The Study on the Genesis of Underground Brine in Laizhou Bay Based on Hydrochemical Data

Abstract

The Laizhou Bay area contains a large amount of Quaternary brine resources, which have been gradually depleted by long-term high-intensity mining. The local brine genesis is still controversial, and the dominant theory of the evaporation of brine formation limits the exploration of brine resources to the land area, while the confirmation of freezing brine formation would greatly expand the brine storage range. In this research paper, the genesis of underground brines was analyzed based on the major ion concentrations of 62 samples of underground brines at different depths at 24 stations along the south coast of Laizhou Bay. The results show that the underground brines originated from seawater; however, their chemical components were changed during the evolution process after formation. The hydrogeochemical modeling results show that the mixing of seawater and fresh groundwater also affects the chemical composition of brines. The large-scale exploitation of brine resources in recent decades has also led to significant changes in the chemical composition of the underground brine in Laizhou Bay compared with the earlier period. The special geographic environment and the development of the brine chemical industry have led to the evolution of underground brines in Laizhou Bay being affected by a variety of factors, which makes the traditional analysis of brine genesis in this region not applicable. Furthermore, although evaporation and concentration are the dominant factors in the formation of brine, there is still a lack of evidence to rule out the existence of the freezing pathway.

1. Introduction

Most researchers believe that subsurface brines in coastal areas mainly originate from seawater concentration, with freezing and evaporation being the two main pathways [1]. Evaporation is one of the earliest mechanisms of brine generation agreed upon by academics; however, the formation of brine is not limited to evaporation. Brine can also be generated by freezing seawater in arid and freezing environments, and it is commonly believed that water is detached as ice in polar regions to concentrate seawater. However, during geological history, freezing to brine was much more widespread and could occur in most of the Northern Hemisphere’s high latitudes [2,3,4]. Significant differences have been found between the products of the two seawater concentration pathways, evaporation and freezing [5,6], and many scholars have attempted to establish criteria for distinguishing between brines formed by different seawater concentration pathways, among which the characteristic ionic curves of seawater freezing versus evaporation [6] have been widely used.

In most cases, subsurface brines in coastal areas are produced by mixing processes, with mixing including fresh groundwater, seawater, and atmospheric precipitation [7]. The composition of brines can also be altered by reacting with minerals in the surrounding rocks, such as dolomitization, sulfate reduction, plagioclase sodium feldsparization, and potassium/aluminum silicate formation [8]. Whether the chemical composition of subsurface brines collected in the field mainly reflects their original state at the time of formation or is controlled by later water–rock and mixing interactions is still debated [9].

The south coast of Laizhou Bay in Shandong Province, China, contains a large amount of Quaternary brine resources, which have been gradually explored and utilized since the 1960s [10]. Research on the genesis of these Quaternary brines began in the 1980s. Han and Wu compared the chemical compositions of underground brines and modern seawater, combined them with paleogeographic and paleoenvironmental studies, and concluded that these Quaternary brines were formed by the concentration of seawater by evaporation [11]. Further studies by many scholars found that the brine was not the result of a simple evaporation and concentration of seawater, but that there were interactions with the surrounding rocks accompanied by ion exchange during the evolution process [12,13,14]. Wang et al. simulated the geochemical evolution of the subsurface brine in the area and agreed with this view [15].

Most scholars’ studies concluded that the subsurface brines in the Laizhou Bay area were formed by seawater evaporation [16,17,18,19], yet there is no conclusive evidence to deny the existence of freezing-induced brines. Han et al. concluded that the Quaternary Ice Age climate in the Laizhou Bay area was cold and that the seawater had conditions for freezing and concentration [20]. Hu et al. constructed a multilayer perceptron (MLP) seawater concentration pathway prediction model based on seawater freezing and evaporation experimental data, and the results of their study indicate the presence of freezing-induced subsurface brines in the Laizhou Bay area [7].

The gradual depletion of underground brine resources due to long-term high-intensity mining in the Laizhou Bay area has seriously threatened the sustainable development of the local economy. The study of seawater concentration pathways in Laizhou Bay, especially the existence of seawater freezing pathways, is of great significance for expanding local brine resource reserves and maintaining sustainable mining and utilization [7]. The purpose of this research paper is to focus on the influence of the mixing of different ratios of freshwater and seawater on the methods of determining the genesis of subsurface brines by using hydrogeochemical simulation methods based on the comparison of the hydrochemical characteristics and ionic ratios of subsurface brines, subsurface freshwater, and seawater.

2. Materials and Methods

2.1. Study Area

Laizhou Bay is located in the northern part of Shandong Province, China, and the southern part of the Bohai Sea; it is one of the three bays in the Bohai Sea (Figure 1). The study area is located on the coastal plain along the southern shore of Laizhou Bay, where alluvial, floodplain, and marine sediments are distributed sequentially in the horizontal direction from the south to the north [16], and the sediments gradually grade from gravels and coarse sands to fine sands, silty sands, and clays [21]. The thickness of the Quaternary sediments gradually increases from 30 to 50 m from the south to the north, with a maximum thickness of up to 300 m [22], and the interaction of the marine strata with the terrestrial sandy and muddy strata results in a complex multi-framework water-bearing system [19]. Since the Late Pleistocene, the region has experienced three large-scale transgressive/regressive phases, resulting in the formation of three subsurface brine layers [21]. The Holocene brine layer is a submerged/micro-submerged brine layer, with a burial depth of 0–14.8 m; the late and early Late Pleistocene brine layers are pressurized brine layers, with burial depths of 33.2–42.3 m and 58.6–74.1 m [16]. Underground freshwater flows into Laizhou Bay from the south to the north, and the recharge sources mainly include atmospheric precipitation, lateral and vertical infiltration of rivers, and irrigation return flow [16]. Since the 1980s, the increase in groundwater extraction in the study area has led to the formation of underground funnels in local areas, which has become an important factor affecting the dynamics of freshwater [16], leading to the occurrence of regional saltwater intrusion [22]. The overexploitation of underground brine has also led to the gradual movement of the underground brine layer to the deeper part of the stratum [23]. At the same time, tidal action and the density difference between brackish and freshwater contribute to the exchange of brine and freshwater [24]. The combined influence of many factors makes the hydrogeological system of this region more complex than those of other regions.

2.2. Sampling and Analysis

Underground brine samples were collected from June 2015 to March 2016 on the south coast of Laizhou Bay, including 62 samples of underground brine samples and 4 groups of freshwater samples at different depths in 24 wells (Figure 1); the depth of the wells and the layers of brine collection are shown in Figure 2. As the nearshore seawater in the study area has been affected by the local raw salt production and wastewater discharges from the chemical industry, the chemical composition of seawater has changed considerably, which makes it difficult to reflect the state of the brine at the time of brine formation. Therefore, the seawater samples in this paper followed the average water chemical composition of the early Bohai seawater, which was less affected [15].

We sampled brines in layers at 24 sites and sealed the top of each brine layer with a water-expandable material to prevent the exchange and leakage of water between the different layers. We used a pumpless drilling technique for the submerged brine layer, while the lower pressurized brine layer was drilled with saturated brine. The drill holes were all 110 mm in diameter, and the return footage was kept within 3.0 m to ensure effective groundwater stratification collection. Filtering and extraction were performed 3–4 times every hour on the groundwater samples from the target layers to ensure that information on the concentration of halide ions and other ions could be accurately extracted from the groundwater samples. Finally, the collected brines were sealed and sent to the laboratory for chemical analyses. These analyses were performed at the First Institute of Oceanography, Ministry of Natural Resources, and their concentrations of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, and Br⁻ were determined using ion chromatography (we pre-collected a large number of data on brines of a known cause to determine the type of indicator to be used).

2.3. Research Methods

Groundwater is a mixed solution with a complex chemical composition. Through detailed statistics and analysis of its composition, it is possible not only to determine the source of groundwater but also to understand the process of its formation [25]. During the evolution of groundwater, specific ionic components and ratios show certain regular changes. Ion ratio analysis is an effective method that can be used to resolve hydrogeochemical effects as well as groundwater genesis [26]. This analysis enables the identification of the main drivers influencing the changes in groundwater chemistry [27].

Experimentally obtaining brine samples and mixing them with seawater and freshwater in certain proportions is a challenging task that can be efficiently solved using forward geochemical modeling. In this research work, the mixing of brine samples of known genesis with freshwater and seawater from Laizhou Bay was simulated based on the mix module of the geochemical simulation software PHREEQC 3.7.3.

3. Results

3.1. Statistical Analysis

The statistics of hydrochemical indicators of underground brine, seawater, and freshwater in the study area are shown in

Table 1. The concentration of each ion in the sample.

Parameters	Average Value (mg/L)			Percentage of Meq (Brine)	Brine/ Seawater	Brine/ Freshwater
	Brine	Seawater	Freshwater
Na+	29,275	10,760	104.80	37.26%	2.72	279.34
K+	513	387	18.90	0.38%	1.33	27.16
Mg2+	4475	1294	37.14	10.77%	3.46	120.48
Ca2+	1044	413	67.26	1.52%	2.53	15.52
Cl−	55,214	19,353	105.10	45.57%	2.85	525.37
SO42−	7255	2712	97.89	4.42%	2.68	74.11
Br−	185	67	0.32	0.07%	2.76	581.41

. The concentration sequence of major ions in the brine was Cl⁻ > Na⁺ > SO₄²⁻ > Mg²⁺ > Ca²⁺ > K⁺ > Br⁻. Among them, Cl⁻ was the major anion with an average milligram equivalent percentage of 45.57% and concentrations ranging from 8226 mg/L to 102,135 mg/L (average concentration of 55,214 mg/L). Na⁺ was the major cation with a milligram equivalent percentage of 37.26% and concentrations ranging from 4346 mg/L to 54,940 mg/L (average concentration of 29,275 mg/L).

The concentration sequence of major ions in the Bohai Sea seawater was Cl⁻ > Na⁺ > SO₄²⁻ > Mg²⁺ > Ca²⁺ > K⁺ > Br⁻ [15]. Among them, Cl⁻ was the dominant anion with a concentration of 19,353 mg/L, and Na⁺ was the dominant cation with a concentration of 10,760 mg/L. The ratio of the average concentration of Cl⁻ between brine and seawater was 2.85, and the ratios of the average concentrations of Na⁺, Ca²⁺, SO₄²⁻, and Br- of the two were all close to the ratio of Cl⁻, whereas the ratios of the average concentrations of K⁺ of the two were relatively low (1.33), and the ratio of the average concentration of Mg²⁺ was relatively high (3.46). The concentrations of major ions in the freshwater of Laizhou Bay were as follows: Cl⁻ > Ca²⁺ > SO₄²⁻ > Na⁺ > Mg²⁺ > K⁺ > Br⁻. Among these, Cl⁻ was the major anion with a mean concentration of 105.10 mg/L, and Ca²⁺ was the major cation with a mean concentration of 67.26 mg/L. The ratios of the average concentrations of Cl⁻ and Br⁻ in brine to freshwater were close, with the ratios of 525.37 and 581.41, respectively, and the ratio of the average concentration of each of the other ions in the two ranged from 27.16 to 279.34.

3.2. Relationship between Major Ionic Ratios and Cl

Cl is considered to be the most conserved element in groundwater [28] and other ionic ratios are compared to Cl to facilitate the comparison of the chemistry of brines with other waters; this is also an important means of examining the role of water rocks [16,29]. The rNa/rCl, rMg/rCl, rSO₄/rCl, and rCa/rCl of the brines in the study area were distributed near the seawater line and far from the freshwater line. The rNa/rCl of seawater (0.8573) was smaller than that of freshwater (1.5376), and 82% of the samples were located below the seawater line, with a variation ranging from 0.6288 to 0.8813 (mean value 0.8154) (Figure 3a). The rMg/rCl of seawater (0.1950) was much smaller than that of freshwater (1.0307), and 98% of the brine samples were located above the seawater line and below the freshwater line, with variations ranging from 0.1791 to 0.3717 (mean value 0.2376) (Figure 3b). The rSO₄/rCl of seawater (0.1034) was smaller than that of freshwater (0.6874), and the brine samples were symmetrically distributed along the seawater line, with a range of 0.0270 to 0.2365 (mean value 0.1004) (Figure 3c). The rCa/rCl of seawater (0.0378) was much smaller than that of freshwater (1.1321), and 65% of the brine samples were located below the seawater line, with variations ranging from 0.0162 to 0.1562 (mean value 0.0367) (Figure 3d).

The rK/rBr and rK/rCl of most of the brine samples were smaller than that of seawater and also much smaller than that of freshwater. The rK/rCl for seawater and freshwater were 0.0181 and 0.1630, respectively, and the rK/rCl for all brine samples was located below the seawater line, with a total variation ranging from 0.0008 to 0.0146 (mean value 0.0081) (Figure 3e). The rK/rBr of seawater (11.8034) was much smaller than (121.6433), and 89% of the brines had rK/rBr located below the seawater line, with a range of variation from 0.3049 to 85.0087 (mean value 8.1432) (Figure 3f).

Compared to other ratios, rBr/rCl and rMg/rCa were higher for seawater than for freshwater. The rBr/rCl of the freshwater (0.0013) was close to that of the seawater (0.0015), and the brine samples as a whole were symmetrically distributed along these two lines, with variations ranging from 0.0001 to 0.0029 (with a mean value of 0.0015) (Figure 3g). The rMg/rCa of the seawater (5.1657) was much higher than that of the freshwater (0.9104), and 81% of the brines were located above the seawater line and below the freshwater line, with a range of variation from 1.4808 to 21.2686 (mean value 7.7966) (Figure 3h).

3.3. Na/Cl-Br/Cl and SO₄/Cl-Br/Cl

Na/Cl-Br/Cl and SO₄/Cl-Br/Cl are commonly used to track the evolution of marine brines [30,31]. During the concentration of seawater, the evaporation and freezing processes cause Na/Cl and SO₄/Cl in seawater to evolve along different paths in the direction of increasing Br/Cl (Figure 4). The Na/Cl-Br/Cl distribution of subsurface brine samples in the study area was below the seawater point and became left/right symmetric, and the distribution of samples close to the seawater point was concentrated and was at the angle of the two paths, which makes it difficult to differentiate them; some of the samples were distributed along the evaporation line, and there was no distribution of samples along the freezing line after the two paths were significantly separated (Figure 4a).

The distinction between SO₄/Cl and Br/Cl for the seawater concentration paths was more pronounced than for Na/Cl-Br/Cl. The brine samples were left/right symmetric along the seawater point, to the right of the seawater point, and most of the brine samples were distributed around the evaporation line, with four samples with low SO₄/Cl moving away from the evaporation line toward the freezing line, and one of these points was located on the freezing line (Figure 4b).

3.4. Na/Cl-SCF and Ca/Mg-SCF

In seawater evaporation and freezing concentration experiments, Na/Cl and Ca/Mg in seawater evolve along different trends with a change in seawater concentration factor (SCF). Therefore, Na/Cl-SCF and Ca/Mg-SCF can be used to determine the formation mechanism of brine [32]. The ratio of Cl⁻ concentration between the brine in the study area and the seawater in the Bohai Sea was chosen as the SCF because Cl⁻ is highly stable and also the most concentrated ion in the brine [32].

In the seawater concentration experiments, the experimental data of evaporation and freezing started to diverge at an SCF of about 3.5, the Na/Cl trends of the evaporation and freezing experimental data were roughly the same at an SCF < 3.5, and the evaporation data remained unchanged while the freezing data showed a significant decreasing trend at an SCF > 3.5. The Na/Cl ratios of brine samples in the study area were smaller relative to the experimental data, with more individuals located near the freezing region than the evaporation region (Figure 5a).

The experimental data Ca/Mg ratios for evaporation and freezing followed roughly the same trend at an SCF < 3.5. When SCF was > 3.5, the experimental data for evaporation and freezing began to diverge similarly to the Na/Cl ratios. However, unlike the Na/Cl ratios, the freezing data remained unchanged, while the evaporation data showed a significant downward trend. The Ca/Mg values of the brine samples in the study area were much more dispersed, with no sample points located in the freezing region (Figure 5b) or near the evaporation line.

3.5. Mixing Simulation

Since the 1980s, the high-speed development of the coastal area of Laizhou Bay and the over-exploitation of groundwater have led to the emergence of underground funnels in the strata of the study area, which accelerated the mixing between the fresh groundwater and brine in the south. However, in the northern part of the brine layer, seawater is also present in the brine layer and infiltrates the brine layer [23,24]. To investigate the influence of this mixing effect on the method of determining the seawater concentration pathway through the ionic ratio, brines obtained from seawater concentration experiments (data from evaporation experiments were obtained from [33]) and brines with known seawater concentration pathways (frozen brine data were obtained from [31], and evaporated brine data were obtained from [34,35]) were simulated by mixing them with seawater and freshwater from Laizhou Bay, respectively. Based on the minimum concentration of Cl⁻ in the brine samples in the study area (8226 mg/L), the maximum value of the dilution factor of seawater and freshwater was determined to be 30-fold, and the mixing dots of seawater and freshwater were substituted for the multiples of mixing from the smallest to largest, respectively, as follows: 1, 2, 3, 10, 20, and 30 (Figure 6 and Figure 7). The results of the mixing simulations show that the chemical composition of the brine from the freezing and evaporation experiments is oriented towards seawater (when mixed with seawater) and towards freshwater (when mixed with freshwater).

4. Discussion

4.1. Sources of Brine Chemical Components

The concentration sequence of the main ions in the underground brine in the study area is the same as that of the seawater in the Bohai Sea (Table 1); rNa/rCl, rK/rCl, rMg/rCl, rCa/rCl, rSO₄/rCl, and rBr/rCl are distributed in the vicinity of seawater, and there is a large difference from the underground freshwater, which indicates that the underground brine in the study area is closely connected with the seawater.

The rNa/rCl can be used to determine the genesis of groundwater and the intensity of metamorphism [36]. The rNa/rCl of the brine is closer to the Bohai Sea seawater and distant from the underground freshwater in the Laizhou Bay area, which indicates that it belongs to the derived water of seawater. Furthermore, 82% of the brine samples have smaller rNa/rCl than that of the Bohai Sea seawater, which indicates that the metamorphism of underground brine in the study area is not significant (Figure 3a).

Typically, rMg/rCa in fresh groundwater and seawater differ greatly; thus, rMg/rCa can be used to measure the extent and degree of seawater intrusion [37]. The rMg/rCa of the seawater in the Bohai Sea is much higher than that of the underground freshwater in the Laizhou Bay area, while the mean value of rMg/rCa of the underground brine (7.7966) is closer to that of seawater (5.1657), which suggests that the underground brine in the study area originates from seawater, but is not a simple concentration of seawater (Figure 3h).

The higher the degree of evaporation and concentration of seawater, the smaller the rK/rBr of the corresponding brine. In general, the rK/rBr of the sedimentary brine should be smaller than that of seawater. That which is higher than seawater is usually dissolved and filtered with potassium salt brine [36]. The rK/rBr of freshwater in Laizhou Bay is much higher than that of seawater. A total of 89% of the brine samples have rK/rBr below the seawater in the Bohai Sea, and only a few samples have rK/rBr close to that of the freshwater in Laizhou Bay. Additionally, the degree of concentration is not high, which is likely to be the result of the leaching of potash salts (Figure 3e).

After the formation of underground brine, it is endowed in the form of interstitial water within the aquifer, and processes such as interaction and material exchange occur when the liquid phase is in contact with the solid phase for a long period [38]. The brine layer in the study area is mainly composed of clay [21], and Ca²⁺ in clay minerals can exchange cations with K⁺, Na⁺, and Mg²⁺ in brine. For Na⁺ and K⁺ in brine, cation exchange results in a lower content of both, which is reflected by the fact that rNa/rCl and rK/rCl are lower than the seawater line for most of the brine samples (Figure 3a,b). In contrast, Mg²⁺ in brines may be related to dolomitization in the deep subsurface in addition to ion exchange with clays [14]. The vast majority of samples have rMg/rCl greater than the seawater, which indicates that the degree of input of dolomitization petrochemistry in the brine in the study area is higher than the degree of the cation exchange output for the Mg²⁺ content (Figure 3b). Moreover, the fact that the rCa/rCl of most of the brine samples is smaller than that of seawater also proves that there exists a stronger dolomitization petrochemistry between the brine and the surrounding rocks (Figure 3d).

4.2. Seawater Concentration Pathways

Seawater produces significantly different mineral sequences during evaporation and freezing, yielding brines with different chemical compositions [6]. Freezing concentration first precipitates mannite (Na₂SO₄·10H₂O) around an SCF = 3.5, the concentrations of Na and SO₄ decrease, and the higher Na/SO₄ of seawater induces a freezing effect on the decrease in the concentration of SO₄ significantly more than that of Na. On the other hand, evaporation concentration first precipitates gypsum (CaSO₄·2H₂O) around an SCF = 3.5, the concentration of Ca decreases, and the smaller Ca/SO₄ of seawater Ca/SO₄ minimizes the effect of evaporation on the increase in the SO₄ concentration. Then, rock salt starts to precipitate only around an SCF = 10 [5,6]. The phenomenon of differentiation in the ionic composition of brines formed by different seawater concentration pathways makes the characteristic ion ratio an important means of distinguishing between the two.

At an SCF > 3.5, the ionic composition of brines formed by both evaporation and freezing pathways is specified in the percentage of Na, SO₄, and Ca. Assuming that the chemical composition of the brine is only affected by the seawater concentration pathway, directly utilizing the relationship between Na/Cl, SO₄/Cl, and Ca/Cl and SCF is an effective way to distinguish the mechanism of brine genesis. Most of the brine samples in the study area with an SCF > 3.5 had similar Na/Cl ratios to those with an SCF < 3.5. However, four samples (w19-1, w24-1, w8-2, and w9-2) had significantly lower Na/Cl ratios than the other samples. In addition, three samples (w1-4, w1-3, and w2-2) had significantly smaller SO₄/Cl ratios than the other samples (Figure 8). These samples are more consistent with seawater freezing if the metamorphism of the brine burial process is not considered.

The NaCl-SCF results show the presence of both evaporated and frozen brines in the study area (Figure 5a), whereas the Ca/Mg-SCF model’s results show the presence of only evaporated brines (Figure 5b), which may be related to the relatively low SCF of the brines in the study area, which itself may mainly be due to the dilution of the brines by the mixing of seawater or freshwater at a later stage [21,39].

4.3. Effects of Mixing

In the Na/Cl-Br/Cl relationship, at the same dilution factor, seawater changes the original chemical components of the brine to a large extent, and the direction of dilution tends to be similar to that of seawater; however, freshwater changes the chemical components of the brine to a weaker extent (Figure 8). In practice, the input of fresh groundwater is often accompanied by diagenetic filtration, and the dissolution of rock salts results in higher Cl⁻ and lower Br/Cl in the brine [40]. The Br/Cl of 56% of the brines in the study area was lower than that of seawater, and the mean Cl⁻ concentration of these brines was 55,383 mg/L, which is slightly larger than that of the remaining 27 groups of brines with a mean Cl⁻ concentration of 54,994 mg/L. It can be seen that the dissolution and filtration of freshwater did not change the Br/Cl of this part of the brines significantly, and the main reason for the decrease in Br/Cl in the study area might be affected by artificial bromine extraction [32]. Frozen brine from the Canadian Shield [31] is near the evaporation line in the figure after being mixing with seawater another 30 times, which suggests that brines of a frozen origin were determined to be of an evaporative origin due to later mixing with seawater (Figure 8), and the same is true for SO₄/Cl-Br/Cl.

Na/Cl-Br/Cl and SO₄/Cl-Br/Cl can be successfully used to determine the genesis of highly concentrated brines, e.g., evaporated brines from Mississippi and Israel [41] and frozen brines from Canada and Finland [1,6,31]. However, these brines have a higher Br/Cl, which corresponds to a better differentiation between evaporated and frozen states in the graphs and allows for a clearer differentiation of the brine genesis. Most of the brine samples in the study area have an SCF between 2 and 4, with relatively small Br/Cl ratios, which are not conducive to the identification of frozen brines (Figure 4a). In addition, the burial depth of these brines is often as high as several hundred meters or even thousands of meters, and the burial environment is relatively closed, which is mainly affected by the mixing of freshwater in the ground [1]. Moreover, freshwater has a relatively small effect on the identification of this method. The depth of the brine layer in the Laizhou Bay area is not greater than 80 m, and the input of seawater changes its chemical components greatly, which has a more significant impact on the identification of the genesis.

In the NaCl-SCF relationship, the direction of the dilution routes is equally directed towards seawater and freshwater, respectively (Figure 7). The concentrations of both seawater and freshwater are smaller than that of the brine; thus, the mixing effect moves the brine in the direction of decreasing SCF. The slope of the Na/Cl curve of the brine obtained from the evaporation experiments is always smaller than that of the freezing experiments; hence, the dilution effect causes the evaporated brine located in the figure with an SCF > 3.5 to gradually change into frozen brine, which affects the judgment of the cause of the brine. Meanwhile, the SCF, as the independent variable of the model, is sensitive to the dilution effect. Moreover, the mixing of seawater and freshwater in equal amounts can change the SCF of the brine to a large extent. In addition, the brine produced by the evaporation and freezing experiments did not have a large change in its Na/Cl and Ca/Mg ratios compared to the original seawater in the course of the concentration process. Further, in practice, the brine was affected by the cation exchange and dephosphorization. In reality, under the influence of the cation exchange and dolomite removal, the Na/Cl and Ca/Mg of the brine in the study area have shown a more obvious decreasing trend compared with seawater (Figure 5), which is also unfavorable to the identification of the method.

4.4. Impact of Other Factors

In recent decades, the underground brine in Laizhou Bay has been heavily mined and utilized, and its water table has been continuously lowered [42], resulting in the appearance of underground brine descent funnels of varying sizes, which cause changes and movements of the brine interface, leading to the mixing of different water bodies. Additionally, the concentration of underground brine has been on a decreasing trend for many years. Taking the underground brine of Yangkou Salt Farm as an example, the concentration of the extracted brine decreased from 12° Be′~14° Be′ before 2000 to 8° Be′~10° Be′ in 2021 [43], and the Br/Cl of the brine was close to that of the seawater at its earliest time of formation. With the wide-scale extraction of bromine from the brine and the infiltration of bromine-poor brine, a gradual decreasing trend appeared from the 1960s to the early part of this century. While the Na/Cl was relatively stable, the Ca/Cl was also relatively stable, and the Ca/Cl was relatively stable as well. Although Cl was relatively stable, Ca/Mg showed more obvious fluctuations (Figure 9). Hence, human activities affect the chemical composition of subsurface brines in the study area.

The evolution of brine is a long and complex process, and the chemical composition of primary brine is often altered during the buried phase of the formation due to a variety of chemical and physical processes, such as ion exchange, interactions with the surrounding rocks, and the input of seawater, freshwater, or other water sources. While the evaporation and freezing characteristics of ion curves reflect the chemical characteristics of primary brines generated after the concentration of seawater by an evaporation or freezing process, the brines used in this study are no longer the initial products of seawater concentration, but instead underwent changes in their chemical composition in the process of later evolution. The real difficulty in distinguishing the seawater concentration pathway lies in obtaining the original subsurface brine samples or excluding the interference of various chemical reactions after brine formation [18].

The problem is further complicated by the presence of multiple layers of brines in the Laizhou Bay area, as well as their interaction and superposition with fresh groundwater layers. The shallow depth (0–80 m) of the brine-bearing layer makes it a transition zone for the interaction of seawater and fresh groundwater [39], and the chemical composition of the brine is now a product of the combined influence of many factors. Together, natural and anthropogenic factors change the evolution of underground brines in the Laizhou Bay area. With regard to the problem of the genesis of underground brines in Laizhou Bay, the traditional hydrochemical methods based on the evaporation and freezing characteristics of ionic curves have limitations, and new methods are needed to study the problem of brine genesis in the future.

5. Conclusions

There are large-scale underground brine deposits in the coastal area of Laizhou Bay, and the rapid development of local large-scale raw salt production and the brine chemical industry has gradually depleted these resources. The existence of frozen brine greatly expands the local brine reserves; however, most of the current studies on brine genesis in the Laizhou Bay area believe that only the evaporation of brine exists. In this paper, we simulated the mixing of brines formed by different seawater concentration pathways with seawater and fresh groundwater in different ratios and then explored the adaptability of the characteristic ion ratios commonly used to judge the seawater concentration pathways in the Laizhou Bay area. The main conclusions drawn are as follows:

The major ion concentration sequences and characteristic ion ratios of the Laizhou Bay subsurface brine show that it originated from seawater; however, its chemical composition has changed compared with seawater. The Ca/Mg-SCF results show that only evaporated brines exist in the Laizhou Bay area, while NaCl-SCF shows that both evaporated and frozen brines exist in the study area.

Our mixing simulation results show that mixing brine with seawater and freshwater can change the chemical components of subsurface brines and affect the judgment of seawater concentration pathways to some extent based on the characteristic ion ratios. The unique hydrogeological conditions and history of brine evolution in the Laizhou Bay area, especially the existence of mixing with seawater after the formation of local brine, make the traditional method of distinguishing the seawater concentration pathway limited in this region.

Human activities are continuously affecting the chemical composition of subsurface brines in the study area. Moreover, superimposed on the original complex groundwater system in the area, new methods to determine the existence of frozen brine in the Laizhou Bay area are also required.