Groundwater Quality and Human Health Risk

1. Introduction

Access to potable water is one of the most significant challenges that humanity will have to deal with in the years to come. Contamination of an aquifer with hazardous chemical species can render it unusable for several decades [1]. The residence time of contaminants in groundwater bodies can vary from a few weeks to several decades, depending on the physico-chemical characteristics of the compounds and the aquifer properties [1,2,3,4]. Moreover, the expansion of human activities often conflicts with the measures necessary to safeguard groundwater supplies for future usage [5,6]. The use of groundwater as a source of drinking water is preferred over the use of surface water, because groundwater often requires less treatment. However, in order to make use of groundwater as a sustainable resource, it is necessary to conserve and preserve the groundwater sources suitable for use. The lithosphere, hydrosphere, biosphere, atmosphere, and anthroposphere are all complicated systems where geochemical and earth processes take place. These processes determine the composition of water and its suitability for various purposes. Due to the fact that groundwater moves through earth materials (rocks, sediments, and soils), it has the potential to pick up trace elements and radionuclides that naturally exist in the earth. Groundwater contamination can come from a variety of lithological [5,6] or anthropogenic sources [4,7], the most prevalent of which are agriculture, industry, and waste from both humans and animals. There are a number of factors that influence the interactions between contaminants in the subsurface system. These factors include the characteristics of earth materials, the activities of microbiological organisms, and the hydrogeological settings.

In contrast to physical hazards such as earthquakes, volcanoes, and landslides, chemical hazards are challenging to accurately identify through spatial and temporal means. Groundwater may include environmentally hazardous chemical substances that pose a risk to human health.

The evaluation of the danger to human health and epidemiological studies are the primary instruments utilised in determining the potential detrimental effects of contaminants. A meticulous investigation of the transfer of contaminants from the land surface to the water table is crucial to accurately identify the mechanisms that affect the groundwater quality. Drinking water standards have already been developed by the majority of national agencies and stakeholders, and these standards include limitations for several common elements. Therefore, the question that needs to be answered is whether the drinking water requirements require additional revisions. The importance of studying issues related to groundwater quality and human health risk has been recognised worldwide by many researchers [4,8,9,10,11,12,13,14].

A bibliographic study was performed via a search in Scopus, using terms such as “groundwater quality” and “human health risk”; the initial search resulted in 47,246 (from 1933 to August 2024) and 1,242,767 (from 1931 to August 2024) documents, respectively. Further, the relevant documents were selected when they met the criteria related to “groundwater quality and human health risk”. The number of documents covering the time period from 1973 to August 2024 for “groundwater quality and human health risk” was 2,292 (Figure 1).

Figure 1 shows the rapid expansion of published documents after the year 2017, depicting the increased concern of the scientific community on issues related to groundwater quality and human health risk.

2. An Overview of the Published Articles

Soares et al.’s (contribution 1) study revealed the existence of trace element contents in the groundwater surrounding the Marituba Landfill in Brazil. Low contents of heavy metals in the groundwater of the area studied, including Al, Cd, Hg, Pb, and Ni, were detected below the permissible limits set by Brazilian regulations. However, the Al concentration exceeded the limit at three sampling sites during the wet season and seven points during the dry season. Upon chemical examination, it was determined that the groundwater was fit for human consumption. The Groundwater Quality Index values in the rainy and dry seasons were 14.766 and 13.279, respectively. These values are deemed excellent for human consumption. The Heavy Metals Assessment Index and Heavy Metal Pollution Index measurements revealed minimal contamination levels. The risk assessment indicates that the data collected from the studies and the calculations do not show any harmful effects on human health. However, the presence of toxic metals, even in small amounts, is a clear indication of contamination caused by human activities, as these metals are not naturally found in the environment. They verified that the landfill is the sole source of pollution, as no other industrial operations are nearby. Despite being located upstream of the surrounding neighbourhoods, the low quantities suggest contamination occurs due to leachate from the free aquifer. Enhanced monitoring is necessary to minimise the effects, safeguard these resources, and regulate the contaminants to ensure that the water remains safe for consumption and to prevent people from experiencing illnesses caused by these pollutants.

Zhao et al. (contribution 2) examined the spatial and temporal variations and patterns of a local water ecosystem by analysing the monitoring data on hydrology and water quality and the geographical placements of the monitoring sections in the study area, the Cixi plain in eastern China. The study focused on analysing the fate and diffusion mechanisms of surface water contaminants in different sections of the river, considering the calculation of the runoff generation and concentration in the coastal plain river network, as well as the characteristics and impact evaluation of regional water contamination attributed to different sources and types of contaminants entering the river. The outcome of the study performed by Zhao et al. (contribution 2) indicates that the coastal plain’s water environment and water resource insecurity primarily cause eutrophication in water bodies. The main contributors to eutrophication are nitrogen and phosphorus inputs from terrestrial sources, which include the production and release of domestic sewage, nutrient loss in aquaculture water bodies, and the impact of agricultural activities. The review also indicates that the expansion of coastal areas and the rapid growth of seaside towns, coupled with the increasing population in these areas, as well as industrial development, are the key drivers for changes in water quality.

The article by Wang et al. (contribution 3) elucidates the potential threat to groundwater in locations where in situ mining of oil shale occurs. Wang et al. analysed the presence of five water quality parameters: Cr, Fe, Mn, N-NH₃, and SO₄²⁻. The content of these parameters in groundwater was assessed applying a health risk assessment approach and an enhanced Nemero comprehensive index method. The findings indicate that the average values of Fe, Mn, and SO₄²⁻ in the leaching solution from Fuyu oil shale surpassed the Class III groundwater quality standard (GB/T 14848-2017) employed in the People’s Republic of China. However, the Cr and N-NH₃ contents were below the standard, and the leaching solution fell within Class V for groundwater quality. The Fe and Mn concentrations in the Fushun oil shale leaching solution exceeded the Class III norm. However, the contents of Cr, N-NH₃, and SO₄² in this oil shale were below the limit. Overall, the leaching solution was classified as Class IV in terms of the groundwater quality. The Nemero evaluation technique assigns the highest weighting value to the heavy metal Cr due to its potential to negatively impact the groundwater quality. The weight assigned to sulphate is the lowest due to its low level of danger. The chemical carcinogen Cr poses the most significant threat to human health. The health danger posed by the non-carcinogenic chemical Mn is higher than that of Fe and N-NH₃. A significant amount of Mn will be discharged into the groundwater when high pyrolysis temperatures are employed. Hence, it is imperative to enhance supervision and control. The findings given in this study can serve as a benchmark for the thorough assessment of groundwater hazards resulting from in situ oil shale mining.

The fourth article published in this Special Issue presents the research conducted by Naz et al. (contribution 4), evaluating 24 physicochemical factors at more than 1094 locations to calculate the Water Quality Index (WQI) in the upper and central areas of Punjab, Pakistan. Before calculating the WQI, an Analytical Hierarchy Process was used to investigate the precise weights for each water quality indicator. The WQI was classified into several categories by creating a pairwise matrix using Saaty’s scale to determine their relative significance. In addition, the integration of the WQI and geostatistical techniques was used to determine the groundwater quality status for drinking and irrigation uses in different zones of the area studied. The results depicted noticeable trace element contents in the Lahore division and increasing microbiological pollution throughout the area studied, possibly caused by untreated industrial waste discharge and poorly maintained sewerage systems. Conversely, the index for the remaining divisions fell within the moderate range, indicating the groundwater’s appropriateness for consumption. The scenario analysis for designing mitigation solutions revealed that implementing basic treatment before wastewater disposal could restore 9% of the area studied. This would be followed by secondary treatment, which could rehabilitate 35%, and tertiary treatment, which could rehabilitate 41% of the study area. The primary obstacle faced by water delivery agencies is the prevalence of microbiological pollution, which accounts for 27% of the challenges encountered. Considering the present decline in water quality, the availability of safe drinking water is likely to become a significant issue of public concern. Therefore, Naz et al. (contribution 4) recommend that government entities take the necessary actions to improve the overall quality of groundwater and promote sustainable development.

In the fifth article, Fernandez-Rojo et al. (contribution 5) studied the utilisation of granular ferric iron–(oxy)hydroxides, a widely recognised and efficient technique to reduce arsenic levels in various water sources. Nevertheless, to directly apply this technique in contaminated groundwaters, it is necessary to create novel injectable adsorbents for aquifers that can counteract acidic substances. Within this framework, the objective was to enhance the adsorption of As(III) and As(V) at various pH levels and regulate the pH by dissolving calcite. A granular ferric hydroxide-calcite (GFH-C) adsorbent was subjected to sonication, reducing the size to 0.4–50 µm. Batch experiments were performed to investigate the adsorption isotherms and kinetics of As(III) and As(V), as well as the kinetics of calcite dissolution, employing two different sizes of GFH-C (granular and sonicated). The findings indicated that the sonicated adsorbents did not substantially enhance their ability to bind arsenic.

In contrast, the sonication process enhanced the adsorption kinetics of As(III) and As(V), similar to the improvement observed in the kinetics of calcite dissolution. The leaching of calcite from the adsorbent resulted in an elevation of the water’s pH to approximately 9.2–9.4. The sonicated adsorbent is smaller, allowing for faster arsenic adsorption and effective neutralisation of acidic water. This makes it advantageous to remediate groundwater contaminated with arsenic.

The research from Alkathiri et al. (contribution 6) reported that adenoviruses are recognised as highly enduring gastrointestinal infections capable of infiltrating entire aquifer systems. Continuous monitoring of irrigation water is crucial in order to minimise any public health hazards. The study examined the influence of climatic conditions (temperature, relative humidity, and wind speed) on the prevalence and genetic variation of Human Adenoviruses (HAdV) in groundwater bodies. This was performed by focusing on a specific section of the Hexon gene. The materials were concentrated using the PEG technique. HAdVs were identified through the use of polymerase chain reaction (PCR), and their genetic sequences were determined using Sanger sequencing. Subsequently, a phylogenetic analysis was conducted. The HAdV sequences were discovered to have a close relationship with species F (specifically type 41), with the 00-2B64 sequence being the most prevalent at 86.4%. The phylogenetic analysis revealed a strong correlation between isolate 00-2B64 from this study and isolates from Brazil and Saudi Arabia. On the other hand, isolate 08-2B64 was found to be closely linked to a sequence obtained from the AnNazim landfill in Riyadh, Saudi Arabia, with a genetic distance of 0.00. The prevalence of HAdV was high when the temperature ranged from 19 to 28 °C, the wind speed ranged from 16 to 20 km h⁻¹, and the relative humidity varied between 15 and 25%. There was no substantial impact of meteorological changes on the prevalence of HAdVs. The study yielded information on the prevalence of HAdV and the dominant species in the irrigation water of Riyadh, Saudi Arabia. Additionally, it provided insights into the environmental consequences of HAdV persistence, which will facilitate the development of disease control measures.

The study by Shao et al. (contribution 7) investigated the groundwater–surface water exchange and spatial distribution of As in the Aksu River, a typical river located in Northwestern China. Groundwater and surface water samples were collected within the river basin of the study area. In total, 18.64% of the groundwater samples showed As contents higher than the allowed limit for drinking water (of 10 μg L⁻¹). Similarly, 39.02% of the surface water samples presented As concentrations that exceeded this threshold. The water bodies in the Aksu River Basin have somewhat alkaline pH values, with the content of dissolved minerals in the surface water being much higher than in the groundwater. The chemical compositions of the surface water and groundwater exhibited remarkable similarities, with Cl⁻ and SO₄²⁻ being the primary anions and Na⁺ being the major cation. The primary factor affecting the water chemistry of surface water and groundwater in the Aksu River Basin is the dissolving of silicate and carbonate minerals, with the dissolution of salt rocks being of secondary importance. Human activities significantly influence the quality of water in the river. The occurrence of groundwater with high As content is completely contained within the geographical distribution of the surface water. The interaction of groundwater and surface water is crucial for the enrichment of the As content in the surface water.

The study conducted by Liu et al. (contribution 8) aimed to investigate the sources and enrichment method of F⁻ in the groundwater of the Weigan River Basin, China, where the fluoride content exceeds the permissible range. Sets of shallow groundwater samples were collected and studied, using information retrieval and regional geological environment studies. The research findings indicated that the F⁻ concentration in the drainage basin of the study area varied between 0.2 and 5.46 mg L⁻¹, with an average of 1.14 mg L⁻¹. The test results indicated that 40.43% of the water sample test results exceeded the national drinking water guidelines of 1.0 mg L⁻¹. The mean pH value of the water in the study site was 8.37, indicating a somewhat alkaline nature. The dominant hydrochemical type was Na⁺–Cl⁻/(SO₄²⁻). It was observed that groundwater with an elevated percentage of F^- exhibited a significant presence of Na⁺ and a minimal amount of Ca²⁺. The primary process responsible for the excessive accumulation of fluoride in the groundwater was the ongoing dissolving of minerals containing F⁻, specifically fluorite, in the study area. The primary source of F^- was the breakdown of silicate rocks and evaporites due to weathering. The primary variables governing the enrichment of F⁻ were evaporation–crystallisation and cation exchange. These findings suggest that physicochemical processes play a significant role in regulating the enrichment of F⁻ and can be valuable for analysing the presence of F^- in groundwater in arid and semi-arid regions.

Adenova et al. (contribution 9) examined groundwater quality in Kazakhstan’s Zhambyl region. The fieldwork and data processing were conducted in three distinct phases. The first phase involved a survey of the existing water wells. In the second phase, more detailed hydrogeological investigations were carried out, which included measuring the flow rates, pH, temperature, and electrical conductivity of water samples. The third phase consisted of processing and analysing the field data samples in chemical laboratories. Kazakhstan’s drinking water standards are far less stringent than the standards set by the European Union (EU).

Tasleem et al. (contribution 10) evaluated the groundwater quality in aquifers of Saudi Arabia to determine the presence of heavy metals. It was found that the concentration of chromium in Madinah was significantly high. In order to understand the function of chromate reductase (ChrR) in the conversion of Cr(VI) to Cr(III), a comprehensive sequence analysis was conducted. Models for both the wild-type and mutant forms of ChrR were developed using various homology modelling approaches. Tasleem et al. (contribution 10) presented an innovative approach to analyse the three-dimensional configuration and interaction between ChrR and Cr(VI) to transform it into a less dangerous form (III). In addition, it offers stable mutants, namely Arg83Trp, Gly124Ile, and His127Trp, that exhibit a strong affinity for Cr(VI). These mutants can be potentially utilised in protein engineering to develop enzymes that are both stable and effective in converting Cr(VI) into a less harmful form.

Rehman et al. (contribution 11) focused on assessing the appropriateness of groundwater in recently established residential areas of the Gujranwala district (Pakistan). The research involved measuring the quality parameters, identifying pesticide contaminants, and analysing the health risks for adults and children consuming groundwater. The detection of pesticides and plasticisers was conducted using gas chromatography–mass spectrometry (GC/MS). The findings indicated that the levels of dissolved Cr and Pb surpassed the permissible threshold set by the World Health Organization (WHO) in over 20% of the locations where samples were taken. Nevertheless, the measured physico-chemical characteristics and amounts of Fe and Zn were below the allowable thresholds. The contaminants that were found in the highest levels were plasticisers, with a total of 30 discovered. This was followed by herbicides, with 21 detected, and fungicides, acaricides, and insecticides, with 16 detected. Lastly, seven different types of plant growth regulators were detected. Distinct variations were noted in the hazard quotient (HQ) and hazard indices (HI), with some exceeding the established limitations set by the WHO. For adults and children, the hazard quotient decreased in the following order: Cl > Zn > Mg > Cr > Pb. Essentially, the water in the recently created housing societies of the researched region is of low quality and may represent a risk to the safety and well-being of the occupants if consumed.

An et al. (contribution 12) devised a novel method for determining the best locations for observation wells and accurately identifying the parameters of groundwater contamination sources. The methodology incorporated Bayesian principles, Markov Chain Monte Carlo, Bayesian design, machine learning, surrogate modelling, and information entropy. The selection of the most effective locations for observation wells was decided using information entropy. This method was used to extract meaningful information about unknown characteristics of groundwater contamination sources from measurements of contaminant concentration based on Bayesian design principles. The applied surrogate model significantly expedited the determination and identification processes. Finally, two hypothetical numerical case studies were conducted to validate the effectiveness of the suggested approach. These case studies involved both homogeneous and heterogeneous scenarios. The findings demonstrated that the suggested approach enables precise and effective implementation of the best design of observation well sites and high-precision detection of groundwater contamination source parameters. In conclusion, the article by An et al. emphasises that combining Bayesian and machine learning methods offers a potential alternative for accurately identifying factors related to the cause of groundwater contamination.

Lalumbe and Kanyerere’s study (contribution 13) analysed groundwater quality data from geothermal springs and boreholes to investigate the hydro-geochemical processes that affect the groundwater composition of the Soutpansberg region. The investigation revealed that most samples were categorised as fresh groundwater with a predominant composition of calcium bicarbonate (Ca-HCO₃) and a mixture of calcium magnesium chloride (Ca-Mg-Cl). The statistical and hydrochemical analysis of the indicated that rock dominance processes primarily influence the groundwater quality in the study area. These processes include the weathering of silicates, the dissolution of carbonates and halite minerals, and the ion exchange processes. The geothermal spring had a significant concentration of F⁻ due to the dissolution of minerals containing fluorite. The content of NO₃⁻ in the groundwater was attributed to the use of fertilisers in the cultivated land. This study proposes that the findings may be helpful to policy makers and stakeholders for influencing and supporting policy change and groundwater allocation in arid to semi-arid rural contexts. This would lead to the sustainable management of water resources. This study suggests implementing appropriate de-fluoridation and de-nitrification methods to strengthen the potability of groundwater.