SALTWATER INTRUSION FROM ABOVE: AQUIFER SALINIZATION FROM STORM OVERWASH

Water quality in coastal aquifers can be threatened not only by upconing and saltwater intrusion, but also by overwash during severe storms. Storm surge causes flooding that recharges water-table aquifers with saline water. In 1993, storm surge from Hurricane Emily inundated Hatteras Island, North Carolina (USA), with salt water from Pamlico Sound, a large estuary on the north side of the island. Winds produced by winter extratropical storms (northeasters) and hurricanes cross as much as 70 km of open water in the sound, raising water levels as much as 3 m. In extreme cases, overwash has penetrated the interior of Hatteras Island by more than one km. Flooding was most severe in the vicinity of the island's well field that exploits the shallow Buxton Woods Aquifer (BWA). Most of the floodwater eventually recharged the BWA, raising chloride levels from approximately 40 mg/L prior to flooding to nearly 280 mg/L within a week of flooding. By January 1997, chloride levels still had not reached pre-flooding concentrations.

One dimensional numerical solutions and transient simulations with the USGS finite-element model SUTRA were used to determine the spatial and temporal extent of elevated chloride concentrations within the BWA. Simulations were performed along two cross-sections: (1) Transect 1 at the western edge of the study area, where the island is 2600 m in width and nearly 50% of the BWA was inundated; and (2) Transect 2 in the center of the island, where the island is 3400 m in width and only 25% of the BWA was inundated. Simulated and field chloride levels along Transect 1 rise during the summer months in response to higher pumping rates and limited aquifer recharge. Simulated chloride concentrations return to pre-storm levels by January 1997. Modeling results are different for Transect 2 because of less extreme overwash and a larger island width. Simulated chloride levels return to pre-storm levels by January 1996 along Transect 2.