Transport Mechanisms

Channels or contaminants are transported through soil principally by 3 mechanisms:

1. Massflow of dissolved chemicals within moving solution
2. Liquid diffusion within soil solution
3. Gaseous diffusion within soil air voids.

1. Massflow: Refers to the passive transport of dissolved solution within moving soil water which is approximated as the product of the volume flux of water times the dissolved solute concentration.

2. Liquid diffusion: Refers to the transport of the dissolved solutes within solution by intermolecular collision which moves the solutes from regions of higher solute density to lower solute density(concentration)

q_c = Ddc/dx

3. Vapour diffusion: Chemical Vapour Molecules in the soil air spaces also undergo molecular collision and spread out by vapour diffusion which is expressed as the product of the vapour density or concentration gradient and a proportionality coefficient called the soil vapour diffusion coefficient

6.4 Loss Pathways of Contaminants in Soil

Contaminants added to a soil from the surface may leave the zone of incorporation by one of these three loss pathways.

1. Leaching takes place principally by mass flow and refers to the downward movement of the contaminants

2. Volatilization: Refers to the loss of contaminants in vapour form to the atmosphere through the soil surface

3. Degradation: Refers to the biological or chemical transformation of the contaminant to a different form with properties distinct from those of the contaminant prior to transformation. (Subsurface) Transformation Processes

Transformation processes change the chemical structure of a compound which might result in one or more of the following

1. Detoxification: An irreversible change in a substance from a toxic to non-toxic form. This occurs most commonly when an organic substance breaks down into its inorganic constituents with water and CO₂ being the main by-products

2. Transtoxification: Inverse the conversion of one toxic compound to another toxic compound. In the process, toxicity may remain the same, increase or reduce 3. Toxification: Is the conversion of non-toxic compound to a toxic substance

6.5 TOXIC ELEMENTS IN THE ENVIRONMENT

Included among the toxic elements are heavy metals such as Ag, Cd, Cr, Co, Cu, Fe, Hg, Mo, Ni, Pb, Sn, Zn as well as lighter elements such as Al, Ag and Sc.

Under certain environmental conditions, these elements may accumulate to a toxic concentration and cause ecological drainage.

6.5.1 MAJOR SOURCES

1. Naturally occurring contaminations, natural emissions of toxic elements to the atmosphere can take place by volcanic outputs or outgassing, and by vapour-phase of relative volatile elements such as As, Hg and Se. We can have contamination from metalliferous sites.

2. Anthropogenic sources of toxic elements

Agricultural practice: From long-term use of pesticides sewage sludge (ie by-product of secondary treatments of municipal sewage)

Pollution from (metal) mining and processing: Pollution from toxic elements is association with various aspects of mining and processing industries. Mining produces ore as a product along with a considerable amount of metal-contaminated wastes pollution around mines is caused by dumping of contaminated overburden, excavating wastes etc

Oil pollution: The contaminations of soil and ground water with mineral oils, hydrocarbons or mineral oil-based products are among the most common negative effect of the industrial society.