Modelling submarine groundwater discharge: an example from the western Baltic Sea

Abstract We investigate (a) the submarine groundwater discharge (SGWD) defined as the net groundwater discharge to the sea and (b) the typical characteristics of the spatial distribution of the groundwater outflow at the sea bottom. The investigation concerns the Eckernforde Bay in the western Baltic Sea. A large-scale groundwater model was established in order to model groundwater flow toward the sea. Due to insufficient field data, different scenarios were simulated in order to approximate the value of SGWD. It is found that the probable range of SGWD in the study area per kilometer of the land–sea interface is from 0.05 to 0.07 m 3 /s. The distribution of the groundwater outflow rates at two sea bottom sites (pockmarks) was investigated using two approaches. First, density effects were neglected. Under this condition, the resulting discharge distribution at one site is approximately uniform whereas at the other site it is strongly non-uniform with high outflow rates at the edges of the pockmark. These differences are due to different hydraulic conductivity distributions of the aquifer. Second, the investigation by means of a density-driven flow model shows that the main effect of the saltwater is to displace the groundwater outflow from the central part of the pockmark to its edges. The approximately uniform distribution estimated by neglecting the density effects does not reflect the conditions at the sea bottom whereas the strongly non-uniform distribution does. The strongly non-uniform distribution of the outflow rates at the sea bottom indicates that locally measured outflow rates can hardly be used for the estimation of mean outflow rates over large parts of the sea bottom.