The exposure assessment requires an understanding of fate and transport of the substance of concern among others. This type of analysis will involve the specific environment in which the spill occurs such as soil, water, air or a combination of the three. The spill must be characterized to identify its specific constituents and the potential for any of them to undergo either biodegrading or chemical reaction hence potential derivatives. The effects of advection, diffusion and dispersion of the spill is important in trying to answer to question of the form: where is it going? to what extend, What direction? and to what degree or concentration.
A good understanding of the spill is critical because it lays the basis for exposure assessment. For instance non-aqueous phase liquid (LNAPL) will behave in a certain way in containing medium compare to a Dense non-aqueous phase liquid (DNAPL) due to distinct physical and chemical properties. LNAPL tends to remain on upper layer of water while DNAPL tends to sink to bottom of water table.
Also the toxicity level will be different containing short and long chain-organic compounds, and possibly aromatics hydrocarbon compounds.
The geographical area is important factor as well for considering the population proximity and obtaining a time frame before reaching potential receptors. Through the investigation a monitoring plan is essential to understand the transport patterns taking place and also to obtain initial and changing measurements in concentration in air, water and soil. Once enough data has been collected to provide enough resolution modeling tools become useful in predicting movement of the spill and constituent concentrations at receptors of concern.
The main routes of exposure for a given population are; ingestion, inhalation and skin absorption. if the contaminant is to reach the population it's important to determine the thresholds at which the contaminants adversely effect the human health.
References:
Transport Phenomena by R. Byron Bird
Fate of Spill Oil in Marina Waters by The American Petroleum Institute
http://www.epa.gov/region4/sesd/reports/1999-0219/nbsect5.pdf
http://www.clu-in.org/conf/itrc/dnaplpa/dnapl_handbook_final.pdf
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