Biogeochemical processes affecting volatile organic compounds (VOCs) in interstitial soil gases from geogenic sources and municipal waste landfills

The present PhD research project was aimed to investigate biodegradation mechanisms controlling the VOC composition in interstitial soil gases in areas affected by extra-atmospheric diffuse seepages to improve the scientific knowledge about the behaviour of geogenic and anthropogenic trace organic compounds within the soil. This goal was achieved by performing comprehensive empirical measurements of different geochemical parameters (i.e., chemical composition of inorganic and organic volatiles associated to isotopic analysis of 13C-CO2 and 13C-CH4 and flux measurements of CO2 and CH4) in soil gases and source fluids from five selected study areas located in Italy pertaining to four distinctive environments, as follows: Solfatara di Nepi and Caldara di Manziana (Viterbo and Roma, Central Italy, respectively) as medium-to-high enthalpy hydrothermal systems, Vulcano Island (the Aeolian Archipelago, southern Italy) that includes volcanic-related and high temperature hydrothermal systems, Terre Calde di Medolla (Modena, Central Italy) as a low temperature sedimentary basin and Belvedere Inc.’s municipal waste landfill (Pontedera, Pisa, Central Italy). Laboratory experiments were also performed to increase the knowledge regarding the genetic processes of VOCs in landfill systems and biodegradation processes for VOCs in landfill cover soils. Punctual and high-flux emissions in the investigated environmental contexts, considered as the best representatives of gas sources, were characterised by peculiar VOC features and abundances, which were dependent on the characteristics and physicochemical conditions of the corresponding reservoirs, including (i) temperature, (ii) contents of organic matter and (iii) high fugacity of heteroatoms, such as sulfur, halogens and oxygen. CO2-dominated gases released from medium-to-high enthalpy hydrothermal systems were dominated by alkanes followed by aromatics, cyclics, S-substituted, O-substituted, halogenated and alkene compounds and phenol. Fumarolic gases released from active volcanic and volcanic-related high temperature hydrothermal systems were characterised by high contents of alkanes and lower amounts of aromatics, alkenes, S-substituted and halogenated compounds, phenol and furans. CH4-dominated gas exhalations from the low temperature sedimentary basin were dominated by alkanes, cyclics and alkenes, whilst aromatics, O-substituted compounds, phenol and terpenes occurred at low amounts. Landfill gas, mainly consisting of CH4 and CO2, included high contents of aromatics followed by alkanes, cyclics, halogenated and terpenes; S-substituted compounds, phenol, benzothiazole and O-substituted compounds were found at lower amounts. The amounts of hydrocarbons in the investigated natural systems were proportional to the abundance of degrading organic matter contained in the source rock(s) and inversely related with the reservoir temperature. Overall, interstitial soil gases exhibited VOC compositional features different from those revealed in the relative gas sources, clearly indicating the action of secondary processes at shallow depth. On the whole, biogeochemical processes acting in soil preferentially degraded alkylated and high-molecular weight organic molecules pertaining to alkanes, aromatics, alkenes and cyclics, furans and S-dimethyl compounds, whilst the non-alkylated and low-molecular weight homologues as well as six-carbon-atom molecules were relatively recalcitrant. O-containing organic compounds (aldehydes, ketones, esters, organic acids and phenol) were metabolites of the ongoing VOC degradations and their contents relatively increased upwards the soil. Halogenated and terpene compounds were particularly resistant at shallow conditions. The proceeding of biodegradation processes in soil appeared only partly dependent on the residence time of gases within the soil and on the contents of free O2. Site-specific soil factors (i.e., soil physicochemical properties and soil gas mixture composition) likely played a pivotal role in controlling the extents of VOC degradation in soil. Consistently with the field findings, the laboratory experiments stated that several different VOCs are released by the degradation of organic fractions of municipal waste, with the odorigenous terpenes and highly polluting short-chain alkanes and benzene being recalcitrant to biodegradation processes acting in soil. Despite, biodegradation processes within the soil undoubtedly reduce the diffuse emission of VOCs from areas affected by VOC diffuse seepages, further efforts are needed to understand the role of physicochemical and biological parameters on the attenuation capacity of soil.