Scoping Review on Soil Contamination from Lead-Zinc Slag and Environmental Assessment Methods

1. Introduction

Soil contamination by heavy metals represents a pervasive global environmental challenge, affecting agricultural productivity, ecosystem health, and human well-being [1]. Heavy metals such as lead, zinc, cadmium, and arsenic, persist in soils due to their non-biodegradable nature, leading to bioaccumulation in food chains and posing risks of chronic toxicity, including neurological disorders, renal damage, and carcinogenicity [2]. According to global estimates, over 50 million hectares of agricultural land worldwide are contaminated by heavy metals [3], with Asia bearing the heaviest burden due to rapid industrialization and intensive mining activities [4,5]. In Europe and North America, legacy pollution from historical smelting operations continues to impact soils decades after facility closures [6]. These contaminants enter soils through atmospheric deposition, wastewater irrigation, agrochemical applications, and direct industrial discharges [4].

Industrial activities, particularly non-ferrous metal smelting and mining, are the primary anthropogenic sources of this pollution, with metallurgical slag — a byproduct of ore processing — making a significant contribution [7,8]. Lead-zinc smelting generates vast quantities of slag, often disposed of in landfills or reused without adequate environmental safeguards, resulting in the leaching of Pb, Zn, Cd, and As into surrounding soils [9]. Slag from Pb-Zn operations typically contains 1–5% Pb, 10–30% Zn, and variable Cd levels, far exceeding natural soil background concentrations [10]. Globally, the annual production of Pb-Zn slag exceeds 10 million tons, stockpiled near smelters in China, India, Australia, and Central Asia [11]. In mining districts of southern China, soil Pb concentrations near slag heaps reach 2,000 mg/kg — more than 100 times regulatory limits — while Zn levels surpass 5,000 mg/kg [12,13].

The geochemical instability of slag under varying environmental conditions exacerbates contamination. Acidic rainfall lowers soil pH and enhances metal solubilization [14], while organic matter increases metal bioavailability [15]. Redox fluctuations in waterlogged soils release bound metal ions into groundwater [16]. In arid regions, dust storms disperse fine slag particles over hundreds of kilometers, contaminating remote farmlands [17,18]. These processes create diffuse pollution plumes that infiltrate aquifers and river systems, extending contamination well beyond point sources.

Environmental risks are multifaceted. In agroecosystems, elevated Pb and Cd inhibit seed germination and reduce crop yields by 20–50% [19]. Plants such as rice and wheat hyperaccumulate Zn, transferring it to edible parts and posing dietary risks [20,21]. Soil microbial communities suffer enzyme activity declines of up to 70%, disrupting nutrient cycling [21,22]. Human exposure occurs via ingestion of contaminated produce, dust inhalation, and dermal contac [23,24]. Children are particularly vulnerable, with blood Pb levels above 5 μg/dL linked to irreversible IQ deficits [25]. Chronic Cd exposure induces itai-itai disease, characterized by severe bone pain and kidney failure [26].

Lead-zinc slag pollution exemplifies these risks globally [27,28]. In Yunnan province, China, decades of Pb-Zn smelting have contaminated over 1,000 km² of farmland [12,29]. In Kabwe, Zambia, childhood blood Pb levels average 50 μg/dL — ten times WHO thresholds — due to slag recycling in informal economies [30,31,32]. In Kazakhstan, the Shymkent lead plant has generated millions of tons of slag over several decades, with documented soil Pb concentrations exceeding 3,000 mg/kg near the facility — more than 900 times the maximum permissible concentration — yet this site remains largely absent from the international peer-reviewed literature [33]. Climate change intensifies these threats: increased rainfall enhances leaching, while rising temperatures accelerate slag weathering and metal release [17,34,35,36].

Current environmental assessment methods include chemical analysis, GIS-based spatial mapping, bioindicators, and predictive modeling [37,38,39]. Sequential extraction procedures such as BCR differentiate metal fractions and inform bioavailability assessment [40,41]. Geostatistical tools including kriging interpolate contamination hotspots, while ecological risk indices quantify environmental threats [42,43,44]. However, these methods face limitations: high costs restrict adoption in low-resource settings; site-specific variability requires calibration; and insufficient integration of multi-media exposure pathways underestimates holistic risk [45,46]. Emerging tools such as diffusive gradients in thin films provide labile metal proxies, but standardization lags [47,48,49].

Despite extensive global research on heavy metal contamination near smelting areas, studies specifically addressing lead-zinc slag and employing systematic environmental assessment remain fragmented [10,11,12,50]. Existing research is concentrated in China, Europe, and parts of South America, with Central Asia — particularly Kazakhstan — entirely unrepresented in the peer-reviewed literature despite hosting major Pb-Zn smelting infrastructure [51]. Furthermore, no scoping review has systematically mapped the diversity of assessment methods applied across slag-contaminated sites globally, limiting methodological standardization and evidence synthesis.

This scoping review addresses these gaps by systematically mapping the global evidence base on soil contamination from lead-zinc slag and the environmental assessment methods employed in primary studies. Following PRISMA-ScR 2018 guidelines [50], the review pursues three objectives: (1) to characterize the scope and distribution of soil contamination from Pb-Zn slag and smelting activities reported in the literature; (2) to identify and categorize environmental assessment and predictive modeling methods applied across study contexts; and (3) to identify geographic, methodological, and thematic research gaps with particular relevance to Kazakhstan and Central Asia.

2. Materials and Methods

2.1. Study Design

This scoping review was conducted in accordance with PRISMA-ScR 2018 guidelines [50]. The review protocol is publicly registered at OSF: https://osf.io/ktg86 (registered 16 March 2026). The review was funded by grant AP25795537 of the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan.

2.2. Eligibility Criteria

Studies were selected using the Population, Concept, and Context (PCC) framework. The Population comprised soils contaminated by lead-zinc slag or associated smelting and mining activities. The Concept encompassed ecological assessment methods and predictive modeling approaches for evaluating soil contamination. The Context included industrial sites globally, with particular relevance to the Shymkent region of Kazakhstan.

Inclusion criteria were: (1) peer-reviewed journal articles published between January 2010 and March 2025; (2) studies reporting soil contamination directly linked to lead-zinc slag, smelter emissions, or metallurgical waste; (3) studies employing at least one environmental assessment or predictive modeling method; and (4) publications available in English.

Exclusion criteria were: (1) studies focused exclusively on construction material applications of slag without environmental assessment; (2) wastewater treatment studies without soil contamination data; (3) pyrometallurgical metal recovery processes; (4) solidification/stabilization technology studies without field environmental data; and (5) publications prior to 2010.

2.3. Information Sources and Search Strategy

A systematic literature search was conducted on 16 March 2026 across three databases: Dimensions (n = 351), PubMed (n = 10), and OpenAlex (n = 49), yielding 410 records in total. The following Boolean search string was applied uniformly across all databases:

("lead-zinc slag" OR "Pb-Zn slag" OR "zinc slag" OR "lead slag") AND ("soil contamination" OR "soil pollution" OR "heavy metals" OR "environmental assessment" OR "remediation")

2.4. Study Selection and Screening Process

All retrieved records were imported into Rayyan (rayyan.ai) for systematic management [52]. After automated and manual duplicate removal (n = 27), a total of 383 records underwent title and abstract screening by one reviewer against the predefined eligibility criteria. Records clearly not meeting inclusion criteria were excluded (n = 253), yielding 130 records for full-text assessment. Following full-text screening, 56 studies met all inclusion criteria and were retained for data extraction and evidence mapping. The study selection process is illustrated in the PRISMA-ScR flow diagram (Figure 1).

2.5. Data Extraction and Evidence Mapping

Data were extracted from the 56 included studies using a standardized charting form implemented in Elicit (elicit.com) and Microsoft Excel. Extracted variables included: bibliographic details (authors, publication year, journal); geographic scope (country, region); contamination source (slag type, smelter, tailings, mine); heavy metals analyzed; environmental assessment methods employed; predictive or transport models used; number of soil samples collected; and key findings regarding contamination levels and risks.

The study selection process and the number of records included at each stage are illustrated in the PRISMA-ScR flow diagram.

3. Results

3.1. Study Selection and PRISMA Flow

The systematic search across Dimensions, PubMed, and OpenAlex identified 410 records. Following the removal of 27 duplicate records, 383 records underwent title and abstract screening. Of these, 253 records were excluded as they did not meet the inclusion criteria, primarily because they focused on construction material applications of slag, wastewater treatment, or pyrometallurgical metal recovery processes without environmental soil assessment. The remaining 130 records proceeded to full-text assessment, of which 56 studies met all inclusion criteria and were included in the final scoping review. The study selection process is presented in Figure 1.

3.2. Characteristics of Included Studies

The 56 included studies were published between 2010 and 2025, with a marked increase in publication frequency after 2020, reflecting growing global concern over the environmental impacts of metallurgical waste. Specifically, 15 studies were published between 2010 and 2017, compared with 41 between 2018 and 2025, representing a 2.7-fold increase. Study designs were predominantly empirical field investigations, with the majority collecting soil samples from sites adjacent to active or abandoned lead-zinc smelters, slag heaps, or tailings deposits. Peer-reviewed journal articles constituted all included publications. The characteristics of included studies are summarized in Table 1.

Figure 2. Bibliometric characteristics of included studies (n = 56): (a) annual publication trends, 2010–2025; (b) geographic distribution by country; (c) frequency of environmental assessment methods applied; (d) distribution of study design types.

Figure 2. Bibliometric characteristics of included studies (n = 56): (a) annual publication trends, 2010–2025; (b) geographic distribution by country; (c) frequency of environmental assessment methods applied; (d) distribution of study design types.

3.3. Geographic Distribution of Studies

The 56 included studies exhibited a pronounced geographic concentration in regions with established lead-zinc smelting histories. China dominated the literature with 20 studies (35.7%) [53,54,55,56,57,58], reflecting the country's status as the world's largest lead and zinc producer [59]. Poland contributed five studies (8.9%) [60,61], primarily examining legacy contamination from historical Pb-Zn mining districts. Brazil accounted for four studies (7.1%) [62,63,64], with particular focus on the Santo Amaro [63] former smelter site in Bahia state. The United States and Australia each contributed two studies (3.6%), while Bangladesh [65], India [66], Indonesia [67,68], France, Greece [69,70], South Korea [71], and the Republic of Macedonia [72] each contributed 1–2 studies [58,73,74]. The geographic distribution is presented in Figure 3.

Critically, no studies were identified from Kazakhstan despite the presence of one of Central Asia's largest lead-zinc smelting complexes in the Shymkent region [75], representing a significant research gap with direct relevance to this review. Central Asia as a whole was absent from the identified literature, highlighting a major geographic underrepresentation in the global evidence base.

3.4. Environmental Assessment Methods Used in the Literature

The 56 included studies employed a diverse range of environmental assessment methods, which were categorized into five primary groups: pollution indices, ecological risk assessment, human health risk assessment, spatial analysis, and advanced analytical and modeling approaches. Many studies applied multiple methods simultaneously, reflecting a trend toward integrated assessment frameworks.

3.4.1. Pollution Indices

Pollution indices were the most frequently applied assessment tools, used in 41 studies (73.2%). The most common indices included the Single Factor Pollution Index (PI), the Nemerow Synthetic Pollution Index (NSPI), the Geoaccumulation Index (Igeo) [76], and the Enrichment Factor (EF) [77]. These indices provide standardized quantitative measures of contamination relative to background or regulatory threshold values, enabling comparison across study sites and metals.

3.4.2. Ecological Risk Assessment

Ecological risk assessment methods were applied [78] in 38 studies (67.9%). The Potential Ecological Risk Index (PER/RI) [62,79], originally developed by Hakanson (1980), was the most widely used approach, calculating risk based on metal toxicity response coefficients and contamination factors. Risk grades ranged from low to significantly high ecological risk, with cadmium (Cd) consistently identified as the primary driver of high ecological risk across studies due to its high toxicity coefficient [80,81].

3.4.3. Human Health Risk Assessment

Human health risk assessment following the United States Environmental Protection Agency (US EPA) [82] framework was applied in 29 studies (51.8%). These assessments calculated non-carcinogenic hazard quotients (HQ) and hazard indices (HI), as well as carcinogenic risk indices (CR/TCR) for adults and children separately, considering exposure pathways including ingestion, dermal contact, and inhalation [83,84]. Six studies employed Monte Carlo simulation for probabilistic risk assessment (10.7%) [85,86].

3.4.4. Spatial Analysis Methods

Geospatial analysis was employed in 32 studies (57.1%) [71,87,88]. Geographic Information Systems (GIS) with kriging interpolation were used for spatial distribution mapping [89,90] and contamination hotspot identification. Principal Component Analysis (PCA) and Cluster Analysis (CA) were applied in 28 studies (50.0%) [91] to identify sources and differentiate anthropogenic from geogenic metal contributions. Positive Matrix Factorization (PMF) [92] and APCS-MLR receptor models were used in 12 studies (21.4%) for quantitative source apportionment [93].

3.4.5. Advanced Analytical and Modeling Approaches

Advanced analytical techniques were applied across multiple studies. Portable X-ray fluorescence (pXRF) was used for rapid field screening in eight studies (14.3%) [94,95,96]. Lead isotope ratio analysis (²⁰⁶Pb/²⁰⁷Pb) for source tracing was applied [97]. Sequential extraction procedures (BCR, Tessier) for metal fractionation and bioavailability assessment were employed in nine studies [98] (16.1%). Predictive transport modeling including Hydrus-1D simulations and multisurface geochemical modeling was used in four studies (7.1%) [99,100,101]. Biological and ecotoxicological methods including microbial community analysis (16S rRNA), phytotoxicity bioassays, and enzymatic biomarkers (dehydrogenase activity) were applied in 14 studies (25.0%) [102,103]. The distribution of assessment methods is summarized in Table 2.

Assessment Method	n	%
Pollution indices (PI, Nemerow, Igeo, EF)	41	73.2
Ecological risk index (PER/RI)	38	67.9
GIS + spatial mapping (kriging)	32	57.1
Human health risk (US EPA, HQ/HI)	29	51.8
PCA + Cluster Analysis	28	50.0
PMF source apportionment	12	21.4
Sequential extraction (BCR/Tessier)	9	16.1
Microbial community (16S rRNA)	8	14.3
pXRF field screening	8	14.3
Monte Carlo simulation	6	10.7
Phytoremediation + BCF/TF	6	10.7
Pb isotope tracing	5	8.9
Predictive transport models (Hydrus-1D)	4	7.1
Ecotoxicology (PICT, bioassays)	4	7.1
Geophysical methods (GPR, radionuclides)	3	5.4
LCA / Material flow analysis	2	3.6
Note: PI = Pollution Index; Igeo = Geoaccumulation Index; EF = Enrichment Factor; PER/RI = Potential Ecological Risk Index; GIS = Geographic Information System; PCA = Principal Component Analysis; PMF = Positive Matrix Factorization; BCR = Bureau Communautaire de Référence; pXRF = portable X-ray fluorescence; BCF = Bioconcentration Factor; TF = Translocation Factor; GPR = Ground-Penetrating Radar; LCA = Life Cycle Assessment; PICT = Pollution-Induced Community Tolerance.

3.5. Evidence Mapping of Research Themes

Analysis of the 56 included studies revealed four dominant research themes: (1) contamination characterization and spatial distribution; (2) ecological and human health risk assessment; (3) environmental assessment methodology and validation; and (4) remediation and ecological restoration approaches.

Contamination characterization was the most prevalent theme, addressed in 45 studies (80.4%), with all studies reporting elevated concentrations of Pb, Zn, and Cd exceeding national or international regulatory thresholds [67,104]. Ecological and human health risk assessment was addressed in 38 studies (67.9%) [67,73], with Cd consistently identified as the primary ecological risk driver and Pb as the dominant human health concern, particularly for children through soil ingestion [64,81,105]. Methodological studies validating or comparing assessment approaches appeared in 12 studies (21.4%), including comparisons of pXRF versus ICP-MS [106], and evaluations of different extraction schemes for bioavailability assessment [78]. Remediation-oriented studies including phytoremediation, electrokinetic remediation, and geophysical contamination mapping appeared in 14 studies (25.0%) [60,107].

A notable temporal trend was observed: studies published before 2018 predominantly focused on contamination characterization (88%), while post-2018 studies increasingly integrated multiple assessment methods and predictive modeling (61%), reflecting methodological maturation in the field [88].

4. Discussion

This scoping review synthesizes evidence from 56 studies published between 2010 and 2025, mapping the global landscape of environmental assessment methods applied to soils contaminated by lead-zinc slag and associated smelting activities. The findings reveal significant advances in methodological diversity, a pronounced geographic concentration of research in Asia, and critical knowledge gaps relevant to Central Asia and Kazakhstan specifically.

Figure 4 illustrates the integrated assessment framework.

4.1. Research Trends in Soil Contamination from Lead-Zinc Slag

The marked increase in publications after 2018, with 41 of 56 studies (73.2%) published in this period, reflects growing global regulatory pressure and scientific concern over the environmental legacy of metallurgical waste [10,108]. Early studies predominantly focused on basic characterization of total heavy metal concentrations, whereas post-2018 research increasingly integrates multi-method frameworks combining spatial analysis, probabilistic risk modeling, and microbial assessment [109,110]. This methodological maturation aligns with international recognition that single-method approaches are insufficient for capturing the dynamic, spatially heterogeneous nature of slag-derived contamination [111].

The dominance of China in the literature (n = 20, 35.7%) reflects the country's position as the world's largest lead and zinc producer, with extensive documented contamination around smelting complexes in Hunan, Yunnan, Guizhou [112], and Henan provinces [12,56,58,113]. European contributions, primarily from Poland and legacy sites in the Linares district of Spain [114], emphasize long-term historical contamination and remediation challenges [114,115]. The near-complete absence of studies from Central Asia, and Kazakhstan in particular, represents a critical evidence gap given the scale of Pb-Zn smelting operations in the Shymkent region, one of the largest in the former Soviet Union [116,117].

4.2. Advances in Environmental Assessment Methods

The diversity of assessment methods identified across 56 studies reflects substantial progress in the field. Pollution indices including the Single Factor Pollution Index, Nemerow Synthetic Pollution Index, and Geoaccumulation Index were the most widely applied tools (73.2%), providing standardized, comparable measures of contamination severity relative to background values [118,119,120,121]. Their widespread adoption across different national regulatory contexts — including Chinese GB standards, US EPA thresholds, and European ANZECC guidelines — underscores their utility as universal screening tools, though direct cross-study comparison remains limited by differences in reference values [122,123].

Ecological risk assessment using the Potential Ecological Risk Index [124,125] was applied in 67.9% of studies, consistently identifying cadmium as the primary ecological risk driver due to its high toxicity response coefficient [126]. This finding has particular implications for slag-contaminated sites, where Cd co-occurs with Pb and Zn as a product of polymetallic ore processing and exhibits high mobility under acidic soil conditions [127,128].

Human health risk assessment following US EPA guidelines was applied in 51.8% of studies, with children consistently identified as the most vulnerable population group due to higher soil ingestion rates and developing physiological systems [129]. The adoption of Monte Carlo [85] simulation for probabilistic risk assessment in 10.7% of studies represents a methodological advance over deterministic approaches, providing uncertainty quantification essential for evidence-based remediation planning [130,131].

Geospatial methods including GIS-based kriging interpolation and source apportionment models (PMF, PCA) were applied in over 50% of studies, enabling spatial visualization of contamination gradients and quantitative differentiation of anthropogenic versus geogenic metal sources [93,132]. The integration of portable XRF for rapid field screening, validated against ICP-MS [90] in multiple studies, demonstrates the potential for cost-effective large-scale soil surveys in resource-limited settings [133].

Emerging approaches including predictive transport modeling (Hydrus-1D), multisurface geochemical modeling, lead isotope tracing, and biological assessment methods (16S rRNA microbial analysis, PICT ecotoxicology, enzymatic biomarkers) appeared in a smaller proportion of studies but represent important methodological frontiers [53,134]. Their limited application in slag-specific contexts highlights opportunities for methodological transfer from adjacent fields.

4.3. Research Gaps and Future Directions

Several critical research gaps emerge from this review. First, the absence of studies from Kazakhstan and Central Asia represents the most geographically significant gap. The Shymkent lead plant, one of the largest in Central Asia, has operated for decades generating substantial slag deposits, yet no peer-reviewed environmental assessment studies meeting international methodological standards were identified [33,135]. This gap is particularly concerning given documented soil lead concentrations exceeding 3,000 mg/kg near the plant—more than 900 times the maximum permissible concentration—with documented health impacts on local populations [136].

Second, predictive modeling approaches remain underrepresented in the slag-specific literature. Only four studies (7.1%) employed transport or fate models to forecast metal migration under varying environmental conditions such as acid rain, climate variability, or land-use change [137,138]. Given the long-term persistence of slag deposits and projected changes in precipitation patterns across Central Asia, predictive modeling represents an urgent research priority.

Third, standardized multi-method assessment frameworks are lacking. The heterogeneity of methods, reference values, and reporting standards across studies severely limits cross-site comparability and policy translation [139]. Development of internationally harmonized protocols for slag-contaminated site assessment, analogous to existing frameworks for mine tailings, is warranted.

Fourth, biological and ecotoxicological assessment methods — including microbial community analysis, phytotoxicity bioassays, and enzymatic biomarkers — remain underutilized relative to chemical methods. These approaches provide integrative measures of ecosystem health that chemical indices alone cannot capture [140,141].

The identified research priorities are summarized in Figure 5.

4.4. Relevance to Kazakhstan and Central Asia

The findings of this review have direct implications for environmental management in Kazakhstan. The Shymkent region hosts one of Central Asia's largest lead-zinc smelting complexes, with slag deposits accumulated over several decades of intensive industrial activity [33,142]. However, the methodological toolkit used globally — including GIS-based spatial risk mapping, PMF source apportionment, US EPA health risk frameworks, and predictive transport modeling — has not been applied systematically in this context.

The geographic bias toward Chinese and European study sites, while reflecting genuine research capacity disparities, also suggests that established assessment methods developed in these contexts may require adaptation to the specific soil types, climate conditions, and regulatory frameworks of Kazakhstan and Central Asia. The semi-arid climate of the Shymkent region, for example, differs substantially from the humid subtropical conditions prevalent in Chinese study sites [143,144,145], potentially affecting metal mobility, bioavailability, and remediation efficacy in ways not captured by existing models [146].

4.5. Limitations of the Review

This review has several limitations that should be acknowledged. The literature search was restricted to three databases — Dimensions, PubMed, and OpenAlex — which, while complementary, may have excluded relevant studies indexed in Russian-language databases such as eLIBRARY.ru or regional Central Asian repositories. This limitation is particularly relevant given the research gap identified in Kazakhstan, where substantial technical literature may exist in Russian but was not captured by the English-language search strategy employed.

The screening and data extraction process was conducted by a single reviewer, which introduces the possibility of selection bias despite the use of predefined eligibility criteria. Additionally, the reliance on abstract-level assessment for studies without accessible full texts may have resulted in misclassification of borderline cases. Future reviews should employ dual-reviewer screening and explicitly search Russian-language databases to improve coverage of Central Asian literature.

4.6. Policy Implications

The findings of this scoping review carry several practical implications for environmental policy and governance, particularly in Kazakhstan and Central Asia. First, the documented absence of peer-reviewed environmental assessments for the Shymkent slag-contaminated area underscores an urgent need for nationally mandated baseline monitoring programs aligned with international standards. Regulatory authorities in Kazakhstan should prioritize the development of soil quality standards specifically applicable to metallurgical slag sites, drawing on the Chinese GB 36600-2018 and European ANZECC frameworks identified across included studies.

Second, the methodological diversity documented in this review suggests that a tiered assessment approach would be most appropriate for resource-limited Central Asian settings. Rapid screening using portable XRF, followed by confirmatory ICP-MS analysis and GIS-based spatial risk mapping, represents a cost-effective and internationally validated pathway for initial site characterization [147,148,149]. This approach has been successfully deployed in comparable industrial contexts in China, Poland, and Brazil [148,150,151].

Third, the consistent identification of children as the highest-risk population, absorbing significantly more lead than adults [152], across human health risk assessments reinforces the need for targeted public health interventions near slag-contaminated sites, including blood lead level surveillance programs [153] and restrictions on agricultural land use within defined contamination radii. Such measures are directly supported by the evidence synthesized in this review and align with WHO guidelines for preventing lead exposure, which recommend identifying and terminating exposure sources [154].

Finally, the review highlights the importance of integrating environmental assessment data into remediation planning and land-use decision-making. Policymakers should require that comprehensive multi-method environmental assessments—encompassing chemical, spatial, and biological indicators—should be completed prior to any residential, agricultural, or industrial development on or adjacent to former smelting sites.

5. Conclusions

This scoping review systematically mapped the global evidence base on soil contamination from lead-zinc slag and associated smelting activities, identifying 56 peer-reviewed studies published between 2010 and 2025 that employed environmental assessment methods relevant to this context. The findings address three core objectives: characterizing the scope of contamination, mapping assessment methodologies, and identifying research gaps with particular relevance to Kazakhstan and Central Asia.

The review confirms that lead-zinc slag and smelter emissions are persistent and widespread sources of soil contamination, with elevated concentrations of Pb, Zn, Cd, and As that consistently exceed national and international regulatory thresholds across all study regions. Cadmium emerged as the primary ecological risk driver across studies due to its high toxicity coefficient and mobility under acidic conditions, while lead posed the greatest human health risk, particularly for children through soil ingestion. These findings align with global assessments of metallurgical waste hazards and underscore the urgency of systematic environmental monitoring at slag-affected sites [56,155].

The methodological landscape has diversified substantially over the review period. Pollution indices, ecological risk assessment, human health risk frameworks, and GIS-based spatial analysis constitute the dominant assessment toolkit, applied in 50–73% of included studies. The increasing integration of source apportionment models (PMF, PCA), probabilistic risk assessment (Monte Carlo simulation) [85,156], and predictive transport modeling (Hydrus-1D) reflects a maturation toward more holistic, policy-relevant assessment frameworks [157]. Emerging biological assessment methods — including microbial community analysis, phytotoxicity bioassays, and enzymatic biomarkers — complement chemical approaches by providing integrative measures of ecosystem health that indices alone cannot capture [140,158,159].

Despite this progress, three critical gaps were identified. First, Central Asia and Kazakhstan are entirely absent from the peer-reviewed literature, despite hosting one of the region's largest Pb-Zn smelting complexes in Shymkent, with documented soil lead concentrations that exceed regulatory limits by orders of magnitude [51,116]. This represents both a scientific gap and a public health concern requiring immediate attention. Second, predictive modeling of metal transport and fate under varying environmental conditions remains underrepresented in slag-specific contexts, limiting the capacity for evidence-based remediation planning and climate change adaptation. Third, the absence of standardized international protocols for slag-contaminated site assessment impedes cross-study comparability and policy translation across jurisdictions [108,160].

These findings carry direct implications for environmental governance in Kazakhstan. The methodological frameworks documented in this review — spanning spatial risk mapping, probabilistic health assessment, isotope-based source tracing, and biological monitoring — provide a validated toolkit that can be systematically applied to Pb-Zn contaminated sites in the Shymkent region. Future research should prioritize: (1) baseline environmental assessment of slag-contaminated soils in Kazakhstan using internationally standardized methods; (2) development of predictive models calibrated to the semi-arid climatic conditions of Central Asia; (3) integration of Russian-language literature and regional databases to capture existing but inaccessible evidence; and (4) establishment of long-term monitoring programs to track metal mobility and ecosystem recovery over time.

This scoping review provides a structured evidence map that can guide researchers, environmental managers, and policymakers in designing comprehensive assessment programs for lead-zinc slag-contaminated sites, with particular relevance to the underserved Central Asian context.