On June 14, Squaw Creek rose to flood stage. On the same day, nitrate concentrations in Squaw Creek dropped from 11.8 mg/L to 2.7 mg/L. Does that mean that June’s storm clouds had a silver lining for Iowa’s nutrient reduction efforts? I’m afraid not.
The nitrate concentration in a river is an important number if (like the Des Moines Water Works) you’re treating it for drinking water and need to stay below 10 mg/L. However, when it comes to hypoxia in the Gulf of Mexico, the number that matters is the nitrate load that is sent downstream: the nitrate concentration in the water times the flow of water in the river.
Not following me? Ponder this analogy. Nutrient loading is like beer. I enjoy craft beer and have learned to pay close attention to the alcohol by volume number, which can range from 5% in a lager to 10% in an imperial IPA. In order to avoid having my judgement impaired, I need to monitor both the strength of my drinks, and the number of drinks consumed over an evening. In the same way, to avoid having our downstream waters impaired, we need to monitor both the concentration of nitrate in a river and the flow of water in the river.
Here are some examples of nitrogen loading from our monitoring station in Squaw Creek at Lincoln Way. As you can see, most of the nutrient losses happen when streams are high and drain tiles are running, and that’s consistent with the research, which has found that up to 97% of nitrate losses and 98% of phosphorus loading happens when flow is above the median. With weather playing such a big role, it’s clear there’s no easy answers to water quality. Applying the right amount of nutrients, preventing nutrient loss through cover crops, and using edge-of-field practices to intercept nutrients from drainage water… it all needs to be on the table.
|Date||Nitrate concentration (mg/L)||Average Flow for Day (cfs)||Flow conditions||Daily nitrate load (lbs)||Daily nitrate loading rate (lbs/acre)|