The effect of accidental sulphuric acid leaking on metal distributions in estuarine sediment of Patos Lagoon

In August of 1998 the tanker BAHAMAS belonging to the Chem Oil Company cont aining 12000 t of concentrated sulphuric acid, had an accident on board, af ter which estuarine water entered one of the compartments of the tanker, re sulting in a vigorous exothermic reaction. The reaction of acid with the me tallic interior hull of the ship and the accompanying heat and H-2 producti on resulted in an imminent risk of explosion. To avoid an explosion, given the fact that neutralization was not possible, some of the cargo was discha rged into the surrounding water. Neutralization was done in January 1999, a fter the acid concentration in the tanker had decreased and the concentrati ons of Fe, Cr and Ni remained elevated. Metal concentrations in bottom sedi ments showed significant modifications. Leached mercury migrated and redepo sited downstream, reaching approximately 76 times the background values. Su ch an anomaly has a well expressed barrier character. The mechanism for red eposition of Hg and other metals probably followed the pattern: Downstream as a result of dilution and mixing with seawater the pH of acid-water incre ases, favouring adsorption and/or precipitation of metals. The leading edge of a geochemical barrier, at positions 7-9 of sampling sites (Fig. 1), is confirmed by pH variations in the water. The reestablishment of normal pH o ccurred after a short time due to the high buffering capacity of seawater a nd large natural dilution process. The concentration of metals in estuarine water during and after the accident showed insignificant anomalies. (C) 20 01 Elsevier Science Ltd. All rights reserved.