Hydrogeochemical responses to rainfall inputs in a small rainforest basin: Rio de Janeiro,Brazil

This work provides basic information about hydrogeochemical contributions to different pathways of water in a small tropical rainforest catchment (3.5 km²) in Rio de Janeiro, Brazil. Precipitation and discharge measurements were conducted at the basin outlet, and at five other stream sites that were selected according to their hydrological behaviour and lithology. Precipitation and streamflow samples were collected weekly, over seven months, and continuously during one storm event. These streamflow samples were analyzed for Ca²⁺, Mg²⁺ Na⁺, K⁺ and SiO₂. Precipitation, throughfall and topsoil water samples were also collected after various storm events, over six months, within a small sub-catchment and were analysed for Ca²⁺, Mg²⁺, K⁺, Na⁺ and pH. Results from this research showed that rainfall was acid, with low cation content. Throughfall was neutral and cations enriched. Analysis of topsoil water indicated that litter was an important source of Ca²⁺ and K⁺ by means of saturation overland flow. In relation to throughfall, streamflow output was characterized by lower concentrations of K⁺, Ca²⁺, and Mg²⁺. Biotite gneiss weathering was the main source of Mg2+ to streamflow. During stormflow periods SiO₂ and Na⁺ concentrations decreased due to Hortonian overland flow contributions to the stream channel. The increasing concentrations of Ca²⁺ and K⁺ were due to litter leaching, especially in the zones producing saturation overland flow. Streamflow hydrogeochemistry was not only a product of weathering processes, but also of biotic processes.