Lead (Pb) is a naturally occurring metallic element; trace concentrations are found in all environmental media and in all living things. However, certain human activities, especially base metal mining and smelting; combustion of leaded gasoline; the use of Pb in hunting, target shooting, and recreational angling; the use of Pb-based paints; and the uncontrolled disposal of Pb-containing products such as old vehicle batteries and electronic devices have resulted in increased environmental levels ofPb, and have created risks for Pb exposure and toxicity in invertebrates, fish, and wildlife in some ecosystems.
The accurate measurement of Pb in water is extremely difficult due to pervasive environmental contamination with atmospheric Pb, which necessitates ultra-clean conditions for collection, storage, and analysis of samples. Analyses that have followed such precautions indicate that much of the Pb in natural waters is associated with particulate material. Dissolved Pb concentrations are typically <1 ngml-1 in uncontaminated waters, but may reach or exceed 20 ng ml~ in contaminated waters. Total Pb concentrations in uncontaminated waters are typically <20ngml~ , but may be much greater in samples with high suspended sediment loads. In solution, Pb-contain-ing salts dissociate and the free divalent ion, Pb2+, can form stable complexes with carbonates, hydroxides, chlorides, other inorganic ions, and with naturally occurring organic substances. It is also readily adsorbed onto the surface of particulate material. In uncontaminated sediments, concentrations of total Pb in the <63 p.M fractions are typically <4-20 mgg1 dry weight, but may reach several thousand mgg~ in sediments containing mine tailings.
Toxicity of inorganic Pb to aquatic organisms is associated primarily with the concentration of the free dissolved ion. The toxicity of Pb2+ declines with increasing concentration of hardness ions (calcium, magnesium), and is also affected by other dissolved organic and inorganic substances that form stable complexes with Pb2+ (e.g., hydroxides, carbonates, chlorides, and dissolved organic compounds). Consequently, waterborne Pb is more bioavailable and toxic in soft freshwater than in either hard freshwater or saltwater. Acidic conditions such as those produced by acid rain and acid mine drainage can increase aqueous Pb2+ concentrations, but geochemical processes such as complexation and co-precipitation with iron and manganese oxides may remove Pb from solution.
The toxicity of waterborne Pb varies over several orders of magnitude depending on taxon, route and duration of exposure, water quality, and other factors. Daphnids and larval fishes are among the most sensitive freshwater organisms, and algae are the least sensitive. Amphibians are also not particularly sensitive to Pb except in combination with other metals, low pH, or both. However, differences among taxa are difficult to ascertain due to differing test protocols (duration of study, chemical form, route of exposure, endpoint, etc.) among studies. Diatoms and mysid shrimp are among the most sensitive marine organisms to inorganic Pb exposure, but organic Pb compounds (including those formerly used in leaded gasoline) are also highly toxic to the few marine organisms against which they have been tested.
Most of the Pb in sediments is associated with organic material, mineral sulfides, carbonates, and oxide coatings of silt and finer-sized particulates (<63 mM). Under anaerobic conditions, Pb concentrations in sediment pore waters are reduced by binding to insoluble acid-volatile sulfides. In sediments where Pb concentrations exceed the binding capacity of acid-volatile sulfides, Pb in sediment pore waters may reach concentrations that are toxic to benthic macroinvertebrates and other organisms. Particulate Pb in sediments may also be toxic to aquatic organisms through ingestion and incorporation into aquatic food chains.
Consensus-based guidelines for Pb concentrations in sediments at which effects on aquatic organisms are considered possible (threshold effect concentration = 35.8 mgg 1 dry weight) or probable (probable effect concentration = 128 mgg1 dry weight) have been established. These values are commonly exceeded, especially in urban areas and in waters contaminated by industrial discharges and mining.
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