Trace metal solubility in salt marsh sediments contaminated with sewage sludge

As part of a study to investigate the effect of nutrient and metal pollution on salt marshes, a sewage sludge fertilizer has been applied to experimental plots in Great Sippewissett Marsh, MA, since 1974. The concentration of nutrients, soluble sulfides, and metals were measured in porewater from these plots every 4–6 weeks from April to December in 1980. Metal and nutrient concentrations in these plots were consistently greater than in corresponding control plots. Nutrients stimulated growth of Spartina alterniflora, the dominant vegetation on these plots, and higher grass production increased sediment oxidation. Concentrations of soluble sulfide in fertilized plots were an order of magnitude lower than in surrounding areas. For much of the year sulfides could not be detected in porewater from surface sediments of fertilized plots.The solubility of metals in sediments in fertilized plots was greatly increased by the decrease in sulfide concentrations. For much of the year, the top 4 cm of the sediments in fertilized plots were undersaturated with respect to all metal sulfide minerals. This undersaturation may account for the large loss of metals observed on these plots. It appears that in the surface sediments of these plots the retention of metals may be governed in part by adsorption onto iron oxyhydroxides.Precipitation of metal sulfides may be important in limiting the penetration of metals deeper into the sediment. At 6 cm, Zn and Cd were always close to the solubility of their respective sulfide minerals. Below 4 cm, iron was undersaturated with respect to all iron monosulfide minerals but supersaturated with respect to pyrite. Copper was supersaturated with respect to CuS and Cu₂S in all samples where sulfide was above the detection limit. Gel filtration experiments indicated that significant amounts of iron and copper were organically complexed in the porewater and may have been partially responsible for the large supersaturations.