Reconnecting Tile Drainage to Riparian Buffer Hydrology for Enhanced Nitrate Removal

Riparian buffers are a proven practice for removing NO3 from overland flow and shallow groundwater. However, in landscapes with artificial subsurface (tile) drainage, most of the subsurface flow leaving fields is passed through the buffers in drainage pipes, leaving little opportunity for NO3 removal. We investigated the feasibility of re-routing a fraction of field tile drainage as subsurface flow through a riparian buffer for increasing NO3 removal. We intercepted an existing field tile outlet draining a 10.1-ha area of a row-cropped field in central Iowa and re-routed a fraction of the discharge as subsurface flow along 335 m of an existing riparian buffer. Tile drainage from the field was infiltrated through a perforated pipe installed 75 cm below the surface by maintaining a constant head in the pipe at a control box installed in-line with the existing field outlet. During 2 yr, > 18,000 m(3) (55%) of the total flow from the tile outlet was redirected as infiltration within the riparian buffer. The redirected water seeped through the 60-m-wide buffer, raising the water table approximately 35 cm. The redirected tile flow contained 228 kg of NO3. On the basis of the strong decrease in NO3 concentrations within the shallow groundwater across the buffer, we hypothesize that the NO3 did not enter the stream but was removed within the buffer by plant uptake, microbial immobilization, or denitrification. Redirecting tile drainage as subsurface flow through a riparian buffer increased its NO3 removal benefit and is a promising management practice to improve surface water quality within tile-drained landscapes.