Numerical modeling of salinity distribution and submarine groundwater discharge to a coastal lagoon in Denmark based on airborne electromagnetic data

The beneficial use of large-scale geophysical surveys in combination with numerical modeling for assessing water resources problems in coastal areas is demonstrated. A 5,000-year long historical evolution of the regional distribution of salinity beneath a coastal lagoon in Denmark is simulated in a stage-wise approach using a two-dimensional variable-density flow and transport model and compared with an interpreted resistivity distribution from transient electromagnetic data. A sequence of multi-layer unconfined/confined aquifers with non-continuous aquitards is needed to match observations in terms of complexity in resistivity/salinity distribution, deep-seated low resistivity zones (trapped residual saltwater), and presence of groundwater discharge tubes with high resistivities indicating both near and off-shore discharge of fresh groundwater. Refreshening of the lagoon system is ongoing and simulations show that this process has been most rapid during the last similar to 300 years, but will continue at a slower rate for the next many hundreds of years. The development of the lagoon over the last 5,000 years, the associated changes in salinity and the present-day control of lagoon salinity are responsible for these processes. Finally, simulation results show that the groundwater influx to the lagoon is significant. The estimated fluxes correspond to 168 % of net precipitation on the lagoon or 17 % of the discharge from the largest river into the lagoon.