A survey of Li isotope content in freshwater on a basaltic island of the Galapagos Archipelago

Li isotopes in continental surface water fractionate during chemical weathering processes, but their utility as a global paleoenvironmental proxy remains in question. Here we investigate factors controlling Li isotope fractionation in groundwater and streams that drain a single lithology on a basaltic island. Isla San Cristobál, Ecuador has no evidence of hydrothermal activity and is the only island in the Galápagos Archipelago that has permanent sources of fresh water. Multiple perennial streams are located on the windward side of the island, many of which are fed by groundwater springs. We investigate the hydrologic control on Li isotopes in groundwater and streams on San Cristobál to affirm the applicability of Li isotopes as tracers of groundwater residence times and chemical weathering intensity. Water samples were taken from groundwater springs and streams throughout 5 different catchments for Li isotopic analysis. Preliminary analyses show that high elevation (>500m) groundwater springs have an average pH and TDS much closer to that of rainwater than low elevation (200-300m) groundwater springs. This points to a comparatively longer groundwater residence time for low elevation springs with larger Li isotope fractionation. Observing the evolution of the dissolved Li isotopic signal from groundwater through streams to the ocean will provide a valuable comparison to studies conducted on catchments draining similar basaltic lithologies.