Carbon and Sulfur Isotope Methods for Tracing Groundwater Contamination: A Review of Sustainable Utilization in Reclaimed Municipal Landﬁll Areas

: Reclaimed landﬁll areas are excluded from various development options including construction, while contaminated zones around such places have no such restrictions. The successful reclamation of landﬁlls means that the old landﬁll visually ﬁts in well with its surroundings, but soil and water contamination problems remain valid. Former landﬁlls were built without properly preparing the land, which resulted in the migration of contaminants in groundwater for a long period after these landﬁlls were closed, further resulting in the limited use of such areas, at least for some purposes. Due to the development of cities, landﬁlls formerly located in suburbs are becoming a part of these cities. In order to optimally and safely use these spaces, knowledge regarding the quality of the soil and water environment is necessary. This article presents methodological considerations regarding the use of carbon and sulfur isotope methods to assess groundwater contamination around former municipal waste landﬁlls, especially reclaimed municipal landﬁlls. It has been shown that natural groundwater is characterized by low values of both δ 13 C DIC and δ 34 S ( δ 13 C DIC from − 20 to − 10‰ and δ 34 S at approximately − 5‰), whereas leachate-contaminated groundwater is characterized by high values of both parameters ( δ 13 C DIC from − 10 to + 5‰ and δ 34 S from +5 to +20‰). The aim of this article is to demonstrate that carbon and sulfur isotope methods extended via SWOT analysis are universal and reliable methods for assessing the migration of pollutants, thus facilitating decisions regarding management.


Introduction
The appropriate selection of locations and ground protection means that new landfills do not pose any problems for the environment.Additionally, efforts are carried out to reduce the amount of waste through selective waste collection and reuse [1].Every year, the amount of municipal waste recycled and composted in each European country systematically increases.The most substantial achievements have been recorded in the case of Lithuania, where the recycling rate (taken as the percentage of municipal waste generated that is recycled, composted, and anaerobically digested and also includes the preparation for reuse) during the 2016-2004 period increased from 2% to 48%.During this period, there was an increase from 5% to 35% in Poland and from 5% to 25% in Latvia [2].In many countries such as Germany, Slovenia, Austria, Switzerland, the Netherlands, and Belgium, the amount of recycled waste was high in this period, and currently exceeds 50%.With the continuation of environmental stewardship, the landfill target for 2035 has been set at 10% [3].Although there are frequent discussions about "zero waste", in the current economic and social circumstances, this seems to be difficult to achieve [3,4].Regardless of the restrictions imposed, landfills will continue to be in operation.Landfills constructed in recent years meet the appropriate requirements [5,6], but monitoring and control tests should still be carried out in case the materials used to protect the substrate turn out to be insufficient for some compounds [7,8].Modern landfills are designed with liners and leachate collection systems to prevent leachates from migrating into groundwater [9].The effectiveness of these projects should be monitored through appropriately and individually designed monitoring methods.A large number of research studies have already been carried out with respect to studying the migration of leachate plumes through landfill liners.The older, closed landfills are typically unlined and do not use leachate collection systems.For the oldest and most problematic sites, excavating the landfill and repositioning at a safer location may be a good solution.However, this only applies to exceptional situations, and on-site remedial measures are usually carried out.One literature review showed that the formation of leachates and their migration in an aquifer, despite the reclamation of old landfills, were serious problems observed in many countries (e.g., the United States of America, China, Italy, and Poland, e.g., [10][11][12][13][14][15]). Despite reclamation, the processes occurring in the stored waste, depending on many complex factors (moisture, temperature, redox conditions, pH, and types of microorganisms), lead to the formation of leachates and biogas, which affect the surrounding environment for a long time [16].One of many examples is an unlined, closed municipal landfill on the alluvial plain in Norman (Oklahoma, USA), where the leachate plume extends at least 225 m downgradient in the unconfined, 12-meter thick alluvial aquifer [17] with a linear plume velocity of 40.2 m/year [18].Groundwater is a medium that transports contaminants, even over long distances of up to 2000 m [19].
It is a well-known fact-proven by numerous studies-that the migration of gases and leachates causes the formation of polluted zones around landfills [20].However, the state of contamination varies greatly, depending on natural factors such as the geological structure and permeability of rocks at the base of the landfill, precipitation, and the technical aspects including the condition of the ground protection, the type and size of the landfill, and the amount, type, and duration of waste storage.The issue of pollutant migration is so important that numerous studies have been undertaken and aimed at a detailed diagnosis of the problem, which concerns not only regarding the cognitive dimension, but also rational space management.
This most often concerns landfills that were operated in the 1950s and 1960s, when environmental protection standards were much lower than today.In many cases, reclamation involved shaping a block, constructing infrastructure for degassing and collecting leachate, covering the surface with a layer of soil, and planting vegetation.It is important to note that in the past, landfills were located on the outskirts of cities, while the dynamic development of urban infrastructure has currently placed them close to inhabited areas.These visually blend into the surroundings, but medical research shows that the proximity of a landfill may be associated with the risk of adverse health effects [21].It is therefore important to safely restore economic utility to reclaimed landfill areas [22,23].Old and historic landfills are managed carefully, but the areas around them are not given thorough consideration.To make these places fully safe for investment development, methods are being improved that will allow for a clear assessment of the potential impact of the landfill on the surrounding environment and human health.When recultivating landfills and developing their surroundings, public safety should be taken into account.In order to restore the economic utility of reclaimed landfill areas and use them for forestry, agricultural, water, municipal, recreational, and construction purposes, one should be sure of the safety of these places in terms of human health.Currently, many requalification plans still exhibit a lack of knowledge regarding cost analyses, long-term emission analyses, legal implications, and reuse designs that look beyond grasslands [22].Opinions on the development of such places should be based on reliable methods that provide clear and irrefutable results.It is worth noting that a historic landfill is a site where there is no environmental permit in force; environment agencies are not the regulators of historic landfills.Even though places with old, historic landfills are managed carefully, the areas around them are not considered as thoroughly.
Old landfills are difficult to study due to their temporal and spatial variability.The problem is intensified by the fact that a landfill is considered to be a typical anaerobic environment, and with infiltrating rainwater, at least locally and periodically, it becomes an aerobic environment, changing the course of hydrogeochemical reactions in the environment.Depending on the redox conditions, various types of microorganisms begin to become active, which actually translates into the formation of biogas and leachates with variable compositions.In the case of old facilities, there are additional difficulties in conducting research, selecting the methodology, and interpreting the results, not only because it is a very complex environment in terms of the processes taking place, but also because the functioning of such facilities in the past was not recorded.Therefore, the amount and type of deposited waste are often only approximately known.An additional complication is the differentiation of these processes depending on the geographical settings (climatic factors and hydrogeological conditions).Therefore, there is a justified need to study such places and develop a quick, easy, and cheap method to assess the suitability of such a place for a specific type of development.Such attempts have been carried out for a long time, starting from the 1960s [22,24,25], and have been systematically continued after 2000 [26], introducing new experiences and knowledge, some of which are presented below.
This article presents the methodological considerations of the use of carbon and sulfur isotope methods to assess the groundwater contamination around reclaimed municipal landfills.The aim of the article was to demonstrate that selected isotope tests are a universal and reliable method.Particular attention was focused on research that concerned the practical use of groundwater for economic and drinking purposes, and due to its aggressiveness toward concrete and metals, its use as an element of underground structures (e.g., garages).The formation of the gas phase, which is an element of the transformation cycles taking place in the landfill; the aeration zone and the contaminated aquifer are also indirectly mentioned.

Methods for Detecting Contamination
Reclaimed landfills comprise places in which the identification of hydrochemical processes and the control of their activity are difficult.Attention should be focused on the contamination of the soil and water environment with leachates and gases that have the potential to migrate.

Groundwater Contamination
The cause of the contamination of the zone around a landfill is the migration of leachates, which mix with groundwater in the zone located below the landfill; this results in pollution zones of various sizes and compositions.All efforts are aimed at selecting a method that will allow for the reliable assessment of the state of the soil and water environment around the reclaimed landfill.This is carried out so that investments made in its vicinity do not damage buildings or human health.The evolution of δ 13 C DIC and δ 34 S in the leachate is affected by a number of physical, chemical, and biological processes [17,27]; therefore, performing only chemical analyses may be insufficient in the most complex situations.Because of this, the best solution will be to obtain multi-species dynamic models based on carbon and hydrogen balances [28].
Geophysical methods, often combined with hydrogeochemical methods, are used to assess the extent of groundwater pollution from different sources.In the case of old landfills, geophysical methods have many applications: identifying the size of the landfill and the structure of waste; assessing the tightness of the impermeable cover of the landfill; assessing the extent of the impact of the landfill on the soil and water environment; and forecasting the activity of the landfill; and locating biogas production zones.Geophysical methods use various physical fields in a geological medium; thus, they complement each other, allowing for a comprehensive consideration of many issues related to the assessment of the pollution zone around the landfill.
A method often used to analyze contaminant migration is the Mise-à-la-Masse method (e.g., [31,42]).Geophysical methods are particularly useful when direct testing is unfeasible or involves the risk of damaging seals and creating new migration routes for contaminants (covers of reclaimed hazardous waste landfills and ground seals in operating landfills).
When collecting a water sample is possible, the simplest way is to measure the physicochemical composition of water (i.e., its specific electrical conductivity (SEC)), mineralization, the concentration of major ions (HCO 3 − , Cl − , NO 3 − , Ca 2+ , Mg 2+ , Na + , and K + ), other components (e.g., heavy metals), and some minor and trace elements (multivariate analysis).These are then compared with the hydrogeochemical background range, the quality standards for groundwater, or assessments of human health risk [13,14,[43][44][45][46][47][48][49][50].Evidence of contamination is the presence of BTEX, PAHs, pesticides, micro-and nanoplastics, xenobiotic organic compounds, pharmaceutical compounds, specific substances (per-/polyfluoroalkyl substances), and other components that do not form in the natural environment [19,47,[51][52][53][54][55].An assessment of water aggressiveness based on the estimation of saturation indices is also a useful method for identifying the groundwater contamination around landfills [56].To assess the status of groundwater near the landfill, the leachate pollution index (LPI) (a mathematical equation that yields a single aggregated value of multiple physicochemical and biological leachate concentrations in a landfill) [57] or modifications of this method-revised leachate pollution index r-LPI [58] or m-LPI [59]-can be used.However, this conventional approach may not be effective in environments where the background levels of chemical indicators are already high such as wetland areas, sites with contamination that pre-dates the landfill, or sites contaminated with constituents originating from a source other than the landfill.For this reason, stable isotope analysis is increasingly used in studies of environmental pollution.The relatively distinct isotopic ratios or signatures of substances, created through biogeochemical processes, offer great potential for tracing contaminants such as leachates around the landfill [14].The most frequently used environmentally stable isotopes are as follows: δ 13 C DIC (CO 3 2− and HCO 3 − ), δ 34 S (SO 4 2− ), δ 2 H, and δ 18 O of water molecules; δ 15 N and δ 18 O (NO 3 − ) [14,27,44,46,[60][61][62][63][64][65][66][67][68][69][70][71][72]; and radioactive isotopes 3 H, 14 C, 87 Sr, 36 Cl, and 11 B [73][74][75][76][77][78][79].Satisfactory results are obtained on the basis of carbon and sulfur isotope tests, which are easy and friendly to use and are characterized by appropriate precision.

Gas Pollution
Particularly important is the presence of hydrogen sulfide in the air due to its smell, and under the surface (in the aeration zone) due to its easy migration and reactivity, which pollutes groundwater.Methane, which poses a serious threat in operating landfills due to its explosiveness and easy spontaneous combustion, is less important in old, historic landfills.In the case of zones around old landfills, the presence of methane is rare for two reasons.The first is the lower activity of the landfill with respect to age, resulting in lower methane production, and the second is the oxidation possibility of the potentially produced methane.
Theoretically, the assessment of landfill gas emissions is a good indicator of the pollution within the aeration zone and, indirectly, also of the saturation zone [12,40].The biogas produced in the landfill could migrate into the soil and water environment [80,81].The composition of biogas varies, among other things, depending on the age of the landfill.In old, historic landfills, the share of methane is reduced in favor of carbon dioxide [80], and the relative proportion of reduced sulfur compounds (hydrogen sulfide (H 2 S), methyl mercaptan (CH 3 SH), dimethyl sulfide ((CH 3 ) 2 S), carbon disulfide (CS 2 ), and dimethyl disulfide (CH 3 ) 2 S 2 )) is substantially smaller than in the active landfill [82].Landfill emissions of reduced S gases can be a nuisance with respect to odor and a potential health hazard for surrounding communities at some sites.H 2 S and volatile organosulfur compounds (VOSCs) such as methyl mercaptan and dimethyl sulfide are considered to be the main factors responsible for the odor of landfills [83].
Methane is a gas that is slightly soluble in water, but is oxidized in the presence of oxygen [20,80,[84][85][86] and dissolves into carbonic acid, bicarbonate ions, and/or carbonate ions (depending on the pH), which are marked by differences in the isotopic composition of groundwater around landfills, as described below.
An attempt to identify the contamination of the environment around an old landfill can be achieved using many independently functioning methods.For the purposes of controlling gas emissions (as the expected concentration in the air), a simple, quite general calculation method proposed by the EPA [87] can be used, which is based on the knowledge of the release rate of substances and the maximum average ground level concentration for the unit mass release rate.There are more advanced methods (e.g., LandGEM model) recommended by the Environmental Protection Agency and used by many researchers (e.g., [12,87]).LandGEM is a very useful model for predicting emission rates for total landfill gas including CH 4 , CO 2 , and non-methanic organic carbons (NMOCs).The CALMIM method, which is based on known engineering and climate factors, can be a useful tool in making site-specific engineering decisions regarding CH 4 emissions from landfills.Background information on CALMIM components, theoretical underpinnings, structure, inputs/outputs, graphics, default values, supporting laboratory studies, field validation, and previous applications are given by Spokas et al. [88].

Results and Discussion
Carbon and isotope analyses are used to identify the type of groundwater contamination and assess its extent and concentration.These methods are also suitable for determining the origin of contamination in the presence of other objects that potentially pose a threat to the environment.Research conducted on a multi-year scale can also be used to predict changes in hydrochemical conditions in stored waste and their impact on the surrounding areas.Such broad application possibilities with respect to carbon and sulfur isotope tests result from the isotopic diversity of individual origin sources, which is characterized below.

Carbon and Sulfur Isotope Methods
The analysis included the isotopic determinations of carbon and sulfur, which are the major components of groundwater and undergo isotopic changes in the soil and water environment.The methodological justification for the use of environmental isotope determinations is the differences in the amount of 13 C/ 12 C and 34 S/ 32 S isotopic fractionation [68,89,90] during processes occurring within the aquifer following Reactions (1) and (2) [60]: The guidelines for the preparation of field (sample collection and initial preparation) and laboratory tests are presented in [91][92][93].
Generally, acetate fermentation and sulfate reduction in a landfill with microbes cause significant carbon and sulfate isotopic fractionation.This results in the depletion of 13 C in CH4, as indicated by a decrease in δ 13 C CH4 to about −50‰ and −60‰, while the produced δ 13 C CO2 is enriched in 13 C, with δ 13 C values between −10‰ and 20‰ [94].The fractionation of light stable isotopes, mainly δ 13 C and δ 34 S in landfills, has been demonstrated in a number of studies (i.e., [70,[95][96][97]).The isotopic fractionation problem is intensified by the fact that a landfill that is considered to be a typical anaerobic environment, together with infiltrating rainwater (containing O 2 and NO 3 − ), at least locally and periodically, becomes an aerobic environment, changing the course of hydrogeochemical reactions as well as isotopic fractionation, as seen in Figure 1.
Hydrogeochemical conditions in each landfill vary greatly, even in reclaimed ones that are expected to have no impact on the adjacent environment.After reclamation, especially in old landfills with permeable covers, there is a predisposition relative to the development of aerobic conditions at the top of the landfill while reducing conditions prevail.Landfill gas emissions vary depending on the physical-chemical properties of the cover soil (e.g., cover thickness, moisture content, compaction ratio, uneven distribution of soil), organic material content and age of buried refuse, and seasonal environmental conditions (such as temperature) [12,98].As CH 4 diffuses through covering materials, it is also oxidized by methanotrophs into CO 2 and water vapor.The rate of methanotrophic CH 4 oxidation in landfill soils can vary by several orders of magnitude depending on the soil moisture and temperature (i.e., [98]).The age of the landfill is also important (the biodegradation phase of waste).Research has shown that methane oxidation is more intense in old landfills.Methane oxidation, estimated using stable isotope methods, range from 2.6 to 38.2%, with mean values of 9.5% and 16.3% for active and closed landfills, respectively [27,99].The change in hydrochemical conditions also causes the transformation of sulfur compounds (Figure 1).
The major forms of sulfur in the landfill subsurface environment include sulfates (SO 4 2− ), sulfides (HS − ), and hydrogen sulfides (H 2 S).Sulfates are usually the dominant form of sulfur and are one of the most common sulfur species in groundwater environments.The geochemistry of sulfur is complicated by its wide range of oxidation states.However, the key reaction in the global sulfur cycle (i.e., the reduction of sulfate (SO 4 2− ) to hydrogen sulfide (H 2 S)) may be identified by the distribution of sulfate concentrations and its sulfur isotopic composition.When sulfate ions reach the anaerobic zone within the saturated soil in the landfill subsurface, the sulfate-reducing bacteria start producing sulfides (S 2− ); consequently, the concentration of dissolved SO 4 2− in water decreases, and the considerable fractionation of the 34 S/ 32 S isotope occurs.The residual SO 4 2− becomes progressively enriched in the heavy 34 S isotopes [100].
Depending on the redox conditions, various types of microorganisms begin to become active, which actually translates into the formation of biogas and leachates with variable compositions in different parts of the landfill [70].The transformations of carbon and sulfur compounds with the participation of microorganisms are accompanied by various (sometimes quite significant) isotopic fractionations (Figure 2), which are the basis for identifying the origin of these ingredients in the environment.A landfill is a large dynamic system where, over time, one group of microorganisms is replaced by another, and this is reflected in the dominance of certain ongoing processes [27,28].The use of stable carbon and sulfur isotopes to trace biogeochemical organic carbon cycling in old landfills relies on an understanding of isotopic fractionation at each transformation stage resulting from condition changes inside the body of stored waste.Isotopic fractionation is characteristic of anaerobic processes because reducing bacteria favor the reduction of lighter sulfate isotopes over heavier isotopes; for example, sulfate-reducing bacteria (SRB) favor the reduction of the lighter sulfate isotope 32 SO 4 2− over the heavier isotope 34 SO 4 2− in the majority of environmental conditions.The consequence of these processes is isotope fractionation, resulting in changes in isotopic compositions and rendering the proposed methods useful for tracing groundwater contamination.It is well-known that SO 4 2− reduction is an important process in contaminated aquifers where sulfates (or nitrates) affect the supply of electron acceptors for the biodegradation of organic matter, thus participating in the carbon cycle (Figure 2).It is important to assess δ 13 C CH4 to obtain better source attribution and understand characteristic methane emission pathways [99].
Acetate fermentation in a landfill during the methanogenic phase results in a depletion of 13 C in CH 4 , as indicated by a decrease in δ 13 C CH4 to about −50‰ and −60‰.The δ 13 C CO2 produced during methanogenesis is enriched in 13 C, with δ 13 C values between −10‰ and 20‰.This is crucial in distinguishing anthropogenic carbon (coming from the decomposition of organic substances in a landfill) from natural carbon (in a peatland).In reducing conditions, which are typical for contaminated zones around landfills, carbon compounds are transformed, and sulfates are decomposed, but re-oxidation is also possible after changing the conditions to more oxidizing ones.Although the evolution of δ 13 C DIC and δ 34 S in the leachate is affected by a number of physical, chemical, and biological processes, the isotopic composition of groundwater provides a clear view of the genesis of the component.A model that takes into account the dynamics of isotopes such as 13 C in dissolved inorganic carbon in landfill leachate can be used to interpret landfill processes, particularly methane emissions and oxidation (Figure 2).
The data in Table 1 show that the isotopic composition of groundwater contaminated by landfill leachates makes it possible to trace the contamination of groundwater in the aquifer around reclaimed municipal waste landfills.The natural background value of δ 13 C DIC in uncontaminated groundwater most often ranges from −20 to −10‰, but extreme values range from −24.2‰ (in Germany) to −4.48‰ (in Australia) (Table 1, Figure 1).The isotopic composition of inorganic carbon in groundwater under natural conditions in various parts of the world ranges from −28 to −3‰ [109].Leachates are enriched in heavy carbon isotopes, and δ 13 C DIC values range from −5.5‰ (in Germany) to +25.9‰ (in Germany), with the most common values ranging from 0 to +20‰.Grossman [101] showed that in landfill leachates, δ 13 C DIC was strongly enriched in 13 C, with values of δ 13 C DIC up to 38‰.Water contaminated with leachates reaches intermediate values between unpolluted groundwater and leachates (from −10 to +5‰).Therefore, the analysis of carbon isotopes in leachates and groundwater can be used to monitor the oxidation-reduction status of landfills, which will determine the development possibilities of areas around the landfill.
Results of the isotopic analyses of sulfate in landfill leachates are not very common in the literature (Table 1).Based on the collected data, it can be concluded that groundwater contaminated with leachates shows significant variations in δ 34 S values most often ranging from +5 to +20‰, with values even exceeding +50‰ (Oklahoma) [110].Krouse and Mayer [90] also confirmed that the bacterial reduction of sulfates in groundwater could result in a significant decrease in their concentration and enrichment in heavy sulfur isotopes, even to a δ 34 S value that exceeds +46‰ in extreme cases.In terms of isotopic sulfur compositions, uncontaminated groundwater is the least well-studied.The value found in pristine groundwater near the Norman landfill (Oklahoma) was −5‰ (Table 1).The most frequent values from −20 to −10 from −10 to +5 from 0 to +20 −5 from +5 to +20 even +50 >+10 - The cross-plot of δ 13 C DIC versus 1/DIC (Figure 3) confirms that the δ 13 C DIC values of the leachates (from 0 to +20‰) were strongly enriched relative to the uncontaminated groundwater (from −20 to −10‰).Intermediate values (from −10 to +5‰) were achieved in water contaminated with leachates, which predisposes this method to the identification of contamination.The most frequent values from −20 to −10 from −10 to +5 from 0 to +20 −5 from +5 to +20 even +50 >+10 -
Despite few studies of isotopic sulfur compositions, it can be observed that contaminated groundwater differs in terms of the sulfur isotopes and sulfate concentrations from leachates and uncontaminated groundwater (Figure 4 and Table 1).The comparison of the isotopic composition and sulfate concentrations in the Norman landfill (Oklahoma) showed a clear difference between the leachates and groundwater contaminated with them.Despite few studies of isotopic sulfur compositions, it can be observed that contaminated groundwater differs in terms of the sulfur isotopes and sulfate concentrations from leachates and uncontaminated groundwater (Figure 4 and Table 1).The comparison of the isotopic composition and sulfate concentrations in the Norman landfill (Oklahoma) showed a clear difference between the leachates and groundwater contaminated with them.
The results of this study suggest that groundwater influenced by landfill leachates can be distinguished from uncontaminated groundwater by their distinct δ 13 CDIC and δ 34 S values.These distinct isotopic signals could therefore provide a useful technique for identifying groundwater contamination using landfill leachates and for tracing contamination sources in aquifers near landfill sites.In old landfills, the intensity of the methanogenesis  Cross-plot of δ 34 S versus 1/SO4 for the leachates and leachate contaminated groundwater [110,125].
The results of this study suggest that groundwater influenced by landfill leachates can be distinguished from uncontaminated groundwater by their distinct δ 13 C DIC and δ 34 S values.These distinct isotopic signals could therefore provide a useful technique for identifying groundwater contamination using landfill leachates and for tracing contamination sources in aquifers near landfill sites.In old landfills, the intensity of the methanogenesis process is gradually decreasing, which should result in lower DIC concentrations (CO 2aq , HCO 3 − , and CO 3 2− ions) and lower δ 13 C DIC values (close to natural conditions).Differences in DIC concentrations in both contaminated and uncontaminated groundwater result from the addition of carbon from different isotopic sources and different process intensities (DIC is formed via methanogenesis, the oxidation of organic matter, the mixing this type of water, and the dilution of leachates), even within the same landfill.
In old landfills where the acetate fermentation and sulfate reduction process weakens over time, the values of δ 13 C DIC and δ 34 S may vary little.Moreover, when carrying out a collective comparison, the line between contaminated and pristine water is not always clear.In this situation, an individual approach may introduce the expected results.In complicated cases, when not only the landfill provides carbon and sulfur compounds, an excellent solution would be to use a mixing model based on isotope research.
A simple method (based on a two-component mixing model) can be employed to estimate the proportion of carbon derived from different sources of the DIC pool [66,109].By interpreting the results of the inorganic carbon isotope tests using the mixing model, it is possible to determine the share of carbon originating from the decomposition of organic pollutants in the landfill and other sources (the decomposition of organic substances of natural origin and the decomposition of rocks and carbonate minerals).In uncontaminated aquifers, the two most significant sources of dissolved inorganic carbon (DIC) in groundwater are the dissolution of carbonate minerals and the oxidation of organic matter (either in the soil zone or in subsurface organic deposits).Each has a distinct δ 13 C signature, with DIC produced from the dissolution of carbonate minerals having a more enriched δ 13 C value (~0‰) and DIC produced from the oxidation of organic matter with a more depleted δ 13 C value (~−25‰) [108,126].In strongly anaerobic environments where the organic substrate is more abundant than the available oxidants, decomposition via fermentation and methane production occurs.In a polluted environment such as a landfill, carbonates usually precipitate, which means that the two-component mixing model takes into account organic substances of natural (sedimentary organic matter) and anthropogenic origin (waste in the landfill) [66,109].In more complex conditions, three-component (e.g., [127,128]) and even four-component mixing models can be used (e.g., [129]).
Two-, three-, or four-component mixing models can be used to assess the genesis of sulfur compounds in groundwater within the vicinity of the landfill due to the fact that the sulfur isotopic signature shows different values for different sources.For example, the δ 34 S values originating from atmospheric deposition are between −3‰ and +12‰; for fertilizers, the value of δ 34 S lies between −7‰ and +21‰; for detergents, it ranges from −3.2% to +25.8%; the δ 34 S values for evaporates, pyrites, and sewage range from −14‰ to 35‰, −15‰ to 4‰, and 2‰ to 12.5‰, respectively [90,130].Zhang et al. [131] proposed the use of environmentally stable isotopes and a Bayesian isotope mixing model to identify and quantify multiple sources of SO 4 2− in groundwater.

SWOT Analysis
Managing reclaimed municipal landfills and the areas around them involves complex decision-making situations that require an understanding of various factors including environmental and technical factors, urban quality of life, and health factors.
A helpful way to analyze these factors is to conduct SWOT analyses (strengths, weaknesses, opportunities, and threats).In SWOT analyses, strengths and weaknesses concern internal factors that are related to the lack of negative impact or the impact of the landfill, while opportunities and threats concern external factors understood as opportunities for development in the areas located around the landfill and the possible side effects.
SWOT analyses take into account all of the relevant criteria: (1) general diagnosis of the problem based on data analysis including the size of the landfill, waste records (type and quantity of stored waste), duration of the operation of the landfill, geological and natural conditions, and method of reclamation; (2) detailing the problem based on geophysical research (recognizing the geometry and dimensions of the landfill, outlining of the contaminated zone, and selecting places for groundwater abstraction); (3) potential threat analysis (a-contamination of the aeration zone with gases and threat of gas accumulation in underground garages; b-pollution of surface waters, restriction of the use of these waters, and restriction of recreation and fishing; c-contamination of the aquifer, limitation of the use of groundwater, and impossibility of drilling wells); (4) assessment of the transformations of the land and water environment based on carbon and sulfur isotope tests in groundwater; and (5) making a decision on the development of the former landfill and the area around it.The decision may be based on the results of soil and water environment research as well as SWOT analysis.
Numerous arguments can be provided regarding both the strengths and weaknesses of the development of sites after landfill reclamation as well as the opportunities and threats.Each case requires an individual approach with a similar procedure (Table 2).Closed landfills and their surrounding areas can be reclaimed as land resources for many purposes provided that the environmental quality is adequately controlled.The development of the space around reclaimed landfills is influenced by three key aspects, which are ordered from the most to the least important: lack of negative impact on health and quality of life, functionality and esthetics of the place, and economic considerations.Consequently, these results may provide guidance for decision-makers (municipalities, planners, developers, etc.) when making decisions regarding the development of the space around reclaimed landfills.Each case requires an individual approach and analysis of all factors, and the primary goal is to maintain the comfort of life and health of all people, which can be ensured by a reliable assessment of the safety of such a place in terms of environmental quality.The basis for such an assessment are carbon and sulfur isotope methods, among others.
The numerous possibilities for the reutilization of a landfill area include the following: (1)"natural" biotopes without significant maintenance such as public parks, thematic parks, botanic gardens, education centers, and forest areas; (2) agricultural uses such as grasslands; (3) sports areas such as a golf courses and bike or motorcycle cross tracks; and (4) industrial areas (e.g., energy use) such as the realization of photovoltaic fleets, cultivation of energy crops for biofuel production and/or lignocellulosic plants, and windmills for electricity production [22].There are examples of the good management of areas around reclaimed landfills.A good example of such an approach is Górka Szczęśliwicka in Warsaw, which was converted into a year-round ski slope [12].
Problems related to the development and construction of landfills have been observed and solved for many years, both from geotechnical [25,132] and environmental perspectives [26,133].Substantial research is being carried out on developing safe solutions for the management of reclaimed landfills, which directly concerns the problem of the long-term settling of municipal waste as a result of waste compaction and the burden of post-reclamation infrastructure (e.g., [132]).Indirect concerns include the migration of gaseous pollutants and liquids in the ground and water environment.The settling process is related to the decomposition of waste in the area where it was stored in the past, but the formation of gases and leachates accompanying the decomposition of organic substances has a wider impact as they can move beyond the boundaries of the landfill due to the migration of pollutants in groundwater, in accordance with the flow direction within the aquifer.Therefore, the settlement problem is indirectly related to groundwater contamination, and geotechnical knowledge is an excellent complement to hydrogeological and environmental research.
In order to select the appropriate method for developing the areas around reclaimed landfills, conducting SWOT analyses on the social and economic demands based on collected information and carrying out reliable tests on the soil and water environment using carbon and sulfur isotope analyses are recommended.

Conclusions
Public safety should be focused on implementing landfill reclamation.In order to optimally and safely use the space around a reclaimed landfill, it is necessary to have knowledge regarding the quality of the soil and water environment.Due to the specificity of landfills (caused by the size, type of waste, age, geographical settings, and climatic and hydrogeological conditions) and the need to adapt to individual cases, any method of contamination detection is valuable.It was observed that the isotopic composition of carbon and sulfur in groundwater contaminated by leachates makes it possible to track the contamination of groundwater around reclaimed municipal landfills with respect to sustainable urban development.
Based on the analysis carried out, it has been shown that: (1) The determination of δ 13 C DIC and δ 34 S provides a powerful tool for identifying a zone with natural and leachate-contaminated groundwater.Natural groundwater is characterized by low δ 13 C DIC and low δ 34 S values, whereas leachate-contaminated groundwater is characterized by high values.In the study area, these values were as follows: in natural groundwater, the δ 13 C DIC ranged from −20 to −10‰, and the δ 34 S values were approximately −5‰, while in the leachate-contaminated groundwater, the δ 13 C DIC ranged from −10 to + 5‰, and the δ 34 S ranged from +5 to +20‰.(2) The decomposition of organic compounds in the landfill results in the formation of carbon compounds (in the forms of CH 4 and CO 2 ) and sulfur compounds (in the forms of H 2 S, CH 3 SH, (CH 3 ) 2 S, CS 2 , and (CH 3 ) 2 S 2 ) that undergo numerous transformations during migration; moreover, their concentrations outside the landfill may be quite low.However, even low concentrations can have an impact on people's health during long stays in such places.(3) The use of combined carbon and sulfur isotope methods leads to a better understanding of the geochemical processes occurring in a landfill and its surroundings, is a good method for diagnosing the activity of the landfill, and facilitates the concept of developing this site without unnecessary risk and financial losses.
(4) The relationship between the measured δ 13 C DIC and δ 34 S is appropriate for detecting groundwater contamination within the vicinity of old municipal landfills and facilitates decision-making with respect to the management of these sites.(5) The δ 13 C DIC methods of dissolved inorganic carbon and the δ 34 S method of dissolved sulfates (which is carried out much less frequently) are recommended for the identification of groundwater contamination around reclaimed municipal waste landfills and to assess the safety of the development of these sites after reclamation.It is suggested that sulfur isotope tests be used more frequently in the case of old landfills.(6) The evaluation of groundwater quality near landfills (with SWOT analysis) is an essential part of land management.SWOT analyses may provide guidance for decisionmakers (municipalities, planners, developers, etc.) regarding the development of spaces around reclaimed landfills.The results of carbon and sulfur isotope research extended with SWOT analyses can be used to justify investments in the development of spatial planning strategies and schemes at the local and regional levels.

Figure 1 .
Figure 1.Transformation of carbon and sulfur compounds leading to the formation of landfill gas and leachates [27,93,101-108].Figure 1. Transformation of carbon and sulfur compounds leading to the formation of landfill gas and leachates [27,93,101-108].

Figure 1 .
Figure 1.Transformation of carbon and sulfur compounds leading to the formation of landfill gas and leachates [27,93,101-108].Figure 1. Transformation of carbon and sulfur compounds leading to the formation of landfill gas and leachates [27,93,101-108].

Table 1 .
Inorganic carbon isotopic composition of leachates and groundwater (δ13C DIC ) and the sulfur isotopic composition of leachates and groundwater (δ 34 S) in the vicinity of waste landfills.n.a.-not analysed.

Table 2 .
SWOT analysis for sustainable urban space management in the area of a reclaimed landfill.