Ground heating and methane oxidation processes at shallow depth in Terre Calde di Medolla (Italy): Numerical modeling

Abstract

The area known as Terre Calde (literally "hot lands") in the plain of the Po River (Italy) is well known for unusual ground temperatures, and up to now, the cause o/f the heating has not been fully investigated. These higher-than-average temperatures are commonly associated with diffuse methane seepage. A detailed study of shallow stratigraphy, temperature profile, and associated gas concentrations and flow rates recently suggested that the observed anomaly could be related to the exothermic oxidation of biogenic methane, possibly rising from a shallow peat layer. In this work, a porous media flow simulator (Transport of Unsaturated Groundwater and Heat 2) was applied to verify a conceptual model of this phenomenon. The model describes a layered system, with a shallow unsaturated zone, where methane is continuously supplied along the base and heat is generated as a result of its oxidation above the water table. To mimic the oxidation process, heat sources are placed within the layer where oxidation takes place, and the heat generation is computed as a function of methane flux entering the layer. Numerical simulations were carried out imposing different methane flow rates along the base of the model. The simulations also explored the efficiency of methane oxidation, considering different heat generation rates and accounting for seasonal effects. The good match between observed and simulated temperature profiles suggests that the main features of the process are captured by the model and that the conceptual model devised on the base of available data is plausible from a physical point of view.

Poroelasticity and Fluid Flow Modeling for the 2012 Emilia-Romagna Earthquakes: Hints from GPS and InSAR Data

“…This means that, in order to simulate the evolution of porepressure induced by earthquakes, a 3D fluid flow modeling can be very important. 13 Geofluids…”

“…The four layers of the poroelastic half-space have different D values representing a decrease of permeability over depth (Table 1), as proposed by Ingebritsen and Manning [33]. The chosen shallow permeability values are constrained by evidences obtained by drilling [13,18,35]. The resulting dependence of permeability on depth can be expressed as log k ≈ − 12 − 2 9 log z , where k is the permeability (m 2 ) and z is the depth (km).…”

“…During the seismic sequence, several fluid-related effects were observed, such as the increase of the level of some water wells [9,10], episodes of soil liquefaction [11], and the presence of bubbling wells due to the methane seepage [12,13]. These evidences suggest that the Emilia-Romagna seismic sequence affected the groundwater equilibrium.…”

Diversity of Methane-Oxidizing Bacteria in Soils from “Hot Lands of Medolla” (Italy) Featured by Anomalous High-Temperatures and Biogenic CO₂ Emission

“…The anomalously warm areas are easily recognizable during winter because snow melts within them and during summer when soil heating leads to the death of corn plants (6). Previous studies conducted in 2012 reported a clear correlation between soil heating and gas seepage (CH 4 and CO 2 ) (29). As shown in Fig.…”

“…2A) indicate that the marked increase in the CO 2 /CH 4 flux ratio is due to the biological oxidation of rising CH 4 , suggesting the occurrence of biological methane oxidation processes in the most aerated layers. Conceptual modeling and numerical simulations have indicated that the exothermic nature of the CH 4 →CO 2 conversion (800 KJ mol −1 of CH 4 ) represents a heat source with the ability to produce the soil thermal anomalies observed in TCM soil (29). This previous finding and thermodynamic prediction prompted us to verify the hypothesized correlation between methanotrophs and soil hot temperatures by examining microbial methanotrophic diversity in TMC soil samples using a cultivation-independent approach.…”