Laboratory Studies on The Effect of MSW Leachate on Soil by Using One-Dimensional Soil Column Method

To understand the soil-pollutant interaction geo-environmental engineer design a hydraulic barrier to control the transport of pollutant through the soil and minimize the pollution of affecting the soil properties. Hence, in this study one-dimensional soil column method was stimulated similar to the eld conditions. The extraction of leachate from the municipal solid waste was manipulated based on the annual precipitation data. The test result showed that the concentration of extracted leachate was increased up to 45 days and slightly decreased by increasing the addition of water. The interaction of extracted leachate changes the soil characteristics (i.e., increasing tendency by period). The result showed that the LL and PL of the soil was increased by increasing the addition of leachate. The UCS was increased from 167.80 kPa to 176.54 kPa up to 75th days and then decreased gradually at 95th to 105th days. Conversely, hydraulic conductivity was decreased at the initial stage and then gradually increased at the end of the experiment. These changes in the soil characteristics are mainly due to the alters of soil particles into dispersive structure with more effective pore space, which reduces the soil strength and increases the permeability value.


Introduction
Due to fast population development and industrialization, large quantities of waste have been produced in the last year and these wastes is simply disposed of in the land ll without proper precaution. When the rainwater enters into the municipal solid waste it undergoes the decomposition process and leaches out the highly contaminated liquid, which pollutes the soil properties and ground water system (1,2). To understand the soil-pollutant interaction is important for the geo-environmental engineer to design a hydraulic barrier to control the migration of pollutant through the soil and changing the soil properties.
Extensive literature has been available on the groundwater pollution due to the effect of MSW leachate (3,4). However, little effort was made to determine the impact of MSW leachate on soil properties. In addition, most researchers have used various types of chemical to understand the geotechnical behavior of soil pollutant interaction. But there was no attempt to simulate the contamination scheme close to the situation of the eld, because the condition of the eld is considerably different from the conditions controlled. To achieve this goal, in this module one-dimensional soil-column method has been used to simulate the eld conditions in the laboratory studies over a long-term period. For this purpose, the leachate has been extracted from the municipal solid waste similar to the eld condition by calculating the annual precipitation data. The extracted leachate has been passed into the soil column to determine the changes in the geotechnical behavior of soil.

Materials And Methodology
To determine the soil-pollutant interaction, the available leachate is not su cient to perform the laboratory experiment. The municipal solid waste was therefore collected from the dumping sites in order to prepare the leachate, and uncontaminated natural soil is also collected in an airtight polythene bag.

Municipal Solid Waste
The collected MSW was carried to the laboratory and placed under ordinary laboratory temperature in the air-tight container until it was actually used. Municipal solid waste involves all kinds of waste such as cooking, paper, cardboard, clothing, plastic and glassware etc.

Natural clay soil
The uncontaminated natural soil was collected from the dumping site and stored until its real use in the air-tight polythene bag. Air is dried and crushed into the wooden mallet before undertaking the laboratory testing of the soil sample and then testing was performed as per IS 2720. The geotechnical properties of soil sample as shown in Table 1. The extraction of leachate was done by taking ve barrels with 68cm height and 35cm diameter. In the bottom of each container a metal wire mesh has been provided. Above the wire mesh 10cm thickness gravel layer was placed to prevent the blocking of the waste which enabled to ow the leachate continuously. The weight of the MSW lled in each barrel has been calculated depending on the real density of the municipal solid waste, which is approximately 700 kg/m 3 in the container (without compaction) and 1,200 kg/m 3 in the land ll, the average is 950 kg/m 3 . The weight of the municipal solid waste introduced to each barrel has been calculated by the following equation (1) Weight = Density X V olume …… (1) Where, From the equation the weight of each sample was calculated as 57 kg and was placed into the barrel without compaction.

Amount of water needed to extract leachate from MSW
The amount of water needed to extract leachate from MSW has been calculated by the annual precipitation that is taken in the Puducherry region by using the following Eq. 2. (2) Where, L = Depth of the leachate water, mm P = Precipitation, mm The leachate coe cient (C) in this study was taken as 0.15 depending on the average daily precipitation in Puducherry (Indian meteorological department).
Depending on this data the quantity of water required for each container has been calculated by using the Where, W = amount of the water to be added, mL L = Depth of the leachate water, mm A = Area of the container or Cylinder, m 2 .
The quantity of water was distributed within 60 days in each container. Since the average rainy days take place in Puducherry region is between 45 days − 60 days (Department of Agricultural and farmer welfare).
The rainfall and evaporation take place in winter season and the laboratory experiments were performed in the month of February and March as shown in Table 2.

MSW Leachate effect on soil using a one-dimensional technique of soil columns
The experimental set-up comprises of seven PVC pipe soil columns. Figure 1 indicates the seven-soil columns schematic perspective. Each column was made by uni cation of 90 mm outer diameter and 80 mm height with an anti-corrosive coated hopper portion with a wire mesh placed between the soil-column and hopper portion. An outlet control valve was installed in the hopper portion to regulate the outlet ow, which is connected to a drain outlet (8 mm dia).
Page 6/13 The leachate extracted from the MSW was percolated into the soil column by feeding through the feeding tank (140 mm dia x 450 mm height) tted with the inlet control valve at the bottom and the inlet feed tube

Characterization of extracted leachate
The characterization of extracted leachate is shown in the Table 3. The pH value of the leachate was slightly decreased from 7.22 to 6.83 at the end of 45 days and then increased at last 15 days. The decrease in pH value signi es that the degradation process has taken place between the municipal solid waste and the water. The pH value reduced as the time period increased due to the biodegradation process of the carbohydrate present in the leachate changed into fatty acids is the major impact in the decrease of pH, EC and TDS values was increased from 5328mg/L to 5832mg/L at the end of 45 days and then slightly decreased at last 15 days (5). The increase in EC and TDS values shows that the presence of inorganic content in the leachate. Further addition of water a dilution process has been taken place and decreases the values. Similar explanation was stated (6), the uctuation in EC and TDS values principally related to the concentration of inorganic matter present in the municipal solid waste leachate, further addition of water the dilution process has been taken and reduces the concentration of inorganic matter during the short experimental period of time. COD values has been increased from 2130mg/L to 4166 mg/L and then slightly decreased. The increase of COD value signi es that the presence of organic content in the leachate. Similarly, other parameters such as Ca, Mg, Cl and Alkalinity were initially increased and then decreased. The increase of chloride is responsible for the dissolved solid present in the leachate. The results of the test show that the amount of leachate obtained is signi cantly greater than the leachate of the dumping site. Since, the concentration of fresh leachate is higher than the matured leachate.

Effect of geotechnical properties of soil due to MSW leachate
The geotechnical characteristic of soil is highly contaminated due to the effect of leachate. Onedimensional soil column process is stimulated to assess the effect of leachate on the soil sample. Seven soil columns were taken and each column was lled with soil and the leachate was passed continuously through the soil by the inlet valve pipe for 105 days and every 15 days the soil sample were taken and analyzed. The long-term interaction of leachate with the soil changes the soil, the behavior and properties due the chemicals present in the leachate.

Variation in Consistency limits
The liquid limit, plastic limit and plasticity index was found to be increased for 75 days and after that to a lesser extend at last 30 days as shown in the Fig. 2. The changes in liquid limit, plastic limit and plasticity index are principally related to the chemicals in the leachate which stimulate the aggregation of particles and increase the content of clay. Hence further addition of leachate and time period alter the soil particle into dispersive and reduce the diffuse double layer thickness of soil, thus may led to decrease the consistency limit of the soil. Conversely shrinkage limit was decreased by increasing the addition of leachate. The decrease in shrinkage value mainly due to chemical present in the leachate increases the soil particle occulation, thus resulting in higher value of shrinkage limit (7).

Variation in Proctor Compaction
Page 8/13 The proctor compaction test has been conducted to assess the MDD and OMC changes take place in the contaminated soil. The test result shows that there is a small reduction in maximum dry density. It was changed from 1.77 g/cc to 1.64 g/cc and optimum moisture content was changed from 17.05-18.45% as shown in the Fig. 3. The reduction in maximum dry density is related to the changes in the microstructure of the soil particles.

Variation in Uncon ned compressive strength
The variation in UCS is shown in the Fig. 4. The UCS was increased from 167.80 kPa to 176.54 kPa up to 75 days and then the value was slightly decreased at the end of 90th to 105th days. The increase in UCS is principally due to the interaction of MSW leachate into the soil changes. The soil particle aggregation to a occulated structure and also depends upon the percentage of clay content. However, the further addition of leachate and interaction time period changes the soil particles into dispersion thus leading to the reduction in UCS of the soil (8).

Variation in Hydraulic Conductivity
The hydraulic conductivity was decreased gradually at the beginning of contamination time and then slightly increased and stabilized at the end of the experiment. The test outcome indicated that the hydraulic conductivity was changed from 8.36x10 − 5 cm/s to 1.28x10 − 4 cm/s as shown in the Fig. 5. It noted that the reduction in hydraulic conductivity was primarily due to the bio-clogging impact of the bacteria present in the leachate. However further addition of leachate and time period collapse the bioclogging effect and changes the soil structure which affect the permeability of the soil, thus may increase the hydraulic conductivity value (9).

Scanning Electron Microscope Imagery (SEM)
A microscope experiment was used to observe modi cations in the contaminated soil microstructure. The Fig. 6 represents the SEM-micrograph of the unaffected soil sample exhibit layered structure and they are uniformly arranged. After contamination the soil particle is formed by agglomeration of small grains with dispersive microstructure as shown in the Fig. 7. The dispersive structure in the soil sample is leading cause for increasing the permeability value (10).

Conclusion
A laboratory experiment was performed using a one-dimensional column set-up technique to determine the soil-pollutant interaction. The following ndings are drawn on the basis of the research.

I. Characteristics of prepared leachate from the MSW
The test outcome indicates that the leachate level increased up to 45 days and then started to slightly decrease over the 50th to 60th day due to leachate dilution. However, the concentration of prepared leachate differs slightly from the leachate of the land ll. The pH value is 6.86, that is slightly acidic and the other parameter such as EC, TDS, chloride, calcium, magnesium, alkalinity and COD exceeds the permissible limit of Standard leachate disposal. The elevated TDS and COD concentration exhibits the existence in the leachate of both organic and inorganic elements.

II. Soil Characteristics
The soil sample long term effect of MSW leachate increases the soils liquid limit, plastic limit, and plasticity. Conversely, after contamination, it decreases the shrinking limit value. The uncon ned compressive strength increased up to 75 days from 167.80 kPa to 176.54 kPa and reduced over the 95th to 105th day. Conversely, in the original phase, hydraulic conductivity decreased and gradually increased in the nal phase. These changes in soil characteristics are primarily due to modi cations in soil particle microstructure and the effect of leachate chemicals. The chemicals present in the soil particle leachate into a dispersive structure with an e cient pore space that reduces soil strength and increases the signi cance of permeability.

Declarations
Availability of data and materials All the data generated or analyzed during this study are fully available in the supplementary materials. Variation in Consistency limits Scanning uncontaminated soil sample electron microscopy Scanning contaminated soil sample electron microscopy