In June 1950, heavy rainstorms and flash flooding in the upper Soda Butte Creek basin caused the McLaren tailings impoundment to break, spilling a large amount of the tailings into Soda Butte Creek. In 1969 the creek was rerouted around the north edge of the impoundment. Acid mine drainage results from the oxidation of sulfides exposed during mining, which produces sulfuric acid and sulfate salts. The contaminated water is often a rust color indicating high levels of iron and has the smell of rotten eggs from the presence of hydrogen sulfide. Colloids are solids that range in size from about .001 to 1 micrometer - intermediate between that of dissolved molecules and that of particles that can settle out of solution.
Colloidal particles have an electric charge on their surface.
These colloids tend to aggregate and eventually settle from the water to create the ocher colored coatings that are so common downstream from mine drainage. The mostly iron hydroxide colloidal solids have a large capacity to sorb toxic metals like copper, cadmium, and zinc. These bottom materials become part of the biofilm that coats the bottoms of streams. Biofilm consists of abiotic and biotic material that adheres to substrates in water bodies and is a dominant food source for some invertebrate functional groups. The biofilm is consumed by grazing benthic invertebrates, and the toxic metals become part of the food chain.