An Overview of Heavy Metal Contamination in Water from Agriculture: Origins, Monitoring, Risks, and Control Measures

Abstract

Agricultural activities are widely recognized as major sources of water pollution, primarily due to the introduction of heavy metals (HMs) through fertilizers, pesticides, manures, sewage sludge, and irrigation water. Owing to their persistence and non-biodegradability, these metals pose substantial risks to ecosystems and public health. While certain HMs such as cobalt, copper, and zinc are essential micronutrients for crops at low concentrations, others—like arsenic, cadmium, lead, and mercury—enter agricultural systems as contaminants and serve no biological function in plants. This paper explores the complex issue of HM contamination in water resulting from agricultural practices. It reviews the primary sources and pathways through which HMs enter aquatic systems, discusses their ecological and health impacts, and examines analytical methods used for HM detection and monitoring. In response to this challenge, several mitigation strategies are highlighted, including the optimized use of agrochemicals, adoption of sustainable farming practices, and implementation of phytoremediation and bioremediation techniques. Additionally, the importance of community education and regulatory enforcement is emphasized as part of an integrated approach to pollution control. Ultimately, this paper underscores the need for balanced solutions that safeguard water resources while maintaining agricultural productivity.

1. Introduction

Over time, the definition of the term heavy metal has been controversial and ambiguous, sparking numerous discussions from various perspectives. In many contexts, the term is often associated with toxicity, a notion sometimes based more on perception than on a scientifically grounded definition [1].

Another common definition refers to elements with high atomic weight or density, typically greater than 5 g cm⁻³. In the scientific literature, heavy metals generally include both metals and metalloids (such as arsenic) that are characterized by high density and known environmental and human toxicity [2].

In line with the latter understanding [2], this paper will use the term heavy metals to refer specifically to arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), mercury (Hg), and nickel (Ni).

Heavy metals (HMs) are naturally occurring elements found in the Earth’s crust, but their harmful environmental presence is often linked to anthropogenic activities such as mining, industrial processes, and agriculture, as well as natural phenomena like volcanic eruptions and soil erosion [3].

HM pollution arising from agricultural practices has become a critical issue, especially in light of the increasing demand for high crop yields. This type of pollution affects the quality of crops, contaminates surface and groundwater resources, and ultimately poses risks to human health.

Mitigating the environmental effects of HMs from agriculture requires regulatory enforcement, the promotion of sustainable and precision farming practices, and, importantly, education and public awareness campaigns. The impacts of HM contamination in water are often not immediate but can have long-lasting consequences that affect entire ecosystems [4].

Due to bioaccumulation and biomagnification, water pollution with HMs poses a significant danger, especially to communities that rely on the consumption of contaminated fish and shellfish [5].

Another important aspect is the interaction between HM ions and antibiotics, which are also commonly found in aquatic environments as pollutants. According to Khurana et al. [6], antibiotics such as tetracyclines, fluoroquinolones, and sulfonamides, organic molecules capable of acting as ligands, can form stable complexes with HM ions. These antibiotic–metal complexes are often more persistent and more toxic than the parent compounds. Furthermore, microplastics can adsorb HM ions on their surfaces, effectively serving as carriers that facilitate the transport of HMs across aquatic systems. These adsorbed metals can subsequently be released back into the surrounding water, increasing the exposure risk to aquatic organisms not only through ingestion of contaminated particles but also through direct contact with metal-contaminated water [7,8].

This paper addresses the critical issue of heavy metal contamination in agricultural water sources, which poses significant risks to environmental and human health globally. It provides in-depth insights into the sources and impacts of heavy metals in agriculture, alongside a comprehensive review of advanced detection and monitoring methods. By highlighting the advantages and limitations of each technique, this paper underscores their essential role in accurate assessment, timely intervention, and effective pollution management.

2. Prospects of HMs Resulting from Agriculture

In agricultural environments, HMs are present as a result of both natural and anthropogenic processes. Their concentrations in soils can vary widely. The average values of HMs in soils worldwide are approximately cadmium (Cd)—0.06 mg kg⁻¹, chromium (Cr)—20–200 mg kg⁻¹, copper (Cu)—20 mg kg⁻¹, nickel (Ni)—40 mg kg⁻¹, lead (Pb)—10–150 mg kg⁻¹, and zinc (Zn)—10–300 mg kg⁻¹ [9].

HMs can be classified into two categories based on their importance to plants: (i) essential HMs, which play significant roles in plant growth and metabolism—such as copper (Cu), iron (Fe), molybdenum (Mo), nickel (Ni), and zinc (Zn); and (ii) non-essential HMs, such as arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb), which have no known biological function in plants and are typically harmful to both plant health and the environment. For example, copper (Cu) is involved in photosynthesis and respiration and is a component of various proteins, while zinc (Zn) contributes to protein synthesis and seed formation and promotes RNA synthesis [10]. Cobalt (Co) also has a positive influence on the growth of leguminous plants [11].

Despite their well-known beneficial roles at trace levels, HMs can become toxic to plants when present in excessive concentrations, leading to environmental pollution. Additionally, the accumulation of non-biodegradable HMs such as arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni), and lead (Pb) in soils—primarily through various agricultural practices—significantly contributes to the contamination of crops and water sources. Their long persistence in the environment further exacerbates their harmful impact.

2.1. Agricultural Inputs as Sources of HMs

The rapid growth of the global population has led to an increased demand for food, prompting the intensification of agricultural practices to achieve higher crop yields. This has been accomplished through the widespread use of agricultural inputs such as fertilizers and various phytosanitary products (Figure 1).

While these inputs have effectively enhanced productivity, they have also introduced unintended environmental consequences [12,13].

The presence of HMs in mineral fertilizers is well documented [14,15,16]. Phosphate fertilizers, in particular, are known to contain contaminants such as arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), and nickel (Ni) [17,18,19] (

Table 1. HM levels in wastewater used for irrigation, soil, and vegetable samples.

Sample	Cd	Cr	Cu	Ni	Pb	Zn
WW, mg L−1	0.8	0.87	0.4	0.38	0.36	0.28
	0.64	0.82	0.37	0.31	0.33	0.23
	0.07	0.11	0.18	0.16	0.09	0.08
SS, mg kg−1	3.20	63	35.40	78	74.50	31.50
	3.35	56	38	85.50	70.75	33
	1.25	34.25	16.75	36.50	46.25	15.75
VS, mg kg−1	4.53	21.15	19	28.4	20.5	14.9
	5.69	22.24	17.35	31.87	22.70	12.27
	2.35	15.89	11.38	17.15	7.33	8.64

WW = wastewater used for irrigation; SS = soil sample; VS = vegetable sample.

).

The repeated application of these fertilizers contributes to the gradual accumulation of HMs in agricultural soils. Notably, the cadmium content in phosphate rock sources can vary significantly; for example, a high concentration of 507 mg kg⁻¹ Cd has been reported in carbonate fluorapatite from Morocco [20].

Furthermore, the processing of phosphate rocks generates phosphogypsum (PG), which contains varying levels of HMs such as arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), and zinc (Zn), as well as natural radionuclides [21,22].

The issue of using PG in agriculture is further overshadowed by the fact that over 85% of the PG produced annually is stockpiled or discharged into water bodies, resulting in significant environmental pollution affecting soil, water, and air quality [23].

In addition, micronutrient fertilizers have also been identified as a source of HMs [17,24] (Table S1). Studies have reported the presence of variable concentrations of HMs—including arsenic (As), cadmium (Cd), chromium (Cr), nickel (Ni), lead (Pb), and mercury (Hg)—in various fertilizer inputs. Notably, limestone, commonly used in soil amendments, has also been shown to contain trace levels of these metals [25]. Nitrate fertilizers contain moderate Cd (0.05–8.5 mg kg⁻¹) and Pb (2–120 mg kg⁻¹) levels [26], while lime may have Pb up to 1250 mg kg⁻¹ [26]. Hg has been detected in phosphatic fertilizers at levels up to 0.35 mg kg⁻¹ [27], and the highest As concentration, 9.8 mg kg⁻¹, was also reported in this category [28].

HMs present as contaminants in mineral fertilizers and liming materials can subsequently be detected in plant tissues and water bodies. Among these, cadmium (Cd) is considered the most mobile heavy metal in the environment. It is more readily released from soils into groundwater compared to other metals [29].

When used in agriculture, certain pesticides can contaminate soil, crops, and water bodies with HMs that are either intentionally included in their formulations or introduced as impurities during the manufacturing process [16]. For example, the application of copper-based fungicides—such as Bordeaux mixture and copper oxychloride—in vineyards can lead to elevated copper concentrations in soils. In regions with intensive viticultural practices, this may also contribute to water pollution due to runoff and leaching [30].

Other pesticide compounds, such as those based on mercury, are no longer in use but persist in the environment due to their high stability. As a result, they continue to be detected in soils and water bodies, where they can accumulate in aquatic organisms [31].

Some herbicides contain arsenic as an active ingredient in their chemical formulations. Examples include calcium arsenate (Ca₃(AsO₄)₂), sodium arsenite (NaAsO₂), and cacodylic acid ((CH₃)₂AsO(OH)) [16].

The impact of arsenic pollution from various sources, including agricultural activities, has been evaluated in a study [32]. According to the findings, groundwater arsenic contamination affects nearly 106 countries worldwide. In Asia, the issue is particularly severe, with Bangladesh, India, and Vietnam being among the most heavily impacted. In contrast, Europe is relatively less affected by arsenic contamination in groundwater.

A study [33] investigated the concentrations of HMs in surface water and groundwater samples collected near a former pesticide factory in El Salvador. The analysis revealed the presence of arsenic (As), cadmium (Cd), copper (Cu), and chromium (Cr) in both types of water sources. For instance, the maximum concentrations of arsenic detected were 0.013 mg L⁻¹ in groundwater and 0.026 mg L⁻¹ in surface water. Cadmium was found at lower levels, with a maximum concentration of 0.004 mg L⁻¹ in groundwater.

Chromium was detected at a concentration of 1.5 mgL⁻¹ in groundwater and 0.0068 mg L⁻¹ in surface water.

HMs have been identified as contaminants in various pesticide formulations. In particular, numerous glyphosate-based herbicides have been found to contain concentrations of arsenic (As), chromium (Cr), nickel (Ni), and lead (Pb) that exceed permissible limits for water quality [34]. For example, in two herbicides—Saturn-G and Ordram—cadmium (Cd), nickel (Ni), and lead (Pb) were detected at concerning levels: 1.48 and 1.38 mg kg⁻¹ for Cd, 12.25 and 14.25 mg kg⁻¹ for Ni, and 10.00 and 7.50 mg kg⁻¹ for Pb, respectively [17]. These contaminants, whether present independently or in combination, contribute significantly to water pollution and pose environmental and health risks.

Livestock manures are commonly applied in agriculture due to their high content of essential nutrients beneficial for crop growth. However, improper management of these organic inputs can lead to negative environmental impacts, including the degradation of water quality. The concentrations of HMs in livestock manures and composts (Table S2) can vary significantly, influenced by factors such as the mineral content of commercial animal feeds [16], the farming system employed, and even the country of origin [35]. HMs may be intentionally added to animal feed due to their essential biological roles—such as cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), and zinc (Zn)—or may be present as undesirable contaminants, including arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb) [35]. Heavy metal concentrations vary significantly across different types of animal manure [36,37,38,39], additional data being presented in Table S2.

Long-term irrigation of agricultural land with untreated wastewater has led to the accumulation of HMs in soils, crops, and water bodies. The concentration and composition of these contaminants in wastewater largely depend on the industrial profile of the region from which the wastewater originates [40].

An analysis of wastewater used for irrigating vegetable crops in Pakistan revealed concentrations of cadmium (Cd), chromium (Cr), and cobalt (Co) at 0.014 mg L⁻¹, 0.240 mg L⁻¹, and 0.780 mg L⁻¹, respectively—exceeding the maximum permissible limits of 0.01 mg L⁻¹ for cadmium (Cd) and 0.1 mg L⁻¹ for both chromium (Cr) and cobalt (Co) [41].

Similarly, irrigation water used for vegetable cultivation in peri-urban areas near Boumerdes city, Algeria, was found to be contaminated with elevated levels of chromium (Cr) (0.09–0.26 mg L⁻¹), copper (Cu) (0.31–1.17 mg L⁻¹), and nickel (Ni) (0.02–1.98 mg L⁻¹) [42].

Wastewater discharged into the Ganges River and subsequently used for irrigating agricultural crops has been analyzed, revealing elevated concentrations of HMs that exceed statutory limits for irrigation water. Specifically, chromium (Cr) levels ranged from 0.07 to 0.60 mg L⁻¹, nickel (Ni) from 0.04 to 0.21 mg L⁻¹, and cadmium (Cd) from 0.016 to 0.23 mg L⁻¹ [43].

Another study [44] investigated the levels of HMs in irrigation wastewater and their accumulation in vegetable crops, assessing the associated health risks from consuming contaminated plants. The concentrations of HMs in the wastewater were as follows: arsenic (As) 0.01–0.2 mg L⁻¹, cadmium (Cd) 0.004–0.13 mg L⁻¹, chromium (Cr) 0.06–2.92 mg L⁻¹, copper (Cu) 0.02–0.24 mg L⁻¹, nickel (Ni) 0.02–0.19 mg L⁻¹, lead (Pb) 0.01–1.17 mg L⁻¹, and zinc (Zn) 0.01–1.95 mg L⁻¹. The same metals were detected in vegetables, with concentrations ranking as Cd > Zn > Pb > Cr > Cu. The health risk assessment conducted by the authors indicated a risk order of Cu > Zn > Cr > Ni > Cd > Pb > As.

A similar study [45] quantified the concentrations of HMs in municipal and industrial wastewater, soils, and vegetable crops to determine the transfer factor, daily metal ingestion, and health risk index. The contamination levels in vegetables grown on soils irrigated with contaminated wastewater took the following order: nickel (Ni) > chromium (Cr) > lead (Pb) > copper (Cu) > zinc (Zn) > cadmium (Cd). The detailed concentrations of HMs are presented in Table 1 [46].

The application of sewage sludge to improve soil fertility may also contribute to environmental pollution due to the presence of various HMs in its composition. The concentrations of these metals vary considerably depending on the sludge’s origin. For instance, sludge from rural wastewater treatment plants in Poland contained cadmium (Cd) at 0.6–9.5 mg kg⁻¹, copper (Cu) at 9.3–524 mg kg⁻¹, nickel (Ni) at 4.8–90.0 mg kg⁻¹, lead (Pb) at 8.8–275.2 mg kg⁻¹, zinc (Zn) at 575–1732 mg kg⁻¹, chromium (Cr) at 7.5–170 mg kg⁻¹, and mercury (Hg) at 0–3.8 mg kg⁻¹ dry matter. In contrast, sludge from urban wastewater treatment plants in Poland showed higher levels for the same metals: Cd 1.07–16.7 mg kg⁻¹, Cu 32–195 mg kg⁻¹, Ni 1.3–128.9 mg kg⁻¹, Pb 21.2–322.4 mg kg⁻¹, Zn 20–5351.1 mg kg⁻¹, Cr 12.7–2759.8 mg kg⁻¹, and Hg 0.1–1.55 mg kg⁻¹ dry matter [46].

Similarly, municipal sewage sludge from a wastewater treatment plant in Croatia was reported to contain Cd (0.02 mg kg⁻¹), Cu (240 mg kg⁻¹), Ni (14.2 mg kg⁻¹), Pb (54.7 mg kg⁻¹), Cr (165 mg kg⁻¹), Zn (508 mg kg⁻¹), and Hg (0.74 mg kg⁻¹) dry weight [47].

An analysis of sewage sludge from the wastewater treatment plant in the Municipality of Alexandria, Romania, was conducted by Marin and Rusănescu [48]. The results revealed concentrations of 1.15 mg kg⁻¹ cadmium (Cd), 26.9 mg kg⁻¹ chromium (Cr), 238 mg kg⁻¹ copper (Cu), 24.7 mg kg⁻¹ nickel (Ni), 485 mg kg⁻¹ zinc (Zn), 3.19 mg kg⁻¹ lead (Pb), and 0.588 mg kg⁻¹ mercury (Hg). The sewage sludge was subsequently used in an experiment, which demonstrated that application rates of 15–20 t ha⁻¹ can be safely used without adverse effects on soil quality, crop yield, or the environment.

Compost derived from municipal sewage sludge also contains significant concentrations of HMs on a dry weight basis, with reported levels of cadmium (Cd) at 4.2 mg kg⁻¹, copper (Cu) at 130 mg kg⁻¹, nickel (Ni) at 27 mg kg⁻¹, lead (Pb) at 80 mg kg⁻¹, and zinc (Zn) at 1367 mg kg⁻¹ [49].

The origin of the compost greatly influences its heavy metal load; for instance, compost produced from organic household and green waste typically contains lower concentrations of HMs compared to compost derived from municipal solid waste [50].

Considering these factors, the presence of HMs in agricultural inputs is undeniable. Consequently, long-term agricultural practices that rely on these inputs contribute to the accumulation of HMs in soils, increasing the likelihood of their transfer into surrounding water sources.

2.2. HMs Transport Pathways from Agricultural Fields to Water Bodies

Once HMs enter the soil, their mobility and transport to water bodies are governed by a complex interplay of chemical, physical, and biological factors (Figure 2).

Key factors influencing their behavior include soil pH, organic matter content, soil texture, and redox conditions. A thorough understanding of these parameters and how to manage them is essential for predicting heavy metal transport and effectively managing contaminated environments.

Soil pH plays a crucial role in the solubility and mobility of HMs. In acidic soils, HMs tend to be more soluble and, consequently, more mobile, increasing the risk of leaching into groundwater and surface waters. Conversely, in alkaline soils, metals often form insoluble compounds that limit their mobility. However, the extent to which pH influences mobility varies among different HMs [51].

The addition of soil amendments, such as lime, can alter soil pH and consequently affect the solubility and mobility of HMs. Soil organic matter, including humus, can bind metals by forming complexes, which generally reduces their mobility. Organic matter, along with oxides and clay minerals, has a high sorption capacity for metals, further influencing their retention in soils [52].

However, the decomposition of organic matter can release bound metals into the soil solution, thereby increasing their mobility [53].

Additionally, HMs can differentially affect the rate of organic matter decomposition. For instance, arsenic (As) and copper (Cu) strongly inhibit organic matter decay, while chromium (Cr) and lead (Pb) exhibit weaker inhibitory effects [54].

Soils with a high clay content readily adsorb HMs, reducing their mobility and limiting their transfer into water bodies [55].

Minerals such as kaolinite, montmorillonite, and bentonite have demonstrated effectiveness in removing contaminants like arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), and lead (Pb) from soils and water. Numerous studies highlight the environmental significance of clay-based adsorbents for remediation purposes [56].

Additionally, soils with a high cation exchange capacity (CEC) retain greater amounts of heavy metal cations, thereby reducing their potential for leaching.

Under aerobic conditions, HMs are more likely to exist in their free ionic forms, which increases their solubility and mobility in water [51]. Furthermore, the presence of chloride ions (Cl⁻) in the soil solution can influence cadmium (Cd) solubility by forming soluble complexes, enhancing its mobility [40].

The movement of HMs from soil to water is a complex process involving multiple pathways, including leaching, surface runoff, tillage practices, and sedimentation (Figure 2). HMs in soluble forms can percolate or leach through the soil profile under the influence of rainfall or irrigation, eventually infiltrating into groundwater sources.

Surface runoff occurs when water flows over the soil surface, washing HMs into rivers, lakes, or ponds. Additionally, soil particles contaminated with HMs may be transported to surface waters through erosion. Under certain conditions, mercury (Hg) can volatilize into the atmosphere and subsequently be deposited into water bodies via atmospheric deposition [57].

In conclusion, understanding the factors that affect the mobility of HMs in soils and their transport pathways to water bodies is critical for preventing water pollution caused by agricultural inputs. To this end, the literature includes soil pollutant migration models [58] that focus on transport within runoff–soil–groundwater systems, as well as emerging models addressing the lateral migration of HMs in soils, which hold promise for improving contamination predictions and management strategies [59].

3. Methods Used for Detection of HMs in Water Samples

3.1. Laboratory Techniques for the Detection of HMs in Water Samples

Advanced spectroscopic techniques are essential tools in environmental monitoring and water quality assessment due to their high sensitivity, low detection limits, and ability to simultaneously quantify HMs in complex water matrices. A variety of laboratory-based methods are available, each with specific strengths. Selecting the appropriate technique depends on factors such as analytical sensitivity, multielement capabilities, sample preparation requirements, and cost.

(a) Atomic absorption spectrometry (AAS) is widely used for a broad range of metals such as Cd, Co, Cr, Cu, Ni, Pb, and Zn. Detection limits are generally in the µg L⁻¹ range, suitable for routine environmental monitoring. It typically requires acid preservation and filtration of samples. Some methods employ preconcentration or coprecipitation steps to enhance sensitivity.

Multielement coprecipitation is a preconcentration and separation technique designed to extract heavy metal ions from a sample matrix before their analysis by AAS. This method is particularly effective for trace-level detection, as it enhances the concentration of metals while simultaneously removing interfering components from the sample [60,61]. For example, a study [60] demonstrated the use of 8-hydroxyquinoline in the presence of Cu(II) as a carrier agent for the coprecipitation of Cd(II), Co(II), Cr(III), Ni(II), and Pb(II) from surface water prior to AAS analysis. Under optimized experimental conditions, quantitative recoveries of the metal ions exceeded 85%. The use of 8-hydroxyquinoline as a precipitating ligand significantly enhanced the separation of metal ions from the complex water matrix. Following treatment, the concentrations of Co(II), Ni(II), Cr(III), and Pb(II) in the water samples were found to be 0.014 mg L⁻¹, 0.030 mg L⁻¹, 0.040 mg L⁻¹, and 0.050 mg L⁻¹, respectively.

In a similar approach, Pb(II), Cd(II), Cr(III), Ni(II), and Mn(II) ions were simultaneously coprecipitated using a Cu(II)-dibenzyldithiocarbamate complex [61]. Following dissolution of the precipitate, the concentrations of the HMs were determined by AAS. The method exhibited detection limits of 0.87 mg L⁻¹ for Pb(II), 0.34 mg L⁻¹ for Cd(II), 0.75 mg L⁻¹ for Cr(III), and 0.45 mg L⁻¹ for both Ni(II) and Mn(II). This procedure was applied for various types of water samples—including tap, sea, spring, and river water—demonstrating the method’s reliability and applicability for monitoring HMs in diverse aquatic environments.

(b) Graphite furnace atomic absorption spectrometry (GFAAS), also known as electrothermal atomic absorption spectrometry (ETAAS), is a highly sensitive analytical technique widely employed for the determination of trace and ultra-trace levels of HMs in water samples. It enables the detection of elements at concentrations as low as a few micrograms per liter, using very small sample volumes. GFAAS is particularly effective for the analysis of metals such as Pb, Cd, As, Cr, and Ni, making it well-suited for applications in environmental monitoring and drinking water quality assessment. The method generally requires minimal sample preparation, mainly filtration and digestion [62,63,64]; however, the use of matrix modifiers is often necessary to enhance accuracy and precision. Matrix modifiers played an important role in the GFAAS determination of arsenic, lead, and cadmium in tap and bottled water samples [65]. A palladium–magnesium (Pd-Mg) mixture was found to be the most effective matrix modifier for arsenic (As) determination, while nickel (Ni) was employed as the modifier for both lead (Pb) and cadmium (Cd). The method achieved detection limits of 2.0 mg L⁻¹ for As, 0.036 mg L⁻¹ for Cd, and 0.25 mg L⁻¹ for Pb. Cadmium levels in all analyzed samples were below the detection limit, whereas lead concentrations ranged from non-detectable to 12.66 mg L⁻¹. The highest recorded arsenic concentration was 11.54 mg L⁻¹.

The concentrations of Cd, Cu, Fe, Mn, and Pb in groundwater, mixed water, and wastewater used for irrigation were determined using GFAAS by Assubaie [66]. The results indicated that heavy metal levels were lowest in groundwater and highest in wastewater. In wastewater, the average concentrations of Cd and Pb were both 0.091 mg L⁻¹. Cu and Zn were also detected at relatively elevated levels, with mean concentrations of 0.353 mg L⁻¹ and 0.232 mg L⁻¹, respectively.

Since the determination of trace levels of lead in aqueous samples is challenging due to its typically low concentration, it is essential to incorporate a preconcentration/extraction step prior to analysis. In this context, a study [67] explored the use of continuous-flow microextraction for the enrichment of trace lead from water samples, employing 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone as the complexing agent. After optimization of the factors influencing extraction efficiency, the procedure demonstrated satisfactory precision, with a relative standard deviation (RSD) of 6.8%. The detection limit for lead (Pb) was determined to be 12 ng L⁻¹.

(c) Cold vapor atomic absorption spectroscopy (CVAAS) is particularly well-suited for mercury analysis, as it employs a cold vapor generation technique that reduces ionic mercury (Hg²⁺) to elemental mercury (Hg⁰), which is then detected via atomic absorption. CVAAS offers high sensitivity and selectivity, making it effective for detecting trace levels of mercury in a wide range of sample matrices, including aqueous environments. The method is capable of achieving detection limits in ng L⁻¹, making it highly suitable for environmental monitoring.

A study [68] presents the assessment of Hg from natural water samples, which was performed using CVAAS, coupled with a preconcentration procedure based on 2-aminothiazole-modified silica gel (SiAT). Here, 2-aminothiazole acts as a chelating agent, and the preconcentration step using the modified silica enhances the sensibility to analyze samples with low mercury concentrations. The mercury (Hg²⁺) levels ranged from 216 to 218 ng L⁻¹ without UV photocatalysis, and from 288 to 294 ng L⁻¹ with UV photocatalysis.

Another study [69] presented Hg concentrations determined by CVAAS, where tap water contained 0.33–1.32 ng L⁻¹ of Hg, while groundwater samples ranged from 0.1 to 1.25 ng L⁻¹. The procedure for sample preparation for Hg assessment by CVAAS is summarized in Table 2.

(d) Inductively coupled plasma mass spectrometry (ICP-MS) is highly effective for detecting a wide range of metals, making it particularly valuable for analyzing trace and ultra-trace levels of these elements in various environmental matrices. The detection limits typically fall within the ng L⁻¹ range, depending on factors such as matrix effects, instrument sensitivity, and other operational parameters [70]. The assessment of HMs in water samples collected from lakes and rivers in Cluj County, Romania, was reported by Voica et al. [71]. The study found concentrations of 12–34 ng L⁻¹ for Pb, 1–3 ng L⁻¹ for Cd, 133–288 ng L⁻¹ for Zn, and 285–766 ng L⁻¹ for Ni. Furthermore, the mean concentrations of HMs achieved by ICP-MS in water samples collected from six different sampling sites on the Pasur River were 0.052, 0.039, 0.0325, 0.001, 0.033, 0.001, and 0.005 mg L⁻¹ for Pb, Cd, Cr, Cu, Ni, Co, Zn, and Mn, respectively. The average metal concentrations were recorded in the order Pb > Cd > Ni > Cr > Zn > Co > Cu > Mn [72]. Considering the importance of determining HMs in water, there are studies [73] that present the quantification of these metals, including in bottled water, using ICP-MS, due to their potential effects on consumers.

(e) Inductively coupled plasma optical emission spectrometry (ICP-OES) is extensively used for multielemental analysis and is highly effective in detecting HMs across various matrices. Although it has lower sensitivity compared to ICP-MS, it remains a reliable technique for heavy metal testing in environmental samples. ICP-OES allows for the simultaneous analysis of multiple elements, making it efficient for large-scale analyses. While ICP-MS delivers superior sensitivity, ICP-OES provides a more affordable alternative for routine monitoring when extreme sensitivity is not essential [70]. The preparation of water samples for heavy metal analysis using ICP-OES involves several key steps to ensure accurate metal quantification and effective management of the sample matrix (Table 2). The initial acidification of the samples with HNO₃ to a pH lower than 2 prevents metal precipitation, keeping the metals in a dissolved form and reducing the risk of inaccurate results. If the water sample contains particulate matter (such as from lakes or rivers), digestion is required to ensure that all metals are in their dissolved form for analysis. For ICP-OES, samples are typically digested (using open-vessel or microwave digestion) with concentrated HNO₃, HCl, and sometimes H₂O₂ to facilitate the extraction of metals into solution. Finally, the samples are diluted to a consistent volume, optimizing metal concentrations within the instrument’s detection range and minimizing matrix effects [74,75,76].

In certain cases, preconcentration of trace metals from water samples is necessary before quantification by the ICP-OES technique. To address this need, a novel complexing adsorbent material has been developed [77] by coating silica particles with polyhexamethylene guanidinium (PHMG) chloride and subsequently saturating them with sulfonated nitrosonaphthol reagents. This study investigates the adsorption of various metal ions, including Cu, Fe, Co, Ni, Zn, Pb, Mn, and Cr, under different conditions such as pH, sample volume, and the presence of interfering ions. The developed method effectively preconcentrates metals and achieves sub-ppb detection limits when combined with ICP-OES for analysis. The preconcentration factors for metals range from 20 to 80, making it a highly sensitive and efficient approach for detecting trace metals in natural water samples.

(f) Atomic fluorescence spectrometry (AFS) is a highly sensitive and selective technique used for the determination of trace metals in various samples, including water. It is based on the principle of atomic absorption and fluorescence emission. The assessment of As and Hg levels in surface and groundwater samples collected from East China was reported [78], with analysis performed using AFS. The level of As in groundwater ranged from 0.06 to 12 mg L⁻¹, while Hg levels ranged from 0.0104 to 0.0497 mg L⁻¹. In surface water, the As level was higher, reaching 71.3 mg L⁻¹, and the Hg level was 0.0372 mg L⁻¹.

(g) Neutron activation analysis (NAA) is a highly sensitive and versatile technique used to determine the concentration of elements across a broad range of sample types. It is especially effective for detecting trace elements, such as HMs, and does so without requiring complex sample preparation. NAA is capable of identifying elements at very low concentrations, is non-destructive, and typically involves minimal sample handling. For water samples, the process generally includes evaporation to reduce the volume, followed by drying or concentration before irradiation. After the samples are irradiated, gamma radiation is measured to quantify the elements of interest (Table 2). A study [79] evaluated the concentrations of hazardous HMs (Cu, Zn, As, Cd, and Cr) in water samples from wells and rivers located around the Adipala Cilacap Steam Power Plant, using NAA. The results revealed that the metal concentrations varied, with copper (Cu) showing the highest level at 0.057 mg L⁻¹ and chromium (Cr) the lowest at 0.0006 mg L⁻¹. The concentration of HMs in well water was found to follow the order Cu > Zn > As > Cd > Cr.

(h) Laser-induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy-based technique used for the detection of metals in various sample types—including solids, liquids, and gases—without causing significant damage to the samples. Among its key advantages are rapid analysis, minimal sample preparation, the requirement of only a small sample volume, non-destructive testing, portability, the capability for remote analysis, and a detection limit typically in the range of 1–10 ppm [80]. One of the main limitations of LIBS is the difficulty in detecting HMs in drinking water due to their typically low concentrations, which are often below the direct detection capability of the technique.

To overcome this challenge, a study [81] demonstrated an improvement in LIBS sensitivity by integrating it with a chelating resin commonly used in water purification. This resin enables rapid enrichment of HMs, significantly lowering the detection limits of a conventional LIBS system. The combination of chelating resin with LIBS offers a cost-effective, rapid, and sensitive method for detecting trace levels of HMs in drinking water. For cadmium, the achieved limit of detection was 3.6 µg L⁻¹.

Table 2 serves as a helpful guide for selecting the appropriate method based on metal targets, sensitivity requirements, and available equipment.

Table 2. Comparison of analytical techniques for HMs detection in water.

Detection Method	Target Metals	Sample Preparation/Concentration	Detection Range	Advantages (A) and Limitations (L)	Ref.
AAS	Cd, Co, Cr, Cu, Ni, Pb, Zn	- preservation of water samples was ensured with ultrapure HNO3 - prior to analysis, water samples were filtered	mg L−1-μg L−1	A: high precision, very low detection limits L: expensive equipment, relatively longer analysis times	[82,83]
	Cd, Cr, Co, Cu, Mn, Ni, Pb, Zn	- water samples were concentrated before quantification (excepting Fe, Zn)			[84]
	Cr, Fe, Mn, Ni, Pb	- microwave digestion was performed of water samples with an acid mixture (67% HNO3: 98%H2SO4: 37%HCl: 40%HF = 2:1:1:1)			[62]
	Cd, Co, Cr, Ni, Pb	- 10 mL water sample was treated with 2 mL 8-hydroxyquinoline and 0.2 mL Cu(II) solution (coprecipitation procedure) - then, centrifugation was performed, and the resulting precipitate was dissolved in 1 mL HNO3			[60]
	Cd, Cr, Mn, Ni, Pb	- water samples were filtered (0.45 μm pore size) and the pH adjusted to 9 - we coprecipitated targeted ions with the aid of Cu(II)-dibenzyldithiocarbamate precipitate, which was then dissolved in 0.5 mL concentrated HNO3			[61]
GFAAS	Hg	- a 2% solution of KMnO4 was added to the water sample to convert mercury from organo-mercuric compounds into its ionic form and help prevent evaporation and loss of metals - measurements were conducted 24 h later, after adding a 20% solution of SnCl2 to the sample	μg L−1	A: high sensitivity for trace metals; small sample volumes required; L: expensive, use of matrix modifiers; useful for detection of Pb and Hg, in particular	[84]
	Cd	- microwave digestion was performed of water samples with an acid mixture (67% HNO3: 98%H2SO4: 37%HCl: 40%HF = 2:1:1:1)			[63]
	Pb	- water samples were filtered (filter membrane with 0.45 μm pore size) - continuous-flow microextraction was conducted with the addition of chelation reagent 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone			[67]
	Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn	- 3 L of water sample was concentrated at 80 °C till reaching a final volume of 50 mL - 4 mL 98% H2SO4 was added and digested by a Digesdahl apparatus (3 min) - 10 mL 30% H2O2 was added and heated until oxidation was finished - the resulting mixture was filtered and diluted to 50 mL with deionized water			[63]
	As, Cd, Cr, Pb	- water samples were filtered (0.45 μm pore size) and we adjusted the pH to lower than 2 - 50 mL acidified sample was treated with 5 mL HNO3 and boiled at 130 °C till the volume was 25–30 mL			[64]
CVAAS	Hg	- water samples spiked with Hg+2 ions were mineralized in a photoreactor thermostatted at 25 °C with a luminous intensity of 3.81 mW cm−2 in the presence of 100 mgTiO2 and 0.01 mol L−1 potassium persulfate - we used a preconcentration system composed of a mini-column packed with 100 mg of 2-aminothiazol-modified silica gel - we carried out elution with 100 μL 2 mol L−1 HCl	ng L−1	A: highly sensitive for mercury detection L: limited to mercury; possible interferences	[68]
	Hg	- 500 mL water was treated in a separatory funnel with 2.5 mL 20 N H2SO4 and 1.5 mL 0.5% KMnO4 - the mixture was neutralized with 5 mL 10N NaOH and 1.5 mL 10% NH2OH∙HCl and then allowed to rest for 20 min - chelating agent was added (1.5 mL 10% EDTA) - mercury extraction was performed with 10 mL 0.01% dithizone-toluene - toluene was evaporated from the dithizone-toluene phase, leaving behind mercury for further analysis - samples resulting after evaporation were digested (2 mL of HNO3:HClO4 = 1:1 and 5 mL of H2SO4) - finally, 1 mL SnCl2 solution was added as a reductant			[69]
ICP-MS	Cd, Cr, Cu, Pb, Zn	- after sampling, a drop of 50% HNO3 was added to maintain a pH < 2 - 5 mL aqua regia (1:1) was added to 20 mL water sample	μg L−1-ng L−1	A: extremely sensitive; multielement analysis; speciation analysis (able to distinguish between different oxidation states); low detection limits L: expensive technique; trained personnel; expensive maintenance for apparatus	[78]
	As, Cd, Co, Mn, Ni, Pb, Zn	- water samples were stabilized with ultrapure 0.5% HNO3			[71]
	As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn	- water samples were filtered (filter membrane with 0.45 μm pore size) - HNO3 was added to acidify the samples for preservation at pH < 2 to prevent oxidation and bacterial growth			[85]
	Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn	- 90 mL water samples were digested with 10 mL concentrated HNO3 at 100 °C - the resulting digestate was filtered and diluted with 0.01 N HNO3			[72]
ICP-OES	As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn	- water samples were pre-acidified with HNO3 (5 mL L−1) - a 10 mL aliquot was subjected to digestion (0.2 mL concentrated HNO3 and 0.5 mL concentrated HCl) - the mixture was heated at 90–95 °C until the final volume was reduced to 3–5 mL - the digested sample was then diluted to a final volume of 10 mL with deionized water	μg L−1	A: multielement analysis; fast analysis L: less sensitive than ICP-MS	[74]
	As, Cd, Cr, Cu, Mn, Ni, Pb, Zn	- water samples were acidified with 65% HNO3 until reaching pH 1–2 in order to prevent precipitation and retention on the walls of the vessels			[75]
	Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn	- 100 mL water samples were acidified with HNO3:HCl = 3:1 - the acidified sample was heated until the final volume was 25 mL - the samples were filtered and diluted to 100 mL final volume			[76]
	As, Cd, Hg, Pb	- prior to analysis, water samples were filtered using a 100 mm Whatman filter			[86]
AFS	As, Hg	- after sampling, a drop of 50% HNO3 was added to maintain a pH < 2 - 5 mL aqua regia (1:1) was added to a 20 mL water sample	μg L−1-ng L−1	A: high sensitivity for Hg, As; selective detection L: limited element range; need to train personnel	[78]
NAA	Cr, Cu, Fe, Mn, Zn	- 1 L of water sample was evaporated to 20 mL - 0.5 mL was stored in a polyethylene vial - measurement of γ-ray was carried out with a high-purity germanium detector	μg L−1-ng L−1	A: non-destructive; highly sensitive; used for a wide range of samples; not affected by the errors typically associated with sample preparation L: expensive; requires access to a neutron source or nuclear reactor	[79]

AAS—atomic absorption spectrometry; GFAAS—graphite furnace atomic absorption spectrometry; CVAAS—cold vapor atomic absorption spectrometry; ICP-MS—inductively coupled plasma–mass spectrometry; ICP-OES—inductively coupled plasma–optical emission spectroscopy; AFS—atomic fluorescence spectrometry; NAA—neutron activation analysis.

Table 2. Comparison of analytical techniques for HMs detection in water.

Detection Method	Target Metals	Sample Preparation/Concentration	Detection Range	Advantages (A) and Limitations (L)	Ref.
AAS	Cd, Co, Cr, Cu, Ni, Pb, Zn	- preservation of water samples was ensured with ultrapure HNO3 - prior to analysis, water samples were filtered	mg L−1-μg L−1	A: high precision, very low detection limits L: expensive equipment, relatively longer analysis times	[82,83]
	Cd, Cr, Co, Cu, Mn, Ni, Pb, Zn	- water samples were concentrated before quantification (excepting Fe, Zn)			[84]
	Cr, Fe, Mn, Ni, Pb	- microwave digestion was performed of water samples with an acid mixture (67% HNO3: 98%H2SO4: 37%HCl: 40%HF = 2:1:1:1)			[62]
	Cd, Co, Cr, Ni, Pb	- 10 mL water sample was treated with 2 mL 8-hydroxyquinoline and 0.2 mL Cu(II) solution (coprecipitation procedure) - then, centrifugation was performed, and the resulting precipitate was dissolved in 1 mL HNO3			[60]
	Cd, Cr, Mn, Ni, Pb	- water samples were filtered (0.45 μm pore size) and the pH adjusted to 9 - we coprecipitated targeted ions with the aid of Cu(II)-dibenzyldithiocarbamate precipitate, which was then dissolved in 0.5 mL concentrated HNO3			[61]
GFAAS	Hg	- a 2% solution of KMnO4 was added to the water sample to convert mercury from organo-mercuric compounds into its ionic form and help prevent evaporation and loss of metals - measurements were conducted 24 h later, after adding a 20% solution of SnCl2 to the sample	μg L−1	A: high sensitivity for trace metals; small sample volumes required; L: expensive, use of matrix modifiers; useful for detection of Pb and Hg, in particular	[84]
	Cd	- microwave digestion was performed of water samples with an acid mixture (67% HNO3: 98%H2SO4: 37%HCl: 40%HF = 2:1:1:1)			[63]
	Pb	- water samples were filtered (filter membrane with 0.45 μm pore size) - continuous-flow microextraction was conducted with the addition of chelation reagent 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone			[67]
	Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn	- 3 L of water sample was concentrated at 80 °C till reaching a final volume of 50 mL - 4 mL 98% H2SO4 was added and digested by a Digesdahl apparatus (3 min) - 10 mL 30% H2O2 was added and heated until oxidation was finished - the resulting mixture was filtered and diluted to 50 mL with deionized water			[63]
	As, Cd, Cr, Pb	- water samples were filtered (0.45 μm pore size) and we adjusted the pH to lower than 2 - 50 mL acidified sample was treated with 5 mL HNO3 and boiled at 130 °C till the volume was 25–30 mL			[64]
CVAAS	Hg	- water samples spiked with Hg+2 ions were mineralized in a photoreactor thermostatted at 25 °C with a luminous intensity of 3.81 mW cm−2 in the presence of 100 mgTiO2 and 0.01 mol L−1 potassium persulfate - we used a preconcentration system composed of a mini-column packed with 100 mg of 2-aminothiazol-modified silica gel - we carried out elution with 100 μL 2 mol L−1 HCl	ng L−1	A: highly sensitive for mercury detection L: limited to mercury; possible interferences	[68]
	Hg	- 500 mL water was treated in a separatory funnel with 2.5 mL 20 N H2SO4 and 1.5 mL 0.5% KMnO4 - the mixture was neutralized with 5 mL 10N NaOH and 1.5 mL 10% NH2OH∙HCl and then allowed to rest for 20 min - chelating agent was added (1.5 mL 10% EDTA) - mercury extraction was performed with 10 mL 0.01% dithizone-toluene - toluene was evaporated from the dithizone-toluene phase, leaving behind mercury for further analysis - samples resulting after evaporation were digested (2 mL of HNO3:HClO4 = 1:1 and 5 mL of H2SO4) - finally, 1 mL SnCl2 solution was added as a reductant			[69]
ICP-MS	Cd, Cr, Cu, Pb, Zn	- after sampling, a drop of 50% HNO3 was added to maintain a pH < 2 - 5 mL aqua regia (1:1) was added to 20 mL water sample	μg L−1-ng L−1	A: extremely sensitive; multielement analysis; speciation analysis (able to distinguish between different oxidation states); low detection limits L: expensive technique; trained personnel; expensive maintenance for apparatus	[78]
	As, Cd, Co, Mn, Ni, Pb, Zn	- water samples were stabilized with ultrapure 0.5% HNO3			[71]
	As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn	- water samples were filtered (filter membrane with 0.45 μm pore size) - HNO3 was added to acidify the samples for preservation at pH < 2 to prevent oxidation and bacterial growth			[85]
	Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn	- 90 mL water samples were digested with 10 mL concentrated HNO3 at 100 °C - the resulting digestate was filtered and diluted with 0.01 N HNO3			[72]
ICP-OES	As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Zn	- water samples were pre-acidified with HNO3 (5 mL L−1) - a 10 mL aliquot was subjected to digestion (0.2 mL concentrated HNO3 and 0.5 mL concentrated HCl) - the mixture was heated at 90–95 °C until the final volume was reduced to 3–5 mL - the digested sample was then diluted to a final volume of 10 mL with deionized water	μg L−1	A: multielement analysis; fast analysis L: less sensitive than ICP-MS	[74]
	As, Cd, Cr, Cu, Mn, Ni, Pb, Zn	- water samples were acidified with 65% HNO3 until reaching pH 1–2 in order to prevent precipitation and retention on the walls of the vessels			[75]
	Cd, Co, Cr, Cu, Mn, Ni, Pb, Zn	- 100 mL water samples were acidified with HNO3:HCl = 3:1 - the acidified sample was heated until the final volume was 25 mL - the samples were filtered and diluted to 100 mL final volume			[76]
	As, Cd, Hg, Pb	- prior to analysis, water samples were filtered using a 100 mm Whatman filter			[86]
AFS	As, Hg	- after sampling, a drop of 50% HNO3 was added to maintain a pH < 2 - 5 mL aqua regia (1:1) was added to a 20 mL water sample	μg L−1-ng L−1	A: high sensitivity for Hg, As; selective detection L: limited element range; need to train personnel	[78]
NAA	Cr, Cu, Fe, Mn, Zn	- 1 L of water sample was evaporated to 20 mL - 0.5 mL was stored in a polyethylene vial - measurement of γ-ray was carried out with a high-purity germanium detector	μg L−1-ng L−1	A: non-destructive; highly sensitive; used for a wide range of samples; not affected by the errors typically associated with sample preparation L: expensive; requires access to a neutron source or nuclear reactor	[79]

AAS—atomic absorption spectrometry; GFAAS—graphite furnace atomic absorption spectrometry; CVAAS—cold vapor atomic absorption spectrometry; ICP-MS—inductively coupled plasma–mass spectrometry; ICP-OES—inductively coupled plasma–optical emission spectroscopy; AFS—atomic fluorescence spectrometry; NAA—neutron activation analysis.

For highly precise quantification of metals, ICP-MS and GFAAS are recommended. These methods offer superior sensitivity, allowing for the detection of even trace concentrations of metals in water samples. If there is a need to analyze multiple metals simultaneously, ICP-MS and ICP-OES are the ideal choices. Both techniques enable the detection of a broad range of elements in a single run, making them well-suited for comprehensive and multielement analysis.

Sample preparation is another key consideration. While the specifics can vary, most methods involve some form of sample filtration and acidification to prevent metal precipitation or contamination. More complex techniques may also require sample digestion to break down organic materials and release metals in measurable forms.

In terms of cost and equipment, advanced methods such as ICP-MS, GFAAS, and NAA tend to be more expensive and require specialized equipment, which can make them less accessible for routine or field analysis. In contrast, AAS and ICP-OES are relatively more affordable and accessible, though they may have limitations in terms of sensitivity or range.

The selection of appropriate HM detection methods in water monitoring depends not only on technical performance but also on practical considerations that vary significantly between developed and developing countries. The affordability of a method is a key factor in its adoption by both developed and developing countries (Figure 3).

In conclusion, the choice of detection method depends on the specific goals of the analysis, whether high sensitivity, multielement analysis, or cost-effectiveness is prioritized. Each method offers unique advantages and limitations, which must be carefully considered in light of the analytical requirements.

3.2. Techniques for In Situ and On-Site Measurement of HMs

The determination of HMs in water through in situ and on-site methods is important for accurate, real-time, and representative environmental monitoring. These approaches minimize the risk of contamination, sample degradation, or changes in metal speciation that can occur during transport and storage. In situ methods are suited for direct analysis within the natural environment, preserving real-time chemical conditions. On-site measurements facilitate rapid decision-making in the field, as well, which is essential for managing pollution events, assessing water quality according to regulatory standards. In this regard, a range of portable devices can be used, operating based on techniques such as ASV, XRF, or various types of sensors (electrochemical, optical, biosensors, nanomaterial-based sensors).

(a) Anodic stripping voltammetry (ASV) is an electrochemical analytical technique primarily used for detecting and quantifying of a variety of trace metals in solutions, including environmental water samples. The detection limits achieved by ASV typically fall within the sub-ppb range, enabling sensitive quantification of trace metals in aqueous samples [87]. The method involves a preconcentration step, during which metal ions are reduced and deposited onto the working electrode surface by applying a negative potential. This is followed by an oxidation step, where the metals are stripped back into the solution, generating an oxidation peak. The height of this peak is measured and is proportional to the metal concentration. ASV is widely used for detecting HM ions in drinking water due to its high sensitivity and selectivity. For example, the detection of total arsenic in aqueous solutions using gold macroelectrodes has been reported by Zhang and Compton [88]. By varying the deposition potential, it is possible to selectively detect As(III) or total arsenic: total arsenic is measured at a higher potential (−1.3 V), whereas As(III) is detected at a lower potential (−0.9 V). This method demonstrated detection limits as low as 0.8 μg L⁻¹ for each species, making it suitable for monitoring total arsenic levels in drinking water below the World Health Organization (WHO) guideline of 10 μg L⁻¹. In addition, an overview of heavy metal voltammetric analyzers, including descriptions of various types of electrodes used, was reported by Zhang and his team [89].

(b) X-ray fluorescence (XRF) is a non-destructive analytical technique used for elemental analysis, and it is particularly useful for detecting metals in various sample matrices, including water. In the context of water analysis, XRF offers the advantage of rapid, in situ measurements without the need for extensive sample preparation. It is not typically the most common or optimal method for routine water analysis, especially when compared to techniques like ICP-MS or ICP-OES. However, XRF can still be useful in specific cases depending on the sample matrix, the level of precision required, and the application.

A study [90] proposes and develops practical techniques for detecting trace amounts of HMs in natural waters and pure salts using X-ray fluorescence (XRF), enhanced by preconcentration methods. These methods enable the analysis of metals at concentrations significantly below the permissible thresholds. For the preconcentration of Cd, Cu, Ni, Pb, and Zn in water samples, a procedure was employed where 0.5–1.0 L of the sample was combined with 10 mL of 30% tartaric acid in a separatory funnel. After adding 0.1% phenol red, 25% ammonia, and 5 mL of 10% sodium diethyldithiocarbamate, along with 5 mL of chloroform, metal carbamates were formed. These carbamates were then extracted into the chloroform phase, effectively concentrating the metals for further analysis.

The portable X-ray fluorescence (pXRF) instrument determines the chemical composition of a sample by measuring the characteristic fluorescent (secondary) X-rays emitted when the sample is excited by a primary X-ray source. Each element in the sample produces a unique set of fluorescent X-rays, effectively serving as its fingerprint [91]. A study [92] reported the quantification of copper and lead in polluted water, with detectable concentrations of 21 mg L⁻¹ for copper (Cu) and 28 mg L⁻¹ for lead (Pb). The results were compared with certified reference concentrations and showed good agreement. However, this method has certain limitations, including relatively high detection limits and the risk of water damage to the instrument.

(c) Sensors are increasingly used for the detection of HMs in water due to their high sensitivity, portability, and rapid response. Various types of sensors, including electrochemical, optical, and biosensors, offer real-time or near real-time monitoring of toxic metal ions such as lead, cadmium, and mercury.

Detection of HMs in water can be effectively performed using two-dimensional (2D) nanomaterials, particularly MXenes, which are highly sensitive sensors. MXenes are a class of 2D transition metal carbides, nitrides, or carbonitrides, first discovered in 2011. They exhibit excellent hydrophilicity, enhanced ion-exchange capacity, and favorable redox properties, making them easily integrable into electrodes for electrochemical sensing applications [93].

For instance, a sensor based on an alkali-modified Ti-MXene decorated with bismuth and sulfur (S–Bi@TiMX) was developed for the ultra-sensitive electrochemical detection of Pb(II) [94]. This sensor demonstrated an excellent detection limit of 0.0002 mg L⁻¹ for Pb(II), along with good repeatability and reproducibility. Moreover, it exhibited minimal interference from competing ions. Real water sample testing across various Pb(II) concentrations, with recovery rates ranging from 99.2% to 99.6%, confirmed that the S–Bi@TiMX/PCE sensor is a promising candidate for on-site monitoring of lead (Pb) contamination.

Furthermore, another sensor based on MXenes (Ti₃C₂T_x, where T_x = –O, –OH, –F) was synthesized by depositing bismuth nanoparticles (BiNPs) onto Ti₃C₂T_x sheets [95]. This sensor proved suitable for the simultaneous electrochemical detection of Pb(II) and Cd (II) in water samples, with detection limits of 10.8 nM for Pb(II) and 12.4 nM for Cd(II). Both HM ions were tested in two different water types (tap water, lake water) at concentrations of 150 nM. The recovery rates for Cd(II) and Pb(II) in tap water were 98.3% and 101.2%, respectively, while in lake water they were 101.5% and 106.3%, respectively. Additionally, results obtained via electrochemical analysis were in good agreement with those from ICP-MS, confirming the sensor’s reliability and efficiency. However, one noted drawback of this sensor was its susceptibility to interference from Cu(II) ions.

An optical sensor, water-soluble and cationic porphyrin-based, was used for the detection of Cd(II), Cu(II), Hg(II), and Pb(II) in water samples [96]. Additionally, a novel chemosensor was designed and synthesized [97] to selectively detect Hg(II) in natural water. It integrates a ferrocene unit and a rhodamine block via a carbohydrazone binding unit. Upon coordination with metallic ion, it exhibits significant fluorescence enhancement, with a detection limit as low as 1 μg L⁻¹.

Furthermore, platforms integrate sensors for the detection of HMs in water. For example, a portable and sensitive Hg(II) detection system was developed using a smartphone-based colorimetric aptamer nanosensor [98]. The sensor showed an excellent performance in detecting mercury in tap and river water, with a detection range of 1–32 μg L⁻¹, a strong linear correlation (R² = 0.991), and a low detection limit of 0.28 μg L⁻¹. The assay is rapid, completing in just 20 min, making it ideal for on-site environmental mercury monitoring.

There are several important aspects to consider regarding the potential of sensors for heavy metal detection. While many sensors offer low detection limits, sometimes comparable to those of laboratory-based techniques, their performance can be affected by various environmental factors. Interferences may arise from the presence of specific ions in water samples. For example, a sensor based on MXenes (Ti₃C₂T_x, where T_x = –O, –OH, –F) has shown significant susceptibility to Cu(II) ions, which can lead to false readings or necessitate additional sample pretreatment. Complex water matrices containing competing ions and organic matter can further compromise sensor selectivity and accuracy.

Although these sensors are highly valued for their portability and suitability for in-field applications, it is important to note that their fabrication, often involving noble metal functionalization or complex molecular modifications, can be both costly and technologically demanding.

In conclusion, on-site and in situ detection methods are essential for accurate, real-time monitoring of HMs in water, minimizing sample contamination and preserving natural conditions. Techniques like ASV provide high sensitivity for trace metals, while pXRF offers rapid elemental analysis with some limitations. Advances in sensors, especially those using novel materials like MXenes, show great potential for sensitive and selective heavy metal detection in complex water samples.

Integrated sensor platforms, including smartphone-based nanosensors, enable fast, portable, and low-cost heavy metal monitoring. These innovative approaches highlight the growing capability of portable devices to deliver timely and reliable water quality assessments in the field.

As part of the final considerations, Figure 4 provides a summary of the key points from Section 3.1 and Section 3.2, improving readability and aiding overall comprehension.

3.3. Integration of Artificial Intelligence and Internet of Things in HM Detection

The integration of artificial intelligence (AI) with Internet of Things (IoT) technologies has revolutionized environmental monitoring by enabling real-time and remote detection of heavy metal contamination in water systems. By combining IoT-enabled sensors that continuously collect data with AI algorithms that analyze and interpret complex datasets, this approach significantly enhances the accuracy, sensitivity, and responsiveness of detection methods.

A study [99] presents a self-powered, automated sensing system integrated with a robotic hand for real-time, on-site detection of toxic HMs (As(III), Cr(III), Pb(II)) in water. Powered by a thermoelectric generator and utilizing copper oxide nanowire sensors with ion-selective membranes, the device operates wirelessly, enabling remote monitoring while minimizing exposure risks. As demonstrated in lake water, this IoT-enabled system offers a safe and efficient solution for environmental monitoring and pollutant detection in hard-to-access areas.

A recent study by Lahari et al. [100] presents a novel electrochemical sensor system based on a gold nanoparticle-modified carbon electrode for the simultaneous detection of Cd(II), Pb(II), Cu(II), and Hg(II) in water using differential pulse voltammetry. The system demonstrated detection limits ranging from 0.62 to 1.38 μM within a concentration range of 1–100 μM. As validated with lake water samples from Hyderabad under acidic conditions, the sensor exhibited strong selectivity and reliability. What sets this platform apart is its integration with Internet of Things (IoT) connectivity and deep learning algorithms, notably a convolutional neural network (CNN) that enhanced heavy metal classification based on DPV signals. The system supports real-time data acquisition, remote monitoring, and intelligent data analysis, making it highly suitable for incorporation into environmental monitoring networks.

Furthermore, an advanced IoT-enabled sensor platform for real-time water quality monitoring has been developed [101], featuring a novel heterostructure of dendritic ReS₂ and graphene functionalized with dithiothreitol-coated gold nanoparticles. This configuration enables rapid detection of As(III) within 2 s, with a detection limit as low as 10 pM.

A comparison between laboratory methods and in situ/on-site methods is presented in Figure 5.

Among the advantages of integrating AI with IoT for detecting HMs, real-time continuous monitoring stands out, enabling rapid detection and prompt response to contamination events. Additionally, AI algorithms can efficiently process large volumes of sensor data, improving accuracy and reducing false positives. Automation further helps decrease operational costs and minimizes human error.

However, despite these benefits, several challenges remain. The initial costs associated with advanced sensors and AI infrastructure can be significant. One of the key challenges in implementing IoT-based sensors for heavy metal monitoring in water systems is ensuring data privacy and security. Existing studies [102,103] highlight risks related to managing large datasets and potential network vulnerabilities, which can compromise data integrity and system reliability.

Moreover, the complexity of these integrated systems requires specialized technical expertise for their development, operation, and maintenance, which may limit accessibility in some settings.

4. Regulatory Standards and Legislative Measures for HMs in Water

4.1. Regulations and Legislation Related to HM Levels in Water

Considering the adverse effects of HMs on human health and the environment, various organizations have established standards for surface water and drinking water quality. In the European Union (EU), water quality is primarily regulated through a combination of directives that set contaminant limits in water bodies, aligning with the recommendations of the World Health Organization (WHO) and other international agencies. One of the United Nations’ Sustainable Development Goals (SDGs), specifically SDG 6, aims to ensure universal access to safe drinking water and sanitation, promote the sustainable management of water resources, and protect aquatic ecosystems. Moreover, target 6.3 of SDG 6 seeks to improve water quality by 2030 through reducing pollution, eliminating dumping, and minimizing the release of hazardous chemicals and materials, including HMs [104].

In the EU, the main legislation act addressing water pollution is the Water Framework Directive (WFD) [105], which establishes the framework for safeguarding inland surface waters, transitional waters, coastal waters, and groundwater, including controlling pollutants like HMs in surface water. The WFD is composed of several targeted directives [106,107,108,109,110,111,112] aimed at achieving integrated water resource management across the EU (Figure 6).

Among these, the Environmental Quality Standards Directive [109] includes provisions for setting Environmental Quality Standards (EQS) for surface water pollutants, including HMs, and requires member states to reduce pollution levels and protect water bodies from degradation.

Furthermore, it is important to note that the Water Framework Directive (WFD) does not establish specific limits for HMs; instead, these are regulated under the Environmental Quality Standards Directive [109].

These standards are designed to protect both aquatic ecosystems and human health from the harmful effects of heavy metal pollution. The WFD has been incorporated into the legislation of EU member states and, based on biological and physicochemical monitoring, is linked to the ecological status of water bodies—classified as high, good, moderate, poor, or bad. Additionally, the WFD requires EU member states to strive for achieving a good ecological status for all surface water and groundwater bodies by 2027.

Table 3. Standards for HMs in water (concentrations are expressed as μg L⁻¹).

Pollutant	EQS (Surface Water) [107]			Drinking Water
	AA-EQS Inland Surface Waters	AA-EQS Other Surface Waters	MAC-EQS Inland Surface Waters	EU Legislation [107]	WHO [113]	USEPA [114]
Arsenic (As)	-	-	-	-	10	10
Cadmium (Cd) *	≤0.08 (Class 1) 0.08 (Class 2) 0.09 (Class 3) 0.15 (Class 4) 0.25 (Class 5)	0.2	≤0.45 (Class 1) 0.45 (Class 2) 0.6 (Class 3) 0.9 (Class 4) 1.5 (Class 5)	5	3	5
Copper (Cu)	-	-	-	2000	2000	1300
Chromium (Cr)	-	-	-	25	50	100
Lead (Pb)	7.2	7.2	NA	5	10	15
Mercury (Hg)	-	-	-	1	6	2
Nickel (Ni)	20	20	NA	20	70	100

EQS = environmental quality standards; AA-EQS = annual average—environmental quality standards; MAC-EQS = maximum allowable concentration—environmental quality standards; NA = not applicable. * For cadmium, the EQS values vary depending on the hardness of the water as specified in five class categories (Class 1: <40 mg CaCO₃/L, Class 2: 40 to <50 mg CaCO₃/L, Class 3: 50 to <100 mg CaCO₃/L, Class 4: 100 to <200 mg CaCO₃/L, Class 5: ≥200 mg CaCO₃/L).

presents regulatory concentration limits for various HMs and pollutants in both surface water [109] and drinking water [107]. For surface waters, the table includes Annual Average Environmental Quality Standards (AA-EQS) and Maximum Allowable Concentrations (MAC-EQS), which vary based on water body type and ecological status (for example, Class 1 to Class 5 for cadmium).

For drinking water, the table compares threshold values established by three major health and environmental authorities: the European Union (EU) [107], the World Health Organization (WHO) [113], and the United States Environmental Protection Agency (USEPA) [114].

4.2. Challenges in Enforcement

Despite existing regulations, significant gaps remain in legislation concerning HMs in surface and drinking water. For instance, the maximum allowable concentrations for lead, cadmium, and mercury vary between countries, which can lead to inconsistent interpretation and enforcement [115].

Additionally, some toxic metals are not routinely monitored nor included as mandatory parameters under EU water quality legislation. A notable example is the contamination of drinking spring water by thallium in the Apuan Alps (Tuscany, Italy) [116].

Moreover, inadequate monitoring infrastructure in certain countries or regions—combined with shortages of trained personnel, high analytical costs, the diffuse nature of agricultural pollution sources, and the lack of real-time monitoring systems—can hinder continuous surveillance of HMs. As a result, there is an increased risk of failing to detect hazardous metal contamination, even when relevant laws and regulations are in place.

5. Impact of HMs on Water Quality and Aquatic Organisms

The quality of water bodies is adversely affected by the presence of HMs, which can be toxic even at low concentrations and disrupt the balance of aquatic ecosystems. This toxicity stems from their persistence in the environment, combined with bioaccumulation and biomagnification processes. Studies have demonstrated the complex interactions between HMs and nutrients in aquatic systems, providing valuable insights into the migration and transformation processes occurring at the sediment–water interface [117]. When HMs are present in drinking water sources, they pose significant risks to human health.

5.1. Impact of HMs on Water Quality

The presence of HMs in water also affects its aesthetic qualities, causing discoloration and a murky appearance, which renders the water unsuitable for recreational activities and, if used as a potable source, unsafe for drinking. Beyond these visible impacts, concentrations of HMs in drinking water that exceed regulatory limits (Table 3) pose significant health risks. Exposure can lead to a wide range of adverse effects and diseases, some of which can be fatal, as extensively documented in the literature. Notably, arsenic (As) is associated with skin lesions and cardiovascular diseases; cadmium (Cd) with renal, hepatic, and pulmonary dysfunctions; lead (Pb) with nervous system damage [118]; chromium (Cr) with stomach and lung cancers [119]; mercury (Hg) with neurological dysfunctions and behavioral abnormalities [120]; and nickel (Ni) with reproductive system disturbances and skin damage [121].

In recent years, the extent of water pollution by HMs has been evaluated using various indices, including the following:

(a)

Heavy metal contamination load (CL) can be calculated using the following equation [118]: CL = HC x Q x 86.4, where CL = heavy metal contamination load (kg day⁻¹), HC = heavy metal content in contaminated water (mg L⁻¹), and Q = flow rate (m³ s⁻¹);

(b)

Heavy metal pollution index (HPI) is used to evaluate the overall water quality as a function of HM contents. It was computed in several steps by [122,123,124,125,126,127], and on the basis of obtained values, the quality of water is classified;

(c)

Pollution index (PI) is calculated on the basis of individual metal concentration. The calculation for this parameter and water classification according to PI values are reported by Goher et al. [128];

(d)

Heavy metal evaluation index (HEI) is an index calculated on the basis of each heavy metal level and maximum admitted concentration in water according to legislation, as reported by [125,128].

5.2. Impact of HMs on Aquatic Organisms

Furthermore, HMs have the capacity to bioaccumulate in aquatic organisms and undergo biomagnification throughout the aquatic food chain (Figure 7). Bioaccumulation refers to the buildup of pollutants, including HMs, within an organism as a result of direct uptake from the environment [5]. In aquatic organisms, metal bioaccumulation occurs primarily through ingestion—herbivores accumulate metals by consuming contaminated plants, while carnivores acquire them by eating already contaminated prey. Other pathways for bioaccumulation include dermal absorption and maternal transfer [129].

Once HMs enter the fish body, they accumulate in organs and tissues, causing histopathological alterations in vital organs [130] along with various other manifestations (Figure 8) [128,129,130,131,132,133]. In particular, invertebrates serve as bioindicators for biomonitoring heavy metal pollution in aquatic ecosystems [5].

Biomagnification is the process whereby pollutant concentrations increase at successive trophic levels within a food chain. This primarily occurs with pollutants like HMs that are not easily broken down or excreted by organisms [5]. Consequently, carnivorous fish, positioned at the top of the food chain, tend to exhibit higher levels of HMs compared to other species [5,129]. This biomagnification can lead to severe health consequences for all species consuming contaminated fish.

Numerous studies have assessed heavy metal concentrations in fish, focusing on accumulation in various organs and tissues [5,131,132]. The detected metal levels vary widely, depending on the degree of aquatic pollution, exposure duration, and fish species [132]. Moreover, the heavy metal content found in fish is often correlated with contamination levels in water and sediments. The metals of greatest environmental concern frequently detected in fish organs and tissues include arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), mercury (Hg), manganese (Mn), nickel (Ni), and zinc (Zn).

6. Approaches to Mitigate HM Contamination Resulting from Agriculture

The escalating issue of HM contamination resulting from agricultural practices requires the adoption of effective control measures. Since completely eliminating the use of fertilizers and pesticides is nearly impossible, implementing sustainable agricultural practices offers a viable solution to mitigate the adverse effects of pollution.

6.1. Best Agricultural Practices

Promoting practices such as reduced input use and organic farming is effective in minimizing heavy metal runoff into water sources and accumulation in the soil. Additionally, the application of high-quality organic fertilizers—such as compost and farmyard manure—can reduce the availability of HMs due to the organic matter’s ability to bind and retain them. For instance, studies have shown that the application of chicken and swine manure can decrease the levels of available cadmium (Cd) and lead (Pb) in the soil [134].

The effects of farmyard manure, commercial inorganic fertilizers (nitrogen, phosphorus, and potassium—NPK), and a combination of farmyard manure with nitrogen have been investigated in relation to heavy metal concentrations in soil and plants following wastewater irrigation. The results indicated that the phytoavailability of metals showed a different trend compared to total metal levels. Among all treatments, the application of farmyard manure led to reductions of 9.58% in copper (Cu), 7.93% in lead (Pb), 18.23% in nickel (Ni), and 7.18% in chromium (Cr) levels. Moreover, the concentrations of Cu, Pb, Ni, and Cr in plants grown on soil treated with farmyard manure were lower compared to both the control and other treatments, and the translocation ratio of metals was also reduced. In contrast, the highest phytoavailability of HMs was observed in soils treated with NPK fertilizers, likely due to decreased soil pH [135].

In conclusion, organic fertilizers help reduce both the availability of HMs in soil and their uptake by plants. Incorporating straw into contaminated soils has also been reported as an effective measure for immobilizing HMs, as the decomposition of straw results in organic matter that can bind these metals [136]. However, some studies highlight the opposite effect, indicating that straw may increase metal mobility under certain conditions [137].

Soil pH is another critical factor influencing heavy metal availability. The use of inorganic amendments, such as lime, can effectively reduce heavy metal mobility by increasing soil pH—though arsenic is an exception, as its mobility may increase with higher pH levels [136]. Another effective material for immobilizing HMs is biochar, which acts through various mechanisms, including electrostatic attraction, complexation, ion exchange, precipitation, and physical adsorption. It also indirectly enhances metal immobilization by improving the soil’s adsorption capacity [138,139]. Due to its strong adsorption properties, biochar is increasingly used for the efficient removal of HMs from both soil and water. Recently, the incorporation of nanoparticles into biochar has gained attention for further improving its effectiveness [139].

Moreover, the adoption of precision farming techniques represents a promising strategy for mitigating heavy metal contamination from agricultural sources. By using advanced technologies such as drone-based monitoring and GPS-assisted applications, farmers can apply inputs more accurately, thereby reducing the risk of runoff and leaching of HMs into water bodies. Precision farming also enables continuous data collection and analysis, allowing for optimized input use and improved monitoring of water quality outcomes [140].

6.2. Supplementary Metal Pollution Control Strategies

Controlling soil erosion is a vital component of soil management that significantly contributes to the reduction in water pollution. Soil particles contaminated with HMs can be transported into water bodies, thereby spreading pollution and harming aquatic ecosystems. Erosion control is a complex task that involves various approaches such as terracing, vegetative stabilization, and the use of chemical stabilizers [141,142].

The restoration of ecosystems affected by heavy metal pollution can also be achieved through phytoremediation, a process that uses hyperaccumulator plants to remove contaminants from soil and water through various mechanisms. Certain species—such as Hordeum vulgare, Typha domingensis, Brassica juncea, and Lupinus luteus—exhibit phytostabilization capabilities, meaning they immobilize HMs in the rhizosphere by converting them into non-toxic forms, which are not taken up by the plant. Other plants—such as Brassica napus, Helianthus annuus, Pisum sativum, and Spinacia oleracea—are known for their ability to phytoextract HMs, absorbing and accumulating them in their tissues [143].

In aquatic environments, phytoremediation can be carried out using various plant species. Free-floating aquatic plants such as water hyacinth (Eichhornia crassipes), duckweeds (Lemna minor, Spirodela intermedia), water lettuce (Pistia stratiotes), and watercress (Nasturtium officinale) have been reported to effectively accumulate HMs including Pb, Hg, Cd, Cu, Cr, Ni, and Zn. Additionally, submerged aquatic plants like parrot feather (Myriophyllum spicatum), pondweed (Potamogeton crispus), and American pondweed (Potamogeton pectinatus) are used to scavenge HMs from water, mainly accumulating them in their leaves and contributing to the purification of entire water surfaces [144].

Recently, bioremediation, a microorganism-assisted process for pollutant removal, has gained attention for its effectiveness in heavy metal cleanup [145]. Microorganisms contribute to the removal of HMs through several mechanisms, including biosorption, bioaccumulation, biomineralization, and bioreduction, making this a promising approach for decontaminating polluted environments [146].

Bioremediations and phytoremediation are good candidates for HM removal, but their large-scale application in agricultural landscapes presents several limitations, such as the time required for effective remediation, site-specific variability (soil composition, climate), bioavailability of HMs, and biological constraints.

Controlling soil erosion and applying phytoremediation with hyperaccumulator plants are effective strategies to reduce heavy metal contamination in soils and water bodies. Additionally, bioremediation using microorganisms offers a sustainable solution for detoxifying polluted environments through natural processes like biosorption and bioaccumulation.

6.3. Public Awareness and Community Involvement

In addition to scientific approaches to pollution control, public education and awareness play a critical role. It is important for people to understand that once pollution is generated—especially by structurally complex and diverse pollutants like HMs—it cannot be entirely eliminated and is extremely difficult to manage. Therefore, responsible behavior must be fostered through education on the sources of heavy metal pollution, their environmental and health impacts, and the importance of preventive measures.

Promoting sustainable agricultural practices—such as reduced input use, organic farming, and the adoption of precision agriculture—can offer more environmentally friendly alternatives [13]. Although these methods may sometimes result in lower crop yields, they significantly reduce the risk of long-term environmental damage.

In conclusion, water pollution caused by HMs from agricultural activities is an increasing concern. However, with appropriate management strategies, public awareness, and responsible behavior, the associated risks can be effectively minimized.

7. Conclusions and Future Perspectives

Water pollution caused by the use of agricultural inputs—such as pesticides, fertilizers, and improper waste management—has intensified due to the growing pressure on agriculture to meet global food demands. This demand has led to intensified farming practices, often at the expense of environmental health, particularly water quality.

Monitoring water quality has become essential, particularly considering the long-term ecological and health effects of HMs. As agricultural practices evolve, so too must the methods for detecting and monitoring pollutants across complex environmental systems. High-performance techniques such as AAS, GFAAS, CVAAS, ICP-MS, and ICP-OES have been widely used for heavy metal detection. These methods require meticulous sample preparation—preconcentration, acidification, filtration, and digestion—to ensure accurate and sensitive quantification.

While these methods provide highly accurate and sensitive results, they also come with limitations—such as high analytical costs, the need for highly trained personnel, and complex sample preparation procedures—which hinder their widespread application, especially in low-resource settings.

Emerging technologies—such as remote sensing via satellites [147,148], nanosensors [149], biosensors [150,151], automated analyzers [152], and real-time monitoring stations—are transforming the landscape of water quality assessment. These tools, particularly when integrated with AI and IoT, offer real-time data analysis and early warning capabilities that enhance pollution control and decision-making.

IoT-enabled systems facilitate continuous, remote monitoring of water quality, enhancing responsiveness and enabling timely interventions [153]. These advances, along with improvements in detection technologies and AI, are making environmental monitoring more precise and accessible, supporting effective, evidence-based pollution management.

Future directions for monitoring and mitigating water contamination by HMs resulting from agricultural practices should encompass both research priorities and policy.

Therefore, the following research priorities should be considered:

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Development of high-sensitivity and high-selectivity sensors, along with other smart monitoring tools, for accurate and real-time field measurements;

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Integration of smart technologies (IoT-enabled systems, remote sensing) into environmental monitoring to enhance detection and early warning capabilities;

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Development of precise, field-ready, and affordable methods for heavy metal monitoring to ensure widespread accessibility, real-time risk assessment, and effective pollution management;

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Incorporation of digital tools and AI-powered systems into precision agriculture to enable smarter input management and improved environmental monitoring;

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Advancement of bioremediation and phytoremediation strategies to enhance their efficiency, flexibility, and affordability in removing heavy metals from the environment;

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Design of multifunctional soil amendments (e.g., biochar, organic amendments) that simultaneously improve soil fertility and immobilize heavy metal contaminants to prevent their leaching into water bodies.

In terms of policy recommendations, the following is essential:

-

Enforce stricter quality standards for agricultural inputs to limit heavy metal content;

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Reinforce regulatory frameworks to ensure compliance with environmental protection measures;

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Expand farmer education programs focused on input management and sustainable agricultural practices to prevent environmental pollution.

In conclusion, while the complete elimination of agricultural pollution is unrealistic, source-level controls and sustainable practices offer effective alternatives. A combined approach, driven by scientific innovation, smart land use, and community involvement, can significantly reduce the risks of heavy metal contamination.

Continued innovation and research remain essential to enhance monitoring systems and mitigate the environmental impacts of intensive agricultural practices.