The retaining wall of a tailings reservoir collapsed at a zinc mine in Spain last spring, releasing into the watershed possibly nearly as much zinc as the mine produces annually. The accident happened April 25,1998, at the Los Frailes mine near Aznalcollar in southern Spain, sending ∼5 x 106m3 of acid sludge into the Guadiamar River. Based on a suite of water and river bank sediment samples collected downstream of the spill on May 1–3, 1998, an estimated 40,000 to 120,000 tons of Zn was added to the watershed. This is comparable to the annual production capacity of the mine of 125,000 tons. While the scale of the accident was certainly very large, an equivalent amount of Zn has been reaching the adjacent Tinto-Odiel estuary every 1–2 years as a result of mining since the middle of the 19th century. Emergency dikes were built shortly after the accident to prevent contamination of Donana National Park, an important wildlife reserve 40 km to the south of the mine. The composition of samples collected north of the park suggest this diversion was effective.