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• Published: 26 July 2023

Assessment of groundwater hydrochemistry, water quality, and health risk in Hainan Island, China

• Qingqin Hou 1 , 2   na1 ,
• Yujie Pan 3   na1 ,
• Min Zeng 4 ,
• Simiao Wang 5 ,
• Huanhuan Shi 6 ,
• Changsheng Huang 4 &
• Hongxia Peng 1 , 7 , 8  

Scientific Reports volume  13 , Article number:  12104 ( 2023 ) Cite this article

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• Environmental chemistry
• Environmental impact
• Environmental sciences

Groundwater is an important source of water for human sustenance. The determination of groundwater quality at island sites is an urgent priority in China, but there are lacking systematic reports relating to them. Here, 63 groups of groundwater samples were collected and analyzed of Hainan Island. The groundwater in the study area is weakly alkaline, mainly comprising hard and soft freshwater. The predominant anions and cations are HCO 3 − , and Ca 2+ and Na + , respectively, and the main water chemistry types are HCO 3 –Cl–Na and HCO 3 –Cl–Na–Ca. The chemical evolution of groundwater is mainly affected by water–rock interactions, cation exchange, and human activity. The groundwater is mostly of high quality and, in most areas, is suitable for drinking and irrigation. Contrastingly, the water quality in the west of the island is relatively poor. The spatial distribution of the risk coefficient (HQ) is consistent with the spatial variation in the NO 3 − concentrations in the groundwater. Notably, there are unacceptable health risks for different groups of people, with infants having the greatest level of impact, followed by children, teenagers, and adults. This study provides a valuable reference for the development and utilization of groundwater resources, as well as the improvement of aquatic ecological conditions on Hainan Island and other island areas worldwide.

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Introduction.

Groundwater is an important source of water for consumption, irrigation, and industrial use 1 , 2 , 3 , 4 . However, the improvement in people’s living standards and the degree of industrialization has resulted in a perpetual increase in the demand for water resources 5 , 6 , 7 , consequently leading to the over-exploitation of global groundwater resources, deterioration in water quality, and worsening of water security problems 8 , 9 , 10 , 11 . In particularly, groundwater resources face severe challenges, especially in island areas with more fragile natural ecosystems 12 , 13 , 14 . Most of the island areas have low rainfall, large evaporation, serious water and soil loss, and relatively lack of surface water resources. Therefore, the exploitation and utilization of groundwater is extremely important for the production and life of residents. The island groundwater system is an independent circulating system with limited supply sources. If water quality pollution is caused by natural factors and human activities, it may cause irreversible losses.

The chemical composition of groundwater is the result of its long-term interactions with the surrounding environment 15 . During groundwater formation and migration, physical and chemical interactions occur with the surrounding media, which affect the chemical composition of the water 16 , 17 . Simultaneously, groundwater is increasingly being affected by human factors 18 , 19 . Harmful substances produced by humans may enter groundwater, and the resulting pollution, which is spread via groundwater flows, can penetrate deep into the ground. In particular, NO 3 − produced by industrial production, agricultural activities and domestic sewage will enter shallow or even deep groundwater with rainwater or surface water, affecting the water quality and hydrochemical evolution process 20 . At present, nitrate pollution has become one of the major pollutants in the global groundwater and has caused potential health risks to residents 21 , 22 . The water quality index (WQI) simplifies the complex water quality index into a single value, which can be used to evaluate the water quality more intuitively and effectively 23 . At present, it has been widely used in the evaluation of drinking and irrigation water, and even innovatively used by some scholars to evaluate the suitability for industrial use. Nsabimana et al. and Li et al. used a new industrial water quality index (Ind WQI) model to determine the overall industrial water quality 24 . According to the determination of the main influencing factors in water quality assessments, the health risk assessment model and the assessment standard provided by the U.S. Environmental Protection Agency (USEPA) can be used to further assess risks to human health 25 .

Hainan Island is China’s second largest island, with one of the largest special economic zones and free trade ports, serving as the country’s major development strategy. Due to its location advantages and policy support, Hainan Island has undergone rapid modernization and the establishment of numerous industrial and agricultural parks. This rapid economic development has also increased pressure on residential water supplies and caused a series of ecological and environmental issues, including pollution, soil salinization, and seawater intrusion 26 , 27 . However, current research on groundwater on Hainan Island remains focused on hydro-chemical exploration, with a lack of comprehensive research on water quality evaluation and human health risk.

To address this research gap, the main goals of this study were to (1) analyze the groundwater hydro-chemical characteristics and the main controlling factors on Hainan Island; (2) evaluate the suitability of groundwater for drinking and irrigation; and (3) assess the risk of the nitrates in groundwater to human health. Our findings will help to better understand the chemical characteristics and quality of groundwater in this area of China, and provide a reference for the development and utilization of groundwater resources and the improvement of the aquatic ecological environment.

Materials and methods

Hainan Island is located at the southernmost tip of China (18° 10′–20° 10′ N and 108° 37′–111° 03′ E; Fig.  1 ). Hainan is surrounded by the sea, located in the tropics and subtropics, and receives abundant precipitation; however, the problems of regional and seasonal precipitation are prominent. High temperatures throughout the year drive strong evaporation. The long-term annual average rainfall in Hainan is 1750 mm, but it is unevenly distributed. The central and northeastern parts of the island receive more rain than the southwest regions. Hainan Island is low and flat, with raised topography in the center of the island, which limits the options for surface water storage. As China’s largest provincial-level special economic zone, the island has a growing population density and is experiencing rapid industrial development, which has resulted in high water demand, leading to a scarcity of water resources. At the end of the 1970s, the amount of groundwater exploitation in Hainan Island was only 290 million m 3 /a. After the construction of Hainan Island as a province, the amount of groundwater exploitation gradually increased due to economic development and technological progress. In 2004, the amount of groundwater exploitation increased to 515 million m 3 /a. Since then, due to the introduction of the national underground water pipe control policy and the enhancement of the development and utilization capacity of surface water, the mining capacity has gradually decreased, and the water supply of underground water in 2015 is still 274 million m 3 /a.

Groundwater sampling points on Hainan Island, China. The map was created using ArcGIS 10.8 ( https://www.esri.com/software/ArcGIS ).

According to the type of water-bearing medium and occurrence conditions, the groundwater in Hainan Island can be divided into five types: bedrock fissure water, pore-confined water of loose and semi-consolidated rock, volcanic rock pore fissure water, carbonate rock fissure karst water, and loose rock pore phreatic water. The main aquifer strata are Quaternary, Neogene, Cretaceous and Triassic. The groundwater in the area is mainly supplied by atmospheric rainfall, and some sections are supplied by surface water. Its runoff generally follows a complete hydrogeological unit, its flow direction is perpendicular to the contour line, and flows from high to low. With Wuzhi Mountain, Limu Mountain, Diaoluo Mountain, etc. as the core, the groundwater runoff flows around and radiates, and discharges along the coast. Generally, it is discharged to rivers, lakes, or discharged to the ground in the form of springs and scattered wetlands. Artificial drainage of groundwater has also become an important form of discharge.

Sample collection and testing

A total of 63 groups of groundwater samples were collected from civilian wells, pump wells and springs, with the sampling well depth between 3.5 and 340 m. According to the sampling depth, it is divided into shallow groundwater (0–20-m deep), middle groundwater (20–50-m deep) and deep groundwater (> 50-m deep), which are respectively from phreatic water, middle confined water and deep confined water. The water temperature, pH, conductivity, dissolved oxygen, total dissolved solids, and redox potential were measured on-site using a portable water-quality analyzer (HQ-40d, HACH, America). The collected water samples were filtered using 0.45-μm microporous filter membranes and then packed in 500-mL polyethylene plastic sample bottles that had been rinsed with deionized water at least three times. The samples used for the determination of cations were acidified to pH < 2 with approximately 3 mL of 65% HNO 3 . The samples for anion detection, without any modification, were sealed and stored in a refrigerator at 4 °C. The processed samples were tested by the Changsha Mineral Resources Supervision and Testing Center, Ministry of Land and Resources. The contents of K + , Na + , Ca 2+ , and Mg 2+ were determined using a plasma-generation spectrometer (ICP-MS; 7700X, Agilent Technologies, Japan); the contents of SO 4 2− , Cl − , and NO 3 − were measured by ion chromatography (ICS-1100, Thermo Scientific, America); and the content of HCO 3 − was determined by titration. The detection limit of each ion was 0.01 mg/L and the measurement error was less than 0.1%. After all the analysis procedures were completed, the charge balance error (CBE) was calculated by the following formula:

In the study, the average value of CBE is less than 5%, indicating that the analysis result is reasonable.

Data processing

Water quality index (wqi).

The WQI is an effective tool for appraising the overall quality of groundwater 28 , and is calculated as:

where i represents the sample number; \({W}_{i}\) and \({w}_{i}\) are the relative weight and weight of each index, respectively (Table S1 ); \({C}_{i}\) and \({S}_{i}\) are the measured concentration and permissible value of each index, respectively; and EW i is the effective weight of each index. WQI values can be categorized as non-drinkable ( WQI  ≥ 300), very poor (200 ≤  WQI  < 300), poor (100 ≤  WQI  < 200), good (50 ≤  WQI  < 100), and excellent ( WQI  < 50).

Irrigation water quality

Understanding the properties of groundwater is essential in areas where it is used as a source of irrigation. For example, an excessive salt content can result in sodium and salinity hazards 29 , 30 . In this study, the irrigation water quality was assessed based on the sodium adsorption ratio (SAR), soluble sodium percentage (% Na), and residual sodium carbonate (RSC), as follows:

where all the ionic concentrations of the respective ions are expressed in milliequivalents per liter (meq/L).

Health risk assessment

The health risk assessment models and standards provided by the USEPA have been widely used to quantitatively assess potential hazards. Previous studies suggested that the oral ingestion of groundwater pollutants is more harmful to human health than inhalation and skin contact, and several factors that are human health risks induced by skin-contact pollutants are relatively uncertain. Therefore, we assessed the threat of nitrate pollution to human health through drinking using 28 , 29 :

where HQ is the non-carcinogenic risk coefficient; E and RfD are the exposure dose and reference dose, respectively; C is the measured nitrate concentration; IR is the daily water consumption; EF is the exposure frequency; ED is the exposure duration; BW is the average body weight; and AT is the average lifetime. Table S2 shows the parameters of the health risk assessment model used to assess the risk from groundwater nitrates on Hainan Island 31 , 32 , 33 , 34 .

Monte Carlo simulation is a random number based calculation method used to simulate probability distribution functions, suitable for simulating highly complex phenomena that traditional analytical methods are difficult to solve. Its basic idea is to simulate a set of random variables that conform to the probability distribution function through random sampling, and perform numerical calculations or statistical analysis based on these random variables to obtain statistical quantities or numerical results. This method can to some extent reduce the impact of exposure parameter uncertainty in health risk models. The concentration of nitrate (C), adult weight (BW), and ingestion rate (IR) were considered as variable parameters, and distribution functions were shown in Table S3 21 . Monte Carlo simulation was performed using Crystal Ball 11.1.2.4 and iterated 10,000 times to ensure the robustness of the study.

Results and discussion

Groundwater chemical characteristics, descriptive statistics.

The pH of groundwater on Hainan Island ranged from 5.11 to 9.37, with an average of 7.47, indicating weak alkalinity (Table 1 ). According to the TDS content, the underground water can be divided into fresh water (TDS < 1000 mg/L) and brackish water (TDS > 1000 mg/L). According to the TH content, the underground water can be divided into soft water (TH < 150 mg/L) and hard water (TH > 150 mg/L). The range and mean value of TDS were 30.95–1077.30 and 287.41 mg/L, respectively, with only one water sample exceeding 1000 mg/L. The range and mean TH were 5.31–495.72 and 121.91 mg/L, respectively, indicating that the island’s groundwater is a combination of both hard and soft freshwater.

The relative abundances of the major cations in the sampled groundwater were in the order of Ca 2+  > Na +  > Mg 2+  > K + , whereas those of the anions were in the order of HCO 3 −  > NO 3 −  > Cl −  > SO 4 2− . The dominant cation was HCO 3 − , which accounted for 42.42% of the total anion concentration, whereas the dominant cations were Ca 2+ and Na + , which accounted for 36.83% and 32.07% of the total cation concentration, respectively. The coefficient of variation of the main ions in the groundwater ranged from 0.14 to 1.61, with the values of Mg 2+ , K + , Cl − , SO 4 2− , and NO 3 − exceeding 1. This indicates that the spatial distribution of these ions was significantly different, with a high degree of local enrichment.

The nitrate concentration in the groundwater samples was 0–226.26 mg/L, with an average of 38.92 mg/L. Based on these results, 41.27% of the samples exceeded the class III value of 20 mg/L, as specified in China’s groundwater quality standard (2017), which was mainly attributable to the discharge of domestic sewage and industrial and agricultural activities.

Hydro-chemical classification of groundwater

Groundwater chemistry is closely correlated with water quality 35 . Piper diagrams are often used to examine the general chemical characteristics and types of groundwater. The Piper diagrams for Hainan Island showed that the predominant cations comprised Ca 2+ and Na +  + K + terminal members, and the predominant anions comprised HCO 3 − terminal members, which may be mainly related to the rich rainfall and the dissolution of carbonate minerals in the study area (Supplementary Fig. S1 ). According to the Schukalev classification, the hydro-chemical types of the regional groundwater were, therefore, HCO 3 –Cl–Na and HCO 3 –Cl–Na–Ca. In the Piper diagram, the groundwater sample points in the study area are relatively scattered and there are many types of hydrochemical types, indicating that the groundwater chemical characteristics vary greatly and may be affected by natural and human factors.

Factors controlling groundwater chemistry

Natural factors.

Gibbs diagrams, which divide formation mechanisms into natural factors, including precipitation, rock weathering processes, and evaporation, are widely employed to explore groundwater formation mechanisms. These diagrams have, indeed, been applied by many scholars to assess groundwater evolution 36 . The groundwater samples from Hainan Island were mainly distributed in the “rock dominance” area, with a few falling in the “precipitation dominance” area (Fig.  2 a,b). This suggests that rock weathering processes dominated the groundwater chemistry in the study area. Precipitation also had some influence on groundwater chemistry, whereas evaporation (and crystallization) appeared to have little influence. Some of the shallow groundwater sample points fell outside of the model block diagram, indicating a stronger influence of human activities.

Gibbs diagrams of groundwater hydro-chemistry for ( a ) total dissolved solids (TDS) versus Na + /(Ca 2+  + Na + ); ( b ) TDS versus Cl − /(Cl −  + HCO 3 − ); ( c ) Mg 2+ /Na + versus Ca 2+ /Na + ; and ( d ) HCO 3 − /Na + versus Ca 2+ /Na + .

The effect of rock weathering on the hydro-chemical evolution of groundwater can be further explored using endmember diagrams. Weathering sources can be divided into carbonate weathering 37 , silicate weathering, and evaporite dissolution based on the ratios of Mg 2+ /Na + , Ca 2+ /Na + , and HCO 3 − /Na + . The groundwater samples from the study area were mainly plotted between the silicate and carbonate weathering endmembers, with only a few samples plotted between the silicate weathering and evaporite dissolution endmembers (Fig.  2 c,d). In contrast to the shallow groundwater samples, the middle and deep groundwater samples tended toward the carbonate mineral endmembers. This implies that the weathering of silicate and carbonate minerals plays a major role in the evolution of groundwater on the island, with a weaker contribution from evaporite dissolution.

Figure  3 shows the Pearson correlation coefficient matrix between the measured groundwater chemical parameters. A strong significant, positive correlation can be observed between Na + and Cl − (r = 0.92), indicating that the two ions had similar sources. The ratio of Na + and Cl − can also indicate the sources of Na + and K + in groundwater 38 . Most of the Hainan samples were plotted to the left of the 1:1 equivalent line (Supplementary Fig. S2 a), indicating that the excess Na + and K + in the groundwater may have originated from the weathering of silicate rocks or cation exchange.

Correlation matrix between water chemistry variables. p < 0.10, **p < 0.05, ***p < 0.01.

Ca 2+ and Mg 2+ were significantly, positively correlated with HCO 3 − (r = 0.78 and r = 0.70, respectively), indicating a common source. The sources of Ca 2+ and Mg 2+ can be determined by (Ca 2+  + Mg 2+ )/HCO 3 − : > 1, indicating that the dissolution of carbonate rocks is likely dominant, and < 1, indicating that the dissolution of silicate and evaporite rocks are considered dominant 39 . For Hainan Island, most of the middle and deep groundwater samples were plotted to the lower right of the 1:1 line (Supplementary Fig. S2 b), indicating that the Ca 2+ and Mg 2+ in these waters were mainly derived from the dissolution of silicates and evaporites. In contrast, the ratios of 71.15% of the shallow groundwater samples were > 1, the dominance of the dissolution of carbonate rocks.

The ratio of Cl −  + SO 4 2− and HCO 3 − can also be used as an index to distinguish the relative contributions of the weathering of different types of rocks. Both the middle and deep groundwater samples were plotted to the upper left in Supplementary Fig. S2 c, indicating that the dissolved ions in these waters were mainly affected by evaporite rocks. In comparison, the shallow groundwater samples were distributed on both sides of the 1:1 line, indicating inputs from both evaporite and carbonate rocks.

Through their long-term interaction, the negative charges carried by rock surfaces can adsorb cations from and release cations to groundwater, i.e., alternate cation adsorption can occur. The possibility of alternate cation adsorption can be determined by the relationship (Mg 2+  + Ca 2+ –SO 4 2– HCO 3 − )/(Na +  + K + –Cl − ), whereby ratios closer to − 1 indicate cation exchange 32 . Most of the shallow, middle, and deep groundwater samples from Hainan Island were plotted around the − 1 ratio line (Supplementary Fig. S2 d), indicating alternate cation adsorption.

The direction and intensity of alternate cation adsorption can be further expressed using the Chloron–Alkaline Index (CAI). In this case, when the Ca 2+ and Mg 2+ in groundwater are exchanged with Na + and K + in the aquifer, both CAI-I and CAI-II are negative, and when reverse ion exchange occurs, CAI-I and CAI-II are positive 40 , 41 . For the Hainan Island samples, 88.89% of the CAI values were negative (Supplementary Fig. S2 e). This indicates that reverse cation exchange is dominant and likely acts to increase the Na + and K + and decrease the Ca 2+ and Mg 2+ concentrations in groundwater. These processes act as an important source of sodium.

Anthropogenic inputs

Nitrate has good solubility in water 42 . Therefore, NO 3 − in wastewater, waste gas, and waste produced through human activities can enter shallow and deep groundwater via rainwater or surface water, thereby affecting groundwater quality and water chemistry 43 . The relationship between Cl − /Na + and NO 3 − /Na + can reflect the influence of groundwater by human activities; the higher the ratio, the stronger the effect of human activities on groundwater chemistry 44 . The ratios of Cl − /Na + and NO 3 − /Na + were relatively high in the samples from Hainan Island (Supplementary Fig. S2 f). Indeed, most of the water samples showed bias towards agricultural activities, with only a few points plotting between carbonate rock and salt rock. This indicates some degree of agricultural pollution on the island. Simultaneously, NO 3 − and K + were strongly correlated (r = 0.71), indicating that agricultural fertilizers, such as potassium fertilizer, that are not fully absorbed by crops enter surface waters or penetrate the groundwater system with irrigation water, resulting in nitrate pollution. The amount of fertilizer applied in Hainan Province is 511,400 tons, including 152,700 tons of nitrogen fertilizer, 40,500 of phosphate fertilizer, 91,100 tons of potassium fertilizer and 227,100 tons of compound fertilizer (Fig. S3 ). The higher application rate of chemical fertilizer can also support our conjecture.

The groundwater samples with NO 3 − concentrations higher than the class III limit specified by China’s groundwater quality standard (20 mg/L) were mainly obtained from Dongfang City and Danzhou City in the west of the island; Sanya City, Ledong County, and Lingshui County in the south; and Wenchang City and Qionghai City in the northeast and coastal areas (Supplementary Fig. S4 a). The high hydrochloride content of the groundwater in these areas may present a certain health risk to the locals; therefore, it cannot be considered suitable as a direct source of drinking water. Given the ongoing development of “tropical agriculture” on Hainan Island, the use of chemical fertilizers needs to be carefully controlled to reduce the impact of agricultural pollution on groundwater quality and avoid damage to the ecological environment.

Water quality evaluation

Adaptability of groundwater for drinking.

Groundwater quality assessment is very important for determining regional drinking water safety 45 . In this study, the WQI was used to evaluate the drinking water quality in the study area. The WQI of the groundwater on Hainan Island ranged from 9.96 to 266.10, with an average of 61.37; and 60.32% of the samples were classified as “excellent”, 19.05% as “good”, 14.29% as “medium”, and 4.76% as “poor”. The overall water quality was “good”. The average Ew i for NO 3 − and pH were the highest, at 43.67% and 29.73%, respectively, indicating that these parameters had the greatest impact on the WQI (Table S1 ).

The groundwater in the study area showed strong spatial variability (Supplementary Fig. S4 b). The samples with “good” water quality were mainly distributed in the middle of the island, whereas the samples with “poor” water quality were mainly obtained from the coastal areas of Dongfang City and to the west of Danzhou City. This may partially reflect the west of the island being on the leeward slope of the southeast monsoon. The southeast monsoon is blocked by Wuzhi Mountain; thus, the air in the southwest is relatively dry, with low precipitation. In addition, the rich mineral resources and convenient transport links in the west of the island make its industry develop rapidly, but also affect the quality of groundwater. Thus, a combination of natural and human factors has a major impact on the groundwater quality of the island.

Adaptability of groundwater for irrigation

Groundwater is the main water source of water for agricultural irrigation; however, high salinity and sodium content in irrigation water lead to salinization, which reduces soil quality and crop yields 46 . The SAR and %Na values can be used to evaluate these potential effects, and RSC indicates the potential for removing Ca 2+ and Mg 2+ from soil solutions. For Hainan Island, the SAR and RSC values were 0.18–3.6 and − 4.16 to 0.99, respectively (Table 1 ). This indicates that all the groundwater sampling points were suitable for irrigation. However, the %Na values ranged from 10.28 to 82.36, with 14.29% of the samples exceeding the acceptable limit for irrigation by 60.

Wilcox and USSL plots can help to evaluate the quality of irrigation water 47 , 48 . Wilcox plots are divided into five areas—“excellent to good”, “good to permissible”, “permissible to doubtful”, “doubtful to unsuitable”, and “unsuitable” 49 . Most of the Hainan Island samples were distributed in the excellent to permissible categories, with one sample (from Changjiang County) falling into the “permissible to doubtful” category (Supplementary Fig. S4 c, Fig.  4 a). In the USSL diagram (Fig.  4 b), the samples fell into the S1 region, with most distributed in the S1C1 and S1C2 regions and 14.29% in the S1C3 region. All sampling points, except for two, were obtained from Danzhou City, Changjiang County, Dongfang City, and Baisha County in the west of the island (Supplementary Fig. S4 d). This may reflect serious seawater intrusion and the widespread distribution of salt fields in the western coastal area of Hainan Island.

Wilcox ( a ) and USSL ( b ) diagrams for irrigation water quality assessment.

Table 2 shows a comparison of irrigation suitability between Hainan Island and other coastal countries. Hainan Island, the Muda Basin in Malaysia 50 , and KwaZulu-Natal in southern Africa 51 have SAR values below 10, indicating good irrigation suitability. In comparison, 20.83% and 10.34% of northern Algeria 52 and Dar es Salaam 53 in Tanzania have SAR values above 10, indicating poor irrigation suitability. Based on %Na, the irrigation suitability of the groundwater on Hainan Island was slightly lower than that of Tamil Nadu in India, but higher than that of a few other regions. The average EC value for the Hainan Island samples was also higher than that of KwaZulu Natal in southern Africa and the Muda Basin in Malaysia, but far lower than those of Tamil Nadu in India, northern Algeria, and Dar es Salaam in Tanzania. Overall, Hainan Island exhibited good groundwater irrigation suitability compared with other regions, with lower salinity and good water quality.

When the nitrate content in groundwater exceeds the safe limit, it poses a potential threat to human health 54 . Previous studies have shown that NO 3 - is the main factor affecting water quality. While water quality assessment can indicate whether groundwater is suitable for drinking at the regional scale, it does not reflect the potential health risks caused by local pollution. Therefore, the model recommended by the U.S. Environmental Protection Agency was used to evaluate the impact of groundwater NO 3 − on human health on Hainan Island.

The HQ ranges for infants, children, adolescents, and adults were 0–10.68, 0–9.03, 0–4.16, and 0–3.66, respectively, with the greatest risk for infants and children. The average HQ of infants and children in all groundwater samples was 1.84 and 1.55, respectively, with 46.03% and 44.44% of the samples exceeding the acceptable value of 1. The average HQ for teenagers and adults was 0.72 and 0.63, respectively, with an over standard rate of 25.4%. The uncertainty analysis results of Monte Carlo simulation showed that the mean values of infants, children, adolescents, and adults were 1.85, 1.56, 0.74, and 0.64, respectively, and the probability of exceeding the threshold was 46%, 44%, 21%, and 17%, respectively (Fig.  5 ). These results are similar to the average value of traditional health risks and the proportion of exceeding acceptable risks, indicating that the results of the two methods are consistent and suitable for human health risk assessment. The NO 3 − health risks reflect the spatial distribution of the NO 3 − concentrations, which were notably high in the west of Danzhou City and Dongfang City, and central Weifang City (Fig.  6 ).

Cumulative probability distribution diagram.

Spatial distribution of the health risks (HQ, hazard quotient) associated with the use of nitrate-contaminated groundwater for drinking by infants ( a ), children ( b ), teenagers ( c ), and adults ( d ). The map was created using ArcGIS 10.8 ( https://www.esri.com/software/ArcGIS ).

Numerous studies have shown that groundwater nitrate pollution is mainly caused by human activities, such as the unqualified discharge of domestic sewage and industrial wastewater, agricultural fertilization, and runoff from livestock breeding 55 . With an increase in population and the development of industry and agriculture, nitrate pollution is becoming increasingly common worldwide. This study shows that nitrate pollution in Hainan Island is currently at a medium level, with a lower average value than that in the central and western regions of Jiaokou in northern China 56 , Songnen Plain in northeastern China 4 , and Shandong Peninsula in eastern China, but higher value than that in the North China Plain 57 and Nanchong in the southwest 58 (Supplementary Fig. S5 ). Hainan Island has a higher average concentration of nitrate in its groundwater than that in some regions of other countries, including Essaouira in Morocco 59 , Haryana in India 60 , South Africa, and Malaysia 61 , but lower than that in Tunisia 62 , and Nanganur and Mothkur in southern India 63 . Given the importance of nitrate groundwater pollution for the safety of regional drinking water, timely monitoring is essential for minimizing the risk to human health.

Conclusions

In conclusion, the groundwater on Hainan Island is mainly weakly alkaline freshwater, characterized as HCO 3 –Cl–Na and HCO 3 –Cl–Na–Ca. The chemical characteristics of groundwater are mainly affected by water–rock interactions, followed by cation alternating adsorption, and human activity. The WQI of 60.32% water sample points is less than 50, and the %Na of 85.71% is less than 60. The overall water quality is good, which is more suitable for drinking and irrigation, although the water quality is different in space. Compared with other areas, the water quality in the western part of the island is poor. Nevertheless, compared with other coastal areas, the average EC value of these samples is only 444.82, which is lower overall and more suitable for irrigation. The nitrate concentration range is 0–226.26 mg/L, and the nitrate pollution level is medium compared with areas of mainland China and other parts of the world. However, the non-carcinogenic risk of nitrate to infants is 36.51% higher than the acceptable value 1, which should be paid attention to. An appropriate level of development and management of water resources is essential, which must enable social development while maintaining use within the environmental carrying capacity. Simultaneously, the utilization efficiency of water resources needs to be improved by raising awareness of water quality and sustainability issues.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on request.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Number 41877297) and the China Geological Survey (12120114029601).

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These authors contributed equally: Qingqin Hou and Yujie Pan.

Authors and Affiliations

School of Geography and Information Engineering, China University of Geosciences, Wuhan, 430074, China

Qingqin Hou & Hongxia Peng

The second Institute of Resources and Environment Investigation of Henan Province, Henan, 471023, China

Qingqin Hou

College of Environmental Sciences and Engineering, Peking University, Beijing, 100000, China

Wuhan Center of Geological Survey of China Geological Survey, Wuhan, 430000, China

Min Zeng & Changsheng Huang

School of Mechanical Engineering and Automation, Northeastern University, Liaoning, 110819, China

Simiao Wang

School of Environmental Studies, China University of Geosciences, Wuhan, 430074, China

Huanhuan Shi

School of Geography and Information Engineering, China University of Geosciences, No. 68, Jincheng Street, East Lake New Technology Development Zone, Wuhan, 430078, Hubei, China

Hongxia Peng

Hubei Key Laboratory of Regional Ecology and Environmental Change, China University of Geosciences, Wuhan, China

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Q.H.: Methodology, Formal analysis, Software, Methodology, Writing—original draft. Y.P.: Investigation, Data curation, Supervision, Writing—review and editing. H.P.: Conceptualization, Investigation, Resources, Funding acquisition. S.W.: Data curation, visualization, investigation. M.Z.: Methodology, Supervision, Formal analysis, Funding acquisition. C.H.: Investigation, Supervision. H.S.: Formal analysis. All authors reviewed the manuscript.

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Hou, Q., Pan, Y., Zeng, M. et al. Assessment of groundwater hydrochemistry, water quality, and health risk in Hainan Island, China. Sci Rep 13 , 12104 (2023). https://doi.org/10.1038/s41598-023-36621-3

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DOI : https://doi.org/10.1038/s41598-023-36621-3

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Past, Present, and Future of Groundwater Remediation Research: A Scientometric Analysis

1 College of Environmental Science and Engineering, Taiyuan University of Technology, Jinzhong 030600, China

Guilian Fan

2 Faculty of Business Administration, Shanxi University of Finance and Economics, Taiyuan 030006, China

3 Shanxi Province C&M Operation Center for Xishan Yellow River-lifting Irrigation Project, Taiyuan 030002, China

Jianguo Cui

Associated data.

In this study, we characterize the body of knowledge of groundwater remediation from 1950 to 2018 by employing scientometric techniques and CiteSpace software, based on the Science Citation Index Expanded (SCI-E) databases. The results indicate that the United States and China contributed 56.4% of the total publications and were the major powers in groundwater remediation research. In addition, the United States, Canada, and China have considerable capabilities and expertise in groundwater remediation research. Groundwater remediation research is a multidisciplinary field, covering water resources, environmental sciences and ecology, environmental sciences, and engineering, among other fields. Journals such as Environmental Science and Technology, Journal of Contaminant Hydrology , and Water Research were the major sources of cited works. The research fronts of groundwater remediation were transitioning from the pump-and-treat method to permeable reactive barriers and nanoscale zero‑valent iron particles. The combination of new persulfate ion‑activation technology and nanotechnology is receiving much attention. Based on the visualized networks, the intelligence base was verified using a variety of metrics. Through landscape portrayal and developmental trajectory identification of groundwater remediation research, this study provides insight into the characteristics of, and global trends in, groundwater remediation, which will facilitate the identification of future research directions.

1. Introduction

Groundwater is an important natural resource that supports socioeconomic development and maintains ecological balance in modern societies [ 1 ]. It provides 36% of drinking water, 42% of water for agriculture, and 24% of water for industry [ 2 , 3 ]. The quality of groundwater resources globally is threatened by the natural geochemical background and anthropogenic pollution [ 4 , 5 ]. To clean polluted groundwater and ensure the sustainability of groundwater resources, a variety of remediation technologies (e.g., pump-and-treat, biodegradation, chemical oxidation, and reduction to adsorption) have been developed and applied [ 6 , 7 , 8 , 9 ]. Each treatment option has associated merits and demerits, depending on remedial goals and site conditions. There are several challenges for selecting sustainable remediation technologies and designing remediation strategies today, including evolving groundwater treatment goals, complex geophysical–chemical characterization, current understanding of available technologies, contaminant mixtures, and economic considerations [ 8 , 10 , 11 , 12 ].

Groundwater remediation research has been reviewed from a variety of perspectives, and the extant review articles focus on technologies for the remediation of contaminated groundwater and their applications. Examples include natural attenuation processes [ 13 ], permeable reactive barriers [ 14 ], sustainability appraisal tools [ 15 ], nanoscale zero‑valent iron particles [ 16 ], and iron sulphide particles for groundwater remediation [ 17 ]. Nevertheless, little attention has been devoted to quantitative analyses of the evolution of groundwater remediation research. In short, they did not provide an overall landscape of the groundwater remediation literature. In a recent article, Zhang, Mao, Crittenden, Liu and Du [ 8 ] used social network analysis and bibliometric technology to evaluate publications related to groundwater remediation from 1995 to 2015, and the results provided valuable insight into groundwater remediation. However, they did not identify and evaluate hotspots and there is, to date, no knowledge base for groundwater remediation.

This study has used scientometric approaches to describe the development trajectory and landscape of groundwater remediation research quantitatively and systematically, and the research frontiers and emerging trends of the groundwater remediation literature were detected and identified using the visualization tool CiteSpace. The results will provide a useful reference for academics, researchers, and policy decision makers.

2. Data Acquisition and Methods

2.1. data acquisition.

It is essential for researchers to quickly and accurately locate publications, using the search strategy and screen methods. The article retrieval source for analysis was the SCI-E databases, which are frequently used in scientific research [ 18 ]. Several topic terms, including “groundwater restoration”, “groundwater reclamation”, and “groundwater remediation”, were used to retrieve publications that contained these words in publications’ titles, keyword lists, and abstracts. These terms helped to locate the majority of groundwater-remediation-related publications. Though there may be other groundwater-remediation-related terminology, they account for a small percentage of publications and may have marginal relation to groundwater remediation research [ 19 ]. The search results have been refined or filtered by web of science categories, research areas, and document types. To do this, several categories needed to be excluded, such as (i) unrelated categories—physiology, pharmacology pharmacy, genetics heredity—(ii) document types—book chapter, data paper, proceedings abstract—and (iii) research areas—imaging science photographic technology, business economics. The search resulted in 2867 publications dated from January 1950 to September 2018. The entire records, including the titles, abstracts, keywords, and references, were then exported for subsequent analyses. Based on the frequency of groundwater remediation research over the past seven decades, we reviewed numerous studies published between 1950 and 2018 (see the Supplementary Materials ).

2.2. Scientometric Analytical Methods

Scientometrics, created by Tibor Braun [ 20 ], has been defined as the “quantitative study of science and technology” [ 21 ]. As a branch of informatics, scientometrics is used to analyze patterns in scientific literature quantitatively, to understand the knowledge structure of emerging trends in a research field [ 22 ]. As a scientometric approach, CiteSpace is used to clarify multidisciplinary relationships, assess research status, map knowledge domains, reveal research frontiers, and predict emerging trends, by analyzing the bibliographic records of associated publications [ 23 ]. In the net knowledge maps generated by CiteSpace, a node represents one item (e.g., country, author, subject, term, journal, or reference), and links describe co-citations or co-occurrences between these nodes [ 24 ]. Furthermore, each node is described as a series of tree rings of different colours, with blue representing the oldest and orange the most recent [ 25 ]. A purple ring around an item indicates good centrality.

To date, CiteSpace has been employed in studies of, for example, nonpoint source pollution [ 26 ], information sciences [ 27 ], psychological science [ 28 ], and global green innovation [ 29 ]. In this study, we produced and analyzed co-occurrence networks of subject categories and countries, and co-citation networks of journals, authors, and references using CiteSpace.

3. Results and Discussion

3.1. characteristics of publication output.

To give an overview of research in groundwater remediation, the annual number of articles published from 1950 to 2018 (total, 2867) is shown in Figure 1 . In 1950, only one article, titled “Ground-Water Pollution in Michigan”, was published [ 30 ]. Subsequently, the annual number of publications was fewer than 10 until 1990. The number of articles increased significantly after this period, from 23 in 1991 to 207 in 2016.

Publication output performance during the period 1950–2018.

3.2. Co-Operations of Countries/Territories and Institutions

3.2.1. co-operation among countries/territories.

Running CiteSpace, we obtained a countries/territories distribution with 78 nodes and 296 links ( Figure 2 ); this map can help researchers find their colleagues elsewhere in the world and establish collaborations. Each node represents a different country or territory, and the size of the node represents the number of publications. Similarly, the lines connecting countries/territories indicate their co-operation, while the thickness of the line represents the degree of co-operation [ 8 ]. The United States (U.S.) was the hub of the co-operation network, and the leader in groundwater remediation research, in collaboration with other productive countries/territories. The discovery of hazardous waste at Love Canal in Niagara, New York, and many other places in the United States, heralded a new era in hazardous waste problems by the end of the 1970s. Subsequently, civil and environmental engineers, hydrologists, hydrogeologists, and other scientists became involved in the identification, evaluation, and remediation of groundwater-contaminated hazardous waste sites [ 31 ]. Groundwater remediation research was distributed among 78 countries/territories, and the top 10 countries/territories published 2607 articles, accounting for 90.9% of the total ( Table 1 ). The U.S. and China published 1152 and 464 articles, respectively, ranking first and second, and accounting for 56.4% of total articles. Thus, the U.S. and China were two major research powers in groundwater remediation and far ahead of other countries/territories.

Distribution of co-operation among countries/territories.

Distribution of 10 co-operative countries/territories and institutions.

3.2.2. Co-Operation among Institutions

Institutional co-operation was also analyzed using CiteSpace ( Figure 3 ). The top 10 productive institutions are shown in Table 1 . These 10 productive institutions worked closely with organizations in geographical proximity, e.g., the University of Waterloo and University of Regina in Canada, the University of Illinois and the University of Arizona in the U.S., and China University of Geosciences and Jilin University in China. Therefore, it is necessary to strengthen international co-operation in the future.

Distribution of co-operation among research institutions.

The top 10 research institutions issued 455 articles, accounting for 15.9% of the total. According to Table 1 and Figure 3 , the first major research echelon was led by the University of Waterloo, where hydrologists first used zero-valent iron (Fe 0 ) to treat contaminated groundwater in situ approximately three decades ago [ 32 , 33 ]. Of these top 10 institutions, three were in the U.S. and three in China, confirming that the U.S., Canada, and China have considerable capabilities in groundwater remediation research, and strong expertise in research and development.

3.3. Co-Occurrence of Subject Categories

Based on co-occurrence analyses of subject category, the disciplines involved in groundwater remediation can be detected. In this study, we selected the top 30 subject categories with the largest number of reoccurrences each year for category characteristic analysis. The information on subject categories was extracted from the SCI-E databases using CiteSpace and analyzed. Figure 4 shows the co-occurrence network from 1950 to 2018, where one node represents a subject category, and the edge connecting two nodes represents the co-occurrence of two subject categories. The top three popular research categories were environmental sciences and ecology, environmental sciences, and engineering. Of the top 10 subject categories, engineering had the central position and played an important role in groundwater remediation. Material science was second, followed by water resources, and chemistry, physical. Therefore, groundwater remediation is a multidisciplinary research field, involving an extensive range of interests.

Disciplines involved in the field of groundwater remediation, shown as a network of subject categories.

3.4. Journal Citation Analyses

“Core journals” usually refer to top-ranking journals with high citation frequencies. We produced a groundwater remediation journal co-citation map with 199 nodes and 1149 links, using CiteSpace software ( Figure 5 ). The top 10 most productive journals, with >500 citations, are listed in Table 2 . The total of 12,090 citations from the 10 journals accounts for 34.74% of the total citation count. Thus, the citation distribution was concentrated. In addition, these 10 journals were defined as the “core journals” in the field of groundwater remediation.

Journal co-citation knowledge map.

Distribution of 10 “core journals” and IF in 2017.

Environmental Science and Technology and Water Research were the core journals in groundwater remediation research, with 1850 and 1204 citations, respectively ( Table 2 ). Water Research and Environmental Science and Technology also had the highest IFs, at 7.051 and 6.653, respectively.

3.5. Author Citation Analyses

White and McCain [ 34 ] first proposed the author co-citation concept in the U.S. Author co-citation maps, which reflect the closeness of the research directions and importance of the authors, and have been widely used to assess scientific research ability and relevance. Herein, one node represents a cited author, and an author co-citation knowledge map was created using CiteSpace ( Figure 6 ).

Author co-citation map/the co-operation network of productive authors.

The largest node corresponded to Blowes DW, whose articles were cited 251 times; this was followed by Gillham RW (233), Wilkin RT (204), Matheson LJ (190), Scherer MM (177), Phillips DH (165), Su CM (157), and Noubactep C (145). Thus, these authors’ works had a marked impact on groundwater remediation research and development, and they represent the core research strength in the field.

As mentioned above, the first major research echelon, led by the University of Waterloo, was composed of Gillham RW, Blowes DW, and other authors, suggesting that this group had the greatest impact on groundwater remediation research.

3.6. Documents Co-Citation Analyses

The co-citation network was divided into many clusters of co-cited references in CiteSpace, so that references are closely connected within the same cluster but loosely connected among different clusters ( Figure 7 ). The 10 major clusters are listed in Table 3 by size, which represents the number of members in each cluster. The silhouette score of a cluster reflects its quality, i.e., homogeneity or consistency. If the silhouette value of a cluster is close to 1.0, then it was homogenous [ 22 ]. All the clusters in Table 3 were highly homogeneous, as indicated by their high silhouette scores. Noun phrases from the terms (e.g., titles or abstracts) used in articles in the cluster were used to label each cluster. Labels selected by the log-likelihood ratio (LLR) test were used in subsequent discussions [ 35 ].

The synthetic network of co-cited references.

Major clusters of co-cited references.

MI = mutual information, LLR = log-likelihood ratio, Ave = average.

We can identify the average year of the publications in a cluster by their recentness, i.e., Cluster #6 on aquifer remediation had an average year of publication of 1985. The recently formed clusters, Clusters #2 and #3 (nano-zero‑valent iron and metallic iron, respectively), had an average year of publication of 2009 and 2008, respectively.

3.6.1. Analyses of Research Fronts

Price [ 36 ] proposed the concept of the research front, and postulated that a research front can characterize the momentary nature of a research field. Garfield [ 37 ] defined a research front as “a cluster of co-cited articles and all articles that cite the cluster”. Chen [ 38 ] defined a research front as “an emergent and transient grouping of concepts and underlying research issues”. CiteSpace shapes the network knowledge map of research fronts, with mutant terms that can be extracted from the index terms, abstracts, titles, and record indicators of the references. Specific methods include selecting a cited reference as the net node, the g-index ( k = 10) as threshold willing, and the key pathfinder algorithm. We obtained 17 clusters by selecting “Find clusters” and abstracted the names of the clusters by selecting “Label clusters with indexing terms”. Figure 7 shows the net knowledge map generated.

There were 558 nodes, 874 links, and 17 clusters. Clusters #2, #3, and #4 had a high concentration of nodes with citation bursts, which echoed the fact that these were the most recently formed clusters.

If a cluster has a larger area, it has more bibliographic entries, and large clusters generally indicate main research directions, i.e., each cluster corresponds to a research front. The research fronts and major trends in groundwater extraction, in situ groundwater remediation, permeable reactive barriers, metallic iron, and nanoscale zero‑valent iron particles are shown in Figure 7 and Table 3 .

3.6.2. Timeline of Research Fronts

Figure 8 shows timelines of the 17 distinct co-citation clusters and their interrelationships. All timelines run from left to right [ 39 ], show the times at which research fronts appear and disappear, and display structural information about the research front clusters [ 40 ]. Analysts can visually identify various characteristics of a cluster, such as its citation classics, historical length, citation bursts, and connection to other clusters.

Timelines of co-citation clusters. Major clusters are labelled on the right.

The following paragraphs provide an interpretation of the research fronts’ timelines. Groundwater remediation research has a long history ( Figure 8 ). The earliest research front, “aquifer remediation”, provided basic information for subsequent research. Next, technologies to deal with contaminated groundwater were developed. The containment and/or control of contaminated groundwater can generally be accomplished using one, or a combination, of several available techniques, which can be broken down into aquifer rehabilitation, physical containment measures, and withdrawal, treatment, and use [ 41 ].

The second research front, “groundwater extraction”, began around 1979, and pump‑and‑treat as a groundwater extraction technology began at selected sites in 1982 [ 42 ], in response to groundwater pollution control and contamination remediation. Earlier pump‑and‑treat systems, which did not consider the presence of geologic heterogeneity, poor definition of initial condition in source zones, did not clean aquifers to the required level. Many of the original systems worked adequately for a period of time, but, after they were switched off, the contaminant levels at many sites reached values higher than those before remediation [ 31 ]. Subsequent to the pivotal 1989 article by Mackay [ 43 ], the research front became inactive. A number of new technologies for groundwater remediation are under development, and these may accelerate contaminant removal from the subsurface (e.g., injection of steam, surfactants) or destroy the contaminant in situ [ 43 ]. Hence, research fronts are discontinuous, and start and end abruptly when scientists move from one puzzle to the next [ 40 ].

In the 1990s, scientists and engineers had to prepare to deal with recent puzzles, which included residual oils, source zones with non-aqueous phase liquids (NAPLs), and vapours in the unsaturated zone [ 31 ]. During this period, four research fronts, “anionic surfactant remediation (1995)”, “decision analyses (1997)”, “laboratory column test (1999)”, and “in situ groundwater remediation (1999)”, were created in response to growing concern over efficient and cost-effective clean‑up solutions. Among these research fronts, “in situ groundwater remediation” was worthy of note. This research front experienced a period of stability and extends to the present. A growing number of researchers focused on the development of in situ remediation technologies, e.g., in situ chemical oxidation [ISCO]. ISCO, a type of advanced oxidation process technology, has proven useful for in situ remediation technology for the most prevalent organic contaminants in groundwater. The development of in situ remediation technologies led to the formation of three research fronts in the new century: “permeable reaction barriers (PRBs) (2002)”, “metallic iron (2008)”, and “nano zero‑valent iron (nZVI) (2009)”.

The research front, “PRB”, which dates to 1989, had a median publication date of 2002. Over the last two decades, PRBs have been emerging as an effective alternative passive in situ remediation technology. In the 1990s, research on PRBs increased considerably, which led to many new approaches for suitable reactive materials, target contaminants, and PRB design.

“nZVI” is the latest research frontier, showing rapid growth and a professional pattern. Gillham and O’Hannesin [ 44 ] discovered that halogenated aliphatic compounds in groundwater can be reduced using bulk ZVI. This characteristic of iron led to the advanced Fe-PRB, in which vertical trenches were filled with granular ZVI, placed in the flow path of the underground contaminant plumes [ 45 , 46 ].

A report [ 47 ] by the Chinese Academy of Sciences indicated that the third top research front in ecology and environmental sciences, entitled “Activation of persulfate for degradation of aqueous pollutants by transition metal and nanotechnology”, is receiving much global attention. The combination of persulfate ion activation technology and nanotechnology will improve the efficiency of polluted water treatment, reduce energy consumption, and promote recycling.

3.6.3. Analyses of the Intelligence Base

Chen [ 38 ] defined “the intellectual base of a research front as its citation and co-citation footprint in the scientific literature, an evolving network of scientific publications cited by research-front concepts”.

(1) Most‑Cited Articles

The most‑cited articles are generally considered landmarks, owing to their ground‑breaking contributions [ 22 ]. Cluster #7 had three of the top 10 landmark articles, and Clusters #3 and #10 each had two ( Table 4 ). The most-cited articles in the databases were by Blowes (2000), with 154 citations, followed by Gillham (1994), with 144 citations and Matheson (1994), with 135 citations, and the most recent was a review article by Fu (2014). Interestingly, the titles of the most‑cited articles contained the terms “permeable reactive barriers”, “zero-valent iron”, “nanoscale iron particles” ( Table 4 ), which were in accordance with the research fronts noted above.

The top 10 most-cited references.

Gillham and O’Hannesin [ 44 ] investigated the potential of Fe 0 in the dehalogenation of ethanes, ethenes, and 14 chlorinated methanes. The results demonstrated biotic reductive dechlorination, in which iron serves as the source of electrons. In response to the rapid degradation rates, an application for in situ remediation of contaminated groundwater was proposed.

Blowes, et al. [ 48 ] was cited the most frequently. This paper reviewed the recent research progress in PRBs for the remediation of inorganic contamination of groundwater.

(2) Betweenness Centrality

The betweenness centrality measure that Freeman [ 49 ] proposed is used to give prominence to potential pivotal points in the synthesized network shifts over time. The betweenness centrality of nodes in a network is indicative of the importance of the location of the nodes. We are especially interested in the nodes located between different node groups, because they probably offer insight into emerging trends [ 22 ]. Table 5 shows 10 structurally crucial references in the network, and three of these nodes were in Cluster #3, and five in Cluster #7. These references can be identified as landmark works in the field of groundwater remediation.

Cited citations with the highest between centrality.

(3) Citation Bursts

A reference citation burst may indicate an emergent research front, and the citation-burst-detection algorithm of Kleinberg [ 50 ] is adapted for identifying emergent research front concepts. Table 6 lists the references that had the strongest metric of citation bursts across the entire database during the period 1950–2018. Among the articles with strong citation bursts ( Table 6 and Figure 9 ), Mackay and Cherry [ 43 ] is worthy of note. Their article explored the reasons for the difficulty of groundwater clean-up, noted some implications, and suggested that achieving stringent health-based clean-up standards is unlikely, and the ultimate cost of clean-up is high in many cases. Thus, they suggested that site characterization and remediation have much room for improvement, by both the development of new tools and ongoing training of staff [ 43 ]. Subsequently, the development of permeable reactive barrier technology using zero-valence iron filings has proceeded from recognition, evaluation, technology conceptualization, and proof of concept, to commercialization.

The top 20 references with the strongest citation bursts.

The top five references with the strongest metric of citation bursts.

The structural centrality and citation burstness of cited references can be measured by the Sigma metric measure, i.e., the Sigma value of a reference that is strong in both measures will be higher than that of a reference that is strong in only one of the two measures [ 22 ] ( Table 7 ). The pioneering article by Fu, et al. [ 51 ] had the highest Sigma of 101,578.09, indicating it to be structurally indispensable in the field, due to its strong citation burst. This article reviewed the recent advances of ZVI and the progress made in groundwater remediation using ZVI technology.

Structurally and temporally significant references.

4. Conclusions

4.1. summary.

This study offers a comprehensive scientometric review of groundwater remediation research. There were 2867 journal articles related to this field published from 1950 to 2018, and the increasing annual number of publications suggests a continued research interest and a globally urgent need to remediate contaminated groundwater, since 1991. The U.S. and China contributed 56.4% of the publications and were the major powers in groundwater remediation research. Groundwater remediation research is a multidisciplinary research field and covers an extensive range of interests, from environmental sciences and ecology to environmental sciences, engineering, and water resources. Furthermore, journals such as Environmental Science and Technology , Water Research , and Journal of Contaminant Hydrology were the main sources of cited works in groundwater remediation research. The research fronts of groundwater remediation were transitioning from the pump-and-treat method to PRBs and nanoscale zero‑valent iron particles. The combination of persulfate ion activation technology and nanotechnology shows promise. Meanwhile, based on the visualized networks, the intelligence base was verified using a variety of metrics. Our study provides a valuable reference for researchers in the field of groundwater remediation, and others with interests in this area.

4.2. Future Outlook

(1) Development of treatment trains. Great advances have been made in the field of groundwater remediation research over recent decades. As the “One Size Fits All” remedy technology does not work effectively at most contaminated sites, the groundwater remediation technologies used are generally parts of a “treatment train”. Hence, tailored approaches and remediation techniques on a site-by-site basis are needed. In addition, research into technologies for pollution remediation of fractured bedrock aquifers, low permeability formations, and green remediation technology, is needed. Hence, these topics will remain areas of active research for many years.

(2) Optimization of groundwater remediation design under uncertainty. The technical and environmental challenges in designing optimal groundwater remediation systems are the spatial variability of natural aquifers, uncertain aquifer parameters, and complex site characteristics, which affect both the cost and efficiency of remediation. Investment in data collection and accurate site characterization may minimize uncertainty. Simultaneously, effort has been made to include uncertainty analyses in optimal groundwater remediation designs, using optimization methods and a coupled simulation–optimization approach. Owing to the complexity and inherent uncertainty of groundwater remediation technologies, the success of their field application is limited. This suggests the need to incorporate new methods and means of quantitatively analyzing uncertainty into the design of optimal groundwater remediation technologies.

(3) Development of green and sustainable remediation. Green and sustainable remediation (GSR) is a new movement in the land and groundwater remediation field that has drawn much attention globally in recent years, and it requires consideration of the environmental, economic, and social dimensions of sustainability. GSR technologies for contaminated groundwater, including biochar materials, green synthesis of engineered nanoparticles [ 52 ], sustainable PRB [ 53 ] and sustainably released long-term green remediation materials, have made rapid progress. However, case studies indicate that public participation must be improved to promote social sustainability, and region-specific factors should be considered when implementing GSR.

Acknowledgments

The authors would like to thank Prof. Guishen Fan for his thoughtful review of a later draft.

Supplementary Materials

Publications list for scientometric analysis (topic searched results in SCI-E database) includes title of the article, journal name, authors, year of publication, volume, page range. The following is available online at https://www.mdpi.com/1660-4601/16/20/3975/s1 , Spreadsheet: Publications list.

Author Contributions

Methodology, G.F.; Q.C.; literature search and data analysis, Q.C.; W.N.; writing—Original draft, Q.C.; J.L.; writing—Review and editing, Q.C.; H.L.; supervision: J.C.

This study was funded by Shanxi Provincial Water Conservancy Science and Technology Research and Promotion Project of China, the Shanxi Provincial Natural Science Foundation of China (Grant No. 201601D102037).

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Trend Analysis of Groundwater Levels and Assessment of Regional Groundwater Drought: Ghataprabha River Basin, India

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• Published: 05 October 2018
• Volume 28 , pages 631–643, ( 2019 )

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• Abhishek A. Pathak 1 &
• B. M. Dodamani 1  

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Groundwater drought is a relatively new concept, particularly in the Indian subcontinent, where groundwater levels are declining rapidly. The present study focuses on understanding the trends in groundwater levels and evaluates regional groundwater drought characteristics in the drought-prone Ghataprabha river basin, India. Cluster analysis was performed on the long-term monthly groundwater levels to classify the wells, and the Mann–Kendall test was accomplished to investigate the annual and seasonal groundwater-level trends. Standardized Groundwater level Index (SGI) was used to evaluate groundwater drought. Significant decreasing trends were observed in more than 61% of the wells in the study area with average decline of 0.21 m. Results of the SGI analysis showed that the wells of clusters 1 and 2 experienced recurrent droughts, which can be attributed to diminishing rainfall and over-exploitation of groundwater resources. The outcome of this study provides valuable information about the long-term behavior of regional groundwater levels which, in turn, helps to establish an operative groundwater management strategy for upcoming droughts.

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Acknowledgments

The authors would like to express their gratitude to the Senior Geologists, Department of Mines and Geology of Belgaum, Vijayapura and Bagalkot Districts of Karnataka State, India, for providing long-term monthly groundwater-level data for the study.

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Pathak, A.A., Dodamani, B.M. Trend Analysis of Groundwater Levels and Assessment of Regional Groundwater Drought: Ghataprabha River Basin, India. Nat Resour Res 28 , 631–643 (2019). https://doi.org/10.1007/s11053-018-9417-0

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