Spatial and temporal variations of Na, Mg, were monitored through 37 sampling stations to determine the main aspects influencing the surface-water quality in the Salí River watershed (Tucumán Province, NW Argentina). The influence of the regional geological setting on water chemistry allows to distinguish three subbasins. The interaction with sedimentary rocks was found to be dominant in the northern and centraleastern sub-basin as well as in the southern subbasin, whereas the metamorphic-granitic basement of the Pampean Ranges was noted in the central subbasin. In addition, anthropogenic activities affect the spatial variation of K, P, Mn, Rb, and Pb as well as dissolved oxygen concentrations and Eh. Temporal water-quality variation is related to the spatial distribution of precipitation and to the seasonal character of the main local industries (sugar cane, alcohol, citrus), increasing P and K concentrations and decreasing dissolved oxygen concentration and Eh in winter. Cl -, Na, SO 4 2-, Al, As, B, Fe, Mn, Se and U concentrations exceed the regulated drinkingwater thresholds at several sampling stations.