In the anoxic hypolimnion of Lake 227, Experimental Lakes Area, northwestern Ontario, ΣH2S exhibits a mid-depth maximum, while Fe2+ increases with depth. At the mid-depth ΣH2S maximum and below, saturation with respect to amorphous FeS is reached, and the concentration of ΣH2S is limited by the high Fe2+ concentrations, in accord with the FeS solubility product. Values for pKsp[Formula: see text] for FeS determined from the ΣH2S maximum and below averaged 3.16 in 1979 and agree well with other in situ and laboratory measurements. In the top 10 cm of sediment, pore water ΣH2S and Fe2+ are in equilibrium with amorphous FeS. Analyses of cores confirms the existence of an iron sulfide phase. Fe2+, which is produced in the pore water from the decomposition of organic matter, increases to concentrations at which siderite may form, although the presence of siderite has not been verified. Comparison of calculated pore water fluxes of Fe2+ with the observed increase of Fe2+ in the anoxic hypolimnion reveals that about 90% of the observed flux originates at the sediment–water interface, while the remainder is derived from pore water flux.
Fine-scale in-situ measurement of lead ions in coastal sediment pore water based on an all-solid-state potentiometric microsensor