Levels of Organic Pollution Indicators in Groundwater at the Old Landfill

applied sciences

Article Levels of Organic Pollution Indicators in Groundwater at the Old Landﬁll and Waste Management Site

Eugeniusz Koda, Anna Miszkowska and Anna Sieczka *

Department of Geotechnical Engineering, Faculty of Civil and Environmental Engineering, Warsaw University of Life Sciences, Nowoursynowska 159 St., 02-776 Warsaw, Poland; [email protected] (E.K.); [email protected] (A.M.) * Correspondence: [email protected]; Tel.: +48-22-593-5222

Academic Editor: Samuel B. Adeloju Received: 19 May 2017; Accepted: 16 June 2017; Published: 20 June 2017

Abstract: The aim of this paper was to assess groundwater quality in a landfill and waste management site, with special regard to levels of organic pollution indicators: chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total organic carbon (TOC). Analysis of specific indicators was conducted for piezometers located in the area of the Radiowo landfill, the composting plant and the facilities adjacent to the landfill. The article discusses the temporal and spatial changes of selected organic pollution indicators. Based on the results of groundwater monitoring, several maps of COD, BOD and TOC areal distribution were provided. Statistical distribution of monitoring data was presented using box-and-whisker plots. Pearson’s correlation coefficients between selected pollution indicators were measured with a significance level set at p < 0.01 and p < 0.05. The strongest correlation was observed between BOD and COD. The maximum BOD/COD ratio was observed at the level of 1.561 before the closure of the vertical barrier, whereas, at present, average values of this ratio are below 0.18. The results indicate significant improvement of groundwater quality in the landfill site after the closure of the vertical barrier. In particular, this refers to BOD values, which decreased even 160 times in the 1998–2016 monitoring period.

Keywords: landﬁll; pollution; monitoring; biochemical oxygen demand; chemical oxygen demand; total organic carbon

1. Introduction Nowadays, landfilling is the most common practice of waste disposal all over the world [1,2]. In 2015, a total of 10.9 million tons of municipal solid wastes were collected in Poland, of which 44.3% were landfilled in 347 operating landfills [3]. Landfill leachate is considered the main source of groundwater and surface water contamination. In recent decades, the influence of leachates on water resources has attracted a lot of attention because of its high environmental significance [4,5]. Leachates mainly contain immense amounts of organic matter, ammonium, heavy metals, and salts [6–8]. According to Christensen et al. [1], the components of leachates can be subdivided into the following groups: organic matter expressed as chemical oxygen demand (COD) and total organic carbon (TOC), specific organic compounds, inorganic compounds, and heavy metals. Leachates are commonly characterized by high chemical and biochemical oxygen demand values (COD and BOD, respectively) that reflect concentrations of soluble organic matter. The organic composition of leachates is variable depending on the characteristics of the landfilled wastes, age of the landfill or climatic conditions [8,9].

Appl. Sci. 2017, 7, 638; doi:10.3390/app7060638 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 638 2 of 22

In order to protect the natural environment from leachates, each landfill must be properly secured, for example by using artificial sealing based on cohesive soil liners [8]. Municipal landfills contain a high content of organic wastes, with great impact on the biogeochemical processes in the landfill body and leachate generation with a substantial content of ammonium, dissolved organic carbon and organic compounds [9]. It is important to stress that the content of organic matter affects the level of dissolved oxygen and can be determined by measurements of the biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The BOD is also used as a value to indicate the leachate “quality”. In cases when leachate is discharged directly into a watercourse, it will absorb oxygen from the water to complete its decomposition. As a result, the level of oxygen in water may fall below a minimum supporting aquatic life [10]. Because of the decomposition of organic matter, leachate derived from landfills comprises primarily dissolved organic carbon [11]. The solubility of organic contaminants in wastes can be slightly enhanced through the presence of high levels of organic carbon in the leachate [12]. The composition of landfill leachate varies over time. Leachates generated during the early stages of anaerobic degradation are characterized e.g., by high BOD values and a high BOD/COD ratio. According to Getinby [13], BOD values are the highest in the first five years of landfill operation and decrease with time (for example BOD equal to 2000 mg O2/L after five years and BOD equal to 70 mg O2/L after 40 years of landfill operation). Similarly, five-year leachates are characterized by a COD value at a level of 8000 mg O2/L, but, after 30 years, this value decreases to 500 mg O2/L. BOD and COD indicators have a significant meaning in environmental studies because of their ability to indicate the pollutant strength of polluted water [14,15]. In recent years, the impact of landfills on groundwater quality has enhanced numerous studies relating to the assessment of the soil-water environment near landfills and waste management facilities [6,16–24]. Owing to the fact that groundwater quality within the landfill site should be controlled by a monitoring system based on a network of piezometric wells and sampling points located along surface streams near the landfills, this study focuses on detailed analyses using data from a monitoring period of almost 20 years (1998–2016). The principal aim of this paper is to evaluate the impact of the landfill and the waste management facilities on the groundwater quality within the landfill site. With particular emphasis on the extent of groundwater contamination by organic substances, concentrations of BOD, COD and TOC were determined in the collected samples. This can complete and add additional information to Radiowo landfill monitoring data results which have been published [21]. An attempt was also made to answer the following questions: (1) “Is groundwater quality significantly affected by leachate percolation from the landfill?” (2) “Does a vertical barrier limit the spread of pollutants to the soil-water environment?”

2. Materials and Methods

2.1. Site Characteristics The Radiowo Landfill (52◦16047.80” N, 20◦52045.30” E) is located partly in the Stare Babice commune and partly in Warsaw, the capital of Poland (Figure1). It covers an area of ca. 16.2 ha and its elevation is almost 60 m. From the north, the landfill lies adjacent to a composting plant and a waste treatment plant located in the Warsaw–Bielany District. A Mechanical-Biological Treatment (MBT) installation located in the study area was built in 2012–2014 as a result of the reconstruction of the former compost plant, which operated on the DANO technology. The area of the MBT installation covers almost 10 ha. The composting plant is situated on the western side from the railway siding and since 2013 has been working as an element of the Regional Municipal Waste Treatment Installation. The basic role of this installation is the production of a stabilat, which contains a reduced content of organic fraction and can be disposed on non-hazardous Appl. Sci. 2017, 7, 638 3 of 22 and inert waste landfills. The capacity of this installation, measured as the mass of mixed processed wastes,Appl. isSci. approximately 2017, 7, 638 1000 tons per day. From the south and east, the landfill is surrounded3 of 22 by the Forest Park “Bemowo” and two Nature Reserves (Kalinowa Ł ˛akaand Łosiowe Błota). To the westB occursłota). To a the railway west occurs siding, a andrailway industrial siding, and and indu servicestrial facilities. and service A facilities. paved storage A paved yard storage is located yard ca. 200 mis located from the ca. toe 200 of m the from landfill the toe slope, of the and landfill to the slope, northwest and to liesthe anorthwest fuel storage lies base.a fuel Thestorage Zaborowski base. The Zaborowski Canal watercourse (connected with the Lipkowska Woda stream) occurs to the north Canal watercourse (connected with the Lipkowska Woda stream) occurs to the north from the landfill, from the landfill, at a distance of about 350 m from the composting plant (400 m from the landfill). at a distance of about 350 m from the composting plant (400 m from the landfill). The area of the The area of the Kampinos National Park is situated approximately 3 km to the northwest of the KampinosRadiowo National landfill Park[21]. is situated approximately 3 km to the northwest of the Radiowo landfill [21].

Figure 1. Location of the study area in relation to protected areas. Figure 1. Location of the study area in relation to protected areas. From 1962 to 1991, the Radiowo landfill was a disposal site for the municipal wastes from Warsaw.From 1962 Since to 1991,1992, only the Radiowo non-comp landfillosted wastes was a from disposal the Radiowo site for the compostory municipal plant wastes are fromstored Warsaw. on Sincethe 1992, landfill, only and, non-composted since 2012, only wastes wastefrom from the MBT Radiowo installation compostory is stored plant there. are Since stored 1994, on remedial the landfill, and,works since 2012,have been only wastecarried fromout on the the MBT landfill installation site, including is stored forming there. Sinceand planting 1994, remedial of slopes, works havereinforcement been carried of out their on thestability, landfill clay site, mineral including capping, forming construction and planting of aof bentonite slopes, reinforcementcut-off wall, of theirperipheral stability, claydrainage, mineral and capping,installation construction of a leachate of re-circulation a bentonite cut-offsystem. wall, peripheral drainage, and A vertical barrier was designed to minimize the spread of pollutants to the groundwater system. installation of a leachate re-circulation system. This construction provides favorable hydraulic conditions by forcing a lowered groundwater level A vertical barrier was designed to minimize the spread of pollutants to the groundwater system. inside the peripheral cut-off wall in relation to the natural water level around the landfill site [25]. This construction provides favorable hydraulic conditions by forcing a lowered groundwater level inside2.2. the Geological peripheral and Hydrogeological cut-off wall in Conditions relation to the natural water level around the landfill site [25].

2.2. GeologicalThe landfill and Hydrogeological subsoil consists Conditions of sandy soils with a thickness of 2–15 m. The upper part is represented by dense sands underlain by well-graded sands. The first groundwater level is at the depthThe landfill0.3–2 m subsoilbelow the consists surface of level sandy and soilsits fluctuations with a thickness depend ofon 2–15the weather m. The and upper the local part is representeddrainage byconditions. dense sands underlain by well-graded sands. The first groundwater level is at the depth 0.3–2The deeper m below part the of the surface subsoil level consists and itsof lo fluctuationsw permeable depend boulder onand the varved weather clays and(thickness the local drainage25–40 conditions.m, average permeability coefficient at 10−9 m/s). Drillings have revealed the occurrence of fluvioglacialThe deeper sands part ofon thethe subsoilsouth and consists southwestern of low si permeablede of the vertical boulder barrier, and varvedand boulder clays clays (thickness in 25–40the m, remaining average sections permeability of the barrier. coefficient These at deposits 10−9 m/s). occur Drillingson glaciotectonic have revealed landforms the composed occurrence of of fluvioglacialPliocene clays. sands Due on theto the south differentiation and southwestern of soils occurring side of the in verticalthe soil barrier,profile, the and subsoil boulder can clays be in the remainingsubdivided sectionsinto five geotechnical of the barrier. layers These (Table deposits 1). occur on glaciotectonic landforms composed of Pliocene clays. Due to the differentiation of soils occurring in the soil profile, the subsoil can be subdivided into five geotechnical layers (Table1). Appl. Sci. 2017, 7, 638 4 of 22

Table 1. Main geotechnical characteristics of the subsoil.

Layer Soil IL or ID Stratigraphy ρ k I clSa 0.4 Quaternary (Pleistocene) 2.1 9.2 × 10−6 II Sa 0.7 Quaternary (Pleistocene) 2.0 5 × 10−5 III saclSi/Cl 0.2 Quaternary (Pleistocene) 2.1 1.2 × 10−7 IV saCl 0.1 Quaternary (Pleistocene) 2.2 3 × 10−8 V Cl/saCl <0.25 Tertiary (Pliocene) 2.1 <10−9 3 Notes: IL: liquidity index (-), ID: density index (-), ρ: bulk density of soil (g/cm ), k: coefﬁcient of permeability (m/s).

The groundwater flow direction is from the southeast to the northwest. Nevertheless, changes in groundwater levels, the presence of drainage systems and linear embankments, surface development, and the shape of the landfill force local changes of the flow direction. Low hydraulic gradients in the southern and eastern part of the area adjacent to the landfill result in very low velocity of the groundwater flow (inflow to the landfill). The influence of the vertical bentonite barrier on the groundwater flow was modeled in our previous studies using the numerical modeling techniques [26]. The FEMWATER model, using 3D finite elements method, was used to solve the issue of groundwater flow described by Richard’s equation. According to that modeling studies, it was noticed the groundwater level increase from approximately 0.25 m in the southern part to more than 0.75 m in the north-western part of the landfill site. Small increase (ca. 0.2 m) of the groundwater level was observed in the area close to the south part of the landfill. Flow velocity on the western side of the landfill (beyond the railway siding) is significantly higher. Along the outflow direction from the landfill, the velocity of groundwater flow in sandy soils is about 5.8 × 10−7 m/s (for a hydraulic gradient at i = 10‰), but this value, as mentioned above, may undergo changes depending on the flow direction, and the hydrogeological and meteorological factors.

2.3. Technical Characteristics of the Vertical Barrier The 0.6 m wide vertical barrier was installed at a depth of 2 m below the top of the clayey soils, at an average level of 3.5–22 m below the surface, which had resulted from the variable depths with regard to the top of the impermeable layer. The permeability coefﬁcient for the cut-off wall is below 10−9 m/s [19], making the subsoil environment practically impermeable. The characteristics of the vertical barrier are presented in Table2.

Table 2. Characteristics of the vertical barrier.

Parameter Unit Value Geometric Parameters Thickness m 0.6 Length m 1687.2 Minimum depth m 3.0 Mean depth m 8.6 Maximum depth m 22.0 Lateral surface area m2 14529.8 Filtration Parameters Permeability coefﬁcient after 28 days m/s 2.5 × 10−8 Permeability coefﬁcient after 60 days m/s 1.0 × 10−9 Strength Parameters Compressive strength after 28 days MPa min. 0.53 Compressive strength after 60 days MPa min. 0.91

2.4. Monitoring Network The monitoring system of the landﬁll site includes three basic elements: (1) groundwater; (2) surface water; and (3) leachates. Shallow piezometers and one deep well were installed in the Appl. Sci. 2017, 7, 638 5 of 22 landﬁll site in order to control groundwater quality (Figure2). Moreover, three sampling points were selected along the Lipkowska Woda watercourse to monitor surface water quality. Appl. Sci. 2017, 7, 638 5 of 22

Figure 2. Location of the groundwater monitoring network in the study area. Figure 2. Location of the groundwater monitoring network in the study area. Leachates were monitored in three sampling points in order to examine the amount of generated Leachatesleachate and were its monitoredquality. Fifteen in threeindicators sampling of pollution points were in examined order to for examine the leachates, the amount surface water of generated and groundwater. leachate and its quality. Fifteen indicators of pollution were examined for the leachates, surface water Quality tests were carried out focusing on the following indicators: electrical conductivity (EC), and groundwater.pH, lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), chromium (Cr), mercury (Hg), total organic Qualitycarbon tests (TOC), were polycyclic carried aromatic out focusing hydrocarbons on the following (PAH), ammonium indicators: (NH electrical4+), chemical conductivity oxygen (EC), pH, leaddemand (Pb), cadmium(COD), biochemical (Cd), copper oxygen (Cu),demand zinc (BOD), (Zn), chlorides chromium and sulfates. (Cr), mercury (Hg), total organic + carbon (TOC),Moreover, polycyclic continuous aromatic measurements hydrocarbons of the (PAH), precipitation ammonium and groundwater (NH4 ), chemical level were oxygen carried demand (COD),out biochemical as part of monitoring oxygen demand program (BOD),for the landfill. chlorides The andaverage sulfates. value of a rainfall for analyzed area is 550 mm. Groundwater level in all piezometers shows seasonal pattern resulting from precipitation. Moreover, continuous measurements of the precipitation and groundwater level were carried It is also observed that due to the high groundwater level, lower concentration of pollution indicators out as partis noted of monitoring and conversely. program Therefore, for the the physicochemical landﬁll. The average analyses valueof the groundwater of a rainfall samples for analyzed can be area is 550 mm.coupled Groundwater with measurements level in allof ground piezometerswater level shows changes seasonal in piezometers. pattern resulting from precipitation. It is also observed that due to the high groundwater level, lower concentration of pollution indicators is noted and conversely. Therefore, the physicochemical analyses of the groundwater samples can be coupled with measurements of groundwater level changes in piezometers. Appl. Sci. 2017, 7, 638 6 of 22

The monitoring network of the Radiowo landfill consists of: piezometer P-2A located on the outflow, to the east of the landfill; piezometers P-4 and P-6 located on the inflow (background), to the southeast of the landfill; piezometers P-7A, P-9, P-10A, P-11A, P-12 and P-17 located in the outflow; piezometers P-14A and P-15 located in the outflow, to the north from the composting area; and deep well S-1 located in the compostory plant area. Monitoring studies are conducted quarterly and annual reports are submitted to the administrative bodies responsible for the environment-related issues.

2.5. Sampling and Experimental Analysis Owing to the fact that about 30% of the monitoring results may be biased due to inappropriate sampling and sample transportation [27], all samples were collected with great care to eliminate potential errors. Water samples were collected in accordance with the procedure outlined in EN ISO 5667-3 [28] and ISO 5667-11 [29]. Before sampling, the top water in each piezometer was carefully pumped out. Calculations of the pumping time were based on the volume of water in each piezometer and the pump efﬁciency. To assure sampling appropriateness, each pumping was accompanied with simultaneous measurements of pH, temperature and electrical conductivity. Groundwater samples were taken four times in each year (March, June, September and November). Chemical analysis of COD, BOD and TOC were conducted according to PN-ISO 15705:2005P [30], PN-EN 1899-2:2002P [31] and PN-EN 1484:1999P [32], respectively. Additionally, the values of pH and EC were measured in accordance with the recommendations presented in PN-EN ISO 10523:2012 [33] and PN-EN 27888:1999 [34]. Analysis of changes of the pollution indicators was carried out in a temporal and spatial aspect. Graphs of BOD, COD and TOC concentrations were presented for piezometers located in the landﬁll area, composting plant, and the adjacent service facilities. The obtained results were compared with the groundwater quality standard presented in the Regulation of the Minister of the Environment of Poland [35] (third class representing good groundwater quality).

2.6. Statistical Analysis The monitoring data were analyzed using the Statistica 12.0 software package. Pearson’s correlation coefficients (r) were calculated between each of the pollution indicator levels in groundwater, with a significance level set at p < 0.05 and p < 0.01. The purpose of the statistical analysis was to present the existing correlations between pollution indicators with respect to the specific locations of piezometers from which the groundwater samples were taken. In order to display the entire statistics of the dataset, box-and-whisker plots were used. The minimum, quartile 1 (25%), median, mean, quartile 3 (75%), and the maximum values were determined.

3. Results

3.1. Pollution Level in Groundwater Due to Localization of Piezometers Based on the monitoring studies, the most vulnerable to contamination were the areas closest to the landfill (0.5 km zone), mainly to the northwest and southwest. Monitoring results from piezometer P-2A, located at the outflow on the eastern slope of the landfill, indicate significant changes in the levels of pollution indicators, in particular caused by organic waste storage. Slight increase of the TOC level observed in this piezometer may be a result of its localization in a forest area. A similar trend in the TOC level, caused by forest surroundings, was also noted in piezometer P-4. Monitoring results from piezometer P-6A, localized in a local depression with periodically stagnant water on the surface, show pollution concentrations exceeding the acceptable values for the third water quality class. Exceeded values primarily refer to the TOC level, which can be regarded as typical of a forest area. For piezometer P-7A, the most visible changes concern the COD level. Exceeded values of this parameter result from its localization in an overgrown area. Appl. Sci. 2017, 7, 638 7 of 22

A similar trend in the COD level changes is observed for the monitoring results from piezometer P-9, which is very much dependent on the putrefaction of vegetal remnants along the railway siding situated near to this piezometer. In the case of piezometer P-9, the fluctuations of some pollutant concentrations (for example the TOC level) also result from its localization. Particular influence on the concentration of selected pollution indicators has the nearest area where the production and service facilities are situated. Higher concentration levels of contaminants in piezometer P-11A result from a local depression where the piezometer is located. Because of this, water from the railway siding and the composting area can flow down easily to that point and directly cause groundwater pollution in this piezometer. Fluctuations in the concentrations of pollution indicators (especially TOC) and exceeded standards for the third class of groundwater quality were also observed in piezometer P-12. The main reason for this is runoff of contaminants from the composting plant area. Groundwater in piezometer P-14A meets the standards for the third class of water quality. Only periodic fluctuations in TOC and COD concentrations may result from seasonal changes of groundwater level, which means that increased values of these parameters correspond to lower groundwater levels. Apart from the sporadic excesses above reference values, the TOC and COD concentrations in groundwater from piezometer P-15 located in the area of the MBT installation comply with Polish standards. Seasonal fluctuations of these indicators can be attributed to runoff from the compostory plant and seasonal changes of groundwater level. Seasonal changes in pollution indicator levels observed in groundwater from piezometer P-17 are determined mainly by runoff of contaminants from Estrady St. and the industrial facilities located nearby. Based on the obtained results, it can be concluded that the concentrations of the analyzed parameters have decreased with the distance from the landfill, which can be linked to biodegradation and dilution. A similar trend proving that concentrations of contaminants in groundwater decrease with increased distance from the pollution source was presented by Ling and Zhang [23], Aderemi et al. [36] and Mor et al. [37]. Regarding the influence of precipitation on contaminant concentration we can also claim that the concentrations are linked to the groundwater level which is mainly supplied by rainfall. According to groundwater level monitoring data (not presented in this article) it is noted that groundwater level in all piezometers show seasonal pattern. It should be noted that in accordance to high precipitation and resulting from this high groundwater level, lower concentration of indicators are observed.

3.2. Areal Distribution and Temporal Changes of Organic Pollution Indicators (BOD, COD, TOC) in Groundwater

3.2.1. Biochemical Oxygen Demand (BOD) in Groundwater

As presented by Hazelton and Murphy [38], typical BOD values are: 150–300 mg O2/L for raw sewage, 200–600 mg O2/L for storm water runoff from residential areas, and 20–30 mg O2/L for treated sewage. Unpolluted natural waters are characterized by BOD values below 5 mg O2/L. It can also be stated that a BOD level between 1 and 2 mg O2/L indicates very clean water, 3.0 to 5.0 mg O2/L indicates moderately clean water and BOD > 5 mg O2/L indicates a nearby pollution source. At BOD levels of 100 mg O2/L or higher, the water supply is considered as very polluted with organic waste. Monitoring data of BOD in groundwater show that two years before the closure of the vertical barrier, the BOD values exceeded 500 mg O2/L in some parts of the monitored area (Figure3). Such high concentrations referred to the western and northern part of the landﬁll site, strictly surrounding piezometers P-9, P-7 and P-11A. In the eastern part of the study area, the BOD concentrations were in the range of 100–260 mg O2/L. As a result, it can deﬁnitely be stated that a large part of the study area was at that time contaminated by organic compounds. Appl. Sci. 2017, 7, 638 8 of 22 Appl. Sci. 2017, 7, 637 8 of 23

FigureFigure 3. 3.Distribution Distribution of of the the mean meanvalues values ofof thethe biochemicalbiochemical oxygen demand demand (B (BOD)OD) in in groundwater groundwater in in 1998, 1998, 2004 2004 and and 2016. 2016.

Appl. Sci. 2017, 7, 637 9 of 22 Appl. Sci. 2017, 7, 638 9 of 22 Worth emphasizing is the fact that, as compared to the survey outcomes from 1998, BOD concentrationsWorth emphasizingwere even seven is the times fact that,lower as four compared years after to the the survey closure outcomes of the vertical from 1998, barrier BOD (2004). The concentrationshighest concentrations were even of seven BOD times recorded lower fourat that years time after in the closuremonitored of the area vertical did barriernot exceed (2004). 70 mg O2/L,The but highest still referred concentrations to the presence of BOD recorded of organic at that contaminants time in the monitored in groundwater area did (Figure not exceed 3). 70 mg O /L, but still referred to the presence of organic contaminants in groundwater (Figure3). For2 actual monitoring data (2016), the results show a low BOD level (1.0–7.0 mg O2/L), which means For actual monitoring data (2016), the results show a low BOD level (1.0–7.0 mg O2/L), which that groundwater has not been contaminated with fresh leachate. Increased values of BOD (6–7 mg O2/L) means that groundwater has not been contaminated with fresh leachate. Increased values of BOD concentrations were measured only in the area of the composting plant and the MBT installation. (6–7 mg O2/L) concentrations were measured only in the area of the composting plant and the MBTSimilar installation. results reporting low BOD concentrations in piezometers located at the landfill site were reported,Similar for instance, resultsreporting by Ngang low and BOD Agbazue concentrations [15], Bandara in piezometers and Hettiaratchi located at the [39], landfill and site Sugirtharan were and reported,Rajendran for instance,[40]. As byopposed Ngang andto these Agbazue exampl [15], Bandaraes, incomparably and Hettiaratchi higher [39 ],values and Sugirtharan of BOD were recordedand Rajendran at a municipal [40]. As solid opposed waste to theselandfill examples, site in incomparablySri Lanka (BOD higher = 3590 values mg of O BOD2/L) were [41]. recorded atAdditionally, a municipal solid valuable waste is landfill observation site in Sri of Lanka temporal (BOD changes = 3590 mg of O the2/L) BOD [41]. level from 1998 to 2016 (Figures Additionally,4–6). Apart from valuable the changes is observation before of the temporal closure changes of the ofvertical the BOD barrier level (November from 1998 to 2000), 2016 the monitoring(Figures 4data–6). show Apart stabilization from the changes with before some theexcept closureions of in the case vertical of piezometers barrier (November located 2000),in the area of adjacentthe monitoring facilities. data However, show stabilization in piezometers with some P-10, exceptions P-17 and in P-14, case of data piezometers show very located low invalues the so evenarea some of adjacentchanges facilities. are acceptable However, and in should piezometers not be P-10, disquieting. P-17 and P-14, It is dataalso show worth very emphasizing low values that so even some changes are acceptable and should not be disquieting. It is also worth emphasizing changes of pH can affect the microorganisms that consume the organic matter and then affect the that changes of pH can affect the microorganisms that consume the organic matter and then affect level of pollution caused by organic compounds. The microorganisms (their amount and activity) have the level of pollution caused by organic compounds. The microorganisms (their amount and activity) the significanthave the significant impact impacton BOD on BODvalues. values. In the In thecase, case, when when the the amountamount of of microorganisms microorganisms is small, is small, the processesthe processes of biochemical of biochemical breakdown breakdown do do not occuroccur oror intensity intensity of of the the biochemical biochemical breakdown breakdown is is insignificant.insignificant. In natural In natural conditions, conditions, such such effect effect is is often causedcaused by by the the presence presen ofce toxicof toxic compounds compounds (e.g.,(e.g., heavy heavy metals) metals) that that adversely adversely affect affect the the enzymatic activityactivity of of the the microorganisms. microorganisms. In thatIn that case, case, it mustit must be considered be considered that that the the changed changed values values of of BO BODD do do not not reflect reflect thethe actualactual level level of of water water pollution. pollution.

800 closure of the vertical barrier (November 2000) 700 piezometer P-2A 600 piezometer P-9 /L]

2 500 piezometer P-7 400 300 200 BOD [mg O BOD 100 0

FigureFigure 4. Temporal 4. Temporal changes changes of of the the BOD BOD level inin piezometers piezometers located located in thein the landﬁll landfill area. area. Appl. Sci. 2017, 7, 638 10 of 22 Appl. Sci. 2017, 7, 637 10 of 22 Appl. Sci. 2017, 7, 637 10 of 22

700 closure of the vertical barrier (November 2000) 700 closure of the vertical barrier (November 2000) 600 piezometer P-12 600 piezometer P-12 500 piezometer P-15 500

/L] piezometer P-15 2

/L] 400 piezometer P-11 2 400 piezometer P-11 300 300 200 BOD [mg O BOD 200 BOD [mg O BOD 100 100 0 0

Figure 5. Temporal changes of the BOD level in piezometers located in the area of composting plant. Figure 5.5. Temporal changeschanges ofof thethe BODBOD levellevel inin piezometerspiezometers locatedlocated inin thethe areaarea ofof compostingcomposting plant.plant.

100 closure of the vertical barrier (November 2000) 100 90 closure of the vertical barrier (November 2000) 90 piezometer P-10 80 piezometer P-10 80 70 piezometer P-17

/L] piezometer P-17 2 70 /L] piezometer P-14 2 60 60 piezometer P-14 50 50 40 40

BOD [mg O BOD 30

BOD [mg O BOD 30 20 20 10 10 0 0

Figure 6. Temporal changes of the BOD level in piezometers located in the area of adjacent facilities. Figure 6.6. Temporal changeschanges ofof thethe BODBOD levellevel inin piezometerspiezometers locatedlocated inin thethe areaarea ofof adjacentadjacent facilities.facilities. 3.2.2. Chemical Oxygen Demand (COD) in Groundwater 3.2.2. Chemical Oxygen Demand (COD) in Groundwater 3.2.2. Chemical Oxygen Demand (COD) in Groundwater In this study, the chemical oxygen demand (COD) was used for assessing organic pollution In this study, the chemical oxygen demand (COD) was used for assessing organic pollution levelsIn in this groundwater study, the (Figure chemical 7). oxygen Based on demand the monitoring (COD) was results used from for 1998, assessing it should organic be noted pollution that levels in groundwater (Figure 7). Based on the monitoring results from 1998, it should be noted that levelsthe highest in groundwater COD level (Figurewas observed7). Based near on pi theezometer monitoring P-7A results (COD fromvalue 1998,at 700 it mg should O2/L). be Lower noted COD that the highest COD level was observed near piezometer P-7A (COD value at 700 mg O2/L). Lower COD thevalues highest were COD obtained level for was groundwa observedter near taken piezometer from piezometers P-7A (COD located value in at the 700 northern mg O2/L). partLower of the values were obtained for groundwater taken from piezometers located in the northern part of the CODstudy values area, beyond were obtained the area forof the groundwater MBT installation taken fromand the piezometers composting located plant. inIn thepiezometers northern located part of study area, beyond the area of the MBT installation and the composting plant. In piezometers located thein the study area area, of the beyond biological the areaproc ofessing the MBTof wastes, installation average and values the composting of COD were plant. in the In range piezometers of 100– in the area of the biological processing of wastes, average values of COD were in the range of 100– located150 mg inO2 the/L. In area the of eastern the biological and western processing part of of the wastes, study averagearea (vicinity values of of piezometer COD were P-2A in the and range P-9, 150 mg O2/L. In the eastern and western part of the study area (vicinity of piezometer P-2A and P-9, ofrespectively), 100–150 mg the O2 /L.COD In level theeastern was almost and westernequal to part400 mg of theO2/L. study The arealowest (vicinity level of of COD piezometer was detected P-2A respectively), the COD level was almost equal to 400 mg O2/L. The lowest level of COD was detected andin the P-9, northern respectively), part of the the COD study level area was (COD almost at the equal level to 400of 10 mg mg O 2O/L.2/L). The Monitoring lowest level data of from COD 2004 was in the northern part of the study area (COD at the level of 10 mg O2/L). Monitoring data from 2004 detectedand 2016 inclearly the northern indicate parta decrease of the studyof COD area concentr (CODations at the in level the ofpiezometers. 10 mg O2/L). The Monitoring most significant data and 2016 clearly indicate a decrease of COD concentrations in the piezometers. The most significant fromchange 2004 had and occurred 2016 clearly near piezometer indicate a decrease P-7A (COD of COD level concentrations decreased from in 703 the piezometers.mg O2/L in 1998 The to most 132 change had occurred near piezometer P-7A (COD level decreased from 703 mg O2/L in 1998 to 132 signiﬁcantmg O2/L inchange 2004 and had 100 occurred mg O2/L near in 2016). piezometer P-7A (COD level decreased from 703 mg O2/L in mg O2/L in 2004 and 100 mg O2/L in 2016). 1998 to 132 mg O2/L in 2004 and 100 mg O2/L in 2016). Appl. Sci. 2017, 7, 638 11 of 22 Appl. Sci. 2017, 7, 637 11 of 22

FigureFigure 7. Distribution of of the the mean mean values values of ofthe the chemical chemical oxygen oxygen demand demand (COD) (COD) in groundwater in groundwater in 1998, in 1998,2004 and 2004 2016. and 2016. Appl. Sci. 2017, 7, 638 12 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 12 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 12 of 22 FiguresFigures8 8–10–10 show show stabilization stabilization in in the the COD COD level level after after November November 2000 2000 in in almost almost all all piezometers piezometers Figures 8–10 show stabilization in the COD level after November 2000 in almost all piezometers withwith thethe exceptionexception ofof piezometerspiezometers P-9,P-9, P-11P-11 andand P-10.P-10. In piezometer P-9, located in the area of landfill, landﬁll, with the exception of piezometers P-9, P-11 and P-10. In piezometer P-9, located in the area of landfill, thethe mostmost visiblevisible changechange inin CODCOD levellevel hashas beenbeen observedobserved afterafter 2006.2006. ItIt dependsdepends onon thethe putrefactionputrefaction ofof the most visible change in COD level has been observed after 2006. It depends on the putrefaction of vegetalvegetal remnantsremnants along along the the railway railway siding. siding. In piezometerIn piezometer P-11, P-11, located located in the in area the ofarea composting of composting plant, vegetal remnants along the railway siding. In piezometer P-11, located in the area of composting signiﬁcantplant, significant increase increase of COD of level COD was level observed was observed in 2005, in 2009 2005, and 2009 2012. and Apart 2012. from Apart these from changes, these plant, significant increase of COD level was observed in 2005, 2009 and 2012. Apart from these monitoringchanges, monitoring data show data stabilization. show stabilization. There is There no doubt is no that doubt development that development of an adjacent of an adjacent area is area the changes, monitoring data show stabilization. There is no doubt that development of an adjacent area mainis the causemain ofcause COD of changesCOD changes in groundwater in groundwater in piezometer in piezometer P-10. P-10. is the main cause of COD changes in groundwater in piezometer P-10. InIn Poland,Poland, there there are are no no standards standards established established for the for COD the levelCOD in level groundwater in groundwater or water intendedor water In Poland, there are no standards established for the COD level in groundwater or water forintended human for consumption, human consumption, but general but standards general stan fordards discharge for discharge of environmental of environmental pollutants pollutants mention theintended value offor COD human equal consumption, to 250 mg Obut/L general as the stan highestdards acceptable for discharge limit. of For environmental instance, according pollutants to mention the value of COD equal to2 250 mg O2/L as the highest acceptable limit. For instance, mention the value of COD equal to 250 mg O2/L as the highest acceptable limit. For instance, theaccording National to Agencythe National for Food Agency and Drugsfor Food Administration and Drugs Administration and Control (NAFDAC), and Control the (NAFDAC), recommended the maximumaccording permissibleto the National level Agency of COD for is givenFood and as 294 Drugs mg OAdministration/L [42]. and Control (NAFDAC), the recommended maximum permissible level of COD is given2 as 294 mg O2/L [42]. recommended maximum permissible level of COD is given as 294 mg O2/L [42]. 1800 closure of the vertical barrier (November 2000) 1800 closure of the vertical barrier (November 2000) 1600 piezometer P-2A 1600 piezometer P-2A 1400 piezometer P-9 1400 piezometer P-9 /L] 1200 2 piezometer P-7 /L] 1200 2 1000 piezometer P-7 1000 800 800 600 COD [mg O 600

COD [mg O 400 400 200 200 0 0

Figure 8. Temporal changes of the COD level in piezometers located in the landfill area. Figure 8.8. Temporal changeschanges ofof thethe CODCOD levellevel inin piezometerspiezometers locatedlocated inin thethe landﬁlllandfill area.area. 400 closure of the vertical barrier (November 2000) 400 piezometerclosure of the P-12 vertical barrier (November 2000) 350 piezometer P-12 350 piezometer P-15 300 piezometer P-11P-15 300 piezometer P-11 /L]

2 250 /L]

2 250 200 200 150 150 COD [mg O

COD [mg O 100 100 50 50 0 0

Figure 9. Temporal changes of the COD level in piezometers located in the area of composting plant. FigureFigure 9.9. Temporal changeschanges ofof thethe CODCOD levellevel inin piezometerspiezometers locatedlocated inin thethe areaarea ofof compostingcomposting plant.plant. Appl. Sci. 2017, 7, 638 13 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 13 of 22

400 closure of the vertical barrier (November 2000) 350 piezometer P-10 300 piezometer P-17 /L] 2 250 piezometer P-14 200 150

COD [mg O 100 50 0

FigureFigure 10.10.Temporal Temporal changes changes of of the theCOD CODlevel levelin inpiezometers piezometerslocated locatedin inthe the areaareaof of adjacentadjacentfacilities. facilities.

3.2.3.3.2.3. BOD/CODBOD/COD Ratio TheThe BOD/COD ratio ratio is considered is considered a valuable a valuable indicator indicator of organic of organic matter matterdegradation degradation in a landfill in a[43]. landfill According [43]. According to Ngang and to Ngang Agbazue and [15], Agbazue if the [BOD:15], if COD the BOD:ratio in COD water ratio is found in water to be is foundhigher tothan be higheror equal than to a or value equal of to 0.8, a value the water of 0.8, is thesaid water to be ishighly said to polluted. be highly polluted. BasedBased onon ourour studies,studies, itit cancan bebe statedstated thatthat beforebefore thethe closureclosure ofof thethe verticalvertical barrier,barrier, thethe highesthighest valuesvalues ofof BOD/COD ratios ratios were were observed observed in in piezom piezometereter P-9 P-9 indicating indicating groundwater groundwater pollution pollution in that in thatarea. area. Mean Mean values values of ofBOD/COD BOD/COD ratios ratios detected detected in in the the remaining remaining piezometers piezometers vary fromfrom 0.3790.379 (piezometer(piezometer P-10A)P-10A) toto 0.5390.539 (piezometer (piezometer P-4). P-4). ForFor comparison, comparison, thethe BOD/COD BOD/COD valuesvalues obtainedobtained inin studiesstudies conductedconducted atat aa landfilllandfill sitesite locatedlocated inin FinlandFinland [[44]44] werewere in in the the range range of of 0.44–0.53. 0.44–0.53. AccordingAccording toto UzUz etet al.al. [45[45],], thesethese valuesvalues cancan bebe comparedcompared toto untreateduntreated domesticdomestic wastewater,wastewater, forfor whichwhich thethe BOD/CODBOD/COD ratio varies from 0.4 to 0.8. AfterAfter thethe closureclosure of of the the vertical vertical barrier barrier (November (November 2000), 2000), all all of of the the mean mean BOD/COD BOD/COD ratioratio valuesvalues becamebecame lowerlower thanthan 0.18.0.18. ChangeChange ofof thethe BOD/CODBOD/COD ratiosratios ofof severalseveral ordersorders ofof magnitudemagnitude indicatesindicates significantsignificant improvementimprovement ofof thethe groundwatergroundwater qualityquality inin thethe studystudy area.area. OnOn thethe otherother hand,hand, CODCOD indicatesindicates thethe totaltotal organicorganic mattermatter contentcontent ofof anan effluent,effluent, bothboth biodegradablebiodegradable asas non-biodegradable,non-biodegradable, whereaswhereas BOD BOD only only measures measures the the biodegradable biodegradable fraction. fraction. Thus, Thus, the the lowest lowest ratio ratio BOD/COD BOD/COD indicatesindicates thatthat itit hashas a a lower lower proportion proportion of of biodegradability. biodegradability. ComparisonComparison ofof thethe minimum, minimum, maximum, maximum, mean mean and and standard standard deviation deviation of of the the BOD/COD BOD/COD ratiosratios obtainedobtained forfor allall piezometerspiezometers during during the the entire entire monitoring monitoring period period are are presented presented in in Table Table3. 3. Table 3. BOD/COD ratio for groundwater samples collected in the study area. Table 3. BOD/COD ratio for groundwater samples collected in the study area. BOD/COD Ratio before the Closure of the BOD/COD Ratio after the Closure of the Piezometar BOD/COD RatioVertical before Barrier the Closure of the BOD/COD RatioVertical after the Barrier Closure of the Piezometar Min MaxVertical BarrierMean STD MinVertical Max Barrier Mean STD P-2A 0.370Min 0.663 Max Mean0.490 STD0.132 Min0.003 Max0.352 Mean0.119 STD 0.119 P-4P-2A 0.279 0.370 0.6631.055 0.4900.539 0.1320.448 0.0030.001 0.3520.481 0.1190.105 0.119 0.127 P-6AP-4 0.260 0.279 1.0550.673 0.5390.493 0.4480.206 0.0010.011 0.4810.469 0.1050.118 0.127 0.123 P-7AP-6A 0.370 0.260 0.6730.498 0.4930.415 0.2060.072 0.0110.007 0.4690.384 0.1180.144 0.123 0.121 P-7A 0.370 0.498 0.415 0.072 0.007 0.384 0.144 0.121 P-9 P-90.560 0.560 2.062 1.5611.561 0.7080.708 0.0050.005 0.6100.610 0.1400.140 0.155 0.155 P-10AP-10A 0.368 0.368 0.390 0.3790.379 0.0160.016 0.0040.004 0.4870.487 0.1390.139 0.136 0.136 P-11AP-11A n.d. n.d. n.d. n.d.n.d. n.d.n.d. 0.004 0.004 0.4450.445 0.1330.133 0.139 0.139 P-12P-12 n.d. n.d. n.d. n.d.n.d. n.d.n.d. 0.011 0.011 0.6710.671 0.1460.146 0.137 0.137 P-14A 0.270 0.560 0.454 0.150 0.016 0.543 0.155 0.119 P-14AP-17 0.270 n.d. 0.560 n.d. 0.454 n.d. n.d.0.150 0.0120.016 0.4390.543 0.1710.155 0.132 0.119 P-17 n.d. n.d. n.d. n.d. 0.012 0.439 0.171 0.132 Note: n.d. (not detected). Note: n.d. (not detected).

Appl. Sci. 2017, 7, 638 14 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 14 of 22

3.2.4. Total OrganicOrganic CarbonCarbon (TOC)(TOC) inin GroundwaterGroundwater TOC (total organic carbon) is considered as the most relevant parameter for quantifying organic pollution in water [[46].46]. The Regulation of the MinisterMinister of the Environment of Poland on the criteriacriteria and method method of of evaluating evaluating the the underground underground water water condition condition sets sets the theupper upper acceptable acceptable limit limitfor TOC for TOCin groundwater in groundwater at the at thelevel level of 10 of 10mg mg C/L. C/L. Comparison Comparison to tothe the results results obtained obtained in in our studiesstudies (Figures(Figures 1111–14)–14) indicatesindicates thatthat thethe groundwatergroundwater isis contaminatedcontaminated byby organicorganic chemicals.chemicals. The highesthighest values are observed nearest the landﬁlllandfill (piezometers P-7A (in 2007), P-9 (in 2015) and P-2A (in 2006)). However, FigureFigure 12 12 shows shows that that level level of TOCof TOC in piezometerin piezometer P-7 wasP-7 stablewas stable during during analyzed analyzed period period (from 1998(from to 1998 2016) to as 2016) opposed as opposed to a piezometer to a piezometer P-9. In piezometerP-9. In piezometer P-2A, after P-2A, 2009 after monitoring 2009 monitoring data indicate data stabilization.indicate stabilization. In 2016, the In concentrations2016, the concentratio were lower,ns were but still lower, exceeded but still the maximumexceeded allowablethe maximum limit. Theallowable maximum limit. level The ofmaximum TOC is currently level of TOC observed is currently around observed piezometers around P-9 and piezometers P-11A but P-9 in piezometerand P-11A P-11Abut in afterpiezometer 2009 increase P-11A inafter level 2009 of TOCincrease was observed.in level of WorthyTOC was of noteobserved. is a fact Worthy that piezometer of note is P-11Aa fact isthat located piezometer in a local P-11A depression, is located and in consequently, a local depression, water fromand theconsequently, railway siding water and from the compostingthe railway areasiding can and ﬂow the down composting easily to area that can point flow and down directly easily cause to that groundwater point and pollutiondirectly cause in this groundwater piezometer. Inpollution case of piezometersin this piezometer. located In in case the areaof piezometers of adjacent located facilities, in afterthe area 2009 of decrease adjacent of facilities, the TOC after level 2009 was observed.decrease of Due the toTOC the lacklevel of was monitoring observed. data Due from to the the lack period of monitoring before theclosure data from of the the vertical period barrier,before the overallclosure assessmentof the vertical of the barrier, impact the of overall the cut-off assessment wall construction of the impact on the of the TOC cut-off in groundwater wall construction cannot beon made.the TOC in groundwater cannot be made.

Figure 11. Distribution of the mean values of the totaltotal organic carbon (TOC) in groundwater in 2004 and 2016. Appl. Sci. 2017, 7, 638 15 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 15 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 15 of 22 Appl. Sci.500 2017, 7, x FOR PEER REVIEW piezometer P-2A 15 of 22 450500 piezometerpiezometer P-9P-2A 500 piezometer P-2A 400450 piezometerpiezometer P-7P-9 450400 piezometer P-9 350 Polishpiezometer standard P-7 of water quality class 400 piezometer P-7 300350 Polish standard of water quality class 350 250300 Polish standard of water quality class 300 200250

TOC [mg C/L] 250 150200

TOC [mg C/L] 200 100150 TOC [mg C/L] 150 10050 100 500 500 0

Figure 12. Temporal changes of the TOC level in piezometers located in the landfill area. Figure 12. Temporal changes of the TOC level in piezometers located in the landﬁll area. Figure 12. Temporal changes of the TOC level in piezometers located in the landfill area. 100 Figure 12. Temporal changes of the TOC levelpiezometer in piezometers P-12 located in the landfill area. 100 90 piezometerpiezometer P-15P-12 100 piezometer P-12 8090 piezometerpiezometer P-11P-15 90 piezometer P-15 piezometer P-11 7080 Polish standard of water quality class 80 piezometer P-11 Polish standard of water quality class 6070 70 Polish standard of water quality class 5060 60 TOC [mgC/L] 4050 50 TOC [mgC/L] 3040 TOC [mgC/L] 40 2030 30 1020 20 100 10 0 0

Figure 13. Temporal changes of the TOC level in piezometers located in the area of composting plant.

FigureFigure 13. Temporal 13. Temporal changes changes of the of the TOC TOC level level in in piezometers piezometers located in in the the area area of ofcomposting composting plant. plant. Figure100 13. Temporal changes of the TOC level in piezometpiezometerers located P-10 in the area of composting plant. 10090 piezometerpiezometer P-17P-10 10090 piezometer P-10 80 piezometerpiezometer P-14P-17 9080 piezometer P-17 70 Polishpiezometer standard P-14 of water quality class 80 piezometer P-14 6070 Polish standard of water quality class 70 5060 Polish standard of water quality class 60 4050 50 3040 TOC [mg C/L] 40 2030 TOC [mg C/L] 30

TOC [mg C/L] 1020 20 100 100 0

Figure 14. Temporal changes of the TOC level in piezometers located in the area of adjacent facilities. Appl. Sci. 2017, 7, 638 16 of 22

3.3. Statistical Analysis of the Monitoring Data

3.3.1. Pearson’s Correlation Analysis Pearson’s correlation analysis between selected pollution indicators were conducted for the monitoring data from piezometers located within the landfill, the composting plant, and the area of the adjacent facilities. Out of a total of 30 correlations (10 for each piezometer) calculated between the selected pollution indicators for piezometers located within the landfill area, 13 were found to be significant at a level of p < 0.05 (six for piezometer P-2A, four for piezometer P-7A and three for piezometer P-9). The most significant correlation (r > 0.8) was detected between BOD and COD, and EC and COD for piezometer P-2A (r = 0.859 and r = 0.813, respectively). According to the classification of the correlation coefficient (r) presented by Evans [47], it can be stated that strong correlation exists between BOD and COD, COD and TOC, and COD and EC for groundwater samples taken from piezometer P-7A (r = 0.621, r = 0.652, r = 0.679, respectively). High values of COD compared to BOD and no correlation between them observed in piezometer P-9 can indicate that the major part of organic compounds is not biodegradable. Strong correlation (r = 0.772) was also observed between COD and TOC for piezometer P-9, and very strong correlation was observed in groundwater from this piezometer between COD and EC, and TOC and EC concentrations (r = 0.815 and r = 0.826, respectively) (Table4). Positive correlations between BOD versus COD (r = 0.652), and TOC versus COD (r = 0.743) were also obtained by Maitera et al. [48], indicating that both are products of organic matter oxidation.

Table 4. Pearson’s correlation analysis between selected pollution indicators based on monitoring data from piezometers located within the landﬁll area.

Variable BOD COD TOC pH EC PIEZOMETER 2A BOD 1 0.859 * 0.601 * −0.246 0.649 * COD 1 0.696 * −0.154 0.813 * TOC 1 −0.338 ** 0.645 * pH 1 −0.214 EC 1 PIEZOMETER 7A BOD 1 0.621 * 0.378 ** 0.052 0.576 * COD 1 0.652 * 0.268 0.679 * TOC 1 0.054 0.354 ** pH 1 0.246 EC 1 PIEZOMETER 9 BOD 1 0.190 0.018 −0.255 0.137 COD 1 0.772 * −0.004 0.815 * TOC 1 0.063 0.826 * pH 1 −0.057 EC 1 * correlation signiﬁcant at p < 0.01 (two-tailed), ** correlation signiﬁcant at p < 0.05 (two-tailed).

For piezometers located in the area of the composting plant and the Municipal Waste Treatment Installation (P-11A, P-12, and P-15), pollution indicators show weaker mutual correlation (Table5). Moderate positive correlation was observed between BOD and COD (r = 0.599), and TOC and EC (r = 0.424) for piezometer P-11A. Moderate negative correlation was observed in this piezometer between BOD and TOC (r = −0.517), and BOD and EC (r = −0.410). In piezometer P-12, the strongest correlation was calculated between COD and TOC (r = 0.717). Moderate correlation exists between COD and EC (r = 0589), and TOC and EC (r = 0.413). Other parameters detected for this piezometer have revealed weak (r < 0.39) or very weak (r < 0.19) correlation. For piezometer P-15, only the relation between BOD and COD was found to be strong (r = 0.666). Moderate negative correlation was Appl. Sci. 2017, 7, 638 17 of 22 observed in piezometer P-15 between BOD and EC (r= −0.407). Weak or very weak correlation was calculated among the rest of the selected parameters.

Table 5. Pearson’s correlation analysis between selected pollution indicators based on monitoring data from piezometers located within the composting area.

Variable BOD COD TOC pH EC PIEZOMETER 11A BOD 1 0.599 * −0.517 * 0.102 −0.410 * COD 1 −0.080 0.303 ** −0.005 TOC 1 0.022 0.424 * pH 1 −0.201 EC 1 PIEZOMETER 12 BOD 1 0.308 ** −0.153 0.188 −0.227 COD 1 0.717 * −0.060 0.589 * TOC 1 −0.263 0.413 * −0.305 pH 1 ** EC 1 PIEZOMETER 15 BOD 1 0.666 * 0.127 0.072 −0.407 * COD 1 0.124 0.146 0.141 TOC 1 −0.168 0.282 pH 1 0.017 EC 1 * correlation signiﬁcant at p < 0.01 (two-tailed), ** correlation signiﬁcant at p < 0.05 (two-tailed).

Among 30 correlations calculated for parameters measured in the piezometers located in the area of the facilities adjacent to the landﬁll (Table6), only one pair of variables has revealed a very strong relation (r = 0.878, measured between BOD and COD in piezometer P-14A). In piezometer P-10A, moderate correlation was observed between BOD and COD (r = 0.462), and COD and TOC (r = 0.468). Correlations between other selected indicators were found to be weak or very weak for this piezometer. With the exception of the correlation between BOD and EC (r = 0.494), a similar tendency was observed for parameters analyzed in piezometer P-17.

Table 6. Pearson’s correlation analysis between selected pollution indicators based on monitoring data from piezometers located in the area of the adjacent facilities.

Variable BOD COD TOC pH EC PIEZOMETER 10A BOD 1 0.462 * 0.311 ** −0.089 0.517 * COD 1 0.468 * 0.298 ** 0.321 ** TOC 1 0.140 0.369 ** −0.380 pH 1 ** EC 1 PIEZOMETER 14A BOD 1 0.878 * 0.052 −0.265 −0.015 COD 1 0.211 −0.237 0.147 TOC 1 −0.313 ** 0.352 ** pH 1 −0.133 EC 1 PIEZOMETER 17 BOD 1 0.352 ** 0.014 0.137 0.494 * COD 1 0.193 0.006 0.232 TOC 1 −0.078 0.004 pH 1 −0.144 EC 1 * correlation signiﬁcant at p < 0.01 (two-tailed), ** correlation signiﬁcant at p < 0.05 (two-tailed). Appl. Sci. 2017, 7, 638 18 of 22

3.3.2. Box-and-Whisker Plots The box-and-whisker plots displayed in Figures 15–17 show the minimum, median and maximum tendency, and the extreme results of monitoring data from 1998 to 2016 collected four times a year, usually in March, June, September and November. Appl. Sci. 2017, 7, x FOR PEER REVIEW 18 of 22 The results show that the minimum BOD levels are in the range of 0.5 mg O2/L for all piezometers but the maximumBOD concentrations 1 are observed 0.352 ** in piezometers 0.014 P-9, P-7A, 0.137 P-11A 0.494 and * P-2A (800, 650, COD 1 0.193 0.006 0.232 600 and 260 mg OCOD2/L, respectively). However,1 these values0.193 were obtained0.006 from the0.232 period before the TOC 1 −0.078 0.004 closure of the verticalTOC barrier. 1 −0.078 0.004 pH 1 −0.144 A similar situation was observed in the case of COD concentrations in groundwater. During the EC 1 1998–2016 monitoring* correlation significant period, the at p maximum < 0.01 (two-tailed), COD levels** correlation were observedsignificant at in p piezometers< 0.05 (two-tailed). P-7A (1758 mg O2/L), P-9 (1157 mg O2/L), P-2A (705 mg O2/L) and P-11A (377 mg O2/L). 3.3.2.In the Box-and-Whisker case of TOC concentrationsPlots in groundwater, the minimum levels were observed in piezometersThe P-10A,box-and-whisker P-12, and plots P-15 (1displayed mg C/L), in andFigu theres maximum15–17 show levels the minimum, in piezometers median P-9, and P-2A, andmaximum P-7A (437, tendency, 359 and and 129 the mg extreme C/L, respectively). results of moni Thistoring conﬁrms data from that 1998 the to BOD,2016 collected COD, and four TOC concentrationstimes a year, in usually groundwater in March, are June, determined September by and the November. location of these piezometers.

900

800

700

600 /L] /L] 2 2 500

400 BOD [mg O BOD [mg O 300

200

100 Median 0 0 25%-75% 12 15 11 2 9 7 10 17 14 Minimum-Maximum piezo meters Mean piezo meters Mean FigureFigure 15. 15.Box-and-whisker Box-and-whisker plots plots ofof the BOD level level in in particular particular piezometers. piezometers.

2000

1800

1600

1400 /L] /L] 2 2 1200

1000

800 COD [mg O COD [mg O 600

400

200 Me dia n 0 0 25%-75% 12 15 11 2 9 7 10 17 14 Minimum-Maximum piezometers Me a n piezometers Me a n FigureFigure 16. 16.Box-and-whisker Box-and-whisker plots plots ofof the COD level level in in particular particular piezometers. piezometers. Appl. Sci. 2017, 7, 638 19 of 22 Appl. Sci. 2017, 7, x FOR PEER REVIEW 19 of 22

500

400

300

200 TOC [mg C/L]

100

Me dia n 0 25%-75% 12 15 11 2 9 7 10 17 14 Minimum-Ma ximum piezometers Me a n FigureFigure 17. 17.Box-and-whisker Box-and-whisker plots plotsof of thethe TOC level in in particular particular piezometers. piezometers.

The results show that the minimum BOD levels are in the range of 0.5 mg O2/L for all 4. Conclusions piezometers but the maximum concentrations are observed in piezometers P-9, P-7A, P-11A and P- Leachate2A (800, 650, containing 600 and large 260 mg concentrations O2/L, respectively). of organic However, compounds these values can result were in obtained many environmental from the problems,period mainlybefore the associated closure of withthe vertical groundwater barrier. contamination. This study has shown that trends pointingA to similar the deterioration situation was orobserved improvement in the case of of waterCOD concentrations quality can bein groundwater. determined During based the on the measurement1998–2016 resultsmonitoring coming period, from the longer maximum time COD spans. levels Assessment were observed of the in results piezometers of groundwater P-7A (1758 mg O2/L), P-9 (1157 mg O2/L), P-2A (705 mg O2/L) and P-11A (377 mg O2/L). monitoring in a landfill site is based on the analysis of pollution indicator parameters. The results of In the case of TOC concentrations in groundwater, the minimum levels were observed in monitoring data presented in this paper clearly indicate the crucial impact of cut-off wall application piezometers P-10A, P-12, and P-15 (1 mg C/L), and the maximum levels in piezometers P-9, P-2A, on theand protection P-7A (437, of 359 the soil-waterand 129 mg environment. C/L, respectively). The obtained This confirms results that in somethe BOD, piezometers COD, and still TOC exceed standardconcentrations of the third in groundwater water quality are class determined established by the by location Polish of law. these In piezometers. particular, it is clearly visible for concentration of the total organic carbon which are still observed at the level greater than 10 mg C/L.4. Comparing Conclusions the environmental condition from the 1998 to the present state, it can be claimed that a significantLeachate improve containing of groundwater large concentrations quality is observed. of organic compounds can result in many Theenvironmental obtained resultsproblems, show mainly that associated the construction with groundwater of a vertical contamination. bentonite barrierThis study has has significantly shown improvedthat trends water pointing quality to in the almost deterioration all piezometers. or improvement For some of samplingwater quality points, can be decrease determined in the based value of the parameterson the measurement was observed results bycoming a few from orders longer of magnitude.time spans. Assessment Especially of visible the results is the of decreasegroundwater of BOD levelmonitoring which is now in a 160landfill times site lower is based than on inthe 1998. analysis of pollution indicator parameters. The results of Basedmonitoring on thedata monitoring presented in studies,this paper it clearly can be indi statedcate the that crucial the mostimpact vulnerable of cut-off wall to contaminationapplication wereon the the areas protection closest of the to thesoil-water landfill environment. and the area The of obtained the composting results in some plant. piezometers It is also still worth exceed noting standard of the third water quality class established by Polish law. In particular, it is clearly visible that external sources can have a great impact on groundwater contamination by organic compounds. for concentration of the total organic carbon which are still observed at the level greater than 10 mg As was emphasized in this article, the forest surroundings, putrefaction of vegetal remnants or location C/L. Comparing the environmental condition from the 1998 to the present state, it can be claimed that of piezometersa significant in improve the overgrown of groundwater area can quality contribute is observed. to increased organic pollution indicator levels observedThe in groundwater obtained results samples show that taken the from construction such location. of a vertical bentonite barrier has significantly Differentimproved water correlations quality in between almost all parameters piezometers. obtained For some for sampling the landfill points, area decrease and the in the composting value plantof may the parameters indicate that was more observed factors by havea few aorders direct of impact magnitude. on pollution Especially indicators visible is the in thedecrease case of the compostingBOD level plant. which Only is now a detailed 160 times analysis lower than would in 1998. allow us to find how specific factors influence the correlationsBased that on we the obtainedmonitoring and studies, certainly it can webe stated should that take the most into considerationvulnerable to contamination such analysis were in our furtherthe studies. areas closest to the landfill and the area of the composting plant. It is also worth noting that Worthyexternal ofsources note iscan also have the a factgreat that impact seasonal on gr changesoundwater of groundwatercontamination contaminationby organic compounds. level can be As was emphasized in this article, the forest surroundings, putrefaction of vegetal remnants or strictly associated with the groundwater level changes. It was not analyzed in detail for the purpose of location of piezometers in the overgrown area can contribute to increased organic pollution indicator this article but based on the monitoring data we can claim that increased values of organic pollution levels observed in groundwater samples taken from such location. indicator levels correspond to lower groundwater levels. For further studies, it would be valuable to analyze the seasonal pattern of groundwater level changes and resulting in that the variability of measured parameters on an annual basis. Appl. Sci. 2017, 7, 638 20 of 22

At present, the levels of BOD, COD and TOC in groundwater point to contamination by organic substances in the study area. Thus, the authors point out the need of a re-analysis in the future based on monitoring data covering a longer time span, which should aid in the assessment of the effectiveness of groundwater self-puriﬁcation.

Author Contributions: Anna Miszkowska, Anna Sieczka and Eugeniusz Koda conceived the subject of the paper, prepared and wrote the manuscript. Anna Miszkowska and Anna Sieczka were responsible for data analysis and figures preparation. Eugeniusz Koda provided the expertise on the research. Conflicts of Interest: The authors declare no conflict of interest.

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