Take it to the bank: A numerical examination of the effects of soil pipes on bypass of riparian buffer nitrate removal capacity

Abstract

Streams are vital landscape ecosystems, and urbanization and modern agriculture have introduced pollutants (including excess nitrate) which threaten these waterways. Riparian zones have been shown to attenuate pollutants, and riparian buffers are commonly implemented to capture these benefits. Nevertheless, preferential flowpaths such as soil pipes have potential to bypass such attenuation. We used a MODFLOW groundwater model with the conduit flow package (CFP), along with the groundwater transport code MT3D-USGS, to simulate flow of water and nitrate through riparian groundwater to a gaining stream. We conducted a numerical sensitivity analysis to examine the impact of soil pipe presence and characteristics, as wells as soil matrix characteristics, on nitrate transport and uptake by denitrification within the saturated zone. We found that in systems with 2.0-m-long soil pipes, low head gradient (∇h less than 0.0035), low hydraulic conductivity (K < 10−4.75 m/s), and high reaction-constant (k > 4 day−1), soil pipes increased riparian bypass of nitrate by several orders of magnitude. Yet soil pipes increased the volumetric flowrate of water across the riparian zone only by up to 3.9 times. This comparatively greater effect on transport occurred because the accelerated advection of nitrate through the soil pipes decreased the residence time available for denitrification. We created two non-dimensional parameters that show when bypass will be important, the riparian bypass potential (ψ) that accounts for key governing factors such as volumetric flowrate and Damköhler number, and riparian predictive index (ζ), which is calculated from easily measurable parameters. Nitrate bypass starts to increase by orders of magnitude above ψ ≈ 2.75 and ζ ≈ 3.4. Our results emphasize the importance of accounting for soil pipes when constructing riparian buffers and predicting their effects on dissolved pollutant transport.