[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-03] Methane dynamics in a Brazilian riverine aquifer: hydrochemical controls, seasonality and implications for atmospheric emissions

*Vinicius Rogel Paulino de Oliveira Oliveira¹, Reginaldo Antonio Bertolo¹, Priscila Ikematsu², Shin-ichi Onodera³, Mitsuyo Saito³, Ricardo Hirata¹ (1.University of Sao Paulo, Brazil, 2.Technological Research Institute of the State of São Paulo, Brazil, 3.Graduate School of Advanced Science and Engineering, Hiroshima University, Japan)

Keywords:Methane, Aquifer, Riparian, Brazil

Methane is a trace atmospheric gas with a global warming potential 20 times greater than carbon dioxide. In confined environments, it becomes explosive at concentrations exceeding 10 mg/L or 5–15% in air. Riparian zones frequently exhibit elevated methane concentrations in groundwater, with potential atmospheric transfer via river-aquifer interactions. Nevertheless, methane emissions from these systems as a greenhouse gas source remain understudied and often overlooked. Biogenic methane production occurs through acetate fermentation or carbon dioxide reduction (the dominant pathway in aquifers), facilitated by anaerobic microbial activity in shallow aquifer zones. Here, bacteria utilize electron acceptors to oxidize labile organic matter derived from biogeochemical cycling. This study aims to: (i) quantify methane concentrations across aquatic matrices; (ii) assess seasonal influences on methane variability; and (iii) evaluate physicochemical controls on methane production. The Bauru Forestry Garden (Horto), a reforested and preserved area in southwestern Sao Paulo (Brazil), was selected as the study site. Groundwater samples were collected from 17 monitoring wells, and surface water samples were taken from three points along the Agua Comprida River (traversing the reserve). Parameters analyzed include methane, Kjeldahl nitrogen (KN), dissolved organic carbon (DOC), and field measurements (temperature, electrical conductivity (EC), dissolved oxygen (DO), pH, and redox potential (Eh). Sampling occurred during winter (May) and summer (December) 2025. In winter, groundwater methane concentrations near the river ranged from 5.17 to 28.60 mg/L (mean: 19.14 mg/L), while riverine concentrations varied between 0.0230 and 0.0260 mg/L (mean: 0.0245 mg/L). During summer, groundwater methane levels increased to 8.58–41.50 mg/L (mean: 29.94 mg/L), and riverine concentrations rose to 0.347–0.670 mg/L (mean: 0.5323 mg/L), exceeding national potability thresholds. Statistical analyses revealed strong positive correlations between methane and DOC , KN, temperature, and EC. Conversely, Eh and DO exhibited negative correlations (both coefficients < 0.5). Groundwater methane concentrations significantly surpassed surface water levels, underscoring river-aquifer exchange and atmospheric emissions. Higher summer concentrations likely reflect elevated temperatures and increased organic inputs. Strong linkages between methane, DOC, KN, temperature, and EC emphasize the role of labile carbon in methanogenesis, organic nitrogen cycling, optimal metabolic conditions, and ionic regulation. Results further highlight the importance of reducing environments (low Eh) and anoxic conditions (low DO).
This work was supported by the Sao Paulo Research Foundation (FAPESP) through the SACRE Project (Grants 2020/15434-0 and 2022/02220-7), CNPq (Process 423950/2021-5), and JSPS Fund for the Promotion of Joint International Research (Fostering Joint International Research (A) (PI: Mitsuyo Saito, No. 20KK0262).