PHYSICAL, CHEMICAL AND BIOLOGICAL INDICATORS OF GROUNDWATER
SEEPAGE THROUGH THE SEA BOTTOM (SOUTHERN BALTIC, POLAND)
H. PIEKAREK-JANKOWSKA
Institute of Oceanography, University of Gdansk, 81-378 Gdynia, av.Pilsudskiego 46, Poland
e-mail: ocehj@univ.gda.pl
Groundwater seepage from Cretaceous, Tertiary
and Quaternary subbottom aquifers
induces various physical-chemical alterations in the
marine environment of the
Gdansk Basin.
The groundwater seepage decreases the chlorinity
of interstitial waters with vertical gradients from 0.01
to over 0.05 g dm-3 cm-1. The comparison of the
oxygen and hydrogen isotope composition
of underground, interstitial and near-bottom waters
indicated a reduction of the content of heavy isotopes
in the interstitial water and thus gave evidence
of fresh water seepage through the sea floor.
Anomalies observed in the thermohaline and
chemical stratification of the water column indicate a
decline in salinity and increase in temperature in the
near-bottom layer. Turbulent convection zones are
found over the seepage area, with a Rayleigh
thermohaline number in the range 1015 - 1016. Owing
to strong thermohaline convection, the pycnocline is
forced and an intrusion of more saline water ascends
into the less saline surface layers. Simultaneous with
the observed convection effects, a redistribution
of the accumulated chemical components below the
pycnocline can be seen in the water column.
Biological indicator of groundwater discharge is
occurrence of the typical fresh water diatoms in the
surficial sediments.
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