Characterization of Fluid Flow and Seawater Infiltration Through a Beach Saltwater/Freshwater Mixing Zone, Cape Henlopen, Delaware

There is increasing evidence that sandy beaches serve important biogeochemical roles in limiting the discharge of nutrients through groundwater pathways to coastal ecosystems. Seawater, carrying fine and organic-rich particles, infiltrates the beachface due to wave swash and tidal forcing. These particles enter a saline recirculation cell and mix with through-flowing freshwater containing nutrients derived from upland sources, potentially driving denitrification and other diagenetic reactions. Thus, beaches can serve as natural biogeochemical sand filters that mitigate eutrophication of coastal and estuarine ecosystems. The flow systems in two beachfaces at Cape Henlopen, DE, were studied to characterize seawater infiltration caused by tides and waves and the subsequent subsurface mixing between freshwater and seawater. The variation in the subsurface salinity distribution over semidiurnal and spring/neap cycles was characterized at these sites. In addition, pressure and moisture sensors were buried in the beachface to quantify seawater infiltration due to waves.  The circulation cell experiences only very minor changes in the course of a day, but the cell freshens during neap tides compared to the spring tide period.  The depth and areal extent of seawater infiltration into the beach through wave and tidal action can be determined from analysis of the pressure and moisture fields and provide the basis for calculating longer-term and average infiltration rates. Coupled with reactive particle characterization, this approach will help to determine the role of sandy beaches in moderating nutrient loads to estuarine and coastal ecosystems.