4!!!9zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Pergamoss PII: S0043-1354(97)00186-3 Wat.Res,Vol.32,No. 1,pp. 193-199,1998 (CJ1998ElsevierScienceLtd. Allrightsreserved Printed in Great Britain 0043-1354/98$19.00+0.00 OXIDIZING EFFECT ON METAL REMOBILIZATION AND DAPHNIA SIMILIS TOXICITY FROM A BRAZILIAN RESERVOIR SEDIMENT SUSPENSION PAULO SERGIO MARTINS de CARVALH ‘, ELIETE ZANARDI, a SANDRA VALERIA BURATINI, MARTA CO DE LAMPARELLI and MAGALI COIMBRA MARTINS CETESB(CornpanhiadeTecnologiade SaneamentoAmbiental),Dah–Divisaode Ecossistemas,av. Prof. Frederico Herrnann Jr, 345, ,%o Paulo 05489-900,Brazil (Received July 1996; accep~edin revisedform June 1997) Abstract-Anoxic sedimentsuspensionassays were performedon BillingsReservoir(SP-Brazil)samples. The effectof oxidationon metal remobilizationcompared to control suspensionsmaintained in reducing conditions was evaluated. Suspensionsamples from the oxidizingand reducing system were sampled during a 15-dayperiod. Sampleswere centrifugedand filtered, and the water phase obtained was used in Daphnia similis 48 h toxicitytests and metals analysis.The reducingsystem(Eh < –65 mV,pH - 6.5) indicated high Fe and Mn values in water with toxicity ranging from 3 to 11.8toxic units. In contrast, in the oxidizingsystem (Eh -500 mV,pH < 3.0),Zn,Cd,Cr,Ni andPbweremobilizedondays7–15, corresponding to a drasticincreasein toxicity(357toxicunits).Thesedata indicatethat laboratory oxygenationofhighlyacidproductive field-collected sedimentscausesincreasedmetalmobilizationand toxicity,suggesting that similarresponsesmayoccurin the fieldafterdredgingactivitiesor natural resuspension ofsediments.~ 1998EleevierScienceLtd.Allrightsreserved Key words-metals,sediment, oxidation,remobilization, Daphnia toxicity,reservoir INTRODUCTION Since the recognition that sediments may be considered as not only sinks but also sources of potentiallyhazardous chemicalsto the water column, many authors have addressedproblemsrelated to the remobilization of metals from sediments and the main factors controllingthis process(Forstner, 1987; Salomonset al., 1987;Calmano et al. 1987).Among the important high-energymechanicaldrivingforces of this process are dredging activities and resuspen- sion processes(Forstner et af., 1989).Contaminated dredgedsedimentsare depositedon spoilingareas on Mnd or directly reintroduced in the water. As a consequence,the contact with air or oxygenouswater causes a change from reducing to oxidizing conditions, which may cause metal remobilization (Salomonset al., 1987;Zhuang et af., 1994).In this process, two master variables appear to dominate: pH and redox potential (Calmano et al., 1993). To simulate resuspension and oxidation of a freshwater anoxic, metal contaminated sediment, a laboratory experiment based on sediment suspen- sions was designed to evaluate the short-term mobility of Cd, Cu, Co, Cr, Fe, Mn, Ni, Pb, V and Zn and the possibleacute biologicaleffectsgenerated *Author to whom all correspondence should be addressed [Fax: 55-11-30306402, E-mail: paulom@cetesb.br]. in the water phase, compared to control suspensions maintained in reducing conditions. Although these kinds of experiments reduce the complexityof natural sediments, they enable easier investigation into relevant processes and clear-cut interpretation of experimental results (Wallmann, 1992)and can be used for the study of short-term metal remobilizationeffects(Forstner et al., 1993).In weakly bufferedsediments, an increase of the redox potential may lead to a decrease of the pH value, which has consequencesin increasing nobilities of most heavy metals (Calmano et al., 1993). Data addressing this issue on sediments from freshwater systems are lacking (Forstner et al., 1989), even though sedimentsand water from theseenvironments have characteristically lower buffer capacity than those from marine and estuarine systems(Stummand Morgan, 1981), which are much more studied (Salomonset al., 1987).The acid-producingpotential of a sediment should be characterized (Calmano et al., 1993),as this parameter may be used to predict possiblemetal remobilizationproblemsthat wouldbe derivedfrom oxidation of anoxicmetal contaminated sediments. Study site Billings Reservoir was built in the 1920s for electricitygeneration and has been receivingsewage 193