Field Scale DNAPLs Transport Under Nonequilibrium Sorption Conditions

The purpose of this work is to study the desorption of dense nonaqueous phase liquids (DNAPLs), TCE in particular, from solid particles in field scale heterogeneous aquifers upon their remediation. A computer program, capable of simulating the fate and transport of NAPLs in porous media, has been developed to work under nonequilibrium sorption conditions. The model has been applied to a field scale site at Hill Air Force Base, Utah, which has been contaminated by DNAPLs. The simulated domain was 155 ft (47.25 m) long, 60 ft (18.29 m) wide, and 15.5 ft (4.72 m) thick. This thickness represents only the saturated zone of the aquifer. Changes in permeability, grain size distribution, and sorptive properties throughout the site have been incorporated into the model. Immediately after the aquifer cleanup, the DNAPL concentration in the aqueous phase was assumed to be zero, and this was considered the start-off time for the simulation. Results show that, with an increase in time, the TCE diffused out of the solid particles, forming a plume. The rate of contaminant diffusion was observed to be very fast at the start, followed by a very slow stage, with a number of years required for substantial desorption of the contaminant from the solid particles. There were local variations in contaminant concentration in the fluid phase across the site due to aquifer heterogeneity. A comparison between numerical results and water samples taken from the site after the end of the cleanup operation is also presented.

Key Words:

• Nonaqueous phase liquids
• Intraparticle diffusion
• Sorption kinetics
• Contaminant transport.

ACKNOWLEDGEMENT

A.A. Ahmed's PhD study was supported by an Overseas Research Studentship (ORS), and a research grant (GR/N03112) from the Engineering and Physical Science Research Council (EPSRC) in the UK. The authors are grateful for the kind assistance from Dr Jon Ginn in collecting the soil samples from the site, and Kyle Gorder for the data he provided. Special thanks are also due to John Ewing and Charles Holbert.

Notes

^a Ahmed et al.[ ²⁷ ]

^b Grathwohl[ ²⁹ ]

^c Brown et al.[ ³⁰ ]

^d Ahmed et al.[ ²⁷ ]

^e Ahmed et al.[ ²⁷ ]

^f Ahmed et al.[ ²⁷ ]