The Marbel River-Matingao River catchment, on the island of Mindanao in the Philippines, drains the Mt. Apo geothermal field and a power development project site. The headwaters and upper reaches of both rivers, as well as local drinking water springs, contained very low levels of As and other contaminants of possible geothermal origin (B, Li, Hg, Sb and Se). The concentrations of As, Li, B and Sb were elevated in the lower reaches of the rivers by the input of natural hot springs. The river is no longer used for drinking water, but residents of the catchment area may still be ingesting As through consumption of aquatic plants and animals.