Modelling saline water migration during dewatering of excavation sites; the role of density-driven flow during risk-assessments of upwelling saline water

Abstract

In coastal areas, aquifer salinization is a major risk during large-scale dewatering operations. This risk is enhanced when extraction wells are used to pump large amounts of water from coastal aquifers. Underneath an extraction well, the salt-fresh water interface may rise towards the surface. To prevent salinization of the sub-soil, the rising salt cone needs to be analyzed, and quantified. To predict the amount and extent of the saline water rise, multiple numerical and analytical methods are available. This report will assess analytical and numerical solutions to solve this problem, and to review which method suits best for each operation. To solve the numerical solutions, a modelling study will be carried out using the MODFLOW code, and MODFLOW extensions: MT3DMS and SEAWAT2000 v4. These methods will be compared with a focus on (1) the role of density-dependent flow and (2) the effects of dispersion to the mixing zone. A method will be provided that uses the numerically light MT3DMS code to predict results of the numerically heavier SEAWAT2000 v4 model. Using this method, time can be spared when assessing whether the risk of salinization is significant or not. A correction factor for the MT3DMS model results can be applied, which is dependent on the vertical distance of the saline water to the extraction well.