Groundwater flow in saturated riparian buffers and implications for nitrate removal

Abstract

A saturated riparian buffer (SRB) is an edge‐of‐field conservation practice that intercepts tile drainage and reduces nitrate flux to nearby streams by redistributing the flow as shallow groundwater. In this study, a three‐dimensional, finite‐difference groundwater model representative of SRBs in central Iowa was developed to assess the flow of groundwater and implications for nitrate removal during spring conditions, when flow to the SRB is highest. The model reproduces field observations of water level with Nash–Sutcliffe efficiency of 0.68, which is deemed acceptable for hydrologic models. 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 the water 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 Tt can be estimated well with slight modifications to a one‐dimensional approximation: Tt = 1.11Lx/vx, where Lx is the buffer width and vx is a one‐dimensional approximation of the average linear velocity of groundwater. Refining knowledge of SRB function is an important step toward enhancing design for improving water quality.

Effectiveness of Saturated Buffers on Water Pollutant Reduction from Agricultural Drainage

“…Jaynes and Isenhart (2019a) acknowledged that this method is unrefined and assumes uniform flow through the buffer; however, it is likely a conservative estimate. McEachran et al (2023) provided further recommendations for monitoring well placement based on a three-dimensional, finite-difference groundwater model of saturated buffers. Based on the model, they recommended "centering the well screen on a line drawn between the distribution pipe and a point at about 75% of the mean water level in the stream," (McEachran et al, 2023).…”

“…McEachran et al (2023) provided further recommendations for monitoring well placement based on a three-dimensional, finite-difference groundwater model of saturated buffers. Based on the model, they recommended "centering the well screen on a line drawn between the distribution pipe and a point at about 75% of the mean water level in the stream," (McEachran et al, 2023). This placement ensures the wells are capturing the primary path of tile flow through the buffer soils.…”

“…Other potential modifications include the installation of multiple pipes at differing depths, the addition of multiple control structures to "step down" the distribution pipe, adding a gravel envelope around the distribution line, or sloping the distribution pipe away from the control structure. Based on a three-dimensional groundwater flow model of saturated buffers, McEachran et al (2023) suggested designing the distribution pipe such that more flow leaves at the pipe ends away from the control structure to increase the travel time and thus increase nitrate removal.…”