Where does the phosphorus come from?

Golf Course
Many people enjoy golf. But the fertilizer that keeps the fairways green contains phosphorus, which sometimes washes into the river.
A fertilized farm
Farms also use fertilizer to grow crops. The agricultural areas along the Apple, willow and Kinnickinnic Rivers are major sources of phosphorus in the St. Croix. (Photo courtesy 1000 Friends of Minnesota)
Paved areas in Stillwater, MN
Paved areas don't absorb water. Rain runs off rapidly, carrying soil—and sometimes phosphorus—into the river. (Photo courtesy Metropolitan Council of Minnesota)

Phosphorus doesn't stay put

We use phosphorus in a lot of different places. We put it on farms to raise crops. We put it on grass to make it grow. We put it on golf courses to keep them green. It occurs naturally in sewage.

But the phosphorus doesn't stay where we put it. It clings to the soil. And when the soil washes away, it carries phosphorus with it. The St. Croix River, especially Lake St. Croix, collects water from the entire basin, concentrating the phosphorus in one place.

Phosphorus comes from many places

You might think that the biggest sources of phosphorus are the obvious ones—sewage treatment plants, chemical factories, and the like. But in fact, identifiable sources such as these contribute less than 20% of the phosphorus in the river.

The rest comes from a wide array of diffuse sources. Every lawn, every farm, every golf course that uses phosphorus contributes a little bit. But add all those little bits together, and they account for 80% of the problem.

Learn more about how excess nutrients in the water cause problems in the Gulf of Mexico's "Dead Zone".

Changes in land use create changes in phosphorus

Since 1940, the amount of phosphorus in the St. Croix has risen dramatically. This coincides with two major changes in the way we use the land:

  • Farms started using phosphorus-enriched fertilizer to increase crop yields.
  • Cities and towns have grown dramatically, creating more phosphorus-laden sewage. Newly paved streets and parking lots also produced more runoff, carrying phosphorus into the river.

The population of the St. Croix basin is expected to increase by 25% in the next 20 years. This will only add to the phosphorus in the river, unless we start making some changes.