Spatial and temporal changes in water chemistry were evaluated using surface water extracted from a Mediterranean river basin, i.e., The Mataquito River, central Chile. The processes that control water chemistry dynamics in the basin were determined by assessing the relationship between hydrologic events (rainfall, river flow, runoff rates), physicalchemistry properties, and major ion concentrations/distributions along the river. During the rainy period, major ion levels were controlled predominantly by weathering (rainfall). During the dry period, seawater intrusion was the main factor controlling water chemistry. Unusually high chlorine and sulfate concentrations suggested anthropogenic sources of the ions entering the basin. Thus, pollution from industrial and agricultural activities in the basin might be quantitatively important and should be considered in future studies of this area.