Impacts of runoff from sulfuric soils on sediment chemistry in an estuarine lake.

The impact of runoff from sulfuric soils in the heavily drained Cudgen Lake floodplain, eastern Australia on water quality and downstream coastal lake sediments has been examined. The oxidation of sulfidic soils and the transformation into sulfuric soils leads to changes not only in the upper soil profile but also affects drainage water quality and the chemistry of bottom sediments in receiving waters. Oxidation transforms the soil from a sink for sulfur and metals to a significant source for downstream environments. Sulfuric soils within the Cudgen Lake catchment contain 9.18 x 10(5) mol H+ per hectare as well as elevated concentration of metals (e.g. Al, Fe, Mn) and sulfate. These products of sulfidic soil oxidation are transported efficiently from the soil profile by the constructed drainage network and into the downstream lake system. The acid volatile sulfur (AVS), chromium reducible sulfur (CRS), total sulfur, organic carbon, and reactive iron contents present in the solid phase of the lake sediments are reported. The AVS/CRS, DOP and DOS values observed in the lake sediments show that natural monosulfide formation in the near surface sediments has been enhanced due to increased inputs of organic matter, sulfate, ferrous iron and other metals following development of the catchment. There are elevated concentrations of metals (e.g. As, Al, Cd, Cr, Hg, Zn and Pb) in the upper layer of monosulfidic lake sediments compared with the underlying pyritic sediments some of which exceed sediment quality guidelines. These metals could be released by dredging or through re-suspension during high flow conditions or enter the food chain.