Intertidal mixing zone dynamics and swash induced infiltration in a sandy beach aquifer, Cape Henlopen, Delaware
Date
2011
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Publisher
University of Delaware
Abstract
Submarine groundwater discharge has been shown to be an important source of nutrients, heavy metals, and organic compounds to the coastal ocean. Physical flow and mixing dynamics in the intertidal zone may influence these contaminant fluxes; however the mechanisms that contribute to the spatial and temporal dynamics of mixing of saltwater and through-flowing freshwater are not well understood. Flow systems in two shallow beach aquifers at Cape Henlopen, DE were characterized over multiple time scales using multilevel porewater samplers, pressure transducers, and moisture sensors. Porewater samples indicate the presence of a saline groundwater circulation cell beneath the beachface that is formed by seawater infiltrating across the upper beachface, which then circulates downward and seaward before discharging farther offshore. The circulation cell deepens and its areal extent increases when forced with a larger tidal amplitude. Longer term monitoring shows that higher inland water tables during spring and the resultant increase in freshwater forcing cause the circulation cell to freshen while lower inland water tables associated with winter result in a well-developed density inversion. A study of swash-induced infiltration was performed to quantify the effects of swash zone width and tidal elevation on the flux of seawater into the beach aquifer. High-frequency pressure and soil moisture measurements from shore-perpendicular arrays across the beachface were used to infer influx rates. Direct measurement of moisture content in the unsaturated zone and capillary fringe is needed to infer fluxes due to moisture conditions above the water table that are not measured by the pressure sensors. Infiltration occurred during the rising tide at the leading edge of the swash zone and increased in magnitude from low tide to high tide. Infiltration rates were on average 2.2 times greater near mean higher high water than near mean lower low water. Measurements revealed that swash zone width influences infiltration: influx rates associated with a beach with a wider swash zone were 2.7 times higher than those of a beach with a narrower swash zone. These insights into beach groundwater dynamics have implications for estimating fluid and chemical fluxes to coastal waters.