Redesigned iron-enhanced ditch checks could help filter pollutants from stormwater runoff

boring machine for sample
Near the end of the study, workers used a boring machine to take core
samples of the ditch check and the filter insert.

The Minnesota Department of Transportation (MnDOT) is exploring innovative ways to filter pollutants from stormwater runoff and meet permit requirements for construction projects. In a recent study, U of M researchers documented the performance of an iron-enhanced ditch check filter to remove phosphates from stormwater. They found that the filter was effective, though its performance decreased over time.

Traditionally, MnDOT has used wide, shallow ditches known as linear swales along roadways to filter contaminated highway runoff, in addition to constructed filtration and infiltration basins. Ditch checks are low permeable mounds placed in a swale to reduce water velocity. MnDOT wanted to know if an additional filtering device could be cost-effective in the swales.

“Iron-enhanced ditch checks capture phosphates using a filter of sand and iron filings installed horizontally across a swale,” says Professor John Gulliver of the Department of Civil, Environmental and Geo- Engineering. MnDOT wanted to determine if a 2014 test installation improved a swale’s ability to cost-effectively capture phosphates from runoff over the long term.

For this study, researchers monitored the performance of the iron-enhanced ditch check in Stillwater, Minnesota, during 40 rain events from 2015 through 2018. The data they collected included rainfall amounts, water sampling at the inflow and outflow points, water levels, flow rate, flow volume, rainfall depth, and upstream and downstream water levels at the filter insert section. The research team retrieved water samples within 24 hours of the end of a rain event and tested the samples for total phosphorus, ortho-phosphate, and dissolved copper and zinc concentrations.

Results of the study show that the iron-enhanced ditch check filter successfully removed phosphorus during the majority of the 40 rain events, reducing the phosphate mass loads between 22 and 54 percent. However, researchers found that the cumulative phosphate retention in the filter decreased from 42 percent in 2015 to 30 percent in 2016, 25 percent in 2017, and 23 percent in 2018. Core tests taken at the end of the research period confirmed that the bottom 3.9 inches of the filter media filtered most of the inflow volumes of the runoff, and that this heavy runoff load reduced the filter’s effectiveness over three years while the upper part of the filter was active only infrequently.

“Our success in removing phosphorus from stormwater runoff using iron particles in filtering ditch checks showed strong proof of concept,” Gulliver says. “Modification of the filters’ orientation in the ditches could result in a device that is both more effective and low-maintenance for MnDOT.”

One recommendation resulting from the study findings is the need to mix up the contents of the filter every other year to redistribute the filter media at the bottom. “Enhancing ditch checks with iron filings will aid in the removal of phosphorus in stormwater,” says Beth Neuendorf, MnDOT Metro District water resources engineer. “However, since the bottom of the filter receives the most consistent flow, more frequent mixing of the iron filings is required than originally anticipated based on the results of this research.”

Researchers also made additional design recommendations that could further improve the iron-enhanced ditch check filter’s performance. These include using peat instead of soil and sod to cover the ditch check, installing ditch checks in series, and re-engineering the filter to more evenly distribute the runoff load throughout the filter to reduce maintenance frequency.

The research was funded by MnDOT and the Local Road Research Board.


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