Submarine seepage of fresh groundwater: disturbance in hydrological and chemical structure of the water column in the Gdansk Basin

Anomalies observed in the thermohaline and chemical stratification of the w ater column in the Gdansk Basin indicate a decline in salinity and an incre ase in temperature in the near-bottom layer. We suggest that these disturba nces were caused by fresh groundwater drainage from Cretaceous water-bearin g horizons. Turbulent convection zones are found over the seepage area, wit h a Rayleigh thermohaline number in the range 10(15)-10(16). Owing to very strong thermohaline convection, the pycnocline is forced and an intrusion o f more saline water ascends into the less saline surface layers. Simultaneo us with the observed convection effects, a redistribution of the accumulate d nutrients below the pycnocline can be seen in the water column and an imp rovement of oxygen conditions in the near-bottom water. (C) 1999 Internatio nal Council for the Exploration of the Sea.