Tide-induced Groundwater Level Fluctuations In Coastal Areas

Coastal zone is the active and intensive area of human activities. The interaction between ocean and groundwater affects the groundwater resources and ecological environment of coastal area directly, and it includes tide-induced groundwater fluctuation. The study area is located（Beihai city, Guangxi） in the northern coast of the Haicheng zone. An unconfined aquifer and three confined aquifers underlie the study area. The aquifers are related to each other through clearstory and leakage layers that form the aquifer leakage system consisting of the coastal unconsolidated media. The groundwater fluctuation is influenced mainly by the tide. The groundwater fluctuation characteristics induced by the tide are described in the thesis. They mainly include similarity, period, hysteresis and attenuation. At the same time, the imitation of water level fluctuation and the ratio of storage coefficient and transmissivity are discussed in the article.The study results show that there exists strong similarity between groundwater level fluctuation and the tide. The period of tide and the coastal groundwater is calculated according to the method of the spectrum analysis, they have three periods. The approximate value respectively are 341 h（14.2 d）、24.6 h、12.3 h. The time of coastal groundwater lag tide is discussed through the relevant analysis. The time lag of water levels at observation wells SK1-1, SK1-2, SK1-3, SK1-4, SK2 and SK3 to the tide are 2.75 h, 2.5 h, 2.5 h, 4 h, 5.5 h and 7 h, respectively. The time lag of water levels at observation wells increases roughly linearly with the distance from the coast. The equation is xL0076.0t =. The tidal coefficient of six observation wells are 0.0926, 0.1268, 0.1596, 0.0509, 0.08 and 0.05, respectively. The tidal coefficient roughly decreases exponentially with the distance from the coast. The equation is TE-e^-0.004x.The trend item is simulated and predicted by a linear equation, the data removed the trend item is simulated with the Fourier progression. The sum of them is the mathematics model which reflects the fluctuation curves perfectly. Three tidal methods which estimate the ratio of storativity to transmissivity are presented in this article, including those with one observation well, two observation wells and multiple observation wells. Six observation wells in the coastal aquifer near Beihai are used to illustrate the application of these methods. The estimated ratio of storativity to transmissivity of the confined aquifer ranges from 0.39×10^-6 to 5.25×10^-6 d/m².