Metals in Soils Soils are formed from the decomposition of parent material (bedrock and organic material) over time. Naturally, soils are composed of different elements, particles, and components that are held together in a matrix. Soil properties vary according to the formation processes, protolith(parent material), and local climate in which they were formed. By their composition, soils have a natural background level of these elements that can be compounded by several different factors that include natural (earthquakes, landslides, runoffs from rich metal deposits) and anthropogenic disturbances (improper mining practises, waste disposal, pesticides). (see figure 1)
Figure 1. Heavy metal pathway cycles
© Dr Mark Hill (2011). Permitted usage
According to Shayler et al (2009), certain chemical elements occur naturally in soils as components of minerals, yet may be toxic at some concentrations. Other potentially harmful substances may end up in soils through human activities. In some regions of the United States, naturally occurring concentrations of certain chemicals may be higher than those in other areas. “For example, typical levels of arsenic in the soils of some regions of New York State can exceed recommended values” (Shayler et al, 2009).
Contaminants in Soils “The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations.”(Lenntech, 2011).
Soil pH, texture, and organic matter content, affects the toxicity of a heavy metal in an environment. For an organism, the adverse effect of an exposure to heavy metals is dependent on the amount and duration of exposure, i.e. “the volume of contaminated soil or food consumed over time.” (Grubinger & Ross, 2011).
Figure 2. Bioavailability of heavy metals
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Once contaminants are introduced into soil, their dispersal rate is dependent on so many factors. Unlike organic contaminants that are more likely to break down into compound or materials that are less toxic, heavy metals are not easily broken down but their chemistry can be altered. “The heavy metals associated with Mn and Fe oxides may be remobilized under reducing condition” (Borba et al., 2003). Bioavailability of contaminants is a major concern because bioaccumulation can lead to heavy metal concentration in organisms within the food web (see figure 2). “The bioavailable portion is the amount of a substance that can cause direct effects on plants, animals or humans because it can be taken up by their bodies” (Shayler et al, 2009).
Effects of metals in soils High levels of metals in soils can adversely affect plant growth, microbial soil processes, and the general chemistry of the soil. If the metals are soluble, a good example being Arsenic, leaching of the soil exposes the groundwater to these contaminants through porous rock units in the region.
Mitigation process A good proposition is the use of bioremediation techniques to remove the heavy metals from the soil (See figure 3).
Figure 3. Adsorption of heavy metals by plants.
(http://www.snf.ch/E/Pages/Impressum.aspx (Permitted usage) Strict care and measures should be employed when handling these metals. Even though heavy metals have a long life span, there are measures that can be applied to lessen the risk these