Differences in the Composition of Leachate from Active and Non-Operational Municipal Waste Landfills in Poland

Abstract

Leachate formation is one of the many environmental hazards associated with landfilling. The leachate may migrate from the landfill to surface water and groundwater, posing a potential threat to aquatic ecosystems. Moreover, its harmful effect on human health and life has been proven. Due to the risks that landfill leachates may pose, it is necessary to control the state of the environment in their surroundings. The paper presents an example of the application of selected statistical methods (basic statistics, statistical tests, principal component analysis) to assess the impact of individual pollution indicators on the quality of landfill leachates. The conducted analysis showed the existence of significant differences between the surveyed active (Legnica, Jawor) and non-operational (Wrocław, Bielawa) landfills in Poland. These differences were especially visible in the cases of the following: electric conductivity (EC) (non-operational landfills 1915–5075 μS/cm, active 5093–11,370 μS/cm), concentrations of total Kjeldahl nitrogen (TKN) (non-operational landfills 0.18–294.5 mg N/dm³, active 167.56–907.4 mg N/dm³), chemical oxygen demand (COD), organic nitrogen (ON), ammonium nitrogen (AN), total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), sulfates, chlorides, sodium, potassium, calcium, magnesium and nickel. Selected indicators should help to determine the progress of decomposition processes inside the landfill and the potential impact of leachate on the environment, and should be used in the mandatory monitoring of landfills.

1. Introduction

Leachate formation is one of the many environmental hazards associated with landfilling [1]. A leachate can be defined as a fluid that seeps through a landfill and is discharged from or contained in a landfill [2].

The leachate contains soluble organic and inorganic compounds, suspended particles and heavy metals [3]. As a result of the physical, chemical and microbiological processes taking place inside the landfill, the leachate takes over a number of substances, and as a result it becomes a highly polluted wastewater [4,5]. The composition of the leachate is dynamic and variable in time, depending on, among other things, the nature of the deposited waste and the chemical and biochemical decomposition processes taking place in it [6], the stabilization level of the deposited waste, the collection system, as well as the location of the landfill and hydrological factors [7,8].

As landfill leachate is one of the main pollutants of the soil and water environment, knowledge of its composition is important in determining the long-term environmental impact of landfills [9]. Leachate from municipal landfills usually contains dissolved organic and inorganic compounds, such as ammonia, calcium, magnesium, sodium, chlorides, iron and heavy metals (cadmium, chromium, copper, lead, nickel and zinc) [5,10,11,12].

The leachate may migrate from the landfill to surface water and groundwater, posing a potential threat to aquatic ecosystems [10]. Moreover, its harmful effect on human health and life has been proven [3]. Due to the risks that landfill leachates may pose, it is necessary to control the state of the environment in their surroundings [13,14].

Therefore, the European Union has introduced EU-wide regulations aimed at preserving and improving the quality of the natural environment, i.e., Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste [2]. For all EU countries, uniform general tasks for the monitoring of landfills in the operational and after-closure phases have been established. Their aim is to determine the potential impact of landfills on the environment and to select appropriate measures to regulate this impact. One of the most important provisions of the Directive on monitoring the water environment in the vicinity of municipal waste disposal sites is the requirements concerning the frequency of testing the volume and composition of leachate. They oblige member states to regularly monitor the leachate condition. Despite the introduction of these regulations, the scope of the parameters to be monitored is not specified. It is only specifies that the analyzed substances should be selected on the basis of the composition of the deposited waste. Due to such a situation, the scope of seepage water testing varies significantly between EU member states. This can be observed in the example of several selected EU countries.

In Luxembourg, landfill leachate monitoring is carried out under the Grand-Ducal Regulation of 24 February 2003 on the landfill of waste [15]. During the operational and after-closure phase, the parameters tested in the seepage water are determined after prior verification, in accordance with the expected composition of the leachate and the quality of groundwater, but not to a lesser extent than indicated in the Regulation. This includes 23 indicators, as follows: pH, electric conductivity, temperature, chlorides, fluorides, sulfates, nitrates, nitrites, sodium, potassium, ammonium nitrogen, free cyanides, chemical oxygen demand, heavy metals (copper, zinc, lead, cadmium, chromium, mercury, arsenic), total organic carbon, phenols and hydrocarbons.

In Portugal, the monitoring of seepage water during the operational phase of a landfill site and after its closure involves a total of 36 parameters analyzed at different intervals, as follows: once a month—pH, electric conductivity, chemical oxygen demand, chlorides, ammonium nitrogen; once every 3 months—carbonates/bi-carbonates, cyanides, arsenic, total chromium, chromium (VI), cadmium, mercury, lead, potassium, phenolic index; once every six months—total organic carbon, fluorides, nitrates, nitrites, sulfates, sulfides, aluminum, barium, boron, copper, iron, manganese, zinc, antimony, nickel, selenium, calcium, magnesium, sodium, adsorbed halogenated compounds (AOX), total hydrocarbons [16].

In Hungary, the parameters to be examined within the framework of seepage water monitoring are determined each time on the basis of the decision of an environmental protection authority, taking into account the composition of waste and hydrogeological properties of the landfill site [17]. Similarly, in Ireland, the parameters to be analyzed in landfill leachate are defined individually in the landfill permit. This range depends on the type and composition of the deposited waste [18].

In Poland, the scope, time, frequency, method and conditions of monitoring are specified in the Regulation of the Minister of the Environment of 30 April 2013 on the landfill of waste [19]. On its basis, the state of the water environment in the vicinity of the municipal waste landfill site is assessed at particular stages of its operation. Landfill leachate tests are carried out, covering 10 indicator parameters, i.e., pH, conductivity, sum of polycyclic aromatic hydrocarbons (PAHs), total organic carbon (TOC) and heavy metals content, including copper, zinc, lead, cadmium, chromium (VI) and mercury. Where a municipal landfill is equipped with a leachate treatment system, samples for physicochemical composition testing are taken at each point of leachate discharge from the landfill, in order to control the effectiveness of the treatment process.

The research on the volume and composition of leachate is carried out in both the operational and post-operational phase. In the operational phase, the tests are carried out every 3 months, while in the post-operational phase, every 6 months. No studies are required to establish a set of parameters for the monitoring of landfill leachate. It is possible to select additional parameters, but due to the costs, landfill managers most often order tests only within the binding scope.

The aim of the study was to analyze the differences between the compositions of leachates from active and non-operational municipal landfills. The indicators selected in this way should help to determine the progress of decomposition processes inside the landfill and the potential impact of leachate on the environment, and should be used in the mandatory monitoring of landfills.

2. Materials and Methods

2.1. Study Area

The research was conducted in the years 2018–2019 on four municipal waste landfills located in Lower Silesia (Figure 1). The oldest one was established in Wroclaw, in 1966. The area occupied by the landfill is 11.7 ha, and the capacity of the landfill is about 2 million m³. It was in operation until 2000. Another large landfill site is located in Legnica. The area occupied by the landfilled waste is 14.12 ha, and the total capacity of the facility is 2.34 million m³. The landfill has been operating since 1977 and consists of 6 sectors.

The research also covered two smaller landfills. The Bielawa landfill was put into operation in 2001 and closed in 2011. The area occupied by the landfilled waste is 0.86 ha, and its capacity is 37.8 thousand m³. The landfill site in Jawor has been in operation since 1997, and the area occupied by waste is 3.37 ha. Total capacity of the landfill is 231.3 thousand m³.

All landfills are equipped with leachate removal systems. Samples for physicochemical analyses were taken at leachate collection points (wells or tanks). Figure 2 shows the leachate collection points.

2.2. Physicochemical Composition of Leachate

The analysis of the quality of landfill leachates was carried out in 2018–2019. A total of 7 sets of leachate samples were tested, taken at the sites of their collection (tanks or leachate wells). In the leachate samples, the following were determined: pH, electrical conductivity (EC), and chemical oxygen demand (COD), and concentrations of total Kjeldahl nitrogen (TKN), organic nitrogen (ON), ammonium nitrogen (AN), total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), sulfates, chlorides, sodium, potassium, calcium, magnesium, iron, manganese, copper, zinc, chromium, lead, nickel and cadmium. Samples were taken every 3–4 months.

Immediately after being taken, the samples were transported in refrigerated conditions to the Environmental Research Laboratory of the Institute of Environmental Engineering, Wroclaw University of Life Sciences. pH, EC, COD, TKN, AN, TDS, TSS, sulfates, chlorides, sodium, potassium, calcium, magnesium, iron, manganese, copper, zinc, chromium, lead, nickel and cadmium were determined in accordance with the methodology presented in ISO standards [20,21,22,23,24,25,26,27,28,29,30,31,32,33]. The ON has been calculated on the basis of the equation ON=TKN-AN [34]. TS was calculated as the sum of TDS and TSS [35].

The results of landfill leachate composition tests were subjected to statistical analysis using Statistica 13.1 (StatSoft Poland, StatSoft, Inc., Tulsa, OK, USA). In the presented studies, non-parametric statistics were used, which are most suitable for small sample numbers (n < 100) and for those which do not show compliance with normal distribution [36]. To characterize the results of the physicochemical analyses of leachate from four landfills, descriptive statistics were used—median, maximum and minimum values—which allow us to present a data set regardless of its distribution. The analysis of differences between active (Legnica, Jawor) and non-operational waste landfills (Wrocław, Bielawa) was carried out using the Mann–Whitney U test (equivalent to the parametric t test) for independent groups, often used in many studies [37].

Based on the principal component analysis (PCA), the pollution indicators (variables) that best characterizes the composition of the tested landfill leachate were identified. The PCA can be used to reduce the number of variables describing the phenomenon under investigation. The determined components are a linear combination of the variables, among which those which have the greatest impact on the individual principal components can be indicated. This influence can be represented by eigenvectors and factor loadings. Eigenvalues indicate what proportion of total variance is explained by a given principal component. The factor loadings correspond to the correlation coefficients between the individual components and the variables under investigation [38]. This method has been used in numerous studies, e.g., to assess groundwater pollution by landfill leachate, to analyze the test results of the leachate itself or to choose the location of landfills [39,40,41,42].

3. Results

In

Table 1. Characteristic values (range: minimum–maximum, median) of pollution indicators of leachate from non-operational municipal waste landfills.

Pollution Indicators	Unit	Non-Operational Landfills
		Bielawa		Wrocław
		Range	Median	Range	Median
pH	-	7.80–9.10	8.80	7.60–8.50	8.30
EC	μS/cm	3048–5075	3657	1915–2553	2358
COD	mg O2/dm3	954–4270	2370	113–237	183
TKN	mg N/dm3	32.20–294.50	131.20	0.18–5.34	1.81
ON	mg Norg/dm3	14.60–182.80	80.50	0.16–5.34	1.54
AN	mg NNH4/dm3	17.6–231.2	109.4	0.002–0.580	0.020
TS	mg/dm3	2580–8745	3135	2225–2515	2280
TDS	mg/dm3	2140–3050	2605	2185–2460	2240
TSS	mg/dm3	105–5995	780	25–105	55
Sulfates	mg SO4/dm3	139.1–1884	256.7	58–1457	1234
Chlorides	mg Cl/dm3	5.5–765.0	636.0	0.82–125	104
Sodium	mg Na/dm3	132.3–285.8	188.8	109–208	113
Potassium	mg K/dm3	188.8–265.6	236.5	55–98	66
Calcium	mg Ca/dm3	69.5–194.3	103.2	166–339	281
Magnesium	mg Mg/dm3	61.3–133.4	74.2	51.4–68.2	60.8
Iron	mg Fe/dm3	1.60–18.03	8.33	0.019–4.780	3.900
Manganese	mg Mn/dm3	0.05–2.43	0.84	0.020–0.150	0.060
Copper	mg Cu/dm3	0.021–4.716	0.352	0.020–6.670	3.210
Zinc	mg Zn/dm3	0.263–1.572	0.693	0.123–1.515	0.338
Chromium	mg Cr/dm3	0.0005–0.497	0.065	0.0005–1.429	0.017
Lead	mg Pb/dm3	0.0005–0.179	0.066	0.0005–0.123	0.029
Nickel	mg Ni/dm3	0.0005–0.052	0.032	0.0005–0.530	0.017
Cadmium	mg Cd/dm3	0.0005–0.008	0.004	0.0005–0.013	0.004

and

Table 2. Characteristic values (range: minimum–maximum, median) of pollution indicators of leachate from active municipal waste landfills.

Pollution Indicators	Unit	Active Landfills
		Legnica		Jawor
		Range	Median	Range	Median
pH	-	8.0–8.9	8.7	7.4–8.6	8.2
EC	μS/cm	8417–11,370	10,411	5093–10,305	7427
COD	mg O2/dm3	1585–3800	3490	843–4110	2880
TKN	mg N/dm3	167.56–907.4	300.91	285.52–613.3	346.27
ON	mg Norg/dm3	4.54–121.31	99.85	32.33–248.27	112.44
AN	mg NNH4/dm3	66.07–786.09	460.91	148.57–460.91	256.08
TS	mg/dm3	6210–8245	7780	4975–22,750	7775
TDS	mg/dm3	6195–7830	7155	3470–5155	4630
TSS	mg/dm3	15–1870	330	275–19,280	3280
Sulfates	mg SO4/dm3	80.6–396.6	106.1	33.7–335.7	216.4
Chlorides	mg Cl/dm3	21.95–2811	2323	11.5–1481	1074
Sodium	mg Na/dm3	175.2–329.2	198.9	176.3–266.4	196.4
Potassium	mg K/dm3	238.2–317.2	281	231.1–308.8	257.3
Calcium	mg Ca/dm3	43.9–113.81	67.1	70.8–264.2	138.9
Magnesium	mg Mg/dm3	70.2–133.82	78.1	77.5–233.45	85.3
Iron	mg Fe/dm3	2.6–10.62	3.81	4.52–38.73	26.23
Manganese	mg Mn/dm3	0.2–0.55	0.31	0.24–14.48	1.61
Copper	mg Cu/dm3	0.07–4.03	1.44	0.142–4.07	0.954
Zinc	mg Zn/dm3	0.145–2.03	0.56	0.232–4.82	0.991
Chromium	mg Cr/dm3	0.023–0.58	0.3	0.025–0.59	0.157
Lead	mg Pb/dm3	0.0005–0.26	0.04	0.0005–0.44	0.059
Nickel	mg Ni/dm3	0.095–0.38	0.17	0.024–0.34	0.071
Cadmium	mg Cd/dm3	0.0005–0.01	0.0005	0.002–0.07	0.007

, the general physicochemical properties of leachate taken from the landfills covered by the study in 2018–2019 are presented. The pH of all tested samples was in the range 7.4–9.1. The highest range of variability was observed in the leachate from the non-operational landfill in Bielawa, and the lowest were found in the case of the non-operational landfill in Wrocław and the active landfill in Legnica. All analyzed leachate samples had an alkaline character, regardless of the phase of operation and the amount of accumulated waste.

Greater differences occurred in the case of EC. The leachate from non-operational landfills was characterized by EC in the range of 1915–5075 μS/cm, with lower variability in the leachate from an older landfill, non-operational since 2000 (Wrocław). Higher variability was observed in the case of the Bielawa landfill, which (despite being closed in 2011) may still undergo intensive processes of waste decomposition and leaching. The leachate from active landfills showed EC levels about twice as high. A larger range of fluctuations occurred in the case of the landfill in Jawor, while the landfill in Legnica was characterized by a more stable leachate EC level.

The situation was slightly different in the case of COD. The lowest values were observed at the non-operational landfill in Wrocław. On the other hand, the COD values observed at the second non-operational landfill (in Bielawa) remained at a level similar to that of the active landfill in Jawor. It can be assumed that the waste decomposition processes at both the non-operational and the active landfill sites are not yet stabilized, which causes such large fluctuations.

In the case of TKN, higher values were recorded in leachate water from active landfills. The leachate from the Legnica landfill showed the highest variability. A much smaller range of fluctuations occurred in the case of the landfill in Jawor. The leachate from the non-operational landfills contained lower amounts of TKN (the lowest in Wrocław, slightly higher in Bielawa). The AN concentrations were similar.

The highest concentrations of ON were found in the leachate from the active landfill in Jawor. The values for the leachate from the second active landfill (in Legnica) were significantly lower. Differences occurred also in the case of the ON concentrations in the leachate from non-operational landfills. As in the case of other parameters, the lowest concentrations were found in leachate from the Wrocław landfill. On the other hand, the values for leachate from the Bielawa landfill were quite high, exceeding those found in the case of the active landfill in Legnica.

The highest content of TS and the highest variability of the results were shown by leachate water tests conducted at the active landfill in Jawor. The second active landfill (in Legnica) was also characterized by quite high TS contents, yet they were much more stable. A very stable TS content was found in the case of leachate from the Wrocław landfill, while a slightly higher and more varied TS content was found in leachate from the Bielawa landfill.

The tests showed a stable level of TDS. The highest concentrations were found in leachate from active landfills. Lower, and at the same time stable, contents of TDS were found in the leachate from non-operational landfills. Much greater variability was observed in the case of TSS. The highest range of fluctuations was found in the case of the active landfill in Jawor, and the lowest in the case of the landfill in Wrocław.

The analysis of sulfate content in the leachate showed a larger concentration range in the case of inactive landfills. The greatest fluctuation occurred in the case of leachate from the Bielawa landfill, while the samples taken at the Wrocław landfill showed slightly lower fluctuation. The range of variability for active landfills was much smaller, and for both sites the sulfate concentrations during the study period did not exceed 400 mg SO₄/dm³.

The chloride content of leachate from the landfills covered by the study was different. The highest concentrations were found in samples taken at active landfills. The chloride content in the leachate from non-operational landfills was lower.

The sodium and potassium contents of the tested leachates showed similar variability. The lowest values were found in the case of the oldest landfill in Wrocław, and the minimum, maximum and median values for the remaining facilities did not differ significantly.

Analyses of calcium content in landfill leachate showed the highest characteristic values for the landfills in Wrocław and Jawor. Samples taken at the remaining landfills showed about half of the calcium concentrations. The highest concentrations of magnesium during the research period were found in leachate from the Jawor landfill. The leachate from the Bielawa and Legnica landfills showed similar magnesium concentrations, while the lowest concentrations of this component were found in leachate from the Wrocław landfill.

The concentrations of iron and manganese in the analyzed landfill leachate showed similar trends. The highest variability was observed at the active landfill in Jawor and at the non-operational landfill in Bielawa. Samples from other landfills showed smaller ranges of variability in the concentrations of the discussed components.

The analysis of the heavy metal content showed that the highest maximum values and the highest variability of the results were characteristic of the two landfills—leachate from the non-operational landfill in Wrocław showed the highest variability in concentrations of copper, chromium and nickel, while leachate from the active landfill in Jawor showed the highest variability in concentrations of zinc, lead and cadmium. The ranges of variability in the concentrations of heavy metals in leachate from the remaining landfills were much smaller.

Table 3. Analysis of differences between the leachate compositions of closed and active landfills (the highlighted results are significant with p < 0.05).

Variable	Sum of Ranks		U	Z	p	Z Corrected	p
	Closed	Active
pH	206.5	199.5	94.5	0.138	0.89037	0.138	0.89019
EC	105.0	301.0	0.0	−4.480	0.00001	−4.480	0.00001
COD	145.0	261.0	40.0	−2.642	0.00824	−2.642	0.00824
TKN	111.0	295.0	6.0	−4.204	0.00003	−4.204	0.00003
ON	136.0	270.0	31.0	−3.056	0.00225	−3.056	0.00225
AN	113.0	293.0	8.0	−4.112	0.00004	−4.113	0.00004
TS	117.0	289.0	12.0	−3.929	0.00009	−3.929	0.00009
TDS	105.0	301.0	0.0	−4.480	0.00001	−4.481	0.00001
TSS	155.0	251.0	50.0	−2.183	0.02907	−2.183	0.02905
Sulfates	248.0	158.0	53.0	2.045	0.04089	2.045	0.04086
Chlorides	129.0	277.0	24.0	−3.377	0.00073	−3.378	0.00073
Sodium	137.0	269.0	32.0	−3.010	0.00262	−3.010	0.00262
Potassium	126.0	280.0	21.0	−3.515	0.00044	−3.515	0.00044
Calcium	253.0	153.0	48.0	2.274	0.02294	2.274	0.02294
Magnesium	137.5	268.5	32.5	−2.987	0.00282	−2.989	0.00280
Iron	164.0	242.0	59.0	−1.769	0.07690	−1.769	0.07690
Manganese	161.0	245.0	56.0	−1.907	0.05654	−1.907	0.05651
Copper	209.0	197.0	92.0	0.253	0.80049	0.253	0.80049
Zinc	198.0	208.0	93.0	−0.207	0.83619	−0.207	0.83619
Chromium	161.0	245.0	56.0	−1.907	0.05654	−1.907	0.05651
Lead	194.0	212.0	89.0	−0.391	0.69613	−0.400	0.68939
Nickel	126.0	280.0	21.0	−3.515	0.00044	−3.520	0.00043
Cadmium	185.5	220.5	80.5	−0.781	0.43474	−0.782	0.43405

U-test statistic for a small sample size, Z-test statistic for a small sample size, p-significance level for the value of test statistics Z.

presents the results of the analysis of differences between the values of selected indicators of leachate contamination from operational and closed landfills. Statistically significant differences were characterized for EC, COD, TKN, ON, AN, TS, TDS, TSS, sulfates, chlorides, sodium, potassium, calcium, magnesium and nickel.

No significant differences were found between the concentrations of other heavy metals (copper, zinc, chromium, lead, cadmium), in spite of the changes observed in the literature following the age of the landfill [1].

Table 4. The results of the PCA for leachates from active landfills.

			Number of Principal Component	Cumulative Eigenvalue	Cumulative % of Total Variance
			1	7.193	31.273
			2	11.456	49.807
			3	14.496	63.028
			4	16.902	73.485
			5	18.973	82.491
			6	20.171	87.701
			7	21.204	92.193
			8	21.938	95.384
			9	22.519	97.907
			10	22.777	99.033
Factor loadings (correlations between variables and components)
Variable	Component 1	Component 2	Component 3	Component 4	Component 5
pH	0.518	−0.212	0.177	−0.280	−0.623
EC	0.796	−0.347	0.236	−0.100	−0.053
COD	−0.470	−0.010	0.749	0.143	0.007
TKN	−0.048	−0.450	−0.037	−0.533	−0.560
ON	−0.710	−0.108	−0.077	0.161	−0.545
AN	0.194	−0.356	0.136	−0.739	−0.355
TS	−0.421	0.483	0.005	0.315	−0.603
TDS	0.719	−0.150	0.627	0.034	0.058
TSS	−0.563	0.460	−0.181	0.272	−0.530
Sulfates	−0.449	0.394	0.497	−0.354	0.249
Chlorides	0.714	−0.318	0.115	0.460	−0.176
Sodium	0.367	−0.324	−0.596	−0.331	0.045
Potassium	0.273	0.095	0.661	−0.062	−0.192
Calcium	−0.861	−0.107	−0.035	−0.364	0.110
Magnesium	−0.435	0.210	0.299	−0.664	0.174
Iron	−0.858	−0.067	−0.067	−0.118	0.149
Manganese	−0.675	−0.642	−0.057	0.210	0.040
Copper	0.516	−0.144	−0.653	−0.073	0.178
Zinc	−0.767	−0.513	0.218	0.055	0.184
Chromium	−0.039	−0.720	0.404	0.396	0.008
Lead	−0.346	−0.785	−0.342	0.003	−0.095
Nickel	0.185	−0.769	0.191	0.113	0.154
Cadmium	−0.680	−0.661	0.017	0.071	−0.022

presents the results of the PCA for the chemical composition of leachate from active municipal waste landfills. The analysis showed the presence of five components that explained a total of over 80% of the variability of the test results. These components showed a strong correlation with EC, COD, ON, AN, TDS, chlorides, calcium, iron, zinc, chromium, lead and nickel.

The first principal component, explaining 31.273% of the total variability of the parameters characterizing the composition of leachate from active landfills, showed a strong correlation (correlation coefficient r from 0.7 to 0.89) [43]) with EC and contents of ON, TDS, chlorides, calcium, iron and zinc.

Moderate correlation (0.4 < r < 0.69) was found for pH, COD, TS, TSS, sulfates, magnesium, manganese, copper and cadmium concentrations. A strong correlation with the concentrations of other heavy metals (chromium, lead and nickel) occurred in the case of the second principal component, which explained 18.53% of the total variation in the leachate test results. In the cases of the third and fourth component, single cases of strong correlation with the analyzed variables (COD and AN) were found. These components explained 13.22% and 10.46% of the total variability of the results, respectively. In total, the principal components showing strong correlations with individual indicators of leachate contamination from active landfills explained 73.49% of the variability of the results.

Table 5. The results of the PCA for leachates from closed landfills.

			Number of Principal Component	Cumulative Eigenvalue	Cumulative % of Total Variance
			1	9.317	40.508
			2	12.571	54.656
			3	14.662	63.746
			4	16.497	71.725
			5	18.232	79.271
			6	19.703	85.667
			7	20.703	90.013
			8	21.515	93.544
			9	22.093	96.057
			10	22.544	98.020
Factor loadings (correlations between variables and components)
Variable	Component 1	Component 2	Component 3	Component 4	Component 5
pH	−0.565	0.042	−0.608	0.040	−0.465
EC	−0.817	−0.428	0.196	−0.240	−0.033
COD	−0.849	0.392	0.109	0.081	0.130
TKN	−0.876	−0.182	0.171	−0.299	−0.182
ON	−0.814	−0.254	−0.054	−0.264	−0.290
AN	−0.722	−0.180	−0.193	−0.351	0.057
TS	−0.711	−0.056	0.168	0.468	−0.365
TDS	−0.633	−0.426	0.506	−0.196	0.025
TSS	−0.651	0.020	0.088	0.545	−0.400
Sulfates	0.305	0.559	0.367	−0.159	−0.267
Chlorides	−0.746	−0.535	−0.047	0.143	0.263
Sodium	−0.553	0.083	−0.467	−0.055	0.383
Potassium	−0.951	−0.062	0.148	−0.092	0.155
Calcium	0.857	−0.035	0.188	−0.111	−0.360
Magnesium	−0.611	0.467	0.454	−0.371	−0.047
Iron	−0.628	−0.053	−0.315	0.388	0.257
Manganese	−0.671	0.610	−0.172	−0.043	0.142
Copper	0.270	−0.459	−0.538	−0.360	−0.168
Zinc	−0.294	0.693	0.068	−0.107	0.323
Chromium	0.243	0.068	0.035	0.147	0.535
Lead	−0.495	0.622	−0.190	0.318	−0.324
Nickel	0.232	−0.300	−0.068	0.215	−0.031
Cadmium	0.025	0.474	−0.476	−0.513	−0.181

presents the results of the PCA for the chemical composition of leachate from closed municipal waste landfills. The analysis showed the presence of five components that explained a total of almost 80% of the variability of the test results. These components showed a strong correlation with EC, COD, TKN, ON, AN, TS, chlorides, potassium and calcium.

In the case of non-operational landfills, the first principal component explained 40.51% of the total variability of the parameters characterizing the composition of leachate from active landfills. This component was the only one to have strong correlations with leachate composition (with EC, COD, TKN, ON, AN, TS, chlorides and calcium contents). There was one case of a very strong correlation (0.9 < r < 1.0) [43])—between the first principal component and the potassium content of the leachate.

A moderate correlation (0.4 < r < 0.69) was found for pH as well as TDS, TSS, sodium, magnesium, iron, manganese and lead concentrations. The other principal components did not show strong correlations with the parameters characterizing the leachate from inactive landfills. Moderate correlations between these components and leachate properties were found for pH, EC, TS, TDS, TSS, sulfates, chlorides, sodium, magnesium, manganese and heavy metals (except nickel).

The results of PCA showed that, for the description of leachate properties from operational and closed landfills, pollution indicators such as EC, COD, ON, AN, chlorides and calcium were best. These parameters are specified in the literature as characteristic for the leachate from municipal waste landfills [44], but for the most part the parameters fall outside the obligatory monitoring range.

4. Discussion

The leachate properties are strongly linked to the waste decomposition phase [45]. The pH values of the leachate from municipal landfills are in the range between 4.5 and 9 [5]. Leachate from “young” landfills (that have been in operation for less than 10 years) is characterized by a lower pH, which is related to a high concentration of volatile fatty acids. This indicates an early acetogenic phase of waste decomposition, when the pH value usually does not exceed 6.6. Leachate from mature landfills, that have been in operation for more than 10 years [46], reaches values higher than 7.5 as volatile fatty acids are converted into methane and carbon dioxide [10,47]. The alkaline nature of the leachate is characteristic of the methanogenic phase and leachates from mature landfills [48]. The pH values of the analyzed leachates were in the range 7.4–9.1, which indicates the methanogenic phase and that they came from mature landfills. Even at active facilities the reaction was alkaline, which may indicate a high proportion of old and stabilized waste in the total mass accumulated at the landfill [49].

With the aging of landfills, the susceptibility of leachate to biological degradation processes decreases, and their composition is less fluctuating [50]. As a result, leachate from old landfills is characterized by lower COD values (within the range of several hundred to several thousand mg O₂/dm³) [51], which was confirmed by the results of the research. The lowest COD value was recorded in leachate from the landfill in Wroclaw (the oldest of the studied landfills), and the highest value in leachate from the operating landfill.

AN concentrations in the leachate from active landfills may vary between 10 and 100 mg/dm³, but values reaching over 10,000 mg/dm³ were also recorded [52,53]. In the analyzed leachate from active landfills, the AN concentration was relatively high, reaching the maximum value of 786.09 mg N_NH4/dm³. Similar trends can be observed in the case of TDS content, which changes with the age of landfills. High contents of soluble solids are reflected in the high EC values of leachate. The concentrations of organic and inorganic soluble solids decrease with the age of the landfill [53], which was confirmed by the study results.

The age of the landfill is also one of the factors influencing the concentration of heavy metals in leachate. At “young” landfills, which have been in use for a short time, higher concentrations of metals and organic compounds are usually observed in comparison with stabilized landfills. This is related to the occurrence of an acidic environment that facilitates the dissolution and migration of metal ions [54]. However, the research has shown the occurrence of larger concentration ranges of heavy metals (e.g., Cr, Cu) in leachate from the non-operational landfill in Wrocław in comparison to active landfills. Low concentrations of heavy metals (<1 mg/dm³) in the leachate from active landfills may indicate that the landfilled waste is mainly municipal waste, not containing these components [3]. The research confirms that the content of heavy metals is not currently the most serious problem in handling landfill leachate [5,14].

Heavy metals, often mentioned in the scientific literature as characteristic of landfill leachate [3,44,55,56], were useful in the PCA method only for characterizing leachate from active landfills. In the analysis of leachate properties from non-operational landfills, the heavy metals no longer played a significant role, due to their low concentrations, which have also been recorded in the literature [45,54,57]. In this case, indicators such as TKN, TS and potassium were more useful.

The studies [54,58,59] pointed out the need to control the state of the environment in the vicinity of landfills as well as the problem with determining the scope of this monitoring. The research showed that the most useful parameters indicating groundwater pollution by landfill leachate include EC, TOC [60] boron, iron, dissolved solids, and chlorides, but mainly ammonium nitrogen [61]. Similarly, Christensen et al. [5] and Thomsen et al. [62], on the basis of the pollutant mass load, considered AN as one of the main factors of water pollution in the landfill area. Similarly, Przydatek [54] pointed out that the deterioration of surface water quality in the area of the landfill site is mostly due to AN.

The studies also pointed out the need to verify and modify the existing scope of monitoring studies [58,59], recommending that the list of indicators required by law to control the quality of water in landfills should include determinations of AN, chlorides [59] and dissolved oxygen [54].

5. Conclusions

The comprehensive assessment of the properties of leachate from active and non-operational municipal landfills enabled the following conclusions to be drawn:

• The results of the study of the physicochemical properties of leachate showed high variability. Statistically significant differences occurred between the values for active and non-operational landfills for most of the analyzed parameters. The leachate from active landfills was characterized by higher EC and COD values and TKN, ON, AN, chlorides, TS and TDS concentration. The leachate from non-operational landfills contained more sulfates.
• The age of the landfill also influenced the variability of the results—the values characterizing older landfills (non-operational in Wrocław and active in Legnica) were more stable. The results of research on leachate from landfills put into operation at the turn of the 20th and 21st century (Bielawa, Jawor) showed greater variability, which sometimes blurred the differences resulting from the exploitation methods. This trend was visible, among others, in the case of COD, ON, TS, iron and manganese.
• The concentrations of heavy metals in the analyzed leachates were characterized by relatively low variability. The lack of significant differences between active and non-operational landfills could result from changes in the method of municipal waste disposal, due to which less waste containing these components is now sent to landfills.
• The conducted analyses showed the existence of significant differences between the surveyed active and closed landfills. These differences were especially visible in the cases of the following: EC, COD, TKN, ON, AN, TS, TDS, TSS, sulfates, chlorides, sodium, potassium, calcium, magnesium and nickel. No significant differences were found between the concentrations of other heavy metals (Cu, Zn, Cr, Pb, Cd) analyzed as part of the monitoring.
• EC, COD, ON, AN, chlorides and Ca appear to be particularly useful for monitoring purposes. These parameters are specified in the literature as characteristic of leachates, and in the conducted tests they also clearly showed differences between the tested landfills. They can complement monitoring due to their clear differentiation and widespread occurrence in landfill leachate.