Explicit consideration of preferential groundwater discharges as surface water ecosystem control points

Abstract

| BACKGROUNDHeterogeneities in sediment and rock permeability induce preferential groundwater flow from the scale of pore networks to large basins. In the unsaturated zone, preferential flow is frequently conceptualized as an infiltration process dominated by macropores, resulting in stronger delivery of surface-derived solute than would be predicted via diffuse percolation alone (Beven & Germann, 2013). In the saturated zone, preferential flow occurs in bedrock fractures and karst, along geologic contacts and fault zones, and through unconsolidated materials of relatively high connectivity (Winter, Harvey, Franke, & Alley, 1998). Focused flow paths emanate on the land surface as preferential groundwater discharges, observed throughout stream, lake, wetland, and estuary systems. The prevalence, and perhaps dominance, of spatially focused discharges to surface water contrasts with the spatially diffuse flow often assumed in various conceptual and predictive process-based models. This simplification is not made out of an unawareness of preferential groundwater discharge; rather, the ability to reliably measure focused flow across a range of scales is hampered by a reliance on (relatively) sparse point measurements. Additionally, realistic distributions of <1-to 100-m-scale preferential groundwater discharges are computationally expensive to simulate at scales relevant to decision making. If we accept that preferential discharge of groundwater to surface water is an ubiquitous process, fundamental questions facing contemporary hydrogeology include (a) When does spatially focused groundwater discharge matter to the process we would like to predict? Followed by (b) If we determine when preferential discharge "matters" and should not be simplified to diffuse inflows, how do we measure it at the spatial and temporal scales needed to inform process-based models?A lopsided emphasis on hyporheic exchange processes has dominated groundwater/surface water exchange research for the past 15 years, as reviewed thoroughly by Boano et al. (2014) and Cardenas (2015). Within this body of research, there is growing recognition of the need to characterize surface water "ecosystem control points" that exert strong influence on the physical exchange and reaction of elements throughout river networks (Bernhardt et al., 2017). However, the process of preferential groundwater discharge is often not explicit in these discussions. The more universal definitions of hyporheic flow involve bidirectional exchange of surface water with near-stream saturated sediments. This definition is distinct from unidirectional preferential groundwater discharge to the land surface. The streambed is often incorrectly characterized as a zone of substantial mixing of groundwater and surface water; however, multiple studies indicate that groundwater and surface water mixing is limited to relatively narrow subsurface convergence zones (e.g., Hester, Young, & Widdowson, 2013). Popular stream-based transient storage models conceptually reduce groundwater discharge...

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