Offshore continuation of coastal groundwater systems; predictions using sharp-interface approximations and variable-density flow modelling

Analytical sharp-interface solutions and numerical variable-density flow and transport modelling are used to study how far into the offshore the influence of meteoric continental groundwater systems extends under steady state flow and transport conditions. Results show that factors which favour the development of an extensive fresh-water wedge in the offshore are: (1) a high head or fresh-water discharge at the coastline, (2) a thick, semi-confined, high-permeability, submarine aquifer, (3) optimum, but not necessarily maximum and minimum values for the thickness and permeability of the confining layer near the seafloor, respectively. Sharp-interface models tend to overpredict the seaward extent of fresh-water considerably because, in distal parts, upward seepage rates through the overlying confining layer are too small to prevent salinisation by diffusion from the overlying sea and ensuing convective mixing by density differences within underlying aquifers. The overprediction is particularly large for relatively low heads at the coastline and low permeabilities of the confining layer. Under favourable conditions, fresh water connected to continental flow systems is predicted to occur up to several tens of kilometers offshore. Comparison of model predictions with observational data demonstrates that relatively fresh, submarine pore waters off the coasts of New Jersey (US), and Suriname (South America) occur too far offshore to be explained by active flow systems, indicating that they are paleo-waters which formed during the Pleistocene when sea levels were lower than at present.