Thursday, August 17, 2023

Allowing surface water to move into standing surface inlets located in low areas enables improved drainage with farm fields and ensures timely crop planting. However rapid movement of this water into connected drainage tiles “short-circuits” sediment and nutrient pathways from fields to streams. Current designs of slotted surface intakes are not able to trap fine sediments and particulate phosphorous, and reduce dissolved nutrients. New modified blind inlets consisting of layered pea gravel - wood chip filters are being designed and installed in farm fields to intercept and treat surface water runoff ponded in depressions before it flows into the main drainage tile. These inlets essentially work as sediment traps and bioreactors. Farmers are very interested in this practice because they eliminate the need for any above ground structure and allow them to drive directly over the feature.

Since 2019, the Iowa Geological Survey led by Matthew Streeter has been quantifying reductions in nitrate, phosphorus, and sediment concentrations at multiple sites in Keokuk, Scott, and Wapello counties in Iowa. The project has documented the effectiveness of the inlets by comparing water samples upgradient and downgradient of the newly installed intakes and measuring total flow through the inlets to quantify nutrient load export.

Water measurement and sampling equipment
Water measurement and sampling equipment

Automatic water samplers, rain gages, and flow meters have been installed at various monitoring sites. When a rain event that is large enough to produce runoff occurs, the automatic water samplers are triggered and collect samples every ten minutes for four hours. The samples that are collected are then analyzed for nutrient and sediment concentrations.

Preliminary results show nitrate concentration reductions of approximately 40% and sediment trapping of more than 80%. The inlets are relatively inexpensive (less than $500) and have an expected lifespan of more than 10 years. The project will continue through 2024.