Characteristics and Causes of Coastal Water Chemistry in Qionghai City, China

Abstract

The coastal zone area of Qionghai City is one of the important coastal zones in the South China Sea, and its water environment has been affected by human activities such as urbanization and industrialization. In order to protect the water resources and ecological environment of this area, the water chemistry characteristics of the main watersheds and their causes in the coastal zone area of eastern Hainan Island were investigated to provide a scientific basis for environmental protection and sustainable development. In this study, the characteristics and sources of water chemical ion components were analyzed using a Piper trilinear diagram, Gibbs diagram, and correlation analysis with the coastal zone area of Qionghai city as the research object. The results show the following: (1) the dominant cation of water chemistry in the coastal zone of Qionghai City is Na⁺ with a mean value of 35.001 mg·L⁻¹, and the dominant anion is Cl⁻ with a mean value of 30.69 mg·L⁻¹; (2) the dominant cation content in the coastal zone of Qionghai City is Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, and the dominant anion content is Cl⁻ > SO₄²⁻ > HCO₃⁻ > CO₃²⁻; (3) at the five collection sites in the study area, the ion concentrations showed different trends, with the highest ion concentration in the water samples collected from aquaculture ponds, and the main water chemistry type was Na-Cl; the lowest ion concentration was in the water samples collected from the rivers, and the main type of water chemistry was Ca·Mg-HCO₃. The source of water chemistry ions in the study area mainly included seawater, rock weathering, atmospheric precipitation, and evaporation concentration. The results of this study can provide a scientific basis for the development, utilization, and management of local water resources and provide basic data for environmental protection and sustainable development.

1. Introduction

In recent years, water quality studies for the coastal zone area of Qionghai City have gradually increased, including the monitoring and analysis of seawater, river water, lake water, and other water bodies. Some bays and estuaries in the region have water pollution problems, mainly from agricultural, industrial, and urban sewage sources. Water chemistry plays an important role in ecological environmental protection, water resource utilization, water pollution control, and water safety assurance. The water chemistry of watersheds can reflect the effects of rock weathering, atmospheric deposition, and human activities in the watershed [1]. By analyzing the characteristics and spatial and temporal changes in river water chemistry, the basic processes that proceed from water chemistry component formation could be effectively discerned [2]. In addition, the analysis of the water chemistry characteristics of coastal zone water bodies is a very critical part of the evaluation of the quality of water resources in the basin, which has non-negligible value for the scientific development and natural protection of water resources in the basin [3]. The water chemistry composition characteristics of watershed water bodies are mainly influenced by physical geography [4]; however, more and more studies show that the water chemistry composition of watershed waters is also influenced by human activities and has become an important issue affecting human survival and social development [4,5,6].

For the above-mentioned studies, many results have been achieved both in China and abroad, and research methods tend to be diversified [7,8]. Since the middle of the 20th century, a large amount of theoretical knowledge and a great number of technical means have been used to study the characteristics and evolution of the chemistry in both surface water and groundwater. Among them, there are many mathematical methods to deal with water quality data, generally including cluster analysis, principal component analysis, correlation analysis, factor analysis, etc. [9,10]. Based on the analysis of water quality data and the further use of piper trilinear plots [11], Gibbs [12], the water quality simulation method, the isotope analysis method [13,14], etc., investigations into the ion chemical characteristics of rivers and the influence of major weathering processes in watersheds have been undertaken [15]. Following this, the GIS visualization function appeared as a clear display of water quality results [16]. Gibbs’ analysis of the water chemistry components of various global water bodies (precipitation, seawater, lakes, and rivers) suggests that rock weathering, atmospheric precipitation input, and evapotranspiration–crystallization are the three major controlling factors of global surface water chemistry components [12]. Kattan’s analysis of the water chemistry of the Euphrates River in Syria showed that the ionic fraction in the river was influenced by the dissolution of rock weathering, water temperature, and evaporation [17]. Different study areas were located in different environmental contexts, including a variety of factors such as geological formations, climate, and human activities. These factors affect the ion concentrations and chemical composition of water bodies, resulting in significant differences in the water chemistry characteristics of different regions. Therefore, when conducting water resources management and environmental protection, the influence of these factors needs to be fully considered when developing management measures that are appropriate for the region [18,19,20].

Qionghai City, located on the east coast of Hainan Island, is a coastal city with rich coastal ecological resources. The quality of the water environment in the area is important for maintaining the local ecological environment and developing tourism [21]. Water chemistry is the study of the distribution, transformation, and transport patterns of substances such as dissolved inorganic compounds, organic matter, and biological elements in water bodies and their impact on the ecological environment.

Therefore, what type of water chemistry is found in Qionghai City? Additionally, what are the sources of water chemistry ions? In this paper, the main water chemistry indicators, including the total dissolved solids (TDS), pH value, and anion and cation concentration, are analyzed to reveal the chemical composition of the area’s water bodies, aiming to provide a scientific basis for environmental protection and sustainable development in the region.

2. Materials and Methods

2.1. Study Area Overview

Qionghai City is located in the center of the eastern Hainan Province [22], located at 110°07′~110°41′ E, 18°59′~19°29′ N, east of the South China Sea, west of Tunchang County and Qiongzhong County, north of Dingan County and Wenchang City, and south of Wanning City, with a total area of 1692 km². Qionghai City belongs to the tropical monsoon and marine humid climate zone, which is greatly influenced by monsoons, abundant light, high temperatures, rain, frequent typhoons, four indistinct seasons, and distinct dry and rainy seasons. The average annual rainfall is 2072 mm, the average annual sunshine is 2155 h, the average annual temperature is 24 °C, and there is no frost and snow all year round. There are many rivers in this territory, abundant water resources, and the total water resources of Qionghai City reach 2.216 billion m³, ranking third in Hainan Province, with available water resources at 8.40 million m³ (37.9% of the total water resources). In 2020, the regional GDP reached CNY 112.98 billion, an increase of 3.1% year on year. At the same time, Qionghai City completed the preliminary results of the “Qionghai City Coastal Zone Protection and Utilization Comprehensive Plan”, which aimed to strengthen the protection of environmental resources and the development and utilization management of the coastal zone.

2.2. Sample Collection and Determination

The water quality of civil wells, reservoirs, aquaculture ponds, rivers, and canals is a key area for water quality in Qionghai City. The specificity of Qionghai City needs to be considered in order to consider the evolution of water chemistry in the coastal zone area of Qionghai City in an integrated way. A total of 177 samples were collected from civil wells, reservoirs, aquaculture ponds, rivers, and canals in Qionghai between May and June 2022. A total of 5 samples were sent for testing, and the distribution of the sampling sites is shown in Figure 1. Water samples were generally collected below 10 cm from the water surface and then filtered through a 0.45 μm filter membrane, and those for cation (Ca²⁺, Mg⁺, K⁺, and Na⁺) analysis were acidified with nitric acid at a pH < 2, and those for anion analysis (Cl⁻, CO₃²⁻, HCO₃⁻, and SO₄², the anion samples (Cl⁻, CO₃²⁻, HCO₃⁻, and SO₄²⁻)) were not added with the reagents; the samples were sealed with paraffin and stored away from light. The samples were sent to the laboratory of the China Geological Survey Haikou Marine Geological Survey Center for the determination of K⁺ and Na⁺ by flame emission spectrometry, Ca²⁺ and Mg²⁺ by disodium ethylenediaminetetraacetate titration, Cl⁻ and SO₄²⁻ by ion chromatography, and HCO₃⁻ and CO₃²⁻ by the hydrochloric acid volumetric method. The test results met the quality requirements.

2.3. Analytical Methods and Quality Assurance

In this paper, the water chemistry data were counted by Excel and mathematical statistics, Pearson correlation analysis, Piper’s trilinear diagram, and Gibbs diagram were used to analyze the water chemistry characteristics and control factors in combination with hydrogeological conditions in the study area. Among them, Piper trilinear plots and Gibbs plots were developed using Origin 2020, and inter-ion correlations were analyzed using SPSS 22 statistics.

The Pearson correlation coefficient, also known as the product difference correlation coefficient, is a statistical indicator that expresses the degree and direction of linear correlation between the two variables. The correlation coefficient of a sample is denoted by the symbol r and is calculated by the following formula [23,24]:

$$\begin{array}{c}r=\frac{{\displaystyle \sum }\left(X-\overline{X}\right)\left(Y-\overline{Y}\right)}{\sqrt{{\displaystyle \sum }{\left(X-\overline{X}\right)}^2{\displaystyle \sum }{\left(Y-\overline{Y}\right)}^2}}=\frac{l_{XY}}{\sqrt{l_{XX}{l}_{YY}}}\end{array}$$

(1)

$$\begin{array}{c}{l}_{XX}={\left(X-\overline{X}\right)}^2={\displaystyle \sum }{X}^2-\frac{{\left({\displaystyle \sum }X\right)}^2}{n}\end{array}$$

(2)

$$\begin{array}{c}{l}_{YY}={\left(Y-\overline{Y}\right)}^2={\displaystyle \sum }{Y}^2-\frac{{\left({\displaystyle \sum }Y\right)}^2}{n}\end{array}$$

(3)

$$\begin{array}{c}{l}_{XY}={\displaystyle \sum }\left(X-\overline{X}\right)\left(Y-\overline{Y}\right)={\displaystyle \sum }XY-\frac{\left({\displaystyle \sum }X\right)\left({\displaystyle \sum }Y\right)}{n}\end{array}$$

(4)

where $l_{YY}$ denotes the off-average sum of squares for X; $l_{YY}$ denotes the off-average sum of squares for Y; $l_{XY}$ denotes the off-average sum of squares for X and Y; Pearson’s correlation coefficient is a dimensionless statistical indicator with a range of −1 ≤ r ≤ 1. A correlation coefficient less than 0 is a negative correlation, greater than 0 is a positive correlation, and equal to 0 indicates no correlation.

The larger the absolute value of the correlation coefficient, the closer the correlation between the two variables. To determine whether there is a linear relationship, the researcher needs to look at the scatter plot of the two variables; if the scatter plot is roughly a straight line, it indicates a linear relationship, and if it is not a straight line, there is no linear relationship.

The cluster analysis K-means algorithm, also known as the K-means clustering algorithm, is a widely used clustering algorithm that is the basis for other clustering algorithms [25,26]. Assuming that the input samples are S = X₁, X₂, ···, X_m, the algorithm steps are as follows: (1) select the initial k category centers μ₁, μ₂, … μ_k; (2) for each sample X_i, mark it as the closest category to the category center (distance can be calculated using Euclidean distance); (3) update each category center to the mean of all samples belonging to that category; (4) repeat the last two steps until the centroid is unchanged or a predetermined number of iterations is reached, and the algorithm terminates.

The objective function of the K-means algorithm is the within-cluster sum of squared errors (SSE), also known as cluster inertia.

$$\begin{array}{c}SSE={\displaystyle {\displaystyle \sum }_{i=1}^n}{\displaystyle {\displaystyle \sum }_{j=1}^k}{w}^{\left(i,j\right)}\Vert x^i-{\mu }^j\Vert {}^2\end{array}$$

(5)

where $w^{\left(i,j\right)}=1$ if $x^i$ belongs to cluster j. Otherwise, it is 0.

3. Results

3.1. Basic Physical and Chemical Properties of Water Bodies

The minimum, maximum, mean, and standard deviation of each indicator of the analyzed water samples in terms of the results are shown in

Table 1. Analysis results of major ion components in the Qionghai coastal basin.

Ions	Maximum Value (mg·L−1)	Minimum Value (mg·L−1)	Average Value (mg·L−1)	Standard Deviation (mg·L−1)	Coefficient of Variation (%)
Ca2+	86.47	10.61	34.43	17.72	52.00
Mg2+	66.84	2.14	22.47	13.04	58.00
Na+	90.90	20.20	35.00	13.39	38.00
K+	11.85	0.45	3.75	2.81	75.00
Cl−	107.00	4.01	30.69	25.68	84.00
SO42−	69.30	0.34	18.49	15.05	81.00
CO32−	5.56	0.12	1.34	1.05	78.00
HCO3−	55.03	1.34	15.36	9.90	64.00
TDS	852.00	12.00	203.60	150.44	74.00
Total hardness	3.51	0.14	0.95	0.66	70.00
pH	9.08	2.41	6.93	0.98	14.00

. The content of the dominant cations in the water in the study area showed Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and the average concentration of the dominant cation Na⁺ was 34.432 mg·L⁻¹. The dominant anion content showed Cl⁻ > SO₄²⁻ > HCO₃⁻ > CO₃²⁻, and the average concentrations of the dominant anions Cl⁻, SO₄²⁻, and HCO₃⁻ were 30.69 mg·L⁻¹, 18.49 mg·L⁻¹, and 15.40 mg·L⁻¹, respectively. The mean equivalent concentration of Cl⁻ accounted for 46% of the total anions, and the mean equivalent concentration of SO₄²⁻ accounted for 28% of the total anions. The coefficient of variation could characterize the stability of ionic components, and the larger the coefficient of variation, the more susceptible the ionic component was to the influence of the external environment.

3.2. Spatial Variation Characteristics of Major Ion Components

The trend of all types of ion concentrations is given in Figure 2. The trend of all ion concentrations is the trend of water chemical ions. Figure 2 shows the temporal trends of the water chemical ion fraction content in the coastal zone area of Qionghai City, and it was found that the overall trend showed the ion concentration to gradually decrease. The trends of different ions also showed their different characteristics, and ion concentrations were affected by many disturbing factors due to the different environments and conditions in which different collection sites were located.

The spatial distribution map shows (Figure 3) that the ionic concentration content of the cation Na⁺ was higher at 40.90 mg·L⁻¹, while the anion Cl⁻ had the highest ionic concentration content of 41.10 mg·L⁻¹. Among the selected collection sites, the water column ion concentration was highest in the aquaculture ponds and lowest in the rivers. In general, the trend of ion concentration was the result of the combined influence of many factors.

Through the mathematical and statistical analysis of the water chemistry indexes of water bodies in the coastal zone of Qionghai City, we could obtain a preliminary understanding of the enrichment degree and the changing pattern of the water chemistry of water bodies in the study area [27]. Figure 4 shows that the average pH value of water bodies in the area was 6.93, with a coefficient of variation of 14%. In addition, the concentration of total dissolved substance (TDS) values showed a gradual decrease and varied between 12 and 852 mg·L⁻¹, with an average value of 203.6 mg·L⁻¹.

3.3. Water Chemistry Type

The Piper trilinear diagram is a graphical method used to describe the chemical components of water. By analyzing the distribution and combination type of ion content in the Piper trilinear map, the water chemistry type and water quality characteristics of the surface water could be reflected [28,29]. The Piper trilinear diagram of the major ions in the surface water of the coastal zone area of Qionghai City is shown in Figure 5. From the figure, the main water chemistry type of surface water in the aquaculture ponds was the Na-Cl type, which accounted for the largest proportion; this was followed by the Ca-Mg-Cl type water, while the SO₄ type water accounted for a smaller proportion. The main water chemistry type of the canal was the Na⁻Ca⁻Cl type, the main water chemistry type of the civil well was the Ca·Mg-Cl type, the main water chemistry type of the reservoir was the Ca-Mg-SO₄ type, and the main water chemistry type of the river was the Ca·Mg-HCO₃ type. The surface water in the area of the aquaculture ponds was mainly influenced by seawater, which had a high content of Na and Cl, while the content of other ions was relatively low, so the ion content in the surface water could also be dominated by Na⁺ and Cl⁻. In addition, the high content of Na also reflected the high salinity of groundwater in the area, and there could be some degree of seawater intrusion. Finally, we also drew an ilr-ion plot (Figure 5f) of all five types [30].

3.4. Analysis of Major Ion Sources and Control Factors

The Gibbs diagram is a graphical method that can describe the source of water chemical components. By analyzing the distribution of each ion in the Gibbs diagram, it is possible to qualitatively determine the influence of factors such as atmospheric rainfall, rock weathering, and evaporation concentration on the source of ions in the surface water [31]. In the Gibbs plot of the coastal zone area of Qionghai City (Figure 6), it can be seen that the cation Na⁺/(Na⁺ + Ca²⁺) was mainly between 8% and 60%, and the anion Cl⁻/(Cl⁻ + HCO₃⁻) was mainly between 9% and 50%. This indicates that the main sources of ions in surface water in the region are atmospheric precipitation and seawater, while the contribution of rock weathering is small. In this region, the influence of seawater was high, so the proportion of Na⁺/(Na⁺ + Ca²⁺) was relatively high, while the influence of evaporation concentration was relatively small, so the proportion of Cl⁻/(Cl⁻ + HCO₃⁻) was also not high.

According to the results of the Pearson correlation coefficient analysis in

Table 2. Correlations between conventional indicators.

	Ca2+	Mg2+	Na+	K+	Cl−	SO42−	HCO3−	CO32−	TDS
Ca2+	1
Mg2+	0.526 **	1
Na+	0.887 **	0.536 *	1
K+	0.125	0.524 *	0.128	1
Cl−	0.791 **	0.619 *	0.844 **	0.116	1
SO42−	0.689	0.199	0.743 **	0.147	0.674 **	1
HCO3−	0.140	0.635 *	0.535 **	0.141	0.450 *	0.507 **	1
CO32−	0.296	0.133	0.372	0.117	0.161	0.136	0.140	1
TDS	0.712 **	0.569 *	0.897 **	0.327	0.830 **	0.730 **	0.568 **	0.177	1

*. Correlation is significant at the 0.05 level (two-tailed); **. Correlation is significant at the 0.01 level (two-tailed).

, it can be seen that there was a significant correlation (p < 0.05) between TDS and Na⁺, Ca²⁺, Cl⁻, and SO₄²⁻ in the surface water of the coastal zone of Qionghai City, indicating that there is a significant linear relationship between the content of these ions in surface water and the content of the total dissolved solids (TDS). Thus, both Na⁺ and Cl⁻ are major contributors of ions to the water of this region. In addition, there was a significant correlation between Na⁺ and Cl⁻, suggesting that these two ions have the same source and may be influenced by seawater. In addition, Table 2 showed a significance of 0.14 between Ca²⁺ and HCO₃⁻, probably because Ca²⁺ was derived from rock weathering processes, while HCO₃⁻ was mainly derived from atmospheric precipitation and evaporative concentration. It can be further speculated that the sources of surface water ions in the coastal zone of Qionghai City are mainly seawater, rock weathering, atmospheric precipitation, and evaporation concentration, and other factors [32,33,34,35,36].

4. Discussion

This study provides an in-depth analysis of the water chemistry characteristics of the main watersheds in the coastal zone area of Qionghai City and their causes. Using Piper’s trilinear diagram, Gibbs diagram, and correlation analysis, this study drew significant conclusions about the water chemistry characteristics and sources in this region. The results show that the characteristics of water chemistry ion fractions in this region were similar to many other coastal zone regions.

Firstly, the dominant cation was Na⁺, and the dominant anion was Cl⁻, which is consistent with the characteristics of being located in the coastal zone area. Secondly, the content of dominant cations showed Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, while the content of dominant anions was Cl⁻ > SO₄²⁻ > HCO₃⁻ > CO₃²⁻, and these results indicate that the influence of seawater was very significant in this region. In addition, the ion concentrations showed different trends in the water samples from different collection points, mainly because of the different sources of water and environmental factors.

For example, the highest ion concentration in the water samples collected from the farming ponds, and the main water chemistry type, was Na⁻Cl, which might be related to the development of the surrounding farming industry, seawater backflow, and other factors. Therefore, measures must be taken to limit the scale of the farming industry and reduce the phenomenon of seawater backflow to reduce the risk of water pollution. On the contrary, the water samples collected in the river had the lowest ion concentration and the main water chemistry type was the Ca·Mg-HCO₃ type; this is because water bodies in the area may be affected by factors such as rock weathering and atmospheric precipitation.

Yang et al. [37] studied the water chemistry characteristics of the southwest coastal zone area of Hainan Province and found that Na⁺ and Cl⁻ were the main water chemistry ions and that water bodies in the area were influenced by human activities, with agricultural activities contributing more to NO₃⁻ and SO₄²⁻ ions in the water bodies. In addition, Xi et al. [38] studied the water chemistry of groundwater in the east coastal zone area of Hainan Island and found that Ca²⁺ and Mg²⁺ were the main cations, HCO₃⁻ and Cl⁻ were the main anions, and the water chemistry of the area was influenced by different hydrogeological conditions and human activities. These findings are consistent with the results of this paper.

The results of this study also indicate that the sources of water chemical ions in the region resulted from various factors such as seawater, rock weathering, atmospheric precipitation, and evapotranspiration concentration effects. This conclusion provides a scientific basis for the development, utilization, and management of local water resources, which could help to protect water resources and the ecological environment of the region and promote the sustainable development of the area.

However, due to the limited scope of this study, other possible factors were not explored in depth. Our future study will further expand this topic, collecting more water quality samples in Qionghai to explore the details and characteristics and possible spatial and temporal variability of the sources of water chemistry ions in the region.

5. Conclusions

Based on the analysis of the chemical composition of surface water in the coastal areas of Qionghai, this study found significant variations in the ion concentrations and chemical types among different sampling sites. The dominant cations followed the order of Na⁺ > Ca²⁺ > Mg²⁺ > K⁺, while the dominant anions were Cl⁻ > SO₄²⁻ > HCO³⁻ > CO₃²⁻. Specifically, the water chemistry in aquaculture farms was dominated by the Na⁻Cl type, while the main chemical types in canals, wells, reservoirs, and rivers were Na·Ca-Cl, Ca·Mg-Cl, Ca-Mg-SO₄, and Ca·Mg⁻HCO₃, respectively.

Furthermore, significant correlations were observed between TDS and the major ions Na⁺, Ca²⁺, Cl⁻, SO₄²⁻, and HCO₃⁻, indicating their contribution to the overall water chemistry of the region. The correlation between Na⁺ and Cl⁻ suggests that they have a common source that is possibly influenced by seawater. Additionally, the correlation between Ca²⁺ and HCO₃⁻ may be related to different geological processes. Human activities and natural factors can influence water chemistry characteristics, as demonstrated in the coastal areas of Qionghai. Thus, long-term monitoring is needed in future studies to ensure the protection of natural water quality. Moreover, surface water and groundwater are often closely related, and further research is required to investigate their interactions in the coastal areas of Qionghai.