In the attached Figure 1 (DNAPL Spill), identify and briefly describe at least e

Question

In the attached Figure 1 (DNAPL Spill), identify and briefly describe at least eight chemical, physical and biological processes that may be acting to spread or to control the spread of the contamination through the subsurface. Assume that the DNAPL is a cleaning solvent (trichloroethylene, TCE: solubility = 1, 100 mg/L; h.c.= 0.0091 atm-m^3/mol; Sp. Grav. = 1.46; density, rho = 1460 Kg/m^3; Log K= 2.5). Example: Volatilization: Because TCE is volatile, vapours are generated in the vadose zone. As a consequence, the mass of residual DNAPL is reduced. However, migration of the vapours can then result in the spread of TCE through the Vadose Zone by other processes. Figure 1. DNAPL spill

Explanation / Answer

How does contamination move from the surface to the subsurface?

Most contaminants are introduced to the subsurface by percolation through soils. The interactions between a soil and a contaminant are important for assessing the fate and transport of the contaminant in the groundwater flow system. Contaminants that are highly soluble, such as salts (e.g. sodium chloride, NaCl) move readily from surface soils to saturated materials below the water table. This often occurs during and after rainfall events. Those contaminants that are not highly soluble may have considerably longer residence times in the soil zone. Some contaminants adsorb readily onto soil particles and slowly dissolve during precipitation events, resulting in dissolve fraction concentrations of contaminants migrating to groundwater. This mode of transport is common for trichloroethylene.

Liquids spilled onto surface soils can migrate downward or can evaporate, which limits their potential for reaching the water table. Once below the water table, contaminants are also subject to dispersion (mechanical mixing with uncontaminated water) and diffusion (dilution by concentration gradients). For additional information concerning the fate and transport of contaminants in soils, refer to Contaminant movement through soils, an overview provided by the the U.S. Department of Agriculture. The U.S. Geologic Survey also provides a helpful summary of the interaction between soils, streams, and groundwater.

Physical Pollution

Of the different types of pollution, physical pollution may be the most recognizable. Simply stated, physical pollution is the introduction of discarded materials into the environment. Physical pollution is what you might refer to as trash and is the direct result of human actions. In other words, nature does not produce physical pollution because in natural systems, all byproducts or wastes are eventually recycled back into the environment. For example, in nature, a fallen tree will degrade and eventually return nutrients to the soil.

However, physical pollutants, such as discarded water bottles and plastic bags along with waste materials from industrial or manufacturing processes, do not naturally degrade and can accumulate or leach chemicals into the ground or water supplies as they breakdown. Physical pollutants are often sent to landfills, which are designated areas for trash disposal in which the waste is dumped and then covered by soil.

Landfills keep physical pollutants confined to one area, and many modern landfills are lined with layers of clay or plastic to prevent leakage. However, as buried waste products and organic matter decompose, they can release methane gas, carbon dioxide and other gases that are harmful to the environment.

Chemical Pollution

Chemical pollution is another type of pollution. It is defined as the introduction of chemicals into the environment. Chemicals may not be seen by the naked eye, but they can cause problems in all areas of the environment, from the air we breathe to the freshwater we drink to the soil we use for growing crops.

Agricultural practices are one example of a chemical pollution source. Pesticides used to control insects and fertilizers used to make soil more fertile contain nitrogen, phosphorus and other chemicals. These chemicals can run off of a farmer's field and enter waterways. Nitrogen and phosphorus fertilize tiny plant life in the body of water, causing rapid growth and eventually depleting oxygen levels in the water to the point where fish and other species of life cannot survive. Chemical pollution from pesticides and fertilizers can also contaminate soil if used in excess. Other sources of soil contamination include the leaking of chemicals from mines and landfills.

Chemical pollution is also seen in the air. The burning of fossil fuels, such as coal, oil and natural gas, release chemical pollutants into the atmosphere. These fossil fuels may be used in our vehicles or by utilities or industries. These chemical pollutants are referred to as greenhouse gases, which are gases in the atmosphere that absorb infrared radiation and trap heat.

For example, the car you drive to work most likely runs on fossil fuels. As this fuel is burned, the greenhouse gas carbon dioxide is released into the atmosphere. Another example is the release of sulfur dioxide into the atmosphere when coal is burned. Sulfur dioxide contributes to acid rain, which is the phenomenon by which impurities combine with water vapor and fall to the earth.

Biological Pollution

Up to this point, we have looked at pollution caused by non-living materials or chemicals. However, biological pollution is the introduction of living organisms, which grow and become invasive, into the environment. Like physical and chemical pollution, biological pollution is mainly due to human interference. Not all foreign organisms will thrive in a new environment; however, biological pollutants not only thrive but they adapt to their new surroundings, multiply and spread at a rapid rate.

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