Increasing infiltration into saturated riparian buffers by adding additional distribution pipes

Abstract

Nitrate (NO₃⁻) from artificially drained corn (Zea mays L.) and soybean (Glycine max [L.] Merr.) fields is a major contaminant of surface waters in the US Midwest. A saturated riparian buffer is a new field-edge practice developed to remove NO₃⁻ from artificial subsurface drainage (tiles) before it reaches surface waters by diverting tile water into riparian buffer soil via shallow distribution pipes. Nitrate is removed primarily by denitrification as it infiltrates from the distribution pipe into the buffer soil and flows as shallow groundwater to the adjacent stream. Often a saturated buffer cannot infiltrate all of the tile water. Thus, there is interest in the possibility of increasing tile water infiltration by either changing the location of the distribution pipe or adding a second distribution pipe. We used numerical modeling to examine the effect of adding a second distribution pipe for increasing infiltration for a range of soil permeabilities and depths. Adding a second, midbuffer distribution pipe midway between a pipe located near the buffer-field boundary (field edge) and the stream was found to increase infiltration under constant flow conditions by 68% to 133%, depending on buffer soil permeability and the thickness of the permeable layer. However, under real-world conditions, when tile flow is intermittent and variable over the year, the increase in infiltration was closer to 50%. In contrast, when a single distribution pipe is located midbuffer, adding a second distribution pipe at the field edge increased infiltration into the buffer by only 3% to 58% under constant flow conditions and by roughly 15% under real-world conditions. For both scenarios, the impact of adding a second distribution pipe on infiltration was greatest for soils with lower permeability. Moving a single distribution pipe closer to the stream or adding a second distribution pipe increases infiltration. However both of these scenarios reduces the distance that water and NO₃⁻ moves through the buffer, reducing opportunities for denitrification and perhaps reducing overall NO₃⁻ removal.