1. Introduction

2. Study Area

3. Materials and Methods

4. Results and Discussions

4.2. Chemical Types of Groundwater

The chemical conditions of the groundwater in the study area are complex, with as many as 56 chemical types of groundwater. The Ca•Na-HCO₃ type of water accounted for the largest share of 10%, followed by the Ca-HCO₃ and Na•Ca-HCO₃•Cl types, both accounting for 7%, while the remaining 53 chemical types all accounted for less than 5%. By grouping and simplifying the chemical types and retaining only one of the most dominant cations and anions, the groundwater chemical types were simplified from 56 to 11[23]. Among them, the Ca-HCO₃ type water dominated with 37%, followed by the Na-HCO₃ type water (21%) and the Na-Cl type water (11%), respectively, and the remaining 8 chemical types were all within 10%. It is worth mentioning that the NO₃ type water accounted for 11%, indicating a more pronounced influence by human activities[24].

Figure 3. Chemical types and spatial distribution of groundwater in the study area.

Figure 3. Chemical types and spatial distribution of groundwater in the study area.

In the study area, the HCO₃ type water accounted for over 60% of the total area and was mainly distributed in Haiwei Town, Shiyuetian Town, and the adjacent area of Changhua, Sigeng, Sanjia Towns. The SO₄ type water ranked the second, accounting for approximately 20% of the area. It was primarily distributed in the northern part of Changhua Town, the adjacent area of Wulie, Sanjia, Shiyuetian Towns, the western and southern parts of Sigeng Town, and the central-eastern part of Haiwei Town. The Cl type water and NO₃ type water each accounted for less than 10% of the area. Specifically, the NO₃ type water was mainly distributed in the urban areas adjacent to Wulie, Sanjia, and Shiyuetian Towns, indicating a close correlation with human activities, such as urbanization, in the study area[3,25].

4.3. Analysis of Groundwater Chemical Controlling Factors

4.3.1. Water-Rock Model Analysis

The study area is located in a tropical climate zone and is greatly influenced by seasonal rainfall. Both the infiltration of atmospheric rainfall and the river water runoff contribute to groundwater recharge[16,17,21]. Additionally, reactions between groundwater and the rock formations in the aquifers can also cause chemical changes in groundwater. Therefore, it is essential to explore the groundwater chemical characteristics and the controlling factors in the study area[26]. Gibbs diagram is commonly used to study the impact of water-rock interactions on groundwater chemistry. It provides a macroscopic understanding of the controlling factors for the major ions in groundwater, including atmospheric precipitation, rock leaching, and evaporation-crystallization[22,27,28].

As shown in Figure 4a,b, the ratio of cation mass concentration Na⁺/(Na⁺+Ca²⁺) ranges from 0.20 to 0.80, and the ratio of anion mass concentration Cl^-/(Cl^-+${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) ranges from 0.10 to 0.80. Water samples with moderate total dissolved solids and low ratios are located in the rock weathering control area, i.e., the middle-left part of the graph, indicating the rock weathering effect. This suggests that water-rock interactions dominate the hydrogeochemical processes in the study area. Some water samples fall within the region influenced by evaporation concentration, indicating a certain degree of impact from evaporation concentration. No water samples are located in the bottom-right region of the graph, which represents the area influenced by atmospheric precipitation. This suggests that atmospheric precipitation may have an influence, but its effect is not significant.

The Na⁺ end-member method was used to further explore the influence of rock weathering on the groundwater hydrochemical evolution process in the study area. According to the concentration ratio of (Ca²⁺/Na⁺)/(${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$/Na⁺) and (Ca²⁺/Na⁺)/(Mg²⁺/Na⁺), the main weathering sources of groundwater ions are divided into three types, namely carbonate rock, silicate rock, and evaporite rock[29,30]. It can be seen from Figure 4c,d that the shallow groundwater samples from the Changhua River Basin are mainly concentrated in the middle of the silicate rock end-member, and a small number are biased toward the carbonate rocks and evaporite rocks control end-members, indicating that the groundwater in this area is mainly affected by silicate rocks weathering, followed by the carbonate rocks and evaporite rocks weathering.

4.3.2. Analysis of Ion Ratio Relationship and Source of Main Components

Previous research commonly used correlation analysis between various chemical components of groundwater to reveal their sources[31,32]. Based on the correlation analysis results of various chemical components in Table 3, TDS shows a significant positive correlation with K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^-, ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$, ${\mathrm{S}\mathrm{O}}_4^{2-}$, and ${\mathrm{N}\mathrm{O}}_3^{-}$ (P < 0.01). This indicates that these chemical components have a significant contribution to the TDS. Among them, the correlations between TDS and Na⁺, Ca²⁺, Mg²⁺, and Cl^- are the most significant, with correlation coefficients of 0.797, 0.842, 0.802, and 0.837, respectively. This suggests that these four components are the primary factors influencing the TDS[33].

${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ shows significant correlations with Na⁺, Ca²⁺, and Mg²⁺, indicating that ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ shares a common source with Na⁺, Ca²⁺, and Mg²⁺, possibly originating from the weathering and dissolution of silicate rocks or carbonate rocks. The significant correlation of Na⁺ with Cl^- and ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ suggests the presence of weathering and dissolution of sodium-bearing silicate minerals like sodium feldspar, which could also come from atmospheric precipitation and the dissolution of evaporite rocks. The correlation coefficient of 0.83 between Na⁺ and Cl^- indicates a common source for both ions. The study area is close to the ocean, whereas the average Cl^-/Na⁺ (mEq/L) ratio of 0.85 is lower than the world average seawater ratio (Cl^-/Na⁺=1.16), suggesting that their chemical components may not be significantly influenced by seawater intrusion[31,34]. Furthermore, the correlation coefficient between K⁺ and Cl^- is 0.169, 0.421 between K⁺ and ${\mathrm{N}\mathrm{O}}_3^{-}$, and 0.136 between Cl^- and ${\mathrm{N}\mathrm{O}}_3^{-}$, respectively. Potassium chloride (KCl) is a common source of K⁺ and Cl^-, and potassium nitrate (KNO₃) is a common source of K⁺ and ${\mathrm{N}\mathrm{O}}_3^{-}$, suggesting that a portion of K⁺, Cl^-, and ${\mathrm{N}\mathrm{O}}_3^{-}$ could come from agricultural activities such as fertilizer usage and the discharge of domestic wastewater[35].

In this section, we further investigate the controlling factors of the chemical characteristics of the shallow aquifers downstream of the Changhua River. Due to the reaction between groundwater and aqueous media, the proportional relationship of various ions in groundwater is conducive to clarify the sources of major ions and hydrogeochemical processes. Therefore, a comparison of the main indicators of the 100 groundwater samples in the study area was conducted using milliequivalent ratios, as shown in Figure 6.

In general, the relationship between n(Na⁺+K⁺) and n(Cl^-) in a groundwater system can provide insights into the sources of Na⁺+K⁺ and Cl^-. For example, a ratio of n(Na⁺+K⁺)/n(Cl^-) = 1 suggests dissolution from evaporite rocks, while a ratio of n(Na⁺+K⁺)/n(Cl^-) > 1 indicates weathering from silicate rocks[4,36]. From Figure 5(a), it can be observed that the majority of groundwater sampling points at downstream of the Changhua River are located above the 1:1 line, indicating that the main source of Na⁺ and K⁺ in this area is likely from the leaching of silicate rocks. Meanwhile, some points are distributed below the 1:1 line, confirming that groundwater chemical composition is influenced by the leaching of carbonate rocks. The ratio of n(${\mathrm{S}\mathrm{O}}_4^{2-}$+Cl^-)/n(${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) reflects the dissolution of carbonate rocks in the study area. When n(${\mathrm{S}\mathrm{O}}_4^{2-}$+Cl^-)/n(${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) > 1, it suggests that ${\mathrm{S}\mathrm{O}}_4^{2-}$and Cl^- are derived from the weathering of evaporite rocks. Conversely, if n(${\mathrm{S}\mathrm{O}}_4^{2-}$+Cl^-)/n(${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) < 1, it indicates that ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$ is sourced from the weathering of carbonate rocks[37,38]. In Figure 5b, all the water sampling points are located above the 1:1 line, and some samples have much higher concentrations of (${\mathrm{S}\mathrm{O}}_4^{2-}$+Cl^-) compared to ${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$, indicating that the groundwater is mainly influenced by the weathering of evaporite rocks such as gypsum, rock salt, and mirabilite.

The sources of Ca²⁺ and Mg²⁺ can be determined based on the milliequivalent concentration ratio of (Ca²⁺+Mg²⁺)/${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$. When this ratio is greater than 1, it indicates that Ca²⁺ and Mg²⁺ are primarily sourced from the dissolution of carbonate rocks. Conversely, if the ratio is less than 1, it suggests that Ca²⁺ and Mg²⁺ are mainly derived from the dissolution of silicate rocks and evaporite rocks[39,40]. As shown in Figure 5c, most groundwater sampling points in the study area are located above the ratio line of 1 and only a small portion is located below, indicating that Ca²⁺ and Mg2+ in the groundwater primarily come from the dissolution of carbonate rocks. From the milliequivalent concentration ratios of (Ca²⁺+Mg²⁺)/${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$and (${\mathrm{S}\mathrm{O}}_4^{2-}$/${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$), we can further analyze the involvement of carbonic acid and sulfuric acid in the dissolution of carbonate rocks. When the ratio is 2, it suggests that sulfuric acid participates in the dissolution process of carbonate minerals. When the ratio is 1:1, it indicates that carbonic acid is involved in the dissolution of carbonate minerals[39,40]. From Figure 5d, it can be observed that most water sampling points are located near the ratio line of 1:2 and the upper left corner, indicating that both carbonic acid and sulfuric acid in the water are involved in the dissolution of carbonate minerals, but the carbonic acid contributes significantly more than the sulfuric acid.

4.3.3. Analysis of Cationic Exchange Adsorption

Under certain conditions, the exchange of certain cations adsorbed on the rock and soil surfaces with the cations in groundwater is referred to as cationic exchange adsorption. It is commonly represented by the ratio of n(Ca²⁺+Mg²⁺${-\mathrm{S}\mathrm{O}}_4^{2-}$${-\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$)/n(Na⁺+K⁺−Cl^-), where if cationic exchange occurs, the ratio of n(Ca²⁺+Mg²⁺${-\mathrm{S}\mathrm{O}}_4^{2-}$${-\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$)/n(Na⁺+K⁺−Cl^-) is approximately equal to -1[15,28]. In the case of the shallow groundwater in the lower reaches of Changhua River, there is a strong correlation between n(Ca²⁺+Mg²⁺${-\mathrm{S}\mathrm{O}}_4^{2-}$${-\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$) and n(Na⁺+K⁺−Cl^-), and the ratio is around -1 (Figure 6a), indicating that cationic exchange is occurring in the study area.

The direction and strength of cationic exchange can be further represented by the Chloro-Alkali Index (CAI)[5,23]. Typically, when the Ca²⁺ and Mg²⁺ in groundwater undergo cationic exchange with the Na⁺ and K⁺ adsorbed on the surface of the aquifer particles, both CAI-Ⅰ and CAI-Ⅱ are negative values. Conversely, if there is an anionic exchange process, CAI-Ⅰ and CAI-Ⅱ will be positive values.

Figure 6. Cationic exchange adsorption.

Figure 6. Cationic exchange adsorption.

From the relationship between CAI and TDS (Figure 6b), it can be observed that 81% of the water sampling points have negative CAI values. This indicates that the shallow groundwater in the study area primarily undergoes cationic exchange, where Ca²⁺ and Mg²⁺ in the pore water exchange with Na⁺ and K⁺ in the aquifer minerals, leading to a reduction in Ca²⁺ and Mg²⁺ concentrations and an increase in Na⁺ and K⁺ concentrations in the water.

4.3.4. Analysis of the Impact of Human Activities

In this study, the downstream area of the Changhua River in Hainan Island is the main tropical crop planting region with a wide rural area. Agricultural fertilization, and untreated domestic sewage and waste can generate a large number of pollutants, such as chloride ions and nitrates, which can enter the shallow groundwater with rainwater or surface water, thereby affecting the hydrochemical evolution of the groundwater[8,9].

The higher the ratios of Cl^-/Na⁺ and ${\mathrm{N}\mathrm{O}}_3^{-}$/Na⁺ in groundwater, the more significant the impact of human activities[41]. From Figure 7a, we can observe that the both ratios are relatively high, and some samples are close to or fall within the agricultural pollution end-member, indicating that shallow groundwater has been influenced to some extent by agricultural pollution. The relationship between ${\mathrm{S}\mathrm{O}}_4^{2-}$/Ca²⁺ and ${\mathrm{N}\mathrm{O}}_3^{-}$/Ca²⁺ is commonly used to analyze the influence of human activities on the main ions in groundwater. When ${\mathrm{S}\mathrm{O}}_4^{2-}$/Ca²⁺ > ${\mathrm{N}\mathrm{O}}_3^{-}$/Ca²⁺, it indicates a greater impact from industrial and mining activities, whereas the opposite suggests a greater influence from agricultural activities and domestic sewage, Figure 7b.

5. Conclusions and Outlook

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