Marine Chemistry, 16 (1985) 173--186 173 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands SULFIDE CONTROL OF CADMIUM AND COPPER CONCENTRATIONS IN ANAEROBIC ESTUARINE SEDIMENTS ROBERT J. DAVIES-COLLEY*, PETER O. NELSON and KENNETH J. WILLIAMSON Civil Engineering Department, Oregon State University, Corvallis, OR 97331 (U.S.A.) (Received December 19, 1983; revision accepted January 5, 1985) ABSTRACT Davies-Colley, R.J., Nelson, P.O. and Witliamson, K.J., 1985. Sulfide control of cadmium and copper concentrations in anaerobic estuarine sediments. Mar. Chem., 16:173--186. Equilibrium concentrations of the toxic trace metals copper and cadmium were calcu- lated for the physico-chemical conditions characterizing pore waters of anaerobic estuarine sediments using available thermodynamic data and assuming simple sulfide minerals control solubilities. Polysulfide complexes are responsible for the solubility of copper in the cuprous (Cu(I)) oxidation state. Predicated copper concentrations, assuming covellite (CuS) is the controlling solid phase, are in reasonable agreement with copper analyses in a wide range of sulfidic waters and sediment pore waters. In the absence of thermodynamic data, no account could be taken of possible polysulfide complexes of cadmium. However, bisulfide complexes appear to account satisfactorily for bbserved solubilities assuming the existence of greenockite (CdS) as the controlling solid phase. Anaerobic estuarine sediments may act as a sink for copper and cadmium in the common situation in which free sulfide concentrations are controlled by the coexistence of iron sulfide and iron oxide minerals. However, where free sulfides reach high concentrations of 10-3M or more, the concomitant increase in concentration of bisulfide and polysulfide complexes may result in the sediments acting as a source of copper and cadmium. INTRODUCTION Metals and other trace toxicants tend to associate strongly with sedimentary particles by adsorption or precipitation/coprecipitation (Leckie and James, 1974). Since estuaries act as sediment traps (McDowell and O'Conner, 1977), these ecosystems may be susceptible to adverse impacts from such toxicants (Odum, 1970). Within the estuarine environment anaerobic (subsurface) sediments are known to be the main reservoir for toxic trace metals. As such there is considerable interest in understanding the behavior of metals in anaerobic sediments since release of only a small portion of this sink of toxic metals to the overlying water or associated aerobic sediments could cause significant environmental damage. Elevated concentrations of a number of trace metals have been observed in the pore waters of anaerobic marine and estuarine sediments (Brooks et al., 1968; Presley et al., 1972; Duchart et al., 1973; Lindberg and Harriss, *Present address: Water and Soil Division, Ministry of Works and Development, Hamilton, New Zealand. 0304-4203/85/$ 03.30 © 1985 Elsevier Science Publishers B.V.