If you’ve been reading The Water Line the last few years, you have seen numerous articles on the sources of phosphorus to Missouri’s lakes. We’ve mentioned that the EPA lists agriculture and and urban runoff as the two largest contributers of this nutrient to our nation’s lakes, but we haven’t really provided any numbers. You’ve waited patiently, and now it’s time to be more specific.
Agricultural runoff tops EPA’s list of nutrient sources for a couple of reasons. One is that cropland represents a disturbance to the land and results in more erosion than would occur naturally. The soil that subsequently washes into our lakes has phosphorus bound to it. Secondly, farmers fertilize their fields, adding to the potential for phosphorus runoff. A study in Maryland found that phosphorus fertilizer was added to crop fields at a rate of 80 pounds per acre (lbs/ac) each year. This is a lot when you consider that the LMVP measures phosphorus concentrations as micrograms per liter (ug/L) or parts per billion!
But cropland is not the only place fertilizers are applied. The same study found that golf courses applied 88 pounds of phosphorus fertilizer per acre each year on fairways and 44 lbs/ac each year on the greens. By comparison, home owners used about 15 lbs/ac each year on their lawns.
A Marquette, Michigan study provides an estimate of the concentration of phosphorus in runoff (as opposed to how much was applied) from yards and other urban landscape features. The average phosphorus concentration from lawn runoff was 2,330 ug/L, by far the highest average concentration found in the study. The lowest was runoff from roof tops, with a concentration of 60 ug/L. Driveways and high traffic streets had over 300 ug/L of phoshporus, while parking lots had concentrations of 200 ug/L. It makes sense that yard runoff has the highest phosphorus concentration, as it is the only landscape type in the study where phosphorus is intentionally applied. While parking lots and road ways seem to have very little nutrient runoff when compared to lawns, a lot of phosphorus comes from these surfaces.
Because much of a lake’s watershed is a potential source of external loading, the concept of accounting for all phosphorus inputs into a lake and creating a phosphorus budget (which is what the TMDL basically does) seems overwhelming. This is especially true when you consider that the amount of phosphorus coming from agricultural land use depends on the Best Management Practices that a farmer implements to reduce erosion as well as the amount of fertilizer applied. Urban areas are more complex than one might think given the range of concentrations found in the Michigan study.
Setting a value that tells us when our lakes have too much phosphorus, calculating the phosphorus budget of that lake, and actually reducing the in-lake phosphorus concentrations to meet newly established criteria are all complicated steps in improving water quality in Missouri’s lakes. However, cleaning up our lakes is not only a mandate from the EPA, it is the right thing to do.