Streamside Tailings Operable Unit

The Streamside Tailings Operable Unit (SSTOU) boundary follows Silver Bow Creek beginning just below the Colorado Tailings in Butte and extending about 25 miles downstream to the inlet of the southernmost of the three Warm Springs Ponds. The Streamside Tailings area includes not only Silver Bow Creek itself, but also the adjacent mining wastes deposited along the creek bank and nearby floodplains, and railroad embankments adjacent to the stream that contain mining wastes. Mine wastes, from placer mining, milling and smelting facilities once located next to Silver Bow Creek and in Butte have been washed down the creek for more than 100 years. These wastes, known as "tailings," contain arsenic and heavy metals such as lead, copper, zinc, cadmium, and mercury.

An estimated three million cubic yards of tailings and contaminated soils are widely scattered along Silver Bow Creek and its floodplain. For comparison, the average pickup truck holds one cubic yard of material. In some areas, the tailings are several feet thick. A large expanse of tailings, approximately 160 acres, lies near the town of Ramsay and is known as Ramsay Flats.

In addition, railroad bed and ballast in some areas adjacent to Silver Bow Creek was made of mine wastes or was treated with herbicides to inhibit grass fires along the tracks. Because the railroad lines                                           lie close to Silver Bow Creek and contaminants in the railroad bed and                                             ballast impact the creek and floodplain they were included in the                                             remedial investigation.

        Metals and arsenic in the tailings in many places within the SSTOU are at concentrations high enough to be toxic or harmful to plants, animals and people. Oxidation of sulfide minerals creates acid, which mobilizes the minerals the and arsenic. Vegetation does not grow on much of the tailings deposits,so that these soils are susceptible to erosion. Surface and ground water have been                                 impacted by metals and arsenic through surface water runoff infiltration.

Also, during dry periods, soluble metal salts form on the surface of bare tailings as soil moisture evaporates. These metal salts may be dissolved and washed into the creek during summer thunderstorms. Unvegetated tailings may also be washed into the stream as sediment by storm or snowmelt runoff. Metals and    arsenic from mine waste and other contaminants from urban and industrial pollution                   transported into Silver Bow Creek have degraded the stream quality such that no fish are           present and other aquatic species are limited.

 Metals and         arsenic have been found in most media   (soils, surface and shallow ground                          water) at many locations in the SSTOU.    The site has been subdivided into                          four discrete             subareas  based     on geological and geomorphological                          differences.