UNSATURATED TRANSPORT PROCESSES IN UNDISTURBED HETEROGENEOUS POROUS-MEDIA .1. INORGANIC CONTAMINANTS

Prolonged disposal of organic and inorganic waste in shallow land buri al sites throughout the USA has prompted detailed investigations of su bsurface contaminant transport processes. The fate and transport of co ntaminant leakage from pits, trenches, and cribs into the vadose zone via storm events is not well understood. The objective of this study w as to investigate the thermodynamic and kinetic processes controlling the transport of inorganic contaminants in unsaturated, heterogeneous subsurface media. Large undisturbed columns were isolated from a propo sed waste site consisting of fractured saprolite (weathered interbedde d shale and limestone), and steady-state nonreactive and reactive solu te transport experiments were performed at a variety of pressure heads . Observed breakthrough curves (BTC) for binary and ternary mixtures i n the Co-Sr-Ca system were delayed relative to nonreactive Br BTC, ind icating that the former tracers were adsorbed by the solid phase. Tran sport of the binary mixtures Co-Ca and Sr-Ca was predicted reasonably well with the equilibrium convective-dispersive (CD) equation using in dependent measurements of all model parameters. However, application o f the nonequilibrium or kinetic CD model to the observed Sr and Co bin ary data resulted in an improved description of contaminant transport. Cation-exchange equilibria relationships on homogenized subsurface ma terial, using both shake batch and miscible displacement methods, adeq uately described the thermodynamic processes that were prevalent durin g contaminant transport. These results suggest that preferential trans port of reactive contaminants is negligible for the unsaturated condit ions used in this study, and that the structured saprolite within the subsurface media is a chemically active constituent during reactive so lute transport. Although the transport of contaminants in the ternary Co-Sr-Ca system tem exhibited many of the same features as the binary transport studies, an attempt to simulate the transport of contaminant s in the ternary system as independent species was less than adequate.