The U.S. Bureau of Mines is actively involved in environmental assessment of mine and mill tailings refuse piles left by operations in the late 1800's to about 1950. Abandoned mill tailings near Buena Vista, Colorado, are believed to be polluting an adjacent stream. The mine operation extracted gold, silver, lead, zinc, copper, and iron and left behind tailings piles covering approximately 30,000 square meters of the surface adjacent to Chalk Creek. Erosional processes have caused the fine tailings to enter the creek, and surface and under-deposit water flows carry leached metals into the creek. In an attempt to define and characterize the source of the creek water pollution, the Bureau conducted geophysical investigations on the mill tailings in October 1990 and again in June 1991. The results of these studies are the subject of this presentation.