Quantitative Analysis of Transient Intertidal Submarine Groundwater Discharge in Coastal Aquifer of Western Japan

Abstract

The work assessed effect of diurnal tidal fluctuation on transient groundwater dynamics with special focus on trace metal mobilization using tide-aquifer interaction technique in the Saijo plain, Japan. Fluctuation of trace metal concentration in groundwater during intertidal phase obtained through geochemical analysis is validated with numerical simulation using two different codes (PHREEQC and SEEP/W) to observe saturation index for different minerals in aquifers and dynamics of submarine groundwater discharge (SGD) respectively. Result for saturation index shows that most of the samples are strongly undersaturated with respect to FeS, goethite, siderite and scorodite, unlike pyrite where it approaches towards saturation. Also during lower low tide situation, water samples relatively getting more undersaturated with pyrite suggesting it as a source for dissolved iron. For numerical simulation, problem domain consists of 368 elements and 3 layers, is an anisotropic unconfined aquifer with horizontal hydraulic conductivities (KX) ranging from 0.00001 to 0.01 m/P. Coastal side of the domain considered as variable head boundary keeping different diurnal tidal amplitude into account. Simulation result shows that during low tide situation, value of SGD is maximum i.e. 2.1644e−005 and 3.3704e−005 m3/s for lower boundary towards sea at 2 and 4 m below mean sea level respectively. It suggests that tidal height affects the amount and position of SGD to play a positive role for trace metal mobilization through oxidation–reduction process. Strong relation between diurnal fluctuations of the simulated results (SGD) versus observed result oxidation–reduction potential firmly supports and validates the results from chemical analysis.