Recent stream water geochemical studies of tropical small mountainous rivers (SMRs) have shown these systems deliver a disproportionately high flux of solutes to the global ocean. However, the origins of these solutes from within the weathering zone are poorly understood. In this context, we evaluated the relative importance of deep and shallow flowpaths in controlling solute fluxes through a first order stream in the Luquillo Mountains of Puerto Rico. Sampling before, during, and after a 3 hour/53 mm storm event in June 2011 revealed a rapid transition in δ18O, 87/86Sr isotopic ratios, major elemental concentrations, and Ge/Si elemental ratios. At storm flow, stream water δ18O becomes more negative (-6.39 to -12.63), typical of precipitation values, suggesting the rapid delivery of rainwater to the stream channel during the storm. Corresponding changes observed in 87/86Sr isotope ratios (0.70446 to 0.70643) and Ge/Si elemental ratios ( 1.21 to 0.8) confirm a quick response in the origination of solutes between a near surface end member dominated by a combination of dust and leaf litter and a baseflow end member suggestive of deep saprolite/bedrock. These results allow for the calculations of the relative source of storm water derived solutes within the critical zone and the relative role of silicate weathering.