@article{leenheer_1980,  title={Origin and nature of humic substances in the waters of the Amazon River Basin},  volume={10},  ISSN={0044-5967},  url={https://doi.org/10.1590/1809-43921980103513},  DOI={10.1590/1809-43921980103513},  number={3},  journal={Acta Amazonica},  publisher={Instituto Nacional de Pesquisas da Amazônia},  author={Leenheer, Jerry A.},  year={1980},  month={Sep},  pages={513–526} }