Landfill Leachate and Water Resources: An Evaluation of Chemical and Microbiological Characterization, Contamination Pathways, and Risk-Oriented Monitoring

2.1 Review design and questions

This study was conducted as a structured narrative review with transparent, replicable procedures for search, screening, and synthesis. The review questions were: (i) how do landfill age, waste composition, climate, and hydrogeology control leachate characteristics; (ii) through what pathways does leachate affect groundwater, surface water, soil, ecosystems, and human health; and (iii) what prevention, treatment, monitoring, and remediation strategies are supported by current evidence? Given the diversity of study designs, analytical methods, climatic settings, landfill types, and reporting units, a structured narrative approach was selected because the evidence does not support a defensible quantitative meta-analysis.

2.2 Search strategy

The search strategy was conducted in Scopus, Web of Science Core Collection, ScienceDirect, PubMed, and Google Scholar. The strategy used three concept blocks: (A) the source, limited to the title field (leachate*), combined with landfill, dumpsite, open dump, or MSW in the title, abstract, and keyword fields; (B) the receiving medium or receptor (groundwater, aquifer, surface water, river, stream, lake, wetland, soil, vadose zone, sediment, drinking water, crops, food chain); and (C) contaminant classes, outcomes, and management responses (heavy and trace metals, potentially toxic elements, ammonium and ammoniacal nitrogen, nitrate, PFAS, microplastics, pharmaceuticals, CECs, ARGs, pathogens, ecotoxicity, toxicity, health risk, risk assessment, pollution index, plume, attenuation, monitoring, treatment, remediation, composition, and characterization). Blocks A, B, and C were combined as A AND (B OR C), and truncation was used where the database syntax allowed. We deliberately limited the source block to the title field to improve precision, in keeping with the exclusion of studies that did not present leachate-specific evidence (Section 2.3). The main evidence window was January 2020 to December 2025, and results were limited to research articles and reviews published in English. Table 1 summarizes the search strategy and the number of records retrieved, and Appendix A provides the search strings for each database.

Because ScienceDirect limits a query to eight Boolean operators, block C was divided into five sub-queries (SD-1 to SD-5). Google Scholar was used as a supplementary source with six short queries anchored on intitle: leachate; its results were screened for relevance to identify records not retrieved from the bibliographic databases, and because the depth of that screening was not recorded during the original review, no fixed record cut-off is reported. Because retrieval counts from the original searches were not archived, the complete strategy was re-executed on 20 September 2026 to document the counts in Table 1; institutional access to Web of Science was unavailable at the time of re-execution, so its count is not reported.

Table 1. Search design, retrieval counts, and characterization of the evidence base

(a) Search design and retrieval (searches re-executed on 20 September 2026)

(b) Evidence base retained for synthesis

Notes: Concept blocks: A = source (leachate* in the title AND landfill*, dumpsite*, "open dump*", or "municipal solid waste"); B = medium or receptor; C = contaminant classes, outcomes, and management; C′ = PFAS, microplastics, and antibiotic resistance only. Full query strings are given in Appendix A. ¹ Institutional access unavailable at re-execution. ² Approximate totals reported by the engine; Google Scholar served as a supplementary source screened by relevance, without a recorded cut-off. ³ “Primary” assigns each source to one domain (sum = 75).

The 2020–2025 window was selected because it spans the period in which multi-country syntheses of leachate chemistry and microbiology became available [2, 8] and in which the literature on PFAS, microplastics, and ARGs in leachate expanded rapidly; the review therefore reflects current characterization and monitoring practice rather than a historical account. Earlier landmark studies were retained where they provide foundational definitions, long-term field observations, analytical frameworks, or widely used indicators that recent publications have not replaced [5, 9]. Because PFAS, microplastics, and antibiotic resistance are fast-moving topics, articles published online ahead of print during screening but subsequently assigned to 2026 issues were retained (two sources, e.g., [14]); these were retrieved by the main search and are distinct from the sources added by the update search described below. A supplementary update search restricted to these three topics was executed on the same date (Scopus, n = 50; PubMed, n = 24; 52 unique records). These records were screened by title and abstract, and eight further sources published in 2026 were added through this update search (these eight, together with the two retained ahead-of-print sources, make up the ten 2026 sources reported in Table 1): a global synthesis of leachate composition and its groundwater impacts [18]; a global compilation of PFAS in leachate [19] and a multi-regional study of PFAS in raw and treated leachate [20]; multi-plant evidence on the persistence of ARGs through treatment [21]; reviews of ARG transfer to groundwater [22], of microplastics in leachate [23], and of leachate pathways in tropical urban aquifers [24]; and a study of PFAS sorption onto aquifer media [25].

2.3 Eligibility and screening

Peer-reviewed research articles and reviews published in English that reported leachate composition, contaminant transport, environmental or health effects, treatment performance, risk assessment, or monitoring and management practice were eligible. Studies were excluded when they addressed general solid-waste management without leachate-specific evidence, consisted of non-scholarly web material, were inaccessible in full text, or duplicated other records. Studies were also excluded for insufficient methodological or contextual information, defined as the absence of one or more of the following: (i) the location of sampling points relative to the landfill and a description of the hydrogeological or hydrological setting; (ii) sampling dates, frequency, or number of samples; (iii) the analytical methods used and any quality-assurance information; (iv) the units and reporting basis of concentrations (for example, NH₄⁺ versus NH₄⁺-N, or NO₃⁻ versus NO₃⁻-N); and (v) a background, control, or reference value against which the reported concentrations could be interpreted. Titles and abstracts were screened first, followed by full-text assessment, and the reference lists of major reviews were examined to identify additional foundational and recent studies. In a final relevance screening, sources that were only tangentially related to the review questions or that duplicated stronger evidence were removed. Consistent with narrative-review practice, flow counts in the style of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) were not recorded during the original screening; the evidence base is instead characterized in Table 1 by the number of full texts assessed, the number of sources retained for synthesis, and their distribution across topic domains and evidence types.

2.4 Data extraction and synthesis

For each retained source, data were extracted on research location and climate, landfill type and age, environmental media sampled, target contaminants, analytical or modeling approach, key findings, reported limitations, and management implications. Evidence was organized in five subject domains: leachate formation and composition; age- and climate-related variation; subsurface and surface-water movement; ecological and human-health impacts; and prevention, treatment, and remediation. Greater interpretive weight was given to multi-site investigations, studies with upstream or background controls, seasonal or long-term monitoring, validated analytical procedures, and explicit uncertainty or statistical evaluation. Because concentration ranges, detection limits, sampling techniques, exposure assumptions, and units varied among studies, the synthesis was qualitative: stronger evidence came from independent sites that agreed, whereas isolated or context-dependent findings are discussed with caution. Throughout the review, concentrations are reported together with the environmental medium and the chemical basis used in the source study, and ranges are not compared directly across media or reporting bases (Section 4).

2.5 Limitations

The review is limited by its restriction to English-language sources, potential publication bias, uneven reporting in the primary studies, and the absence of a registered protocol and a full database export. Retrieval counts in Table 1 derive from a re-execution of the search strategy rather than from the original searches; overlap between databases was not quantified, Google Scholar totals are the engine’s approximate estimates, and the Web of Science count could not be re-established. The conceptual curves of leachate evolution with landfill age presented in Section 4 are normalized rather than measured concentrations.

Throughout this review, concentrations are reported on the environmental medium (raw leachate, groundwater, surface water, soil, or sediment) and the chemical basis used in the source study. Nitrogen species are expressed as nitrogen (NH₄⁺-N, NO₃⁻-N) except where a guideline value is defined as the ion, as for the WHO nitrate guideline of 50 mg/L as NO₃⁻ (11.3 mg/L as NO₃⁻-N) [28]; “ammonium” denotes the chemical species in process descriptions, whereas NH₄⁺-N denotes the measured concentration. Ranges from different media or reporting bases are placed side by side only to indicate orders of magnitude and are not compared directly, because sampling points, detection limits, and units differ among studies.

When industrial waste is added, leachate tends to contain more heavy metals, xenobiotic organic compounds, and salts than landfill leachate, which receives only municipal waste. It also affects metal bonding in the liquid state [17, 37]. Organic waste decomposed in landfills, particularly food or household organic waste, creates leachate that is rich in biodegradable organic matter and ammonium. Adding industrial or electronic waste [31, 38] pollutes the mixture with solvents, PFAS precursors, metals, and pharmaceuticals. Worldwide, landfills that receive more food waste tend to generate leachate with higher organic strength and ammonium concentration because easier-to-hydrolyze materials decompose rapidly in the waste matrix, but food waste contributes only a small part of this variability [2].

Young leachate (<5 years old) is generally quite acidic [5, 6, 39]. It contains high BOD, COD, volatile fatty acids (VFAs), and heavy metals owing to extensive organic decomposition in the early stage [3]. As the landfill ages for ten years and enters the methanogenic phase, the BOD-to-COD ratio drops, indicating reduced biodegradable content. The main concern, however, is ammonium, which tends to be independent of landfill age [2, 3, 11].

Leachate volume and concentration are greatly affected by climatic factors, particularly temperature and rainfall. In tropical regions, heavy rainfall increases COD, five-day biochemical oxygen demand (BOD₅), NH₄⁺-N, and metal leaching [40], whereas in arid regions, low leachate volume is associated with high pollutant concentrations [2, 13]. Ammonium and chloride are common and dangerous macro elements in aquifers, but emerging pollutants such as PFAS, microplastics, and pharmaceuticals are a current concern [19, 23]. These compounds are typically stubborn and difficult to treat, and DOM may further complicate their mobility by adsorbing onto metals, enabling their entrance into groundwater [5, 41].

Table 2 presents the major pollutant groups in landfill leachate, their representative constituents, associated risks, monitoring approaches, and potential control measures for direct comparison.

Table 2. Major landfill leachate pollutant groups, risks, monitoring indicators, and control options

Notes: BOD, biochemical oxygen demand; COD, chemical oxygen demand; VFAs, volatile fatty acids; BOD₅, five-day biochemical oxygen demand; TOC, total organic carbon; UV₂₅₄, ultraviolet absorbance at 254 nm; EC, electrical conductivity; TDS, total dissolved solids; ICP-MS, inductively coupled plasma mass spectrometry; ICP-OES, inductively coupled plasma optical emission spectrometry; AOP, advanced oxidation process; ARB, antibiotic-resistant bacteria; ARGs, antibiotic resistance genes; qPCR, quantitative polymerase chain reaction.

Landfill leachate has become a global phenomenon, a diverse yet systematically ordered complex of pollutants. In raw leachate, global compilations report COD of 3,700–75,900 mg/L and NH₄⁺-N of 300–8,100 mg/L (as N) [8]. Major inorganic ions (Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺) occur at hundreds to thousands of milligrams per liter, far above domestic wastewater levels [5, 43]. Dissolved heavy metals in raw leachate are typically at or below the mg/L level, yet leachate-impacted groundwater frequently exceeds drinking-water and groundwater standards for individual metals [9]. Recent global surveys indicate that leachate is a primary source of PFAS, bisphenol A, N,N-diethyl-3-methylbenzamide (DEET), and other pharmaceuticals and personal care products (PPCPs), which occur in raw leachate at ng/L to µg/L concentrations but remain persistent and toxic [8, 32]. A 2026 compilation of 1,416 leachate samples from 539 facilities in 22 countries reported total PFAS of 1 to 5,542,000 ng/L, with industrial landfills averaging 56–101 times the concentrations of other landfill types and PFAS levels correlated with TOC, COD, and ammonium [19]. These ranges refer to different media and reporting bases and are presented to indicate orders of magnitude, not for direct comparison.

Figure 3 summarizes the conceptual evolution of leachate characteristics with landfill age, and Table 3 compares the principal changes in leachate chemistry across young, transitional, and mature landfill phases, along with their treatment implications.

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Figure 3. Conceptual evolution of leachate characteristics with landfill age [39]

Table 3. Comparative characteristics of landfill leachate by landfill age and biochemical phase

Notes: VFAs, volatile fatty acids; TDS, total dissolved solids. Boundaries are indicative and vary with climate, waste composition, and operational conditions.

4.1 Global trends and driving forces

Patterns associated with development status, climate, and waste composition are evident in a database of 51 countries. Sites in developing countries in dry temperate and moist-to-wet tropical zones have significantly higher concentrations of COD and BOD₅ than sites in industrialized countries, as these landfills receive a higher proportion of food and other easily biodegradable materials [2, 8, 40, 44]. Food waste has a positive correlation with COD, BOD₅, NH₄⁺-N, and some macro-ions, but accounts for a small proportion of the variance (R² = 0.07–0.18), indicating that the food-waste content is a minor contributor [2].

However, the statistical and machine-learning data show that the size and age of a landfill can have a large effect on the volume and composition of the leachate. Larger facilities produce more leachate and higher total nitrogen and ammonium; hotter areas have higher COD but lower BOD₅, indicating more but weaker biological processing [45]. Cluster analysis distinguishes among anaerobic, early and intermediate, and later methanogenic stabilization, showing that the degradation phase is a major organizing variable in leachate chemistry [45]. The same age-related patterns appear in the regional surveys: Fe/Mn and organic pollution are prevalent in young cells, ammonium in intermediate locations, and refractory organics and salts in older facilities [45, 46].

4.2 Age and biogeochemical phase of landfill

Research indicates that leachate composition follows a predictable pattern that undergoes acidogenic, acetogenic, and methanogenic stages (Figure 3). During the first five years, COD and BOD₅ are typically an order of magnitude higher than in mature leachate; pH is acidic to near neutral, and VFAs and soluble metals are pre-eminent [5, 39]. As decomposition continues, methanogenic bacteria consume the VFAs generated during the acid phase, and the leachate shifts from acidogenic to methanogenic conditions. This transition is accompanied by a major decline in leachate biodegradability; BOD/COD ratios are typically >0.3 in fresh leachate but <0.1 in aged, methanogenic leachate [5, 39, 43]. By contrast, NH₄⁺-N is not attenuated under methanogenic conditions and persists in raw leachate at concentrations commonly in the range of 500–2,000 mg/L (as N) with no decreasing trend over time, making it the dominant long-term contaminant [5].

Long-term reviews and field studies confirm that most heavy metals decrease or precipitate into insoluble species over time and that ammonium is the prevailing long-term pollutant [9]. Other metals such as Cd and Pb may decrease but remain above regulatory levels in old landfills, particularly in warmer regions where the decomposition process is not fully complete [46]. Over the multi-year monitoring period, the leachate pollution index (LPI) indicates a steady decrease in total pollution scores as the landfills stabilize. However, the main parameters of COD, NH₄⁺-N, and chloride are usually not optimal [47].

More recent surveys focus on the leachate microbiome as a source and indicator of decay. Proteobacteria, Bacteroidota, Firmicutes, and Chloroflexi were the most common phyla identified globally [8]. The fate of inorganic nitrogen is determined by functional guilds essential to nitrogen cycling, including anammox bacteria (Candidatus Brocadia) and nitrifying bacteria such as Nitrosomonas [8, 56]. Time-series surveillance correlates declining VFAs with the emergence of methanogens, providing indirect evidence of landfill stabilization [5, 39].

The LPI combines parameters such as COD, NH₄⁺-N, and heavy metals. LPI values decline with stabilization but remain above optimal levels over decades [47, 57]. As regional-scale studies point out, hydrogeological environments buffer the expression of leachate characteristics in groundwater effects [46]. Groundwater exceedance rates are lower in landfills in mountainous regions with thick, fine-grained vadose zones than in coarse-gravel aquifers, supporting the idea that leachate characterization should be accompanied by site-specific transport evaluation [44, 52].

Microbial succession drives the biochemical evolution described in Section 4.2: fermentative bacteria dominate the hydrolytic and acidogenic phases, breaking readily biodegradable organic matter down to VFAs, alcohols, hydrogen, and carbon dioxide and producing acidic leachate with high COD and BOD [5], whereas methanogenic archaea dominate the subsequent strictly anaerobic phase, converting these intermediates to methane and carbon dioxide, reducing organic strength and raising pH [9, 58]. Recent breakthroughs in molecular sequencing have revealed that microbial diversity and metabolic activity in landfills vary widely depending on site age, waste content, temperature, moisture regime, and oxygen availability [16, 58, 59]. Metagenomic and microbiome studies over the past few years have revealed that landfill leachate harbors remarkably diverse microbial assemblages that mediate the cycling of carbon, nitrogen, sulfur, and metals [8, 58]. The functional guilds responsible for nitrification, denitrification, sulfate reduction, methanogenesis, and anaerobic ammonium oxidation are crucial to this process, influencing contaminant conversion and, ultimately, the degree to which they are attenuated in the landfill environment. For example, sulfate-reducing bacteria can precipitate dissolved metals as insoluble sulfides, while methanogenic communities can stabilize organic matter under decreased circumstances [9]. At the same time, microbial degradation can generate intermediate chemicals that affect leachate toxicity and mobility. Environmental parameters such as pH, dissolved oxygen (DO), temperature, and salinity substantially regulate microbial activity and the fate of contaminants in landfill plumes [6, 60]. Another area receiving increasing scientific attention is the presence of pathogenic microorganisms, ARB, and ARGs in landfill leachate, particularly at sites receiving commingled municipal, medical, and industrial waste streams [16, 42]. Metagenomic surveys of landfill leachates and hydraulically connected groundwater have recovered hundreds of distinct ARG subtypes encoding resistance determinants to tetracyclines, sulfonamides, and β-lactams, along with genes mediating multidrug efflux [16]. These findings indicate that landfill environments may serve as reservoirs and active sites of horizontal gene transfer, where sustained exposure to sub-inhibitory concentrations of pharmaceuticals, heavy metals, and other toxicants exerts co-selective pressure that favors the maintenance and dissemination of resistance determinants. Recent reviews of ARG transfer from leachate to groundwater, and the biotic and abiotic factors that govern it, conclude that groundwater near landfills remains comparatively understudied and lacks a standardized ARG methodology [22].

Table 4 presents a risk-oriented framework that integrates environmental media, sampling design, core monitoring parameters, sampling frequency, and interpretive objectives. The surface-water category requires hydraulic consideration. DO levels are affected not only by oxygen demand from organic and ammoniacal loads in leachate but also by re-aeration processes, which are influenced by channel geometry, turbulence, and flow over drops, weirs, and stepped structures. These factors can greatly increase the DO concentration [74]. Downstream monitoring stations must therefore be placed in such a way that the load signal from the landfill is not masked by hydraulic re-aeration.

Table 4. Risk-oriented monitoring framework for landfill-affected environments

Notes: Adapt the proposed framework to landfill design, hydrogeology, climate, regulatory requirements, and receptor vulnerability. BOD, biochemical oxygen demand; CECs, contaminants of emerging concern; COD, chemical oxygen demand; DO, dissolved oxygen; EC, electrical conductivity; PFAS, per- and polyfluoroalkyl substances; TDS, total dissolved solids; TOC, total organic carbon. Compiled by the authors (Section 8.1); references indicate the principal sources on which each row is based.

8.1 Derivation of the framework and the risk-tier rule set

The framework in Table 4 was derived from three lines of evidence, not precedent alone. First, the core parameters are those that are elevated by one to three orders of magnitude in leachate relative to natural waters irrespective of climate, waste composition, or landfill age [2, 5, 8] and that behave predictably along the plume: chloride and EC are conservative tracers of dilution; NH₄⁺-N is the persistent long-term contaminant; nitrate, dissolved iron and manganese, and sulfate map the redox zonation of the plume; and COD or TOC quantifies the organic load [9, 46]. Multivariate analyses of leachate datasets consistently identify pH, EC, NH₄⁺-N, TDS, Cl⁻, Fe, and Mn as the most discriminating variables [35, 36]. The parameter set is therefore transferable across settings; what differs between sites is monitoring intensity, not the indicators themselves. Second, the sampling frequency is anchored on the seasonal recharge cycle that drives leachate generation and plume loading [2, 40, 67] and on regulatory practice. The EU Landfill Directive prescribes quarterly monitoring of leachate composition and surface water during the operational phase and six-monthly monitoring during aftercare, with groundwater composition sampled at a site-specific frequency set by groundwater velocity and the time needed for remedial action between two samplings, and with trigger levels verified by repeat sampling [75]. The US Subtitle D criteria require detection monitoring of up-gradient and down-gradient wells at least semi-annually throughout the active life and post-closure period, with statistical comparison to background and escalation to assessment monitoring when a significant increase is confirmed [76]. The “quarterly minimum” in Table 4 is thus the operational-phase frequency of the Directive and the coarsest interval at which wet- and dry-season contrasts can be resolved; semi-annual sampling is the regulatory floor appropriate to low-risk or aftercare sites. Third, station density follows the same regulatory logic of background comparison and plume interception (Section 8.3).

Table 5. Risk-tier rule set for monitoring design (risk tier = highest rating of the three factors)

Notes: Tier A, B, and C analytical sets are defined in Table 6. Trigger values are indicative and should be replaced by site-specific background statistics (mean plus three standard deviations, or a prediction limit) once sufficient baseline data exist. DO, dissolved oxygen; EC, electrical conductivity; RCRA, Resource Conservation and Recovery Act (US).

Table 5 converts the framework into a rule set. Three factors are rated low, moderate, or high: containment and source control, hydrogeological vulnerability, and receptor proximity and use. The site tier is the highest rating of any single factor. This precautionary “maximum rule” is deliberate: an unlined cell above a shallow gravel aquifer with wells down-gradient supply is a high-risk site regardless of how favorably the other factors score, and a scoring scheme that averages the factors would understate such cases. Each tier then specifies parameter priority (the analytical tiers A, B, and C defined in Section 8.4), sampling frequency, station density, and the conditions that trigger intensified monitoring. The trigger values are indicative and should be replaced by site-specific background statistics once four or more rounds of data exist; the verification principle, however, is general: a trigger is confirmed only when it recurs in a consecutive sampling round, after which the site moves up one tier and adds wells or analytes as specified. Where eight consecutive rounds pass without a confirmed trigger, the site may step down one tier.

8.2 Climate-conditioned monitoring design

Climate enters the framework as a modifier of timing and parameter emphasis rather than as a separate tier. In arid and semi-arid settings, leachate generation is episodic and evaporative concentration raises salinity and ionic strength, so the plume is small in volume but highly concentrated [2, 13, 53]. Elevated ionic strength and abundant dissolved organic matter alter sorption and metal complexation, so that mobility rather than load governs risk [41, 48, 49]; for short-chain PFAS, leachate DOM and coexisting ions have been shown to modify sorption onto aquifer media, with low-molecular-weight DOM fractions increasing adsorption by about 32% and Cl⁻–Mg²⁺ pairs exerting a stronger effect than SO₄²⁻–Na⁺ [25]. Monitoring should therefore target peak concentrations: sampling of leachate sumps and near-field wells should be timed to the weeks following rainfall or irrigation-driven recharge above a site-defined threshold; EC, Cl⁻, and Na⁺ should serve as first-line tracers; and vadose-zone monitoring beneath cells (suction lysimeters or moisture sensors) justifies its cost because travel times to deep water tables are long and no other early warning exists [13]. In humid, tropical, and monsoon settings, leachate is generated continuously, concentrations are diluted, but loads are high, and storm runoff and pond overflow create direct surface-water pathways [27, 40, 63]; in a tropical urban aquifer, isotopic tracers indicated leachate-infiltration fractions of 1–33% of local groundwater, with nitrate-driven health risk peaking after the monsoon [24]. Here the priority is load rather than concentration, so continuous flow or level measurement at leachate outlets and surface-water stations adds more information than additional analytes; event-based sampling during and immediately after storms, paired wet- and dry-season campaigns, and DO, BOD, nutrient, and microbial indicators in receiving waters take precedence [67]. In temperate settings with seasonal recharge, paired campaigns before and after the recharge season are the minimum, and the quarterly interval of Table 5 resolves the annual cycle. In all climates, concentration must be reported together with the flow condition under which it was sampled.

8.3 Monitoring network design: Well placement, depth, and rapid plume checks

The framework assumes a network that can establish background, detect a release as early as possible, and verify whether the plume reaches a receptor. The conceptual minimum (Figure 4) is therefore one up-gradient background well, one near-field down-gradient well at the waste boundary or compliance line, and one far-field control well between the landfill and the nearest receptor, or the receptor well itself sampled as a compliance point. Because transverse dispersion in most aquifers is small and plumes are narrow relative to their length [9], lateral wells on both flanks of the flow axis are added in the moderate and high tiers to bound the plume width.

Leachate plumes occupy the uppermost aquifer and, at the sites reviewed here, were typically imaged within roughly the upper 10 m of the shallow aquifers [10, 61, 62]; with distance they may sink beneath fresh recharge, and density differences can cause sinking close to the source [9]. Near-field wells should therefore be screened with short intervals (about 2–3 m) straddling the seasonal range of the water table, where the first arrival of a release is detected; far-field wells should be screened in the expected plume core or, preferably, constructed as multi-level nested wells with screens at two or more depths. Long screens that span both clean and contaminated water blend the sample and can dilute a plume signal below detection. In layered or multi-aquifer settings, nested piezometers in each unit with measured vertical gradients are needed because perched water and leaky aquitards can route leachate downward, and borehole annuli must be sealed so that the monitoring wells do not themselves become pathways.

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Figure 4. Conceptual minimum groundwater monitoring network in cross-section with a plan-view inset (schematic, not to scale; present study, based on study [9])

The background well must be hydraulically up-gradient and outside the zone of influence of the landfill, beyond the leachate mounding that can locally reverse gradients beneath unlined sites and beyond gas migration, while being screened in the same hydrostratigraphic unit as the down-gradient wells and unaffected by other sources such as agriculture, septic systems, or road salt. Flow direction should be established from at least three water-level points and re-checked seasonally, because directions can reverse near rivers, under pumping, or during monsoon flooding [27, 63].

EC and chloride provide a quick, low-cost delineation before or between full sampling rounds. Field EC measured in wells, springs, drains, and temporary hand-augered or direct-push points, together with chloride by titration or test strip, identifies the plume where the down-gradient value exceeds roughly twice the up-gradient value; the boundary is traced by stepping laterally and down-gradient until values return to background (Figure 4). Because chloride is neither retarded nor degraded, its boundary marks the outer limit of leachate influence, and retarded constituents such as NH₄⁺-N lie inside it [9]. Ratios of Cl⁻ to Br⁻ or of Na⁺ to Cl⁻ help distinguish leachate from other salinity sources. Where available, electrical resistivity imaging maps the low-resistivity plume within a day and is the most efficient guide to permanent well placement [10, 61]. Where the budget is tight, the order of priority is: first, inventory and sample every existing down-gradient supply well, spring, drain, and stream, because these are the receptors and cost nothing to install; second, install the three-well minimum with short screens across the water table; third, run field EC, Cl⁻, NH₄⁺-N, NO₃⁻, and water level quarterly (Tier A, Section 8.4); fourth, add lateral and multi-level wells only where the EC/Cl⁻ survey or resistivity imaging shows the plume leaving the axis or sinking.

8.4 Tiered analytical strategy and practical feasibility

Table 2 lists monitoring approaches for each pollutant group but not what they cost, what they detect, or what they require in the field. Table 6 therefore organizes the parameters into three analytical tiers. Tier A comprises routine, low-cost field and basic-laboratory indicators that detect and delineate a plume and can be run at every station in every round. Tier B comprises confirmatory analyses available in regional or university laboratories that quantify hazard. Tier C comprises specialist analyses for PFAS, pharmaceuticals, microplastics, and ARGs that characterize hazards Tier A cannot detect but do not change the plume delineation; they are run at baseline and when triggered. The rationale is that the leachate signature in EC, Cl⁻, and NH₄⁺-N lies one to three orders of magnitude above natural variability, so Tier A alone suffices to detect and delineate a release in any setting [2, 5, 9], whereas the higher tiers answer different questions, namely how hazardous the release is and to whom, and are justified once a pathway to a receptor is plausible or confirmed. The practical constraints of low- and middle-income settings, including holding times relative to the distance to the laboratory, the availability of reference materials, and specialist analyses that often require shipment abroad, are addressed in the preservation and access columns of Table 6.

Table 6. Tiered analytical strategy: Parameters, methods, detection capability, sample handling, and escalation triggers

Notes: LOQ values are indicative orders of magnitude for water samples and depend on matrix and laboratory; leachate matrices generally require dilution. AAS, atomic absorption spectrometry; CFU, colony-forming unit; FAAS, flame atomic absorption spectrometry; IC, ion chromatography; LOQ, limit of quantification; MPN, most probable number; ORP, oxidation–reduction potential; PTFE, polytetrafluoroethylene; HDPE, high-density polyethylene; EC, electrical conductivity; TDS, total dissolved solids; ICP-MS, inductively coupled plasma mass spectrometry; ICP-OES, inductively coupled plasma optical emission spectrometry; qPCR, quantitative polymerase chain reaction. Compiled by the authors (Section 8.4); references in the first column indicate the sources on which each tier is based; analytical methods follow the standards named in the cells (ISO 21675, US EPA Method 1633, ISO/OECD bioassay guidelines).

8.5 Indicator interpretation by medium

The same constituent carries different information in each medium. Table 7 supplements Table 4 by stating, for each core indicator, what a measurement means in leachate, groundwater, and surface water, and what to do next.

Table 7. Indicator–medium–interpretation guide: What is measured, what it means, and what to do next

Notes: Tier C refers to Table 6. EC, electrical conductivity; TDS, total dissolved solids; CECs, contaminants of emerging concern; DOM, dissolved organic matter; ORP, oxidation–reduction potential; qPCR, quantitative polymerase chain reaction. Compiled by the authors (Section 8.5); bracketed numbers give the supporting studies for each statement, and the ‘Next step’ column is the authors’ recommendation derived from Sections 8.1–8.4.

Uncontrolled and aging landfills remain persistent sources of contamination for decades after disposal ceases. Leachate composition depends on waste characteristics, landfill age, and climate, and its migration to groundwater and, through runoff and lateral flow, to surface water generates plumes that alter redox conditions, degrade water quality and soil functions, and expose people through drinking water, crops, and aquatic food webs.

Effective risk reduction requires thorough leachate collection and groundwater-monitoring programs. Engineered containment systems and multi-barrier treatment trains are just as important but will only have the desired effect when linked with strong regulatory enforcement. These initiatives need to be coupled with the closure or safe rehabilitation of existing disposal sites and the phasing out of open dumping. Tracing constituents across groundwater, surface water, soil and the vadose zone, biota, and the atmosphere within a single source–pathway–receptor sequence showed that the same constituent carries different significance according to the medium in which it is measured and the receptor it reaches; risk therefore cannot be inferred from source composition alone. Within that sequence, the microbiological signature proved inseparable from physicochemical characterization: microbial guilds govern the attenuation of nitrogen, sulfur, and metals, while pathogens, ARB, and ARGs are hazards that conventional determinants do not detect. The resulting risk-oriented monitoring framework links environmental medium, sampling design, core parameters, frequency, and interpretive objective. The framework is expressed as a rule set in which containment, hydrogeological vulnerability, and receptor proximity assign a site to a risk tier that prescribes parameter priority, sampling frequency, well-network density, and triggers for intensified monitoring; a tiered analytical strategy confines costly PFAS, microplastic, and resistome analyses to sites where a pathway to a receptor is plausible or confirmed; and an indicator–medium guide translates each measurement into its meaning and the action that should follow. Because the source–pathway–receptor sequence maps onto the barriers of landfill design, the framework also indicates which barrier should carry investment at a given site. Read alongside the comparative evaluation of prevention, treatment, and remediation options, it allows monitoring effort to be allocated by receptor vulnerability rather than uniform routine sampling.

Future studies should focus on the transport, persistence, and toxicity of PFAS, microplastics, and pharmaceutical residues in legacy landfill plumes. More attention is needed on long-term exposure to contaminant mixtures and antibiotic resistance. Research is also needed to evaluate the long-term performance of landfill liners. At the same time, heterogeneous leachate constituents should be treated using low-cost nature-based and advanced technologies.

Environmental policies should be focused on pollution prevention, not remediation. Protecting groundwater and surface-water resources from landfill contamination must be central to sustainable environmental governance and public-health protection.