SPATIAL VARIATION OF DISSOLVED WEATHERING PRODUCTS, NUTRIENTS, AND CARBON IN A RAPIDLY WEATHERING TROPICAL WATERSHED, LUQUILLO MOUNTAINS, PUERTO RICO

Tropical small mountainous rivers (SMRs) play a disproportionately large role in global biogeochemical cycles because of their high primary productivity and rapid weathering. Many studies have identified high solute fluxes from tropical SMRs, but landscape controls on the weathering and biogeochemistry that produce solute loads are less well understood. This study documents the spatial variability in stream and groundwater chemistry for Rio Icacos, a humid tropical forested watershed in the Luquillo Mountains of Puerto Rico with a weathering rate among the highest in the world for granitic terrain. Surface water chemistry differs greatly with location within the watershed, with highest concentrations of dissolved weathering products (silica, DIC, and major cations) and nitrate occurring at highest elevation sampling sites in the main stem. Along a transect of the main stem, concentrations decrease with distance downstream. Tributaries all have lower concentrations of weathering products and nitrate than the lowest concentrations found in the main stem. Deep groundwater contacting bedrock and the active weathering profile contains concentrations of silica and nitrate higher than surface waters. We believe that discharge of deep groundwater at discrete locations in the watershed (namely high on the main stem), along with dilution by groundwater discharge from shallower flow paths, is causing the observed patterns.