© Author(s) 2023. CC Atribution 4.0 License Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes Ocena vpliva odlagališč Gajke in Brstje na stanje podzemne vode z uporabo stabilnih in radioaktivnih izotopov Sonja CERAR 1 , Luka SERIANZ 1 , Polona VREČA 2 , Marko ŠTROK 2 & Tjaša KANDUČ 2 1 Geological Survey of Slovenia, Dimičeva ulica 14, SI–1000 Ljubljana, Slovenia; e-mail: sonja.cerar@geo-zs.si, luka.serianz@geo-zs.si 2 Jožef Stefan Institute, Department of Environmental Sciences, Jamova cesta 39, SI–1000, Ljubljana, Slovenia; e-mail: polona.vreca@ijs.si; marko.strok@ijs.si; tjasa.kanduc@ijs.si Prejeto / Received 17. 11. 2023; Sprejeto / Accepted 20. 12. 2023; Objavljeno na spletu / Published online 21. 12. 2023 Key words: groundwater, monitoring, landfill, stable isotopes, tritium, Gajke, Brstje Ključne besede: podzemna voda, monitoring, odlagališče odpadkov, stabilni izotopi, tritij, Gajke, Brstje Abstract Waste disposal in landfills represents a severe threat to aquatic environments on the local, regional, and global levels. In Slovenia, there are 69 registered landfills where groundwater is regularly monitored. However, isotope techniques are not regularly employed. Therefore, we employed isotope analysis of hydrogen, carbon, and oxygen in combination with total alkalinity to assess the impact of the selected landfill on groundwater and to evaluate the biogeochemical processes at work. The δ 18 O, δ 2 H, δ 13 C DIC , 3 H activity and total alkalinity were determined in October 2020 at 12 sampling points from the surrounding area of the Gajke and Brstje landfills and leachate from the Gajke landfill. The δ 18 O (-9.24 ± 0.3 ‰) and δ 2 H (-64.9 ± 2.7 ‰) in groundwater indicate that the main water source consists in direct infiltration of precipitation, with no significant isotopic fractionation. Total alkalinity in the investigated area ranges from 5.45 to 73 mM and δ 13 C DIC from –14.9 to +6.1 ‰, respectively. Higher values of total alkalinity (up to 73 mM), δ 13 C DIC (up to +6.1 ‰), δ 18 O (-7.64 ‰) and 3 H (209.8 TU) are detected in the leachate, indicating biogeochemical process related to CO 2 reduction or methanogenesis. Methanogenesis could be present at locations GAP-10/13 (Brstje landfill) and G-2 (Gajke landfill) with δ 13 C DIC values ranging from –8.2 to –7.6 ‰ and with dissolved oxygen values around 0 % and elevated 3 H values (from 16 to 18 TU). This study demonstrates the effectiveness of isotopic analysis as a valuable tool for monitoring landfills, revealing shifts in biogeochemical processes within the groundwater there. Izvleček Odlaganje odpadkov na odlagališčih predstavlja resno grožnjo za vodna okolja na lokalni, regionalni in globalni ravni. V Sloveniji je 69 registriranih odlagališč, kjer se redno izvajajo obratovalni monitoringi kemijskega stanja podzemne vode. Kljub temu izotopske tehnike niso rutinsko uporabljene. Zato smo uporabili analizo izotopov vodika, ogljika in kisika v kombinaciji s skupno alkalnostjo, da bi ocenili vpliv izbranega odlagališča na podzemno vodo in ovrednotili biogeokemične procese. δ 18 O, δ 2 H, δ 13 C DIC , aktivnost 3 H in skupna alkalnost so bile določene v oktobru 2020 v 12 vodnjakih v okolici odlagališč Gajke in Brstje in v izcedni vodi iz odlagališča Gajke. Vrednosti δ 18 O (-9,24 ± 0,3 ‰) in δ 2 H (-64,9 ± 2,7 ‰) v podzemni vodi kažejo, da je glavni vir vode neposredna infiltracija padavin, brez bistvene izotopske frakcionacije. Totalna alkalnost na preiskanem območju se spreminja od 5.45 do 73 mM, δ 13 C DIC od –1 4.9 do +6.1 ‰. Višje vrednosti totalne alkalnosti (do 73 mM), δ 13 C DIC (do +6.1 ‰), δ 18 O (-7.64 ‰) in 3 H z 209.8 TU so zaznane v izcedni vodi CERO Gajke (kanal), kar kaže na biogeokemijski proces redukcije CO 2 ali metanogeneze. Metanogeneza bi lahko bila prisotna tudi na lokacijah GAP-10/13 (odlagališče Brstje) in G-2 (odlagališče Gajke) z δ 13 C DIC vrednostima od -8.2 do -7.6 ‰ in z vrednostjo raztopljenega kisika okrog 0 % ter povišano vrednostjo 3 H (od 16 do 18.2 TU). V tej raziskavi smo dokazali, da so izotopi koristna orodja v monitoring raziskavah odlagališč in kažejo na spremembe biogeokemičnih procesov v podzemni vodi. GEOLOGIJA 66/2, 285-299, Ljubljana 2023 https://doi.org/10.5474/geologija.2023.014 286 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Introduction Economic development, population growth, and technological developments are resulting in ever-increasing amounts of deposited waste, mak- ing the importance of ecological waste manage- ment and environmental protection ever more ap- parent. Landfills, which are potential local sources of pollution, are generally considered minor local inconveniences and can also pose problems in larger areas, especially if the pollution spreads from the landfill to the groundwater and surface waters (Bhalla et al., 2013; Abiriga et al., 2020, 2021). Depending on the nature of the contami - nants and their chemical properties, they may re- main or decompose in groundwater for decades or even centuries. Once a waste facility is closed, proper functioning of landfill systems must be en- sured, so operational monitoring of groundwater status and, in some cases, surface water status monitoring must be conducted to determine the status of groundwater during and after landfill op- erations have concluded. Operational monitoring of groundwater status is likely to be conducted in the area of the hydrogeologic target zone, which is a lithostratigraphic unit where contamination could be expected due to indirect or direct dis - charge of contaminants from a source of contam- ination to groundwater. Operational monitoring includes measurements of hydrogeological and chemical parameters, which we use to assess the impact of a landfill on the status of groundwater. An environmental permit is required to operate the landfill, which specifies the scope and content of monitoring; the content and scope are explained in more detail in the groundwater monitoring pro- gramme. In the event that the landfill is deter- mined to have an impact on the status of ground- water based on the chemical analyses performed, it is also necessary to prepare a programme of measures that must include, among other things, an estimate of the discharge of pollutants from the landfill to groundwater and an assessment of the magnitude of the impact on the recipients (Serianz et al., 2017; Cerar et al., 2022). In recent years, several studies have been pub- lished on the determination of chemical parame- ters in landfill leachate (Hussein et al., 2019; Ančić et al., 2020; Baettker, et al., 2020) and their impact on groundwater quality (Kapelewska et al., 2019; Chidichimo et al., 2020). Groundwater quality is usually assessed by defining chemical parameters and comparing the data with standards set in leg- islation. Such an approach provides information only on specific contaminants and provides little information on overall water quality. As differ- ent materials or wastes are introduced into the disposal body, the leachate also has a different chemical composition. As a result, there are also differences in the formation of a groundwater pol- lution plume along the length of the groundwater stream. An important factor in understanding the occurrence of contaminants in groundwater is also their zonation, which is due to the fact that landfill leachate alters the physicochemical properties of groundwater by creating reduction conditions that affect the behaviour of individual contaminants in groundwater (Abiriga et al., 2021). Hackley et al. (1996) already suggested using the isotopes of hydrogen (δ 2 H), carbon (δ 13 C), and oxygen (δ 18 O) of the major landfill constituents of landfill gas and leachate to identify landfill lea- chate contamination. Landfill gases (CO 2 and CH 4 ) and landfill leached products (water and dissolved inorganic carbon) have a characteristic isotope composition with respect to the surrounding en- vironment (Hackley et al., 1996; Kerfoot et al., 2003, Bakkaloglu et al., 2021, Vavilin & Lokshi - na, 2023). Recently, several studies (Adeolu et al., 2011; Castañeda et al., 2012; Wimer et al., 2013; Negrel et al., 2017; Lee et al., 2020; Andrei et al., 2021) have observed that stable isotopes found in landfill leachates, such as δ 13 C, δ 2 H and δ 18 O, are influenced by processes within municipal solid waste (MSW) landfills, mainly on the methano - genesis phase of the landfill. In addition, δ 13 C has frequently been used in environmental monitoring studies of landfills and in the determination of the origin of dissolved inorganic carbon in ground- water (DIC) (North et al., 2006; Porowska, 2015; Nigro et al., 2017; de Medeiros Engelmann et al., 2018). Several studies included tritium ( 3 H) analy- sis as a tool to assess leachate contamination (Ni- gro et al., 2017; Raco & Battaglini, 2022; Gupta & Raju, 2023). In Slovenia, there are 69 registered landfills where the chemical parameters of groundwater in the area of the landfill is monitored as part of the larger operational monitoring of the status of groundwater, while isotopic studies are not rou- tinely performed. The only known case study in Slovenia applying the stable isotope analysis of oxygen (δ 18 O) and hydrogen ( δ 2 H) in water and carbon in the dissolved inorganic carbon (δ 13 C DIC ) in combination with 3 H activity concentrations were conducted at the Puconci landfill (Brenčič et. al., 2013). The combination of techniques used in the investigation of groundwater, sur- face water, and leakage water proved to be use- ful in identifying the influence of leakage water on surface and groundwater, the complexity of 287 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes contamination below the landfill, and also provid- ed a picture of the methane-forming conditions in the landfill. One operational landfill in Slovenia is the Gajke landfill, where in the recharge area about 500 m upstream, there is also the closed Brstje landfill for non-hazardous waste. The re- sults of the operational monitoring of groundwa- ter status at the Brstje landfill showed that it has an impact on groundwater status, as warning lev- els for pollutants have been exceeded for sever - al years (Cerar et al., 2019). When analysing the spatial distribution of pollutants in groundwater, it should be considered that pollutants in ground- water in the area of the Gajke landfill may origi- nate from leachate or may be the result of contact between groundwater and waste or may already flow into the area of the landfill via groundwater from the Brstje landfill upstream. In such cases, it may be very difficult or even impossible to iso- late the individual impacts on the condition of the groundwater. It is therefore essential to consid- er both landfills simultaneously when analysing pollutants in space and time. In this case study, we hypothesise that a mul- ti-parameter isotope approach could be applied to separate the potential impact of two landfills, Gajke and Brstje on the groundwater in the mu- nicipality of Ptuj. By applying in-situ measure- ments in combination with determination of δ 18 O, δ 2 H, δ 13 C DIC , and 3 H in groundwater at all available sampling points and in leachate from Gajke before treatment we characterised the spatial changes of measured parameters in autumn 2020. The results will be useful for improving the management of the landfills and could in future serve as example of improved water monitoring programme, incor- porating isotope measurements, for other landfills in Slovenia. Case study area characteristics Gajke landfill The active Gajke landfill for non-hazardous waste is located in an abandoned gravel pit north of the settlement of Spuhija, in the municipality of Ptuj (Fig. 1). The landfill, including the accom - panying areas, currently covers 7.5 ha. It is sur - rounded by agricultural land on all sides. Waste disposal at the Gajke landfill started in 2003. In total, 572.886 t of waste were deposited from 2003 through 2018. Mixed municipal waste was no longer landfilled with the adoption of new reg- ulations in 2016, as the regulations stipulated the need for post-treatment of this waste. Currently, all municipal waste is transported to Ormož for treatment and disposal. According to the environ- mental permit, the Gajke landfill still has the sta- tus of an active repository. The bottom of the landfill is sealed with three layers of mineral clay (each layer 25 cm thick) and a 2.5 mm thick PEHD plastic film on top. A protective layer of geotextile is laid on the ground above the PEHD film, on top of which a 40 cm thick drainage layer of gravel is placed. A sepa- rating drainage felt is laid over the drainage layer. The collection and discharge of leachate and pre- cipitation water is regulated. Leachate is collected in the leachate basin, then cleaned by a reverse osmosis treatment plant and discharged into the sewage system to the wastewater treatment plant in Ptuj. Precipitation from road and work areas is collected and discharged into the soil collection basin (lagoon), from where it is pumped into the sewer system to the wastewater treatment plant in Ptuj. Clean precipitation and backwater collect in the earth ditches and canals, from where they lead to subsidence chambers and sink into the ground. Brstje landfill The closed Brstje landfill for non-hazardous waste is located in the municipality of Ptuj, ap- prox. 500 m north-west of the Gajke landfill (Fig. 1). The nearest residential houses are 100 m away from the landfill and are located in the Brstje dis- trict. The landfill, including the accompanying ar- eas, covers 6.8 ha. The Brstje landfill consists of the older northern part of the deposition fields and the younger southern part of the deposition fields. The area was filled in several phases and subphas- es, which employed different ways of disposing of the waste and protection measures used to reduce negative environmental impacts. The exact geom - etry and area of the deposited waste is not known, nor is the volume and mass of the deposited waste. The beginnings of waste disposal in the area of the Brstje landfill date back to the 1970s. Exact information on the earliest days of waste disposal there is not known. Data on the amount of waste deposited is only available for the southern land- fill. Thus, the actual amount of waste deposited in the entire landfill is far higher. A total of 66,818 t of mixed municipal waste was deposited at the southern landfill. The youngest part of the land- fill, where waste was deposited between 1996 and 2001, is the southernmost landfill. The landfill is special because it is not located on the embankment, but rather the waste is depos- ited in the former gravel pit at a depth of 5 to 7 m below the surface. On the eastern side, the gravel 288 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ pit is recultivated with a poplar plantation. The old and new parts of the landfill are covered with a less permeable top layer of clay, geocomposite and a layer of soil and humus. In the bottom of the new part, an impermea- ble PEHD foil with a drainage system with suction pipes for pumping out the leachate was installed, which is also a special feature of this landfill. In older deposit fields, the leachate drainage system is not regulated. All water drains gravitational- ly into the groundwater. Precipitation water is drained through the cover layer into the peripheral subsidence ditch. Geographical and hydrological characteristics of the area The Gajke and Brstje landfills are located in Ptujsko polje, which from the morphological point of view consists of two Drava terraces and is sur- rounded on all sides by agricultural land. The larg- est part of the field is occupied by a high terrace with an elevation of 222–224 m, where there are also landfills. In the western part of the field, the terrace is 7–8 m high, in the central part 4.5 m high, and in the eastern part it 2–3 m high. About 500 m upstream northwest of the Ga - jke landfill is the closed Brstje landfill. The near- est surface water is the Rogoznica stream, which flows into the Drava River southeast of the landfill (Fig. 1) (Cerar et al., 2019). Geological and hydrogeological features in the area of the landfills The Gajke and Brstje landfills are located on the Quaternary aquifer of the Ptujsko polje, which consists in the upper part of alluvial deposits from the Drava, Pesnica, and Rogoznica rivers. These are mainly sediments of medium-grained sandy gravel, between which there are lenses of silt and clay with limited extension. The lower part is dom - inated by fine- and medium-grained gravels with more sand and silt, as well as sand layers with silt. The base of the Quaternary sediments is formed by fine-grained sediments of Pliocene age (Fig. 2). The first hydrogeological unit in the landfill area is represented by a slightly permeable (K=10 -3 m/s), open Quaternary aquifer, with groundwater at a depth of about 8–9 m below the surface, freely fluctuating in the range between 2.5 and 3.2 m, depending on the hydrologic conditions. Fig. 1. Overview map of the area of the Gajke and Brstje landfills. 289 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes Groundwater flows in the Quaternary aquifer in the NW–SE direction to the Drava River, into which it discharges at 4 to 10 km (Fig. 3). The di - rection of groundwater flow is approximately the same with respect to the different hydrological conditions, with the largest deviations observed mainly in the south-eastern part of the Gajke de- posit, where the flow is directed further to the northeast during floods. The groundwater flow gradient is 0.002 and depends on the intensity of recharge from precipitation. Groundwater velocity is estimated to be 2 to 3 m/day. Depending on the depth of the deposited waste and the groundwater table, it is an indirect input of pollutants since the landfill is in or above the unsaturated zone. During the flood period, the bottom of the deposit body (the outside of the liner system) is occasionally in contact with groundwa - ter (Cerar et al., 2019). Methods and materials Sampling Sampling for isotope analysis was conducted in 27–28 October, 2020, by Javne službe Ptuj d.o.o. in collaboration with the NLZOH (National Lab- oratory of Health and Food, Maribor), following the prescribed instructions outlined below. Water samples for δ 18 O and δ 2 H analysis were gathered in 60 mL HDPE bottles, which were prewashed twice with the sample and had no headspace. Samples for δ 13 C DIC and total alkalinity (TA ) analysis were filtered through a 0.45 µm membrane filter and transferred into two glass ampoules, each with a volume of 12 ml and no headspace, using gas-tight syringes. For 3 H analysis, 1 L of unfiltered water was collected in an HDPE container. Prior to stable isotope analysis, the samples were stored in the re- frigerator at temperatures ranging from 4 to 6 °C, while samples for 3 H analysis were stored at room temperature. Sampling was conducted at a total of 13 locations (Table 1). Groundwater was collect - ed from 12 wells in the Gajke and Brstje landfill areas and leachate was collected from a channel from the Gajke landfill (Fig. 3, Table 1). Sampling of groundwater from seven wells was performed by the NLZOH concurrently with the regular op- erational monitoring of groundwater conditions in accordance with SIST ISO 5667-11:2010 (referred to as “monitoring” in Table 1). Five other wells and leachate were sampled by the Javne službe Ptuj d.o.o. only for TA and isotope analysis (referred to as “other” in Table 1) and in situ measurements were not conducted. Fig. 2. Simplified hydrogeological cross-section through Gajke and Brstje landfills. 290 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Analysis All water samples received were analysed for isotope analysis at the Jožef Stefan Institute lab - oratories using the procedures described below. Since leachate analysis is not routinely performed, we anticipated problems with the analysis of this water. Ultimately, the only difficulties encountered were in determining the δ 2 H, which could not be eliminated despite repeated analyses, so the result is not reported for this parameter. TA was measured using Gran titration (Giesk- es, 1974) with a precision of ±1 % within 24 hours of sample collection. Approximately 8–10 g of the sample was weighted in a plastic HDPE bottle with a magnetic stirrer. The pH electrode of the Mettler toledo Seven compact pH meter S220 was Sampling point Date and time of sampling Location* N (D96) E (D96) Z ground (m) Z well (m) Type of sampling G-1a 26.10.2020 08:00 upstream 142665 569926 224.70 224.79 monitoring G-2 26.10.2020 08:35 upstream 142842 569978 224.28 224.38 other G-3 26.10.2020 09:00 upstream 142944 570102 224.38 224.23 other G-3a 26.10.2020 10:30 downstream 142556 570352 224.39 224.58 monitoring G-4 26.10.2020 10:00 downstream 142724 570403 223.80 223.97 other G-4b/12 26.10.2020 11:05 downstream 142622 570160 222.48 222.70 monitoring G-5 26.10.2020 09:25 downstream 142388 570332 223.58 223.62 monitoring GAP-7 26.10.2020 13:15 upstream 142911 569273 225.86 226.01 monitoring GAP -8/13 26.10.2020 13:45 upstream 143090 569480 225.69 226.14 other GAP -10/13 26.10.2020 11:45 upstream 142952 569880 224.66 225.19 monitoring V-3/2 26.10.2020 12:15 upstream 142832 569521 225.44 225.44 monitoring cemetery 26.10.2020 12:40 upstream 143094 569409 / 226.03 other leachate 27.10.2020 07:30 laterally 142716 570192 218.30 / other * - according to the direction of groundwater flow in the Gajke landfill area Table 1. Locations of sampling points and time and type of sampling. Fig. 3. Sampling points of groundwater and leachate in the Gajke and Brstje landfill area. 291 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes calibrated using certificate buffers with values of 7.00 and 4.00 ±0.02. With this method we deter - mined the change in pH depending on the volume of added acid with a known concentration, which is added to a solution of unknown concentration (Vreča et al., 2020). δ 13 C DIC was determined using the Europa-Scien- tific 20-20 with TG preparation module. Approxi - mately 200 µL of phosphoric acid (Sigma-Aldrich p.a., ≥85 %) was added to a 12 mL vial and purged with helium (He). Subsequently, the water sample (0.5 –5 mL, depending on total alkalinity) was in - jected into the ampoule, and CO 2 was measured from headspace. For one point normalization of samples, a Carlo Erba solution (8 mg/12mL) with a known value of –10.8 ±0.2 was used to calibrate δ 13 C DIC m e a s u r e m e n t s ( S p ö t l , 2 0 0 5 ; V r e č a e t a l . , 2020). δ 2 H and δ 18 O were determined using the H 2 - H 2 O (Coplen et al., 1991) and CO 2 -H 2 O (Epstein & Mayeda, 1953; Avak & Brand, 1995) equilibra - tion technique. Measurements were performed on a dual inlet isotope ratio mass spectrometer (DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an automated H 2 - H 2 O and CO 2 -H 2 O HDOeq 48 Equilibration Unit (custom built by M. Jaklitsch). All measurements were performed together with laboratory reference materials (LRM) calibrated periodically against primary IAEA calibration standards to VSMOW/ SLAP scale. Samples were measured as independ- ent duplicates and results were normalized to the VSMOW/SLAP scale using the Laboratory Infor - mation Management System for Light Stable Iso- topes (LIMS) programme (https:/ /water.usgs.gov/ water-resources/software/RSIL-LIMS/). For in- dependent quality control, we used internal LRM and USGS commercial reference materials. The overall measurement uncertainties are estimated to be less than 1 ‰ and 0.05 ‰ for δ 2 H and δ 18 O, respectively (Vreča et al., 2020). The results for δ 13 C DIC , δ 2 H and δ 18 O are ex- pressed in a standard δ n o t a t i o n i n p e r m i l ( ‰ ) relative to international standards (Coplen et al., 1991; Coplen, 1994; IAEA, 2018). For 3 H analysis, the samples were distilled prior to tritium enrichment in order to remove dissolved solids and other possible interferences. The 3 H was enriched using electrolysis. After electrolysis, the sample was transferred to stainless steel distilla- tion flasks for a second distillation. Then 10 g of sample solution was mixed with 12 mL of Ultima Gold LLT scintillation cocktail and measured in Quantulus 1220 (Perkin Elmer) liquid scintilla - tion counter for 5 h, together with a tritium-free water sample (dead water) to correct for detector noise and background and according to standards used to determine 3 H detection efficiency. In the STC 131/20 analytical report (Štrok & Svetek, 2020; Appendix 2), results for 3 H activity (As) are expressed in Bqkg -1 . Tritium units (TU = tritium unit) are commonly used in isotope hydrology, where 1 TU represents 1 3 H atom per 10 18 1 H at- oms. Therefore, the results were converted to TU for interpretation of the results, considering 1 TU = 0.1 1 8 BqL -1 (Ingraham, 1 998 ; Ga t et al. , 2 00 1 ) and 1 kg = 1 L. Spatial analysis The spatial distribution of individual param- eters was carried out using GIS software ESRI ® ArcMap™ (v. 10.5.) using the interpolation meth - od of natural neighbours, which uses Thiessen polygons or Voronoi diagrams and weighted aver- ages of neighbouring values to arrive at the most appropriate values. Experientially, this method is most suitable for the given spatial data density. Results and discussion The results of the in-situ measurements (tem- perature (T), pH, electrical conductivity (EC), re - dox potential (Eh) and dissolved oxygen (DO)) re - ceived from the client and performed by NLZOH, of isotope analysis (δ 18 O, δ 2 H, δ 13 C DIC , 3 H) and TA are presented in Table 2. To assess the potential impact of the Gajke and Brstje landfills on groundwater quality sta- tus, maps illustrating the spatial distribution of δ 18 O, δ 2 H, δ 13 C DIC , TA, and 3 H in groundwater were created for the entire study area. To separate the impacts of the two landfills, isotope groundwater data were compared with analytical results from additional sampling of leachate prior to reverse osmosis at the Gajke landfill. Leachate from the Brstje landfill was not analysed due to the unreg- ulated drainage system. These leachates are col- lected at the bottom of the protected deposit field through pipes to the leachate where no changes have been observed for years, as leachate has not been detected since 2005. Monthly inspections of the leachate shaft and meter inventory are carried out, which is also performed several times a year by a representative of the Javne službe Ptuj d.o.o. Only the older parts of the landfill, where there are poplars and plateau, have no soil protection, although there is upper protection in the form of asphalt and poplars. 292 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Field measurements Groundwater temperatures at the Brstje landfill are lower compared to the measured groundwater temperatures in the Gajke area. The groundwater temperature in the Brstje landfill area is between 13.9 °C and 14.9 °C, while in the Gajke landfill area it is between 13.6 °C and 19 °C (Table 2). The G-3a (16.3 °C) and G-4b/12 (19 °C), which are located downstream (Fig. 3) of the Gajke landfill, are outstanding. Variable temperatures are result of different thickness and position of wastes which influences the heat-generating (exothermic) reac - tions. The pH of the groundwater in the area of the Gajke landfill is constant at all sampling points (between 7.0 and 7.1). In the area of the Brstje landfill, the pH value varies between 6.9 and 7.1, and the pH value of the groundwater is comparable to that of the groundwater in the area of the Gajke landfill. EC values in the entire study area fall in the in- terval from 750 to 977 μS/cm, deviating from the sampling point GAP-10/13 (977 μS/cm), which is located on the eastern edge (downstream) of the old deposition field of the Brstje landfill and about 200 m northwest (upstream) of the Gajke landfill (Fig. 3). The reason for the deviating values of the electrical conductivity on GAP-10/13 are the addi - tional pressures on the groundwater caused by the Brstje landfill, which were already identified in the study by Cerar et al. (2019). According to the measured contents of DO in the groundwater in the area of the Gajke and Brstje landfills, constant suboxic conditions prevail at the G-3a and GAP-10/13 sampling points. The meas - ured DO content at these two sites is below the lower limit of quantification (LOQ = 0.5 mg/L). Somewhat higher values were obtained at V-3/2, where they are 1.73 mg/L. At the other monitoring points, DO values are higher, ranging from 5.51 to 7.63 mg/L (Table 2). The Eh indicates the prevailing oxidation to transient oxidation-reduction conditions, ex- pressed as values of 309–343 mV, with the up - stream monitoring point GAP-7 standing out with a value of 426 mV, indicating higher aeration of the groundwater (7.63 mg/L), which is also confirmed by the highest concentration of DO. Isotope composition of oxygen and hydrogen Values for δ 18 O in groundwater in the vicinity of the two landfills vary from -9.69 to -8.56 ‰ (Fig. 4). The lowest δ 18 O values are observed at lo- cations around the Gajke landfill, while the high- est values were detected at locations northwest of Brstje and at G-3a in the south-eastern part of the study area downstream from the Gajke landfill. In leachate, the measured value for δ 18 O is -7.64 ‰ and is slightly higher than in groundwater sam- ples, indicating the influence of secondary pro- cesses on the δ 18 O (Tazioli, 2011), as confirmed by positive δ 13 C DIC and higher TA values (Table 2). Table 2. Results of in-situ measurements (T, pH, EC, Eh, DO), isotope analysis (δ 18 O, δ 2 H, δ 13 C DIC , 3 H) and TA from 27–28 October, 2020 are summarised from Vreča et al. (2020) and Štrok & Svetek (2020). Sampling point T (°C) pH EC (µS/cm) Eh (mV) DO (mg/L) DO (%) δ 18 O (‰) δ 2 H (‰) δ 13 C DIC (‰) TA (mM) 3 H (TU) G-1a 13.9 7.1 788 342 6.59 65.9 -9.57 - 67.0 -14.9 7.72 5.9 G-2 n.d. n.d. n.d. n.d. n.d. n.d. -9.22 -65.3 -8.2 7.12 18.8 G-3 n.d. n.d. n.d. n.d. n.d. n.d. -9.30 -65.1 -10.6 7.70 11.8 G-3a 16.3 7.1 778 309 0.12 1.3 -8.87 -62.1 -14.3 7 .85 5.9 G-4 n.d. n.d. n.d. n.d. n.d. n.d. -9.60 - 67. 5 -13.4 7. 8 3 7. 3 G-4b/12 19 7.0 790 335 5.51 61.3 -9.69 -68.6 -14.4 7.74 4.6 G-5 13.6 7.1 752 343 7.16 71.1 -9.47 -66.5 -13.4 8.11 4.4 GAP-7 14.9 7.1 794 426 7.63 77. 8 -9.17 -65.1 -14.7 7. 8 4 5.2 GAP -8/13 n.d. n.d. n.d. n.d. n.d. n.d. -8.81 -61.3 -14.8 5.5 6.0 GAP -10/13 14.4 6.9 977 329 0.06 0.6 -9.24 -65.2 -7.6 9.84 16.2 V- 3/2 14.2 7.1 750 331 1.73 17.4 -9.37 -65.9 -14.6 7.0 8 6.6 cemetery n.d. n.d. n.d. n.d. n.d. n.d. -8.56 -59.3 -14.4 6.07 7. 5 leachate CERO Gajke (canal) n.d. n.d. n.d. n.d. n.d. n.d. -7.6 4 n.d. +6.1 73 209.8 n.d. - not determined 293 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes The values for δ 2 H in groundwater in the two landfills follow the changes in δ 18 O and vary be- tween -68.6 and -59.3 ‰ (Table 2, Fig. 5). In the leachate, δ 2 H was not measured due to technical problems. All measured δ 18 O and δ 2 H values in ground- water indicate that the main water source con- sists in direct infiltration of local precipitation and does not indicate the considerable influence of evaporation or other secondary processes. The isotope composition was monitored in the peri- od 2016–2018 at Murska Sobota and Sv. Urban, i.e., locations NE and SW of the investigated area (Vreča et al., 2022). The average δ 18 O and δ 2 H val- ues amounted –9.28 ‰ and –65.8 ‰ for Murska Sobota and –8.53 ‰ and –59.2 ‰ for Sv. Urban. The δ 18 O values vary as a function of tempera- ture and are lowest at sites where temperatures are lowest. Unfortunately, water temperature was not measured at all sites where samples were collect- ed. Only G-4b/12 deviates, where water tempera - ture was relatively high (19 °C) and δ 18 O was low- est (-9.69 ‰). The result indicates different water properties at this site (δ 2 H and 3 H activity are also the lowest) and is due to the location of G-4b/12, which is at the edge of the storage field and has a higher temperature compared to the other points. Values for δ 18 O and δ 2 H in groundwater indi- cate that the main source of water is direct infil- tration of precipitation, with no significant isotop- ic fractionation, and that the isotope composition depends on water temperature, which was deter- mined at only 7 sampling points. It is estimat - ed that the δ 18 O and δ 2 H values in groundwater downstream of the Brstje landfill decrease, while they increase again slightly at the G-3a. This indi- cates that the impact of the Gajke landfill cannot be completely excluded. Total alkalinity The TA values in groundwater around the two landfills range from 5.45 to 8.11 mM, deviating GAP-10/13 with slightly higher values of 9.84 mM (Fig. 6). In the leachate the measured value is 73 mM. The spatial distribution shows that the highest values (9.84 mM) appear at the down - stream sampling point of GAP-10/13 and then decrease at sampling points: G-1a and G-4b/12 (around 7.7 mM) downstream of the Gajke landfill. The lowest values (5.45 mM) are located upstream (GAP-8/13) from the Brstje landfill. Fig. 4. Spatial distribution of δ 18 O (‰) in groundwater in the Gajke and Brstje landfill area. 294 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Fig. 5. Spatial distribution of δ 2 H (‰) in groundwater in the Gajke and Brstje landfill area. Fig. 6. Spatial distribution of TA (mM) in groundwater in the area of the Gajke and Brstje landfill area. 295 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes Isotope composition of carbon from dissolved inorganic carbon The δ 13 C DIC values in groundwater in the two landfills vary from -14.9 to -8.2 ‰ (Table 2, Figs. 7 and 8). The spatial distribution of δ 13 C DIC shows that the lowest values were measured upstream of the Brstje landfill at GAP-8/13 (-14.8 ‰), where the impact of the landfill leachate is not expect - ed. Slightly higher δ 13 C DIC values are observed at downstream monitoring point G-4 (-13.4 ‰) and G-5 (-13.4 ‰) from Gajke landfill compared to G-4b/12 with δ 13 C DIC of – 1 4. 4 ‰. T o confirm the influence of leachate, measurements should be re- peated several times during the hydrological year. The measured δ 13 C DIC value in the leachate from the Gajke landfill is +6.1 ‰ (Table 2, Figs. 7 and 8). This value characterizes degradation of organ - ic matter, including methanogenesis under anoxic conditions in landfills. Leachate becomes enriched with a heavier carbon isotope ( 13 C) during this process (Fig. 8). The highest positive δ 13 C DIC values in ground- water at locations GAP-10/13 (-7.6 ‰) and G-2 (-8.2 ‰) indicate a significant impact on the car - bon cycle in groundwater (see Table 2, Figs. 7 and 8). Furthermore, at GAP-10/13, recorded dis - solved oxygen concentrations around 0 % (Table 2) could suggest the presence of methanogenesis in groundwater (North et al., 2006). However, this presence isn’t as pronounced as in the leachate. North et al. in 2006 found δ 13 C DIC ranged from +2.8 to +15.8 ‰ in all analysed leachate samples, indicating the presence of methanogenesis. The maximum values of δ 13 C DIC , TA, and 3 H were detected at monitoring points GAP-10/13 (-7.6 ‰, 9.84 mM, 16.2 TU) and G-2 (-8.2 ‰, 7.12 mM, 18.8 TU) and gradually decreased to - ward monitoring points G-4b/12 and G-4 down - stream of the Gajke landfill. This indicates that no significant impact is expected downstream of the Gajke landfill but occurs in the north-central part of the Gajke landfill. Similar results have already been observed during “benchmarking”. Fig. 7. Spatial distribution of δ 13 C DIC (‰) in groundwater in the Gajke and Brstje landfill area. 296 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Tritium Groundwater 3 H activities in the surrounding area of two landfills range from 4.4 to 18.7 TU (Figs. 8 and 9). The highest activities of 3 H occur in wells GAP-10/13 (16.2 TU), G-2 (18.8 TU), and G-3 (11.8 TU), all of which are upstream of the Ga- jke landfill and may be attributed to the influence of the Brstje landfill, whose leachate was not sam- pled. The lowest activities were found upstream of the Brstje landfill and downstream of the Gajke landfill. Downstream of Gajke, 3 H deviates at G-4 (7.3 TU), and the southward changes show the same trend as the change in boron concentration (Cerar et al., 2019). The spatial distribution of 3 H in groundwater shows similar characteristics to TA and δ 13 C DIC (Figs. 6, 7 and 8). Fig. 8. TA and 3 H versus δ 13 C DIC with associated locations. 0 50 100 150 200 250 0 10 20 30 40 50 60 70 80 -20 -15 -10 -5 0 5 10 3 H (TU) TA (mM) δ δ 13 C DIC (‰) methanogenesis, enrichment with 13 C isotope leachate CERO Gajke (canal) GAP-10/13 G-2 G-3 G-4, G-5 G-1a G-3a, G-4b/12 GAP8/13 GAP-7, V-3/2, cemetery Fig. 9. Spatial distribution of 3 H (TU) in groundwater in the Gajke and Brstje landfill area. 297 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes In the leachate of the Gajke landfill, the meas- ured activity is significantly higher than in the groundwater and is 209.8 TU (Fig. 8). High 3 H activities are characteristic of leachate and can be as high as 1,000 TU (Tazioli, 2011). In monthly precipitation over Slovenia, 3 H activity rarely ex- ceeds 20 TU (Internet 1) and amounts to an annu - al average of less than 10 TU over the past decade (Kern et al., 2020; Vreča et al., 2022), making the 3 H parameter a very good indicator of pollution from landfills. Raco and Battaglini (2022) found 3 H values in leachate ranging from 55 to 923 TU, while Gupta and Raju (2023) found in their study of landfill leachate and groundwater sample 3 H value from 8.11 TU and 3.03 TU, respectively. We could conclude that higher measured activity in GAP-10/13 is the result of pollution from the Brst - je landfill. Conclusions The results of the present study show that iso- tope analysis is a valuable tool for monitoring land- fills, revealing shifts in biogeochemical process- es within groundwater and allowing prediction of contamination plumes in the potential impact area. The results will further improve the picture of the spatial distribution of conservative contam- inants, while also identifying possible scenarios for the input of leachate from the Gajke Landfill into the aquifer. Tritium, which demonstrates high activity in leachate, proved to be the most relia- ble parameter for such a prediction. The analyses performed proved to be an effective method to determine the dispersion of loads from landfills, especially in terms of predicting the spatial distri- bution of the loads and possible scenarios of the load of the aquifer by leachate. However, it should be noted that this paper summarises the results of a single sampling. For a more reliable assessment, we suggest repeating the same analyses in different water conditions (low, medium, high) or establishing monthly mon - itoring of δ 18 O, δ 2 H, δ 13 C DIC , and 3 H in the ground- water at all 12 sampling points in order to allow for adequate isotopic characterization of the water. In addition, we propose including sampling of the Rogoznica stream upstream and downstream of the Brstje landfill in the monitoring, as knowledge of surface water infiltration and surface/ground- water interactions are also important in any eval- uation of the results. Further, systematic research is necessary due to climate extremes that can sig - nificantly impact the flow of groundwater, thus af- fecting the spread of pollution clouds. Acknowledgments This research was funded by Javne službe Ptuj d.o.o., the Slovenian Research Agency, and Innovation (ARIS), under the Research Programmes Groundwater and Geochemistry (No. P1-0020) and Cycling of Sub - stances in the Environment, Mass Balances, Modelling of Environmental Processes, and Risk Assessment (No. P1-0143). The authors would also like to express grat - itude to Barbara Svetek, Klara Žagar, and Stojan Žigon for their valuable help with isotope analysis. References Abiriga, D., Vestgarden, L.S. & Klempe, H. 2020: Groundwater contamination from a municipal landfill: Effect of age, landfill closure, and sea- son on groundwater chemistry. Science of The Total Environment, 737: 140307. ht t p s://doi . org/10.1016/j.scitotenv.2020.140307 Abiriga, D., Vestgarden, L.S. & Klempe, H. 2021: Long-term redox conditions in a landfill-lea - chate-contaminated groundwater. Science of The Total Environment, 755: 143725. ht t p s:// doi.org/10.1016/j.scitotenv.2020.143725 Adeolu, A.O., Ada, O.V., Gbenga, A.A. & Adebayo, O.A. 2011: Assessment of groundwater con - tamination by leachate near a municipal solid waste landfill. African Journal of Environmen- tal Science and Technology, 5/11: 933-940. Andrei, F., Barbieri, M. & Sappa, G. 2021: Appli - cation of 2 H and 18 O isotopes for tracing mu- nicipal solid waste landfill contamination of groundwater: Two Italian case histories. Water, 13/8: 1065. https://doi.org/10.3390/ w13081065 Ančić, M., Huđek, A., Rihtarić, I., Cazar, M., Bačun-Družina, V., Kopjar, N. & Durgo, K. 2020: PHYSICO chemical properties and tox - icological effect of landfill groundwaters and leachates. Chemosphere, 238: 124574. ht t p s:// doi.org/10.1016/j.chemosphere.2019.124574 Avak, H. & Brand, W.A. 1995: The Finning MAT HDO-Equilibration—A fully automated HO/ gas phase equilibration system for hydrogen and oxygen isotope analyses. Thermo Elec- tron. Corp. Appl. News, 11: 1–13. Bakkaloglu S., Lowry D., Fisher R.E., France J.L. & Nisbet E.G. 2021: Carbon isotopic charac - terization and oxidation of UK landfill meth - ane emissions by atmospheric measurements. Waste management, 132: 162–175. ht t p s://doi . org/10.1016/j.wasman.2021.07.012 Baettker, E.C., Kozak, C., Knapik, H.G. & Aisse, M.M. 2020: Applicability of conventional and non-conventional parameters for mu- nicipal landfill leachate characterization. 298 Sonja CERAR, Luka SERIANZ, Polona VREČA, Marko ŠTROK & Tjaša KANDUČ Chemosphere, 251: 126414. ht t p s://doi . org/10.1016/j.chemosphere.2020.126414 Bhalla, B., Saini, M.S. & Jha, M.K. 2013: Effect of age and seasonal variations on leachate char- acteristics of municipal solid waste landfill. International Journal of Research in Engi- neering and Technology, 2/8: 223–232. ht t p:// www.ijret.org Brenčič, M., Ivanuša-Šket, H., Torkar, A. & Vreča, P. 2013: Influences of sanitary landfill on groundwater under the complex hydrogeolog - ical conditions. In: Cossu, R. (ed.): 14th In - ternational waste management and landfill symposium, 30 September - 4 October 2013, Forte Village, S. Margherita di Pula, Cagliari, Sardinia, Italy. Cagliari: CISA, cop. 2013. 10 p. Castañeda, S.S., Sucgang, R.J., Almoneda, R.V., Mendoza, N.D. S. & David, C.P.C. 2012: En - vironmental isotopes and major ions for trac- ing leachate contamination from a municipal landfill in Metro Manila, Philippines. Journal of environmental radioactivity, 110, 30-37. https://doi.org/10.1016/j.jenvrad.2012.01.022 Cerar, S., Serianz, L., Koren, K. & Lapajne, S. 2019: Carrying out a comparative analysis of basic and indicative parameters in groundwater and water balance in the Gajke landfill area. Lju- bljana: Geological Survey of Slovenia. Cerar, S., Serianz, L., Koren, K., Prestor, J. & Mali, N. 2022: Synoptic Risk Assessment of Ground - water Contamination from Landfills. Ener- gies, 15/14: 5150. https://doi.org/10.3390/ en15145150 Chidichimo, F., De Biase, M. & Straface, S. 2020: Groundwater pollution assessment in land- fill areas: Is it only about the leachate? Waste Management, 102: 655–666. ht t p s://doi . org/10.1016/j.wasman.2019.11.038 Coplen, T.B., Wildman, J.D. & Chen, J. 1991: Improvements in the gaseous hydrogen-wa- ter equilibration technique for hydrogen iso- tope-ratio analysis. Analytical Chemistry, 63/9: 910–912. https://doi.org/10.1021/ ac00009a014 Coplen, T.B. 1994: Reporting of stable hydro - gen, carbon, and oxygen isotopic abundances (technical report). Pure and applied chemis - try, 66/2: 273–276. https://doi.org/10.1351/ pac199466020273 de Medeiros Engelmann, P., Dos Santos, V.H.J.M., Barbieri, C.B., Augustin, A.H., Ketzer, J.M.M. & Rodrigues, L.F. 2018: Environmental moni - toring of a landfill area through the application of carbon stable isotopes, chemical parameters and multivariate analysis. Waste Management, 76/591–605. https://doi.org/10.1016/j.was- man.2018.02.027 Epstein, S. & Mayeda, T. 1953: Variation of 18 O content of waters from natural sources. Geo- chimica et cosmochimica acta, 4/5: 213–224. https://doi.org/10.1016/0016-7037(53)90051-9 Gat, J.R., Mook, W.G. & Meijer, H.A. 2001: Atmos - pheric water. In: Environmental Isotopes in the Hydrological cycle. Edited by W.G. Mook, IAEA and UNESCO, Technical documents in Hydrology, 39/II, UNESCO Paris: 63–74. Gieskes, J.M. 1974: The alkalinity – tool carbon dioxide system in seawater. Marine chemis - try, 5 (The Sea). Edited by Goldberg E.D. New York: John Wiley and Sons: 123–151. Gupta, S. & Raju, N.J. 2023: Potential environ - mental pollution study by leachate generation and health risk assessment in the vicinity of bandhwari landfill disposal site, National Capital Region, India. Groundwater for sus- tainable development 23: 101032. ht t p s://doi . org/10.1016/j.gsd.2023.101032 Hackley, K.C., Liu, C.L. & Coleman, D.D. 1996: Environmental isotope charac - teristics of landfill leachates and gases. Groundwater, 34/5: 827-836. ht t p s://doi . org/10.1111/j.1745-6584.1996.tb02077.x Hussein, M., Yoneda, K., Zaki, Z.M. & Amir, A. 2019: Leachate characterizations and pollution indices of active and closed unlined landfills in Malaysia. Environmental Nanotechnolo- gy, Monitoring & Management, 12: 100232. https://doi.org/10.1016/j.enmm.2019.100232 Ingraham, N.L. 1998: Isotopic variations in pre - cipitation. In: Kendall C. & McDonnell, J.J. (eds.): Isotope tracers in catchment hydrology. Elsevier, 87–118. IAEA - International Atomic Energy Agency 2018: Reference Sheet for VSMOW2 and SLAP2 In - ternational Measurement Standards. IAEA, Vienna, Austria: 8 p. Lee, K.S., Ko, K.S. & Kim, E.Y. 2020: Applica - tion of stable isotopes and dissolved ions for monitoring landfill leachate contamination. Environmental geochemistry and health, 42: 1387–1399. https://doi.org/10.1007/s10653- 019-00427-y Kerfoot, H.B., Baker, J.A. & Burt, D.M. 2003: The use of isotopes to identify landfill gas effects on groundwater. Journal of Environmental Moni- toring, 5/6: 896–901. https://doi.org/10.1039/ B310351J Kapelewska, J., Kotowska, U., Karpińska, J., As - tel, A., Zieliński, P., Suchta, J. & Algrzym, K. 2019: Water pollution indicators and chemom - 299 Impact assessment of the Gajke and Brstje landfills on groundwater status using stable and radioactive isotopes etric expertise for the assessment of the impact of municipal solid waste landfills on ground- water located in their area. Chemical Engi- neering Journal, 359: 790–800. ht t p s://doi . org/10.1016/j.cej.2018.11.137 Kern, Z., Erdélyi, D., Vreča, P., Krajcar Bronić, I., Fórizs, I., Kanduč, T., Štrok, M., Palcsu, L., Süveges, M., Czuppon, G., Kohán, B. & Gábor Hatvani, I. 2020: Isoscape of amount-weighted annual mean precipitation tritium ( 3 H) activity from 1976 to 2017 for the Adriatic–Pannonian region – AP 3 H_v1 database, Earth Syst. Sci. Data, 12, 2061–2073. https://doi.org/10.5194/ essd-12-2061-2020 Negrel, P., Ollivier, P., Flehoc, C. & Hube, D. 2017: An innovative application of stable isotopes (δ 2 H and δ 18 O) for tracing pollutant plumes in groundwater. Science of the Total Environ- ment, 578: 495–501. https://doi.org/10.1016/j. scitotenv.2016.10.214 Nigro, A., Sappa, G. & Barbieri, M. 2017: Applica - tion of boron and tritium isotopes for tracing landfill contamination in groundwater. Jour- nal of geochemical exploration, 172: 101–108. https://doi.org/10.1016/j.gexplo.2016.10.011 North, J.C., Russel, D.F. & Van Hale, R. 2006: Can stable isotopes be used to monitor landfill lea- chate impact on surface waters? Journal of ge- ochemical exploration, 88: 49–53. ht t p s://doi . org/10.1016/j.gexplo.2005.08.003 Porowska, D. 2015: Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes. Waste Management, 39: 216–225. https://doi.org/10.1016/j.was- man.2015.01.044 Raco, B. & Battaglini, R. 2022: Tritium as a tool to assess leachate contamination: An exam - ple from Conversano landfill (Southern It- aly), Journal of geochemical exploration, 235: 106939. https://doi.org/10.1016/j.gexp- lo.2021.106939 Serianz, L., Cerar, S., Koren, K., Prestor, J., Mali, N. & Mladenović, B. 2017: Evaluation of the impact of landfills on groundwater status, Wa - ter Congress, Podčetrtek 19.–20. 4. 2017. Spötl, C. 2005: A robust and fast method of sam - pling and analysis of δ 13 C of dissolved inorgan- ic carbon in ground waters. Isotopes in Envi- ronmental and Health Studies, 41/3: 217–221. https://doi.org/10.1080/10256010500230023 Štrok, M. & Svetek, B. 2020: Determination of trit - ium. AP STC 131/20: Client of Public Service Ptuj d.o.o. Ljubljana: Jožef Stefan Institute, Department of Environmental Sciences, 3 p. Tazioli, A. 2011: Landfill investigation using trit - ium and isotopes as pollution tracers. Acquae Mundi, 18: 83–92. Vavilin, V.A. & Lokshina, L.Y. 2023: Carbon and hydrogen dynamic isotope equations are used to describe the dominant processes of waste biodegradation: effect of aeration in methano- genic phase of landfill. Waste management, 166: 280–293. https://doi.org/10.1016/j.was- man.2023.04.027 Vreča, P., Kanduč, T., Žigon, S. & Nagode, K. 2020: Determination of the isotope composition of oxygen and hydrogen, carbon from dissolved inorganic carbon and total alkalinity according to Gran in water. AP GEO 25/2020: Client of Public Service Ptuj d.o.o. Ljubljana: Jožef Ste - fan Institute, Department of Environmental Sciences, 5 p. Vreča, P., Pavšek, A. & Kocman, D. 2022: SLONIP–A Slovenian Web-Based Interactive Research Platform on Water Isotopes in Pre - cipitation. Water, 14/13: 2127. ht t p s://doi . org/10.3390/w14132127 Wimmer, B., Hrad, M., Huber-Humer, M., Watzinger, A., Wyhlidal, S. & Reichenauer, T.G. 2013: Stable isotope signatures for char - acterising the biological stability of landfilled municipal solid waste. Waste Management, 33/10: 2083-2090. https://doi.org/10.1016/j. wasman.2013.02.017 Internet source: Internet 1. SLONIP: Slovenian Net work of Isotopes in Precipitation. Jožef Stefan Institute. Avail - able online: https://slonip.ijs.si/ (accessed on 13 November 2023).