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

Aleksandra Wdowczyk * and Agata Szyma ´nska-Pulikowska

Faculty of Environmental Engineering and Geodesy, Institute of Environmental Engineering, Wrocław University of Environmental and Life Sciences, pl. Grunwaldzki 24, 50-363 Wrocław, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-71-320-5544

 Received: 27 September 2020; Accepted: 5 November 2020; Published: 8 November 2020 

Abstract: Leachate formation is one of the many environmental hazards associated with landfilling. The leachate may migrate from the landfill to surface water and , 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 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 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), of total Kjeldahl nitrogen (TKN) (non-operational landfills 0.18–294.5 mg N/dm3, active 167.56–907.4 mg N/dm3), (COD), organic nitrogen (ON), ammonium nitrogen (AN), total solids (TS), (TDS), (TSS), sulfates, chlorides, , , , magnesium and nickel. Selected indicators should help to determine the progress of processes inside the landfill and the potential impact of leachate on the environment, and should be used in the mandatory monitoring of landfills.

Keywords: municipal waste landfills; leachate monitoring; pollution indicators

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 [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 [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 of the 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

Water 2020, 12, 3129; doi:10.3390/w12113129 www.mdpi.com/journal/water Water 2020, 12, 3129 2 of 15 as , calcium, magnesium, sodium, chlorides, iron and heavy metals (cadmium, chromium, copper, lead, nickel and zinc) [5,10–12]. 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 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, , chlorides, fluorides, sulfates, , nitrites, sodium, potassium, ammonium nitrogen, free cyanides, chemical oxygen demand, heavy metals (copper, zinc, lead, cadmium, chromium, mercury, arsenic), total organic , 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. Water 2020, 12, 3129 3 of 15

The research on the volume and composition of leachate is carried out in both the operational and post-operationalWater 2020, 12, x FOR phase. PEER REVIEW In theoperational phase, the tests are carried out every 3 months, while in3 of the 15 post-operational phase, every 6 months. No studies are required to establish a set of parameters for thefor monitoringthe monitori ofng landfill of leachate. leachate. It is possibleIt is possible to select to select additional additional parameters, parameters, but due but to due the to costs, the landfillcosts, landfill managers managers most often most order often testsorder only tests within only within the binding the binding scope. scope. TheThe aimaim ofof thethe studystudy waswas toto analyzeanalyze thethe didifferencesfferences betweenbetween thethe compositionscompositions ofof leachatesleachates fromfrom activeactive andand non-operationalnon-operational municipalmunicipal landfills.. The indicators selected in this way shouldshould helphelp toto determinedetermine thethe progressprogress ofof decompositiondecomposition processesprocesses insideinside thethe landfilllandfill andand thethe potentialpotential impactimpact ofof leachateleachate onon thethe environment,environment, andand shouldshould bebe usedused inin thethe mandatorymandatory monitoringmonitoring ofof landfills.landfills.

2.2. Materials and MethodsMethods

2.1.2.1. Study Area TheThe researchresearch waswas conductedconducted inin thethe yearsyears 2018–20192018–2019 onon fourfour municipalmunicipal wastewaste landfillslandfills locatedlocated inin LowerLower SilesiaSilesia (Figure(Figure1 ).1). The The oldest oldest one one was was established established in Wroclaw,in Wroclaw, in 1966. in 1966. The The area area occupied occupied by the by 3 landfillthe landfill is 11.7 is 11.7 ha, and ha, theand capacity the capacity of the of landfill the landfill is about is about 2 million 2 million m . It m was3. It in was operation in operation until 2000.until Another2000. Another large landfill large landfill site is locatedsite is located in Legnica. in Legnica. The area The occupied area occupied by the landfilled by the landfilled waste is 14.12waste ha, is 3 and14.12 the ha, total and capacity the total of capaci the facilityty of the is 2.34facility million is 2.34 m million. The landfill m3. The has landfill been operatinghas been operating since 1977 since and consists1977 and of consists 6 sectors. of 6 sectors.

FigureFigure 1.1. Location of research facilitiesfacilities inin LowerLower SilesianSilesian voivodeship,voivodeship, Poland.Poland.

TheThe researchresearch alsoalso coveredcovered twotwo smallersmaller landfills.landfills. TheThe BielawaBielawa landfilllandfill waswas putput intointo operationoperation inin 20012001 andand closedclosed inin 2011.2011. The area occupiedoccupied by thethe landfilledlandfilled wastewaste isis 0.860.86 ha,ha, andand itsits capacitycapacity isis 37.837.8 thousandthousand mm33. The The landfill landfill site site in Jawor has been in operationoperation sincesince 1997,1997, and thethe areaarea occupiedoccupied byby wastewaste isis 3.373.37 ha.ha. TotalTotal capacitycapacity ofof thethe landfilllandfill isis 231.3231.3 thousandthousand mm33.. AllAll landfillslandfills areare equippedequipped withwith leachateleachate removalremoval systems.systems. SamplesSamples forfor physicochemical analysesanalyses werewere takentaken atat leachateleachate collectioncollection pointspoints (wells(wells oror tanks).tanks). Figure2 2 shows shows thethe leachateleachate collectioncollection points.points.

Water 2020, 12, 3129 4 of 15 Water 2020, 12, x FOR PEER REVIEW 4 of 15

a) b)

c) d)

FigureFigure 2.2. Leachate collectioncollection points:points: (a) Wrocław, ((bb)) Bielawa,Bielawa, ((cc)) Legnica,Legnica, ((dd)) Jawor.Jawor.

2.2.2.2. PhysicochemicalPhysicochemical CompositionComposition ofof LeachateLeachate TheThe analysisanalysis ofof the quality of landfill landfill leachates leachates was was carried carried out out in in 2018 2018–2019.–2019. A Atotal total of of7 sets 7 sets of ofleachate leachate samples samples were were tested, tested, taken taken at at the the sites sites of of their their coll collectionection (tanks or leachateleachate wells).wells). InIn thethe leachateleachate samples, samples, the the following following were were determined: determined: pH, pH, electrical electrical conductivity conductivity (EC), (EC), andand chemicalchemical oxygenoxygen demanddemand (COD),(COD), andand concentrationsconcentrations ofof totaltotal KjeldahlKjeldahl nitrogennitrogen (TKN),(TKN), organicorganic nitrogennitrogen (ON),(ON), ammoniumammonium nitrogennitrogen (AN),(AN),total total solidssolids (TS),(TS), totaltotal dissolveddissolved solidssolids (TDS),(TDS), totaltotal suspendedsuspended solidssolids (TSS), sulfates,sulfates, chlorides, chlorides, sodium, sodium, potassium, potassium, calcium, calcium, magnesium, magnesium, iron, manganese, iron, manganese, copper, zinc, copper, chromium, zinc, lead,chromium, nickel andlead, cadmium. nickel and Samples cadmium. were Samples taken everywere taken 3–4 months. every 3–4 months. ImmediatelyImmediately afterafter beingbeing taken,taken, thethe samplessamples werewere transportedtransported inin refrigeratedrefrigerated conditionsconditions toto thethe EnvironmentalEnvironmental Research Research Laboratory Laboratory of the of Institute the Institute of Environmental of Environmental Engineering, Engineering, Wroclaw University Wroclaw ofUniversity Life Sciences. of Life pH, Sciences.EC, COD, pH, TKN, EC, AN, COD, TDS, TSS, TKN, sulfates, AN, TDS, chlorides, TSS, sodium, sulfates, potassium, chlorides, calcium, sodium, magnesium,potassium, iron,calcium, manganese, magnesium, copper, iron, zinc, manganese, chromium, lead, copper, nickel zinc, and cadmiumchromium, were lead, determined nickel and in accordancecadmium were with determined the methodology in accordance presented with in ISO the standardsmethodology [20– 33presented]. The ON in hasISO beenstandards calculated [20–33]. on theThe basis ON has of the been equation calculated ON= onTKN-AN the basis [34 of]. the TS equation was calculated ON=TKN as the-AN sum [34]. of TS TDS was and calculated TSS [35]. as the sumThe of TDS results and ofTSS landfill [35]. leachate composition tests were subjected to statistical analysis using StatisticaThe 13.1 results (StatSoft of landfill Poland, leachate StatSoft, composition Inc., Tulsa, OK, tests USA). were In subjected the presented to statistical studies, non-parametric analysis using statisticsStatistica were 13.1 used, (StatSoft which Poland, are most StatSoft, suitable for Inc., small Tulsa, sample OK, numbers USA). In (n < the100) presented and for those studies, which non do- notparamet showric compliance statistics were with normalused, which distribution are most [36 suitable]. To characterize for small sample the results numbers of the physicochemical(n < 100) and for analysesthose which of leachate do not show from compliance four landfills, with descriptive normal distribution statistics were[36]. To used—median, characterize the maximum results of and the minimumphysicochemical values—which analyses allow of leachate us to present from four a data landfills, set regardless descriptive of its statistics distribution. were Theused analysis—median, of dimaximumfferences and between minimum active values (Legnica,—which Jawor) allow and us non-operational to present a data waste set regardless landfills (Wrocław, of its distribution Bielawa). wasThe carried analysis out of using differences the Mann–Whitney between active U test (Legnica, (equivalent Jawor) to the and parametric non-operational t test) for waste independent landfills groups,(Wrocław, often Bielawa) used in was many carr studiesied out [37 using]. the Mann–Whitney U test (equivalent to the parametric t test)Based for independent on the principal groups, component often used analysis in many (PCA), studies the [37]. pollution indicators (variables) that best characterizesBased on the the composition principal component of the tested analysis landfill (PCA), leachate the werepollution identified. indicators The PCA(variables) can be that used best to reducecharacterizes the number the composit of variablesion of describing the tested landfill the phenomenon leachate were under identified. investigation. The PCA The can determined be used to componentsreduce the number are a linear of variables combination describing of the variables,the phenomenon among whichunder thoseinvestigation. which have The thedetermined greatest impactcomponents on the are individual a linear principalcombination components of the variables, can be indicated. among which This influence those which can behave represented the greatest by 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

Water 2020, 12, 3129 5 of 15 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 by landfill leachate, to analyze the test results of the leachate itself or to choose the location of landfills [39–42].

3. Results In Tables1 and2, 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 undergo intensive processes of waste decomposition and . 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.

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

Non-Operational Landfills Pollution Indicators Unit 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 3 COD mg O2/dm 954–4270 2370 113–237 183 TKN mg N/dm3 32.20–294.50 131.20 0.18–5.34 1.81 3 ON mg Norg/dm 14.60–182.80 80.50 0.16–5.34 1.54 3 AN mg NNH4/dm 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 3 Sulfates mg SO4/dm 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 Water 2020, 12, 3129 6 of 15

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

Active Landfills Pollution Indicators Unit 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 3 COD mg O2/dm 1585–3800 3490 843–4110 2880 TKN mg N/dm3 167.56–907.4 300.91 285.52–613.3 346.27 3 ON mg Norg/dm 4.54–121.31 99.85 32.33–248.27 112.44 3 AN mg NNH4/dm 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 3 Sulfates mg SO4/dm 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 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 Water 2020, 12, 3129 7 of 15 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 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 3 the study period did not exceed 400 mg SO4/dm . 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. Table3 presents the results of the analysis of di fferences 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]. Table4 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, Water 2020, 12, 3129 8 of 15 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. Table5 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.

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

Sum of Ranks Variable UZ 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. Water 2020, 12, x FOR PEER REVIEW 9 of 15

Water 2020, 12, 3129 9 of 15 correlation (0.9 < r < 1.0) [43])—between the first principal component and the potassium content of the leachate. Table 4. The results of the PCA for leachates from active landfills. Table 4. The results of the PCA for leachates from active landfills. Number of Cumulative % Cumulative PrincipalNumber of Cumulativeof Total % 8 CumulativeEigenvalue 31.27% ComponentPrincipal of TotalVariance 7 Eigenvalue Component Variance 1 7.193 31.273 6 1 7.193 31.273 5 2 11.456 49.807 18.53% 2 11.456 49.807 3 14.496 63.028 4 3 14.496 63.028 13.22% 3 44 16.902 16.902 73.485 73.485 10.46% 9.01% 2 55 18.973 18.973 82.491 82.491 5.21% 4.49% 1 6 20.171 87.701 2.52% 6 20.171 87.701 3.19%

Eigenvalues / % of total variance 7 21.204 92.193 0 1.13% 7 21.204 92.193 8 21.938 95.384 -1 8 21.938 95.384 0 5 10 15 20 25 9 22.519 97.907 Number of principal component 9 22.519 97.907 10 22.777 99.033 Factor loadings (correlations between variables10 and components) 22.777 99.033 Variable ComponentFactor loadings 1 (correlationsComponent between 2 variablesComponent and 3 components)Component 4 Component 5 VariablepH Component0.518 1 Component−0.212 2 Component0.177 3 Component−0.280 4− Component0.623 5 ECpH 0.5180.796 0.212−0.347 0.1770.236 −0.1000.280 −0.0530.623 − − − COD −0.470 −0.010 0.749 0.143 0.007 EC 0.796 0.347 0.236 0.100 0.053 TKN −0.048 − −0.450 −0.037 −−0.533 −0.560− COD 0.470 0.010 0.749 0.143 0.007 ON −−0.710 − −0.108 −0.077 0.161 −0.545 TKN 0.048 0.450 0.037 0.533 0.560 AN −0.194 − −0.356 − 0.136 −−0.739 −0.355− TSON −0.7100.421 0.1080.483 0.0770.005 0.315 0.161 −0.6030.545 − − − − TDSAN 0.1940.719 0.356−0.150 0.1360.627 0.0340.739 0.0580.355 − − − TSSTS −0.4210.563 0.4830.460 0.005−0.181 0.272 0.315 −0.5300.603 Sulfates −−0.449 0.394 0.497 −0.354 0.249− TDS 0.719 0.150 0.627 0.034 0.058 Chlorides 0.714 − −0.318 0.115 0.460 −0.176 TSS 0.563 0.460 0.181 0.272 0.530 Sodium −0.367 −0.324 − −0.596 −0.331 0.045− Sulfates 0.449 0.394 0.497 0.354 0.249 Potassium −0.273 0.095 0.661 −−0.062 −0.192 CalciumChlorides 0.714−0.861 0.318−0.107 0.115−0.035 −0.364 0.460 0.1100.176 − − MagnesiumSodium 0.367−0.435 0.3240.210 0.5960.299 −0.6640.331 0.174 0.045 − − − PotassiumIron 0.273−0.858 0.095−0.067 0.661−0.067 −0.1180.062 0.1490.192 Manganese −0.675 −0.642 −0.057 0.210− 0.040− Calcium 0.861 0.107 0.035 0.364 0.110 Copper −0.516 − −0.144 − −0.653 −−0.073 0.178 Magnesium 0.435 0.210 0.299 0.664 0.174 Zinc −−0.767 −0.513 0.218 0.055− 0.184 Iron 0.858 0.067 0.067 0.118 0.149 Chromium −−0.039 − −0.720 − 0.404 0.396− 0.008 ManganeseLead −0.6750.346 0.642−0.785 0.057−0.342 0.003 0.210 −0.095 0.040 − − − NickelCopper 0.5160.185 0.144−0.769 0.6530.191 0.1130.073 0.154 0.178 − − − CadmiumZinc −0.7670.680 0.513−0.661 0.2180.017 0.071 0.055 −0.022 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 − − −

Water 2020, 12, 3129 10 of 15

Water 2020, 12, x FOR PEER REVIEW 10 of 15 Table 5. The results of the PCA for leachates from closed landfills. Table 5. The results of the PCA for leachates from closed landfills. Number of Cumulative % Number of Cumulative Cumulative 10 Principal Cumulative of Total 40.51% Principal Eigenvalue % of Total 9 Component Eigenvalue Variance Component Variance 8 1 9.317 40.508 7 1 9.317 40.508 2 12.571 54.656 6 2 12.571 54.656 5 33 14.66214.662 63.746 63.746 4 14.15% 44 16.49716.497 71.725 71.725 3 55 18.23218.232 79.271 79.271 2 9.09% 7.55% 7.98% 4.35% 66 19.70319.703 85.667 85.667 1 6.40% 2.51% Eigenvaluesoftotal % variance/ 3.53% 7 20.703 90.013 0 1.96% 7 20.703 90.013 8 21.515 93.544 -1 8 21.515 93.544 0 5 10 15 20 25 9 22.093 96.057 Number of principal component 910 22.09322.544 98.020 96.057 Factor loadings (correlations between variables10 and components) 22.544 98.020 Variable ComponentFactor loadings 1 (correlationsComponent between 2 variablesComponent and components)3 Component 4 Component 5 VariablepH Component−0.565 1 Component0.042 2 Component−0.608 3 Component0.040 4 Component−0.465 5 EC −0.817 −0.428 0.196 −0.240 −0.033 pH 0.565 0.042 0.608 0.040 0.465 COD −−0.849 0.392 − 0.109 0.081 0.130− EC 0.817 0.428 0.196 0.240 0.033 TKN −−0.876 − −0.182 0.171 −−0.299 −0.182− COD 0.849 0.392 0.109 0.081 0.130 ON −−0.814 −0.254 −0.054 −0.264 −0.290 ANTKN 0.876−0.722 0.182−0.180 0.171−0.193 −0.2990.351 0.0570.182 − − − − TSON 0.814−0.711 0.254−0.056 0.0540.168 0.4680.264 −0.3650.290 − − − − − TDSAN 0.722−0.633 0.180−0.426 0.1930.506 −0.3510.196 0.025 0.057 TSS −−0.651 − 0.020 − 0.088 −0.545 −0.400 TS 0.711 0.056 0.168 0.468 0.365 Sulfates − 0.305 − 0.559 0.367 −0.159 −0.267− TDS 0.633 0.426 0.506 0.196 0.025 Chlorides −−0.746 − −0.535 −0.047 −0.143 0.263 TSS 0.651 0.020 0.088 0.545 0.400 Sodium −−0.553 0.083 −0.467 −0.055 0.383− PotassiumSulfates 0.305−0.951 0.559−0.062 0.3670.148 −0.1590.092 0.1550.267 − − CalciChloridesum 0.7460.857 0.535−0.035 0.0470.188 0.143−0.111 −0.360 0.263 − − − MagnesiumSodium 0.553−0.611 0.0830.467 0.4670.454 −0.0550.371 −0.047 0.383 Iron −−0.628 −0.053 − −0.315 −0.388 0.257 Potassium 0.951 0.062 0.148 0.092 0.155 Manganese −−0.671 − 0.610 −0.172 −−0.043 0.142 Calcium 0.857 0.035 0.188 0.111 0.360 Copper 0.270 − −0.459 −0.538 −−0.360 −0.168− Magnesium 0.611 0.467 0.454 0.371 0.047 Zinc −−0.294 0.693 0.068 −−0.107 0.323− ChromiumIron 0.6280.243 0.0530.068 0.3150.035 0.3880.147 0.535 0.257 − − − ManganeseLead 0.671−0.495 0.6100.622 0.172−0.190 0.3180.043 −0.324 0.142 − − − NickelCopper 0.2700.232 0.459−0.300 0.538−0.068 0.2150.360 −0.0310.168 Cadmium 0.025 − 0.474 − −0.476 −−0.513 −0.181− Zinc 0.294 0.693 0.068 0.107 0.323 − − AChromium moderate correlation 0.243 (0.4 < r < 0.69) 0.068 was found for 0.035 pH as well as 0.147 TDS, TSS, sodium, 0.535 Lead 0.495 0.622 0.190 0.318 0.324 magnesium, iron, manganese− and lead concentrations. The other− principal components did not −show strong correlationsNickel with the 0.232 parameters characterizing0.300 the leachate0.068 from inactive 0.215 landfills. Moderate0.031 − − − correlationsCadmium between these 0.025 components and 0.474leachate properties0.476 were found for p0.513H, EC, TS, TDS, 0.181TSS, 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 Alandfills, moderate pollution correlation indicators (0.4 < rsuch< 0.69) as EC, was COD, found ON, for pHAN, as chlorides well as TDS, and TSS,calcium sodium, were magnesium,best. Theseiron, parameters manganese are and specified lead concentrations.in the literature as Thecharacteristic other principal for the leachate components from municipal did not show waste strong landfillscorrelations [44], but with for the the most parameters part the parameters characterizing fall outside the leachate the obligatory from inactive monitoring landfills. range. 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).

Water 2020, 12, 3129 11 of 15

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 and [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 3 O2/dm )[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 3 NNH4/dm . 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 [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 mass load, considered AN as Water 2020, 12, 3129 12 of 15 one of the main factors of in the landfill area. Similarly, Przydatek [54] pointed out that the deterioration of surface 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.

Author Contributions: A.W. and A.S.-P.conceived and designed the experiments; A.W. performed the experiments; A.W. and A.S.-P. analyzed the data; A.S.-P. contributed reagents/materials/analyzes tools; A.W. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by a targeted subsidy for research and development of young scientists and PhD students at the Faculty of Environmental Engineering and Geodesy of the Wrocław University of Environmental and Life Sciences—contract no. B030/0104/18. APC is financed by the Institute of Environmental Engineering of Wroclaw University of Life Sciences. Conflicts of Interest: The authors declare no conflict of interest.

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