Temporal and spatial variability of sediment saturation and patterns of groundwater-surface water exchange in the intertidal zone at swash and tidal time scales

Abstract:

The swash zone on sandy beaches is a highly energetic and dynamic region of the coastal zone where wave runup and rundown and underlying subsurface flow result in groundwater-surface water exchange. Fluid flow across the sand surface within this zone is important to the biogeochemistry of coastal aquifers and the mobilization and transport of sediments and potentially contaminants along the coastline. Despite the importance of groundwater-surface water interactions in the swash zone, coupling between surface and subsurface flow is not well understood and spatial and temporal patterns of vertical flow direction and magnitude across the beachface remain unclear. Simultaneous high-frequency measurements of the position of the swash edge, the boundary between the saturated and unsaturated beachface, sediment saturation, and water table elevation were collected on a moderate-energy sandy beach. The measurements provide a unique dataset linking swash zone forcing to groundwater flow. Swash infiltration across the unsaturated beachface leads to an accumulation of water in the unsaturated zone and forms a stable lens of partially saturated sediment that serves as a source of water to the water table. The positions of infiltration, recharge, and discharge zones on the beachface were temporally and spatially variable and were distinguishable only through combined use of surface and subsurface measurements. The zone of infiltration was controlled primarily by swash processes while the width and location of the groundwater discharge zone on the beach surface was controlled by the tide. Saturation dynamics, time scales of flow, and the position and extent of infiltration, recharge, and discharge zones may be important considerations when estimating fluid, solute, and sediment fluxes to, from, and within the intertidal zone.