Temporal variations of groundwater salinity and temperature in a tidal flat in front of a tide pool

Abstract

A tidal flat in an estuary is a complex hydrological system, which is characterized by interactions between surface water in river and groundwater and is particularly driven by tides. Small-scale variability in the discharge or inflow could lead to variable results of surface groundwater salinity and temperature. In particular, there is a high possibility that a hydraulic head difference due to the presence of a tide pool, lagoon, or seep would cause the generation of small-scale spatial submarine groundwater discharge.This study investigates the spatio-temporal variations in surface groundwater salinity and temperature (0–50cm depth) and the groundwater table in a tidal flat in the presence and absence of a tide pool. A tide pool formed in the Ota River diversion channel at the study observation site following the construction of a masonry revetment in the intertidal zone. We established observation sites at three locations to consider the effects of the presence or absence of a tide pool. Specifically, we measured the surface water in river and groundwater salinity, temperature, and level in the presence and absence of a tide pool in 2007 and 2009. Reviewing the past data based on these results, we found the characteristic variation of groundwater salinity around the tide pool during flood event in 2004. Groundwater salinity and temperature were directly measured by setting conductivity-temperature meters in the tidal flat.We conclude that the groundwater table in the presence of the tide pool was 20cm higher than in areas where no tide pool existed. The temporal variation of groundwater salinity in the presence of the tide pool was 5psu lower than those where a tide pool was absent. Moreover, we confirmed the increase in groundwater salinity up to 8psu in the tidal flat during flood, when river water salinity was at 0psu and groundwater salinity in the tide pool was at 10psu.We consider that the high groundwater table, the low salinity, and the increase of groundwater salinity were caused by water discharge from the tide pool, and that the groundwater salinity was increased by groundwater discharge during a flood event.We expect the groundwater flow around masonry revetment has important roles of substance transportation and formation of tidal flat environment. The consideration of this phenomenon will be important for construction of artificial tidal flat and improvement of tidal flat environment.