Fluxes of PAHs from coal tar-impacted river sediment under variable seepage rates.

The flux of several polycyclic aromatic hydrocarbons (PAHs) from coal tar-impacted river sediment was measured under various seepage rates in a laboratory scale. The batch PAH equilibrium data between aqueous phase (C(eq)) and sediment phase was well-explained using a Raoult's law approach. In the flux measurement from column study, the steady-state PAH concentrations (C(infinity)) in the effluent solution were affected by a time-dependent nonequilibrium process; the seepage velocity was inversely proportional to the C(infinity) of PAH in the effluents. With a high seepage velocity of 4.06 cm d(-1), a large initial concentration was observed, possibly due to the greater disparity between C(infinity) and C(eq). This initial-flush export leveled off after a few pore volumes had been displaced. The chemical flux pattern of PAH was conceptually depicted by superimposing two analytical solutions, such as a slug lease followed by rate-limited solute lease. Under quiescent conditions, the sediment pore-water reached dissolution equilibrium with the solid phase. However, under dynamic groundwater seepage conditions, the seepage water flowing through sediment pores did not reach dissolution equilibrium, which was evidence of nonequilibrium dissolution. In conclusion, the mass flux of PAHs from coal tar-impacted sediment is determined by concurrent effect of the groundwater seepage rate and rate-limited PAH concentration in seepage water.