Large scale occurrence of aluminium-rich shallow groundwater in the Pearl River Delta after the rapid urbanization: Co-effects of anthropogenic and geogenic factors

A regional investigation on natural background levels and contamination assessment of shallow groundwater contaminants in a coastal urbanized area

Knowledge on natural background levels (NBLs) is indispensable for evaluating groundwater contamination at regional scales. This study used a combination of modified oxidation capacity method and the box plot method to assess NBLs of common contaminants including iron, nitrite, sodium, total hardness, fluoride, sulfate, and selenium in groundwater of a coastal urbanized area of China (Pearl River Delta [PRD]), discussed geochemical factors controlling NBLs of these related contaminants in various groundwater units via multivariate statistical techniques, and evaluated groundwater comprehensive contamination in the PRD on the basis of groundwater NBLs. Results showed that NBLs of iron, nitrite, sodium, total hardness, fluoride, and sulfate in Groundwater Unit I of the PRD were higher than those in other groundwater units. Conversely, NBLs of selenium in Groundwater Units II and III of the PRD were higher than that in other groundwater units. Both of NBLs of iron and total hardness in Groundwater Unit I were higher than their allowable limits in groundwater quality standards recommended by China, while others were not. The high NBL of iron in Groundwater Unit I was probably attributed to the reductive dissolution of iron minerals in Quaternary sediments induced by the mineralization of organic matters in overlaid marine sediments. By contrast, the irrigation of river water and the leaching of trapped seawater in overlying marine sediment were likely responsible for the high NBL of total hardness in Groundwater Unit I. Using a groundwater contamination evaluation method on the basis of NBLs shows that uncontaminated groundwater accounted for 17.6%, 32.4%, 45.9%, and 53.6% in urban lands, peri-urban lands, agricultural lands, and other lands of the PRD, respectively. Groundwater with moderate to extremely high contamination was mainly distributed in Foshan, Dongguan, southern part of Guangzhou, western part of Shenzhen, northern part of Zhongshan, southern part of Zhuhai, and downstream of these cities. The leakage of industrial wastewater and domestic sewage and the irrigation of contaminated river water were likely main factors controlling groundwater comprehensive contamination in the PRD. PRACTITIONER POINTS: Groundwater natural background levels (NBLs) of seven contaminants in a coastal urbanized area (Pearl River Delta) were assessed. Geochemical factors controlling groundwater NBLs of seven contaminants in the Pearl River Delta were discussed. Groundwater comprehensive contamination in the Pearl River Delta was assessed on the basis of groundwater NBLs.

Appraisal of potential toxic elements pollution, sources apportionment, and health risks in groundwater from a coastal area of SE China

Groundwater is a vital natural resource, but the presence of potentially toxic elements (PTEs) poses significant risks to both groundwater safety and human health. This study collected 120 groundwater samples from a coastal area in southeastern China during wet and dry seasons to assess PTE levels, identify their sources, and evaluate pollution and health risks. Results showed that Mn, Zn, and Al had the highest average concentrations in both seasons, with Mn, Cd, and Zn frequently exceeding safe limits. PTE levels were higher during the wet season. Natural background levels (NBLs) were determined, revealing that most elements met quality standards except for Mn and Cd. Four PTE sources were identified using principal component analysis and the APCS-MLR model: industrial emissions (25.5% dry, 23.8% wet), geological background (21.2% dry, 19% wet), natural sources (27.2% dry, 16.2% wet), and mining activities (20.8% dry, 23.4% wet). Heavy metal pollution was significant (moderate to heavy: 72.73% dry, 45.76% wet), but ecological risks were low (low risk: 92.73% dry, 66.10% wet). Health risk assessments and Monte Carlo simulations indicated low carcinogenic and non-carcinogenic risks, slightly higher in children than adults. Risks were more severe in the southwestern part of the study area. These findings support local groundwater management efforts.

Factors causing groundwater acidification in the high land area of Ho Chi Minh City, Vietnam

This study explores the causes of the acidic nature of the metal-rich, dilute groundwaters in the highland area of Ho Chi Minh City (HCMC), which is populated and mostly used for domestic, agricultural, and industrial purposes. The groundwater is generally very dilute (176 ± 128 µS/cm in electrical conductivity, 70 ± 220 µeq/L in alkalinity), but high in redox potential (343 ± 55 mV), and nitrate concentrations (19 ± 19 mg/L). Since the area corresponds to the highland and, thus, serves as a groundwater recharge zone. However, 53% and 90% of the investigated groundwater samples (n = 58) showed pH lower than 4 and 5, respectively, and, thus, 43%, 21%, and 7% of groundwater samples showed Al, Pb, and Cr concentrations exceeding their respective drinking water standards recommended by World Health Organization. Although nitrification is the most common acidification driver in the agricultural and/or urbanized lands, the nitrate concentration in this study area is strangely low compared to similar acidic groundwaters reported from other agricultural regions. To find out the causes of acidification, this study investigated the geochemical processes from the extensive groundwater chemistry data and performed geochemical simulations by changing water alkalinity and cation exchange capacity (CEC) of sediment and matching the results with the observed water chemistry data to confirm our hypothesis. Based on this approach, we could reveal that groundwaters of this study could become very acidic due to its dilute nature and low sediment CEC. Since groundwaters are generally very dilute in the recharge area, our finding provides another reason for the discreet management of highland areas where groundwater recharge is concentrated.

Key insight of groundwater quality and nutrients contamination in Pulau Kapas, Terengganu

Pulau Kapas is tropical island which dominantly depends on land-supplied and groundwater for freshwater sources. The groundwater quality was monitored monthly, to identify the possible factors effecting the groundwater quality throughout May to October 2022. Physico-chemical parameters were in-situ measured and groundwater were collected for nutrients analysis in the laboratory. The concentration of ammonium, phosphate, nitrite, and nitrate were in the range of 0.07-1.08 mg/L, 0.00-0.06 mg/L, BDL-18 × 10-4 mg/L and 0.01-0.19 mg/L, respectively. The cluster and principal component analysis unveiled the seawater intrusion for freshwater needs was the dominant factor affecting the groundwater. Followed by the dissolution of soil particles surrounds the groundwater table, and the surface run-off by rainfall. In conclusion, the groundwater was affected by geogenic factors as it was not extensively extracted due to movement control order of Covid-19 event. This has provided significant insight for a better management plan in sustaining the groundwater of Pulau Kapas.

A multidisciplinary approach using hydrogeochemistry, δ15NNO3 isotopes, land use, and statistical tools in evaluating nitrate pollution sources and biochemical processes in Costa Rican volcanic aquifers

Nitrate pollution threatens the Barva and Colima multi-aquifer system, the primary drinking water source in the Greater Metropolitan Area of Costa Rica. In addressing nitrate contamination dynamics, this study proposes an integrated approach by combining multivariate statistical analyses, hydrochemical parameters, sewage discharge, and regional land-use and land-cover patterns to assess the extent and degree of contamination, dominant biogeochemical processes, and refine the interpretation of nitrate sources previously derived solely from δ15NNO3 information. Over seven years (2015-2022), 714 groundwater samples from 43 sites were analyzed for nitrate and major ions, including two sampling campaigns for dissolved organic and inorganic carbon, nitrite, ammonium, FeTotal, MnTotal, and δ15NNO3 analyses. The findings presented elevated nitrate concentrations in urban and agricultural/urban areas, surpassing the Maximum Concentration Levels on several occasions, and oxidizing conditions favoring mineralization and nitrification processes in unconfined Barva and locally confined Upper Colima/Lower Colima aquifers. Similar nitrate contents and spatial patterns in agricultural and urban zones in the shallow Barva aquifer suggest comparable contributions from nitrogen fertilizers and urban wastewaters despite the gradual increase in urban land cover and the reduction of agricultural areas. Isotopic analyses and dissolved organic carbon (DOC) indicate a shift in nitrate sources from agricultural to urban areas in both Barva and Colima aquifers. Principal Component and Hierarchical Cluster Analyses link land use, nitrate sources, and water quality. Three distinct sample clusters aligned with forest/grassland, agricultural/urban, and urban land use, emphasizing the impact of anthropogenic activities on groundwater quality, even in the deeper Colima aquifers. The study challenges nitrate isotope mixing models, enhancing accuracy in identifying pollution sources and assessing the spatial extent of contamination by incorporating DOC and other hydrochemical parameters. Similar outcomes, with and without the use of nitrate isotopes, reinforce the usefulness of the integrated approach, providing a practical and cost-effective alternative.

Natural background levels and driving factors of aluminum in shallow groundwater of an urbanized delta: Insight from eliminating anthropogenic-impacted groundwaters

Knowledge on natural background levels (NBLs) of aluminum (Al) in groundwater can accurately assess groundwater Al contamination at a regional scale. However, it has received little attention. This study used a combination of preselection and statistic methods consisting of the oxidation capacity and the boxplot iteration methods to evaluate the NBL of shallow groundwater Al in four groundwater units of the Pearl River Delta (PRD) via eliminating anthropogenic-impacted groundwaters and to discuss driving factors controlling high NBLs of Al in groundwater in this area. A total of 280 water samples were collected, and 18 physico-chemical parameters including Redox potential, dissolved oxygen, pH, total dissolved solids, HCO3 -, NH4 +, NO3 -, SO4 2-, Cl-, NO2 -, F-, K+, Na+, Ca2+, Mg2+, Fe, Mn, and Al were analyzed. Results showed that groundwater Al NBLs in groundwater units A-D were 0.11, 0.16, 0.15, and 0.08 mg/L, respectively. The used method in this study is acceptable for the assessment of groundwater Al NBLs in the PRD, because groundwater Al concentrations in various groundwater units in residual datasets were independent of land-use types, but they were opposite in the original datasets. The dissolution of Al-rich minerals in sediments/rocks was the major source for groundwater Al NBLs in the PRD, and the interaction with Al-rich river water was secondary one. The high groundwater Al NBL in groundwater unit B was mainly attributed to the acid precipitation and the organic matter mineralization inducing the release of Al in Quaternary sediments. By contrast, the high groundwater Al NBL in groundwater unit C mainly was ascribed to the release of Al complexes such as fluoroaluminate from rocks/soils into groundwater induced by acid precipitation, but it was limited by the dissolution of Mg minerals (e.g., dolomite) in aquifers. This study provides not only useful groundwater Al NBLs for the evaluation of groundwater Al contamination but also a reference for understanding the natural geochemical factors controlling groundwater Al in urbanized deltas such as the PRD. PRACTITIONER POINTS: The natural background level (NBL) of groundwater aluminum in the Pearl River Delta (PRD) was evaluated. The dissolution of aluminum-rich minerals in sediments/rocks was the major source for groundwater aluminum NBLs in the PRD. The acid precipitation and organic matter mineralization contribute to high groundwater Al NBL in the groundwater unit B. The acid precipitation contributes to high groundwater Al NBL in the groundwater unit C, while dissolution of magnesium minerals limits it.

Assessing heavy metal and physiochemical pollution load of Danro River and its management using floating bed remediation

River Danro in Garhwa (India) plays a vital role as a significant source of surface water and a crucial tributary of the North Koel River, ultimately joining the Ganga River Basin. Serving both urban-industrial and rural areas, the region faces challenges, including sand mining near Belchampa Ghat. This study aimed to assess physicochemical and heavy metals pollution at nine sampling locations, utilizing the Overall Index of Pollution (OIP), Nemerow Pollution Index (NPI), and Heavy Metal Pollution Index (HPI). OIP values indicated excellent surface water quality (0.71) in non-monsoon and slight pollution (6.28) in monsoon. NPI ranged from 0.10 to 1.74 in non-monsoon and from 0.22 (clean) to 27.15 (heavily polluted) in monsoon. HPI results suggested groundwater contamination, particularly by lead. Principal component analysis (PCA) and geospatial mapping showed similar outcomes, highlighting the influence of adjacent land use on water quality. Recognizing the significance of the Danro River in sustaining life, livelihoods, and economic growth, the study recommends implementing measures like floating bed remediation and regulatory actions for effective river management. The study acknowledges weaknesses in the current practical assessment methods for water contamination. These weaknesses make it difficult to put plans for cleaning up and controlling contamination into action. Because of this, future research on developing new in-place remediation techniques should focus on creating better ways to measure how effective the cleanup is.

Nitrate dynamics in the streamwater-groundwater interaction system: Sources, fate, and controls

Nitrate (NO3-) pollution has attracted widespread attention as a threat to human health and aquatic ecosystems; however, elucidating the controlling factors behind nitrate dynamics under the context of changeable hydrological processes, particularly the interactions between streamwater and groundwater (SW-GW), presents significant challenges. A multi-tracer approach, integrating physicochemical and isotopic tracers (Cl-, δ2H-H2O, δ18O-H2O, δ15N-NO3- and δ18O-NO3-), was employed to identify the response of nitrate dynamics to SW-GW interaction in the Fen River Basin. The streamwater and groundwater NO3- concentrations varied greatly with space and time. Sewage and manure (28 %-73 %), fertilizer (14 %-36 %) and soil organic nitrogen (12 %-28 %) were the main NO3- sources in water bodies. Despite the control of land use type on streamwater nitrate dynamics in losing sections, SW-GW interactions drove NO3- dynamics in both streamwater and groundwater under most circumstances. In gaining streams, streamwater nitrate dynamics were influenced either by groundwater dilution or microbial nitrification, depending on whether groundwater discharge ratios exceeded or fell below 25 %, respectively. In losing streams, groundwater nitrate content increased with streamwater infiltration time, but the influence was mainly limited within 3 km from the river channel. This study provides a scientific basis for the effective management of water nitrate pollution at the watershed scale.

Distribution, sources, and risk analysis of heavy metals in sediments of Xiaoqing River basin, Shandong province, China

The accumulation of heavy metals in river sediment poses a major threat to ecological safety. The Xiaoqing River originates in western Jinan, with higher population density and per capita gross domestic product (GDP) in its basin compared to the Shandong province average. This study analyzed the spatial characteristics, ecological risk, human health risk, and contamination sources of heavy metals by collecting sediment samples from Xiaoqing River. We use the methods such as geo-accumulation index (Igeo), ecological risk assessment based on the interval number sorting method, and health risk assessment to evaluate the risk of heavy metals in sediments. The research finding suggests heavy metals including Pb, As, Ni, and Cr are low ecological risks, while Hg and Cd have reached high and extreme ecological risks. Correlation analysis and principal component analysis were used to analyze the correlation and sources of different heavy metals. The six heavy metals were categorized into three groups. Factor 1, comprising Hg, Cr, and Pb, was identified as a mixed source with a contribution rate of 37.76%. Factor 2 is an agricultural source and comprises Ni, Cd, and As with a contribution rate of 27.05%. Factor 3 includes Pb and Ni contributing to 15.30% as a natural source. This study offers valuable insights for the prevention of heavy metal pollution, as well as promoting sustainable urban development.

Evaluating natural background levels of heavy metals in shallow groundwater of the Pearl River Delta via removal of contaminated groundwaters: Comparison of three preselection related methods

Assessing natural background levels (NBLs) in groundwater is a global concern. Knowledge on groundwater NBLs in urbanized areas is challenging due to the impact of complex human activities. Preselection related methods are common ones for assessing groundwater NBLs. The present study used three preselection related methods to assess groundwater heavy metals (lead, zinc, barium) NBLs in four groundwater units of the Pearl River Delta (PRD) where urbanization continues, and to identify the best one for assessing groundwater NBLs in urbanized areas. Here, methods include a preselection method (method-P), a preselection dominated method (method-PD), and a statistic dominated method (method-SD). Results showed that the method-PD was better than other two methods for assessing groundwater NBLs of heavy metals in the PRD. This is supported by the evidence that differences among heavy metals concentrations in various land-use types in residual datasets formed by the method-PD were insignificant. NBLs of lead in groundwater units I to IV assessed by the method-PD were 2.8 μg/L, 5.9 μg/L, 5.8 μg/L, and 2.6 μg/L, respectively. NBLs of zinc in groundwater units I to IV assessed by the method-PD were 30 μg/L, 180 μg/L, 160 μg/L, and 100 μg/L, respectively. NBLs of barium in groundwater units I to IV assessed by the method-PD were 120 μg/L, 120 μg/L, 90 μg/L, and 50 μg/L, respectively. Compared to the method-PD, the method-SD often underestimates groundwater NBLs of heavy metals because of using the experiential evaluation for residual datasets. The method-P also has an inaccurate evaluation of groundwater NBLs of heavy metals in comparison with the method-PD, owing to both of using the experiential evaluation and the absence of a function for outliers test. The method-P combining with an outliers test would be better than itself for assessing groundwater NBLs. Therefore, the method-PD is the first choice to be recommended for assessing groundwater NBLs in urbanized areas such the PRD. However, this method should not be taken into account for assessing groundwater NBLs in areas where groundwater Cl/Br mass ratios are invalid. Instead, the method-SD and the method-P combining with one outliers test may be choices, because no constraint for these two methods.