Background Concentration of Organic Pollutants

Organic pollutants are compounds of hydrogen and carbon in various bonding configurations and also bonded to other elements that are mostly introduced in the environment by anthropogenic activities. Derivatives of petroleum, especially those used as fuels or solvents, are the most dangerous organic pollutants. The remaining pollutants are dominated by chemically altered petroleum derivatives.

In spite not being acknowledged as much as inorganic substances are, all natural waters contain organic compounds either dissolved or in the colloidal and particulate phases. These compounds have important effects on silicate dissolution and general weathering reactions, and are important transport agents for trace metals with which they can form strong bonds. Therefore, they have an indisputable influence in the inorganic content of aqueous environments. Usually, the content of natural organic compounds do not pose health risks, although exceptions are known (e.g., 20).

The source of organic pollutants is varied and includes spilling and leakage from underground storage tanks and landfills, and several commercial and industrial activities. Background concentration of these pollutants is solely the result of human impact on the water system. For water quality assessment, it is important to have a good knowledge of organic pollutant dispersion and persistence in water systems, and the same basic principles presented also apply in this case, including the definition of background concentration.

Contrary to trace metals for example (the most important group of inorganic pollutants), several of these compounds are biodegradable, which, coupled to other mechanisms such as abiotic decomposition (including hydrolysis, oxidation-reduction, and elimination), adsorption, dispersion, and dilution, contributes to what is called natural attenuation. Given the right environmental conditions, some organic pollutants can be persistent with long decay reaction half-life. Therefore, these compounds may have a complex behavior in water systems. Chlorinated solvents, for example, sometimes behave as conservative solutes that are rapidly transported, but they also undergo several microbial degradation processes causing them to rapidly disappear and be replaced by the lightly chlorinated ethenes.

Some case studies describe the development of dynamic steady-state plumes of hydrocarbons that stop spreading because the rate of input of soluble hydrocarbons is balanced by biodegradation mechanisms that consume hydrocarbons in the plume. Thus, knowledge of these processes is fundamental to evaluate and prevent organic pollutant concentrations to build up above levels considered potentially harmful to humans and ecosystems.

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