Manganese oxide minerals formed environments

Table IL Manganese oxide minerals formed in surface and subsurface environments...

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

Suboxic metabolism, which includes nitrate, manganese and iron reduction, does not generally seem to form a discrete layer due to the relatively low levels of nitrate available under natural conditions and the insoluble nature of manganese oxyhydroxides and iron oxides. These reactions appear to be restricted, where they are apparent at all, to micro-environments, but since, of the reduction reactions in Table 6.1, they are the only ones to generate alkaline conditions, they may be important for the precipitation and preservation of carbonate minerals. 

---