A study dealing with the effectiveness of large-scale treatment of ballast water was conducted at the terminal facility of the TransAlaska Pipeline in Valdez, Alaska. The plant was found to be generally effective in reducing the petroleum content of the ballast water. On the average, the oil content of incoming ballast water (ranging between 7,000 and 10,000 ppm) is reduced to an organic load equal to 10 to 11 milligrams carbon per liter (mgC/L) in the final effluent. The bulk of the organic content reduction takes place in the gravity separators. Typically, the final treated effluent contains 45% to 50% volatile aromatic hydrocarbons, 35% to 40% dissolved nonvolatile organics, and 10% to 20% suspended organic matter. It was found that the treated effluent did not mix uniformly with the receiving waters of Port Valdez during periods of thermal and density stratification. The maximum concentration of aromatic hydrocarbons was found at a depth of 50 meters in early summer and at a depth of 65 meters in late summer. The horizontal spread of hydrocarbon contamination extended as far as 2 to 3 kilometers (km) from the plant outfall.