MONITORING OF LEACHATE QUALITY AT A SELECTED MUNICIPAL LANDFILL SITE IN PODLASIE , POLAND

The paper presents an evaluation of leachate monitoring data as well as investigation of temporal and seasonal variability of leachate quality. The analysis includes 27 sampling carried out at selected municipal landfill during 7-year observation period. In leachate samples, concentration of Pb, Cu, Zn, Cr, Hg, Cd, Total Organic Carbon (TOC), Polycyclic Aromatic Hydrocarbons (PAH) and value of pH and electro-conductivity (EC) were determined. Estimation of landfill leachate demonstrated a declining trend in EC, TOC and Zn concentration with time. Concentration of Pb, Hg, Cr, Cd and PAH remained relatively constant with respect to the sampling time, however, the value of pH showed a growing trend due to the decrease of the concentration of the partially ionized free volatile fatty acids. Average concentration of heavy metals in leachate were observed in trace quantities (<1.0 mg·dm-3). There were no statistically significant seasonal variations in concentration of the analyzed parameters, however, the highest concentration of EC, TOC and PAH were observed in summer. The negative correlation of pH with Pb, Cu, Zn, Cr, Hg, Cd indicated a strong connection of leachate’s pH with heavy metals concentration. A cluster analysis allowed to specify two groups of variables: cluster I with TOC as an indicator of organic pollution in leachate and claster II with the rest of parameters. Further systematic monitoring is necessary, but its scope, according to the author, should be redefined.


INTRODUCTION
The deposition of municipal solid waste in landfills is the most common management alternative in most countries [Kjeldsen et al. 2002, Eurostat 2015].One of the products which are produced during landfill operation is leachate.They are the result of undergoing physico-chemical and biological changes in landfill body, percolation of rainwater through the wastes and inherent moisture content in the waste [Theepharaksapan et al. 2011, Ahmed andLan 2012].Leachate is formed when moisture content of waste exceeds the field capacity, defined as the total amount of moisture that can be retained in a waste sample subject to the downward pull of gravity [Durmusoglu and Yilmaz 2006].Combined physical, chemical and microbial processes in the waste transfer pollutants from the waste material to the percolating water.
Many factors influence leachate composition including the types of wastes deposited in the landfill, moisture content, the particle size, the hydrology of the site, the climate, and age of the fill, and other site-specific conditions including landfill design or type of liners used [Kumar and Alappat 2005].However, the biodegrability of organic content in the MSW and compaction of the waste layers make the landfill all anaerobic environment, giving many similarities to the composition of leachate between different landfills [Kjeldsen et al. 2002].
According to Christensen et al. [2001], focusing on the common type of landfill receiving a mixture of municipal waste, but excluding significant amounts of concentrated specific chemical waste, landfill leachate may be characterized as a water based solution of four groups of pollutants [Christensen et al. 2001]: • dissolved organic matter, expressed as Chemical Oxygen Demand (COD) or Total Organic Carbon (TOC), including CH 4 , volatile fatty acids (in particular in the acid phase of the waste stabilization) and more refractory compounds for example, fulvic-like and humiclike compounds, • inorganic macrocomponents: Ca, Mg, Na, K, NH 4 + , Fe, Mn, Cl, SO 4 2-, HCO 3 -, • heavy metals: Cd, Cr, Cu, Pb, Ni and Zn, • xenobiotic organic compounds (XOCs) originating from household or industrial chemicals and present in relatively low concentrations in the leachate (usually less than 1mg×dm -3 of individual compounds).These compounds include, among others, a variety of aromatic hydrocarbons, phenols and chlorinated aliphatics.
Other compounds may be found in leachate from landfills: e.g.B, As, Se, Ba, Co.But in general they are found in very low concentrations and are not often monitored.
Due to leachate's characterization, they have been identified as one of the major threats to groundwater resources [Mor et al. 2006a].So the monitoring of leachate is among the most important factors to be considered in planning, design and operation of MSW landfills [Umar et 24.07.2006] are often used.None of these methods is fully adequate, so interpretation of leachate analytical data is difficult and may be not-objective because of the lack of clear standard limits.Another problem is the scope of monitoring, focused mainly on the analysis of heavy metals, which does not fully reflect the degree of leachate contamination and their impact on the environment.
The main objective of the present study is to evaluate leachate data of 7-year monitoring period from selected landfills in Poland.The variability of leachate quality and the direction of the analyzed parameters changes during the study period were determined.Particular attention was paid to seasonal variability of leachate.The relationship between the analyzed parameters were also examined and explained by the use of correlation and cluster analysis.

Area of study
The analyzed landfill is located in southeast part of Podlasie Province, Poland (53 0 3'54.3739''N,28 0 8'49.6621''E).The climate in this region is continental with close to 50% rainfall occurring between May and August.The average rainfall is about 550 mm per year.The average temperature is between -5 °C (in January) and 17.3 °C (in July).The average annual temperature is about 7 °C.
The analyzed landfill is one of the biggest landfill sites in Podlasie and it has been operated since 1981.The total landfill area is 40 ha.The landfill is designed for municipal waste with exception of fluid waste, hazardous substances, radioactive and toxic waste.The total amount of solid waste deposited in the landfill at the end of 2011 was estimated at 375 000 m 3 .Remaining free capacity is estimated at about 300 000 m 3 , which will deposit about 200 000 Mg of waste.The landfill consists of four cells, from which the oldest one -cell A-(closed in 2001) is not equipped with a liner system and is sealed with 50cm clay layer to protect groundwater (Figure 1).
The other three other cells are protected with the 2.5-mm-thick geomembrane (PEHD) placed at the bottom.In these cells, the leachate is collected by perforated pipes on top of the liner and flows by gravity to pumping station.Then, pumps transfer the leachate to the retention reservoir.Leachate amount is about 25 000 m 3 per year.
The area around the landfill is covered by a sand formation, which is underlain by a complex of boulder clay.The free groundwater table lays 0.95 to 5.5 m below the land surface.The landfill is underwashed on the west side by groundwater that flows down in north-easterly, south-easterly, and easterly directions.

Data analysis
Leachate samples for analysis were collected according to monitoring regulations -four times a year starting from March 2004 until December 2010.The sample used in this study were collected from the leachate reservoir, then transported to laboratory and stored at 4 °C.The following parameters were monitored: pH, EC, TOC, Pb, Cu, Zn, Cr, Hg, Cd, PAHs.The analyses were done in commercial and accredited laboratory of Regional Inspectorate of Environmental Protection in Bialystok.Determinations were carried out according to the Polish Standards.pH was measured using potentiometric method (according to PN90/C-04540-01), EC -using conductivity method (PN-EN27888:1999).The soluble forms of metals were determined in samples filtered through micropore filtres (f = 0.45 mm).Digestion of metals was performed applying microwave digestion system.The heavy metals -except for Data analysis included basic statistics (mean, standard deviation, median, minimum, maximum), linear regression of chemical variation vs. time to determine change direction, analysis of variance to determine effect of season on leachate quality.Correlation analyses were performed to show the relationship between the chemi-

Characteristics of landfill leachate
The characteristic of leachate samples collected from the landfill site during its monitoring in years 2004-2010 has been presented in Table 1 Kulikowska and Klimiuk [2008] pH of leachate increase with time due to the decrease of the concentration of the partionally ionized free volatile fatty acids.In the methanogene phase, the increasing number of methane bacteria results in a net utilization of intermediate products.During study period the value of pH was within acceptable limits for wastewater introduced into water and soil.The average EC value of the collected samples was found to be 15.20 mS•cm -1 , which is relatively not very high.In Poland, there is no standard limits for this parameter in wastewater.One of the parameters, which are used to describe the content of dissolved organic matter in leachate, is TOC.A typical concentration range of TOC in MSW landfill leachate is 30-30 000 mg•dm -3 [Kjeldsen et al. 2002].the results in the table show that the monitored leachate had a low organic content and the highest TOC concentration in the analyzed leachate did not achieve 5500 mg•dm -3 .However this value exceeds the Polish standard limit for wastewater introduced into water and soil, which is 30 mg•dm -3 .
The highest Pb, Cu, Zn, Hg and Cd concentration in the analyzed leachate did not exceed 1 mg•dm -3 .Only Cr(VI) concentration reached its maximum of 1.87 mg•dm -3 , although its mean was not high -0.32 mg•dm -3 .The presence of Zn in leachate shows, that landfill receives waste from batteries and fluorescent lamp.The presence of Pb in the leachate samples indicates probably the disposal of Pb batteries, chemicals for photograph processing, Pb-based paints [Mor et al. 2006a, Mor et al. 2006b, Moturi et al. 2004].So, the reason for the low concentration of heavy metals in analyzed leachate is not a lack of heavy metals present in waste.Heavy metal balances for landfills have shown that less than 0.02% of heavy metals received at landfills are leached from the  -3 , with mean value of 0.25 mg•dm -3 .PAH standard limit for wastewater discharging into sewage system is 0.2 mg•dm -3 .There were observed two peaks with value exceeding the standard limit for wastewater, which occurred in time of highest precipitation: in June 2006 and September 2008.Despite this, the concentration of PAH can be taken as stabile during seasons and years.

Temporal and seasonal variation
The variation in chemical composition of leachate for the period from March 2004 to December 2010 is shown in Figure 2 and Figure 3.
The evaluation of temporal trends is based on the slope of linear regression between time and concentration of each analyzed parameter.Table 1 lists the trend as either increasing (↑), decreasing (↓) or stable (~), depending on the value and slope of regression line.
Values of pH were relatively constant with low increase tendency.Effect of pH on the mobility of heavy metals was evident in 2009, when the increase in pH caused a decrease in the concentration of Pb, Cd, Hg and Cd in the leachate.Value of conductivity showed great differences than pH.As time goes, its value decreased from 17.02 mS•cm -1 (mean value in 2004) to 7.38 mS•cm -1 (mean value in 2010).The highest values of conductivity are achieved during the summer months (mean value for June is 18.69 mS•cm -1 ) and the lowest in spring (mean value for March -12.96mS•cm -1 ).This may be due to higher precipitation in summer and leaching from waste a number of soluble minerals.About 12% of percolation in Poland falls on May, and 22% -on May and June.The highest concentration of TOC was recorded in 2004 -549 mg•dm -3 .TOC concentration decreases over time, which is result of organic matter decomposition and its weight and volume loss in the waste.The highest concentration of TOC, exceeding of 1000 mg•dm -3 , falls on June and slightly lower -on September.As in the case of conductivity, it is due to both higher values of rainfall in this period and intensified degradation processes of organic matter which is a result of temperature increases in summer months.
The metals: Pb, Cu, Cr, Cd, Hg all displayed relatively stabile trend (Figure 3).Concentration of all heavy metals are low because landfilled waste, especially at the neutral or above neutral pH values, has significant sorptive capacity and enhances metal hydroxide precipitation  -3 .This high concentration of PAH in leachate may come from intensive infiltration and pollutants leaching from waste body at time of high rainfall as well as from washing out from the road pavement contamina-tion coming from car exhausts, from wearing off car tire, and from asphalt rich in hydrocarbon fractions, which all next can flow along with surface run-offs into open leachate reservoir.
The evaluation of seasonal variation was made with use of variation analysis, which results are resented in Table 2. Obtained data have shown that there is no statistically significant difference in observed concentration of analyzed parameters in each season of year.However, the concentration of analyzed parameters differs in each part of the year.Value of PAHs, EC and TOC increased in June (summer), while concentration of Pb, Cu, Zn, Cr(VI), Hg and Cd was relatively stable during the year.Mobility and migration of heavy metals is reduced by the sorption by organic matter.Heavy metals may also be submitted to sorption by hydrated oxides of Fe and Mn, as well as undergo precipitation, especially in the presence of sulfide ions [Christensen et al. 2001

Correlation and cluster analysis
Correlation analysis is a preliminary descriptive technique to estimate the degree of association among the variables involved [Mor et al. 2006a].Such association is likely to lead to reasoning about relationship between the variables.Correlation matrix between analyzed parameters is shown in Table 3.Some parameters were found to bear statistically significant correlation with each other indicating close association of these parameters.Pb had a positive correlation with Hg, which suggests a similar chemical behavior and similar source of these metals.Both of these metals are specific for road runoff, which contains Pb (from vehicle exhaust) and Hg (from the abrasion of car tires).From Figure 3 it is seen that the highest concentration of Pb and Hg occured in time of spring meltdown, which means, that at this time contamination accumulated in the snow during winter time get into the leach- The above-mentioned interdependence is confirmed by the results of cluster analysis, which was used to classify the studied variables in such a way, that the degree of variables relationship in the same group was the largest.Ward's agglomeration method was used for connection of variables and Euclidean distance for a measure of distance.
The conducted analysis allowed to distinguish two clusters (groups) of variables: cluster I with TOC as a indicator of organic pollution in leachate and cluster II with the rest of parameters.Inside cluster (group) II we can isolate three subgroup: The result of the analysis indicate that for the stabilized landfills, in methanogenic phase, it could be reasonable to limit the number of analyzed heavy metals.Example of heavy metals situated close to each other in one cluster are: Pb, Hg from subgroup IIA and Cd, Cu, Zn, Cr(VI) from subgroup IIB.The question is, if for a long time operated landfill, in methanogenic phase with a high value of pH, is reasonable to analyze the concentration of six heavy metals.Especially that some of them show a strong correlation between each other.It should be reasonable to consider replacing the determination of

CONCLUSIONS
The study presents results of leachate quality data from seven years of its monitoring.The high value of pH, low TOC and EC value and low heavy metals concentration pointed out that the landfill is in methanogenic stage.The average concentration of most analyzed parameters does not exceed standards limits for wastewater discharging into sewage system and for water putting into water and soil.Exception is TOC and Cr(VI), which concentration surpass permissible standards.Results for the EC, TOC and Zn indicate a decrease in concentration, with time, while the rest of analyzed heavy metals and PAH remained stable during the sampling period.
There was no observed statistically significant seasonal variation of the analyzed parameters during the 7-year study period, however the highest concentration of EC, TOC and PAH were observed in summer.Correlation analysis indicated a strong association between Cu and Hg, Cr(VI), PAH.It also pointed the negative correlation between pH and the heavy metals concentration in the leachate.The cluster analyzes has specified two groups of analyzed variables: group I with TOC as a indicator of organic pollution in leachate and group II with all heavy metals.The metals situated close to each other in one cluster are: Pb, Hg from group IIA and Cd, Cu, Zn, Cr(VI) from group IIB.Although, the concentration of most analyzed contaminants do not exceed the literature value, the further systematic monitoring is necessary, but its scope, according to the author, should be redefined.

Figure 1 .
Figure 1.Localization of analyzed landfill site

Figure 2 .
Figure 2. pH and EC value and TOC, PAH concentration in leachate vs. time

Figure 3 .
Figure 3. Heavy metals (Pb, Cu, Zn, Cr, Hg, Cd) concentration in leachate vs. time TOC, Cu, Zn, Pb, Cd, Cr(VI), Hg, Polycy-clic Aromatic Hydrocarbons (PAHs).Frequency of leachate monitoring in accordance with regulations is: every three months for operating landfills and every six months for closed landfills.Although monitoring of landfill leachate is required, it does not work satisfactorily.The main problem of leachate monitoring in Poland is the lack of standards limits to which one could compare the leachate data obtained during monitoring.For the interpretation of the results the standards for discharging wastewater into sewage system [Journal of Laws from 14.07.2006] or standards for putting of wastewater into water and soil.[Journal of Laws from . The table includes also the direction of change with time for each analyzed parameter and the standard limits, which are used in Poland in leachate quality interpretation: A -standard limits for industrial wastewater discharging into sewerage system (Journal of Laws from 14.07.2006) and B -standard limits that must be met during releasing wastewater into water and soil (Journal of Laws from 24.07.2006).All values in the table are within the range of leachate quality data presented in literature [Bocanegra et al. 2000, Kjeldsen and Christophersen 2001, Tatsi and Zouboulis 2002, Al.-Yaqout and Hamoda 2003, Renou et al. 2008, Abbas et al. 2009].The pH of leachates was in the range 7.1-8.6,with mean value of 8.1.According to Długosz [2012] and

Table 1 .
[Christensen et al. 2001, Kjeldsen et al. 2002,rend of change directionØygard et al. 2004, Liu andSang 2010].Analyzed heavy metals -except the Cr(VI) -do not exceed the Polish standard limits.Both sorption and precipitation are believed to be a significant mechanism for metal immobilization[Christensen et al. 2001, Kjeldsen et al. 2002, -A -standard limits according to Regulation of Minister of Building concerning the obligations of supplier of industrial wastewater and the conditions for discharging wastewater into sewerage system (Journal of Laws from 14.07.2006).B -standard limits according to Regulation of Minister of Environment concerning the conditions that must be met during the putting of wastewater into water and soil (Journal of Laws from 24.07.2006).Concentration change with time: ↑ increase, ↓ decrease, ~ stable All in mg•dm -3 except EC (mS•cm -1 ) ↓and pH landfill after 30 years [

Table 2 .
The mean value of monitored parameters in each season of the year and results of variance analysis SS -sum of squares between groups; df -degree of freedom; MS -mean squares between groups; F -value of test F; p -probability level • IIA -with Pb and Hg negatively correlated with pH, low pH values cause an increase in the mobility of Pb and Hg; • IIB -with Cd, Cu, Zn, Cr positively correlated with each other and behaving similarly in different environmental conditions; • IIC -with EC and PAH as a general indicator of leachate pollution.

Table 3 .
Correlation matrix for analyzed leachate parameters Cluster analysis heavy metals by other analysis, providing more valuable information about the leachate quality, the stage of waste decomposition as well as the leachate impact on environment.