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 
re mobilized: 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. 
9 
