Preliminary observation of complex salt-fresh water mixing in a beach aquifer

Continuous observations of beach groundwater salinity over a 35-d period from a monitoring well established in the intertidal zone of a coastal harbor provided intriguing data on the interaction in the intertidal zone between the salt and fresh groundwater. During the monitoring period of the study, both semidiurnal variations and longer temporal trends in groundwater salinity were observed. The semidiurnal salinity variations were observed to occur nearly synchronously, but inconsistently with the tides. However, the salinity relationship with the tides was more complex, switching back and forth from being in-sync (higher salinities at high tide) to out-of-sync (higher salinities at low tide) a total of four times during the 35-d test period. The longer temporal trends showed chloride concentration (representing salinity) varying from as low as 50 mg/L to as high as 3600 mg/L over a period of between 9 to 12 d. The observations from the monitoring well reveal a complex pattern likely resulting from a combination of tidal pumping, density-induced convection, and changes in the terrestrial hydraulic gradient. However, these observations are based upon data from only one monitoring well, and are speculative at this point. A more thorough study of the complex fresh water-saline water relationship in the intertidal zone seems to have merit.