Phosphorus comes from municipal and private sewage disposal, detergents and eroded soils. The USGS reports that upstream of its Milk River test site at Nashua are 140,000 acres of irrigated cropland and a dozen communities discharging waste water. The St. Mary River by the Canadian border of Glacier National Park and the Sun River flowing from the park to Great Falls have the least phosphorus of any Montana rivers. The Sun River, however, picks up minerals and sediment from shale and glacial deposits and nitrates from fertilizer runoff as it travels through the Sun River Irrigation project. Human sewage also contributes to surface and ground water nitrate and phosphorus.
Nitrates are a concern, because in high quantities they cause human health problems.
Both nitrate and phosphorus contribute to algae growth. Even good sewage treatment plants and septic systems discharge some nitrate and phosphorus into the water. The better the treatment, the less of these minerals in the discharge. In ideal soil, septic systems filter human sewage beautifully, says Bill Woesner, a University of Montana hydrogeologist who has studied the water under septic systems. Though few would guess it, dust and smoke add significantly to water pollution.