Resumo: Decorrente do seu imenso tamanho e das dificuldades de acesso a áreas remotas é desafiadora a geração de informação sobre a hidrogeoquímica de microbacias na Bacia Amazônica. Um estudo, ao longo de quatro anos, em um igarapé na Flona do Tapajós, apresentou resultados que contribuem para a ampliação do conhecimento sobre os processos hidrogeoquímicos atuantes na região. Estes revelam condições fluviais de baixa carga iônica, característica de solos bastante intemperizados, e uma situação prístina com abundante ciclagem de nitrogênio na microbacia, refletidas em concentrações de amônio e nitrato que superam as de constituintes maiores da hidrogeoquímica fluvial. Abstract: Due to its huge size and the difficulties of accessing remote forest areas it is a challenge to generate data about the hydrogeochemistry of small catchments in the Amazon Basin. A four year study of a stream in the Flona do Tapajós has shown results that contribute to increase the knowledge about the hydrogeochemical processes in the region. These findings reveal stream conditions of low ionic charge, a characteristic of high weathered soils, and a pristine situation with an abundant nitrogen cycling in the catchment, resulting in ammonium and nitrate concentrations that overlap those of the mayor elements of the fluvial hydrogeochemistry. Abstract: Due to its huge size and the diffi culties of accessing remote forest areas it is a challenge to generate data about the hydrogeochemistry of small catchments in the Amazon Basin. A four year study of a stream in the Flona do Tapajós has shown results that contribute to increase the knowledge about the hydrogeochemical processes in the region. These fi ndings reveal stream conditions of low ionic charge, a characteristic of high weathered soils, and a pristine situation with an abundant nitrogen cycling in the catchment, resulting in ammonium and nitrate concentrations that overlap those of the mayor elements of the fl uvial hydrogeochemistry
