Drilling discharges as tracer of sedimentation and geochemical processes in Adriatic Sea.

A sedimentological and geochemical study was carried out on a continental shelf affected by water-based drilling fluid and cutting discharges from an offshore platform. Investigation of the distribution of the cuttings and drilling mud over time and space in proximity to the dumping site provided information on mechanisms of sedimentation, including resuspension and redistribution of the natural and anthropogenic sediment in this marine area. The study area is at a depth of about 15 m in the Northern Adriatic Sea. Surface and subsurface sediment samples were collected during six surveys from 1985 to 1993 by box-corer, altogether 201 box-corer were collected. Grain size, total Ba and Cr content and chemical bonding between Cr and the solid matrix were determined from sediment samples. Coarser sediment fractions, Ba and Cr can be considered as tracers of the discharged drilling sludge. Furthermore they provide information on sedimentation and resuspension processes and are good detectors of the main seasonal, annual and multi-annual sea bottom currents in the investigated area and on continental shelves in general. They showed that local marine hydrodynamics were modified by the platform structure that induced higher turbulence in its vicinity. Out of the influence of the platform these tracers highlighted the presence of drift and tidal bottom currents, directed parallel to the coastline, and storm currents acting mainly perpendicularly to the coastline. Partitioning of Cr in the solid phase showed an ageing process of chemical bonds that confirm the repeated and continuous resuspension and resedimentation processes of the fine sediments.