Aquifer Salinization from Storm Overwash

William P. Anderson, Jr.

Abstract


Overwash processes are not only an agent of change to the morphology of barrier islands; they are also a source of instantaneous saltwater intrusion to coastal aquifers. Hatteras Island, North Carolina, USA is particularly susceptible to overwash processes because of its geography and the frequency with which tropical and extra-tropical storms strike the area. Hurricane Emily inundated the island in 1993 with saline water from Pamlico Sound. The floodwaters recharged the Buxton Woods Aquifer, raising salinity levels from approximately 40 mg/L prior to flooding to nearly 280 mg/L within several weeks of flooding. By 1997, chloride levels still had not returned to pre-storm levels. One-dimensional analytical solutions of the advection-dispersion equation are used to simulate chloride transport within the aquifer utilizing a pulse source implemented with linear superposition. These simulations are matched with chloride breakthrough curves. Initial simulations show that a pulse duration of five days provides the best fit to the data. Simulation of chloride breakthrough at two streamline locations demonstrates that higher gradients advect chloride further into the aquifer, causing higher chloride concentrations and increasing the duration of contamination. The Cape Hatteras region historically is susceptible to several hurricanes in a single season. In order to analyze the effect of multiple overwash events on water quality, predictive simulations show the effect of two overwash events separated by several time lags between storms. Simulations indicate that higher gradients and short time lags between overwash events result in chloride MCL violations that persist for more than four months.


Keywords


Hurricanes; Hurricane Emily; North Carolina; Hatteras Island; saltwater intrusion; storm surge; overwash; barrier island; groundwater modeling; analytical solution; coastal aquifers

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