Hydrogeochemical Properties Providing Links to Methanogenesis in Deep Aquifer of Bhola Island, Bangladesh

Abstract

This paper compares the hydrogeochemical signatures between naturally flowing wells with methane (FWs) and non-flowing wells (require pumping) without methane (NFWs) by analyzing a total of 59 groundwater samples to find possible links for methane gas generation. Relatively higher concentration of HCO3 ⁻ indicates oxidation of organic matter (OM) for both the cases which is corroborated by the dominance of Na-Mg-HCO3 and Na- Mg-HCO3-Cl water type. Low concentrations of redox sensitive parameters i.e., NO3 ⁻, SO4 2⁻, Fe2+ and Mn2+ are indicating prevalence of extremely reducing condition and relatively lower concentrations of SO₄²⁻ in FWs suggest microbial sulphate reduction reaching final stage of redox ladder i.e., methanogenesis. Comparatively higher temperature and subsequent higher electrical conductivity (EC) in FWs possibly attributed by the chemical reactions of methane generation or vice versa. Low geothermal gradient, presence of OM and distribution of the FWs over a paleo valley in the deep confined aquifer zone refer to the existence of favourable environment for biogenic methane production rather than the upward migration or leakage of thermogenic gas from the reservoir below.

Asiat. Soc. Bangladesh, Sci. 45(2): 217-228, December 2019