greater than that of sea-saltwater mixing in the Dagu River Basin, and the salinization efficiency of seawater intrusion in Longkou is the lowest one according to the δ 18 O and Cl − mixing process and Piper diagram ( Figure 6). Accordingly, the groundwater in Laizhou Bay had a higher degree of mixing and faster speed of evolution in 2018-2019 compared with those in 2005. The groundwater evolution in the Dagu River had a high starting level, and the evolution speed is second only to the groundwater in Laizhou Bay. The saltwater in Longkou had the lowest degree of mixing.

Evolutionary III: Water-Rock Interaction
(1) Ion Exchange. In a conservative mixing process of different water, the hydrochemical compositions of mixed water will increase strictly along the mixing line in a fixed ratio with the increase of the mixing degree. However, most mixed water in nature would deviate from the mixing line to varying degrees. The dominant effect contributing to   (Na + +K + )/(Na + /K + +Ca 2+ ) 3 Cl -/(Cl -+HCO -)   (Figure 7(b)). No significant difference exists in the degree of Mg 2+ -Na + exchange in the remaining groundwater samples. Compared with the ion exchange between Ca 2+ -Na + and Mg 2+ -Na + , the degree of Ca 2+ -Na + exchange in groundwater is significantly higher than that of Mg 2+ -Na + . The degree of ion exchange in the freshwater and brackish water is greater than that of saline water, which indicates that salinity has no significant positive correlation with the ion exchange in groundwater.
(2) Enrichment Effect. Ion enrichment is also one of the dominant effects that cause the water samples to deviate from the mixing line, for example, F − enrichment in groundwater.
The fluoride-rich groundwater [37] can be caused by anthropogenic activities. However, the fluoride-rich of groundwater in most areas is due to natural conditions, for example, the dissolution of fluoride-rich minerals [38]. CaF 2 is considered to be the dominant source of fluorine in groundwater. The main mechanism of fluorine enrichment in groundwater is as follow: The solubility of NaF (41,700 mg/L) is much greater than that of CaF 2 (15 mg/L), and F − is easier to bind with Na +compared with Ca 2+ and Mg 2+ . The increase of ðNa + + K + Þ/ Ca 2+ in groundwater will promote the release and enrichment of F − in the surrounding rocks [39]. Meanwhile, the increase of HCO − 3 content in groundwater will also play the same role as ðNa + + K + Þ/Ca 2+ [14]. Saltwater intrusion has a positive effect on the release of F − in groundwater in coastal areas by increasing the ðNa + + K + Þ/Ca 2+ value in groundwater [40].   N = 142), respectively, while the value of HCO − 3 concentration is much larger than ðNa + + K + Þ/Ca 2+ value, which indicates that the enrichment efficiency of HCO − 3 for F − is significantly higher than that of ðNa + + K + Þ/Ca 2+ . There is no significant difference in groundwater HCO − 3 content in the three study areas, while values of ðNa + + K + Þ/Ca 2+ increase with the increase of salinity, and the values of ðNa + + K + Þ/Ca 2+ in groundwater are as follows: Laizhou Bay > Dagu River Basin > Longkou. It can be concluded that the high fluorine in groundwater originates from the dissolution of fluorinecontaining minerals caused by saltwater intrusion. And the palaeosaltwater intrusion in Laizhou Bay has the most significant promotion effect on fluorine enrichment, followed by the sea-saltwater mixed intrusion in the Dagu River, and the last is the seawater intrusion in Longkou.

Types of Intrusion in the Study Areas.
Most groundwater in coastal areas of Shandong has been intruded to varying degrees ( Figure 8). The main intrusion types include modern seawater, palaeosaltwater, and sea-saltwater mixed intrusion. The groundwater in Laizhou Bay has the fastest evolution speed and the highest extent of evolution in the three study areas. The saline water samples of Laizhou Bay in 2018-2019 are scattered near the mixing line of freshwater and brine, with the mixing proportion ranges from 10% to 40%, which indicated that the groundwater in Laizhou Bay is affected by the palaeosaltwater intrusion. The groundwater samples in Longkou are close to the mixing line of freshwater and seawater, and the mixed level is less than 5%. This finding indicated that the brackish water in Longkou is salinized by modern seawater intrusion. The speed and extent of groundwater evolution in the Dagu River Basin are between Laizhou Bay and Longkou. And some water samples are scattered near the mixing line of freshwater and saltwater, and the others are closely distributed to the mixing line of freshwater and seawater. This finding verified that  10 Lithosphere groundwater in the Dagu River Basin is dominated by seasaltwater mixed intrusion. According to the regional survey of Shandong in 2017, the distribution of salt groundwater is approximate 23,000 km 2 (Figure 9). Palaeosaltwater intrusion is mainly distributed in the south and west of Laizhou Bay, accounting for about 22,000 km 2 . Seawater intrusion is distributed along the coastline of Shandong accounting for 700 km 2 . And sea-saltwater mixed intrusion distributed in the east of Laizhou Bay and Dagu River Basin accounts for 300 km 2 .

Lithosphere
With regarding the scope of the intrusion, the area of palaeosaltwater intrusion in the Laizhou Bay area is the largest; the intrusion distance to inland is the longest. The distance of sea-saltwater mixed intrusion to inland in the Dagu River Basin is larger than that of Longkou.
Three large groundwater level dropping funnels are in Changyi City, Shouguang City, and the coastal area on the south of Laizhou Bay, respectively (Figure 9(a)). The dropping funnels in Changyi and Shouguang are fresh groundwater level dropping funnels, while those in the coastal area are brine dropping funnels, which are distributed in a stripe shape along the east-west direction. From the sea to the land, brine, saline water, brackish water, and freshwater are distributed in sequence. Saline water and brackish water are evolution products of brine intruding into fresh groundwater. The width of the saline groundwater zone near the groundwater level dropping funnel in Changyi City is larger. This finding indicates that the decline of the groundwater level is the main factor of palaeosaltwater intrusion. The scope of intrusion increases from 2009 to 2013 and then decreases in the south of Laizhou Bay.
Two large groundwater level dropping funnels are in the northern coastal area of Longkou city (Figure 9(b)). The scope of seawater intrusion is near the dropping funnel, thereby indicating that the main factor of seawater intrusion in this area is the overexploitation of groundwater. From 2005 to 2018, the scope of intrusion has increased, and the increased area is mainly distributed between the two dropping funnels. However, the cutoff wall has decreased the seawater intrusion distance in the north-south direction but increased in the east-west direction.
The scope of intrusion in the Dagu River Basin has decreased from 2009 to 2018 (Figure 9(c)). The intrusion in 2009 and 2018 shows that the intrusion distance on the north of Jiaozhou Bay is greater than that on the east and west of Jiaozhou Bay. However, the cutoff wall has hindered sea-saltwater mixed intrusion towards the north, but the intrusion scope on the east side of the cutoff wall gradually   Table S7 in Supplementary Data). 12 Lithosphere increases. It can be concluded that the cutoff wall hinders the longitudinal intrusion and causes the horizontal expansion of intrusion along the cutoff wall. Moreover, dense villages and farming areas are distributed on the north side; thus, the long-term overexploitation of groundwater causes the sea-saltwater mixed intrusion.

Evolution of Groundwater in Laizhou
Bay. Three largescale groundwater level dropping funnels (one palaeosaltwater level dropping funnel and two freshwater level dropping funnels) are distributed in a stripe shape along the east-west direction on the south of Laizhou Bay. The coastal palaeosaltwater has intruded the fresh groundwater due to the groundwater level dropping funnel, and the degree of intrusion gradually decreases from sea to inland. Therefore, brine, saline, brackish groundwater, and fresh groundwater are distributed in a strip from the sea to the inland. The width of the brine gradually decreased from west to east, while the width of the saline and brackish water gradually increased from west to east. In the Changyi area, the boundary between saline water and brackish water is blurred, and the width of the saline water is large. The intrusion line is consistent with the east-west centerline of the dropping funnel. The palaeosaltwater intrusion cannot cross the center of the groundwater level dropping funnel. Thus, the groundwater level dropping funnel hinders the further development of the intrusion even though it induces palaeosaltwater intrusion.
We  Figure 12(a), the major ion in brackish and saline water in Laizhou Bay remains stable, while Figure 12(b) shows that the ratio of Br/Cl increases from 2006 to 2010 and then decreases, which is consistent with the intrusion characteristics in Laizhou Bay. It can be concluded that the hydrogen and oxygen isotopes in the mixed water enrich gradually when the proportion of brine mixed into freshwater increases, while the decrease of the stable isotopes in the mixed water is delayed compared with the regression of palaeosaltwater intrusion.
The borehole data from Laizhou Bay in 2020 (Table S8 and Figure S9) shows that there is a porewater hydrochemical mutation interface at 40 m depth; the value of the stable isotopes and Br − in the groundwater buried within 40 m is higher than that of the groundwater buried between 40 and 80 m. These results indicate that palaeosaltwater intrusion mainly occurs in aquifers buried within 40 m, while aquifers deeper than 40 m are slightly affected by palaeosaltwater intrusion and anthropogenic activities. In the groundwater system where palaeosaltwater intrusion occurs, the stable isotopes in the aquiclude are more enriched compared with the aquifer, and the salinity ratio of aquiclude to the aquifer is lower than the stable isotopes. Combined with the attenuation hysteresis of the stable isotopes in mixed water, it can be proved that the chemical osmosis [41][42][43] and the electric double layer of clay film allow the stable isotope exchange and enrichment in the aquiclude compared to salinity during the seawater intrusion. The stable isotopes in the aquiclude are released and attenuate after salinity in the aquifer attenuate during the saltwater retreat.
3.4. Anthropogenic Influences on the Local Environment 3.4.1. Fluorine Pollution. Fluoride is highly toxic to living organisms. When the concentration exceeds 25 ppm, fluoride irritates the eyes and respiratory system and harms the liver and kidneys. When the concentration of fluorine reaches 100 ppm, the eyes and nose will be seriously damaged. If the concentration reaches 1000 ppm, then breathing for a few minutes can be fatal. Organic fluorine is biopersistent and can stay in nature for a long time and enrich in the organism.
F − pollution in Laizhou Bay is more serious than that in the Dagu River Basin (Figure 8). The F − concentration in almost all groundwater samples of Longkou is far below safety standards. Approximately 5.13% of the total groundwater samples in the Dagu River Basin exceeded the safety standard (C F < 1 mg/L) set by the National Sanitary Standard for drinking water in China [44]. 28 of the 63 groundwater samples in Laizhou Bay in 2013 exceed the safety standard, accounting for 44.44% of the total water samples. The proportion of groundwater samples exceeding the safety standard in 2014, 2018, and 2019 is 35%, 70%, and 54.84%, respectively. The F − pollution in 2018 is most serious, followed by that of 2019. And the pollution degree in saline water and brine is higher than that of freshwater and brackish water.

Nitrate Pollution.
Nitrate is a common pollutant in groundwater. The nitrate sources are mainly industrial, agricultural, and domestic waste, which is closely related to anthropogenic activities. Nitrate will be reduced to toxic nitrite under certain conditions, and nitrite reacts with the human blood to form methemoglobin. Accordingly, the blood loses its oxygen-carrying capacity, thereby causing health problems.
The safety standard for nitrate in drinking water is less than 20 mg/L, which is set by the Standardization Administration of China (GB/T 14848-2017) [45].  14 Lithosphere Therefore, the nitrate pollution in groundwater in the Longkou area is the most serious, followed by the Dagu River Basin. Nitrate pollution in groundwater in the Laizhou Bay is severe though it has been decreasing from 2005 to 2019. Groundwater nitrate pollution is serious in freshwater and brackish water distribution areas ( Table 4). The afore-mentioned areas are agricultural farming or population gathering areas. Accordingly, the chemical fertilizers and pesticides, industrial waste, and domestic waste are the main causes of nitrate pollution in coastal groundwater of Shandong. In the south of Laizhou Bay, nitrate pollution in saline water and brine samples were serious in 2005.  However, pollution in 2018-2019 has been alleviated. Many saltworks and chemical plants are distributed in the brine area in the south of Laizhou Bay. The sampling wells are generally close to the human activity area. Domestic waste enters the saline groundwater or brine through the leaching infiltration or poor water stop wells, thereby causing nitrate pollution.

Conclusions
The palaeosaltwater mixing would produce a more severe salinization impact compared with seawater mixing; thus it can be used to determine the type of intrusion. The groundwater evolution speed and extent in Laizhou Bay are the highest in the three study areas, the groundwater samples are scattered close to the mixing line of brine and freshwater, and the brine as the initial groundwater distributed in a stripe shape on the south of Laizhou Bay. These findings confirm that the groundwater in Laizhou Bay is dominated by palaeosaltwater intrusion. The groundwater evolution speed and extent in Longkou are the lowest, and the groundwater samples are close to the mixing line of freshwater and local seawater. These findings show that the brackish water in Longkou is salinized by modern seawater intrusion. Meanwhile, the brackish water in Dagu River Basin is between Laizhou Bay and Longkou. Some groundwater samples are scattered near the mixing line of freshwater and local saltwater. And the others are closely distributed to the mixing line of freshwater and local seawater, thereby verifying that the groundwater in Dagu River Basin is dominated by the sea-saltwater mixed intrusion.
The groundwater samples of the three typical intrusion areas are in different stages of evolution and present varying evolutionary trends due to diverse intrusion. The groundwater evolution path of Laizhou Bay in 2005 is Ca-HCO 3 ⟶ Ca-HCO 3 -Cl ⟶ Ca-Na-HCO 3 -Cl ⟶ Na-HCO 3 -Cl ⟶ Na-Cl-HCO 3 ⟶ Na-Cl. The groundwater evolution path is Ca-HCO 3 ⟶ Na(Mg)-HCO 3 -Cl ⟶ Na(Mg)-Cl-HCO 3 ⟶ Na-Cl-SO 4 ⟶ Na-Cl in 2019. The evolution path of groundwater in the Longkou area is Ca-HCO 3 ⟶ Ca-HCO 3 -Cl ⟶ Ca-Cl-HCO 3 ⟶ Ca-Na-Cl-HCO 3 ⟶ (Ca-Na-Cl) ⟶ (Na-Cl). The evolution path of groundwater in the Dagu River Basin is Ca-Na-HCO 3 ⟶ Na-Ca-HCO 3 -Cl ⟶ Na-HCO 3 -Cl ⟶ Na-Cl-HCO 3 ⟶ Na-Cl. The groundwater in Laizhou Bay has the fastest evolution rate and the highest degree of evolution, followed by that of the Dagu River Basin. By contrast, the evolution starting point of groundwater in the Dagu River Basin is high. In Laizhou Bay, the extent and scope of palaeosaltwater intrusion gradually increased from 2005 to 2010 and then slowly eased from 2011 to 2019. The palaeosaltwater intrusion was small in 2005 and large in 2010. Correspondingly, the isotopes are gradually enriched as the intrusion increases, while the decrease of that is delayed compared with the saltwater retreat, which is caused by that the stable isotopes enriched in the aquiclude due to chemical osmosis will be released and attenuate after salinity in the aquifer attenuate during the saltwater retreat.
Anthropogenic activity is the main cause of saltwater intrusion and environmental pollution in the coastal area of Shandong. The F − pollution in groundwater has been attributed to saltwater intrusion. Among the three typical intruded areas, the groundwater F − pollution in the south of Laizhou Bay is the most serious due to the palaeosaltwater intrusion. The F − concentration in most groundwater in the Longkou and Dagu River Basin is below the safety standard. The F − concentration in saline water and brine is higher than those of freshwater and brackish water. Under the influence of anthropogenic activities, the nitrate concentration in groundwater in the Longkou area seriously exceeded the safety standards, followed by the Dagu River Basin. Although the groundwater nitrate pollution in the Laizhou Bay area is gradually mitigating, the pollution situation is still severe.

Data Availability
The data are listed in the Supplementary Materials.

Conflicts of Interest
The authors declare that they have no conflicts of interest.