Alexandra May / Earth & Environmental Sciences / Faculty Mentor: Ricardo Sanchez Murillo
Serpentinization of ultramafic minerals provides pathways for GeoH2 and CH4 production. We hypothesize that in tropical monsoonal environments, rainfall amount and seasonality increase groundwater recharge, leading to significant water-rock interaction and a GeoH2/CH4-rich dissolved phase during baseflow conditions. We present geochemical and hydrological results from a tropical serpentinization system in Costa Rica, where heavy convective rainfall is common during the wet season, followed by a dry season. GeoH2 values are lower than other sites, but CH4 values are the highest reported, indicating favorable conditions for abundant baseflow CH4 production from biotic and abiotic processes. Tritium and δ18O values suggest recent meteoric origin and high connectivity between rainfall, groundwater, and hyperalkaline circulation. Seasonal rainfall and recharge control are supported by longitudinal stream discharge (Q) measurements. During the dry season, Q values indicate large groundwater input to the stream network, with an ~8x increase downstream, coinciding with higher GeoH2 and CH4 values. In May, Q increased only 1.2x downstream, but subsurface storage provides sustained flow, coinciding with hyperalkaline seepages. Our study shows the control of monsoonal rainfall, natural recharge, and baseflow recession on tropical GeoH2 and CH4. Understanding natural GeoH2 and CH4 production can help scale up cleaner GeoH2 production.
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