Water Quality: Heavy Metals

Metals can be present in the environment either as ions, complex molecules, or in combination with other metals or particulates as colloids and precipitates (Ashton et al. 2001). 

There are several factors that determine how toxic a metal is to the biological receptors and how far a metal can travel from its source. Toxicity depends on the type of metal, the chemical interactions of the metal with other metals and the presence of organic compounds which may increase the bioavailability and spread of the toxic metal (Davies and Day 1998).  The flow rate and volume of water, the physical make-up of sediments, water temperature, oxygen, pH and salinity also impact: (1) how toxic a metal is in a given environment (Davies and Day 1998); (2) speciation (the proportion of metals in different forms); and, (3) the mobility (Ashton et al. 2001).

Heavy metals include cadmium, copper, nickel, zinc, chromium, arsenic, mercury, lead, etc. Many of the metals present in the aquatic system are in suspension or are absorbed onto particulate matter, rather than being in solution as free ions (Ashton et al. 2001). Metals absorbed onto particulates can travel long distances along water courses and be detected downstream of the source. 

Mining

Heavy metals are persistent in the environment and can be bioaccumulated in aquatic organisms. Mining is one of the primary sources of metal contamination in the Limpopo River basin. The water quality impacts related to mining, with specific reference to metals, are shown below for impacted sub-basins. Sub-basins that have no record of water quality impacts related to heavy metals from mining are excluded from the table below.

Heavy metal water quality issues associated with mining for sub-basins in the Limpopo River basin.

Effluent from metal mines must be contained in hydrologically isolated waterbodies.

Source: Vogel 2005

( click to enlarge )

Industry and Other Sources

Metal contamination can also originate from industrial activities as well as from urban storm water runoff from roads, parking areas and other impervious surfaces, ending up either in waste water treatment plants, or directly in the river. Toxic metals are also associated with some pesticides (Heath and Claasen 1999).

Sources of metal pollution from industry.