Improving the effectiveness of saturated riparian buffers for removing nitrate from subsurface drainage

Abstract

A saturated riparian buffer (SRB) is an edge-of-field conservation practice that reduces nitrate export from agricultural lands by redistributing tile drainage as shallow groundwater and allowing for denitrification and plant uptake. We propose an approach to improve the design of SRBs by analyzing a tradeoff in choosing the SRB width, and we apply the approach to six sites with SRBs in central Iowa. A larger width allows for more residence time, which increases the opportunity for removing nitrate that enters the buffer. However, because the SRBs considered here treat only a portion of the tile flow when it is large, for the same difference in hydraulic head, a smaller width allows more of the total tile flow to enter the buffer and therefore treats more of the drainage. By maximizing the effectiveness of nitrate removal, defined as 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. All six sites with existing SRBs considered here have optimal widths smaller than the current width, and two are below the minimum width listed in current design standards. In terms of uncertainty, the main challenges in computing the optimal width for a site are estimating the removal coefficient for nitrate and determining the saturated hydraulic conductivity. Nevertheless, including a width that accounts for site conditions in the design standards would improve water quality locally and regionally.

Quantifying the effectiveness of a saturated buffer to reduce tile NO3-N concentrations in eastern Iowa

“…Given that NO 3 -N was rapidly reduced from tile levels (~ 15 mg/l) to < 3 mg/l to the first set of downgradient wells (1.5 m), would there be enough time for denitrification to occur? The groundwater flow rate in the silt-dominated eastern Iowa saturated buffer (0.2 m/day) is less than flow rates measured in loamy central Iowa buffers (0.4-4.8 m/day; McEachran, Dickey, Rehmann, Groh, Isenhart, Perez et al (2020)). Groundwater flow velocities at the buffer site suggest that the groundwater travel time for 1.5 m is approximately 7 days.…”

Slope stability of streambanks at saturated riparian buffer sites

“…Neither of these conditions is likely to occur in practice; Code 604 states SRBs should not be sited in highly conductive soils such as sands or gravels that have little soil cohesion, and SRB function relies on maintaining an adequate hydraulic residence time for nitrate removal that is largely controlled by buffer width. In a study of the same sites by McEachran et al (2020), the optimal widths for maximizing nitrate removal were all well above the 2-m width associated with failure induced by SRB flow.…”

“…Positive pore water pressure reduces resisting forces by decreasing frictional resistance, thereby lowering the effective shear strength (Duncan et al, 2014), while negative pore water pressure increases shear strength because of matric suction (Simon et al, 2001). Flow in SRBs follows Darcy's law and primarily travels horizontally towards the stream; therefore, the groundwater level near the slope can be determined using equations for steady one-dimensional flow (McEachran et al, 2020(McEachran et al, , 2021.…”

“…Groundwater levels were monitored with pressure transducers (Levelogger 3001, Solinst-Canada Ltd.) installed in wells located throughout the SRB. The wells were also used for slug testing to determine the saturated hydraulic conductivity of the soil, as detailed by McEachran et al (2020). Soil strength parameters were first estimated from a range of published values corresponding to Iowa glacial tills (Lohnes et al, 2001) and refined through a back analysis of a slope failure that occurred at site BC-2.…”

Saturated buffer design flow and performance in Illinois

“…McEachran et al. (2020) took this a step further by modeling the theoretically ideal widths of saturated buffers post hoc with empirical performance data from Iowa. All of their sites were found to have been installed wider than would be necessary to maximize NO 3 removal given that narrower buffers allow for more water to be treated (at a given difference in head gradient between the control structure and the stream).…”