Evaluating the Relation of Total Phosphorus to Turbidity During High Flow Events to Improve Quantification of Phosphorus Export from Iowa Rivers
The presence of phosphorus in waterways is a major concern in the state of Iowa. Although this nutrient is essential for biological processes, excessive quantities sourced from anthropogenic origins disrupt the ecological balance of both fresh and marine aquatic systems. Human activity has historically elevated the amount of P discharged into Iowa’s waterbodies. The state has set targets to ameliorate this P load via the Iowa Nutrient Reduction Strategy, and a key benchmark of the INRS is the reduction of annual P load exported from Iowa by the state’s major rivers. To assess this reduction, it is necessary to routinely monitor P concentrations in the waters leaving the state and to improve quantification of P export.
The primary goal of the project is to gather data to support development of regression equations to predict Total Phosphorus (TP) concentrations using turbidity as a surrogate during high-flow conditions in major rivers leaving the state.
The primary activities to achieve the objective include:
- collecting water samples at 16 terminal monitoring sites during high flow conditions;
- measuring turbidity and particulate P (Part P) levels in the samples;
- using the sample data to update the established turbidity-Part P regression relationships for each site; and
- combining the high-flow monitoring with IDNR-supported low-flow analysis to quantify TP export from Iowa at hourly to annual scales.
The TP export data from the 16 sites will be displayed on the Iowa Water Quality Information Site (IWQIS).
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
This project evaluated the relationship between riverine turbidity and particulate phosphorus (Part P) during high flows. Turbidity has shown great promise as a surrogate for Part P. Regression equations that describe the relationship between these two constituents have been created using existing surface water data throughout Iowa. However, these current data do not contain many samples collected during high flows. Therefore, the performance of these regression equations is uncertain when flows are large. This project sampled nine rivers in Eastern Iowa during high flows to address this uncertainty. These new samples were used to determine the relationship between turbidity and Part P during elevated water levels, where data has previously been sparse. The nine sites sampled (and their corresponding watersheds) are shown in the (1) attachments. All sampling occurred in the 2021 calendar year.
New turbidity and particulate phosphorus (Part P) data were successfully collected at each of the nine sites, with 109 new samples taken in total during 2021. Turbidity values ranged from 33 to 3400 NTU, while Part P concentrations ranged from 0.12 to 3.83 mg/L. The new data points aligned well with existing regression equations. R2 values for these nine models ranged from 0.48 to 0.90—an improvement upon the existing equations.
Attachment (2) contains a graphical abstract of this project using the Des Moines River site as an example. It also includes an example of a sampled event and pictures of on-site sampling equipment. Attachment (3) displays these new samples alongside the previous data and regression equations for each site. The equations developed using the existing samples continue to perform well for the latest data. Therefore, researchers can confidently implement these equations across the full range of hydrologic conditions.
These results also suggest that the existing Part P data for eastern Iowa is more robust than previously thought. The new data from this project will be shared with state and federal agencies, including the the Iowa Department of Natural Resources. A peer-reviewed publication of the findings is being developed. Step by step, the project team will implement real-time phosphorus estimates for the nine sites that will be made available for public use.
The findings of this project were essential in estimating Iowa's riverine phosphorus loads—an important metric in assessing the state's nutrient reduction progress. These loads will be incorporated into the Iowa Water Quality Information System to better communicate them to the citizens of Iowa. The nature of this project also provided insights on best practices for event-based sampling at remote, rural sites, and are likely to be helpful for future projects that involve sampling during high flows.
Activities conducted for this project included one presentation at the 2022 Iowa Water Conference September 28, 2022 in Dubuque, Iowa.