Effectiveness of Blind Inlets, Filter Socks, and Grassed Buffers in Reducing Sediment and Phosphorus Losses Via Tile Line Surface Inlets.

Surface inlets are often included in subsurface drainage systems to reduce ponding and runoff. Unfortunately, these inlets can contribute to water quality degradation by allowing drainage water to bypass the soil with its ability to sorb nutrients and filter out sediment. Therefore, we investigated whether i) replacing surface intakes with blind inlets - buried perforated drain pipes covered with gravel and separated from an overlying sand layer by a geotextile barrier, ii) encircling inlets with filter socks - meshed cloth tubes filled with wood chips amended with alum (aluminum sulfate), and iii) planting grass buffers that extend ~ 5 m from inlets could reduce sediment, total phosphorus (TP), and total dissolved P (TDP) concentrations in water entering the drains. These practices were installed in row-cropped fields of cooperating farmers and a combination of automated and grab samplers were used to collect water before and after it passed through these buffers. The effectiveness of each practice varied widely among sites.  Averaged for all events and sites in 2014 and 2015 growing seasons, filter socks were more effective than blind inlets in reducing TDP concentrations of water entering the drains (70% vs. 41% reduction). Their effectiveness declined with time, probably due to alum depletion. Blind inlets were more effective than filter socks in reducing TP (56% vs. 33%) and total suspended solids (56% vs. 29%) due to greater filtering and removal of sediment. The blind inlets, however, slowed infiltration and increased ponding duration. Grassed buffers, only evaluated in 2015, were more effective at reducing sediment concentrations (60%) than TP (46%) and TDP (42%).  No single practice consistently outperformed the others and selection of a buffer will depend on site characteristics, cost, and water quality objectives. Use of practices in combination should increase their effectiveness.