A number of metals, such as manganese (Mn), zinc (Zn), and copper (Cu), are essential to biochemical processes that sustain life (Hatfield 2008). However, these same metals, and a variety of others, can be toxic to aquatic organisms at certain concentrations. Repeated exposure to even low (non-acutely toxic) concentrations can eventually result in toxic effects. Metals can be toxic to humans as well, if they are ingested directly in water, or if they accumulate in organisms that are higher in the food chain and are consumed by humans (Järup 2003).
Dissolved metals are generally more bioavailable and toxic than metals bound in complexes with other molecules or adsorbed to sediment particles. The toxicity and bioavailability of many metals depends on their oxidation state and the form in which they occur. These characteristics of metals—oxidation state, form, solubility, and toxicity—are influenced by chemical characteristics of water such as pH, dissolved oxygen levels, and hardness (CaCO3 concentration).
CALCIUM CARBONATE. SOURCE: WALKER 2005
Sources of Metals
Metals occur naturally in aquatic ecosystems due to weathering of rocks and soils. Erosion and sedimentation can introduce metals into an aquatic ecosystem, although the fate of metals introduced with sediments depends on the chemical characteristics of the water. Other sources of metals include effluent from wastewater treatment plants, industry, and mining operations, and sewage or soils contaminated by previous industrial activity. Metals may be introduced into the atmosphere through burning and ore smelting, and can be deposited in surface waters. Mercury is particularly susceptible to bio-accumulation as methyl mercury, and in high concentrations presents a risk to human health. A common pathway for the accumulation of mercury is by eating fish that have been exposed to high levels of the metal (Hatfield 2008).
