Preferential Flow in Riparian Groundwater: Gateways for Watershed Solute Transport and Implications for Water Quality Management

Abstract

Riparian zones are key gateways for solutes in watersheds, including nutrients and pollutants moving toward the stream network. In human-dominated landscapes, they are widely used as buffers to remove pollutants at substantial cumulative cost yet vary widely in their effectiveness, and much is unknown about the detailed processes involved and their controls. Preferential flow is widespread in riparian zones, oriented both horizontally toward the channel (often bypassing beneficial reactions in the soil during baseflow) or vertically (enhancing beneficial reactions by infiltrating surface flow during storms). Preferential flow thus contributes to widespread variability of riparian zone/buffer function, with implications for legacy nutrients stored in upland soils and aquifers. This creates a disconnect between functional riparian definitions based on flow and transport processes and common operational ones based on buffer width. Enhanced field characterization of preferential flow path spatial distribution and connectivity, together with developments in simulating preferential solute transport in soils, would allow better prediction of spatial and temporal variation in riparian zone function. Such prediction in turn would allow improved buffer function by tailoring buffer design to site specific conditions, thereby reconciling functional and operational viewpoints. 1. Riparian Zones as Watershed Gateways for Solutes Riparian zones are the interface between groundwater aquifers, the terrestrial landscape above them (e.g., fields and forests), and downgradient surface water (e.g., streams, ponds, and wetlands) (Jencso et al., 2009; McGlynn & Seibert, 2003). As water moves toward a stream, riparian zones intercept and filter groundwater coming from aquifers during baseflow conditions but also provide an opportunity for surface runoff during storms to infiltrate and become groundwater (Cey et al., 1998; Fox, 2019; McGlynn & McDonnell, 2003). Thus, shallow and deep groundwater flow paths in riparian zones are a key gateway for solutes including nutrients and pollutants moving toward the stream network (Briggs & Hare, 2018; Groffman et al., 2002) (Figure 1a). The width of the riparian zone gateway, the magnitude of flow crossing it, and importance of baseflow to watershed outflow (e.g., baseflow index) can all vary throughout watersheds and fluctuate over time (Figure 1b) (Jencso et al., 2009; Tesoriero et al., 2013). The conventional view holds that transport of water and solutes across riparian groundwater occurs via homogeneous matrix flow (Bosch et al., 1994; Jordan et al., 1993). Yet preferential flow in riparian groundwater is common (Allaire et al., 2015). 2. Preferential Flow in Riparian Zone Groundwater While frequently not acknowledged in practice, preferential flow is widespread in soil and groundwater. Preferential flow refers to the locally rapid movement of water and solutes (Beven & Germann, 1982, 2013) through areas of higher soil matrix hydraulic conductivity (K) such as gravel veins and paleochannels (...

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