Numerical Study on Tide-Driven Submarine Groundwater Discharge and Seawater Recirculation in Heterogeneous Aquifer

Abstract:

Numerical modeling was performed to quantitatively study the influences of various factors on submarine groundwater discharge (SGD) in a coastal aquifer. Tide and terrestrial hydraulic gradient are the important forces driving fresh and salt water movement in a coastal aquifer. Unlike steady-state flow, dynamic fresh and salt water mixing at near-shore seafloor could form an intertidal mixing zone (IMZ) near the surface. Based on the construction of a general SGD model, the effects of various model components such as boundary conditions, model geometry and hydraulic parameters on SGD were systematically studied. Several important findings were obtained from the study results. (1) Previous studies indicated that a freshwater discharge tube could be formed between classic transition zone and IMZ. However, this phenomenon might become unclear with the increase of heterogeneity and anisotropy of the medium’s conductivity field. (2) SGD and IMZ were both more sensitive to the vertical anisotropy ratio of hydraulic conductivity (K_x/K_z) than to the horizontal anisotropy ratio of hydraulic conductivity (K_x/ K_y). (3) Heterogeneity of effective porosity significant affected SGD and IMZ. (4) Increase of storage coefficient decreased fresh water discharge while increased mixing salt water discharge and total SGD increases. The increase would also change the shape of the IMZ. (5) Variation of dispersivities had little influence on SGD, but would significantly change the distributions of the IMZ and the whole mixing zone. These findings will be helpful to the field study of SGD on sampling design and the application of dynamic SGD model to field sites for model development and calibration.