ABSTRACT Barrier islands, along the mid-Atlantic coast, are subject to some of the fastest rates of geomorphic change in North America (Kochel and Dolan, 1986). Parramore Island, located in the Virginia Coast Reserve (VCR), was chosen for study because of its high concentration of distinct landforms called "pimples." Pimples are sedimentary domes that are topographically higher than the relatively flat barrier island. Pimple topography permits the development of a fresh-groundwater lens providing a supply of water for the surface vegetation. The groundwater lens exhibits a salinity gradient which increases linearly with depth as a result of daily mixing with ocean water (Hayden et al., 1992). Raised salinities during storm surges contaminate the fresh water lens. Therefore, vegetation is dependent on rainwater, which is the only source of replenishment to the fresh water capsule.

This research attempts to determine the impact of natural overwash events on the hydrology of the freshwater lens and the vegetative patterns of the southern most pimple on Parramore Island, called the "dying pimple." It has acquired its name because of rapid erosion and transgression bringing the ocean closer to its eastern rim. The pimple will eventually be affected by a permanent saline intrusion killing its fresh water vegetation.

The southern most pimple on Parramore Island (located approximately 18 to 21 meters from the mean high water shoreline) was selected for study. The topographic zonation of vegetation was expected to correlate with the profile of the fresh water lens, which approximates the topography of the pimple (Schneider, 1984). A unique characteristic about the southern most pimple is that it sporadically experiences a saline intrusion into its fresh water lens which is killing its vegetation and will eventually drive the pimple into extinction. As a result, the center of the pimple is depressed due to the lack of plant root systems which would prevent aeolian erosion.

The main objectives of this research were: (1) to determine the effect of macro- meso- and micro-scale salt water intrusions on the underlying freshwater lens of the pimple and the corresponding influence on vegetation; and (2) to determine the nature of the hydrodynamic variations in the pimple. The area of the pimple was partitioned into three topographic compartments for comparison: front, middle, and back, progressing away from the ocean.

Water table levels, measured in a network of monitoring wells, varied inversely with salinity. Groundwater levels in the pimple varied significantly throughout the summer at three temporal scales of influence: (1) a micro-scale which appears to be controlled by the daily respiration and transpiration cycles of the vegetation on the pimple; (2) a meso-scale dominated by the monthly tidal cycle; and (3) a macro-scale, controlled by seasonal overwash events during hurricanes and other major tropical storms. The micro- and meso-scale influences occurred on a regular basis whereas the macro-scale influences were episodic and irregular in nature.

Between major storms of the summer, there were significant changes in the elevation of the water table driven by the dynamics of the precipitation and evapo-transpiration of the vegetation communities. When water levels declined, it was generally associated with hot weather. Thus evapo-transpiration appeared to be the likely cause. Daily transpiration cycles of the vegetation in the pimple caused systematic fluctuations in the water table of the pimple. During the sunlight hours of the day, the vegetation photosynthesizes at maximum rates, thus transpiration was maximal during the day. Nighttime respiration decreased significantly because of the lack of sunshine to promote photosynthesis allowing the water table to recover. These processes oscillated the freshwater lens significantly.

The monthly astronomical tidal cycles caused the meso-scale influences on the water table levels experienced during a spring tide and neap tide. These varying tidal ranges cause the water table boundary to oscillate on a bi-weekly cycle.

The third scale of influence on the water table levels were the major episodic overwash events. The fall months on the mid-Atlantic coast of the United States can be subject to severe tropical storms and hurricanes. The fresh water lens underlying the pimple was affected by overwash events due to the intrusion of saline water. Precipitation events also had a significant affect on the water levels. The precipitation events did not have as large an influence on the water table as did the overwash events. The magnitude of water table elevation change can be seen in Figure 1.

Continued saline intrusions from overwash events will soon destroy the pimple. The vegetation will die because it can not tolerate high salinities. The low densities of vegetative root systems will make the pimple more susceptible to aeolian erosion, or blowout, which has already resulted in the depressed pimple interior.

The rise and fall of the pimple's water table varied inversely with changes in salinity. Although the magnitude of change varied across the pimple compartments, it followed the same general patterns over time. The front compartment wells had higher salinities and water levels than the back compartment because the freshwater lens dips toward the base sea level. The salinity in the four topographic regions followed the same patterns as the pimple compartments over time (Figures 1 and 2).

The Long Term Ecological Research Project describes the shape of the fresh water lens as following the shape of the topography of the pimple (Hayden et al, 1992). The pimple in which their project concentrated on however, did not have a depressed topographic center. The LTER pimple is further landward than the pimple of study and has not yet been exposed to oceanic overwash and saline intrusion. The LTER pimple remains active with its vegetation growth and fresh water lens development and has not experienced aeolian deflation of its center. The convex topography of the LTER pimple, similar to that of a sand dune, supports a convex fresh water lens. The pimple of the present study however, is subject to erosion and transgression which have significantly lowered the vegetation density. Continuous overwash events contaminate the freshwater lens with salt water which the vegetation can not tolerate. As the vegetation is being destroyed, there are fewer binding agents to prevent aeolian erosion deflation. The center of the study pimple is depressed and the water table has adjusted to mirror the concave topography (Figures 3 and 4).

Hayden, B.P. (1992) VCR-LTER Proposal No. University of Virginia.

Kochel, R. C. and Dolan, R., 1986. The role of overwash on a mid-Atlantic coast barrier island. Journal of Geology, 94: 902-906.

Schneider, R.L. (1984) The relationship of infrequent oceanic flooding to groundwater salinity, topography, and coastal vegetation. Master's Thesis, University of Virginia.

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