1 Introduction

Landfills designated for the disposal of municipal solid waste pose a significant environmental challenge, primarily attributed to the generation of leachate, a concentrated liquid waste resulting from the interaction of external moisture or water with the solid waste material [1] The nature and magnitude of leachate are highly variable, contingent upon factors such as precipitation levels [2, 3], waste composition and characteristics, landfill age, and operational practices [4, 5]. Leachate is characterized by its elevated concentrations of suspended and dissolved solid pollutants, encompassing a diverse array of organic and inorganic chemicals, including ammonia, nitrates, nitrites, sulfides, heavy metals, nitrogen compounds, among others [1]. Given the concentrated and toxic nature of leachate, improper management poses a grave risk to environmental integrity and the quality of water bodies [6]. Moreover, the infiltration of leachates into the soil and groundwater aquifers presents a significant threat to public health [7, 8]. This concern assumes global significance, particularly in arid regions where freshwater resources are scarce [9].

To safeguard groundwater resources and protect human health from leachate contamination, an in-depth comprehension of its physico-chemical and biological attributes is imperative during the design of treatment processes [10]. The degree of contamination inflicted upon water bodies by leachate discharge hinges upon the type and characteristics of the leachate, as well as its short-term and long-term impacts [11]. Therefore, the identification of an optimal treatment strategy for landfill leachates necessitates a comprehensive understanding of the contaminants within the leachate itself [12]. In light of these considerations, effective leachate management emerges as an indispensable imperative, serving as a bulwark against environmental contamination stemming from the unmitigated release of leachate.

In the context of waste management, it is noteworthy that developing countries often generate waste characterized by a higher organic content. This prevalence of organic matter results in the production of high-density waste with elevated moisture levels within landfills [13, 14]. Consequently, the properties of landfill leachate exhibit significant variations from one landfill site to another [15]. The accurate identification and characterization of landfill leachate, therefore, emerge as fundamental prerequisites for the formulation of effective waste management strategies.

Studies investigating the composition of leachate and its consequential effects on groundwater quality have been conducted extensively across the globe, transcending geographical boundaries. Notable investigations encompass diverse regions, including Australia [16], India [17, 18], Brazil [19], Egypt [20], Ethiopia [21], Ghana [22], Nigeria [23], Tunisia [24], South Africa [25], and Namibia [26, 27].

The extant body of literature pertaining to landfill leachate can be categorized into several key areas. These include comprehensive reviews focusing on leachate characteristics (6), reviews centered on factors influencing leachate properties [28], examinations of leachate treatment methodologies [29, 30], and comprehensive reviews that combine these various facets [31, 32]. These reviews encompass data from disposal sites across the globe.

However, it is noteworthy that most leachate studies have concentrated on regions such as China [33], the United States, and European nations situated in temperate climates [32]. As highlighted [12], limited research has been conducted on the comprehensive characterization of municipal landfill leachates in arid urban landfill environments. Additionally, our understanding of the intricate interplay between climate conditions and landfill leachate composition remains limited.

For instance, investigations on the influence of climate and landfill age on leachate properties in semi-arid regions have demonstrated considerable variations in contaminants, including organic compounds, salts, and heavy metals, associated with leachate age. Seasonal analyses of leachates have revealed that rainfall events accelerate waste decomposition rates, consequently affecting the concentration of pollutants in the leachate [34]. Similarly, [35] conducted research in India, finding that fresh leachate exhibited higher values for parameters such as color, conductivity, hardness, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC) compared to aged leachate, indicating a preponderance of slowly biodegradable organic matter in the former. [36] conducted studies in diverse landfill settings worldwide, revealing that the inorganic content of leachate remains relatively stable over time, while organics exhibit substantial changes. Moreover, with regard to climate impact, certain leachate parameters, primarily organics and the BOD/COD ratio, displayed significant differences between landfills in low precipitation areas and those in high rainfall climates. [37] explored Indochina peninsular countries (Thailand, Cambodia, and Laos), demonstrating that basic biological and chemical parameters in fresh leachates exhibited higher concentrations during dry seasons compared to wet seasons. [38] conducted research in Kuwait, revealing the impact of landfill aging through a very low BOD/COD ratio of 0.04 and a BOD degradation rate constant of 0.027/year in the leachate.

The primary objective of this research was to conduct a comprehensive characterization of the physico-chemical properties of the Kupferberg landfill leachate in Windhoek. This investigation sought to elucidate the influence of climate conditions and landfill age on the physical, chemical, and biological attributes of the leachate. Furthermore, the study aimed to provide insights into the intricate dynamics of leachate dilution by rainfall, thereby contributing to a deeper understanding of leachate properties within the context of the Kupferberg landfill. The study was guided by two research questions: What are the seasonal variations in the physico-chemical properties of leachate in relation to landfill age and what are the seasonal variations in the physico-chemical properties of leachate in relation to climatic factors? To systematically address these questions, the study formulated and rigorously tested two hypotheses: Ho1: there exists no discernible relationship between the seasonal variations in physico-chemical properties of leachate and landfill age; H02: There is no meaningful association prevailing between the seasonal variations in physico-chemical properties of leachate and climatic factors. These hypotheses were subjected to stringent evaluation at a 95% confidence level, ensuring statistical robustness in the subsequent analyses. The hypotheses were systematically examined through statistical analyses, employing established methodologies to ascertain the veracity of the formulated assertions. This meticulous scrutiny aimed to either accept or reject the null hypotheses, providing a robust foundation for the subsequent interpretation of research findings. The choice of a 95% confidence level ensured a high standard of statistical rigor, enhancing the reliability and validity of the study's outcomes.

This methodical approach, grounded in well-defined research questions and hypotheses, served as the compass guiding the empirical exploration of seasonal variations in leachate physico-chemical properties, thereby contributing to the scholarly discourse on landfill dynamics. This study carries substantial significance, particularly in light of the existing knowledge gaps related to the seasonal variation, landfill leachate composition, and the impact of landfill age within the confines of the Kupferberg landfill in Windhoek. By addressing these critical gaps, the research endeavors to underscore the pressing need for the development of more tailored and context-specific techniques for the management of leachate in this region. This research serves as a vital stepping stone toward enhancing the sustainability and environmental stewardship of the Kupferberg landfill and analogous waste disposal sites, promoting responsible waste management practices within the region.

2 Study area

This study was conducted in Windhoek, the capital city of Namibia, which, as of 2023, has a population of approximately 477,000. The city has experienced a population increase of 3.47% compared to the previous year, as reported by the [39]. Windhoek is situated in a predominantly arid climate region, characterized by a notable disparity between potential evaporation rates and precipitation levels. This climatic condition results in exceptionally low humidity levels [40].

The climate in Windhoek is consistent with a subtropical semi-arid climate, featuring an annual average temperature of 19.3 °C. Notably, January represents the hottest month, with a mean temperature of 23.5 °C, while July is the coolest month, with temperatures averaging around 13 °C. Annual precipitation in Windhoek averages approximately 370 mm (mm) per year, with March being the wettest month, registering an average of 81 mm, and August being the driest, with virtually no precipitation [41]. The Kupferberg landfill, positioned closer to the Auas Mountains and Regenstein, experiences relatively higher rainfall, averaging around 400 mm annually [42].

The Kupferberg waste disposal facility is strategically situated at coordinates 22.63712°S latitude and 17.02672°E longitude, approximately 11 km southwest of Windhoek, as reported by [43]. This landfill site is nestled in the foothills of the Auas Mountains and falls within the geographic expanse of the Windhoek aquifer, which holds immense significance as a vital water resource for the city of Windhoek [41]. The geological composition beneath the Kupferberg waste disposal facility includes quartz-biotite schist, characterized by very low permeability. Beneath this schist layer lies the aquifer, consisting of micaceous quartzite horizons. This secondary aquifer stores and transmits water primarily through fractures within the brittle quartzite. Recharge to the aquifer predominantly occurs at the outcrop of the quartzite body, located south of the Kupferberg facility. Additionally, there is limited local recharge through weathered material and exposed fractures in both the schist and the quartzite layers. The underlying fractured aquifer within the quartzite is essentially unconfined, which means that while water can only enter through fractures and other structural features, there is the potential for water to infiltrate through the overlying materials and reach the aquifer below [44].

In the Kupferberg area, the soil cover is comprised of a thin layer of coarse-grained to gravelly sand, with residual quartz veins occasionally present. In some locations, Calcrete horizons can also be observed [41]. Generally, the soil cover in the Kupferberg region ranges from 1 to 3 m in thickness. However, weathered material has been identified at greater depths, extending down to 12 to 15 m in most cases and exceptionally reaching depths of up to 31 m, as reported by [44]. Figure 1 illustrates the geographical location of the Kupferberg landfill site.

Fig. 1
figure 1

Location of Kupferberg waste disposal facility: [45]

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The Kupferberg landfill incorporates two critical pollution control dams designed to manage environmental impacts: Stormwater Collection Dam: Positioned west of the general waste cell, this dam serves as the repository for stormwater runoff emanating from the general cell. Leachate Collection Dam: Located north of the upper terrace of the general waste cell, this dam is dedicated to collecting leachate generated primarily from the Hazardous cell. The management of leachate entails either it’s controlled pumping back into the landfill or its natural evaporation within the lagoon.

Open evaporation of leachate, especially from the leachate collection dam, presents potential environmental and health risks. Inhalation of toxic gases such as ammonia (NH3-N) and hydrogen sulfide (H2S) can pose health hazards to individuals in the vicinity [46]. Furthermore, there exists a risk of groundwater contamination through leachate seepage.

In Namibia, the intensity and duration of rainfall are notably less pronounced compared to tropical regions. Consequently, the landfill site has not experienced significant leachate overflow from the dams, except for an exceptional event in 2014 when above-average rainfall resulted in spillage from the stormwater dam.

3 Description of the Kupferberg landfill site

The study site, Kupferberg landfill, encompasses a total area of 310,000 square meters, with approximately 170,000 square meters dedicated to active landfill operations. According to data obtained from the City of Windhoek's landfill site questionnaire in 2019, the annual tonnage of waste deposited at the site is categorized as follows: General waste: 78,000 tons, Hazardous waste: 7680 tons, Building rubble: 89,699 cubic meters, Garden refuse: 90,000 cubic meters, Tyres: 25,00 and Health Care Risk waste: 1260 tons.

As per the 2020 waste audit report published by the City of Windhoek, an analysis of household waste composition revealed distinct waste types and their respective percentages of generation. The most prevalent waste types generated at the household level include: Organic Food: 19%, Garden Refuse: 15%, Glass: 12%, Paper (Plain): 9%, Paper (Carton): 7.4%, Plastic (Soft Bags): 9%, and Plastic (Containers): 7%.

A noteworthy observation is the preeminence of organic waste, which constitutes the highest proportion of waste by weight. In high-income neighborhoods, organic waste accounts for a substantial 45% of total waste composition. This percentage diminishes to 35% and 25% in middle- and low-income areas, respectively. Figure 2, displayed below, provides a graphical representation of these waste types.

Fig. 2
figure 2

Various types of MSW collected at Kupferberg landfill that contribute to leachate generation [47]

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Despite the diversity of waste types generated, recycling rates remain relatively low. Approximately 5% to 8% of recyclable materials are diverted away from landfills for recycling purposes.

In terms of waste generation rates, the report estimates that waste is produced at varying rates across different income brackets. In high-income areas, waste generation averages 0.56 kg per person per day. In middle-income areas, this rate ranges from 0.19 kg to 0.33 kg, while low-income areas fall within a similar range [47].

4 Data and methods

4.1 Sampling and leachate assessment

To fulfill the objectives of this study, a total of 48 samples were collected from two leachate dams over a six-year period, spanning from 2017 to 2022 by the city of Windhoek’s municipality. These collections encompassed both wet and dry seasons, resulting in the compilation of 24 water analysis reports. These reports represent the average values of 14 key physico-chemical parameters for each of the two sampling sites and seasons.

According to the city of Windhoek’s municipality personnel, leachate samples were meticulously collected using a standardized procedure. A scoop, thoroughly rinsed three times with the same water to be studied, was utilized for sample collection. Each sample was then placed in a 1-L container, which had also undergone a three-time rinsing process. Subsequently, the samples were packaged in cooler bags containing ice cubes to maintain their integrity and transported to the laboratory for analysis. The laboratory analyses thereafter focused on 14 physico-chemical parameters, which encompassed: pH, measured using a pH meter, Electrical conductivity (EC), measured using a conductivity meter, Chemical oxygen demand (COD), analyzed through Merck and MN COD cell tests, Ammonia concentration, determined using the Skalar Continuous Flow Analyzer (CFA) with the colorimetric salicylate method, Chloride (Cl), sulfate (SO4), nitrate (NO3), nitrite (NO2), and phosphate (PO4) levels, measured using Ion chromatograph (IC), Total Kjeldahl Nitrogen (TKN), prepared through digestion in copper sulfate and sulfuric acid at specified temperatures and measured on the CFA with the salicylate method, Phenol concentration, assessed using Merck cell tests with a photometric method and Metals including iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), zinc (Zn), and copper (Cu), quantified via inductively coupled plasma mass spectrometry (ICP-MS).

It's essential to note that the collection, preservation, and analysis of samples adhered to rigorous standard procedures, of the American Public Health Association (APHA) guidelines for examining water and wastewater [48].

4.2 Statistical analysis: ANOVA

Statistical Packages for Social Scientists (SPSS) was used to perform the tests on the data provided by Windhoek Municipality. To investigate the variations in leachate characteristics concerning landfill age and climate conditions, an Analysis of Variance (ANOVA) was employed. ANOVA is a parametric statistical test utilized for comparing means across various groups, as articulated by [49]. Given its frequent use in previous leachate studies, ANOVA was the chosen analytical method for this investigation, aligning with the practice of other researchers [50]. All statistical analyses were conducted using IBM SPSS software for Windows version 25. A significance level of 0.05 (p < 0.05) was set, determining the threshold for statistical significance. Prior to executing ANOVA, several preparatory steps were undertaken. It was imperative to assess whether the data met the assumption of normal distribution. In addition, the data underwent comprehensive data treatment procedures, including the removal of outliers and addressing missing data. Standardization, normalization, and data grouping procedures were performed as part of the data pre-treatment phase. These actions were essential to ensure the integrity and comparability of the dataset.

The data collection spanned multiple years, encompassing the period from 2017 to 2022. Two distinct climate conditions, characterized as wet and dry seasons, were evaluated for each year. Seasonal means were calculated to facilitate a comprehensive examination of seasonal variations in groundwater quality. For the analysis of the landfill age effect, the leachate data were classified into different age groups. The age of each landfill was determined by calculating the time elapsed between the initial waste placement (start date of the landfill) and the leachate sampling time, in accordance with the methodology outlined by [51]. These age groups were structured to differentiate between younger and older landfills, taking inspiration from previous studies [15, 52].

4.3 Data limitation

While addressing missing data was a significant concern in this study, it was acknowledged that several crucial steps needed to precede the analyses. These preparatory measures encompassed standardization, normalization, and data grouping. A noteworthy limitation was the restricted availability of leachate parameters from both stormwater and leachate dams, along with data gaps in the wet seasons of 2017 and 2019, and the dry season of 2022. Specifically, parameters such as TOC, BOD, DOC, phosphorus, sodium (Na), potassium (K), and total Kjeldahl nitrogen (TKN), along with certain heavy metal measurements, were conspicuously absent from these datasets. Conversely, the datasets did include measurements for pH, electrical conductivity (EC), chemical oxygen demand (COD), sulfate (SO42-), chloride (Cl), iron (Fe), manganese (Mn), zinc (Zn), and ammonium nitrogen (NH4-N).The absence of data can be attributed to a lack of data collection by the responsible individuals at the specified time, as required. Consequently, no sampling occurred during this period.

5 Results

5.1 Analyzing climate influence: ANOVA findings

Given that Cl− is commonly acknowledged as a conservative pollutant and is frequently used to ascertain dilution factors in leachate investigations [53], investigating variations in its concentration in response to precipitation rates would be valuable. In light of this for this study, the analysis of precipitation's influence on leachate quality focused on EC and Cl−.

The electrical conductivity (EC) within the Kupferberg landfill exhibited dynamic values, showcasing distinct patterns during dry and wet periods. In the stormwater dam, EC values ranged from 520 to 1060 mS/m during dry periods, highlighting a relatively constrained variability. Conversely, in the leachate dam, EC values were more expansive, spanning from 960 to 8140 mS/m in dry periods.

During wet seasons, a notable shift in EC values was observed. In the stormwater dam, the range extended from 710 to 1030 mS/m, reflecting a moderate increase compared to dry periods. In contrast, the leachate dam exhibited a substantial elevation in EC values, ranging from 1090 to 15530 mS/m during wet seasons. Notably, these elevated EC values exceeded Namibian drinking water quality standards of 400 mS/m in both dry and wet seasons.

Illustrated in Fig. 3 is a visual representation of the heightened EC levels during wet periods in both the leachate and stormwater dams of the Kupferberg landfill.

Fig. 3
figure 3

EC mean values from the samples of the (a) Leachate dam and (b) storm water dam of the Kupferberg landfill during wet and dry seasons

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Chloride levels within the Kupferberg landfill exhibited distinct seasonal patterns, with concentrations notably higher during the wet season compared to the dry season. A more detailed analysis of the data uncovered intriguing yearly trends, highlighting unique dynamics within the stormwater and leachate dams.

In the stormwater dam, chloride concentrations displayed a discernible upward trajectory from 2017 to 2022 during the wet season, increasing from 1830 to 3050 mg/l. This trend suggests a progressive rise in chloride levels within the stormwater dam during wet periods over the specified years. Conversely, in the leachate dam, chloride concentrations exhibited a contrasting pattern over the same period during the wet season, showcasing a decreasing trend from 72180 to 27210 mg/l, as visually represented in Fig. 4. This decline in chloride concentrations within the leachate dam during wet seasons stands in stark contrast to the observed increase in the stormwater dam.

Fig. 4
figure 4

Cl mean values from the samples of the (a) Leachate dam  and (b) storm water dam of the Kupferberg landfill during wet and dry seasons

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These divergent trends in chloride concentrations between the stormwater and leachate dams may be attributed to the specific waste composition within the hazardous and general cells of the landfill. The unique characteristics of these waste cells likely contribute to the differential leaching of chloride compounds, resulting in the observed variations in chloride concentrations between the two dam systems. This nuanced understanding underscores the complex interplay of factors influencing leachate dynamics within different compartments of the landfill and emphasizes the importance of considering specific waste characteristics in the interpretation of leachate data.

5.2 T-test analysis

The evaluation of parameter means between dry and wet climate seasons involved independent samples t-tests, providing insights into the variations across distinct climates at the Kupferberg landfill. Notably, during wet seasons, Electrical Conductivity (EC) and Chloride (Cl-) exhibited elevated mean values of 5066 mg/l and 19597 mg/l, respectively, surpassing dry season values (2069 mg/l and 12847 mg/l). Remarkably, these parameters demonstrated the highest values within their respective climate groups, trailing only behind Chemical Oxygen Demand (COD).

The independent-samples t-test further unveiled significant differences in climate means for temperature (t (30) =  − 3.158, p < 0.05), Chemical Oxygen Demand (t (23.095) = − 2.090, p < 0.05), Manganese (t (13.532) =  − 2.393, p < 0.05), and Zinc (t (12.284) = − 2.193, p < 0.05) between the two climates. Specifically, the wet climate exhibited a significantly higher mean temperature (M = 24.69, sd = 5.99) compared to the dry climate (M = 19.45, sd = 3.02). Moreover, both Chemical Oxygen Demand and Manganese concentration were higher in the wet climate. Similarly, the mean Zinc concentration was elevated in the wet climate compared to the dry climate. Conversely, no significant differences were observed in other parameters when comparing the two climates.

These results underscore significant disparities between dry and wet climates, particularly in, Chemical Oxygen Demand, Chloride, and Sulphate. The comprehensive parameter values categorized by climate (dry/wet) are presented in Table 1, offering a detailed overview of leachate data at the Kupferberg landfill.

Table 1 Evaluation of variation in parameter means between climate seasons (dry and wet)
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EC is measured in mS/m, Temp in oC and all other parameters are in mg/L.

5.3 Landfill age effects: ANOVA insights

In a manner analogous to the investigation into the climatic influence (wet/dry) on leachate quality, an examination of the temporal effect based on landfill age was also conducted. Given the volatile nature of leachate, only a subset of parameters could provide insights into the correlation with landfill age.

Therefore, COD, which represents organic content, and NH3-N, a significant leachate pollutant, were chosen for preliminary analysis, aiming to gain initial insights into the impact of landfill age. COD was mainly high in the wet season in both leachate and storm water dams leachate samples. The concentration levels of COD during the wet season was 3 times more than the dry seasons as shown in Fig. 5.

Fig. 5
figure 5

COD mean values from the samples of the a Leachate and b storm water dams of the Kupferberg landfill during wet and dry seasons

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The ANOVA analysis revealed a substantial disparity in temperature across the years [F (5, 21) = 4.493, p < 0.05]. However, the ANOVA alone does not pinpoint the specific years contributing to this difference. To address this, post hoc tests, specifically the Least Significant Difference (LSD) test, were executed to ascertain the pairwise differences between individual years (refer to Table 2). The outcomes of these tests highlight distinct variations in temperature means among the different years, offering a more nuanced understanding of temporal temperature dynamics.

Table 2 Post-Hoc LSD test for means of temperatures across the years: Temp in oC
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Notably, an intriguing trend was observed in Chemical Oxygen Demand (COD) as it exhibited a diminishing pattern in tandem with landfill age. Concurrently, Ammonia–Nitrogen (NH3-N) concentrations within the leachate displayed substantial fluctuations over time. This fluctuation aligns with expectations, considering that the biodegradable organic components within the leachate tend to decrease as the landfill ages, attributed to the ongoing anaerobic decomposition processes within the waste.

Notably, temperature exhibits significant fluctuations at distinct points in landfill age, evident by the escalating standard deviations. Additionally, specific trends become apparent within certain groups for various parameters. For instance, a discernible decreasing trend is observable over time for organic content. In contrast, certain inorganic components, such as Cl− and alkalinity, do not exhibit any discernible trend concerning landfill age. Figure 6 below presents the descriptive statistics of landfill leachate temperature across various landfill age categories.

Fig. 6
figure 6

Descriptive statistics: Mean, Std Dev and Std error for temperature variations from 2017 to 2022

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The present investigation employed the Least Significant Difference (LSD) post hoc test to conduct in-depth pairwise comparisons of annual temperature means. Notably, statistically significant differences were uncovered across several temporal comparisons. Specifically, a marked distinction was observed in temperature means between the years 2017 (M = 19.59, SD = 3.44) and 2022 (M = 26.96, SD = 7.86). Similarly, a significant divergence in temperature means manifested between 2018 (M = 14.75, SD = 0.97) and 2019 (M = 21.94, SD = 3.80). Additionally, a pronounced difference was detected between 2018 (M = 14.75, SD = 0.97) and 2020 (M = 23.21, SD = 2.65). Subsequently, a noteworthy contrast emerged between 2018 (M = 14.75, SD = 0.97) and 2021 (M = 25.25, SD = 4.04). Finally, a statistically significant disparity was identified between 2018 (M = 14.75, SD = 0.97) and 2022 (M = 26.96, SD = 7.86). This detailed exploration provides valuable insights into the nuanced temporal variations in temperature, underscoring the dynamic nature of the climatic conditions under consideration.

6 Discussion

The Electrical Conductivity (EC) levels within both the leachate dam and the stormwater dam demonstrated a consistent trend of higher concentrations during the wet season compared to the dry season. High or elevated level concentrations in this study refers to the acceptable standard limits of drinking water by the WHO or the Namibian drinking water standards. Intriguingly, divergent patterns emerged concerning the relationship between EC concentrations and landfill age in these two environments.

In the leachate dam, EC concentrations exhibited an upward trend with landfill age, suggesting an increase in soluble salt concentrations over time. Conversely, in the stormwater dam, the trend was contrary, indicating a decrease in EC concentrations with advancing landfill age.

This observation aligns with findings reported by [54], who similarly documented elevated EC values during pre-monsoon, monsoon, and post-monsoon periods, indicative of higher concentrations of soluble salts such as sodium, potassium, and chloride. Furthermore, [55] supported this trend by noting substantial EC concentrations during the pre-monsoon season, with a gradual increase during the monsoon season. The heightened EC concentrations were attributed to increased dissolution of pollutants, facilitated by rainwater intrusion.

In this investigation, a discernible pattern emerged as chloride levels exhibited higher concentrations during the wet season in comparison to the dry season within the leachate dam. Intriguingly, an inverse relationship between chloride concentration and leachate age was observed from 2018 to 2022 during the wet season in the leachate dam. Conversely, in the stormwater dam, the trend was reversed. This disparity may be attributed to the distinctive waste compositions within the hazardous and general cells, suggesting a complex interplay of factors influencing chloride dynamics.

This finding aligns with the observations made by [56], where leachate samples displayed markedly elevated concentrations of major anions, with chloride being the predominant species. However, in contrast, [57] reported an increase in major cation concentrations during the dry season compared to the rainy season. The variation in chloride concentrations with leachate age has been a subject of extensive exploration in the literature. [58] documented an increase in chloride concentrations as leachate ages. In contrast, studies by [59] and [60] revealed no discernible change in chloride levels, irrespective of landfill age. Similarly, [61] did not observe any decreasing trend in leachate chloride concentrations within the initial two decades of leaching. This inconsistency is further underscored by [51] observations on a Florida landfill, where chloride concentrations in leachate exhibited relative stability over the landfill's lifespan.

The relationship between environmental factors and Chemical Oxygen Demand (COD) concentrations in leachate at the Kupferberg landfill unveils intriguing dynamics. Elevated levels of rainfall and temperature correlated with higher COD concentrations, particularly evident during the wet season in comparison to the dry season within the leachate dam. However, a noteworthy trend was observed in COD levels concerning landfill age, indicating a decreasing pattern from 2018 to 2022 in the leachate dam and a corresponding opposite trend in the stormwater dam.

This observation aligns with the findings of [62], who asserted that the quantities of organic components in landfill leachate rise alongside precipitation, potentially due to the cumulative scouring effect of heavy rainfall. In our study, COD concentration levels during the wet season at the Kupferberg site were approximately three times greater than those during dry seasons, underscoring the impact of precipitation on leachate composition.

Contrasting perspectives were presented by [63], who suggested that leachate becomes concentrated during dry seasons due to evaporation, and [64], who proposed that dry climates could yield higher concentrations of organics due to limited leachate formation. [65] noted that intense precipitation rates could lead to the flushing out of soluble organics, resulting in a substantial volume of diluted leachate. This implies that excessive rainwater infiltration into landfilled waste amplifies the generation of dilute leachate by dissolving and extracting various compounds.

Conversely, in arid climates, specific parameters like COD concentration can be higher, as detailed by [64]. The presence of elevated COD levels signifies potent organic strength, rendering it an index for assessing organic pollution according to [66].

The literature provides a rich array of perspectives on landfill leachate characteristics, particularly in relation to seasonal variations and landfill age. [37] conducted a comprehensive characterization of landfill leachates, revealing higher parameter concentrations in the dry season than in the wet season across all fresh leachate samples. This finding aligns with [67], who demonstrated that landfill leachate COD was 5–8 times higher in the dry season compared to the rainy season. These results confirm the observation by [63] regarding low COD levels during the rainy season and stand in contrast to the study [65].

[21] further supported these trends by examining a leachate sample and finding increased electrical conductivity, chloride, and COD during dry periods. A contrast to this study’s findings. The findings of statistical tests conducted in regions with high precipitation and elevated temperatures, showing notably reduced organic content in leachate and lower BOD/COD ratios, further underscore the influence of climatic conditions on leachate composition. Conversely, in arid or low precipitation landfills, where rainfall infiltration is minimal, the waste tends to remain dry, according to a study by [68].

The impact of landfill age on leachate composition is also well-documented [69]. demonstrated that the physico-chemical characteristics of waste evolve with its age [60] explored the influence of landfill age on leachate composition, observing a reduction in organic content as the landfill aged. [70] noted a similar trend, with organic matter content in the leachate of a Spanish landfill reaching peak values during initial landfilling years and gradually decreasing over time. The investigation by [71] into leachate composition across 106 landfills in Denmark, spanning a range of 10 to 40 years old, revealed a consistent trend of declining constituent concentrations with landfill age. This observation aligns with the findings of Ehrig (1983), who studied 20 German landfills over a 3-year period in a low precipitation climate. Ehrig noted changes in leachate parameters such as pH, Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), iron (Fe), and calcium (Ca) over the landfill's age. Furthermore, [72] echoed a similar observation, stating that most chemical parameters in leachate notably decreased within the first 4 years of landfill operation. This collective body of research underscores the consistent pattern of decreasing leachate constituent concentrations as landfills age.

Although short term weather events such as precipitation and Temperature play a primary role for landfill processes especially for leachate management and transport, this study found that temperature did not exert discernible influence on all other parameters except for organic content.

The observed decline in leachate parameters over time has important implications for landfill management and environmental impact assessments. Understanding the temporal evolution of leachate characteristics is crucial for designing effective waste management strategies and predicting potential environmental impacts associated with landfill operations.

7 Conclusion

A one-way ANOVA was utilized to compare parameter means across different years, yielding results of significant importance in comprehending leachate behaviour across varying landfill ages and climates.

The ANOVA analysis revealed a notable disparity in temperature trends over the years. It became evident that certain leachate parameters exhibited significant variations corresponding to distinct conditions, such as organic content and pH, while others displayed no clear discernible trends, particularly inorganics and heavy metals. Specifically, COD displayed a declining trend in conjunction with landfill age. However, certain inorganic components, like Cl− and alkalinity, did not manifest any distinct trend with respect to landfill age. The obtained p-value from the one-way ANOVA analysis yielded statistical significance, registering below the conventional threshold of 0.05. This compelling statistical evidence indicates a substantive relationship between landfill age, climate and the observed seasonal variations in the physico-chemical properties of leachate. Consequently, the null hypotheses, postulating no discernible association between these variables, was unequivocally rejected. This rejection underscores the empirical support for an intrinsic link between the temporal evolution of landfill sites and the resultant alterations in leachate characteristics across seasons. The robust statistical foundation provided by the p-value below 0.05 substantiates the rejection of the null hypothesis, thereby affirming the presence of a significant correlation between landfill age, climate and the dynamic physico-chemical attributes of leachate. This pivotal finding contributes nuanced insights to the broader understanding of landfill dynamics and their environmental implications, forming a cornerstone for subsequent analyses and policy considerations in waste management.

To assess variations in parameter means between climate seasons (wet and dry), independent sample t-tests were conducted, to compare leachate parameter means across these two distinct climates. Notably, during the wet seasons, EC and Cl− displayed higher mean values when compared to the dry season. Furthermore, the independent t-test also identified a higher mean temperature in the wet climate compared to the dry climate, along with an elevated COD level.

Generally, both EC and Cl− values exhibit an increasing trend in relation to precipitation. This pattern is consistent with numerous studies conducted on leachate data from a specific landfill under various seasonal conditions such as droughts and heavy rainfalls. Understanding how the climate and landfill age affects the production of leachate is crucial for Namibia's groundwater resource management given the country's arid climate and irregular rainfall patterns. Due to seasonal variance, multiple sampling programs are needed to monitor landfill leachate. Control and management of landfill leachate are the most important concerns to protect the surrounding environment, especially the groundwater.