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Article

Comprehending Spatial Distribution and Controlling Mechanisms of Groundwater in Topical Coastal Aquifers of Southern China Based on Hydrochemical Evaluations

by
Jun He
1,2,
Pan Wu
1,2,
Yiyong Li
1,2,
Min Zeng
1,2,
Chen Chen
1,2,
Hamza Jakada
3,* and
Xinwen Zhao
1,2
1
Wuhan Center, China Geological Survey, Wuhan 430205, China
2
Key Laboratory of Eco-Hydrogeological Process of River and Lake Wetlands in Changjiang Reaches, China Geological Survey, Wuhan 430205, China
3
Department of Civil Engineering, Baze University, Abuja 900211, Nigeria
*
Author to whom correspondence should be addressed.
Water 2024, 16(17), 2502; https://doi.org/10.3390/w16172502
Submission received: 1 August 2024 / Revised: 27 August 2024 / Accepted: 29 August 2024 / Published: 3 September 2024
(This article belongs to the Special Issue Soil and Groundwater Quality and Resources Assessment)
Browse Figures
Versions Notes

Abstract

:
Groundwater quality and availability in coastal aquifers have become a serious concern in recent times due to increased abstraction for domestic, agricultural and industrial purposes. (1) Background: Zhuhai city is selected as a representative coastal aquifer in Southern China to comprehensively evaluate the hydrochemical characteristics, spatial distribution and controlling mechanisms of groundwater. (2) Methods: A detailed study utilizing statistical analyses, a Piper diagram, Gibbs plots, and ion ratios was conducted on 114 surface water samples and 211 groundwater samples. (3) Results: The findings indicate that the pH of most groundwater is from 6.06 to 6.52, indicating a weakly acidic environment. The pH of surface water ranges from 5.35 to 9.86, with most values being weakly alkaline. The acidity in the groundwater may be related to the acidic atmospheric precipitation, an acidic unsaturated zone, oxidation of sulphide minerals and tidal action. The groundwater chemical types are predominantly mixed, followed by Ca-Mg-HCO3 type. Surface water samples are predominantly Na-Cl-SO4 type. The NO3− concentration in groundwater is relatively high, with a mean value of 17.46 mg/L. The NO2− and NH4+ concentrations in groundwater are relatively low, with mean values of 0.46 mg/L and 7.58 mg/L. (4) Conclusions: The spatial distribution of the principal chemical constituents in the groundwater is related to the landform. The chemical characteristics of groundwater in the study area are mainly controlled by the weathering and dissolution of silicate and sulfate minerals, evaporation, seawater mixing and cation exchange. Nitrate in clastic fissure groundwater, granite fissure groundwater and unconfined pore groundwater primarily originates from atmospheric precipitation, agricultural activities of slope farmland and forest land. Nitrate in confined pore groundwater and karst groundwater primarily originates from domestic sewage and mariculture wastewater. Our findings elucidate the processes characterizing the hydrogeology and surface water interactions in Zhuhai City’s coastal system, which are relevant to other catchments with similar geological characteristics.

1. Introduction

Groundwater has become an indispensable freshwater resource to meet the increasing domestic, agricultural and industrial demands of the growing global population [1]. Of the total amount of groundwater extracted globally, about 65% is used for drinking, 20% for irrigation and livestock rearing, and the remaining 15% is used for industrial and mining activities [2,3]. Coastal areas serve as transition zones between terrestrial and marine ecosystems, which are influenced by land and sea changes [4]. These changes are sometimes driven by anthropogenic activities along the coast, including groundwater abstraction. Generally, coastal aquifers are in close proximity to the ocean, and a variety of complex hydrogeological and geochemical processes, such as evaporation, mineral dissolution and mixing of saline and fresh water, define the water chemistry characteristics [5]. Soil properties strongly influence groundwater chemical composition after leaching, through the soil, due to their significant role in nutrient leaching provision water supply to crops [6].
Therefore, characterizing the chemicals in groundwater is crucial for providing reliable hydrogeological descriptions and diagnosing freshwater and saltwater mixing systems [7]. Proper understanding of groundwater’s hydrochemical characteristics is invaluable for assessing water quality suitability for drinking and irrigation [8], and the concentration of certain inorganic and organic substances can adversely affect human health and agricultural production. The main analysis methods for water chemistry include statistical analysis, hydrogeochemistry and isotope hydrology [9,10,11,12,13,14,15,16].
Zhuhai City is located in the coastal area of Southern China, characterized by a dense river network and a close relationship with seawater. The interaction between groundwater and seawater is frequent and significant. A series of groundwater environmental problems has arisen due to unique geological conditions and human engineering activities such as sea reclamation and mariculture [17,18,19,20,21,22]. Numerous research results have been obtained, and the origins and mechanisms of the pollution components have been analyzed. However, there is a relative dearth of research concerning the groundwater’s hydrochemical characteristics, spatial patterns and driving mechanisms beyond seawater intrusion. This paper will analyze the hydrochemical composition using statistical analyses, a Piper diagram, Gibbs plots, and ion ratio methods [23,24], study the formation control mechanism of groundwater in coastal areas [10], identify the ionic sources [25] and examine major hydrogeochemical effects [26], based on the concentration data of chemical components forming a large number of surface water and groundwater samples. The study is focused on the unique geologic environment of coastal zones, including the factors of topography, hydrology and meteorology, filling the current research gaps of Southern China. The research can provide a scientific basis for the utilization and protection of groundwater resources.

2. Materials and Methods

2.1. Study Area

Zhuhai is a coastal city in Southern China with a transitional oceanic climate between the southern subtropics and the tropics. The average annual rainfall is 2472.1 mm. The terrain slopes from northwest to southeast, and there are numerous waterways from the Xijiang River dividing into the South China Sea, intertwined with local rivers and tidal surges. The distribution of groundwater in the region is mainly controlled by a combination of factors, including geological structure, stratigraphic lithology, topography, meteorology and hydrology [22]. Groundwater is categorized into four types: pore groundwater, clastic fissure groundwater, granite fissure groundwater and karst groundwater (Figure 1). Pore groundwater is primarily distributed in the coastal plain area with a well-developed river network, and the aquifer lithology is dominated by coarse and medium sand and gravel (Figure 2). The aquifer thickness ranges from 1 to 10 m, and the groundwater is mostly slightly brackish or brackish. Clastic fissure groundwater is sporadically distributed in Pingsha, Nanshui and Sanzao. The aquifer lithology is dominated by conglomerate, siltstone and quartz sandstone, and the quantity of water is low. Granite fissure groundwater is widely distributed in the hilly and terrace areas, and the aquifer lithology is primarily granite, with low to moderate quantity [19]. Karst groundwater was found in three drill holes sporadically covered by the Quaternary System, which had not been previously identified, and its distribution range remains unclear. Groundwater is primarily recharged through infiltration from rainfall and surface water. Fissure groundwater in the hilly mountainous areas is discharged into the neighboring gullies via spring seepage, or laterally recharged to the pore groundwater of the Quaternary System via a sub-surface flow. The level of pore groundwater in the valleys and flood plains is shallow, and it is locally slightly confined. Groundwater is primarily drained through evaporation, discharged into the rivers and sea, and artificially extracted.

2.2. Sampling and Testing

From April to May 2023, 325 water samples were collected (Figure 1), including 114 surface water samples and 211 groundwater samples. Surface water samples were collected from depths of 30 to 50 cm below the water surface at 34 rivers, 49 channels, 14 ponds and 17 reservoirs. Groundwater samples included 19 clastic fissure groundwater, 63 granitic fissure groundwater, 3 karst groundwater, 97 unconfined pore groundwater and 29 confined pore groundwater samples, which were collected from 28 springs, 52 boreholes and 131 wells. The depth of the groundwater sample is from 0.1 to 58.1 m. pH, electrical conductivity (EC) and total dissolved solids (TDS) were measured on-site using a portable water quality analyzer (Manta2). The collected water samples were filtered through a 0.45 μm membrane for anion analysis. Samples for cation analysis were acidified to a pH below 2 with ultrapure concentrated nitric acid after filtering. All samples were collected in polyethylene sampling bottles and stored in a refrigerated sample preservation box at 4 °C. They were delivered to the laboratory for analysis within one week, and the analysis was performed at the South-Central Mineral Resources Testing Center of the Ministry of Natural Resources. The major cations (K+, Na+, Ca2+ and Mg2+) were determined by ICP-AES (ICAP 6000 Series, Thermo Fisher Scientific, Waltham, MA, USA). The major anions (Cl−, SO42−, NO3−) were determined by ion chromatography (ICS 5000+, Thermo Fisher Scientific, Waltham, MA, USA). Bromides and iodides were determined by ICP-MS (X Series II, Thermo Fisher Scientific, Waltham, MA, USA), and bicarbonate was determined by titration. The accuracy of the test was controlled by certified reference materials or standard addition recovery methods, and the precision was controlled by sample repeatability analysis. The spiked recovery samples were inserted in each of the batch samples for accuracy control. The qualified rate of each index was 100%. More than 20% of the samples were randomly selected for repeatability analysis, and the qualified rate of relative deviation of each batch of repeated analysis results was more than 90%. The charge balance errors (CBEs) of the main cations and anions were calculated to confirm the reliability of the water quality data. The CBEs of the data were within the range of ±5%, indicating reliable water quality data.

3. Results

3.1. Hydrochemical Characteristics

The pH of groundwater in the study area ranges from 3.24 to 8.23 (Table 1), with most samples falling between 6.06 and 6.52, indicating a weakly acidic environment. The pH of surface water ranges from 5.35 to 9.86 (Table 2), with most values being weakly alkaline, typically above 7.
The EC of groundwater ranges from 29.7 to 40,450 μs/cm (Table 1), with the deep confined groundwater generally having higher EC than shallow aquifer. This may be related to recharged by seawater. The EC of the confined pore groundwater ranges from 454.5 and 40,450 μs/cm, with an average of 18,475 μs/cm. The EC of the three karst groundwater samples ranges from 2632 to 22,550 μs/cm, with an average of 14,067 μs/cm. Most of these two types of groundwater are saline. A small number of other types of groundwater with high conductivity are individually influenced by seawater. The conductivity of surface water ranges from 2.5 to 40,070 μs/cm (Table 2), and most of the surface water showing high conductivity is closely related to seawater, except for the reservoir freshwater with EC ranging from 29.9 to 823.6 μs/cm.
The cations in the groundwater are dominated by Na+ and Ca2+, with the overall trend being Na+ > Ca2+ > Mg2+ > K+, and the average concentrations are 457.7 mg/L, 67.93 mg/L, 61.30 mg/L and 25.53 mg/L, respectively. The anions are dominated by Cl− and HCO3−, with the overall trend being Cl− > HCO3− > SO42−, and the average concentrations are 909.9 mg/L, 154.84 mg/L and 65.23 mg/L, respectively. The coefficient variations of the major anions and cations in groundwater is between 1.57 and 2.94, indicating that the chemical characteristics, source of groundwater and the water–rock interactions during transport are relatively complex.
The distribution characteristics of major anions and cations in groundwater are generally consistent across different types, except that the concentrations of various ions in confined pore groundwater and karst groundwater are significantly higher compared to other types. The average concentrations of the two types are 117.06 mg/L and 57.53 mg/L for K+, 2974 mg/L and 1795 mg/L for Na+, 259.5 mg/L and 714.2 mg/L for Ca2+, 381.9 mg/L and 318.4 mg/L for Mg2+, 5918 mg/L and 4500 mg/L for Cl−, 310.8 mg/L and 283.7 mg/L for SO42−, 486.6 mg/L and 117.43 mg/L for HCO3−.
The main cations in surface water are dominated by Na+ and Mg2+, with the overall trend being Na+ > Mg2+ > Ca2+ > K+, and the average concentrations are 1360 mg/L, 173.4 mg/L, 82.80 mg/L and 70.35 mg/L, respectively. The anions are dominated by Cl− and SO42−, with the overall trend being Cl− > SO42− > HCO3−, and the average concentrations are 2299 mg/L, 368.03 mg/L and 128.12 mg/L, respectively. The chemical composition of water in canals and ponds is broadly similar to that of rivers. The concentrations of major anions and cations in reservoir water are much lower compared to other types of surface water. Reservoir water is primarily recharged by atmospheric precipitation and groundwater from surrounding mountains. Rivers, canals and ponds are interconnected with seawater through the water system, and their quality is significantly influenced by seawater.
The TDS of groundwater ranges from 32.2 to 22,500 mg/L, with an average of 1650 mg/L. The TDS of surface water ranges from 12.6 to 25,300 mg/L, with an average of 4435 mg/L. The variation characteristics of TDS are consistent with those of conductivity. The TDS of confined pore groundwater and karst groundwater are higher compared to other types of groundwater. The maximum TDS of the samples from rivers, canals and ponds exceeded 20 g/L, except for reservoir water, which has a maximum TDS of 316 mg/L.
The detection rates of nitrate, nitrite and ammonia nitrogen in the 211 groundwater samples collected in this survey are 97.15%, 68.72% and 75.83%, respectively. The dissolved inorganic nitrogen in groundwater is predominantly nitrate, except in confined pore water where ammonia nitrogen predominates.
The concentration of nitrate in groundwater ranges from 0.02 to 162 mg/L, with an average of 17.46 mg/L. The concentration of nitrite in groundwater ranges from 0.02 to 6.11 mg/L, with an average of 0.46 mg/L. The concentration of ammonia nitrogen in groundwater ranges from 0.02 to 194 mg/L, with an average of 7.58 mg/L. The concentration of nitrate and nitrite in shallow water is higher than in deep water, whereas the concentration of ammonia nitrogen shows the opposite trend

3.2. Water Chemical Types

The Piper diagram of groundwater and surface water shows that the main groundwater chemical types were Ca-Mg-HCO3, Na-Cl-SO4, mixed Ca-Na-HCO3 and mixed Ca-Mg-Cl, accounting for 36.49%, 27.01%, 16.11% and 14.69% of the samples, respectively (Figure 3). The chemical types of different types of groundwater are clearly distinct. Most of the clastic fissure groundwater lacks a dominant cation, with a small amount dominated by Ca2+, and the anions are dominated by HCO3−, mainly forming the Ca-Mg-HCO3 type, with a small amount of Na-Cl-SO4 and mixed Ca-Mg-Cl type.
The chemical composition of the granite fissure groundwater is consistent with that of clastic fissure groundwater. The cations in most samples are dominated by Na+, a small number of samples lack dominant cations or are dominated by Ca2+. The anions are dominated by HCO3−, and the chemical type of the groundwater is predominantly the mixed Ca-Na-HCO3 type, with a small proportion being Ca-Mg-HCO3 type. Two out of the three karst groundwater samples are Na-Cl-SO4 type, and the remaining one is Ca-Mg-HCO3 type. Most of the unconfined pore groundwater samples lack dominant cations, some are dominated by Ca2+ or Na+, and the anions are dominated by HCO3− or lack dominant anions. The water chemical types are Ca-Mg-HCO3 type or mixed Ca-Mg-Cl type. The cations in the confined pore groundwater samples are dominated by Na+, a small amount lack dominant cations or are dominated by Ca2+. The anions are dominated by Cl−, and the primary water chemical type is Na-Cl-SO4 type.
Most of the surface water samples are dominated by Na+, and a small amount of water samples lack dominant cations. The anions in rivers, canals and pond samples are dominated by Cl−, and the anions in reservoir samples are dominated by HCO3−, with a small number of water samples from canals and reservoirs lacking dominant anions. The water chemical types of surface water are predominantly Na-Cl-SO4 type and mixed Ca-Na-HCO3 type, accounting for 71.93% and 17.54% of the surface water samples, respectively. The water samples of reservoirs are predominantly mixed Ca-Na-HCO3 type, accounting for 64.71% of the samples.

4. Discussion

4.1. Spatial Patterns of the Main Chemical Components

The pH of groundwater increases with the decrease in terrain elevation [27]. Most of the groundwater samples from hilly platform areas are strongly or weakly acidic, and a small number of samples are neutral (Figure 4). The main types of groundwater in these areas are clastic fissure groundwater, granite fissure groundwater and unconfined pore groundwater. Most of confined pore groundwater in the coastal plain area is neutral, and the pH values of two water samples exceed 8.0. Groundwater in this area is primarily recharged by river water that is weakly alkaline and connected to surface water and seawater. We can still find that there are some acidic samples in the plain located in the alluvial plain and the intermountain valley, which may be related to the recharge of groundwater from the hilly area. The genesis of acidic groundwater in coastal areas of Southern China is primarily related to atmospheric precipitation, the vadose zone aquifer medium, water–rock interaction, river influence and tidal action [27,28,29].
The major cations and anions in different groundwater aquifers are shown in Figure 5. The spatial patterns of main anions and cations in groundwater are closely related to landform and aquifer type. The main chemical components of confine pore groundwater and karst groundwater in coastal plain areas are significantly higher than those in hilly platform areas, which may be due to the direct or indirect lateral recharge of seawater in these two aquifers. The distribution of nitrate and ammonia nitrogen in groundwater is also related to topography. The nitrate concentration in granite fissure groundwater, clastic fissure groundwater and unconfined pore groundwater distributed in the hilly platform area with higher terrain is higher, while the ammonia nitrogen content in confined pore groundwater and karst groundwater in the coastal plain with lower terrain is higher. Studies have shown that inorganic nitrogen pollution in groundwater is primarily caused by chemical fertilizer, cultivation, urban domestic sewage and industrial activities, in addition to geological background factors [30,31]. The mineralization of organic matter, denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction in different redox environments also affect the distribution of nitrate and ammonia nitrogen [32].

4.2. Mechanism and Influencing Factors of Groundwater Hydrochemical Formation

The soluble ions in groundwater are mainly derived from atmospheric precipitation, lateral recharge of surface water, weathering and hydrolysis of minerals in sediments and soils [33,34], and human activities [35,36]. The chemical composition results from a long-term process of water–rock interaction between groundwater and aquifer medium, recording information about groundwater recharge, migration and circulation [37]. The main control mechanisms and influencing factors of groundwater chemical characteristics can be understood by ion ratios.

4.2.1. Rock Weathering

Gibbs identified three major mechanisms that control of world waters: atmospheric precipitation, rock dominance, and the evaporation–crystallization process, based on analytical chemical data from numerous water samples [38]. The major cations that characterize the end members of the world’s waters are Ca for freshwater bodies and Na for high-saline water bodies, and the presence of chloride and bicarbonate in the world’s waters is similar to the major cations. The chemical compositions of low-salinity waters are controlled by the amount of dissolved salts furnished by precipitation. The precipitation control zone is one end member of this series. The opposite end member of this series is comprised of waters having the rocks and soils of their basins as their dominant source of dissolved salts. The Na-rich, high-salinity, end-member components are the various seawaters of the earth whose compositions cluster near the Na-rich axis. Therefore, the relationship between the equivalent concentration ratios of Na+/(Na+ + Ca2+) and Cl−/(Cl− + HCO3−) is used to determine the origin mechanism of natural water bodies, which is controlled by rainfall dilution, evaporation concentration or rock weathering. Figure 6 shows that most of the equivalent concentration ratios of Na+/(Na+ + Ca2+) and Cl−/(Cl− + HCO3−) are mainly concentrated in the less than 0.5 area and are close to rock weathering zones. The ratios for confined pore groundwater, karst groundwater and most surface water are mainly concentrated in the greater than 0.5 area and near the evaporation and seawater influence area, which further indicates that the groundwater in Zhuhai City is primarily controlled by rock weathering, and the confined pore groundwater and karst groundwater are more affected by evaporation. The confined pore groundwater, characterized by slow runoff, is influenced by a certain degree of evaporation, and it is distributed in the plain areas such as Doumen, Jinwan and Baijiao. According to the geological background conditions of the study area, the confined pore groundwater and karst groundwater are recharged by seawater that has undergone intense evaporation. Therefore, these samples appear in the top right corner of Figure 6. The surface water in Zhuhai, excluding the reservoir water, is recharged by seawater, which relates to the chemical composition of seawater. The reservoir water is primarily recharged by the surface and underground runoff from the surrounding mountains, thus falling within the rock weathering area, and its chemical composition is influenced by the lithology of the surrounding mountains.

4.2.2. Ion Ratios

The equivalent concentration ratio between Na+ and Cl− is a hydrogeological parameter that characterizes the abundance of Na+ [39]. Liu et al. found that the equivalent concentration ratio in natural seawater and atmospheric precipitation is approximately 0.86 [40]. The equivalent concentration ratio between Na+ and Cl− of the samples in the study area is showed in Figure 7. It can be seen that the equivalent concentration ratio of surface water (excluding reservoirs), confined pore groundwater and karst groundwater is mainly near the 0.86 line, indicating that the primary source of these samples is atmospheric precipitation. The ratio of samples with low Cl− concentration is more variable, and the range of variation gradually decreases and approaches 0.86 as the Cl− concentration increases. This further indicates that marine atmospheric precipitation or seawater is the main source of surface water, confined pore groundwater and karst groundwater. However, the Na+ concentration in clastic fissure groundwater, granite fissure groundwater and unconfined pore groundwater is higher than that in atmospheric precipitation and seawater sources, indicating that silicate dissolution or cation exchange occurs in these three aquifers.
The equivalent concentration ratios of Mg2+/Na+, Ca2+/Na+ and HCO3−/Na+ produced by the weathering of carbonate, silicate and evaporite minerals are significantly different, and are often used for ion genesis analysis [41]. Wu et al. and Gaillardet et al. studied the Ca2+ /Na+ and Mg2+/Na+ ratios of silicate, carbonate and evaporite minerals [13,42]. The results show that the Ca2+ /Na+ and Mg2+/Na+ ratios produced by silicate mineral weathering are 0.35 ± 0.15 and 0.24 ± 0.12, respectively; those produced by carbonate mineral weathering are 50 ± 20 and 20 ± 8, respectively; and those produced by evaporite mineral weathering are 0.17 ± 0.09 and 0.02 ± 0.01, respectively. The Figure 8 shows that most of the groundwater samples are located near the weathering and dissolution of silicate minerals, indicating that the weathering and hydrolysis of silicate minerals play a major role in controlling the chemical characteristics of groundwater in the study area. The confined pore groundwater, karst groundwater and most surface water samples are located at the end of the dissolution of evaporite salt rock minerals, indicating evaporation and concentration. In addition, unconfined pore groundwater and some granite fissure groundwater samples are located at the end of carbonate mineral dissolution, indicating that dissolution of carbonate minerals occurs in these samples with low Na+ concentration.
The ratio of the equivalent concentration of (Ca2+ + Mg2+) and (HCO3− + SO42−) is an important parameter commonly used to study mineral dissolution in groundwater [10]. When the ratio of the equivalent concentration is close to 1, it indicates that Ca2+ and Mg2+ mainly originate from the dissolution of carbonate and sulfate minerals. If the ratio is greater than 1, it indicates that dissolution of carbonate and sulfate minerals is accompanied by the dissolution of silicate. If the ratio is less than 1, it indicates that the process of mineral weathering and dissolution is accompanied by ion exchange absorption [14]. The plots showing the relationships between (Ca2+ + Mg2+) and (HCO3− + SO42−) of samples demonstrate that unconfined pore groundwater, clastic fissure groundwater and granite fissure groundwater with low Ca2+ and Mg2+ concentration (<10 meq/L) mainly fall near the 1:1 line, indicating that the Ca2+ and Mg2+ in these groundwaters primarily originate from the dissolution of carbonate or sulphate (Figure 9). The confined pore groundwater and surface water samples with high Ca2+ and Mg2+ concentration (>30 meq/L) mainly fall near the 2:1 line, indicating that Ca2+ and Mg2+ in groundwater are primarily derived from the dissolution of silicate or the cation exchange of Na+ replacing Ca2+ and Mg2+.

4.2.3. Cation Exchange

Cation exchange in groundwater can be verified by the correlation between the equivalent concentrations of (Na+ − Cl−) and (Ca2+ + Mg2+) − (HCO3− + SO42−) [43,44]. A strong negative correlation indicates significant cation exchange in groundwater. The confined pore groundwater, karst groundwater and some surface waters fall along the 1:1 line in Figure 10, indicating that cation exchange occurs in these water bodies. In addition, the (Ca2+ + Mg2+) − (HCO3− + SO42−) values for unconfined pore groundwater and clastic fissure groundwater are close to 0, which further indicates that the Ca2+ and Mg2+ in these two types of groundwaters are primarily originated from the dissolution of carbonate or sulfate minerals.
Two basic exchange indexes, the chlor-alkali indices CAI 1 and CAI 2, were used to determine the replacement relationship between groundwater and cations in an aquifer medium during the cation exchange reaction [45]. Positive values of both indexes indicate that reverse ion exchange occurs, where Na+ and K+ in groundwater replace Ca2+ and Mg2+ in an aquifer medium. Negative values of both indexes indicate that a positive ion exchange occurs, where Ca2+ and Mg2+ in groundwater replace Na+ and K+ in the aquifer medium [24].
CAI 1 = [c(Cl−)-c(Na+ + K+)]/c(Cl−)
CAI 2 = [c(Cl−) − c(Na++K+)]/[2c(SO42−) + c(HCO3−) + c(NO3−)]
Figure 11 shows that the chlor-alkali indices of confined pore groundwater, karst groundwater, river, canal, lake and reservoir water in the study area are primarily positive, indicating reverse cation exchange. The replacement of Ca2+ and Mg2+ in the aquifer medium by Na+ and K+ in the groundwater explains why the equivalent concentration of Na+ in these samples are less than that of Cl−, and the content of Ca2+ and Mg2+ is excessive, exceeding the dissolution line of silicate minerals. The chlor-alkali indices of unconfined pore groundwater, clastic fissure groundwater and granite fissure groundwater are mostly negative, indicating that Ca2+ and Mg2+ in the groundwater displace Na+ and K+ in the aquifer medium. Consequently, there is an excess of Na+ in these groundwaters, and a certain amount of Na+ can be produced through the dissolution of silicate.

4.2.4. Impact of Human Activities

Nitrate has become one of the primary pollutants in groundwater environments globally, originating predominantly from industrial and agricultural activities, as well as domestic sewage [46,47]. The molar concentration ratios of SO42−/Na+, Cl−/Na+ and NO3−/Na+ can be used to identify the effects of agricultural or industrial activities on nitrate in groundwater [40]. In general, high-intensity human activities lead to the increased Cl−, NO3− and SO42− concentrations in groundwater, resulting in high Cl−/Na+ and NO3−/Na+ molar ratios. Figure 12a shows the Cl−/Na+ and NO3−/Na+ molar ratios. Most of the clastic fissure groundwater, granite fissure groundwater, unconfined pore groundwater, pond and reservoir samples are located near the upper right corner of line y = x, suggesting that most of the nitrate in these samples originates from agricultural activities in hills, such as forestry and orchards. The samples of confined pore water, rivers and ponds are primarily distributed near the evaporite and domestic sewage zones, indicating a domestic sewage source and the influence of evaporation. Figure 12b indicates that a small number of samples were mildly affected by industrial activities.
The halides Br− and I− are considered conservative anions due to their minimal interactions with the surrounding substrate during groundwater migration. Bromide may concentrate in brines due to evaporation and clay-membrane effects, which I− tends to concentrate in marine vegetation and can also be concentrated in some brines [48]. Statistical analysis of the Br− and I−/Na+ ratios in samples can help identify the source of nitrate [49]. Figure 13 illustrates that most of the clastic fissure groundwater, granite fissure groundwater, unconfined pore groundwater and reservoir samples fall within the atmospheric precipitation and soil water zone, characterized by high-concentration I− and relatively low Br− concentration. This further indicates that the nitrate in the samples from hilly terraces and intermountain valleys primarily originates from atmospheric precipitation and agricultural activities, with a minor influence from domestic sewage. The confined pore groundwater, river, channel and pond samples are clearly distinct from other samples, falling within the sewage and feces contamination zones. This indicates that the nitrate in these samples primarily originates from domestic sewage and aquaculture wastewater, in conjunction with the specific land use types.

5. Conclusions

Freshwater sources in coastal areas worldwide are increasingly vulnerable due to rising sea levels and extensive abstraction for domestic, agricultural and industrial uses. Accordingly, we assessed the water system in Zhuhai city, Southern China. The chemical characteristics of the water samples and the impact of hydrogeochemical processes on mineral dissolution, evaporation and concentration, and cation exchange were evaluated using analytical and statistical methods. Our key findings are summarized as follows:
(1)
The pH of groundwater in the study area ranges from 3.24 to 8.23, with most of the groundwater being slightly acidic. The pH of surface water ranges from 5.35 to 9.86, with most of it being weakly alkaline. The acidity in the groundwater is linked to acidic atmospheric precipitation, an acidic unsaturated zone, the oxidation of sulphide minerals and tidal action.
(2)
The cations in groundwater are dominated by Na+ and Ca2+, with mean concentrations of 457.7 mg/L and 67.93 mg/L, respectively. The anions are dominated by Cl− and HCO3−, with mean concentrations of 909.9 mg/L and 154.84 mg/L, respectively. The groundwater chemical types are predominantly mixed, followed by Ca-Mg-HCO3 type. Surface water samples are predominantly of the Na-Cl-SO4 type.
(3)
The spatial patterns of major anions and cations in groundwater are closely related to landform and aquifer type. The main chemical components of confine pore groundwater and karst groundwater in coastal plain areas are significantly higher compared to those in hilly plateau areas.
(4)
The samples from rivers, canals, lakes, ponds, confined pore groundwater and karst groundwater are strongly influenced by the dissolution of rock salts, evaporation and concentration, and reverse cation exchange. Unconfined pore groundwater, clastic fissure groundwater, granite fissure groundwater and reservoir water are controlled by the dissolution of carbonate, sulphate and silicate, along with positive cation exchange.
(5)
The nitrate concentration in groundwater ranges from 0.02 to 162 mg/L with a mean of 17.46 mg/L. Nitrate in groundwater primarily originates from atmospheric precipitation and human activities. Nitrate in clastic pore fissure groundwater, granite fissure groundwater and pore unconfined groundwater primarily originates from atmospheric precipitation, agricultural activities on slope farmland and forest land. Nitrate in pore confined groundwater and karst water primarily originates from domestic sewage and mariculture wastewater.
Overall, our findings elucidate the processes that define the hydrogeology and surface water interactions in the coastal system of Zhuhai city, which are relevant for other catchments with similar geological characteristics.

Author Contributions

Methodology, Y.L. and M.Z.; software, P.W.; validation, H.J. and X.Z.; formal analysis, J.H.; investigation, C.C. and P.W.; resources, X.Z.; data curation, Y.L.; writing—original draft preparation, J.H. and H.J.; writing—review and editing, M.Z. and P.W.; visualization, C.C.; supervision, H.J.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by urban geological survey of Zhuhai City, Zhuhai Municipal People’s Government, grant number MZCD-2201-008 and monitoring and evaluation of resource and environment carrying capacity in Guangdong–Hong Kong–Macao Greater Bay Area, China Geological Survey, grant number DD20221729.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Water 16 02502 g001
Figure 1. Simple hydrogeological map of study area and location of samples.
Figure 1. Simple hydrogeological map of study area and location of samples.
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Water 16 02502 g002
Figure 2. Section map of study area (location of the section is in Figure 1).
Figure 2. Section map of study area (location of the section is in Figure 1).
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Water 16 02502 g003
Figure 3. Piper diagram of water samples.
Figure 3. Piper diagram of water samples.
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Figure 4. Relationship between acidic groundwater and landform.
Figure 4. Relationship between acidic groundwater and landform.
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Figure 5. Box diagram of main chemical composition of groundwater.
Figure 5. Box diagram of main chemical composition of groundwater.
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Figure 6. Gibbs diagram of water samples.
Figure 6. Gibbs diagram of water samples.
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Figure 7. Relationship between Na+ and Cl− of water samples.
Figure 7. Relationship between Na+ and Cl− of water samples.
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Water 16 02502 g008
Figure 8. Relationship between HCO3−/Na+, Mg2+/Na+ and Ca2+/Na+ of water samples.
Figure 8. Relationship between HCO3−/Na+, Mg2+/Na+ and Ca2+/Na+ of water samples.
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Water 16 02502 g009
Figure 9. Relationships between (Ca2+ + Mg2+) and (HCO3−+ SO42−) of water samples.
Figure 9. Relationships between (Ca2+ + Mg2+) and (HCO3−+ SO42−) of water samples.
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Water 16 02502 g010
Figure 10. Relationships between (Na+ − Cl−) and (Ca2+ + Mg2+) − (HCO3− + SO42−) of water samples.
Figure 10. Relationships between (Na+ − Cl−) and (Ca2+ + Mg2+) − (HCO3− + SO42−) of water samples.
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Water 16 02502 g011
Figure 11. Chlor-alkali index of water samples.
Figure 11. Chlor-alkali index of water samples.
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Water 16 02502 g012
Figure 12. Relationship between NO3−/Na+, Cl−/Na+ and SO42−/Na+ of groundwater. (a) NO3−/Na+ and Cl−/Na+; (b) Cl−/Na+ and SO42−/Na+.
Figure 12. Relationship between NO3−/Na+, Cl−/Na+ and SO42−/Na+ of groundwater. (a) NO3−/Na+ and Cl−/Na+; (b) Cl−/Na+ and SO42−/Na+.
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Water 16 02502 g013
Figure 13. Relationship between I−/Na+ and Br− of groundwater.
Figure 13. Relationship between I−/Na+ and Br− of groundwater.
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Table 1. Statistical eigenvalues of major macronutrient components of groundwater.
Table 1. Statistical eigenvalues of major macronutrient components of groundwater.
Groundwater
Type		 pHECK+Na+Ca2+Mg2+Cl−SO42−HCO3−TDS
μs/cmmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L
Clastic
fissure
groundwater		max6.86726.714.6061.8066.7013.0092.9083.30219.0190.0
min4.5431.10.653.920.460.582.361.766.303.78
avg5.86178.24.6212.9416.333.3214.7713.8461.3054.46
std0.70203.53.9314.1921.243.0723.4519.9367.8563.23
cv0.121.140.851.101.300.921.591.441.111.16
Granite
fissure
groundwater		max8.003135 55.60463.096.208.14745.060.30291.01480
min4.5329.7<0.073.80<0.020.21<0.06<0.1<0.532.20
avg6.29231.95.9823.2014.681.9324.2611.3175.15158.9
std0.79409.07.3858.2319.871.6693.5013.9374.06194.0
cv0.131.761.242.511.350.863.851.230.991.22
Karst
groundwater		max7.0422,55097.2036701580495.07240420.0339.012,500
min5.88263222.8049418.5032.20500.0150.025.301400
avg6.2914,06757.531795714.2318.44500283.7177.437860
std0.6510,28237.441664794.5250.13542135.0157.065769
cv0.100.730.650.931.110.790.790.480.890.73
Unconfined
pore
groundwater		max7.79392866.40658.0119.0198.01070172.0973.02140
min3.2439.81.383.980.450.293.161.894.7647.30
avg6.18486.713.9733.4835.377.4252.1330.15125.05289.0
std0.71501.011.6370.9523.9920.48121.2623.58132.66267.2
cv0.111.030.832.120.682.762.330.781.060.92
Confined
pore
groundwater		max8.2340,450294.07920814.0102013,8001380158022,500
min4.64454.58.1615.3010.703.9141.203.92<0.5224.0
avg6.8618,475117.062974259.5381.95918310.8486.69801
std0.9012,98380.812330194.6320.34344407.0478.96799
cv0.130.700.690.780.750.840.731.310.980.69
Table 2. Statistical eigenvalues of major macronutrient components of surface water.
Table 2. Statistical eigenvalues of major macronutrient components of surface water.
Surface Water Type pHECK+Na+Ca2+Mg2+Cl−SO42−HCO3−TDS
μs/cmmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L
Rivermax9.8640,070306.007700316.0113013,7002020267.025,300
min6.022.502.2310.600.760.6810.7012.4031.70131.0
avg7.1210,23489.461803104.92233.83082511.9162.85925
std0.7611,55189.55213577.77290.83690543.148.866810
cv0.111.131.001.180.741.241.201.060.301.15
Channelmax9.4234,640273.006980263.090010,1001750304.020,400
min5.355.302.638.730.750.674.723.54<0.556.70
avg6.67784776.50149885.60189.72457396.2135.74472
std0.6710,00486.39204577.60261.03239513.970.625859
cv0.101.271.131.370.911.381.321.300.521.31
Pondmax8.0137,850305.007650565.0101011,9001920214.023,200
min6.2453.25.548.250.990.335.533.8623.8048.90
avg6.9510,96788.931820123.54228.03167453.7126.95961
std0.4913,385104.132489160.84328.64244605.568.967887
cv0.071.221.171.371.301.441.341.330.541.32
Reservoirmax8.61823.632.6046.6042.0012.7083.9038.80153.0316.0
min5.6329.92.044.500.780.353.082.1714.2012.60
avg6.92140.310.6415.917.392.2314.538.2039.8389.38
std0.74202.89.1710.0011.733.1920.3810.0737.6670.83
cv0.111.450.860.631.591.431.401.230.950.79
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He, J.; Wu, P.; Li, Y.; Zeng, M.; Chen, C.; Jakada, H.; Zhao, X. Comprehending Spatial Distribution and Controlling Mechanisms of Groundwater in Topical Coastal Aquifers of Southern China Based on Hydrochemical Evaluations. Water 2024, 16, 2502. https://doi.org/10.3390/w16172502

AMA Style

He J, Wu P, Li Y, Zeng M, Chen C, Jakada H, Zhao X. Comprehending Spatial Distribution and Controlling Mechanisms of Groundwater in Topical Coastal Aquifers of Southern China Based on Hydrochemical Evaluations. Water. 2024; 16(17):2502. https://doi.org/10.3390/w16172502

Chicago/Turabian Style

He, Jun, Pan Wu, Yiyong Li, Min Zeng, Chen Chen, Hamza Jakada, and Xinwen Zhao. 2024. "Comprehending Spatial Distribution and Controlling Mechanisms of Groundwater in Topical Coastal Aquifers of Southern China Based on Hydrochemical Evaluations" Water 16, no. 17: 2502. https://doi.org/10.3390/w16172502

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