EXTENT OF TIDAL FLUCTUATIONS ON GROUNDWATER LEVELS ON A BARRIER ISLAND

Groundwater levels in the surficial aquifer on Bogue Banks barrier island (North Carolina) were monitored over a 7-month period with the aim of assessing the extent of tidal fluctuations on the water table. A total of 29 groundwater monitoring wells were drilled into the Surficial aquifer across the island. Sediment cores were retrieved from each well, whereas automated water level loggers were used to monitor groundwater levels in 17 of the 29 monitoring wells at a sampling interval of 10 minutes. The sediments derived from the cores were predominantly unconsolidated sand with lenses of clay and peat. The groundwater wells were installed at various depths (2 m - 8 m), elevations (0.3 m - 7 m amsl) and distances from the ocean and sound (10 - 600 m). Over the analysis window, data from a nearby tidal gage indicated that Bogue Banks experienced semidiurnal tides with an amplitude as high as 50 cm, and a period of 12 hours. Water table amplitudes acquired from detrended groundwater level data were then compared to the tidal amplitude to assess the extent of tidal variations on groundwater levels.

The results reveal that water table amplitudes recorded in all wells were no larger than 4 cm. In general, the wells closest to the ocean and sounds (< 65 m) experienced the largest amplitudes, whereas the wells farthest away from the large water bodies (>200 m) experienced the smallest amplitudes. However, the wells located between 65 m and 200 m away from large water bodies experienced a mix of both large and small water table amplitudes. These results suggest that heterogeneities in the Surficial aquifer may affect the extent of tidal fluctuations in the aquifer. This study will therefore be useful for assessing hydraulic properties of the Surficial aquifer on Bogue Banks barrier island.