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Human Health Risk Assessment. Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road § Tracy, California r 2.3 DISSOLVED-PHASE HYDROCARBON MIGRATION � 1 As chemicals are released into ground water, several factors contribute to their migration and transport. These factors include advection, diffusion, dispersion, retardation, and decay. Advection, diffusion, and dispersion are the predominant transport pro;:esses for nonreactive or conservative solutes. For reactive or nonconservative solute, concentrations also depend upon chemical and biochemical processes. These processes can be d - ided into two general ' categories: retardation processes and decay processes. The primary difference between retardation and decay is that retardation processes are generally reversible while decay processes ' are generally irreversible. Advective transport involves the movement of solute at the average linear velocity of the ground ' water. If this were the only mechanism governing the transport of sc' rtes, it would behave as an aggregated solid particle traveling through the media without any lengthening or spreading ' (Yeh, 1981). The flow parcel actually travels more slowly near the walls of the pore than in the center; faster in larger pores than in small pores; and travels randomly in different directions (Yeh, 1981). This mechanism of migration is called hydraulic disp�n. Field and laboratory M observations indicate that solute concentrations are not constant throu hcut, plumes g p es and that some degree of mixing takes place at the plume edges. As a result, the body of the solute will spread because the solution does not move uniformly in the porous media (Yeh, 1981). ' Another process causing the growth in size of the solute patches is molecular diffusion. Molecular diffusion is governed by Fick's Law and is caused by the random Brownian motion of molecules in the solution. This mechanism occurs whether the solution in the porous media is stationary or has average motion. This diffusion process is normally small compared to the hydraulic dispersion and its effects are usually combined in the term of dispersion (Yeh, 1981). Retardation is the process by which solute is transferred between the liquid and solid phases ' because of adsorption-desorption reactions, oxidation-reduction reactions, and precipitation- dissolution reactions. The overall effect of retardation is to partition the chemical constituents ' between the solid and liquid phases. Retardation causes the average linear velocity of the solute front to be less than the average linear velocity of ground water. The ra-do of the average linear _ velocity of the ground water to the velocity of the, colutP. frn„t ic tprna-A the - — y -- ---- ----�� _� � _� ..v=mss v� raav lv�µl uuLlV ll 1Ql-LVl. 30-0136-11 � 2-2