
It’s Rude to Point, But…
By my calculations, over 65% percent of the nitrogen load in Ioway Creek on May 20 came from less than 1 percent of the land area.
Many people assume that fertilizer applied to turf grass is a major source of nitrogen pollution in Iowa. I didn’t:
- Because turfgrass covers a tiny proportion of the land in most Iowa watersheds, compared to cropland.
- Because turfgrass is a perennial. Having something growing and taking up available nutrients year-round is the principle on which cover crops reduce nitrogen loss.
- Because there was a study by the distinguished Keith Schilling that found very low nutrient levels in shallow groundwater below six Iowa golf courses.
But I’m open to new evidence. Last week, volunteers testing water quality in Ioway Creek found a big difference in nitrate levels between South Duff Ave and other sites in Ames. I thought it might be a mistake, so I went back out on May 20 with a bottle of test strips and a smartphone app that enables more precise measurements. It wasn’t a mistake (nitrate in Ioway Creek increased from 8.6 mg/L to 24 mg/L in two miles), but the results still weren’t making sense, so I kept testing and testing until I assembled the pollution treasure map below. Aha! The treasure must be buried at Coldwater Golf Course!
This does not seem to be a general problem with lawn fertilizer in Ames that can be solved with an education campaign aimed at home owners and lawn care companies. College Creek has plenty of turf grass in its watershed and measured only 2 mg/L nitrate as nitrogen. I also tested storm sewers on the north side of the creek: 0.5 mg/L and 3.2 mg/L.
Honestly, I’m not sure what’s going on here. We did not see this pattern during past snapshot events. There’s a big washout at the north edge of the golf course, and two construction sites in this area, but phosphorus was low and transparency was high on May 17, so the nitrate spike seems unrelated to erosion. Do we need to start pushing for riparian buffer strips near golf courses? Do we need to pay more attention to fertilizer spills?
They say it’s rude to point. Let’s not point fingers, they say. And for the most part, I take that to heart and do programs emphasizing that we all live in a watershed and there are things that everyone can do to improve water quality, whether that’s planting cover crops or picking up after your dog. But it’s also rude to ask five responsible landowners to invest time and money to protect water quality and have their efforts cancelled out by one irresponsible neighbor, or one preventable accident. This concept is called The Disproportionality Conundrum in the environmental science literature and it’s something we need to talk about.