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Estuaries in NSW

Estuarine Water Quality

Heavy Metals

Heavy metals, such as copper, lead, zinc, calcium, tin and mercury, can be acutely toxic to aquatic organisms. Even when present in sub-lethal levels, heavy metals can have a number of debilitating effects that reduce the health and survival of aquatic organisms. The toxic and sub-lethal effects of heavy metals are affected by a number of factors that include pH, salinity, temperature, DO and carbon dioxide levels, water hardness, etc.

Heavy metals tend to accumulate in fish and shellfish (eg mercury in some fish, arsenic in some crustacea). If sufficiently high, accumulated heavy metals in fish and shellfish are of public health concern and may lead to the closure of fisheries.

Heavy metals find their way into estuaries in a number of ways. They are present in urban stormwater, originating from atmospheric fallout, vehicle emissions (lowered since the introduction of lead-free petrol) and from galvanised iron roofs. They may also be present in industrial discharges and sewage effluent (from industrial wastes discharged to sewer). Heavy metals also originate from processing operations associated with metalliferous mining.

Tin is rarely found in natural waters and then only in trace concentrations. The most likely source of tin in surface waters is via industrial process effluents from alloy manufacture, tin plating operations and from defouling and anti-fouling paints used to maintain boat"s hulls. The purpose of the active ingredients of these paints (often tin salts) is to kill algae and other marine organisms that attach themselves to the hull. The effects of tin on molluscs is of concern. Restrictions on the use of tin in marine paints are currently being introduced.

Lead is a persistent chemical which is biologically accumulated in aquatic organisms (upon digestion, immobilisation of lead leads to its buildup in organic tissues). Lead is acutely toxic to most aquatic organisms, killing them by destruction of gill tissue. At sub-lethal concentrations, lead has a number of chronic ill-effects which include mucus buildup on gill tissue (leading to suffocation and death), general enfeeblement and reduction in disease resistance and growth rates. The ingestion by waterfowl of lead shot (from waterfowl hunting) can cause lead poisoning and death in severe cases.

Zinc is present in natural waters in only trace amounts (3-50 µg/L in uncontaminated estuarine waters). Whilst small amounts of zinc are essential for all forms of life, excessive levels can have toxic and sub-lethal effects on aquatic wildlife. Zinc - like lead - also destroys gill tissues and bioaccumulates in aquatic organisms. Sub-lethal effects of zinc include general enfeeblement, reduced growth rates and pathological changes to many organs. The early stages of many saltwater invertebrates are sensitive to zinc.

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