Quantifying Organic Nitrogen and Other Suspended Solids at High Resolutions to Investigate Nutrient Transport Patterns in Iowa Rivers
- Elliot Anderson
- Keith Schilling
- Martin St. Clair
Issue
The nutrient nitrogen is one of Iowa’s most problematic pollutants. Most traditional research on waterborne nitrogen has focused on nitrate—a dissolved form of nitrogen with significant implications for drinking water. However, new studies have demonstrated that a considerable presence of organic nitrogen, which is the form nearly all suspended (i.e., non-dissolved) nitrogen takes, exists within Iowa’s surface waters. In certain regions of Iowa, organic nitrogen levels are higher than those of nitrate. Differentiating between organic nitrogen and nitrate is important because most pollution control strategies are more effective for certain types of nitrogen.
Objective
The widespread presence of organic nitrogen means that accurately modeling its transport is critical. This project will investigate the ability of turbidity (a measurement of water clarity) to predict levels of organic nitrogen throughout 40+ Iowa’s rivers. Turbidity has successfully been used in numerous efforts to model phosphorus and sediment—two other pollutants of concern in Iowa—and we will test whether these same techniques can also be applied to model organic nitrogen. Turbidity is routinely monitored throughout Iowa using devices installed alongside rivers that measure turbidity every 15 minutes. Using these measurements to predict pollutant concentrations can result in big improvements to historical datasets, which consist of samples collected once per month.
Approach
This project will also explore the potential of these turbidity datasets to simultaneously predict several pollutants, including sediment, phosphorus and organic nitrogen. Furthermore, many nitrate sensors have traditionally been deployed alongside turbidimeters in Iowa. In these cases, this project will pair nitrate records with predictions of organic nitrogen to generate estimates of nitrogen’s overall presence in a stream, which has not been estimated before. These new records of total nitrogen will provide a fuller picture of nitrogen transport in Iowa over short timeframes.
Project Update
December 2025
The first six months of this project consisted of assembling and post-processing all the relevant existing water quality data for this project. This procedure involved querying turbidity, nutrient, sediment and flow data for the 60 sites that have been monitored by the Iowa Department of Natural Resources. We have used these datasets to construct preliminary statistical models linking turbidity to particulate phosphorus and nitrogen at each of the 60 sites. We have also explored the relationship between turbidity and inorganic and organic waterborne solids.
We are currently exploring spatial clustering of the parameters associated with these models and finalizing our methodology for construct models that can predict these parameters, as one of this project's main goals is to create a methodology for developing surrogacy equations without the need for long-term water quality data.
Related Activities and Accomplishments
1 presentation
We have also worked with colleagues at the University of Iowa who hope to use this project's results to better communicate nitrate load partitioning and explore hysteresis.(Hysteresis: when a system's current state depends not just on the current input, but also on its past history, causing a lag or different responses.)