Origin and Implications of Pollution in Coastal Groundwater of the Guangdong Province

Abstract

The groundwater resource is crucial to the urbanization and industrialization in the Guangdong coastal area; the rapid development of Daya Bay has presented a challenge for the management of the groundwater quantity and quality. Therefore, a novel approach to hydrochemical analysis, which, combined with the health risk model and the water quality index (WQI), was used to explain the hydrochemistry characteristics and risks to human health of groundwater in the Guangdong coastal areas in addition to investigating the factors controlling groundwater quality. The results showed that the average concentration of total dissolved solids (TDS) in groundwater was 1935.26 mg/L and the quality of water was weakly alkaline. The dominant hydrochemical types of groundwater were identified to be Mg-HCO₃ and Na-Cl·HCO₃. The main factor influencing the hydrochemical composition was rock weathering, while the result of principal component analysis (PCA) shows seawater intrusion and anthropogenic inputs also have an effect on the water quality. The conclusions of the water quality assessment indicated that most of the groundwater samples were acceptable for drinking. However, both WQI and the non-carcinogenic hazard quotient (HQ) values indicated unacceptable risks in any area of Maoming, Zhanjiang and Shantou, and, according to the hazard index (HI) value, children in the study area are at more danger to health risks than adults. It is suggested that both groundwater salinization and nitrate pollution should be paid attention to when improving groundwater quality and exploring the sustainable utilization of groundwater resources.

1. Introduction

Groundwater is firmly connected to human society, such as in industrial, agricultural, and domestic activities [1]. Due to the benefits of wide distribution, convenient use, and good water quality, groundwater is always considered as an important drinking water source [2]. Nevertheless, the enormous expansion in population has driven the request for freshwater and has caused stress on groundwater resources; over-exploitation has recently led to the depletion of groundwater, which has been a worldwide problem [3]. According to WHO, roughly 780 million individuals all over the globe have no access to harmless drinking water [4]. The groundwater quality has been affected by urbanization, population growth, industrialization, and many other anthropogenic activities, especially in developing countries [5,6].

As a developing country, China depends heavily on groundwater, and more than 70% of urban communities make use of groundwater for domestic and drinking purposes in China [7]. Due to the accelerated progress of the Guangdong-Hong Kong-Macao Greater Bay Area, more and more attention should be paid to developing and utilizing groundwater resources in the coastal zones. In Guangdong, the issue of the quality and quantity of groundwater is turning into a severe matter on account of the developing need. Nevertheless, a rising population followed by urbanization has increased the demand for groundwater supplies in Guangdong. Guangdong has undergone extensive urbanization and has been confronting various groundwater quality issues inferable from overpopulation and industrialization, similar to other urbanized regions [8,9,10,11]. The industrialization and urbanization have sped up the deterioration of groundwater quality in Guangdong over past three decades [12], as well as groundwater contamination [13]. Therefore, we should understand that the chemical characteristics of groundwater are of great importance for revealing the quality assessment and the hydrochemical process of groundwater. In recent years, studies on hydrochemistry in the Guangdong coastal zone have been focused on the Pearl River [14,15,16,17,18], while few investigations have been conducted in the other areas of Guangdong coastal area. The evolution of coastal groundwater resulting from the rapid industrial development over the past decade is not yet well understood. It is necessary to understand the specific ionic sources, which change the chemistry of groundwater and lead to groundwater contamination. Therefore, sustainable development and management of groundwater resources and their protection from pollution activities play a vital role in the Chinese economy.

Groundwater geochemistry, due to the impact of landfill and stormwater runoff from agricultural fields, is vulnerable to non-geogenous inputs [19]. Geochemical ratios can be used to understand the chemical processes that take place between the water and soils/rocks, as well as the water and anthropogenic activities that alter the chemical composition of groundwater [20]. A groundwater quality survey on the Prakasam district in India showed that rock-weathering, mineral dissolution, ion exchange, and evaporation were the dominant hydrochemical processes and the anthropogenic sources (waste waters and agricultural activities) were the secondary activities regulating the aquifer chemistry.

Generally, most of the groundwater chemistry is characterized by geogenic processes and non-geogenic sources, which can alter the chemistry of the groundwater [21]. A groundwater quality investigation was conducted in Visakhapatnam, Andhra Pradesh, India and it was found that the silicate weathering and dissolution, ion exchange and evaporation were the dominant geogenic processes. Domestic waste, septic-tank seepage, irrigation practices, and fertilizers were the artificial activities that control the groundwater chemistry [19,20,21].

In general, individual activities and nature have an impact on ground and surface water quality [22]. Numerous studies have presented that both hydrochemical and water quality are consequences of long-term associations between groundwater and the surrounding environment, which are affected by various factors, such as geology, hydrogeology, hydrodynamic conditions, precipitation, climate, and anthropogenic inputs [20,21,22,23,24]. Usually, freshwater in the coastal region is a vital resource for a few communities, as they are vulnerable to salinization, especially because of their proximity to the ocean and because coastal regions are vulnerable to over-exploration due to their high population density [25,26,27,28,29]. In addition, NO₃⁻ and heavy metal-contaminated groundwater is also a global environmental issue that has a long-term health influence. The elevated contamination of NO₃⁻ in drinking water instigates health diseases such as methemoglobinemia in infants, thyroid dysfunctions, and hypertension [30]. The nitrate contamination in groundwater caused by agricultural surface contamination started to appear during the 1990s [31,32]. At present, nitrate-contaminated groundwater has become a hazard in all nations [33]. With the rapid development of the economy, nitrate pollution in groundwater in China is becoming more and more serious. In addition, the NO₃⁻ concentration in groundwater is significantly elevated due to the domestic sewage, industrial wastewater, septic tanks, and landfill leachate caused by urban expansion [34]. In the past few decades, urbanization has led to population concentration, economic growth, and expansion of construction land; around urban areas, the simultaneous existence of industrial, domestic, and agricultural pollution sources have made the pollution sources of “triple nitrogen” in groundwater very complex [35]. The recognizable proof of pollutants’ origin could assist in the understanding of geochemical processes, elucidate the circulation, and convey a model of the pollutants.

The water quality index (WQI) was usually used for drinking purposes [36,37]. In general, consumption of contaminated drinking water and groundwater can pose a serious risk to humans, primarily through two routes of exposure, the first being drunk water (or the oral route) and the second being a dermal or skin connection [38,39]. Comprehensive hazard quotients (HQ) and the hazard index (HI) are useful for health risk assessment [40,41,42]. This extensive model for human health risk evaluation was initially proposed by the United States Environmental Protection Agency [43]. At present, many investigations have likewise explored heavy metals, volatile organics, pesticides, polycyclic aromatic hydrocarbons, and the human health risk of nitrate pollution on a provincial or national level [44,45,46,47]. In spite of this, few researchers have explored the connection between human health risks and nitrate contamination in the coastal zone.

Therefore, it is of extraordinary importance to concentrate on the significant particle science and water quality assessment of groundwater in the Guangdong coastal zone. Research on groundwater hydrochemistry and water quality in Guangdong is outstandingly restricted. Therefore, the targets of this study are to (1) identify the hydrochemical characteristics of groundwater; (2) evaluate the controlling factors of the water chemistry; (3) assess the health risks due to contamination. This paper will be useful for understanding the hydrochemical composition and characteristics of groundwater in the Guangdong coastal area and will benefit sustainable development of groundwater environments, and will provide a valuable data set for the formulations of future mitigation strategies and policy updates.

2. Study Area

The Guangdong Province is located in the southeast of China (geographical coordinates: latitude: 20°13′~25°31′ N; longitude: 109°39′~117°19′ E) (Figure 1). The coastline of Guangdong is the longest one in the provinces of China, extending for 3368.10 km and including 1288 km of silt coastline and 27,778.30 km² of mangroves [48]. The study area has a southern subtropical monsoon climate. The southwest monsoon and southeast trade winds prevail in summer, which is humid and rainy, with many typhoons. In winter, northerly and northeasterly winds prevail, and rainfall is scarce. The average annual temperature in the area is 21.9 °C, and the average annual temperature in the coastal area is higher than that in the northern inland zone. Influenced by the monsoon climate conditions, the province has strongly seasonal rainfall, rain days, rainfall, and uneven distribution, with an average annual rainfall of 1300–2500 mm.

Under the combined influence of crustal movement, magma movement, folding, and fracture tectonics, the territory has complex types of landforms, including mountains, hills, terraces, plains, and valleys, The mountains in the west, northeast, and north of the region are high, while the central and southern coastal areas are mostly hills, terraces or plains. In general, Guangdong Province is topographically high in the north and low in the south. In the investigation region, the river network area is, for the most part, covered by Quaternary sediments. At present, 80% of the water supply sources in the Zhanjiang, Lufeng, and Huiyang city areas come from groundwater. Due to the long-term, concentrated and massive exploitation of medium and deep pressurized water, the medium and deep pressure water has changed from the original one-way net flow to a runoff pattern of convergence from the funnel around to the center of the funnel, forming an artificially exploited flow field. This regional water-level landing funnel is gradually expanding in the direction of the northwest and southeast recharge zone areas and the direction of the East China Sea drainage area. The rivers in Guangdong are located in the transition area between the low mountains of central Guangdong and the Pearl River Delta. The coastal area is mostly plain, interspersed with low hills and mountains, especially in Zhanjiang, Lufeng, and Huiyang. The terrain slopes from northeast to southwest and the stratified structure of the landscape is obvious. The northern part is dominated by mountains and hills, the central part by terraces, and the southern and western parts by plains. The river system in the territory is well developed, with many rivers of various sizes and a vast water area. The southern river network area is in a tidally influenced area, with high runoff and strong tidal action. Groundwater is essentially renewed by the precipitation of vertical penetration, and when it is in a flood period, the horizontal progression of waterways occurs. The groundwater resources in the coastal cities of Guangdong have been exploited for more than 50 years. Due to the massive exploitation of middle and deep groundwater over a long period, the groundwater level has fallen significantly, triggering a variety of hydrogeological and environmental issues such as water resource depletion, ground subsidence, and seawater intrusion [49].

3. Materials and Methods

3.1. Sampling and Test

Thirty-nine groundwater and two seawater samples were collected from the shallow aquifer (<100 m) in eight cities Zhanjiang (ZJ), Maoming (MM), Yangjiang (YJ), Zhuhai (ZH), Huizhou (HZ), Shanwei (SW), Shantou (ST), and Jieyang (JY), along the Guangdong coastline in October 2019. Twenty-one water samples within 1 km of the coastline were considered to be near-sea samples (in the figures, the abbreviation “N” indicates near-sea samples and “F” indicates far-sea samples). The sampling locations are shown in Figure 1. EC, pH, TDS, and different parameters of the groundwater sample were measured in the field using YSI Professional Plus (YSI, USA). The groundwater samples were collected and stored in polyethylene bottles. To eliminate the effects of stagnant water, the wells were pumped two to three times before being collected in 150 mL polyethylene bottles, which were rinsed 2–3 times with nitric acid, after which they were washed with deionized water and dried. The bottles were thoroughly washed three times with well water from the sampling site before collecting the groundwater samples, after which three separate bottles were collected at each sampling site. During sampling, we filled the entire bottle and sealed it with adhesive tape. One bottle was sealed directly for the major ions and halogenated elements, one bottle was filtered through a membrane and sealed for the hydroxide isotopes, and another bottle was filtered through a membrane and filtered with concentrated nitric acid to pH < 2 for the metal elements. The second group of water samples was used as a blank control after the hydroxide isotopes had been measured to allow the extent of contamination caused by improper sampling to be seen. They were labeled, stored, and transported to the laboratory for chemical analysis.

The indexes, such as Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄²⁻, NO₃⁻, F⁻, Cl⁻ and Br⁻ were tested by ICS-6000 (Thermo Fisher, Waltham, MA, USA), and the detection limit was 0.01 mg/L. HCO₃⁻ was determined by acid-base indicator titration, and the detection limit was 1 mg/L. The oxygen and hydrogen stable isotope values were analyzed by LGR liquid water isotope laser spectroscopy (MAT 253 PLUS, Thermo Fisher, USA) from the Institute of Oceanology, Chinese Academy of Sciences, and calculated according to the Vienna Standard Mean Ocean Water (V-SMOW). The analytical accuracies of the long-term standard measurements of δD and δ¹⁸O were ±0.2‰ and ±0.6‰, respectively. The descriptive statistics of the groundwater chemistry data use the criteria for evaluation indicators of groundwater utilizing their concentrations in mg/L.

3.2. Multivariate Statistical Analysis

Multivariate statistical methods can provide inferred information of cause-and-effect relationships. The method of correlation analysis has been effectively applied to extract variables and the results obtained show that the factors controlling the chemical processes in groundwater are anthropogenic or natural influences [50,51,52]. Descriptive analyses of hydrogeochemical data and graphical representations were used in this paper to investigate hydrogeochemical characteristics, ionic generators, and associated major mechanisms influencing hydrogeochemical processes by using the SPSS software version 26.0 (SPSS, 2019).

Concentrations of some indicators in groundwater above the allowable values probably lead to poor-quality groundwater. We need to know which ones are the main impact indicators. It is known that PCA is a useful tool for data reduction, and parameters on the same PC with positive loadings mean the same origin or similar geochemical behaviors [53,54]. Thus, in this study, PCA was used to reduce the indicators and extract the main impact indicators (in the same PC with results of the groundwater quality assessment) which are responsible for the poor-quality groundwater. In the PCA, the rotation of principal components (PCs) was carried out using the varimax method, and only PCs with eigenvalues that exceed one were retained for analyses. This study selected the maximum absolute PC loading of one index to evaluate the relationships between the PCs and indicators.

The principal components (PCs) issued from different exploratory data analyses can be automatically described by quantitative or categorical variables [55,56,57,58,59,60]. For the purposes of this study, TDS (a measure of groundwater salinity) was entered as a quantitative supplementary variable. In addition to the PCA, ion ratios were used to find the inter-relationships of the chemical parameters for samples in each cluster.

3.3. Analytical Method

Multivariate statistical methods can provide inferred information on cause-and-effect relationships. The method of correlation analysis has been effectively applied to extract variables and the results obtained show that factors controlling the chemical processes in groundwater are anthropogenic or natural influences [50,51,52]. Descriptive analyses of hydrogeochemical data and graphical representations were used in the paper to investigate hydrogeochemical characteristics, ionic generators, and associated major mechanisms influencing hydrogeochemical processes by using the SPSS software version 26.0 (SPSS, 2019).

WQI is a significant parameter for establishing groundwater quality and its suitability for the purpose of consumption. WQI is an approach of rating that provides the composite influence of individual parameters on the comprehensive quality of water for human utilization. The standards for drinking purposes are as recommended by the WHO [4]. The calculation of WQI is in three steps.

Firstly, each of the nine parameters was weighted by their relative significance in the overall water quality for consumption. A maximum weight of 5 was assigned to the parameters, including the nitrate, fluoride, chloride, and total dissolved solids, because of their importance in evaluating water quality. Bicarbonate with a minimum weight of 1 is a negligible factor in the evaluation of water quality. Other items such as sodium, calcium, potassium, and magnesium were assigned weights ranging from 1 to 5 depending on their importance in the determination of water quality.

Secondly, the relative weight ($W_i$) was calculated in the present study as:

$$W_i=w_i/{{\displaystyle \sum }}_{i=1}^n{w}_i$$

(1)

The relative weight is $W_i$. The weight of each parameter $\mathrm{is} w_i$. The number of parameters is n.

Table 1. Relative weights of chemical parameters.

Parameters (mg/L)	WHO Standards (2022)	${\mathit{w}}_{\mathit{i}}$	Relative Weight ${\mathit{W}}_{\mathit{i}}={\mathit{w}}_{\mathit{i}}/{{\displaystyle \sum }}_{\mathit{i}=1}^{\mathit{n}}{\mathit{w}}_{\mathit{i}}$
TDS	500	5	0.128
Bicarbonate	500	1	0.0256
Chloride	250	5	0.128
Sulphate	200	5	0.128
Nitrate	45	5	0.128
Calcium	75	3	0.0769
Magnesium	30	3	0.0769
Sodium	200	5	0.128
Potassium	12	2	0.0523
Fluoride	1.5	5	0.128

shows calculated relative weight ($W_i$) values of each parameter.

Furthermore, by dividing the concentration in each water sample by its own criteria according to the guidelines outlined in WHO [4], a quality rating scale ($q_i$) is assigned for each parameter, and the result is multiplied by 100:

$$q_i=\left(C_i/S_i\right)\times 100$$

(2)

The quality rating $\mathrm{is} “q_i”$. The concentration of each chemical parameter in each water sample in milligrams per liter $\mathrm{is} “C_i”$. “$S_i”$ is the China drinking water standard based on the recommendations of the WHO [4] (Table 1), the units of which chemical parameter are mg/L.

When calculating the WQI, first determined for each chemical parameter is the “SI”, which is then used to determine the WQI according to the following equation:

$$S{I}_i=W_i\times q_i$$

(3)

$$\mathrm{WQI}=\sum S{I}_i$$

(4)

The sub-index of the ith parameter is $S{I}_i$. According to WQI values, five classifications can be allocated: undrinkable (WQI ≥ 300), very poor (200 ≤ WQI < 300), poor (100 ≤ WQI < 200), good (50 ≤ WQI < 100), and excellent (WQI < 50) [53].

The risk assessment investigation of the toxic contaminant of drinking water in terms of a non-carcinogenic health hazard is based on the risk level [26]. In general, the oral pathway of exposure covers a great threat to health and other routes of dermal and oral contact. Taking this into consideration, a health risk assessment of non-carcinogenic contaminants were selected [30,31,32,33,34,35]. According to the research, the health hazard of dermal and oral connection was evaluated individually for children, females and males. The non-carcinogenic risk from dermal and oral connection was calculated and listed hereafter [12,13,14]:

For drinking water intake:

$$\mathrm{CDI}=\frac{\mathrm{C}\times \mathrm{IR}\times \mathrm{ED}\times \mathrm{EF} }{\mathrm{ABW}\times \mathrm{AET}}$$

(5)

$${\mathrm{HQ}}_{\mathrm{Oral}}=\frac{\mathrm{CDI} }{\mathrm{RfD}}$$

(6)

The chronic daily intake is “CDI” (mg/kg/day). The content of nitrate is “C” (mg/L). The daily ingestion rate of water is “IR” (L/day), the values of which are 1 L/day, 2.5 L/day and 2.5 L/day for children, males, and females, respectively. The exposure duration is “ED” (year), the values of which are 64 years for males, 67 years for females, and 12 years for children [38,58]. The exposure frequency is “EF” (days/year), the values of which are 365 days/year for everyone [43]. The average body weight is ABW, the values of which are 65 kg, 55 kg and 15 kg for males, females and children, respectively. The hazard quotient is demonstrated as HQ. The average exposure time is “AET” (days), the values of which are 23,360 days, 24,455 days, and 4380 days for males, females and children, respectively. The reference portion of pollutants is “RfD” (mg/kg/day) with 1.6 mg/kg/day for NO₃⁻ according to US EPA [43].

In the case of a non-carcinogenic hazard resulting from dermal contact, the calculation formula should be chosen as below [5,46]:

$$\mathrm{DAD}=\frac{\mathrm{TC}\times \mathrm{C}\times \mathrm{Ki}\times \mathrm{ED}\times \mathrm{EV}\times \mathrm{CF}\times \mathrm{EF}\times \mathrm{SSA} }{\mathrm{AET}\times \mathrm{ABW}}$$

(7)

$${\mathrm{HQ}}_{\mathrm{Deraml}}=\frac{\mathrm{DAD} }{\mathrm{RfD}}$$

(8)

$${\mathrm{HI}}_{\mathrm{total}}={\displaystyle \sum }_1^{\mathrm{n}}\left({\mathrm{HQ}}_{\mathrm{Oral}}+{\mathrm{HQ}}_{\mathrm{Dermal}}\right)$$

(9)

The dose of absorbed dermal is “DAD” (mg/kg × day). The time of contact is “TC” (h/day). The parameters of dermal adsorption are “Ki” (cm/h), and the conversion factor is “CF”. The values of “TC”, “Ki” and “CF” are regarded as 0.4 h/day, 0.001 cm/h, and 0.001, respectively [43]. The frequency of showering is “EV” (times/day) and its value is considered as 1 time in a day. The area of skin surface is “SSA” (cm²), the value of which is defined as 12,000 cm², 16,600 cm² and 16,600 cm² for children, males and females, respectively (USEPA, 2017). The value of HI can represent the non-carcinogenic health risk, which is a hazard index. A HI > 1 indicates that non-carcinogenic health risk is potential, while HI < 1 means that the health risk impersonated by non-carcinogenic substances is within a satisfactory level [43].

4. Results and Discussion

4.1. Hydrochemical Compositions

The statistical results of the main components of groundwater chemistry in the coastal zone of Guangdong are shown in

Table 2. Statistical characteristics of the major chemical composition of groundwater in Guangdong.

Parameters	Units	Minimum	Maximum	Mean	SD	CV	DWQI
Na+	mg/L	14.03	1590	121.97	296.47	2.43	200
K+	mg/L	1.04	264.8	25.7	50.93	1.98	12
Ca+	mg/L	0.8	282.7	38.87	56.67	1.46	75–200
Mg2+	mg/L	0.46	275.6	20.7	57.77	2.79	30–100
HCO3−	mg/L	15.62	917.6	165.34	161.99	0.98	-
Cl−	mg/L	3.64	18,441.76	1009.5	4076.78	4.04	250–1000
SO42-	mg/L	0.74	2238.06	137.82	464.21	3.37	150–400
NO3−	mg/L	0.12	115.12	30.55	32.89	1.08	45
TDS	mg/L	27.95	31,245	1935.26	6808.76	3.52	500–2000
EC	μs/cm	0.04	48.16	2.98	10.48	3.52	-
ORP	mv	−132.6	−7.5	−75.23	30.76	−0.41	-
pH	/	5.51	8.2	7.18	0.58	0.08	6.5–8.5
δD	‰	−52.68	−27.99	−39.26	10.61	−0.27	-
δ18O	‰	−7.36	−4.33	−5.97	0.67	−0.11	-

. The mean value of Cl⁻ content in Guangdong was 1009.50 mg/L, indicating that the groundwater in the coastal zone of Guangdong is affected by salinization. The pH values of groundwater samples in the study area ranged from 5.15 to 8.20, with the mean pH value of 7.18 in Guangdong Province, which was weakly alkaline, whereas the average TDS concentration was 1935.26 mg/L (Table 2). The average concentrations of pH and TDS in groundwater exceeded China’s permissible groundwater quality standards [4] of 7 and 1000 mg/L, respectively. In accordance with the classification, most of the groundwater was classified as alkaline-freshwater and alkaline-saline water. It is worth noting that the shallow groundwater in the Pearl River Delta is generally characterized by variational TDS and weak acidity, with Na⁺ and Ca²⁺ as the main cations and Cl⁻ and HCO₃⁻ as the main anions in the groundwater. The Pearl River Delta is prone to seawater intrusion, causing the salinization of water bodies. As the climate warms, the sea level rises and salty tides occur dramatically, resulting in acidification of groundwater bodies in the Pearl River Delta region.

4.2. Hydrochemical Type

The Piper diagram [54] can reflect the water chemistry characteristics and types of water bodies. From Figure 2, it can be observed that the main anions and cations in the groundwater of the Guangdong coastal zone are found in the bottom corner of the Piper diagram, and their distribution is relatively concentrated, indicating that the ionic composition of most groundwater has a high consistency. The hydrochemical characteristics in the coastal zone also indicate that the distance from the sea influences the chemical composition in the Guangdong coastal zone.

The main types of groundwater chemistry in the Guangdong coastal zone are Mg-HCO₃ and Na-Cl·HCO₃. From land to sea, the water chemistry of the study area progressively varies from Ca-HCO₃ to Na·Mg-Cl. The freshwater chemistry of the Guangdong coastal zone is Na·Ca-Cl and Ca·Mg-HCO₃, while the saline water chemistry is Na-Cl·SO₄. This is consistent with Huang Guan’s conclusion, who concluded that the main hydrochemical type of the Pearl River Delta was Ca-HCO₃ and Ca-Cl types.

4.3. Factors Controlling the Hydrochemical Characteristics of Groundwater of the Guangdong

Gibbs diagrams [55] were constructed by the comparability proportions of TDS divided by Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻). This methodology has been broadly used to recognize hydrogeochemical evolution processes, which include evaporation, mineral weathering, and precipitation.

As shown in the Gibbs diagram (Figure 3), the result shows that both the cation and anion drawn in the middle of the figure signify that mineral weathering is the main process that controls the groundwater characteristics. For the cation, more than half of the samples are out of the three-control-end membership zone, while for the Cl⁻/(Cl⁻ + HCO₃⁻), approximately 90–95% of the samples fall in the mineral weathering domain and 5–10% of samples fall between the mineral weathering and evaporation region. There is a mixed controlling mechanism here, and we should take into account the element of human activities affecting the area.

In general, atmospheric precipitation varies by altitude, latitude, temperature, and proximity to the coastline, and is generally less than −60‰ (SMOW). When atmospheric precipitation forms groundwater, unlike δ¹⁸O, the rocks contain few hydrogen minerals and have low δD values, so isotope exchange reactions have little effect on the δD values of the water. Since the δD difference between seawater and atmospheric precipitation is large, δD and δ¹⁸O is an important basis for determining the origin of formation of groundwater. The global meteoric water line (GMWL) is a linear correlation line between the hydrogen-oxygen stable isotopes of atmospheric precipitation on a global scale. The hydrogen-oxygen isotope ratio line, compared to the precipitation line, can indicate the origin of local water sources. The values of δD and δ¹⁸O are shown in Table 2, while the saline water and seawater samples had relatively high δD and δ¹⁸O values, as shown in Figure 4. From (Figure 4a), there is a large variation in hydrogen-oxygen isotope content of groundwater in Guangdong and the values are mainly distributed between the global meteoric water line (GMWL) and the local meteoric water line (LMWL), indicating that atmospheric precipitation is the major supplier of groundwater replenishment.

The relationship between δ¹⁸O and Cl⁻ was also used to determine the mixing trajectory and proportions of different waters. Figure 4b demonstrates that the majority of the groundwater samples are distributed as the freshwater states. The water samples in Guangdong are firmly dispersed on the blending line among freshwater and seawater. Furthermore, the seawater blending proportion ranges from 0% to 10% and the saline mixing proportion is from 60% to 90%, which also demonstrates that part of the groundwater samples in this area have been affected by seawater intrusion.

As shown in Figure 5a, most of the water sample points were distributed near the y = x relationship line, implying that the dissolution of halite is the primary hydrogeochemical process influencing the synthetic parts of groundwater [3]. Figure 5b shows that the water samples were mainly drawn in the upper side of the y = x relationship line, indicating that silicate weathering was the main factor driving the hydrochemical characteristics of groundwater. Figure 5c shows that the water samples were mainly located on the upper side of the 1:1 line, indicating that other sources of Ca²⁺ may exist, such as cation exchange. As shown in Figure 5d, the water samples were mainly located around the y = x relationship line, thereby indicating that Ca²⁺ and SO₄²⁻ in the groundwater of Guangdong mainly originated from the dissolution of gypsum. The ratio of Ca²⁺/Mg²⁺ can be utilized to investigate the impact of carbonate (calcite) and silicate weathering on groundwater hydrochemical characteristics [39]. Most water samples fall below the y = 0.5x relationship line, which is shown in Figure 5e, subsequently showing that a great deal of water samples are affected by silicate weathering. Cation exchange occurs widely between groundwater and rock minerals [56]. A remarkable influence of cation exchange on the chemical characteristics of groundwater would result in the relationship of Na⁺ + K⁺ − Cl⁻ to HCO₃⁻ + SO₄²⁻ − Ca²⁺ − Mg²⁺ being 1:1 [57,58]. As shown in Figure 5f, most of the water sample points fell on the left side of the 1:1 line, with a few points falling on the line, thereby showing that although cation exchange had an influence on the groundwater of Guangdong, it was not the main dominant factor and seawater may be a process affecting the hydrochemical characteristics of groundwater in Guangdong. The ion contents of groundwater near the coast are higher, indicating that the source is influenced by seawater intrusion.

4.4. Possible Sources of Chemical Composition

The regional hydrogeochemical characteristics are affected by many factors, while multivariate statistical methods can infer the main mechanisms affecting regional groundwater chemistry based on hydrochemical variables. As a quantitative and independent approach for groundwater classification, principal component analysis (PCA) was utilized for grouping groundwater samples and explaining relationships between groundwater samples and chemical parameters. In this study, PCA was adopted to analyze 12 chemical variables (TDS, EC, ORP, pH, Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃⁻, SO₄²⁻, Cl⁻, and NO₃⁻) in 39 samples in order to explore the main factors and mechanisms affecting the groundwater chemical characteristics and water quality in Guangdong.

As shown in

Table 3. Principal component eigenvalues and variance contribution rates and principal component load matrix.

Component	Initial Eigenvalue			Extract the Sum of Load Squares				Component
	Total	Variance Percentage	Cumulate %	Total	Variance Percentage	Cumulate %		1	2
1	8.519	70.993	70.993	8.519	70.993	70.993	Cl	0.987	0.110
2	2.000	16.663	87.656	2.000	16.663	87.656	NO3	-	−0.722
3	0.684	5.698	93.354				SO4	0.981	0.103
4	0.613	5.111	98.465				HCO3	0.696	-
5	0.077	0.643	99.108				Na	0.959	-
6	0.057	0.473	99.581				K	0.956	-
7	0.029	0.244	99.825				Ca	0.960	-
8	0.015	0.128	99.953				Mg	0.989	-
9	0.004	0.035	99.988				TDS	0.990	0.107
10	0.001	0.012	100.000				EC	0.990	0.106
11	0	0	100.000				ORP	0.471	−0.822
12	0	0	100.000				PH	−0.430	0.854

Extraction method: principal component analysis method.

, the two principal components were extracted. The eigenvalue of principal component 1 was 8.519 with a variance contribution of 70.993%, while the eigenvalue of principal component 2 was 2.000 with a variance contribution of 16.663%. The cumulative contribution of variance was as high as 87.656%. The hydrochemical composition of groundwater in the study area was mainly determined by principal components 1 and 2 (Table 3).

Principal component 1 was mainly TDS, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, and HCO₃⁻, with load matrix of 0.990, 0.959, 0.956, 0.960, 0.989, 0.987, 0.981, and 0.696, respectively, which explained that they are similar in terms of causes, and as discussed above, it can be concluded that principal component 1 can be an explanation of silicate weathering and seawater intrusion. According to the classification results, only three samples in Shantou were saline water, which should be paid attention to.

Principal component 2 has a strong negative loading (>0.70) with NO₃⁻, ORP, and pH with a load matrix of 0.722, 0.822, and 0.854 mm, respectively. NO₃⁻ mainly explains pollution caused by industrial and agricultural activities, while ORP and pH mainly explain fertilizer pollution and chemical plant pollution from agricultural activities. Far less fertilizer is used in urbanized areas than in agricultural areas, and the dose of NO₃⁻ in urban fertilizers is commonly less than 20 mg/L, indicating that the contribution of fertilizers to NO₃⁻ in granular aquifers in urbanized areas is negligible. Therefore, fertilizer is not the main source of NO₃⁻ in granular aquifers, because most of the high- NO₃⁻ groundwater occurred in urbanized areas. Therefore, the principal component 2 explains the degree of pollution caused by anthropogenic activities.

4.5. Anthropogenic Inputs

Based on the statistical results, nitrate pollution has occurred in Shantou, Maoming, and Zhanjiang due to urbanization and industrialization. In accordance with the National Standardization Administration of China (GB/T14848-2017), the safe concentration of nitrate in drinking water is under 20 mg/L. The average of the nitrate compounds is 29.77 mg/L, with a maximum value of 155.12 mg/L and minimum value of 0.01 mg/L in Guangdong. The average value is higher than the standard value of 20 mg/L, indicating the presence of slight pollution.

As Figure 6 shows, nitrate pollution in freshwater groundwater in the study area is more serious than in saline water. The proportion of groundwater samples exceeding the safety standard of nitrate in drinking water is 75.5% in Maoming (3 of 4 groundwater samples), 50% in Shantou (7 of 14 groundwater samples), 66.7% in Zhanjiang and Zhuhai, 33.3% in Yangjiang, 25% in Lufeng, and no pollution in Huiyang. Furthermore, the nitrate concentration in Maoming ranges from 8.67 to 111.56, with an average value of 51.60. The nitrate concentration in the groundwater of Shantou and Zhangjiang ranges from 0.19 to 115.12 with an average value of 34.38, and from 0.49 to 98.77 with an average value of 42.64, separately. NO₃⁻ contamination (>20 mg/L) exists in nineteen samples of all the first ten of them were more than 50 mg/L. The level of nitrate contamination in Guangdong is indicated as below: Shantou < Zhanjiang < Maoming.

Increasing urbanization, industrialization, and growing human populations have resulted in progressively negative consequences on the groundwater environment. Contaminated groundwater commonly displays higher molar proportions of NO₃⁻/Na⁺ and Cl⁻/Na⁺. According to Figure 7, it leads to the conclusion that groundwater NO₃⁻ in Guangdong coastal area mainly affected by agricultural activities and domestic sewage. The relatively higher NO₃⁻ concentration in Guangdong has been identified as caused by agricultural production, which is non-point-source water pollution. The potential non-point sources of nitrogen from agricultural production include manure and fertilizer. Consequently, it is suggested that control of groundwater contamination ought to be a significant point of groundwater management in the Guangdong coastal zone.

4.6. Drinking Water Quality

In this paper, we used the criteria set out by the drinking water standards of WHO [4] to assess the drinking quality of water. The water quality evaluation used 39 Guangdong sample sites and classified the water quality into 5 classes according to the WQI value: Class I water (WQI < 50), Class II water (50 ≤ WQI < 100), Class III water (100 ≤ WQI < 200), Class IV water (200 ≤ WQI < 300), and Class V water (WQI ≥ 300). The results showed that 64.1% of the samples were ranked as Class I, these sampling sites were far away from the coastal zone and less affected by seawater; 28.2% of the samples were ranked as Class II, 3% of the samples were ranked as Class III, and 5.1% of the samples were ranked as Class V water. There are two sample sites (ST1-1 and ST3-4) in Shantou, Guangdong, with WQI values of 2132.57 and 2161.40, which have more serious seawater pollution, and an ST2-1 value of 175.01, which also indicates poor water quality not suitable for drinking (Figure 8). In addition, some of the samples in Maoming, Zhanjiang, and Maoming were not suitable for drinking. More attention should be paid, especially in Maoming, Zhanjiang, and Shantou, where both the salinity and nitrate contamination of some samples exceed the drinking water standards of WHO [4]. It is well-known that high concentrations of NO₃⁻ and Cl⁻ commonly occur in the groundwater of cultivated land due to the amount of application of chemical fertilizers, such as nitrogenous fertilizer and muriate of potash [19,20,21], especially in shallow groundwater with high permeability of the vadose zone. Further, groundwater with elevated NH₄⁺ and NO₃⁻ was also reported in the PRD, and the natural source of groundwater containing NH₄⁺ in granular aquifers and anthropogenic sources of groundwater containing NO₃⁻ have also been discussed. Therefore, the fissured aquifer should be protected and exploited for drinking and irrigation purposes.

4.7. Human Health Risk Assessment

HQ analyzation was adopted for health risk assessment in the Guangdong coastal zone. Only when the values of HQ were less than 1, there were minor hazards to human health. In this study, health risk evaluation of nitrate pollution for children, males, and females was performed depending on USEPA [43] and the consequences are introduced in

Table A1. Nitrate results of non-carcinogenic risk through drinking water intake and dermal contact.

Non-Carcinogenic Risk
Samples	HQOral			HQDermal			HItotal
	Males	Females	Children	Males	Females	Children	Males	Females	Children
HY 1-1	0.0075	0.0088	0.0129	0.0000	0.0000	0.0001	0.0075	0.0088	0.0130
HY 1-3	0.0031	0.0037	0.0054	0.0000	0.0000	0.0000	0.0031	0.0037	0.0054
LF1-1	0.5901	0.6974	1.0229	0.0016	0.0019	0.0049	0.5917	0.6993	1.0278
LF1-2	0.1563	0.1847	0.2708	0.0004	0.0005	0.0013	0.1567	0.1851	0.2721
LF1-3	0.3567	0.4216	0.6183	0.0009	0.0011	0.0030	0.3577	0.4227	0.6213
LF1-4	0.1142	0.1349	0.1979	0.0003	0.0004	0.0010	0.1145	0.1353	0.1989
MM 1-1	2.6817	3.1693	4.6483	0.0071	0.0084	0.0223	2.6889	3.1777	4.6706
MM 1-2	0.5724	0.6764	0.9921	0.0015	0.0018	0.0048	0.5739	0.6782	0.9968
MM 1-3	0.2084	0.2463	0.3613	0.0006	0.0007	0.0017	0.2090	0.2470	0.3630
MM 1-4	1.4993	1.7719	2.5988	0.0040	0.0047	0.0125	1.5033	1.7766	2.6112
ST1-1	0.5007	0.5918	0.8679	0.0013	0.0016	0.0042	0.5021	0.5933	0.8721
ST1-2	0.0046	0.0054	0.0079	0.0000	0.0000	0.0000	0.0046	0.0054	0.0080
ST1-3	0.1601	0.1892	0.2775	0.0004	0.0005	0.0013	0.1605	0.1897	0.2788
ST1-4	1.6700	1.9736	2.8946	0.0044	0.0052	0.0139	1.6744	1.9788	2.9085
ST1-5	1.5776	1.8645	2.7346	0.0042	0.0050	0.0131	1.5818	1.8694	2.7477
ST1-6	1.7130	2.0244	2.9692	0.0045	0.0054	0.0143	1.7175	2.0298	2.9834
ST2-1	0.1430	0.1690	0.2479	0.0004	0.0004	0.0012	0.1434	0.1695	0.2491
ST2-2	0.0712	0.0841	0.1233	0.0002	0.0002	0.0006	0.0713	0.0843	0.1239
ST2-3	0.0139	0.0165	0.0242	0.0000	0.0000	0.0001	0.0140	0.0165	0.0243
ST3-1	1.2113	1.4315	2.0996	0.0032	0.0038	0.0101	1.2145	1.4353	2.1097
ST3-2	1.0750	1.2705	1.8633	0.0029	0.0034	0.0089	1.0779	1.2738	1.8723
ST3-3	2.7673	3.2705	4.7967	0.0073	0.0087	0.0230	2.7747	3.2791	4.8197
ST3-4	0.3183	0.3761	0.5517	0.0008	0.0010	0.0026	0.3191	0.3771	0.5543
ST3-5	0.3425	0.4048	0.5938	0.0009	0.0011	0.0029	0.3435	0.4059	0.5966
YJ 1-4	0.5103	0.6031	0.8846	0.0014	0.0016	0.0042	0.5117	0.6047	0.8888
YJ 2-1	0.1089	0.1287	0.1888	0.0003	0.0003	0.0009	0.1092	0.1290	0.1897
YJ 2-2	0.7180	0.8486	1.2446	0.0019	0.0023	0.0060	0.7199	0.8508	1.2506
YJ1-1	0.1793	0.2119	0.3108	0.0005	0.0006	0.0015	0.1798	0.2125	0.3123
YJ1-2	0.1909	0.2256	0.3308	0.0005	0.0006	0.0016	0.1914	0.2262	0.3324
YJ1-3	0.0029	0.0034	0.0050	0.0000	0.0000	0.0000	0.0029	0.0034	0.0050
ZH 1-1	1.8115	2.1409	3.1400	0.0048	0.0057	0.0151	1.8163	2.1466	3.1551
ZH 1-2	0.1154	0.1364	0.2000	0.0003	0.0004	0.0010	0.1157	0.1367	0.2010
ZH 1-3	1.3877	1.6401	2.4054	0.0037	0.0044	0.0115	1.3914	1.6444	2.4170
ZJ 1-1	1.0505	1.2415	1.8208	0.0028	0.0033	0.0087	1.0533	1.2448	1.8296
ZJ 1-2	2.3743	2.8060	4.1154	0.0063	0.0075	0.0198	2.3806	2.8134	4.1352
ZJ 1-3	0.0002	0.0003	0.0004	0.0000	0.0000	0.0000	0.0002	0.0003	0.0004
ZJ 2-1	0.6988	0.8259	1.2113	0.0019	0.0022	0.0058	0.7007	0.8280	1.2171
ZJ 2-2	0.9894	1.1693	1.7150	0.0026	0.0031	0.0082	0.9921	1.1724	1.7232
ZJ 2-3	0.0118	0.0139	0.0204	0.0000	0.0000	0.0001	0.0118	0.0140	0.0205
Min	0.0002	0.0003	0.0004	0.0000	0.0000	0.0000	0.0002	0.0003	0.0004
Max	2.7673	3.2705	4.7967	0.0073	0.0087	0.0230	2.7747	3.2791	4.8197
Mean	0.7156	0.8457	1.2404	0.0019	0.0022	0.0060	0.7175	0.8479	1.2463

. We can notice that the HQ_Dermal results for NO₃⁻ were very low in three unique groups with some data below 0, while HQ_Oral of NO₃⁻ changes from 0.0004 to 4.7967 with its mean as 1.2404 for children, 0.0003 to 3.2705 with an average of 0.8457 for females, and 0.0002 to 2.7673 with a mean value of 0.7156 for males. This finding discloses that the request for non-carcinogenic hazard quotient (HQ) under dermal and oral radiation was definitely examined as HQ_Dermal < HQ_Oral (Appendix A). We found a high quotient of nitrate occur in Shantou, Maoming and Zhanjiang, which was accorded with the result of WQI.

As noted above, under USEPA risk evaluation instructions, the limit of non-carcinogenic risk we can accept is ≤1, which means HI ≤ 1. Assuming that the worth of the HI is more noteworthy than 1, the likelihood of antagonistic human well-being risk due to openness is very high. For the non-carcinogenic risk, the HI_Total values of NO₃⁻ of groundwater in the investigated region ranged from 0.0004 to 4.8197 (mean of 1.2463), 0.0003 to 3.2791 (mean of 0.8479), and 0.0002 to 2.7747 (mean of 0.7175) for children, females and males, respectively (Figure 9). It should be noted that out of 39 groundwater samples, the nitrate exposure levels in drinking water in 16, 13 and 12 communities for children, females, and males expose these age groups to acute nitrate poisoning in the study area. All the more explicitly, the results demonstrate that the children in the study area have more risk to non-carcinogenic impacts because of the higher level of nitrate in drinking water. This was significantly to numerous different researchers [57,58,59,60] who have profoundly observed that children are more powerless to chronic non-carcinogenic risks on account of their more modest body weight. Excessive intake of nitrates can lead to harmful physiological reactions, even in small orders of magnitude, such as high blood pressure and poisoning [24,29,50]. What is more, unsatisfactory non-carcinogenic values were accounted for partial samples of Zhanjiang, Shantou, Maoming and Zhuhai, which indicated that the groundwater was unsuitable for direct drinking in these areas. Therefore, fundamental measures to limit nitrate pollution should be initiated by local government to ensure a safe drinking water supply.

5. Conclusions

The groundwater in the Guangdong coastal zone is weakly alkaline, with mean value concentrations of TDS of 1935.26 mg/L and Cl of 1009.5 mg/L. The hydrochemical categories of groundwater in the Guangdong coastal zone are Mg-HCO₃ and Na-Cl·HCO₃, and the trend of evolution from land to sea is Ca-HCO₃→Na·Ca-Cl.

The Gibbs diagram shows that the groundwater composition was mainly responsible for mineral weathering, and the ratio of ions indicates that silicate weathering and the dissolution of gypsum were the dominant origins of salt of the groundwater. The result of PCA shows that rock weathering, groundwater salinization, and anthropogenic activities have affected the water quality. According to this investigation, the seawater intrusion occurs in Shantou, while the nitrate pollution mainly occurs in Maoming, Zhanjiang, and Shantou.

Based on water quality index (WQI) and hazard quotients (HQ), most of the samples were labelled as good water quality, except for some spots in Shantou, Maoming, and Zhanjiang. The HQ result exposed that the health risk of direct intake is more serious than that of dermal exposure, and compared with the adult, the HI value for children was much higher, with the average exceeding 1. This indicates that women and men are at a significantly lower health risk than children in the Guangdong coastal areas.

These results indicate that the pollution in the Guangdong coastal area caused by seawater intrusion as well as nitrate input should be taken seriously. More importantly, rational utilization of groundwater resources and the monitoring and evaluation of groundwater quality, in which the control and analysis of groundwater contamination are essential, should be taken seriously by local governments.