Assateague Island study conclusion

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Storm Summary:

• Likely Storm overwash event occurs in March 3-9, 2017. This has large effect on conductivity in WO Ef 41, causing near-ocean conductivities (~40,000 uS/cm) during high tide. The response in well WO Ef 43 doesn’t occur until March 10 to mid-April. This may have implications for maritime trees, since well WO Ef 43 is only 2 feet below the surface but experiences conductivities nearly half that of seawater. During this time period, precipitation events (March 13, March 20) were associated with a slight increase in conductivity in well WO Ef 43.
  • During March overwash event, the temperature of well WO Ef 43 decreased while the temperature of WO Ef 41 increased. This, and the conductivity increases in WO Ef 41 suggest that the water source driving the increases in stage are different. WO Ef 41 from warmer and saltier ocean and WO Ef 43 from cooler and fresher precipitation.
    • Additionally, the temperature in well WO Ef 41 oscillates twice per day (suggesting tidal influence), while the temperature in well WO Ef 43 oscillates once per day (suggesting different source).
  • High reported elevations at tide gauges seems to drive tidal oscillation in wells on September 19-20^th
  • Numerous storms (2, 8, 10, 11, and 12 in the table above) have significant precipitation, though the tidal elevations do not exceed the ground surface elevation for Wo Ef 41 and there is little-to-no tidal signal.

Water Level Summary:

**Note: These water levels are uncorrected for variations in density. Therefore, these levels are best interpreted as freshwater head equivalents. If a well reached the density of seawater, then the true water level would be 3% lower than the fresh water head equivalent reported here**

• WO Ef 41 is much more responsive to precipitation events than the two interior wells (both shallow and deep). The well hydrograph drops off much more steeply following precipitation events.
• For much of the time, all three wells are non-tidally responsive. When the reported elevations of the Ocean City and Chincoteague tide gauges approaches the elevation of the well land surface, sometimes the water level in well WO Ef 41 becomes tidally responsive.
• As an example, November 5 through 11, the ocean surface increases above the ground surface at the Wo Ef 41 well, leading to tidal oscillations in water level (without recorded precipitation). Well Wo Ef 41 shows a tidal oscillation in conductivity as well.
  • Chincoteague Ampliude: 3.33 ft
  • Ocean City Amplitude: 1.977 ft
  • WO Ef 41 amplitude = 1.25 ft, 5 hour lag, asymmetrical shape
  • WO Ef 43 and WO Ef 42 = minimal tidal oscillation
• Looking at the cumulative distribution of water surface elevations (figure 4), the median water surface elevations for all three wells are tightly grouped. However, the WO Ef 41 well shows less variation overall than the other two wells.

Specific conductance summary:

• Shallow pine well (Wo Ef 43) is generally fresher (500 uS/cm to 10,000 uS/cm), deeper pine well (Wo Ef 42) is generally saline (>40,000 uS/cm).
• In Wo Ef 41 well, early in the study period there is a relationship between well head and conductivity. The well appears responsive to what is happening in the ocean. Then after mid-November, it decouples. This change does not occur at a site visit and seems unrelated to the water depth. It is unclear if this is somehow related to the hydrologic regime or freshwater lens.
• There are two periods when the Wo Ef 43 well (shallow well close to pine trees) increases above 5,000 uS/cm. The first is October 24 – November 25, 2017, following two large precipitation events. The second is March 12 through the end of the monitoring period. The conductivity increases sharply with precipitation events and then decreases more slowly. The exact mechanism is uncertain.

Figure 3: Cumulative Distribution of Water Surface Elevations

The water surface elevation (fresh water equivalent) exceeds the ground elevation 8% of the time for wells Wo Ef 43 and Wo Ef 42 and about 0.3% of the time for Wo Ef 41.

Narrative Summary

During the monitoring period, two primary types of responses were observed: overwash-type events, and precipitation-type events. An example of the overwash-type event occurred in early March 2018 (Figure 4). Higher-than typical tidal elevations generated by a pair of Nor’easter events from March 2-3rd and March 6th -8th, 2018 preceded increased water table elevations in the three monitoring wells on March 5th. Observed water table elevations exceeded the land surface elevation in all three wells (land surface elevation shown by dashed horizontal lines). Tidal cycling in observed water levels, and temperature increased in all three wells between March 6th and 8th. At this point in the monitoring period, the shallowest Wo Ef 43 well was consistently fresh (low conductivity) and the deeper Wo Ef 42 well was consistently saline (high conductivity) and these remained unaffected by the event. However, Wo Ef 41, which is deeper but closer to the bay and typically has a brackish water composition (medium conductivity), exhibits large tidal changes in conductivity from brackish to saline. Following the event, March 12th precipitation event and a March 20th – 22nd Nor’easter had a more muted influence on tides, water levels, and specific conductance. However, abrupt increases in conductivity in shallow well Wo Ef 43 are associated with these events.

Figure 4: Overwash Event Occurred in March 4-10, 2018 (All times are in EST)

The period of May 17th –19th, 2018 (Figure 5) is typical of a precipitation-dominated event. The characteristics of the collected timeseries data are different than the overwash-type event described above. On November 18th, precipitation exceeding 40 mm (1.5 inches) led to an increase in water level of nearly 1 foot in all three wells, causing the water level in the wells to rise above the land surface elevation. No change in the daily tidal elevations, temperature, or specific conductance is associated with the precipitation. In some precipitation-dominated events a slight flushing effect (decrease in specific conductance) is also observed.

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