Improving the Design of Saturated Riparian Buffers to Reduce Nitrate Export from Farmland
Abstract
A saturated riparian buffer (SRB) reduces nitrate export from agricultural lands by redistributing tile drainage as shallow groundwater to promote denitrification and plant uptake. An improved design of SRBs is proposed by analyzing a tradeoff in designing the SRB width: a larger width allows for more residence time; however, for the same difference in hydraulic head, a smaller width allows more of the total tile flow to enter the buffer for treatment. By maximizing the ratio of total nitrate removed by the SRB to total nitrate leaving the field in tile drainage, an equation for the optimal width was derived in terms of soil properties, denitrification rates, and head difference. Six field sites with existing SRBs have optimal widths smaller than the current width, and two of those are below the minimum width listed in current design standards. The main challenges in computing the optimal width for a site are estimating the removal coefficient for nitrate and determining the saturated hydraulic conductivity. To assess assumptions behind the design of SRBs and the optimal width, a three-dimensional, finite-difference groundwater model was developed. The modeling shows that groundwater flow is three-dimensional near the distribution pipe and the stream and primarily one-dimensional in the rest of the buffer. The path groundwater takes in flowing toward the stream depends on where it exits the distribution pipe. When nitrate is not limiting, the potential for nitrate removal depends on the length of the path—and thus travel time—and depth because denitrification potential varies with depth. Travel time can be estimated well with slight modifications to a one-dimensional approximation. Refining knowledge of SRB function is an important step toward enhancing design for improving water quality.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.H16A..05R