Seiche Induced Variations in Submarine Groundwater Discharge

Submarine groundwater discharge (SGD) has been shown to vary on a wide range of time scales from seconds due to waves to seasonal changes over months. The focus of research has primarily been on tidal and seasonal variability. Tidal modulation, for example, is commonly observed with high SGD at times of low tides. It is important to recognize shorter term fluctuations, however, in the interpretation of data. Vented benthic chambers (a.k.a. "seepage meters") were deployed in Guam (Tumon Bay) in 2008. Short term fluctuations in groundwater discharge were seen that could not be explained completely by tidal influences. SGD ranged from 0.3 to 246 cm d^-1 with an average value of 60 cm d^-1. SGD was inversely correlated to the tidal elevation. Shorter period variations were superimposed on the tidal modulation and appeared to be a result of seiching. The average period for these variations is 66 minutes. The average amplitude of the changes is 27 cm d^-1, with a maximum of 153 cm d^-1. The average amplitude change corresponds to 45% of average SGD. A submersible pressure sensor was simultaneously deployed with the seepage meters. The pressure sensor had a sampling rate of two seconds. Analysis of the five days of pressure data shows energy in a frequency correlating to a period between 8 and 21 minutes. Assuming an average depth of 2 meters in Tumon Bay, a width of 500 meters and a length of 2,650 meters, the seiche period was calculated to be 4 minutes for a width dominated seiche and 20 minutes for a length dominated seiche. The apparent disparity between the period evident in discharge flow rates and the period determined by analyzing the pressure sensor record is likely due to aliasing by the sampling rate for the SGD measurements. The average sampling frequency is 0.5 mHz, which is much less than the Nyquist rate for the seiche frequencies sampled by the pressure sensor, 2 to 8 mHz. Artifacts due to seiching have been suggested in other areas. Similar phenomena can be seen in earlier data from Brazil where a seiche period of 10 to 20 minutes corresponded modulated SGD. Another study in Mallorca found evidence for nutrient fluxes and changes in plankton community structure as a result of seiche-induced variability in groundwater discharge.