As noted previously, the mean crustal abundance of Cu is approximately 60 mg kg _ , but surface soil tends to be somewhat lower, with a mean of c. 30mgkg_1. Depending on the source rock of the soil, this could be highly variable, but it serves as a baseline for comparison for enrichment of Cu in soils and sediment. Some Cu in surface waters is natural, from rainfall and weathering of soils. Copper concentrations in freshwater can vary widely depending on the hydrology and pollution level, but concentrations of low to subpart per billion (mg l-1) levels would be typical in a region with little or no anthropogenic contribution. Copper concentrations in open ocean seawater are relatively low and probably not substantially influenced by anthropogenic sources. In the central gyres of the major oceans, surface values for copper may be as low as 0.05 mgl-1. In open ocean depth profiles, Cu exhibits what is often referred to as a 'nutrient-like' profile, with low concentrations in the surface waters, and higher concentrations at depth. These result from scavenging of dissolved copper in the surface waters by algae and other particles, and its subsequent downward transport by sinking particles. The particles are dissolved at depth by various processes, and the Cu is returned to the dissolved phase. Deep ocean bottom water copper concentrations reach approximately 0.5 mgl _1.
Not surprisingly, given its widespread mining, refining, and use as a structural material and biocide, the occurrence of copper at concentrations above 'natural' concentrations is ubiquitous near human development.
Mining and refining of copper and other metals can be a significant source of Cu pollution. One particularly well-studied site is near Sudbury, Canada, a site where a large mixed sulfide deposit bearing Cu, Ni, and platinum group metals have been mined and smelted since the late 1800s. Concentrations of Cu in soils as high as 2800 mg kg have been recorded nearby. The contamination, the result of air deposition from the mining and refining operations, was largely restricted to the surface 20 cm. Vegetation from the area was also examined and had concentrations up to 360mgkgin quaking aspen and 220mgkg in forage grass. Typical 'background values' were 10—20 m gkg and 5-10 mgkg 1, respectively.
Surface waters in the region are also significantly contaminated with metals, with concentrations reaching single to tens of mgl_1. The acidic precipitation resulting from sulfur dioxide from smelting operations results in a substantial pulse of acidic water and metals in spring runoff. Since 1978, when emission controls were implemented and sulfur emissions began to decline, the acidity of lakes and streams and their metal concentrations have declined.
Sudbury is by no means unique; it is simply one example of the effects of one of many large-scale metal mining and refining operations throughout the world. Clarks Fork River, in Montana, a tributary of the Blackfoot River, has been heavily impacted by acid and metals pollution from the Anaconda copper mine. The site is being cleaned up under the 'Superfund' law, costing an estimated $100 million dollars, entailing 10 years of effort and cleanup of 120 miles of river, stabilizing 56 miles of river bank to prevent erosion of contaminated material, treatment of 700 acres of soil, disposal of contaminated soil into ponds, and reestablishment of river bank vegetation.
While mining and smelting operations are a classic case of 'point source' pollution, much copper pollution comes from numerous ill-defined sources, 'nonpoint source' pollution. For example, in the typical urban environment, Cu is emitted as airborne particles from fossil fuel combustion and brake wear, into surface waters from runoff from streets, copper roofs and gutters, agrochem-icals, and from copper piping into the water waste stream. Elevated Cu concentrations have been widely observed in urban soils and dusts, storm runoff from streets, and discharges from sewage treatment plants.
Overuse of copper supplementation or pesticides and its deliberate use as an algaecide in water can lead to accumulation of concentrations on land and waters receiving their runoff. This, however, tends to be a localized problem.
Coastal and estuarine sediments are one of the primary sinks for waterborne pollution. Estuaries and harbors, in particular, are the sites of many industrial activities, and traditionally, receiving bodies for many waste streams. Consequently, the sediments of harbors and estuaries are often highly contaminated with Cu (as well as large number of other contaminants). The United States National Oceanic and Atmospheric Administration (NOAA) Program, the National Status and Trends Program, also known as 'the Mussel Watch' monitors contaminants including Cu in coastal sediments and organism tissues. Concentrations of Cu in sediments from some of the more contaminated sites in the US exceed 500mgkg _1.
Surface waters may receive Cu pollution from numerous sources, direct atmospheric deposition, urban or agricultural runoff, sewage discharge, acid mine drainage, or other industrial wastes. Similarly, copper concentrations in some polluted estuaries may reach a few parts per billion.
Cu has a strong tendency to adsorb to organic or inorganic particles. This is often measured by a distribution coefficient:
Where Cs is the concentration of contaminant in the solid phase and Cw is the concentration of the contaminant dissolved in the water. For a given water chemistry and sediment type, kd is approximately constant regardless of total Cu concentrations; thus dissolved and particulate Cu concentrations tend to rise and fall together. Typical values of kd for Cu are of the order of 10 lg_ .
Despite widespread contamination, it is often difficult to demonstrate that pollution by copper is causing environmental problems. One problem with demonstrating harm from elevated concentrations is that pollution rarely consists of a single toxicant. In most cases, pollution results in several to many different toxicants being released. For example, usually several toxic elements are present in the same ores, and are released in same process. In copper mining and refining, As, Cu, Pb, Cd, Zn, and SO2 are common co-contaminants. In such a complex mixture the cause of environmental damage is not necessarily clear-cut and may ultimately be due to complex interactions among the contaminants. Similarly, coastal regions (particular harbors near large urban areas) are usually contaminated with a number of metals and organic contaminants. Careful testing is required to determine which contaminants are exerting the strongest negative effects in such cases.
In aquatic systems, the toxicity of copper is controlled largely by the complexation of copper by inorganic and organic ligands. For the simplest biota such as algae and bacteria, that do not ingest food, but extract nutrients from solution, the uptake and toxicity of copper is controlled almost exclusively by the fraction of copper remaining unbound or 'free' (there are exceptions; a few organic ligands form lipid soluble complexes which are more readily taken up than inorganic copper). For example, in seawater, inorganic complexation of copper can reduce the free copper to approximately 5% of the total copper, and organic complexation can further reduce the fraction of free copper to as little as 0.1% of the total. As a result of inorganic complexation, copper standards to protect aquatic life in freshwater systems are often based on water hardness, a rough measure of the amount of inorganic ions. Free copper ion concentrations of as little as 10_ 13 M can be inhibitory to marine algae, bacteria, or small zooplankton. Given the extent of complexation, these can be produced by total copper concentrations of c. or approximately
0.1 mgl"1. However, there are some copper complexes which are more available and more toxic than uncomplexed copper. These include complexes of 8-hydroxyquinoline and dithiocarbamate pesticides. Concentrations of copper which inhibit some, but not all species of algae can produce dramatic shifts in phytoplankton community composition with little change in total biomass.
In larger aquatic organisms, which may take in copper both from diet and solution, the issue of metal uptake and toxicity is more complex. The relative importance of dissolved copper and copper in the diet may depend on many things, including the availability of forms of copper in the food and water, the rate at which Cu is lost from the organism, and whether that depends on its source.
Some noted cases of high concentrations of Cu have been found, one of the earliest was accumulations of massive levels of Cu in oysters in the estuarine portion of Thames River in England in the mid-1800s. Copper concentrations in oysters were so high, the flesh turned green and produced a bad taste and even illness in people who ate the oysters. This has also happened more recently in Chesapeake Bay and Taiwan, where the sources of copper were linked to local industries. However, it is not known whether the oysters suffered any ill effects from the elevated tissue copper concentrations. Oysters have a remarkable ability to sequester copper and some other heavy metal ions in sulfide granules within their tissues.
One of the factors which control Cu and other metal toxicity in sediments is reduction and complexation by sulfide. Most sediment is anaerobic below a certain sediment depth, when oxygen is used up by the respiration of organic materials. Below that depth, sulfate may be reduced by bacteria to sulfide, a compound which has very strong complexation coefficients for many metals. In the presence of S2~, Cu2+ is reduced to Cu+, and precipitated out as solid Cu2S, or more complicated mixed solids such as FeCuS. Provided that the sum of Cu and other metals complexed by sulfide more strongly than Fe and Mn (e.g., Zn, Cd, Ag) are in lower molar concentration than the total sulfide, it has been shown that there is unlikely to be overt metal toxicity. However, sulfide concentrations are highly variable; depending upon temperature, organic loads, and sediment irrigation; thus the toxicity of such sediment may vary over the course of time independent of metal concentrations. In oxygenated surface sediments, Cu toxicity can be similarly controlled by complexation with sediment organic matter and adsorption on to mineral surfaces.
See also: Acute and Chronic Toxicity; Bioaccumulation; Bioavailability; Ecotoxicological Model of Populations, Ecosystems, and Landscapes; Material and Metal Ecology; Mine Area Remediation; Pollution Indices; Sediment Retention and Release; Trace Elements.
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