Dispersion from NAPL to Solution

Water and hydrocarbons occurring together, in shallow aquifer systems, may be considered immiscible for flow calculation purposes however, each is somewhat soluble in the other. Since groundwater cleanup is the purpose behind restorations, it receives greater attention. Definition of water quality based on samples retrieved from monitoring wells relies heavily upon the concentration of individual chemical components found dissolved in those samples. An understanding of the processes that cause concentration gradients is important for the proper interpretation of analytical results. 

The transfer of chemical molecules from oil to water is most often a surface area phenomenon caused by kinetic activity of the molecules. At the interface between the liquids (either static or moving), oil molecules (i.e., benzene, hexane, etc.) have a tendency to disperse from a high concentration (100% oil) to a low concentration (100% water) according to the functions of solubihty, molecular size, molecular shape, ionic properties, and several other related factors. The rate of dispersion across this interface boundary is controlled largely by temperature and contact surface area. If the two fluids are static (i.e., no flow), an equilibrium concentration will develop between them and further dispersion across the interface will not occur. This situation is fairly common in the unsaturated zone. 

The intensity with which dissolved chemicals are released from liquid hydrocarbon over time is referred to as source strength. Two approaches have been reported to provide an estimate of source strength. Source strength can be expressed as mass/time per unit contact area such that 

Fuel oil, diesel, and kerosene 0.01 mg/m2/s Lube oils and heavy fuel oil 0.001 mg/m2/s 

These mass coefficient values are considered to be maximum values. Field values could be as much as one to two orders of magnitude lower. 

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