Abstract

The purpose of this work was to highlight the potential impact of the leachate generated by the Aképé technical landfill center on the physicochemical quality of groundwater in it vicinity. Thus, physicochemical characterizations of the raw leachate and the leachate from the infiltration basin which constitutes the post-treatment leachate and the water from three piezometers available on the site were carried out. To assess the pollution level of the leachate and groundwater, the statistical tools such Leachate Pollution Index (LPI), Water Quality Index (WQI) and Principal Component Analysis (PCA) were used. Five sampling campaigns were carried between May 2022 to May 2023. Raw leachate (taken just at the entrance to the aeration basin), treated leachate (taken from the infiltration basin) and water from the piezometers are taken during each campaign. Analysis of the raw leachate showed high minerals content, organic and metallic pollutant load (average chloride concentration: 5425.96 mg/L, sulphates: 256.33 mg/L, COD: 7420 mg/L, BOD5: 255 mg/L, copper: 0.20 mg/L, zinc: 0.27 mg/L, lead: 0.13 mg/L and cadmium: 0.03 mg/L) with an average LPI of 128.54. In addition, a significant pollutant load in the leachate of the infiltration basin was noticed (chlorides: 3103.41 mg/L, sulphates: 67.92 mg/L, COD: 2580 mg/L, BOD5: 20.90 mg/L, copper: 0.15 mg/L, zinc: 0.03 mg/L, lead: 0.11 mg/L and cadmium: 0.02 mg/L), with an LPI of 92.97. These leachates were respectively classified as “very high pollution” and “high pollution”. This high pollution load of leachate makes the surrounding groundwater vulnerable. The physicochemical study of water from the piezometers revealed a significant deterioration in ground water quality. Calculation of the water quality index (WQI) for piezometers P1, P2 and P3 gives values of 17.75; 36.13; 269.32 respectively. According to the classification, waters from P1 and P2 are characterized by good quality while P3 is very poor quality. Piezometer P3 is the most polluted with high mineralization (EC = 7466 μS /cm) and high chloride concentrations (1339.90 mg/L), testifying the potential contamination by leachate from the basin of infiltration given to their proximity. The PCA confirmed this pollution showing anthropogenic origin of the majority of chemical elements, especially in piezometer 3.

1. Introduction

Socio-economic evolution and its many advantages have created many environmental problems consecutively to the increase in the quantity of household waste and toxic industrial waste. The most economical and widely used method for municipal solid waste disposal is landfilling, which accounts for up to 95% of waste disposed of worldwide ¹. Over the last few decades, landfill sites, whether uncontrolled or controlled, have caused enormous problems for the environment. Groundwater pollution by heavy metals or by non-biodegradable organic pollutants in the vicinity of landfills has often been reported in the literature [2-6] ². Within the waste piled up, the degradation processes occur due to biological and physicochemical reactions generating leachate, commonly called "landfill juice". Leachate is a major vector of pollutants load and falls into the same category as heavily polluted municipal and industrial wastewater. These leachates are loaded with harmful substances; their discharge into the environment in their raw state, without prior treatment or with inadequate treatment, can lead to the contamination of soil, surface water and groundwater and therefore threaten human health ⁷. The technical landfill of Aképé, located about 15 km from downtown Lomé, receives large quantities of urban waste from different parts of Lomé and its surroundings. It meets international standards for environmental protection and public health ⁸. However, given that it is located in a vulnerable environment, the sedimentary basin, known for its low retention capacity for chemical elements, poor leachate treatment or an accidental leak of leachate can be a potential threaten to the surrounding groundwater resources. This work was carried out to understand the interaction between the landfill and surrounding groundwater aiming to reduce the potential negative impact. Water quality indices and leachate pollution indices are employed to quantify and understand the quality of the water sources. The groundwater suitability for drinking and domestic purposes, the hydrochemical and microbiological results were compared to WHO ⁹ and EU ¹⁰ water quality standards. Principal Component Analysis (PCA) was performed using XLSTAT software. The Piper diagram ¹¹ was used to determine the hydrochemical facies or water types indicating the dominants anion and cation in water samples.

2. Presentation of the Study Area

The technical landfill of Aképé is located northwest of Lomé, near the border with Ghana (Figure 1). The Aképé area extends on the two morphological units which are the continental plateau and the coastal sedimentary basin, at an altitude of around 40 m above sea level and in a topographic context relatively flat. The geology of the area is the same as that of larger Lomé which is the coastal sedimentary basin. From a hydrogeological point of view, three main aquifers stand out in Lomé and its agglomeration: Continental Terminal Sandy aquifer, limestone Paleocene aquifer and the sandy Maastrichtian. These aquifers are mainly fed by rainwater infiltration.

At the local level, the aquifer most concerned and the first encountered is the aquifer of the sandy continental terminal.

The study area located between parallels 6°14' and 6°16' North latitude is subject to a four seasons subequatorial climate: a long rainy season from March to July, a short dry season from July to August, a short rainy season in September-October and a long dry season from November to February.

The rainfall in the area is weak and irregular due to the climatic anomaly in South of Togo and is about 900 mm of water per year. However, the recent climatic changes trend shows a big upset between the four seasons ¹².

Aképé landfill extended on 194 ha is characterized with waste landfilling site stabilized with a specific geo-membrane (HDPE). Raw leachate (RL) is pumped to a treatment basin planted with reed behaving as constructed wetland. After, hydraulic retention time of 2 to 3 days leachate is stocked in infiltration basin, named TL. Activities on the site started in 2018.

3. Methodology

Five sampling campaigns of leachates and water from piezometers were carried out in May, August, November in 2022 and in February and May 2023.

It should be noted that three piezometers are available on the site. Two are located in the technical zone just near the leachate treatment basins and the third at the entrance of the landfill, 1400 m from the technical site. Samples are taken in PET bottles previously washed with nitric acid and rinsed with distilled water.

Leachate sample were subjected to pH, EC, BOD5, COD, Nitrate, sulphate, chloride some traces metallic elements (TME) such as Cu, Pb, Cd and Zn while water samples were subjected to pH, EC, nitrites, nitrates, phosphates et sulphate, chloride, calcium, magnesium, total hardness and the same TME. All parameters were determined in laboratory using the analytical techniques following the standard methods ¹³. The Table 1 presents water and leachate quality parameters and their analytical methods.

Eleven important parameters (pH, COD, BOD5, TKN, NO₃^-, SO₄^2-, Cl^-, Cu, Pb, Cd and Zn) have been selected to calculate the Leachate Pollution Index (LPI). The LPI, is considered to be a powerful tool that can present a comprehensive picture of effluent body pollution. LPI is the rate that reflects the integrated impact of different variables. So, the LPI summarize large quantities of data on the degree of leachate pollution in simple terms such as Excellent, Good, Bad, Very Bad, etc. (Table 2). It was calculated using the weighted arithmetic index method. This method has been used by ^{14, 15} to calculate the water quality index and we have adapted it for the determination of LPI. It is calculated as shown by the equation (1):

(1)

The quality rating scale (Pi) for each parameter is calculated using this expression:

(2)

Where Ci is the concentration of each element in the leachate, and Si is the discharge standard.

In this approach, a numerical value called relative weight (Wi), specific to each physicochemical parameter, is calculated according to the following formula:

(3)

Where, K = proportionality constant and can also be calculated using the following equation:

(4)

Five pollution classes can be identified according to the values of the LPI.

Since the first proposal to evaluate the quality of supply water to ensure safe drinking water by Horton (1965), the mathematical technique of water quality index (WQI) has been widely used throughout the world ¹⁶. It is calculated in three stages. In the first stage, each of the 18 parameters (pH, EC, NO₃^-, NO₂, NH₄⁺, NTK, HCO₃^-, CO₃^2-, Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, Cl^-, Cu, Pb, Cd et Zn) was assigned a weight (wi) according to its relative importance in the overall quality of water intended for consumption. The maximum weighting of 5 was assigned to the EC, NO₃^-, Cl^-, SO₄^2- parameters because of their major importance in assessing water quality ¹⁷. Magnesium and potassium are given the minimum weighting of 1, because of its weak harmful effect. Other parameters were assigned weight between 1 and 5 depending on their importance in water quality determination. In the second step, the relative weight (Wi) is calculated from the following equation:

(5)

Wi is the relative weight, wi is the weight of each parameter.

In the third step, a quality rating scale (qi) is assigned to each parameter by dividing its concentration in each water sample by its respective WHO standard and multiplying the result by 100:

(6)

where qi is the quality rating scale, Ci is the concentration of each chemical parameter in each water sample in mg/L, and Si is the WHO drinking water standard for each chemical parameter in mg/L. WQI is now calculated according to the following equation:

(7)

The calculated WQI values are classified into five types, from "excellent water" to "water unfit for drinking"(Table 3) ^{17, 18}.

Principal Component Analysis was also applied on water parameters to extract information about variables correlation and the potential origin of pollution. Indeed, parameters in the same PC with positive loadings mean a same origin or similar geochemical behaviors ¹⁹. XLSTAT software was used for this purpose.

4. Results and Discussion

The results of analysis of raw leachate (RL) and leachate obtained after wetland treatment (TL) are indicated in Table 4.

Raw (RL) and treated (TL) leachate are characterized by alkaline pH. The average value found for the five campaigns carried out was 8.28 for the raw leachate and 9.33 for the leachate from the infiltration basin. The more alkaline value in the infiltration basin, reflects intense evaporation of the effluent during treatment. Also, the alkaline nature of this leachate indicates the advanced state of waste degradation. Indeed, the alkalinity shows the oldest leachate and the high content in humic and fulvic substances ²⁰. The average value of conductivity recorded in the leachates during our study was 29223µS/cm for the raw leachate and 16770 µS/cm for the treated leachate. These values are very high, indicating a high level of leachate mineralization. High amount of chloride, sulfate, nitrate etc. corroborate this high salinity. High chloride contents are well correlated with electrical conductivity (Figure 2 and 3), a feature common to leachates ²¹. In raw leachate, the correlation coefficient was R² = 0.91 and R² = 0.97 in leachate from the infiltration basin in the five campaigns carried out. The figures below show the correlation between conductivity and chloride ions in raw leachate (RL) and in treated leachate (TL) over the five campaigns.

The chloride ion is a highly mobile element, easily migrating to the underlying water table. It is not affected by adsorption or ion exchange phenomena. Nor is it involved in acid-base or redox equilibrium. Nor is it retained by the clay-humus complexes of soils ²². As a result, it is often used as a pollution index and frequently as a good conservative tracer, enabling us to highlight the impact of leachates on the physicochemical quality of water tables ^{23, 24}.

The COD and BOD5 values ranged between 4000 – 13 000 mgO₂/L (average: 7420 mgO₂/L) and 130 - 380 mgO₂/L (average: 255 mgO₂/L) for raw leachate respectively, while it ranged between 400 – 5000 mgO₂/L (average: 2580 mgO₂/L) and 8 - 57 mgO₂/L (average: 20.90 mgO₂/L) for treated leachate, respectively.

The BOD5/COD ratio indicates the degree of biodegradability and provides information on the nature of the biochemical transformations occurring within the landfill ². For young landfills, this ratio, where biological activity corresponds to the acid phase of anaerobic degradation, reaches a value of 0.83 and decreases to 0 for stabilized leachates [25-27] ²⁵. Taking to account the COD and BOD5 values in raw leachate, the ratio varies between 0.016 to 0.079 with an average of 0.040. The BOD5/COD ratio calculated in the infiltration basin leachate vary between 0.003 and 0.143 with an average of 0.032. This low ratio confirms that the leachate from this pond is stable and hard to biodegrade. The very week ratio value in raw leachate suggested leachate from low biodegradable organic content. Indeed, Fatta et al. ²⁸ reported that BOD5/COD ratio is lower than 0.1 for landfills older than 10-15 years, that is not the case of Akepé landfill started in 2018. The nature of waste, probably more inorganic, could explain the low BOD5 values. Applying directly wetland seems not suitable, thus, prior to wetland, leachate should be subjected to an anaerobic pond increasing the biodegradability. Indeed, the treated leachate characteristics did not meet Ivory Coast standard regarding COD and other minerals such as chloride and nitrate.

The monitoring of traces metallic elements in leachate is very important since its amount revels the heterogeneity of waste and the toxicity of the leachate. Copper, lead, cadmium and zinc were analyzed in leachates. The average concentration for all the examined heavy metals is fairly low compared to WHO standard for wastewater that can be rejected in environment. The low concentration of trace elements in leachate could be attributed to the strong attenuation by both sorption and precipitation which are believed to be significant mechanisms for metal immobilization and the subsequent low leachate heavy metals concentration. ²⁹. It should be noted that leachate is regularly pumped from the landfill to the aeration basin for treatment, while landfilling continues. The mobilization rate of heavy metals could therefore be low, hence their low concentration in the leachate. In addition, the low concentration of metals could be due to the fact that the landfill receives mainly municipal solid waste and very low quantities of industrial waste ²⁸. However, the accumulation of heavy metals in infiltration basin of Akepe landfill could be an environmental treat.

As shown in the Table 5, the raw leachate LPI values varied between 86.53 – 222.43 and the treated leachate LPI varied from 71.61 to 108.65. The average LPI values for the five campaigns carried out on raw leachate (RL) and leachate from the infiltration basin (TL) fall into the "very high pollution" and "high pollution" classes respectively. These results show that leachate destined for discharge could be a source of pollution for the environment, and groundwater in particular.

Three piezometers were installed in the landfill area for groundwater quality control. The physicochemical characteristics of water samples from these piezometers are presented in Table 6. Water samples have slightly acidic pH ranging from 5.88 to 6.60. This acidic pH may be increasing the harmful effect of metal in case of water consumption. Electrical conductivity values vary from 278.80 to 7466.00 µS/cm and this wide range indicate a probably contamination by leachate especially in the piezometer P3 located about 20 m away from infiltration basin. Indeed, only this piezometer is characterized with EC large over WHO recommended value. Other parameters analyzed in water samples of piezometers were below the WHO limits except for P3, which has high concentrations of HCO₃^-, Na⁺, K⁺, NH₄⁺, Ca²⁺, Cl^- and Pb. The high chloride content is well correlated to high conductivity in this piezometer (P3). The percentage noncompliance of chloride content and electrical conductivity is 100 during all sampling campaign. Even, the Aképé landfill is situated in the coastal sedimentary basin (continental Terminal aquifer) subjected to the intrusion of seawater, the high content of chloride in water sample P3 could be explained by leachate infiltration in this piezometer. The metals known as heavy metals or traces metallic elements are undesirable elements in water for many purposes such as drinking, agriculture, cooling etc. Copper, zinc and cadmium are below WHO limits for drinking purpose while lead was found slightly above the WHO limit in water sample P2 and P3 that are close to leachate basins. Groundwater contamination by lead was also reported by Han et al. ³⁰ when studding the impact of an uncontrolled landfill on surrounding groundwater Zhoukou in China and by Fatta et al. ²⁸. The toxicity of heavy metals depends on their concentration: it can be essential to life at very low doses (micro-nutrients for enzymatic transformation) and become inhibitors or toxic for biological systems above a certain concentration threshold. For example, high levels of lead in drinking water are a cumulative poison for humans, passing rapidly through the bloodstream and attaching to red blood cells.

The water quality indexes of individual samples are presented in Table 7. The Water Quality Index (WQI) is used to assess and classify water quality. It shows the impact of each physicochemical parameter on water quality. WQI indexes varied from 14.91 to 22.04; 20.20 to 68.34 and 153.34 to 367.63 for piezometers P1, P2 and P3 respectively. The average values were 17.75, 36.13 and 269.32 respectively. Based on WQI, water sample are classified as excellent water (P1 and P2) and very poor water (P3).

In order to clearly visualize the major trends and probable origins of the chemical elements in the piezometers water, in particular piezometer P3, which has poor quality (WQI= 269.32), a principal component analysis was carried out using XLSTAT software.

PCA is a powerful tool for compressing and synthesising information, and can be used to process and interpret a very large amount of quantitative data ³¹. This method makes it possible to specify the relationships between variables and the phenomena at the origin of these relationships. It is widely used to interpret hydrochemical data ^{15, 32}. The statistical analysis of the physico-chemical data obtained by analyzing the water from piezometer P3 was carried out on a data matrix consisting of seventeen physico-chemical parameters determined in this water over 5 sampling campaigns, i.e., 17 variables and 5 individuals.

Analysis of results shows that most of the information is explained by the first two factorial axes (Figure 4). The F1xF2 factorial design was taken into consideration to describe the correlations between the variables related to spatial structures, which alone account for 83.17% of the total information, with 49.89% for the F1 axis and 33.28% for F2.

The F1 axis is expressed positively by pH, EC, NO₃^-, HCO₃^-, Na⁺, K⁺, TH, Ca²⁺, SO₄^2-, Cl^- and Zn and the F2 axis by NO₂^- NH₄⁺, Mg²⁺ and Pb. The Table 8 shows the values of the different variables that make up the different factorial axes. It can be seen from this table that the values in bold correspond to the values for which the cosine squared is greatest for each axis. The higher the cosine squared (tending towards 1), the more the parameter is linked to the factorial axis.

The correlation between electrical conductivity and the ions nitrate, ammonium, calcium, magnesium and chloride is significant; it explains that the mineralisation of the water in piezometer P3 is largely due to these ions. We should also note the significant correlation between sodium, bicarbonate, sulphate, potassium and zinc and their grouping (Figure 5). This suggests a common source and an identical mechanism for dissolving these elements. This source may be of natural origin: water-rock contacts or anthropogenic input: infiltration of leachate, given the higher concentration of these elements in this piezometer than in the others.

To visualise the trends of the elements in general in the groundwater of the site, the PCA was carried out on the physico-chemical data obtained by analysis of the water from the three piezometers on a data matrix consisting of seventeen physico-chemical parameters, i.e., 17 variables and 3 individuals. Analysis of the correlation circle supported by the correlation matrix shows that the F1-F2 factorial plane accounts for 100% of the total inertia, so the PCA results will be analysed on this plane. The F1 axis represents the main component with 96.05%. All the parameters analysed are situated at the positive pole (figure 4) with a significant correlation coefficient (greater than 0.80). The projection of the individuals in the space of the F1-F2 statistical units (Figure 6) divides the different samples into two groups of water: good quality waters (P1 and P2) are grouped together at the negative pole of the F1 axis and poor quality or polluted water (P3) at the opposite pole. This projection confirms the values found in the calculation of the water quality index for these waters by classifying them into the two groups mentioned above.

Analysis of the chemical results (major cations and major anions) by the hydrochemical method using Piper's triangular diagram shows that the groundwater at the site through the available piezometers is divided into three types of facies (Figure 7) which are:

- Calcium - magnesium chloride - sulphate facies: P1

- Calcium - magnesium - bicarbonate facies: P2

- Sodium - potassium - chloride or sodium sulphate facies: P3.

5. Conclusion

At the end of this study, it emerges that the leachates generated by the Aképé technical landfill center are characterized by highly pollutants contents resulting to high pollution index values. These leachates can be assigned as stabilized or old leachates with slightly biodegradability organic content showing methanogenesis as predominate phenomena occurring in the landfill. Heavy metal contents are below standards, however, its accumulation towards regular discharge will have negative effect on biodiversity and waters resources in the area. Physico-chemical analysis of the water in the site's piezometers reveals significant degradation in sample from piezometer (P3), resulting in high index. The water quality deterioration in P3 piezometer could be assign to its shallow depth (30 m) leading to rapid infiltration of water from inflation basin given their proximity.

References