UNDERSTANDING TIDAL EFFECTS ON THE BIG PINE KEY FRESHWATER LENS WITH TIME-LAPSE ERT

Big Pine Key is one of the few islands in the Florida Keys that retains a freshwater lens year-long. The fresh groundwater of the island is located within two major freshwater lenses that have been determined to change in size from the wet season (June-November) to the dry season (December-May). Previous researchers have observed tidal changes in the well water level by as much as 1 foot close to the coast. These tidal changes have not been quantitatively measured before due to the difficulty of measurement and variability of the tides in the Florida Keys. Electrical geophysical methods can be used for mapping groundwater in island settings due to the contrast in conductivity of the freshwater and saltwater. The method provides a great deal of information while being minimally invasive.

Using electrical resistivity tomography (ERT), hourly measurements were taken over a 12-hour period on a 54-m-long profile, extending inland from the shoreline to monitor the tidally driven changes at the edge of the lens. Measurements of water level, temperature, and conductivity were made throughout the day in shallow wells adjacent to the profile. The experiment was conducted on two separate days, in July 2011 and in May 2012. Both surveys were conducted over the same time in the tidal cycle. ERT data were modeled using a time-lapse inversion scheme. For July 2011, the resistivity models show an abrupt movement of fresh water towards the shore over the period of an hour as the tide level decreased. In contrast, the May 2012 profile showed little change. The different results may be due to variations in groundwater level and precipitation. The water level observed in the wells in May 2012 was higher than the water level observed in July 2011. In addition, there was twice as much rain in the two months prior to the May survey compared to the survey in July. The results suggest that the tide has a larger influence on the saltwater/freshwater mixing at the edge of the lens under drier conditions.