The potential for molecular hydrogen (H-OH^- groundwaters bearing up to 4.05 μmol⋅L^-1 H₂ , 3.81 μmol⋅L^-1 methane (CH₄) and 946 μmol⋅L^-1 sulfate (SO₄^2-) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H₂ oxidation coupled to SO₄^2- reduction. In shallower, oxidized Mg²⁺-HCO₃^- groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ¹³C and δD of CH₄ (up to 23.9 ‰ VPDB and 45 ‰ VSMOW , respectively), indicated microbial CH₄ oxidation, particularly in Ca²⁺-OH^- waters with evidence of mixing with Mg²⁺-HCO₃^- waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.