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 an d transport modelling are used to study how far into the offshore the influ ence 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: (I) a hi gh head or fresh-water discharge at the coastline, (2) a thick, semi-confin ed, high-permeability, submarine aquifer, (3) optimum, but not necessarily maximum and minimum values for the thickness and permeability of the confin ing layer near the seafloor, respectively. Sharp-interface models tend to o verpredict the seaward extent of fresh-water considerably because, in dista l parts, upward seepage rates through the overlying confining layer are too small to prevent salinisation by diffusion from the overlying sea and ensu ing convective mixing by density differences within underlying aquifers. Th e overprediction is particularly large for relatively low heads at the coas tline and low permeabilities of the confining layer. Under favourable condi tions, fresh water connected to continental flow systems is predicted to oc cur up to several tens of kilometers offshore. Comparison of model predicti ons with observational data demonstrates that relatively fresh, submarine p ore waters off the coasts of New Jersey (US), and Suriname (South America) occur too far offshore to be explained by active Row systems, indicating th at they are paleo-waters which formed during the Pleistocene when sea level s were lower than at present. (C) 2001 Elsevier Science B.V. All rights res erved.