UNEXPECTED PHASE ADVANCE IN SALINITY RELATIVE TO WATER LEVELS IN A TIDALLY INFLUENCED COASTAL AQUIFER, HAWAI‘I

Groundwater-fed pools and ponds in Kaloko-Honokôhau National Historical Park on the Island of Hawai‘i support a diversity of plants and wildlife, including threatened and endangered species. Groundwater levels and salinity are monitored in three observation wells in the park to detect long-term trends (years to decades). Data from these observation wells are useful for evaluating potential changes in groundwater conditions near the pools and ponds, which are commonly located adjacent to or within a few hundred meters of the coast. Analysis of continuous groundwater-level and salinity records from the observation wells revealed interesting and counter-intuitive dynamics within the highly permeable coastal groundwater system. Groundwater levels and specific conductance (a measurement of salinity) in the park fluctuate with the ocean tide and changes in air pressure. Of interest was the observation that peak salinity and peak groundwater levels do not occur simultaneously in individual wells. An unexpected phase advance in specific conductance relative to groundwater levels was observed in all three observation wells. The phase advance is a few hours in the wells. A simple analytical model of tidally influenced groundwater flow provides some insight into the phenomenon.