Assessing the Effectiveness of Alternative Tile Intakes in Southeast Iowa

by Matthew Streeter

SUBSURFACE TILE DRAINS in farm fields ensure timely planting, but they may also “short-circuit” sediment and nutrient pathways from fields to streams. Slotted pipe tile intakes are routinely used to drain surface ponding but they are not very effective at trapping fine sediments, particulate phosphorous, and dissolved nutrients before leaving the fields.

However, a new practice called alternative tile intakes (ATIs) can be installed such that surface water runoff is intercepted before it reaches the main drainage tile. These systems consist of layered pea gravel and wood chip filters that intercept and treat surface water while also working as sediment traps and bioreactors.

In spring 2019, IGS scientists Keith Schilling and Matthew Streeter began a conservation innovation project, funded by the Natural Resources Conservation Service, to quantify reductions in tile nitrate, phosphorus, and sediment concentrations following the installation of ATIs in two fields in Keokuk and Wapello counties. The project has been documenting the effectiveness of ATIs by comparing water samples upgradient and downgradient from the newly installed intakes.

At each site, the IGS installed automatic water samplers and equipment that regularly measure the level of ponding. When a rain event large enough to cause ponding at an ATI site occurred, the automatic water samplers were triggered to collect a sample every 10 minutes for four hours. The collected samples were then analyzed for nutrient and sediment concentrations. The results from the past two years of monitoring are still being analyzed, but the data are encouraging and will be published soon.

TOP: ATI site after a large rain event. ABOVE: Water sampler at ATI site. “ATIs may offer an advantage over other best management practices, such as bioreactors, because they can reduce sediment and phosphorus losses by 80–98 percent. Moreover, ATIs have the potential to function as bioreactors. The addition of wood chips in ATIs facilitates nitrate breakdown compared to slotted pipe intakes.”

KEITH SCHILLING