Long-Term Effects of Municipal Solid Waste Leachate on Soil Hydraulic Properties

Abstract

This experimental study examines the effects of landfill leachate contamination on soil hydraulic conductivity over a 12-month period, addressing the current lack of long-term experimental data in this field. Laboratory permeability tests were performed on sandy clayey silt samples contaminated with leachate at concentrations ranging from 5% to 25%. Microstructural and mineralogical analyses were conducted using SEM and XRD to identify the mechanisms behind observed changes. The results identify a critical threshold at 15% contamination, where soil behavior transitions from granular to cohesive characteristics. Hydraulic conductivity increases at low contamination levels (5–10%, up to 1.2 × 10⁻⁷ m/s) but decreases significantly at higher levels (4.172 × 10⁻⁸ m/s at 15%, 8.545 × 10⁻⁹ m/s at 20%). These changes are controlled by contamination level rather than exposure time, with values remaining stable throughout the 12-month period. The study provides essential parameters for landfill design and contamination assessment, demonstrating how leachate concentration affects long-term soil hydraulic properties through mineral formation and structural modification.

1. Introduction

The stability of landfills is highly linked to the geotechnical properties of the waste body; however, the overall stability is directly related to the foundation soil. When these properties are altered by leachate interaction, the risk of structural failure increases significantly, potentially leading to environmental catastrophes. The environmental impact of landfills extends far beyond their physical boundaries, with soil contamination through leachate migration representing one of the most significant long-term environmental concerns. Understanding the evolution of soil hydraulic properties under leachate contamination is crucial for both environmental protection and landfill stability. Recent incidents, such as the Koshe Landfill failure in Ethiopia in 2017 [1], which resulted in 115 fatalities, highlight the critical importance of comprehending how leachate affects soil behavior and stability over time.

Landfill leachate, generated through waste decomposition and water infiltration, contains a complex mixture of organic and inorganic compounds, including biodegradable matter, heavy metals, inorganic salts, and various chemical compounds [2,3]. The interaction between these components and surrounding soil can significantly alter soil hydraulic conductivity, which directly influences both contaminant transport patterns and overall landfill stability. Changes in soil permeability can lead to unexpected leachate migration paths, potentially compromising containment systems and increasing environmental risks [4].

The hydraulic conductivity of contaminated soils plays a pivotal role in several critical aspects of landfill engineering. It determines the effectiveness of containment systems and influences the design of leachate collection systems [5]. Additionally, it affects the stability of landfill foundations, as changes in soil permeability can alter pore water pressure distributions and effective stress states. Also, it impacts the long-term environmental risk assessment, as hydraulic conductivity governs the potential for contaminant migration into surrounding areas and groundwater resources [6].

Previous research has primarily focused on short-term leachate–soil interactions, leaving a significant knowledge gap regarding long-term effects. Studies by Nayak et al. [7] and Khan et al. [4] have demonstrated that leachate can modify soil properties, including permeability, but their investigations typically covered only brief exposure periods. This limitation is particularly concerning given that landfills remain active sources of contamination for decades after closure.

This research addresses this critical knowledge gap by investigating the evolution of soil hydraulic conductivity under various degrees of leachate contamination over a 12-month period. While both chemical and biological processes affect soil characteristics during leachate exposure, this study focuses on chemical interactions. By examining different contamination levels (5–25%) and their temporal effects, this study provides essential insights for:

• Improving the design of containment systems in new landfills;
• Enhancing risk assessment methods for existing facilities;
• Developing more effective monitoring protocols for long-term landfill management;
• Informing remediation strategies for contaminated sites.

The findings of this study directly contribute to the advancement of landfill engineering practices by providing quantitative data on the relationship between contamination levels and hydraulic conductivity changes over time. This understanding is essential for developing more reliable predictive models for leachate migration and improving the long-term performance of waste containment facilities.

2. A Brief Review of Soil–Leachate Interaction

The interaction between landfill leachate and surrounding soil is a complex process involving chemical, biological, and physical mechanisms resulting in several mechanical changes in soil properties.

2.1. Soil Leachate Interaction Processes

2.1.1. Chemical Processes

The behavior of contaminants in soil–leachate systems is governed by three main mechanisms.

First, sorption processes (both adsorption and absorption) are influenced by soil properties and leachate characteristics, with high organic matter content enhancing organic contaminant sorption and clay minerals affecting heavy metal retention [8].

Second, precipitation and dissolution reactions control metal mobility, where metal solubility depends on carbonate, hydroxide, and sulfide precipitates [9], and pH conditions significantly affect these processes—alkaline conditions reduce metal mobility while acidic conditions increase it [10].

Third, redox reactions affect element mobility, particularly for iron, manganese, and sulfur [10], where reducing conditions in mature landfills promote metal immobilization through sulfide formation, while oxidizing conditions increase metal mobility [10].

2.1.2. Physical Processes

The clogging of soil pores is a significant physical process in leachate–soil interactions, potentially altering the hydraulic properties of the soil. Clogging can result from various mechanisms, including:

• Physical clogging due to suspended particles in leachate.
• Chemical clogging from precipitation reactions.
• Biological clogging from biofilm growth and microbial activity.

Li et al. [11] observed that clogging can significantly reduce the hydraulic conductivity of soils, potentially affecting leachate movement and distribution in the subsurface.

2.2. Effect of Leachate on Soil Geotechnical Properties

The interaction between landfill leachate and soil can significantly alter various geotechnical parameters, including shear strength, permeability, compaction characteristics, and consistency limits.

2.2.1. Effect on Shear Parameters

Studies on leachate’s impact on soil shear strength show varying results across different soil types (Figure 1). In laterite soils, some researchers found increased cohesion with decreased friction angle due to increased clay content [12,13], while others reported reductions in both parameters from particle dispersion [14]. For clay soils, studies consistently show significant impacts on shear strength, though the nature of changes varies [15]. Recent research suggests that leachate effects depend on contamination levels and stress conditions, with heavily contaminated soils showing ductile behavior [16,17]. Understanding microscopic changes in mineral compositions and soil structure [18] may be key to explaining these varied effects.

2.2.2. Effect on Permeability

The effect of leachate on soil permeability appears to be time-dependent and influenced by both physical and chemical processes. Initially, many studies reported a decrease in permeability due to pore clogging by leachate particles [22,23,24,25]. However, over time, an increase in permeability is often observed, attributed to the dissolution of soil particles by acidic leachate components (Figure 2).

The dramatic increases in permeability reported by some researchers (e.g., Rowe, [26], claiming up to 1000-fold increase) should be viewed cautiously. Such extreme changes may be specific to certain soil–leachate combinations and may not be generalizable. The transformation of clay minerals (e.g., smectite to illite) proposed by some authors [27,28] as a mechanism for permeability increase is plausible but requires further investigation, particularly regarding the timescales and conditions necessary for such transformations.

The contrasting effects of organic and inorganic leachate components on permeability [29] highlight the complexity of leachate–soil interactions. This complexity highlights the need for a more detailed characterization of leachate composition in permeability studies.

2.2.3. Effect on Soil Compaction

The literature reveals inconsistent findings regarding the effect of leachate on soil compaction characteristics as shown in Figure 3 (Maximum Dry Density, MDD, and Optimum Moisture Content, OMC). While some studies [7,20,30] report a decrease in MDD and an increase in OMC with leachate contamination, others [31,32]) observe the opposite trend.

These contradictions may result from differences in soil types, leachate compositions, or contamination levels. The explanation offered by some researchers that leachate causes mineral dissolution and increased void ratios is logical but fails to account for cases where MDD increases. The alternative explanation of particle aggregation in highly concentrated leachate [31] offers a plausible mechanism for increased MDD but may not apply in all cases.

The lubricating effect of leachate on soil particles, proposed by Karthika [33] to explain decreased OMC, is an interesting concept that warrants further investigation. Future studies should aim to clarify the specific conditions under which leachate leads to increases or decreases in MDD and OMC.

The interaction between landfill leachate and soil results in complex modifications to soil characteristics, with significant implications for geotechnical engineering and environmental management. The nature of these modifications is strongly influenced by the physical and biochemical properties of the leachate components. The presence of suspended solids of varying sizes can lead to physical clogging of soil pores, while dissolved organic matter and chemical constituents can trigger biochemical reactions affecting soil structure. The size distribution of suspended particles in leachate particularly influences their transport and retention in the soil matrix, with finer particles potentially penetrating deeper into the soil structure while larger particles may form surface deposits. Figure 4 represents a summary of the geotechnical modification mechanism in the leachate–soil interaction.

3. Materials and Methods

The current study employed a comprehensive experimental approach to quantitatively evaluate the effects of Municipal Solid Waste (MSW) leachate contamination on the geotechnical parameters of soil, with a particular focus on soil permeability. The experimental design was structured to assess both the immediate and long-term impacts of leachate contamination on soil permeability (Figure 5). While laboratory conditions cannot fully replicate the dynamic nature of field settings, this controlled environment allows for the precise measurement of specific contamination effects and the isolation of key mechanisms driving soil property changes.

The investigation focused on the following key aspects:

• Contamination Levels: Soil samples were prepared with varying degrees of leachate contamination from 5 to 25% to simulate different exposure scenarios.
• Temporal Analysis: To capture the evolution of soil properties over time, tests were conducted at specific intervals: immediately after contamination, and at 1, 3, 6, 9, and 12 months post-contamination.

To further understand the mechanical changes in soil, Scanning Electron Microscopy (SEM) was employed on the samples. XRD tests were used to identify the chemical changes.

3.1. Leachate Characteristics

The leachate used in this study was obtained directly from the leachate collecting system of the Pusztazámor landfill in Hungary. This approach was chosen to preserve the initial characteristics of the leachate, ensuring that the experimental conditions closely mirrored real-world scenarios. The chemical analysis of the leachate revealed that the leachate is characterized by alkaline conditions (pH 7.95) and high electrical conductivity (10,230 µS/cm). The analysis shows elevated concentrations of major ions, particularly sodium (1960 mg/L), potassium (1280 mg/L), and chloride (2140 mg/L), along with substantial hydrogen carbonate content (2867 mg/L). The Chemical Oxygen Demand (COD) value is 544 mg/L, and the ammonium concentration is 442 mg/L. The leachate also contains divalent cations including calcium (54.3 mg/L) and magnesium (47.4 mg/L), as well as iron (2.77 mg/L) and manganese (104 mg/L).

3.2. Soil Characteristics

The tested soil is collected from the same landfill and used as leveling soil under the lining system as it is the naturally available soil in the landfill location (

Table 1. Soil characteristics summary.

Property	Value
Composition
-Silt	32.64%
-Sand	57.50%
-Clay	8.29%
-Gravel	1.57%
Atterberg Limits
-Liquid Limit (LL)	25.5%
-Plastic Limit (PL)	20.7%
-Plasticity Index (IP)	4.9%
Physical Properties
-Natural Water Content	13.3%
-Solid Density	2.66 g/cm3
-Void Ratio	0.85
-Degree of Saturation	0.31
Gradation Parameters
-Uniformity Coefficient (Cu)	17.18
-Curvature Coefficient (Cc)	2.06
Hydraulic Properties
-Hydraulic Conductivity (k)	6.428 × 10−7 m/s
Strength Parameters
-Peak Cohesion (c′)	10.3 kPa
-Peak Friction Angle (φ′)	36.5°
-Residual Cohesion $({\mathrm{c}}_{\mathrm{r}}^{\prime }$)	8.8 kPa
- Residual Friction Angle $({\phi }_{\mathrm{r}}^{\prime }$)	36.5°

). It is a well-graded sandy clayey silt, characterized by 32.64% silt with significant sand content (57.5%), a minor clay fraction (8.29%), and minimal gravel (1.57%). The soil exhibits a low plasticity index (IP = 4.9%) with liquid and plastic limits of 25.5% and 20.7%, respectively. Physical properties include a natural water content of 13.3%, a solid density of 2.66 g/cm³, a void ratio of 0.85, and a degree of saturation of 0.31. The soil’s grain size distribution shows uniformity (Cu = 17.18) and curvature (Cc = 2.06) coefficients indicative of well-graded material. The hydraulic conductivity (k = 6.428 × 10⁻⁷ m/s) indicates low permeability, while direct shear tests revealed moderate strength parameters with peak values of c′ = 10.3 kPa and φ′ = 36.5° and similar residual values (${\mathrm{c}}_{\mathrm{r}}^{\prime }$ = 8.8 kPa; ${\phi }_{\mathrm{r}}^{\prime }$ = 36.5°). All tests were conducted following relevant ISO standards.

3.3. Sample Preparation

The preparation of soil samples for this study was conducted with thorough attention to detail to ensure consistency and reliability of results. The process was designed to simulate various degrees of leachate contamination and to allow for the observation of long-term effects on soil properties.

Soil samples were carefully mixed with MSW leachate to create a range of contamination levels. The concentrations of leachate used were 5%, 10%, 15%, 20%, and 25% relative to the soil’s dry weight. This spectrum of contamination levels was carefully selected to encompass a wide array of potential real-world scenarios, from mild contamination to severe leachate exposure.

Following the preparation, each contaminated soil sample was transferred to an airtight dark container. These containers were designed to isolate the samples from external environmental factors that could influence the chemical reactions within leachate soil interaction. All prepared samples were stored in a controlled laboratory environment maintained at a constant temperature of 25 °C.

As the key aspect of this study was the investigation of the long-term effects of leachate contamination on soil properties, the prepared samples were conditioned over an extended period of 12 months. This permits the complete interaction between the soil particles and leachate components, allowing the potential chemical reactions to reach equilibrium and enabling the assessment of any time-dependent changes in soil microstructure or geotechnical properties.

3.4. Testing Procedures

To assess the impact of MSW leachate contamination on the hydraulic conductivity of the soil, comprehensive laboratory tests were conducted on each sample. These tests were performed in accordance with the relevant ISO standards [35]. The falling head method was employed to determine the hydraulic conductivity of the soil samples, calculated using the following equation:

$$k=(a\times l)/(A\times t)\times \mathrm{l}\mathrm{n}(h_1/h_2)$$

(1)

where

• k is the hydraulic conductivity (m/s);
• a is the cross-sectional area of the standpipe (m²);
• l is the length of the soil specimen (m);
• A is the cross-sectional area of the soil specimen (m²);
• t is the time interval between measurements (s);
• h₁ is the initial hydraulic head (m);
• h₂ is the final hydraulic head (m).

To fulfill the objective of long-term evaluation of the geotechnical change, for each contamination level (5%, 10%, 15%, 20%, and 25% leachate content), these tests were conducted after 1 month, 3 months, 6 months, 9 months, and 12 months.

To further characterize the soil and assess potential microstructural changes due to contamination, Scanning Electron Microscopy (SEM) was employed. Samples were air-dried prior to scanning. To improve the conductivity of electrons for a better resolution, the samples were gold-plated. SEM images were taken of both uncontaminated soil samples and samples exposed to MSW leachate (Figure 6).

X-ray diffraction (XRD) analyses were conducted on both uncontaminated and contaminated soil samples to identify mineralogical changes induced by leachate–soil chemical interactions, providing insights into the mechanisms behind observed mechanical property alterations.

4. Results

The introduction of leachate into the soil matrix resulted in complex alterations to the soil’s hydraulic conductivity, with effects varying considerably depending on the level of contamination and exposure time, showing a constant pattern during the 12 months of curing time (Figure 7).

4.1. Contamination Level Effects

At low contamination levels (5–10%), as shown in Figure 7, these levels led to a slight increase in hydraulic conductivity. This initial increase is attributed to the leachate’s effect on soil structure, possibly causing the dispersion of fine particles and creating preferential flow paths. This suggests that at low concentrations, the leachate may act as a dispersing agent, temporarily enhancing the soil’s ability to transmit water. Despite these fluctuations, permeability for both levels (5–10%) remains elevated above the clean soil value even after 12 months, suggesting a lasting alteration of soil structure at these contamination levels.

A significant behavioral shift occurred at 15% contamination and beyond. At 15%, hydraulic conductivity decreased dramatically to 4.172 × 10⁻⁸ m/s (93.5% reduction from clean soil). Higher contamination levels (20–25%) showed even more severe reductions, respectively, to 8.545 × 10⁻⁹ m/s (98.7% reduction) and 1.65 × 10⁻⁸ m/s (97.4% reduction). These significant decreases suggest severe soil structure alterations, likely due to combined effects of physical pore clogging, and chemical alterations of clay particles induced by high leachate concentrations.

4.2. Time-Dependent Effects

The analysis of hydraulic conductivity evolution revealed that changes are predominantly governed by contamination level rather than exposure duration, with permeability values remaining remarkably stable throughout the 12-month period for each contamination level (Figure 7). This time-independent behavior suggests that the structural modifications affecting hydraulic conductivity occur rapidly upon initial exposure to leachate and establish a new stable configuration that persists over time.

4.3. Microstructural and Mineralogical Analysis

XRD analysis revealed significant mineralogical changes across contamination levels (Figure 8). The clean soil showed characteristic peaks for dolomite, calcite, and clay minerals. At 5% contamination, slight reductions in carbonate mineral peak intensities were observed. At 15% contamination, more pronounced alterations in the diffraction patterns emerged, indicating the substantial modification of existing minerals and the formation of new phases.

SEM imaging showed progressive changes in soil fabric with increasing contamination (Figure 9). Clean soil exhibited a relatively open structure with distinct particle boundaries (Figure 9A,B). At low contamination levels (5–10%), partial surface modifications became evident (Figure 9C–E). Higher contamination levels (≥15%) revealed extensive structural modifications, including the development of connecting bridges between particles (Figure 9F) and the formation of new crystalline phases characterized by fibrous and needle-like structures (Figure 9G–I).

5. Discussion

The introduction of leachate into the soil matrix resulted in complex modifications of soil properties, with distinct patterns emerging at different contamination levels. These modifications manifested through changes in hydraulic conductivity, mineralogical composition, and microstructural characteristics.

The chemical composition of the leachate provides important insights into the observed soil property modifications. The high electrical conductivity (10,230 µS/cm) indicates significant ionic content that can alter soil particle interactions [11]. The elevated concentrations of monovalent cations (Na⁺, K⁺) combined with divalent cations (Ca²⁺, Mg²⁺) likely influence soil structure through cation exchange processes, particularly affecting the behavior of clay particles [15,36]. This explains the observed transition from granular to cohesive behavior at higher contamination levels. The presence of iron and manganese, along with high hydrogen carbonate content, provides conditions conducive to mineral precipitation, as evidenced by the new crystal formations observed in SEM analysis [16]. The relatively high chloride content (2140 mg/L) may contribute to the dissolution of existing minerals [37], particularly affecting soil behavior at lower contamination levels where increased hydraulic conductivity was observed.

At low contamination levels (5–10%), the slight increase in hydraulic conductivity correlates with the partial dissolution of carbonate minerals, as evidenced by decreased peak intensities of dolomite and calcite in XRD analysis (Figure 8). This initial phase suggests that leachate acts as a dispersing agent, temporarily enhancing the soil’s ability to transmit water through the creation of preferential flow paths.

The 15% contamination level emerged as a critical threshold in the soil’s response to leachate exposure. At this concentration, the soil experienced a significant reduction in permeability (93.5% reduction), coinciding with the maximum alteration of carbonate minerals and the formation of new phase minerals revealed by XRD analysis. The microstructural evidence from SEM imaging supports these mineralogical changes, showing the onset of significant fabric modifications.

At higher contamination levels (20%), SEM analysis (Figure 9D–F) reveals extensive particle surface modification and the development of a denser fabric structure, explaining the dramatic reduction in hydraulic conductivity (98.7% reduction). The development of new mineral phases, clearly evidenced in Figure 9G–I, through fibrous and needle-like crystal formations, creates extensive particle connecting and matrix densification. The observed reduction in permeability indicates a fundamental shift in soil behavior: while low contamination levels maintain granular soil characteristics, higher concentrations (>15%) induce a transformation to cohesive soil behavior, evidenced by both the hydraulic response and structural modifications observed in microscopic analysis.

The formation of fibrous, needle-like crystals is consistent with observations by Sunil et al. [13], who identified comparable secondary mineral formations in contaminated soils, particularly at higher contamination levels. The extensive structural modifications observed at higher contamination levels provide physical confirmation of the complex strength evolution patterns, corresponding with observations by Arasan [36]. The transition from an open structure to a more densified fabric with extensive particle bridges corresponds with findings by Shariatmadari et al. [16], who observed similar structural evolution in contaminated soils and linked it to enhanced cohesion development.

XRD analysis revealed the formation of new mineral phases, including secondary clay minerals and aluminum silicate hydrates, explaining these complex strength modifications. The transformation of existing minerals as evidenced by XRD peak shifts, contributed to this strengthening effect. These mineralogical changes align with findings by Kumar et al. [38] and Khodary et al. [32], who reported cohesion increases in similar conditions for silty clays.

The time-independence of hydraulic conductivity values throughout the 12-month period suggests that structural modifications occur rapidly upon initial exposure to leachate and establish a new stable configuration that persists over time. This temporal behavior pattern indicates that contamination level, rather than exposure duration, primarily governs the soil’s hydraulic properties. The relationship between fabric modification and mechanical properties observed in our study parallels the work of Oztoprak and Pisirici [15], who documented comparable structure-property relationships through SEM analysis.

Although this investigation primarily focused on physicochemical mechanisms, the potential contribution of biological processes to the observed permeability modifications warrants consideration. These biological processes could act synergistically with the observed mineralogical and structural modifications, where the development of biofilms could provide additional mechanisms for permeability reduction, complementing the documented chemical and physical alterations observed through SEM and XRD analyses. This biological aspect represents an important consideration for future research, particularly in quantifying the relative contributions of biological, chemical, and physical mechanisms to long-term permeability modifications in leachate-contaminated soils.

6. Conclusions

This research addressed the lack of long-term experimental data on leachate effects on soil hydraulic properties. In a 12-month laboratory study, sandy clayey silt samples were tested under various leachate concentrations (5–25%), using standardized permeability tests combined with microstructural analysis. This investigation revealed the following:

Hydraulic conductivity modifications are primarily governed by contamination level rather than exposure duration. Permeability values remained stable throughout the testing period, indicating the rapid establishment of new structural configurations upon initial exposure.

A critical threshold of 15% contamination marks the transition from granular to cohesive soil behavior. Below this threshold (5–10%), increased permeability (up to 1.2 × 10⁻⁷ m/s) indicates enhanced hydraulic transmission, while higher contamination levels show significant reductions. This behavioral shift coincides with mineralogical alterations and microstructural modifications, evidenced by the formation of new mineral phases and development of particle bridges.

These findings provide quantitative parameters for waste containment facility design and offer a framework for assessing long-term contamination effects on soil hydraulic properties. Future research should address the reversibility of these modifications under various environmental conditions and investigate the potential contribution of biofilm formation to permeability reduction in field conditions.