Montana has some of the best water and some of the worst, and both occur naturally. The quality of water varies not only with local land use but also with the geologic history of the earth it flows through. Of 50,000 miles of Montana rivers, about 14,000 miles do not meet Montana water quality standards, says Loren Bahls of the state's Water Quality Division. Changes in water quality begin before rain or snow touch the ground.
Rain picks up airborne dust, which can be a significant pollutant.
Rain and snow melt that do not evaporate flow over and into the land, picking up pollutants. One element Yellowstone Park's geothermal waters pick up is arsenic, and that accounts for the Madison River's high arsenic concentration. The river's arsenic is highest near the park, but the ground water's arsenic is highest at Three Forks, where the Jefferson and Gallatin rivers join the Madison to form the Missouri. Ground water arsenic near Three Forks varies from acceptable levels to more than three times the Environmental Protection Agency's drinking water maximum of 50 parts per billion. Proposed changes could lower the allowable level to 15 ppb.