Towards the integration of methanogenesis and external methane sources into process-based freshwater biogeochemical models: a literature review and data-analysis

Abstract

Methane emissions from freshwater systems contribute significantly to the total methane flux to the atmosphere. Not much is known, however, about the individual contribution of freshwater environments. Estimating quantities and understanding processes delivering methane from aquifers to rivers and streams is crucial to understand carbon cycling in freshwater systems. In order to assess methane dynamics in freshwater systems and possibilities for modelling on a larger scale, first a literature review was conducted in which production, consumption, transport and modelling of methane are discussed. After the literature review, groundwater methane was identified as an important part of the methane dynamics in freshwater systems. Data on groundwater methane concentrations were assembled and a data-analysis was conducted using statistical software programs SPSS and Microsoft Excel. From the data-analysis, DOC and sulfate were identified as important predictors of dissolved methane in groundwater. Both variables could explain 63.6% of variation of methane concentrations in groundwater. Applying these results to a global biogeochemical freshwater model (IMAGE-DGNM) was unsuccessful because the topsoil DOC concentrations used as model input did not relate to measured groundwater DOC concentrations. In order to improve these results, it is recommended that following research makes a division between confined and unconfined aquifers. Increasing the number and spatial spread of assembled data entries would further increase the reliability of the data-analysis. Using this method to integrate methane into biogeochemical models will, however, not simulate the heterogeneous character of methane concentrations in aquifers. In order to simulate methane delivery from aquifers to surface waters, there is a need for a process-based, numerical model that calculates methanogenesis, methanotrophy and aquifer-surface water exchange.