Analysis of Biogenic Secondary Pollution Materials from Sludge in Surface Waters

International Journal of Environmental Research and Public Health

Article Analysis of Biogenic Secondary Pollution Materials from Sludge in Surface Waters

Laima Cesonienˇ e˙ * , Edita Mažuolyte-Miškin˙ e,˙ Daiva Šileikiene,˙ Kristina Lingyte˙ and Edmundas Bartkeviˇcius Institute of Environment and Ecology, Faculty of Forest Science and Ecology, Vytautas Magnus University, Studentu˛str. 11, LT–53361 Akademija, Kaunas, Lithuania; [email protected] (E.M.-M.); [email protected] (D.Š.); [email protected] (K.L.); [email protected] (E.B.) * Correspondence: [email protected]; Tel.:+370-37752-224

Received: 18 October 2019; Accepted: 20 November 2019; Published: 25 November 2019

Abstract: Many countries of the world, including Lithuania, are making an effort to reduce surface water pollution. State monitoring data show that almost 80% of the lakes in Lithuania have an increased amount of sludge. One of the reasons for this increase in sludge is an excessive amount of biogenic material in the water. It is known that even after the source of pollution is removed, the condition of the lake water does not improve; rather, the condition of the lake water worsens due to the secondary pollution of sludge in the water. A study was conducted to determine the impact of secondary sludge pollution on water. For this study, 5 sludge samples were taken from different lakes in Lithuania. Fresh water was poured on the sludge samples, the concentrations of Nt, NO2-N, NO3-N, NH4-N, PO4-P, Pt, the pH and the changes in the electric conductivity (C) were measured in the water within 28 h. Research has shown that the thickness of the sludge layer influences the total amounts of nitrogen, phosphorus, and organic matter present in the sludge. As the thickness of the sludge layer increases in a lake, the total concentrations of nitrogen, total phosphorus and organic matter increase. Studies have also shown that the concentrations of all biogenic substances in water increase, with the exception of total phosphorus. This finding shows that organic phosphorus is "locked" in sludge, and no secondary pollution occurs from this source. Moreover, the electrical conductivity values of the water influence the release of biogenic substances from sludge in the water.

Keywords: lakes; sampling; nitrogen; phosphorus

1. Introduction Water quality depends mostly on the characteristics and quantities of pollutants entering a body of water. In Lithuania, as in many European countries, the main water polluters are industry, agriculture, and households [1]. The factors determining the quality of surface water can be divided into direct and indirect factors. Direct factors (rocks, soils, biota, and human activity) remove or supply soluble chemical compounds in water. Indirect factors (climate, terrain, water regime, and vegetation, hydrological, and hydrodynamic conditions) produce an environment in which materials can interact with water [2]. The increased nutrients concentration enhanced the eutrophication, and, as a consequence, production of organic matter increases due to increased primary production. This has affected many water bodies, especially since the middle of the 19th century with the development of agricultural intensity and wastewater treatment plants [3]. Despite much research over the last five decades, eutrophication remains a major problem worldwide [4]. Lake ecosystems are particularly susceptible to anthropogenic effects, as they can become a reservoir of water pollutants and a main cause of the deterioration of the ecological status in inland and coastal waters [5–7]. In recent years, eutrophication in lakes has been one of the major ecological problems in the world. Most freshwater

Int. J. Environ. Res. Public Health 2019, 16, 4691; doi:10.3390/ijerph16234691 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2019, 16, x 2 of 17 Int. J. Environ. Res. Public Health 2019, 16, 4691 2 of 17 eutrophication in lakes has been one of the major ecological problems in the world. Most freshwater lakeslakes and and wetlands wetlands are are facing facing water water quality quality degradation degradation and and ecological ecological imbalances imbalances duedue toto growing growing anthropogenicanthropogenic activity, activity, especially especially in in developing developing countries countries [ 8[8].]. NitrogenNitrogen and and phosphorusphosphorus compounds compounds areare considered considered to to be be the the key key factors factors in in the the development development of of eutrophication eutrophication in in lakes lakes [ 9[9,10].,10]. TheThe transformationtransformation of nitrogen compounds compounds into into sl sludgeudge and and subsequent subsequent migration, migration, as aswell well as the as thereactions reactions between between sludge sludge and and lake lake water, water, are are important important processes processes in in the the biochemical cyclecycle ofof lakelake ecosystemsecosystems [ 10[10].]. TheThe nitrogennitrogen andand phosphorusphosphorus areare compoundscompounds thatthat promotepromote thethe primaryprimary productionproduction inin thethe lakeslakes thatthat cancan result result in in the the algal algal blogoms. blogoms. PhosphorusPhosphorus isis usuallyusually thethe limitinglimiting nutrientnutrient inin freshwater,freshwater, and and control control of of the the phosphorus phosphorus input input can can improve improve the the water water quality quality [ 11[11,12].,12]. ItIt hashas beenbeen foundfound that that the the “internal” “internal” pollution pollution ofof phosphorusphosphorus poses poses a a more more serious serious risk risk in in shallow shallow lakes lakes than than in in deepdeep lakeslakes [13[13].]. TheThe finestfinest particlesparticles areare usuallyusually foundfound inin thethe deepestdeepest areasareas ofof lakelake bottoms,bottoms, wherewhere relativelyrelatively intensive intensive sedimentation sedimentation occurs occurs due due to to the the production production of of phytoplankton. phytoplankton. SedimentSediment organicorganic mattermatter intensifiesintensifies biogeochemicalbiogeochemical nutrientnutrient overlaysoverlays onon thethe sludgesludge surface,surface, suchsuch asas mineralization,mineralization, denitrification,denitrification, oror phosphorusphosphorus relaxationrelaxation [14[14].]. AccordingAccording toto ŠaulysŠaulys [15[15],],phosphorus phosphorus entersenterssurface surface waterswaters byby being being flushedflushed fromfrom thethe soil,soil, rinsedrinsed outout ofof rocks,rocks,and and releasedreleased asas aaproduct product ofof vitalvital activityactivity andand thethe decompositiondecomposition ofof aquaticaquatic organisms.organisms. BottomBottom sedimentssediments areare anan importantimportant elementelement ofof lakelake ecosystemsecosystems because because they they are are involved involved in in internal internal nutrient nutrient cycle cycle processes, processes, and and the the main main role role of of bottombottom sedimentssediments is is to to accumulate accumulate nutrients nutrients [ 16[16].]. TheThe depositiondeposition of of phosphorusphosphorus from from sludge sludge to to water water and and its its transfertransfer to to the the trophogenic trophogenic zone zone may may be be more more intense intense than than its its sorption sorption and and deposition deposition [17 ].[17]. The The aim aim of theof the studies studies was was to determine to determine the the impact impact of secondary of secondary sludge sludge pollution pollution on water.on water.

2.2. MaterialsMaterials andand MethodsMethods LakeLake sludgesludge studiesstudies werewere carriedcarried out,out, consideringconsidering thethe depthdepth ofof sludge,sludge, organicorganic mattermatter content,content, nitrogennitrogen and and phosphorus phosphorus concentrations. concentrations. FiveFive lakes lakes were were selected selected for for detailed detailed research. research. The The reasonreason for forthe the choicechoice isis thethe highhigh concentrationconcentration ofof N N andand PP inin thethe sludge.sludge. AA total of 7–10 sludge samplessamples were taken from 5 didifferentﬀerent LithuanianLithuanian lakes:lakes: Kiementas,Kiementas, Sp Spera,˙ėra, Biržulis, Biržulis, Gauštvinis, Gauštvinis, and and Antakmeniai. Antakmeniai.Figure Figure1 1shows shows the the layout layout of of the the lakes. lakes.

FigureFigure 1. 1.The The layoutlayout ofof thethe investigatedinvestigated lakeslakesin inthe theterritory territoryof of Lithuania. Lithuania.

LakeLake Biržulis Biržulis is is locatedlocated inin westernwestern LithuaniaLithuania inin thethe TelšiaiTelšiai district,district, approximatelyapproximately 5 5 km km northeastnortheast ofof VarniaiVarniai (Figure (Figure1 )1) in in Varniai Varniai Regional Regional Park. Park. The The length length of of the the lake lake in in the the north-south north-south direction direction is is 3.63.6 km, km, and and the the width width is is 1.3 1.3 km. km. The The bottom bottom of Lakeof Lake Biržulis Biržulis is covered is covered with with sapropelic sapropelic clay, clay, sludge sludge and

Int. J. Environ. Res. Public Health 2019, 16, 4691 3 of 17 Int. J. Environ. Res. Public Health 2019, 16, x 3 of 17 andclay clay sapropel sapropel in the in westernthe western and northernand northern bays bays and sandand sand along along the east the coast. east coast. In the In west, the west, the Varna the Varnariver flows river fromflows Lake from L ukštas,Lake¯ Lū andkštas, the and Nakaˇciariver the Nakač flowsia river into flows the southinto the of thesouth lake. of the Along lake. the Along Venta thetributary, Venta tributary, the Virvyt thee˙ river Virvyt flowsė river into flows the northinto the of north the lake. of the There lake. are There groundwater are groundwater springs springs on the oneastern the eastern slope ofslope the lake.of the lake. LakeLake Antakmeniai Antakmeniai is locatedlocated inin southeasternsoutheastern Lithuania Lithuania in in the the Trakai Trakai district, district, approximately approximately 4 km 4 kmnortheast northeast of Aukštadvaris.of Aukštadvaris. The The length length of of the the lake lake in in the thewest-southeast west-southeast directiondirection is 2 km, and and the the widthwidth is is up up to to 0.7 0.7 km. km. The The deepest deepest place place is is 12.5 12.5 m. m. GauštvinisGauštvinis Lake Lake is is located located in in the the middle middle of of Lithuania Lithuania in in the the Kelm Kelmėe˙ district, district, approximately approximately 6 6 km km northnorth of of Tytuv Tytuvėenainai˙ on on the the southern southern edge edge of ofthe the Great Great Tyruliai Tyruliai peat peat bog. bog. The The length length of the of lake the lakein the in north-souththe north-south direction direction is 3.1 is km, 3.1 km,the thewidth width is 0.6 is 0.6km, km, and and the the dept depthh is isup up to to 5 5m. m. The The Šimša Šimša and and SpangupisSpangupis rivers rivers flow flow into into lake lake Gauštvinis, Gauštvinis, and and the the Dubysa Dubysa tributary tributary Gryzuva Gryzuva flows flows into intoit from it from the south.the south. LakeLake Sp Spėera˙ra is is located located in in the the Širvintos Širvintos district district of of central central Lithuania. Lithuania. The The maximum maximum lake lake depth depth of of LakeLake Sp Spėera˙ra is is only only 2.7 2.7 m, m, and and the the average average depth depth is is 1. 1.8585 m. m. The The thickness thickness of of the the accumulated accumulated sludge sludge layerlayer is is 6.3 6.3 m, m, and and the the average average layer layer thickness thickness is is 3.0 3.0 m. m. LakeLake Kiementas Kiementas (N55.076553, (N55.076553, E25.267448) E25.267448) is is located located in in the the southwest southwest of of the the Mol Molėetai˙tai district district municipality,municipality, ea eastst of of Giedrai Giedraiˇciai.Thečiai. The lake lake is is of of a a glacial glacial desert desert or origin.igin. The The area area of of the the lake lake is is 98.6 98.6 ha. ha. TheThe length length is is 2.4 2.4 km, km, and the maximum width is 0.7 km. The pool area is 23.323.3 kmkm².2;. A A stream stream flows flows intointo Širvintos Širvintos through through Lake Lake Kiementis Kiementis and and belongs belongs to to the the Šventoji Šventoji Basin. Basin. The The leakage leakage of of the the lake lake is is 198%.198%. The The seabed seabed is is covered covered with with muddy muddy sand sand and and deep deep sludge sludge

2.1.2.1. Methodology Methodology for for Sampling Sampling and and Laboratory Laboratory Analysis Analysis SludgeSludge and and lake lake water samples were takentaken fromfrom variousvarious parts parts of of the the lake. lake. The The sampling sampling scheme scheme is isshown shown in in Figure Figure2. 2.

Lake Birzulis Lake Kiementas Figure 2. Cont.

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Lake Gaustvinis Lake Antakmeniu

Lake Spera

FigureFigure 2. 2.SamplingSampling scheme. scheme.

AA composite composite sample sample is ismade made from from a a7–10 7–10 of of discrete discrete samples samples that that have have been been collected collected from from differentdiﬀerent locations locations in in the the lake lake and and combined combined into into a asingle single sample. sample. This This single, single, composite, composite, sample sample is is representativerepresentative of ofthe the average average sludge sludge properties. properties. Sludge Sludge and andlake lakewater water samples samples were weretaken taken under under the samethe samenatural natural conditions, conditions, namely, namely, sunny sunny days. days.After the After collection the collection of the sludge, of the sludge,all samples all samples were transferredwere transferred to a laboratory to a laboratory and stored and stored in a refrigerator in a refrigerator (5 °C). (5 ◦ C).Sludge Sludge samples samples are are taken taken in inglass glass cylinderscylinders during during daylight daylight hours hours according according to to EN EN ISO ISO standards: standards: LSTLST EN EN 16179: 16179: 2012 2012 Sludge, Sludge, treat treateded bio-waste bio-waste and and soil soil—Guidance – Guidance for for sample sample pretreatment pretreatment [18]. [18 ]. LSTLST EN EN ISO ISO 5667-13:2011 5667-13:2011 Water Water quality quality—Sampling—Part – Sampling – Part 13: Guidance 13: Guidance on sampling on samplingof sludge’s of [19].sludge’s For sludge [19]. collection For sludge was collection used the wasvacuum used sampler. the vacuum All of sampler. the sludge All samples, of the along sludge with samples, the overlayingalong with water, the overlayingwere collected water, with were a glass collected cylinders with ( aΦ-25 glass mm, cylinders length (5Φ m),-25 the mm, sludge length cores 5 m), containedthe sludge 30–40 cores cm contained of sediment 30–40 in cm a oftube, sediment and they in awere tube, covered and they by-overlaying were covered by-overlayingwater to maintain water theto sludge maintain structure the sludge during structure sampling during and sampling transportation. and transportation. LSTLST EN EN ISO ISO 5667-15:2009 5667-15:2009 Water Water quality quality—Sampling-Part –Sampling-Part 15: 15: Guidance Guidance on on preservation preservation and and handlinghandling of of sludge sludge and and sediment sediment samples. samples [20]. [20]. WaterWater samples samples are are taken taken according according to to EN EN ISO ISO standards: standards: LST LST EN EN ISO ISO 5667-14:2016 5667-14:2016—water – water qualityquality—Sampling—Part – Sampling – Part 14. 14 [21]. [21]. Sludge samples were filled with fresh surface water. The ratio of sludge to water by weight was 1:10. In this study, 20 g of sludge was collected, and 200 ml of water was added. The prepared samples were stored at a constant temperature of 18 °C under laboratory conditions.

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Sludge samples were ﬁlled with fresh surface water. The ratio of sludge to water by weight was 1:10. In this study, 20 g of sludge was collected, and 200 mL of water was added. The prepared samples were stored at a constant temperature of 18 ◦C under laboratory conditions. Sludge and water research was conducted at ASU Laboratory in Kedainiai,˙ with a permit issued by the Minister of the Environment on October 15, 2007, by Order no. D1-522 “Description of Procedure for Issue and Investigation of Pollutant Emissions from Pollutants and Pollutants in Environmental Elements”. During the study, the total nitrogen (Nt) and total phosphorus (Pt) concentrations, electrical conductivity (C) µS/cm, NO2–N, NO3-N, NH4-N and pH values in the water were analyzed. Measurements were made 7, 14, 21 and 28 days after freshwater dilution. The checkpoint was measured on the pickup day. The following analysis methods were applied: in sludge—total phosphorus (Pt) studies were performed according to the method LST EN ISO 6878:2004 [22], total nitrogen (Nt) was tested according to the method LST EN ISO 17184:2014 Soil quality- Determination of carbon and nitrogen by near-infrared spectrometry (NIRS) (ISO 17184:2014) EN ISO 17184:2014 [23], organic matter content was tested according to the method LST EN 13039: Soil improvers and growing media—Determination of organic matter content and ash EN 13039:2011 [24]. In water—total nitrogen (Nt) was tested according to the method LST EN 13342- 2002 Determination of nitrogen—Determination of bound nitrogen (TNb), following oxidation to nitrogen oxides EN 12260:2003 [25]; total phosphorus (Pt) studies were performed according to LST EN ISO 6878:2004 [22], electrical conductivity (C) tests were performed according to the method LST EN 27888:1999 [26], pH values—LST EN ISO 10523—2012 Water quality—determination of pH [27], NO2–N—LST EN 26777:1999 Water quality—Determination of nitrite—Molecular absorption spectrometric method (ISO 6777:1984) EN 26777:1993 [28], NO3-N tests were performed according to the method LST ISO 7890-3:1998 Water quality—Determination of nitrate. Part 3: Spectrometric method using sulfosalicylic acid ISO 7890-3:1988 [29], NH4-N tests were performed according to the method LST ISO 7150-1:1998 Water quality. Determination of ammonium. Part 1: Manual spectrometric method ISO 7150-1:1984 [30]. In this study, the lakes were arranged in order from the thinnest to the thickest sludge layer as follows: Biržulis—Antakmeniu˛—Gauštvinis—Spera—Kiementas.˙

2.2. Statistical Analysis Correlation and regression were calculated using the computer program STATISTICA 7 [18,19]. The STATISTICA package Nonlinear Estimation was used to determine the correlation coeﬃcients and to deﬁne the relationships between the indicators surveyed [31]. The symbol * indicates that the data were reliable within a probability of 95%. To predict the percentage of total nitrogen, phosphorus and organic matter (%) in the sludge depending on the thickness of the sludge layer and the variable values, IBM SPSS Statistics for Windows, Version 24.0 (IBM Corp., Armonk, NY, USA), was used. Regression formulas were calculated, and scatter diagrams. Analyses of the selected lake sludge were created.

3. Results Analyses of the 36 lake sludge were carried out by estimating the total phosphorus and total nitrogen concentrations. The results of the study are presented in Figure3. Int. J. Environ. Res. Public Health 2019, 16, 4691 6 of 17

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40000 30000 20000 10000 0 N total mg/l N total Orija Širvio Spėra Bublių Zapsys Ekrano Biržulis Masčio Angirių Talkšos Kavalys Krivėnų Tausalo Dviragis Skaistės Veisiejis Rėkyvos Didžiulis Alsėdžių Kadrėnų Paežerių Kemėšys Latežeris Gelvanės Imbradas Luksnėnų Janušonių Sablauskų Gauštvinis Kiementas Draudenių Bartkuškio Vaitiekūnų J.Kauknoris Stepanonių Antakmenių

1000 800 600 400 200 P total mg/l 0 Orija Širvio Spėra Bublių Zapsys Ekrano Biržulis Masčio Angirių Talkšos Kavalys Krivėnų Tausalo Dviragis Skaistės Veisiejis Rėkyvos Didžiulis Alsėdžių Kadrėnų Paežerių Kemėšys Latežeris Gelvanės Imbradas Luksnėnų Janušonių Sablauskų Gauštvinis Kiementas Draudenių Bartkuškio Vaitiekūnų J.Kauknoris Stepanonių Antakmenių

FigureFigure 3. NitrogenNitrogen and and phosphorus phosphorus concentration concentration in insludge. sludge.

AnalysesAnalyses of of the the selected selected 5 lake5 lake sludge sludge samples samples were were carried carried out out by estimatingby estimating the totalthe total phosphorus andphosphorus total nitrogen and total concentrations, nitrogen concentrations, the organic matter the organic content matter and thecontent sludge and layer the sludge thickness. layer Since the averagethickness. lake Since depth the is average a very importantlake depth indicator,is a very important it is presented indicator, in Table it is1 presentedbelow. in Table 1 below.

Table 1. Depth, sludge layer, Nt,Pt and organic matter content of the sludge samples from the Table 1. Depth, sludge layer, Nt, Pt and organic matter content of the sludge samples from the analyzed lakes. analyzed lakes. Organic Matter Average Depth Sludge Layer N (Sludge) P (Sludge) Number Lake T T OrganicContent % of theAverage Lake, m Thickness m mgNT/L PmgT /L Sludge Layer Matter(Sludge) Number Lake Depth of the (Sludge) (Sludge) Thickness m Content % 1 BiržulisLake, 0.9 m 1.9 14,238.4mg/l mg/l 479.6 32.9 2 Antakmenis 5.8 2.7 12,144.7 725.3(Sludge) 32.4 1 Biržulis 0.9 1.9 14,238.4 479.6 32.9 3 Gauštvinis 5 4.3 8464.5 674 27.6 2 Antakmenis 5.8 2.7 12,144.7 725.3 32.4 4 Spera˙ 1.85 4.7 18,078.0 980.5 43.1 3 Gauštvinis 5 4.3 8464.5 674 27.6 5 Kiementas 4 6.1 21,415.0 898.0 46.9 4 Spėra 1.85 4.7 18,078.0 980.5 43.1 5 Kiementas 4 6.1 21,415.0 898.0 46.9 All the analyzed lakes are at risk of increased eutrophication. According to the thickness of the sludgeAll layer, the analyzed Kiemantas lakes Lake are had at risk the of largest increased layer eutrophication. (6.1 m), while BiržulisAccording Lake to the had thickness the smallest of the layer (1.9sludge m). Accordinglayer, Kiemantas to the Lake average had the lake largest depth, layer Antakmieniai (6.1 m), while Lake Biržulis was Lake the deepest had the (12.5smallest m), layer Biržulis lake(1.9 was m). theAccording shallowest to the (0.9 average m). The lake largest depth, amount Antakmieniai of organic Lake matter was the was d ineepest Lake (12.5 Kiemantas m), Biržulis (46.9%), andlake the was smallest the shallowest was in Gauštvinis(0.9 m). The Lakelargest (27.6%). amount The of organic highest matter concentrations was in Lake of Kiemantas total phosphorus (46.9%), and totaland nitrogenthe smallest were was found in Gauštvinis in Kiemantas Lake (27.6%). and Spera ˙The lakes. highest Assessing concentrations the ecological of total phosphorus status of the and lakes accordingtotal nitrogen to the were physico-chemical found in Kiemantas values and according Spėra lakes. to theAssessing Environmental the ecological Protection status Agencyof the lakes (EPA), according to the physico-chemical values according to the Environmental Protection Agency (EPA), Lake Kiemantas showed a very good ecological status (Nt 0.90 mg/L and Pt 0.02 mg/L). Based on the Lake Kiemantas showed a very good ecological status (Nt 0.90 mg/l and Pt 0.02 mg/l). Based on the total nitrogen concentration (Nt 1.0 mg/L) and the total phosphorus concentration (Pt 0.081 mg/L), whichtotal nitrogen are average concentration ecological status(Nt 1.0indicators, mg/l) and Lakethe total Spera˙ phosphorus also showed concentration a very good (P ecologicalt 0.081 mg/l), status. which are average ecological status indicators, Lake Spėra also showed a very good ecological status. According to the state lake monitoring data and based on the total nitrogen concentration (Nt 2.15 mg/L),

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Lake Gauštvinis showed values corresponding to average ecological status indicators. Based on the total phosphorus concentration (Pt 0.078 mg/L), Lake Gauštvinis had average state values. Based on the total nitrogen concentration (Nt 0.644 mg/L), the values of Antakmeniu˛ Lake corresponded to a very good ecological status, while based on the total phosphorus concentration (Pt 0.075 mg/L) the values of the lake represented average state values. The total nitrogen concentration (Nt 1.25 mg/L) values of Lake Biržulis corresponded to a very good ecological status, while the total phosphorus concentration (Pt 0.096 mg/L) values indicated a poor ecological status. The highest (>10,000 mg/L) concentrations of total nitrogen (Nt) in sludge were found in Tausalas, Kiementas, Spera, Veisejas, Rekyva˙ Lake, Judo Kauknorisy, Biržulis, Antakmeniai Lake, Draudeniai Lake, Sablauskiai Pond. Verylow (<1000 mg/L) nitrogen concentration in sludge—in Bubliai, Bartkuškis, Janušonys ponds, Luksnenai,˙ Orija, Skaiste,˙ Kavalis, Alsedžiai˙ lakes. The highest (>600 mg/L) concentrations of total phosphorus (Pt) in sludge were found in the Spera, Kiementas, Draudeniai, Mastis, Antakmeniai, Gauštvinis, Veisiejai, Vaitiekunai¯ and Stepanioniai ponds. Many of these lakes also contain very high levels of nitrogen and high levels of organic matter. Therefore, ﬁve lakes with high concentrations of n and p in sludge were selected for further studies—Birzulis, Antakmenis, Gaustvinis, Spera, and Kiementas. All the analyzed lakes are at risk of increased eutrophication. Five lakes were selected for detailed research. According to the thickness of the sludge layer, Kiemantas Lake had the largest layer (6.1 m), while Biržulis Lake had the smallest layer (1.9 m). According to the average lake depth, Antakmieniai Lake was the deepest (12.5 m), Biržulis lake was the shallowest (0.9 m). The largest amount of organic matter was in Lake Kiemantas (46.9%), and the smallest was in Gauštvinis Lake (27.6%). The highest concentrations of total phosphorus and total nitrogen were found in Kiemantas and Spera˙ lakes. Assessing the ecological status of the lakes according to the physico-chemical values according to the Environmental Protection Agency (EPA), Lake Kiemantas showed a very good ecological status (Nt 0.90 mg/L and Pt 0.02 mg/L). Based on the total nitrogen concentration (Nt 1.0 mg/L) and the total phosphorus concentration (Pt 0.081 mg/L), which are average ecological status indicators, Lake Spera˙ also showed a very good ecological status. According to the state lake monitoring data and based on the total nitrogen concentration (Nt 2.15 mg/L), Lake Gauštvinis showed values corresponding to average ecological status indicators. Based on the total phosphorus concentration (Pt 0.078 mg/L), Lake Gauštvinis had average state values. Based on the total nitrogen concentration (Nt 0.644 mg/L), the values of Antakmeniu˛ Lake corresponded to a very good ecological status, while based on the total phosphorus concentration (Pt 0.075 mg/L) the values of the lake represented average state values. The total nitrogen concentration (Nt 1.25 mg/L) values of Lake Biržulis corresponded to a very good ecological status, while the total phosphorus concentration (Pt 0.096 mg/L) values indicated a poor ecological status. After analyzing the data from the collected samples, it can be stated that the amount of biogenic substances in sludge does not always inﬂuence the quality of lake water; for secondary pollution to appear from sludge, certain conditions in the water are required. To predict the amounts of total nitrogen, total phosphorus, and organic matter (%) depending on the thickness of the sludge layer, regression equations were calculated in STATISTICA 8. The scatter charts and regression equations are presented in Figure4. Int. J. Environ. Res. Public Health 2019, 16, 4691 8 of 17 Int. J. Environ. Res. Public Health 2019, 16, x 8 of 17

Ntotal = 3209,6391+1212,3175*x; 0,95 Conf.Int. r=0,39, p=0,048 P total = 289,7891+36,613*x; 0,95 Conf .Int. r= 0,34; p=0,049 45000 1100

40000 1000

35000 900 800 30000 700 25000 600

20000 500 Ntotal, mg/l

P total mg/l total P 400 15000 300 10000 200

5000 100

0 0 012345678 012345678 The sludge layer thickness, m The sludge layer thickness, m

Organic matter (%) = 18,5371+2,847*x; 0,95 Conf.Int. r=0,480, p=0,039 80

20 Organic matter content, % 10

0 012345678 The sludge layer thickness, m

Figure 4. Correlation between Nt, Pt and the amount of organic matter in the sludge to sludge layer Figure 4. Correlation between Nt,Pt and the amount of organic matter in the sludge to sludge thickness.layer thickness.

After the analysis, the graph shows a linear trend of a positive tendency between the thickness of the sludge layer in the lake and the total amountamount of nitrogen, phosphorus and organic matter in the sludge of the lake. As As the the sludge sludge layer layer increase increases,s, the concentrations of nitrogen, phosphorus and organic matter also increase. To evaluate the intensity of the release of biogen biogenicic substances from sludge in freshwater, freshwater, the N tt,, NO2–N,–N, NO NO33-N,-N, and and P Pt tconcentrations,concentrations, the the specific speciﬁc electrical electrical conductivity conductivity µSµS // cm and the pH values were analyzed. The The analyzed data data are presented in Figures5 5–8.–8. RegressionRegression equationsequations thatthat cancan bebe used to predict water quality valu valueses depending on time were calculated.calculated. The equations are shown in Tables2 2–5.–5.

Table 2.2. Regression equations for predictingpredicting NNtt and NO2-N-N concentrations. concentrations.

NumberNumber N N total Total mgmg/l/L NO NO22-N-N mg/l mg/L 2 2 1 YY Birzulis == 3.3123.312+ +0.5577x 0.5577x0.0131x − 0.0131x, r =2,0.863 r = YYBirzulis == 0.01670.0167 ++ 0.0008x0.0008x+ 4.6356E+ 4.6356E5x − ;5x r =2;0.98 r = 1 Birzulis − Birzulis − 2 0.863 2 YAntakmeniu = 0.0283 +0.980.0013x 3.14111E 5x ; 2 YAntakmeniu = 1.1729 + 0.2463x 0.0026x ; r = 0.975 − − YAntakmeniu = 1.1729 + 0.2463x− − 0.0026x2; r YAntakmeniu = 0.0283r =+ 0.9710.0013x − 3.14111E − 2 2 3 YGaustvinis = 4.5323 +=0.2396x 0.975 0.0037x ; r = 0.944 YGauštvinis =5x0.02692; r = +0.9710.0009x; r = 0.96 − 2 2 4 YYGaustvinis= 4.6234= 4.5323+ 0.6687x +0.2396x0.0162x − 0.0037x; r = 0.8832; r = Y YGauštvinis= 0.0243 =+ 0.02690.0034x + 0.0009x;9.7668E r5x = 0.96; r = 0.885 3 Spera − Spera − − 2 5 YKiementas = 4.8806 +0.9440.808x 0.0225x ; r = 0.928 YKiementas = 0.023 + 0.0011x; r = 0.969 * − 2 2 YSpera = 4.6234+0.6687x − 0.0162x0; r x= 2.0 YSpera = 0.0243 + 0.0034x − 9.7668E − 5x ; r = 4 ≤ ≤ Note: The symbol0.883 * indicates that the data were reliable within a probability0.885 of 95%. YKiementas = 4.8806 + 0.808x − 0.0225x2; r = YKiementas = 0.023 + 0.0011x; r = 0.969 * 5 0.928 0 ≤x ≤ 2.0 Note: The symbol * indicates that the data were reliable within a probability of 95%.

Int. J. Environ. Res. Public Health 2019, 16, 4691 9 of 17

Table 3. Regression equations for predicting NO3-N and electrical conductivity (C) concentrations.

Number NO3-N mg/L Electrical Conductivity (C) µS/cm 1 Y = 0.0344 + 0.0049x; r = 0.963 Y = 649.2857 + 1.0327x 0.0175x2; r = 0.94 Biržulis Biržulis − 2 Y = 0.0263 + 0.0029x; r = 0.97 Y = 653.1714 + 4.2939x 0.137x2; r = 0.982 * Antakmeniu Antakmeniu − 3 YGauštvinis = 0.0303 + 0.0026x; r = 0.985 * YGauštvinis = 655.8 + 2.4286x; r = 0.938 * 4 Y = 0.0542 + 0.0107x 0.0003x2; r = 0.994 * Y = 628.4 + 2.7x 0.0714x2; r = 0.98 * Spera − Spera − 5 Y = 0.0515 + 0.0111x 0.0003x2; r = 0.975 Y = 653.7429 + 2.8735x 0.07x2; r = 0.942 Kiementas − Kiementas − 0 x 2.0 ≤ ≤ Note: The symbol * indicates that the data were reliable within a probability of 95%.

Table 4. Regression equations for predicting PTotal and PO4-P concentrations.

Number P total mg/L PO4-P mg/L Y = 0.0815 + 0.0003x 2.629E-5 * x2; 1 Birzulis − Y = 0.1227 + 0.0242x; r = 0.987 r = 0.909 Biržulis Y = 0.1267 + 0.0342x 0.0009x2; 2 Y = 0.09 0.0013x; r = 0.998 * Antakmeniu − Antakmeniu − r = 0.958 Y = 0.1112 + 0.0007x 2.2886E 5 * x2; 3 Gaustvinis − − Y = 0.0486 + 0.0879x 0.0016x2; r = 0.908 r = 0.781 Gaustvinis − 4 Y = 0.1061 + 0.0035x 0.0002x2; r = 0.87 Y = 0.067 + 0.0307x 0.0006x2; r = 0.92 Spera − Spera − Y = 0.13 3.3061E 5x 3.207E 6Ex2; 5 Kiementas − − − − Y = 0.1349 + 0.13x; r = 0.82 r = 0.998 * Kiementas 0 x 2.0 ≤ ≤ Note: The symbol * indicates that the data were reliable within a probability of 95%.

Table 5. Regression equations for predicting pH value and NH4-N concentrations.

Number pH NH4-N mg/L 1 Y = 7.264 + 0.0783x 0.0018x2; r = 0.974 Y = 0.0183 0.0106x + 0.0006x2, r = 0.92 Biržulis − Biržulis − 2 Y = 7.1877 + 0.0805x 0.0021x2; r = 0.978 Y = 0.0119 + 0.0029x 0.0001x2, r = 0.39 Antakmeniu − Antakmeniu − 3 Y = 7.2446 + 0.0787x 0.002x2; r = 0.958 Y = 0.0114 0.0074x + 0.0004x2, r = 0.95 Gauštvinis − Gauštvinis − 4 Y = 7.2551 + 0.0544x 0.0009x2; r = 0.942 Y = 0.0004 + 0.0005x, r = 0.88 Spera − Spera 5 Y = 7.302 + 0.078x 0.002x2; r = 0.975 Y = 0.0061 0.0035x + 0.0002x2, r = 0.92 Kiementas − Kiementas − 0 x 2.0 Int. J. Environ. Res. Public Health 2019, 16, x ≤ ≤ 10 of 17

14 0,08

12 0,07

10 0,06

8 0,05 mg/l -Nmg/l 2 total total 6 0,04 N NO

4 0,03

y (Birzulis) y(Birzulis)

2 y (Antakmeniu) 0,02 y(Antakmeniu)

y (Gaustvinis) y(Gaustvinis)

0 y (Spera) 0,01 y(Spera) -5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 y (Kiementas) y(Kiementas) Days Days

Figure 5. Dynamics of Nt and NO2-N concentrations in water. Figure 5. Dynamics of Nt and NO2-N concentrations in water.

0,20 0,30

0,18 0,25 0,16

0,20 0,14

0,12 0,15 -N -N mg/l 0,10 4 -N mg/l -N 3 NH 0,10

NO 0,08 0,05 0,06

0,04 0,00 y (B irzulis ) y (Antakmeniu) 0,02 y y (Gaustvinis ) (B irz ulis ) -0,05 y (Spe ra ) y(Antakmeniu) -5 0 5 10 15 20 25 30 y y (Kiemento) 0,00 (Gaustvinis) Days -5 0 5 10 15 20 25 30 y(Spe ra ) y(Kie mentas ) Days

Figure 6. Dynamics of NO3-N and NH4-N concentrations in water.

0,14 1,6

0,13 1,4

0,12 1,2

0,11 1,0

0,10 0,8 mg/l -P mg/l-P T 0,09 4 P 0,6 PO

0,08 0,4

0,07 0,2 y (Birzulis) y(Birzulis) y (Antakmeniu) 0,06 y(Antakmeniu) 0,0 y (Gaustvinis) y(Ga ustvinis) 0,05 y y -0,2 (Spera) -5 0 5 10 15 20 25 30 (Spera) -5 0 5 10 15 20 25 30 y y (Kiementas) Days (Kiementas) Days

Figure 7. Dynamics of Pt and PO4-P concentrations in water.

Int.Int. J. J. Environ. Environ. Res. Res. Public Public Health Health 2019 2019, 16, 16, x, x 1010 of of 17 17

14 0,08 14 0,08

12 0,07 12 0,07

10 0,06 10 0,06

8 0,05 8 0,05 mg/l mg/l -Nmg/l 2 total total -Nmg/l 2 total total 6 0,04

N 6 0,04 NO N NO 4 0,03 4 0,03 y (Birzulis) y(Birzulis) y (Birzulis) y(Birzulis) 2 y (Antakmeniu) 0,02 y(Antakmeniu) 2 y (Antakmeniu) 0,02 y(Antakmeniu) y (Gaustvinis) y(Gaustvinis) y (Gaustvinis) y(Gaustvinis) 0 0 y (Spera)y 0,01 y(Spera) -5 0 5 10 15 20 25 30 (Spera) 0,01-5 0 5 10 15 20 25 30 y(Spera) -5 0 5 10 15 20 25 30 y (Kiementas) -5 0 5 10 15 20 25 30 y(Kiementas) y (Kiementas) y(Kiementas) DaysDays DaysDays

Int. J. Environ. Res. Public Health 2019, 16, 4691 10 of 17 t 2 FigureFigure 5. 5. Dynamics Dynamics of of N N andt and NO NO-N2-N concentrations concentrations in in water. water.

0,20 0,30 0,20 0,30 0,18 0,18 0,25 0,25 0,16 0,16 0,20 0,14 0,20 0,14

0,12 0,15 0,12 0,15 -N -N mg/l 4

0,10 -N mg/l 4 -N mg/l -N

3 0,10 NH

-N mg/l -N 0,10 3 NH 0,10

NO 0,08 NO 0,08 0,05 0,06 0,05 0,06

0,04 0,00 0,04 0,00 y (B irzulis ) y (B irzulis ) y (Antakmeniu) y (Antakmeniu) 0,02 y (Gaustvinis ) y(B irz ulis ) 0,02 y -0,05 y (Gaustvinis ) y (B irz ulis ) -0,05 y (Spe ra ) (Antakmeniu)y -5 0 5 10 15 20 25 30 y (Spe ra ) y (Antakmeniu) -5 0 5 10 15 20 25 30 y (Kiemento) 0,00 (Gaustvinis) y (Kiemento) 0,00 y(Gaustvinis) Days -5 0 5 10 15 20 25 30 y(Spe ra ) Days y(Spe ra ) -5 0 5 10 15 20 25 30 y(Kie mentas ) Days y(Kie mentas ) Days

Figure 6. Dynamics of NO3-N and NH4-N concentrations in water. FigureFigure 6. 6. DynamicsDynamics of of NO NO33-N-N and and NH NH4-N4-N concentrations concentrations in in water. water.

0,14 1,6 0,14 1,6

0,13 1,4 0,13 1,4

0,12 1,2 0,12 1,2

0,11 1,0 0,11 1,0

0,10 0,8 0,10 0,8 mg/l -P mg/l-P T mg/l 4 -P mg/l-P

T 0,09 P 0,6 0,09 4 P 0,6 PO PO 0,08 0,4 0,08 0,4 0,07 y 0,07 0,2 (Birzulis)y y (Birzulis)y 0,2 (Birzulis) (Birzulis) y (Antakmeniu) 0,06 y y (Antakmeniu) 0,06 (Antakmeniu)y 0,0 (Antakmeniu) 0,0 y (Gaustvinis)y y(Ga ustvinis) (Gaustvinis) 0,05 y(Ga ustvinis) y (Spera) 0,05 -0,2 y (Spera) -5 0 5 10 15 20 25 30 y(Spera) -0,2-5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 y(Spera) -5 0 5 10 15 20 25 30 y (Kiementas) y y (Kiementas) (Kiementas)y Days DaysDays (Kiementas) Days Int. J. Environ. Res. Public Health 2019, 16, x 11 of 17 Figure 7. Dynamics oft P and PO4 -P concentrations in water. FigureFigure 7. 7. Dynamics Dynamics of of P P andt tand PO PO-P4-P4 concentrations concentrations in in water. water.

740 8,2

720 8,0

700 7,8

7,6 680 pH SEC µS/cm

7,4 660

7,2 640 y (B irz ulis ) y(Birzulis) y (Antakmeniu) y(Antakmeniu) y 7,0 (Gaustvinis) 620 y(Gaustvinis) y(Spe ra ) -5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 y(Spera) y(K ie me nta s ) Days y(Kiementas) Days

Figure 8. Dynamics of pH value and SEC concentrations in water. Figure 8. Dynamics of pH value and SEC concentrations in water. The results show that by comparing data from day one to day 28, the NH4-N concentration increasedThe results from 0.017show mgthat/L by to 0.102comparing mg/L, data the from NO2-N day concentration one to day 28, ranged the NH from4-N 0.039concentration mg/L to 0.049increased mg/L, from the NO 0.0173-N mg/l concentration to 0.102 mg/l, increased the NO from2-N 0.021 concentration mg/L to 0.136 ranged mg/L, from and the0.039 Pt concentrationmg/l to 0.049 decreasedmg/l, the NO from3-N 0.198 concentration mg/L to 0.006 increased mg/L. Lowfrom concentrations0.021 mg/l to 0.136 of nitrites, mg/l, nitrates,and the andPt concentration ammonium nitrogendecreased and from high 0.198 concentrations mg/l to 0.006 of totalmg/l. nitrogen Low conc indicateentrations that theof nitrites, sludge isnitrates, rich in organicand ammonium nitrogen. Thenitrogen nitriﬁcation and high process, concentrations which takesof total place nitrogen in water, indicate is a that biological the sludge process is rich whereby in organic ammonia nitrogen. is oxidizedThe nitrification to nitrite process, and the which latter is takes oxidized place to in nitrate. water, is a biological process whereby ammonia is oxidized to nitrite and the latter is oxidized to nitrate. The results of the study indicate that the concentrations of all the parameters studied increased, with the exception of the total phosphorus concentration. The dynamics of the average phosphorus values are shown in Figure 9.

P total (av erage v alue) = 0,13-3,3061E-5*x-3,207E-6*x^2 0,1305

0,1300

0,1295

0,1290

0,1285

0,1280

0,1275 P total (average value), mg/l 0,1270

0,1265

0,1260 -5 0 5 10 15 20 25 30 Days

Figure 9. Dynamics of average phosphorus values.

The average phosphorus values in water decreased from 0.13 mg/l to 0.126 mg/l over 30 days. Therefore, mineral phosphorus (PO4-P form) was emitted, and organic phosphorus was "locked" in the sludge. The same effect can be expected in the lakes that have a low concentration of these elements in the sediment because the lakes were studied don't have a similar water source and the atmospheric conditions in the region are very similar.

Int. J. Environ. Res. Public Health 2019, 16, x 11 of 17

740 8,2

720 8,0

700 7,8

7,6 680 pH SEC µS/cm

7,4 660

7,2 640

y(B irz ulis ) y(Birzulis) y (Antakmeniu) y(Antakmeniu) y 7,0 (Gaustvinis) 620 y(Gaustvinis) y(Spe ra ) -5 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 y(Spera) y(K ie me nta s ) Days y(Kiementas) Days

Figure 8. Dynamics of pH value and SEC concentrations in water.

The results show that by comparing data from day one to day 28, the NH4-N concentration increased from 0.017 mg/l to 0.102 mg/l, the NO2-N concentration ranged from 0.039 mg/l to 0.049 mg/l, the NO3-N concentration increased from 0.021 mg/l to 0.136 mg/l, and the Pt concentration decreased from 0.198 mg/l to 0.006 mg/l. Low concentrations of nitrites, nitrates, and ammonium Int. J. Environ. Res. Public Health 2019, 16, 4691 11 of 17 nitrogen and high concentrations of total nitrogen indicate that the sludge is rich in organic nitrogen. The nitrification process, which takes place in water, is a biological process whereby ammonia is oxidizedThe resultsto nitrite of and the studythe latter indicate is oxidized that the to concentrationsnitrate. of all the parameters studied increased, withThe the exceptionresults of the of the study total indicate phosphorus that the concentration. concentrations The of dynamicsall the parameters of the average studied phosphorus increased, valueswith the are exception shown in of Figure the total9. phosphorus concentration. The dynamics of the average phosphorus values are shown in Figure 9.

P total (av erage v alue) = 0,13-3,3061E-5*x-3,207E-6*x^2 0,1305

0,1300

0,1295

0,1290

0,1285

0,1280

0,1275 P total (average value), mg/l 0,1270

0,1265

0,1260 -5 0 5 10 15 20 25 30 Days

Figure 9. Dynamics of average phosphorus values.

The average phosphorus values in waterwater decreaseddecreased fromfrom 0.130.13 mgmg/l/L to 0.126 mgmg/l/L over over 30 30 days. days. Therefore, mineral phosphorusphosphorus (PO(PO44-P form) was emitted,emitted, andand organicorganic phosphorusphosphorus waswas “locked”"locked" in the sludge. The same effect eﬀect can be expected in the lakes that have a low concentration of these elements in the sediment because the lakes were studied don’tdon't have a similar water source and the atmospheric conditions in the region are very similar.similar. To assess the relationships between the total nitrogen, total phosphorus, nitrates, nitrites, ammonium ions, phosphate, and phosphorus concentrations and the electrical conductivity in water, the STATISTICA 8 software was used to calculate the correlation coeﬃcients based on the data collected, as shown in Table6.

Table 6. Correlation matrix between Nt; NO2 -N; NO3- N; NH4 -NPO4-P, Pt concentrations and C values.

NH -N NO -N NO -N PO -P Parameter P mg/LN mg/L 4 2 3 4 T T mg/L mg/L mg/L mg/L r = 0.945 r = 0.966 r = 0.001 r = 0.559 r = 0.787 r = 0.033 SEC µS/cm − − − − p = 0.000 p = 0.000 p = 0.998 p = 0.150 p = 0.049 p = 0.938

A strong statistically signiﬁcant negative correlation between the SEC values and the total phosphorus concentrations indicates that a higher concentration of dissolved salts results in a lower phosphorus concentration in water. A strong statistically signiﬁcant positive relationship between the SEC values and the total nitrogen and nitrate concentrations indicates that a higher concentration of dissolved salts results in a greater concentration of total nitrogen and nitrates in water. There is no correlation between the SEC values and ammonium nitrogen with phosphates. Int.Int. J. J. Environ. Environ. Res. Res. Public Public Health Health 2019 2019, ,16 16, ,x x 1212 of of 17 17

ToTo assessassess thethe relationshipsrelationships betweenbetween thethe totaltotal ninitrogen,trogen, totaltotal phosphorus,phosphorus, nitrates,nitrates, nitrites,nitrites, ammoniumammonium ions,ions, phosphate,phosphate, andand phosphorusphosphorus concentratconcentrationsions andand thethe electricalelectrical conductivityconductivity inin water,water, thethe STATISTICASTATISTICA 88 softwaresoftware waswas usedused toto calculatecalculate thethe correlationcorrelation coefficientscoefficients basedbased onon thethe datadata collected,collected, asas shownshown inin TableTable 6.6.

t 2 3 4 4 t TableTable 6.6. CorrelationCorrelation matrixmatrix betweenbetween NNt;; NONO2 -N;-N; NONO3-- N;N; NHNH4 -NPO-NPO4-P,-P, PPt concentrationsconcentrations andand CC values.values.

4- 3 4 NH4- N NO3 -N PO4 -P T T NH N 2 NO -N PO -P ParameterParameter PPT mg/lmg/l NNT mg/lmg/l NONO2-N-N mg/lmg/l mg/lmg/l mg/lmg/l mg/lmg/l rr == −−0.9450.945 rr == 0.9660.966 rr == −−0.0010.001 rr == −−0.5590.559 rr == 0.7870.787 rr == −−0.0330.033 SECSEC µS/cmµS/cm pp == 0.0000.000 pp == 0.0000.000 pp == 0.9980.998 pp == 0.1500.150 pp == 0.0490.049 pp == 0.9380.938

AA strongstrong statisticallystatistically significantsignificant negativenegative correlationcorrelation betweenbetween thethe SECSEC valuesvalues andand thethe totaltotal phosphorusphosphorus concentrationsconcentrations indicatesindicates thatthat aa higherhigher concentrationconcentration ofof dissolveddissolved saltssalts resultsresults inin aa lowerlower phosphorusphosphorus concentrationconcentration inin water.water. AA strongstrong statisticallystatistically significantsignificant positivepositive relationshiprelationship betweenbetween thethe SECSEC valuesvalues andand thethe totaltotal Int. J. Environ. Res. Public Health 2019, 16, 4691 12 of 17 nitrogennitrogen andand nitratenitrate concentrationsconcentrations indicatesindicates thatthat aa higherhigher concentrationconcentration ofof dissolveddissolved saltssalts resultsresults inin aa greatergreater concentrationconcentration ofof totaltotal nitrogennitrogen andand nitratnitrateses inin water.water. ThereThere isis nono correlationcorrelation betweenbetween thethe SEC values and ammonium nitrogen with phosphates. SEC Regressionvalues and ammonium formulas were nitrogen calculated with phosphates. with the STATISTICA 8 software. Scatter charts and Regression formulas were calculated with the STATISTICA 8 software. Scatter charts and regressionRegression equations formulas are presented were calculated in Figures with 10–12 th. e STATISTICA 8 software. Scatter charts and regressionregression equationsequations areare presentedpresented inin FiguresFigures 10–12.10–12.

PTotal = 0,1348-0,0002*x; 0,95 Conf.Int. PTotal = 0,1348-0,0002*x; 0,95 Conf.Int. 0,14 NTotal = 1,0891+0,0106*x; 0,95 Conf.Int. 0,14 NTotal = 1,0891+0,0106*x; 0,95 Conf.Int. 10 10 0,12 0,12 9 9 0,10 0,10 8 8 0,08 0,08 7 7 mg/l mg/l

mg/l 0,06

mg/l 6 0,06 6 total total Total Total total total P Total Total P 0,04 N 5 0,04 N 5

0,02 4 0,02 4

0,00 3 0,00 3 2 -0,02 2 0 100 200 300 400 500 600 700 800 -0,02 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 Specific electrical conductivity (SEC) µS/cm Specific electrical conductivity (SEC) µS/cm Specific electrical conductivity (SEC) µS/cm Specific electrical conductivity (SEC) µS/cm

t t FigureFigureFigure 10. 10.10. Correlation Correlation ofof NNtt andand PPt concentrationsconcentrationsconcentrations with with electrical electrical water water conductivity.conductivity.

NO2-N = 0,0255-2,4555E-5*x; 0,95 Conf.Int. NO2-N = 0,0255-2,4555E-5*x; 0,95 Conf.Int. 0,030 NO3 -N = 0,0259+9,27E-5*x; 0,95 Conf.Int. 0,030 NO3 -N = 0,0259+9,27E-5*x; 0,95 Conf.Int. 0,120,12 0,025 0,025 0,100,10 0,020 0,020 0,080,08 0,015 0,015

0,06 -N mg/l -N mg/l -Nmg/l 0,06 2 -Nmg/l 2 3 0,010

3 0,010 NO NO NO

NO 0,04 0,04 0,005 0,005 0,02 0,02 0,000 0,000 0,00 0,00 0 100 200 300 400 500 600 700 800 -0,005 0 100 200 300 400 500 600 700 800 -0,005 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 Specif ic electrical conductiv ity (SEC) µS/cm Specific electrical conductivity (SEC) µS/cm Specif ic electrical conductiv ity (SEC) µS/cm Specific electrical conductivity (SEC) µS/cm Int. J. Environ. Res. Public Health 2019, 16, x 13 of 17 Figure 11. Correlation of NO3-N and NO -N2 concentrations with electrical water conductivity. FigureFigure 11.11. CorrelationCorrelation ofof NONO33-N-N andand NONO22-N-N concentrationsconcentrations withwith electrelectricalical waterwater conductivity.conductivity.

NH4 -N = 0,0179-1,2168E-7*x; 0,95 Conf .Int. 0,10 PO4 -P = 0,455-2,0416E-5*x; 0,95 Conf.Int. 0,8

0,08 0,7

0,06 0,6

0,5 0,04 -N mg/l 4 -P mg/l

4 0,4 NH

0,02 PO 0,3

0,00 0,2

-0,02 0,1 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 Specif ic electrical conductiv ity (SEC) µS/cm Specif ic electrical conductiv ity (SEC) µS/cm

FigureFigure 12.12.Correlation Correlation ofof NHNH44-N-N andand POPO44-P concentrations with electricalelectrical waterwater conductivity.conductivity.

TheThe graphsgraphs inin FiguresFigures8 8–9 and show9 show a positive a positive correlation correlation between between the the concentrations concentrations of nitrates of nitrates and andtotal total nitrogen nitrogen in water in water and and the the electrical electrical conduc conductivitytivity of of the the lake lake water water samples samples and aa negativenegative correlation between the concentrations of nitrite and total phosphorus in water and the electrical conductivity of the lake water samples. An increased concentration of dissolved salts resulted in an increased concentration of total nitrates and nitrogen in water. However, if the concentration of the dissolved salts decreased, the total nitrite and phosphorus concentrations in the water decreased. The regression equations can be used to predict changes in the Y (Pt; Nt) value when the Y (C µS/cm) value varies. The ammonium nitrogen and phosphate concentrations had no relation to the concentration of dissolved salts.

Int. J. Environ. Res. Public Health 2019, 16, 4691 13 of 17 correlation between the concentrations of nitrite and total phosphorus in water and the electrical conductivity of the lake water samples. An increased concentration of dissolved salts resulted in an increased concentration of total nitrates and nitrogen in water. However, if the concentration of the dissolved salts decreased, the total nitrite and phosphorus concentrations in the water decreased. The regression equations can be used to predict changes in the Y (Pt;Nt) value when the Y (C µS/cm) value varies. The ammonium nitrogen and phosphate concentrations had no relation to the concentration of dissolved salts.

4. Discussion One of the main biogenic substances that determine the productivity of a surface water body is phosphorus [11]. The study showed that only nitrogen compounds are liberated and can become a source of secondary pollution from sludge in the investigated lakes. Phosphorus accumulates in sludge. These results coincide with the results of a study of the nutrients from Curonian Lagoon and their impact on the ecosystems of the Curonian Lagoon, which were presented on 10-01-2017 [14]. However, Kowalczewska-Madura et al. [32] found that phosphorus release from sludge continues throughout the year in deeper lake areas. Slightly higher values were observed only at the beginning of summer and autumn, and the emitted phosphorus remained in the water layer near the bottom for a long time. Phosphorus in sediment exists in organic and inorganic forms. Organic phosphorus represents a small fraction of total phosphorus (Pt), which is generally associated with organisms [33–35]. Inorganic phosphorus is typically associated with Fe, Al and Ca compounds [24] and mainly consists of exchangeable phosphorus (Ex-P), ferric phosphate (Fe-P), aluminum phosphate (Al-P), calcium phosphate (Ca-P), and occluded phosphate (O-P) [33]. Fe-P is the most active inorganic phosphorus species, and it has a high release rate. A close correlation between TP release and the Fe-P components in sediment has been reported [35–37]. Occluded phosphate (O-P) is adsorbed to a Fe2O3 or Al2O3 layer to form a mantle around the phosphoric core, which has a high physical and chemical stability and is not readily available to plants. In addition to phosphorus speciation in sediments, the ecological features of a lake could significantly influence phosphorus release and retention [38]. Studies of the release of biogenic substances from lake sludge into freshwater show differences in the concentrations of biogenic substances released into water. The distribution of biogenic substances in water in the form of fine particles results in a dispersive system. The distributed substance is a dispersed phase (biogenic substance), and the substance in which the dispersed phase is distributed is called the dispersed environment (water). The release of biogenic substances may not be the same in water, and the concentration of dissolved salts is low compared to freshwater, in which dissolved salts have a concentration of approximately 1 mg/L. Dispersed environments are dissimilar, and the distribution of materials is different. Studies have shown that an increase in sludge thickness increases the concentrations of nitrogen, phosphorus and organic matter in the sludge. Most data on the internal pollution of water bodies are related to the shallowness of the bodies [17,39,40]; however, little is known about phosphorus release from lakes with greater depths [41], especially those affected by reconstruction [42]. The accumulation of biogenic substances in the bottom sediment depends on the environmental conditions, the morphology of the water body and the hydrology (the fluctuation of the water level and a constant water level). The maximum amount of nutrients accumulates in the deepest part of the water body sediment when the area is adjacent to agricultural property/land [43]. Biogenic substances (nitrogen, phosphorus) accumulate in lake bottoms, and the 10 cm layer of bottom sludge can contain more than 90% of the phosphorus present in the water body itself. Under optimal conditions, nutrients can re-release themselves into the water, which negatively affects the entire lake ecosystem [44–46]. Anaerobic processes take place in the thin layer of a few millimeters to a few centimeters of sludge, where organic and inorganic phosphorus forms in orthophosphate in the bottom sediment, and nitrogen compounds turn into ammonia, nitrite or nitrate [44]. Studies have shown that the concentration of dissolved salts in Int. J. Environ. Res. Public Health 2019, 16, 4691 14 of 17 water affects the release of biogenic substances from sludge. The release of total nitrite and phosphorus concentrations from sludge into water are reduced when the values of electrical conductivity are higher. Factors that promote phosphorus release include temperature, pH, concentration of oxygen in the sludge and water layer above the sludge [47], redox potential, type of chemical compounds containing phosphorus, concentrations of Fe, Mn, Al and Ca [38,43,48], bioturbation by microinvertebrates [49] and structure of the bottom sediment [50]. The deposition of phosphorus from sludge to water and its transfer to the trophogenic zone may be more intense than its sorption and deposition [32].

5. Conclusions This research has shown that the thickness of the sludge layer influences the amounts of total nitrogen, phosphorus, and organic matter present in the sludge. As the thickness of the sludge layer increases in a lake, the total concentrations of nitrogen, total phosphorus, and organic matter also increase. This study on the release of biogenic substances from sludge in freshwater has shown that the concentrations of nitrites (from 0.038 mg/L to 0.049 mg/L), nitrates (from 0.021 mg/L to 0.136 mg/L), phosphates (from 0.317 mg/L to 0.642 mg/L), and total nitrogen (from 7.973 mg/L to 9.06 mg/L) increase, while the concentration of total phosphorus decreases (from 0.198 mg/L to 0.006 mg/L). These results show that organic phosphorus becomes "locked" in sludge, and no secondary water pollution occurs from this source. Previous studies have shown that the concentration of dissolved salts in water affects the release of biogenic substances from sludge. Due to different dispersion environments, the concentrations of nitrates and total nitrogen increase with the increase in electrical conductivity values. The release of nitrite and total phosphorus concentrations from sludge into water is reduced when higher values of electrical water conductivity are measured.

Author Contributions: Conceptualization, L.C.,ˇ E.B. and E.M.-M.; methodology, L.C.;ˇ software, D.Š.; validation, L.C.;ˇ E.M.-M. and K.L.; formal analysis, L.C.;ˇ investigation, L.C.;ˇ resources, L.C.;ˇ data curation, E.M.-M.; writing—original draft preparation, L.C.;ˇ writing—review and editing, L.C. visualization, L.C.ˇ Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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