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 ∼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.