Chemical weathering, silica sources, and water flowpaths in a humid tropical watershed

Tropical ecosystems are responsible for 65% and 50% of the global riverine silica and water fluxes to the oceans, respectively, but are underrepresented relative to temperate watersheds in the flowpath hydrology literature. The purpose of this study was to determine the physical pathways taken by water and the geochemical reactions that occur along those pathways in the Rio Icacos watershed, Puerto Rico. Germanium-silicon ratios (Ge/Si) were employed here as novel hydrologic tracer. This geochemical perspective was complemented by hydrometric data collected in the field and laboratory, and by a physics-based hydrologic model of the watershed. The first part of this study involves developing a detailed understanding of Ge/Si fractionation during weathering of the quartz diorite bedrock underlying the watershed. Ge/Si ratios were determined on unweathered bedrock, saprolite, soil, primary and secondary minerals, plant phytolith, soil and saprolite porewaters, spring water and baseflow stream samples. Mineral weathering reactions are the primary control on the Ge/Si observed in soil porewaters, with secondary affects from plant cycling of silica. Baselow stream chemistry is dominated by the weathering of primary plagioclase with precipitation of secondary kaolinite, suggesting a deep groundwater source.;The second part of the study involves using changes in streamwater Ge/Si ratios during storm events to infer silica sources and water flowpaths. High Ge/Si ratios in storm flow indicate a soil source of solutes, consistent with very shallow throughflow or overland flow. Slightly higher Ge/Si ratios measured during hydrograph recession indicate an additional, slower contribution to storm flow, likely through the soil matrix or macropores.;In the third part of the study, hydrometric data including in situ soil moisture and saturated hydraulic conductivity measurements were incorporated into a three-dimensional coupled saturated-unsaturated flow hydrologic model as an additional tool to infer water flowpaths. The hydrometric data are consistent with the development of a transient perched water table in shallow hillslope soils during storms. The model indicates that the flashy response of the Rio Icacos must involve a significant component of overland flow.