THE RELEASE OF HEAVY METALS 

 FROM REDUCING MARINE SEDIMENTS 



Michael L. Bender 



Richard J. McCaffrey 



J. Douglas Cullen 



Graduate School of Oceanography 



University of Rhode Island 



Kingston, Rl 02881 



ABSTRACT 



We address the hypothesis that metals forming soluble sulfides are released 

 from nearshore anoxic sediments, while those forming insoluble sulfides are 

 retained. As a test, we have studied pore water chemistry, benthic fluxes, and 

 water column distributions of heavy metals in Narragansett Bay, Rhode Island. 

 The results show that metal forming soluble sulfides (Mn and Fe) have high 

 pore water concentrations and are released to the Bay waters, while metals 

 forming insoluble sulfides have low pore water concentrations and negligible 

 benthic fluxes. 



INTRODUCTION 



Several authors have suggested that heavy metal concentrations in reducing 

 marine waters are strongly influenced by sulfide solubility: metals forming 

 relatively soluble sulfides (such as Fe and Mn) will have relatively high 

 concentrations, and those forming relatively insoluble sulfides (such as Cd and 

 Cu) will have very low concentrations (5, 13, 4). A corollary of this suggestion 

 is that benthic fluxes (fluxes of dissolved chemicals from sediments to the 

 overlying water) out of anoxic sediments should be high for those metals 

 forming soluble sulfides, and negligible or even negative for those metals 

 forming insoluble sulfides. These hypotheses, if correct, have important 

 implications. They imply that estuarine and reducing nearshore and continental 

 shelf sediments are not sites where insoluble sulfide-forming metals are readily 

 remobilized: metals reaching these sites by inorganic scavenging or biological 

 removal are, most likely, permanently sequestered as long as sulfide is being 

 produced in the sediment porewaters. Secondly, the hypotheses indicate that, 

 to avoid release of metals forming insoluble sulfides, sewage sludge and dredge 

 spoils polluted with heavy metals should be dumped under sites of high organic 

 productivity, where continued deposition of organic matter maintains the 

 conditions which prevent their release to the overlying water. Of course, other 

 considerations (such as the presence of organic toxins) may show another 

 course of action to be more prudent. 



