Contribution of TDEM and ERT saltwater interface observations to developing decision-support seawater intrusion models

This study explored the assimilation of Time-Domain Electromagnetic (TDEM) and Electrical Resistivity Tomography (ERT) surveys in a sharp-interface model of seawater intrusion. While these models, which do no simulate mixing between freshwater and saltwater, are computationally-efficient alternatives to advective-dispersive models, guidelines for data assimilation are limited. In the Magdalen Islands (Québec, Canada), a sharp-interface seawater intrusion model was developed to support groundwater management. In addition to freshwater head observations, saltwater interface observations were defined as the elevation of the 50% seawater salinity contour and were extracted from deep, open boreholes, TDEM and ERT surveys. Parameter estimation and linear uncertainty analysis were carried out, accompanied by a data worth analysis. The contribution of the TDEM and ERT saltwater interface observations was then explored, both in constraining parameter estimation and in reducing the uncertainty of model forecasts of interest to groundwater management. While they were highly uncertain compared to all well observations, the geophysically-derived interface observations were the most useful observations to reduce predictive uncertainty.