The Karasu Stream is located in the province of Sakarya in Turkey. It flows into the Black Sea with an output of 10 m(3)/s. Chemical compositions of surface and ground waters change following interaction with geological units and chemical weathering at the surrounding rocks. This change may be realized as passing of the ions mobilized from the rocks and soil into the water. Hence, concentrations in natural waters of some ions that may negatively affect human health may increase. Some of heavy-metal concentrations in the Karasu Stream water were slightly higher than the world average values. High ions contents in the Karasu Stream water are related to geological origin. The ions such as Al, Sb, As, Ba, Cr, Se, Pb, Fe and Zn in the Karasu Stream waters increases by chemical weathering of the rocks and water - rock interaction. The amount of aluminum (Al) in the Karasu Stream water (823 mu g/L) is higher than the drinking water limit values (200 mu g/L) of the World Health Organization, Environmental Protection Agency, European Union and Water Intended for Human Consumption. Hence, the Karasu Stream water might need to be treated.