Heavy metals in soil and water: From pollution sources and spatial dynamics to environmental risks and sustainable remediation: Review

1: Introduction

Heavy metals pollution of soil and water has become a major environmental problem due to the persistence, toxicity and potential mobility of these elements in natural ecosystems. Unlike many organic pollutants, heavy metals cannot be biologically degraded and may remain in soils and sediments for long periods of time, accumulating and entering surface water, groundwater, plants, animals and the human food chain. Lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), mercury (Hg), nickel (Ni), copper (Cu) and zinc (Zn) are some of the most studied contaminants due to their persistence in the environment and potential toxicity to living organisms (Hou et al., 2017; Sodango et al., 2018) [1,2] . Heavy metals are derived from natural and anthropogenic sources. Natural sources are mainly due to rock and mineral weathering, volcanic processes and natural erosion, whereas anthropogenic sources include mining, metal processing, industrial emissions, fossil fuel combustion, transportation, agricultural activities, wastewater irrigation, fertilizers, sewage sludge and inappropriate waste disposal (Sodango et al., 2018; Vardhan et al., 2019) [2,3] . Further inputs can also be caused by rapid urbanization and industrial development, leading to localized contamination around industrial areas, roads, mining areas, agricultural areas and points of wastewater discharge. Therefore, it is important to determine the relative contribution of natural and anthropogenic sources for understanding the origin and persistence of heavy metal contamination (Hou et al., 2017; Vardhan et al., 2019) [1,3] . The heavy metal content of soil is connected to characteristics of the source and environmental conditions. Soil pH, organic matter, clay content, cation exchange capacity, redox potential, carbonate content, iron and manganese oxides can strongly influence metal adsorption, desorption, precipitation and dissolution, mobility and bioavailability. Thus, total metal concentration may not be sufficient to characterize the environmental behavior or potential risk of a contaminant (Briffa et al., 2020) [4] . Metals can be tightly bound to soil particles under some conditions, but they become more mobile if the environmental conditions change (particularly pH or redox potential). The spatial variability of heavy metal pollution is a serious problem. Land use, geology, industrial activity and agricultural methods can all cause dramatic changes in concentrations over short distances. Geostatistical and multivariate statistical methods combined with Geographic Information Systems (GIS) are therefore powerful tools for pollution mapping, identification of spatial patterns and potential sources or hot spot of pollution (Briffa et al., 2020; Dhaliwal et al., 2020) [4,5] . This allows environmental scientists to use techniques beyond point sampling and to develop a more complete picture of the spatial distribution and possible source origins of pollutants over time. Heavy metals can be transferred indefinitely between environmental compartments and therefore, contamination of soil and water is of particular concern. Leaching is the process where metals in the soil are transported to the groundwater. Particles with attached metals can be transported to rivers, lakes and other water bodies by surface runoff. Irrigation with surface or ground water contaminated with metals can lead to build-up of metals in agricultural soils. Other processes such as adsorption-desorption, precipitation-dissolution, complexation and sedimentation and resuspension also play a major role in the fate of metals being species into soil-water-sediments systems (Sodango et al., 2018; Cheng et al., 2020) [2,6] . Thus, the heavy metal pollution should not be viewed as two completely different systems of soil and water. Heavy metal contamination: Ecological consequences High concentrations of metals in the soil ecosystems could affect microbes, enzyme activities and nutrient cycling fertility of soil. Excess accumulation in plants impairs nutrient uptake, photosynthesis, germination, plant growth and cellular processes. Sediments and aquatic organisms can accumulate heavy metals from the environment and transfer them through food webs (Cheng et al., 2020; Wang et al., 2020) [6,7] . The processes are especially important for agricultural and aquatic ecosystems because the polluted environmental media may be long-term exposure routes. Another important aspect of heavy metal pollution is human exposure. Humans are exposed via ingestion of contaminated food and water, inhalation of dust containing any As , and direct skin contact with soils and sediments containing As . Health effects are dependent on the metal, its concentration and physical/chemical form, route of exposure (e.g. inhalation, injection), duration of exposure and individual susceptibility. Examples include lead and mercury which are associated with significant neurological effects. Cadmium is strongly associated with renal and skeletal toxicity. Some oxidation states of chromium and arsenic are also relevant since they are carcinogenic.. (Wang et al., 2020) [7] . Environmental assessments are changing to include measures of contamination in ecological and human health risk assessment. Heavy metals are persistent and bioaccumulate in environmental and biological systems, so that remediation is an important aspect of environmental management. soil excavation, soil washing and solidification/stabilization, chemical precipitation and chemical immobilization are traditional technologies. These techniques allow a relatively rapid treatment but are frequently associated with high costs, high energy consumption, generation of secondary waste or disturbance of the contaminated site (Wang et al., 2020) [8] . These limitations have opened the door for biological and environmentally sustainable alternatives. Phytoremediation, microbial remediation, biosorption and bioaugmentation have attracted much interest as less invasive approaches to contaminated environments. Phytoremediation includes phytoextraction, phytostabilization, rhizofiltration and other plant mechanisms depending on the contaminant and environmental conditions (Wang et al., 2021) [10] . Moreover, biochar has been recognized as a potential soil amendment for remediation of heavy metal contaminated soils, due to its porous structure and surface functional groups, which can facilitate the adsorption and immobilization of heavy metals, and thus reduce their mobility and bioavailability (Nedjimi, 2021) [9] . However, the effectiveness of such measures is often limited by much higher plant species and soil amendments content of heavy metals, which have root systems that can uptake bioavailable heavy metals from contaminated soils through passive (absorption) and active mechanisms. The recent studies are leaning toward integrated remediation combining physical, chemical and biological and nature-based measures. Therefore, the selection of suitable technology is not only based on the removal efficiency, but also on economics, ecological approaches, long-term stability and secondary pollution potential and reusability of valuable metals. The knowledge gaps between these strategies are particularly relevant when directly relating to the development of feasible remediation strategies for a range of contamination types (Wang et al., 2020; Nedjimi, 2021; Wang et al., 2021) [8,9,10] . Heavy metal contamination has been widely studied, but there is still much to learn about the connections between potential contamination sources and their distribution in the environment, transport in terrestrial ecosystems, effects on species abundance or ecosystem biodiversity, health risks to humans and pets from exposure to such contaminants, and the effectiveness of clean-up operations. Most studies have been focused on either single environmental compartments or single metals, while most real contaminated sites are characterized by several metals acting on complex systems of soil and water. Therefore, a combined effort of spatial assessment, source identification and risk assessment & remediation research may lead to a more comprehensive framework for environmental management. This review thus offers a thorough overview on heavy metal contamination in soil and water with emphasis on major natural and anthropogenic sources, spatial distribution and controlling factors, environmental and human-health impacts as well as conventional and emerging remediation technologies. Especially the use of GIS and spatial analysis, the methods for pollution and risk assessment, and sustainable solutions for cleanup. In addition, the review proposes major research directions to be addressed and possible efforts for improving monitoring, management and remediation of heavy metal contaminated soil and water systems.

Background and literature review

The increasing throughput of information on heavy metal contamination in terrestrial compared to aquatic systems highlights the pressing need for site specific knowledge of their sources, fate, bioaccumulation and effect on ecological and biological systems. Much research has been done on single metals, sources of pollution, polluted sites, ecological consequences and treatments, but the results are heterogeneous due to the contamination styles, which strongly depend on geographical, geological, climatic or anthropogenic parameters. This means synthesizing existing evidence to identify common trends, major routes of contamination and the overall strengths and weaknesses in current assessment and remediation practices. Paper highlights past research work in heavy metal pollution, sources, spatial distribution. Environmental and human health authors of papers reported assessment approaches as well as the statistics behind them remediation technologies to mitigate heavy metals pollution.

Heavy metal pollution: A literature review

Previous reports show that contamination of soil and water systems by heavy metals is a complicated and an ongoing environmental issue, caused by the collaboration of natural geological processes and anthropogenic activities. Many studies have reported the presence and distribution of metals like Pb, Cd, Cr, As, Hg, Ni, Cu and Zn in various environmental compartments from industrial, agricultural, urban and mining impacted sites. The literature review shows that the heavy metals are random in their origin. The concentrations and shapes of mobilities / environmental behavior depend on geological features of a territory, land utilization, industrial activity, soil properties and hydrological conditions. Other studies were directed toward the need for distinguishing between natural and anthropogenic fluxes to evaluate pollution impact and to identify sources, which can assist in environmental management responses ( [10,11,12,13,14,15,16] . Recently, more attention has been paid to spatial assessment in heavy metal studies. Researchers have applied GIS, geostatistical and spatial interpolation/multivariate statistical techniques to map contamination patterns, identify pollution hot spots and investigate associations between metal concentrations and potential sources. These methods can give a holistic view of contamination, rather than point measurements and may help to support environmental monitoring or land management decisions (Wang et al., 2021; Rana et al., 2021; Bhat et al., 2022) [10,11,17] . High accumulations of heavy metals in plant tissues can occur over relatively short and spatially restricted distances (Huisman et al. 1998; Mackie, Muirhead & Hesketh 2003). Differences in land use associated with atmospheric deposition, traffic density, industrial emissions, irrigation practices and geological background were found to be potential sources of variability in heavy metal concentrations in soil (Roselle et al. In this way, soil biogeochemical properties can vary substantially and give rise to heterogeneities on relatively small scales related to land cover, which may have major implications for heavy metal accumulation in crops grown in the vicinity. The literature review also includes correlations of heavy metal pollution and environmental effects. But they may have separate effects on soil micro-organisms, soil fertility, n plant growth and nutrient cycling. Metals in surface water and groundwater can be detrimental to aquatic life and water quality. The uptake of metals by plants is of particular importance as this may increase the transfer of these substances from polluted soils and irrigation water, into crops and subsequently into the food chain (Wu et [18,19,20,21,22] . In this regard, many studies have combined contaminant measurements with ecological and human-health risk assessments to investigate potential long-term exposure effects. Another one of the possible applications that attracted a lot of interest is remediation, or clean-up, of contaminated environments. Conventional methods like excavation, soil washing and chemical treatment, stabilization and immobilization can minimize pollution, but are expensive for implementing, technically complicated and may creates further environmental damages as well. As a result, biological and green techniques for remediating contaminated sites such as phytoremediation (Sarker et al., 2023) [23] , microbial remediation (Aziz et al., 2023) [24] , biosorption (Yang et al., 2023) [25] , Biochar application (as adsorbents) ( [26,27,28] and other modified adsorbents tailored to the contaminants under investigation have become important research areas. Previous research has focused on integrated approaches where several technologies are combined according to the specific site characteristics and the redox behavior of the metal. Overall, the existing literature has moved from the simple measurement of heavy metal concentrations to more integrated approaches including source identification, spatial modeling, environmental transport fate, and risk assessment and remediation to ecological systems and human-health concern. Differences in sampling strategies, analytical methods and environmental conditions and risk-assessment approaches can create challenges for direct comparison of study results. Table 1 summarizes the description of the studies selected for this review. The table contains aspects of the research such as environmental systems studied, key findings, and the extent to which findings contributed toward understanding/management of heavy metal contamination in soil and water. No.

Heavy-metal contamination review (2024)

Sources, toxicity and remediation Reviewed sources, toxicity, environmental behavior, and remediation methods for heavy metals in soil and water.

Figure 1 shows the distribution of the 25 studies reviewed by their main research focus. The literature shows a major focus on remediation and biochar-based technologies being the predominant group of studies, which demonstrates the growing interest in sustainable solutions to reduce the mobility and bioavailability of heavy metals.

Studies on sources and spatial distribution and on environmental and human-health impacts are also important parts of the literature, emphasizing the necessity to identify contamination sources and to un-derstand its consequences. Moreover, some studies have focused specifically on soil and water contamination, and on phytoremediation and bioremediation. In general, the distribution indicates that the current research is trending more toward an integrated approach including contamination characterization, spatial assessment, risk evaluation, and environmentally sustainable remediation . . [11,12,13,14,15,16] . Mining and industrial activ- . Another important source is the agricultural sector. Continued use of fertilizers, pesticides, sewage sludge and contaminated irrigation water can increase metal concentrations in agricul tural soils. These metals could then be taken up by plants and transferred through the food chain depending on their chemical forms, mobility and bioavailability (Song et [14,21,24] . Runoff, infiltration and leaching may also transfer heavy metals from contaminated soils to surface and ground water. Factors like soil pH, organic matter, texture, clay content, redox conditions and their interactions with soil minerals and organic compounds strongly influence their mobility (Rana et al., 2021; Ahmad et al., 2021) [11,20] . Changes in such environmental parameters can enhance the mobility and transport of metals among soil, water, sediments, and biotic compartments. Spatial analysis can play a key role in locating potential sources of contamination and pollution hot spots. GIS, as well as geostatistical methods including spatial interpolation and multivariate analysis may help in discovering spatial patterns and associations between heavy metal content with land use or (potential) sources of pollution (Wang et [10,22,27,28] . This makes the processes of natural and anthropogenic sources of heavy metal pollution as intertwined, which indicates the integrated source identification, spatial assessment, environmental monitoring and sustainable remediation. (Rana et [11, 22, 27, 28, 30] .

Results of the literature review

On the basis of reviewed literature, we hypothesize that heavy metal pollution of soils and waters is a complex environmental problem driven by both natural geological conditions as well as a multitude of anthropogenic activities. Most of included studies in this review have cited mining and industrial activities, agricultural activities (and/or agrochemical applications), effluent discharges, transporting processes, fossil fuels combustion and excessive waste disposal as the major sources of those metals. They even uncovered processes that scientists also separately identified as important metal background sources, like the weathering and erosion of rock over millennia. The relative contribution from each source differed by location, land use, industry intensity and geology characteristics {ref 11-16} Our review of the studies revealed reasonable spatial variation. Those sites with the higher concentrations of metal related, in most cases to mining activities, industrial areas, urbanized area and they located to the points of high discharges of waste waters and very intensively cultivated agricultural lands. On the other hand, locations distant from major anthropogenic sources generally exhibited milder contamination or concentrations near background natural values. In this work, in addition to the methods described above, we used Geographic Information Systems (GIS), geostatistical techniques and spatial interpolation and multivariate statistical analysis to identify the contaminations hotspots, describe spatial patterns and relate elevated concentrations with potential polluting sources (Wang et [10,22,23,24,25,26,27] . The findings show that spatial assessment is an integral component of environmental monitoring and can be useful for identifying priorities for future research. Another important point visible from these studies is that heavy metals do not stay in the same environmental compartment. Contaminated soil (a long-term reservoir) can release metals into surface water and ground water via runoff, infiltration and leaching. Additionally, contaminants including heavy metals in irrigation water could cause metal accumulation in agricultural soils. Some environmental properties that control the mobility of heavy metals (i.e pH, organic matter content, soil texture and clay content and redox conditions in addition to mineral composition) (Rana et al., 2021; Oladoye et al., 2022; Ahmad et al., 2021) [11,16,20] . Alterations of these parameters can facilitate or impede metal mobility, and thereby the availability of metals to organisms. Environmental effects were reported in terrestrial and aquatic ecosystems, at several levels. Soil microorganisms are very sensitive to certain heavy metals (Li et al., 2021) and regular ingestion of high doses of metals is very toxic or harmful to them (soil microbial properties occupying a superlative position in the hierarchy of soil quality assessment ) such as Arsenic, leads, Cobalt and metals with free radicals effects what can altering all enzymatic activities, disturbing nutrient cycles. Metal accumulation in plants can affect germination, growth, nutrient uptake, photosynthesis and other physiological processes. They become accumulated in sediments and aquatic organisms from where they get transferred via food webs. ( [13,20,21,24] . These results indicate that the ecological significance of contamination depends on not only total concentrations, but also on the chemical form, mobility, persistence and bioavailability of individual metals. Another major issue identified in the literature was human exposure. Contaminated drinking and irrigation water, agricultural products, soil and atmospheric dust can each provide various exposure pathways. Agricultural environments are of particular importance due to uptake of metals accumulated in soil by crops and transfer to humans through food consumption (Ahmad et [20,21,31] . Health effects depend on the metal, concentration, duration and route of exposure. Therefore, a number of studies have suggested that the measurements of environmental contamination could be combined with appropriate ecological and human health risk assessment methods. The review has also identified a significant development in the area of remediation. The conventional physical and chemical methods still have important options for heavily contaminated sites, such as excavation, washing, stabilization, immobilization and chemical treatment (Ghosh et [12,14,23,25] . But the use of these techniques can be limited by high costs, technical requirements, site disturbance or secondary wastes. Hence, the biological and sustainable methods have gained more attention such as phytoremediation, microbial remediation, biosorption and bioaugmentation (Song et [19,21,24,29] .Biochar and modified biochar have attracted special attention among emerging remediation techniques due to their ability to reduce the mobility and bioavailability of heavy metals in contaminated soils . Their efficiency arises from the porous structure, surface functional groups, ion-exchange capacity and the capacity to promote adsorption and immobilization processes (Bhat et al., 2022; Wu et al., 2022) [17,18] . Recent studies have also focused on integrated remediation strategies combining physical, chemical, biological and nature-based technologies based on the characteristics of the contaminated site (Sarker et al., 2023; Yang et al., 2023; Liu et al., 2024) [23,25,33] . This overview of the literature indicates a clear shift away from traditional concentration based assessments towards consideration of sources, spatial distribution, environmental transport, ecological effects, humanhealth risks and remediation. The outcomes of the presented studies in this manuscript indicate that heavy metal pollution is highly site specific and, for effective management, geological conditions, land-use types information regarding potential sources of contamination (pollutants), soil and water properties as well as metal bioavailability must be accounted. We are now integrating GIS and spatial analysis into projects more so than ever before and there is an increasing capacity of identifying contamination hotspots coupled with new emerging innovations in phytoremediation, biochar, microbial technologies, integrated treatment systems amongst others [10 17-19 • 22•27 29 •33], opening up many possibilities for sustainable remediation across multiple scales. The data presented here suggest that future studies should integrate field monitoring, spatial scale, source apportionment and risk assessment/remedial evaluation in a comprehensive manner. The implementation of those ideas will further enhance heavy metal modeling approaches and be important in formulating soil and water protection strategies that are not only more effective but also cost-effective and environmentally sustainable. . .

Discussion

The results presented in Table 1 indicate that the heavy metal pollution of soil and water is a complex environmental problem under the influence of multiple natural and anthropogenic factors. All studies reviewed stated that mining, industrial activities, agricultural procedures, discharge of wastewater, transportation and improper disposal of waste were important sources of heavy metals. The results suggest that contamination is seldom due to one source, but is more likely to be the consequence of a combination of activities and environmental processes (Wang et [10,11,12,13,14,15,16] . There is also a considerable spatial variability of heavy metals concentrations in the literature. Higher levels were often linked to mining areas, industrial zones, urban environments, roads and agricultural areas impacted by waste water or by contaminated irrigation. "Whereas sites further away from heavily polluted areas tended to have lower concentrations." GIS, spatial interpolation, geostatistical methods and multivariate statistical analysis have thus been applied successfully to detect contamination hotspots and associate spatial patterns to potential sources (Wang et [10,16,22,27,28] .An important result of the reviewed studies is that heavy metals can transfer between soil, water, sediments, plants and other compartments of the environment. Contaminated soils can release metals through runoff, infiltration and leaching. Contaminated irrigation water can be a source of accumulation of metals in agricultural soils. Soil pH, organic matter, clay content, mineral composition and redox conditions have a strong influence on the mobility of metals. Therefore, the environmental risk cannot always be assessed only with the total concentration of metals, since the mobility and bioavailability of metals can vary significantly under different environmental conditions (Rana et [11,16,20,24] . The environmental impacts reported in the reviewed studies are more than the mere accumulation of metals. High levels may impact soil microorganisms , nutrient cycling , soil quality , plant growth , and aquatic organisms . Agricultural environments are of particular concern because metals accumulated in soil may be taken up by crops and subsequently transferred through the food chain. This pathway provides a direct link between environmental contamination and possible human exposure, underlining the importance of assessing heavy metal contamination from both ecological and public-health points of view (Wu et [13,20,21,24,31] .The findings also demonstrate that heavy metal contamination is not confined to the soil or water but is an interrelated environmental system. Studies on water contamination showed the persistence of metals in aquatic systems and sediments; studies on contaminated soils revealed the potential for further transfer to ground water and surface water. This interaction emphasizes the need for integrated monitoring of soil and water, rather than monitoring each environmental compartment separately (Ahmad et [20,24,30,33] .The literature reviewed also suggests a clear development in remediation strategies. Physical and chemical removal or immobilization of heavy metals has been extensively studied in previous studies. More recent studies are beginning to focus more on biological, nature-based and sustainable technologies. Conventional techniques can be effective in treating highly contaminated sites, but their application may be associated with high costs, disturbance of contaminated areas and generation of secondary waste (Ghosh et [12,14,23,25] . Phytoremediation is an environment friendly alternative as plants can absorb, accumulate or stabilize heavy metals within contaminated soils. However, the efficiency of phytoremediation depends on plant species, metal properties, soil properties and environmental conditions. The studies reviewed suggest that phytoremediation is more promising when combined with other amendments or biological processes, rather than being viewed as a universal solution for all contaminated sites (Song et [19,21,24,29] .The literature reviewed also highlights another important development: the use of biochar for remediation. Biochar can adsorb heavy metals, exchange ions, form surface complexes and use other immobilization mechanisms to reduce their mobility . The effectiveness depends on the feedstock characteristics, production conditions, application rate and chemical properties of the target metal (Bhat et al., 2022; Wu et al., 2022) [17,18] . Hence, biochar has received great attention as a relatively sustainable amendment for contaminated soils, but its long-term stability and performance in field conditions need further investigation.Recently, integrated remediation approaches combining physical, chemical, biological and nature-based technologies have gained increased attention. These approaches may be complementary to each other in reducing the mobility of metals, improving the soil properties and promoting ecological recovery. Literature, thus, suggests that the remediation method should be chosen depending on the site specific characteristics, level of contamination, domi nant metals, environmental conditions, treatment objectives and sustainability over the long term (Sarker et al., 2023; Yang et al., 2023; Liu et al., 2024; Wu et al., 2024) [23,25,33,34] .The general results of the reviewed literature indicate a trend from simple measurements of heavy metal concentrations to an integrated assessment of contamination sources, spatial distribution, environmental transport, ecological impacts, human-health risks and remediation. The studies summarized in Table 1 collectively demonstrate that heavy metal contamination is highly site-specific and that no one remediation technology can be universally applied. Therefore, effective management requires accurate source identification, spatial monitoring, assessment of metal mobility and bioavailability, appropriate risk evaluation and selection of sustainable remediation strategies according to local environmental conditions (Wang et [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35] .

Conclusions

The present overview suggests that the pollution of soil and water with heavy metals is still one of the major environmental problems, as it is a complex situation depending on natural processes and manmade activities. The main causes of pollution are mining, industry, agriculture, wastewater and transport and disposal. GIS and spatial analysis for the detection of pollution patterns and hotspots. Heavy metals are among the most ubiquitous contaminants, as they can migrate from soil, water and sediments into plants and food chains, posing a potential ecological or human health hazard. In addition, the studies considered provide evidence of an immediate shift toward sustainable remediation technologies, focusing on phytoremediation, biochar applications, microbial technologies and integrated treatment systems. To summarize, efficient control of heavy metals pollution should be implemented with continuous environmental monitoring, proper source identification and spatial profiling, risk assessment and restoration. This provides the basis for further research work of longterm performance assessments under field conditions; studies to evaluate metals bioavailability and interaction between soil metal and other contaminants; and development of low-cost affordable sustainable remediation technologies. .

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