Water-injection dredging: environmental hazard related to remobilization of bioavailable compounds of heavy metals.

GOMEZ, EDUARDO ALBERTO; GRECCO, L. E.; BOTTÉ, S.; MARCOVECCHIO, J. E.; CUADRADO, DIANA G.

Rio de janeiro

Congreso; Congresso Brasileiro de Oceanografia ? CBO-2012; 2012

CBO2012

At the northern coast of the Bahia Blanca estuary it is located the most important deep-harbor system of Argentina, as well as petrochemical and fertilizer industries and populated cities. Great part of the 100 km long navigation channel has to be periodically dredged by means of cutting-suction dredging to maintain navigational depths, while depths at mooring sites are maintained by water injection dredging. At estuaries, pollutants usually concentrate where clay-silty sediments settling takes place: on tidal flats and within the mooring sites. Several samples from these areas located close and faraway from man-influenced zones are analyzed. The total heavy metal contents were under the established hazardous levels in all the analyzed samples when compared with those established by the NOAA Screening Quick Reference Tables. Nevertheless, those potentially man-influenced areas (harbors, industries, cities) show highest, total concentration values as well as percentages of bioavailable fractions, which evidence a low but noticeable anthropogenic input into the environment. However, such impact shows not to be extensive as not far away from the sources, bioavailable compounds are within the order of those in areas without human influence. The relative high proportion in which Cd, Pb, Zn and Cr appear as bioavailable compounds at some sites not-influenced by human activities, indicate a natural source for these elements. It was linked to the weathering of naturally occurring primary minerals (olivine, piroxenes and amphiboles) and volcanic glass coming from the Central Andes and carried to the Atlantic coast by rivers and winds. However, those mooring sites where sewage outfall takes place exhibited almost the greatest concentrations of heavy metals as bioavailable compounds measured in the estuary.  As these places are dredged by water injection twice a year, moving these pollutants and potentially affecting more extensive areas of the estuary, it is highly recommended to relocate these sewage outfalls to places where physical disturbance as resuspension by dredging does not occur.