Résumé : This paper presents the results of a decade-long environmental geochemistry research program on the three watersheds of the arid Coquimbo Region, north central Chile, where mining and agriculture are the principal economic activities. The studies include water and sediment geochemistry. Among the principal conclusions of these studies are the extremely high Cu, and high As and Zn contents of the fine-grained fraction of the fluvial sediments of the northern, Elqui river watershed, which reaches an average of 0.1-0.2% Cu. These contents are related to acid drainage originating in the El Indio (gold-copper-arsenic) district located on the Andean head of the Elqui river basin and surrounded by advanced argillic hydrothermal alteration. It is, however, also due to the erosion of tailing deposits on the flank of major rivers and creeks, products of small to medium scale copper mining. The major copper-(molybdenum) ore deposit of the Region, the world-class Los Pelambres porphyry copper deposit, is located at the head of the southern Choapa river basin. Surprisingly, the Choapa river water and fine-grained sediments do not present high Cu or Mo contents. This fact is interpreted in terms of the dominant mafic rock types of the deposit, and its rather weak hydrothermal alteration, which have the potential to neutralize the generation of metallic - rich acid drainage.