A survey of Maipo and Mapocho basins (Metropolitan Region) to determine the aptitude of the waters for irrigation, was carried out from 1985 to 1988; emphasis upon active pollution processes was given. Surface waters from both basins are qualitatively different; whereas the Maipo waters present high E.C. values and very few trace elements, the Mapocho waters show low E.C. values and large amounts of dissolved Cu. E.C. values of Maipo waters are due to sulfate contents, which normally exceed the maximum tolerance defined by the Chilean guideline for irrigation waters. Cu contents in Mapocho waters are due to San Francisco river and are almost always exceeding the tolerance defined by the guideline. The richness in dissolved Cu and sulfate of San Francisco waters are mainly induced by discharges of liquid residues from a Cu mine. Untreated waste waters from Santiago are significantly increasing the E.C. values of Mapocho waters, which is due to high contents of dissolved organic compounds, detergent residues and others. Although the waste waters include the industrial Iiquid effluents, the measured trace element contents were rather low. The Maipo river is scarcely polluted with untreated waste waters or mining sediments; the major pollution problem is referred to intermitent discharges of highly resistant fibrous residues from a paper mili factory.