A New Mechanism for Anomalously Rapid Transport
Field observations of anomalously fast migration of chemical species have presented regulators and researchers with a paradox. Many hazardous chemical species adsorb very strongly to mineral surfaces and should therefore migrate very slowly if released into the subsurface. Yet travel speeds approaching groundwater velocity -- orders of magnitude larger than expected -- have been observed. As emphasized in a recent report by the National Research Council, these observations reveal shortcomings in our understanding of contaminant fate and migration in the subsurface.
Anomalously rapid transport is likely to be the consequence of a host of interacting, complex phenomena. For example, a possible resolution is the fact that minerals and organic matter can and frequently do exist as colloids, particles small enough to pass through the pore throats in soils and aquifers. Colloids can travel as a suspension at groundwater velocity, transporting with them any solutes sorbed to their surfaces. The colloids themselves can sorb to soil and rock surfaces, however, making colloid transport a complex process and the subject of ongoing research.
An independent and previously unrecognized mechanism for fast transport has recently been discovered [1,2]. Numerical modeling of leakage from waste disposal pits at the Oak Ridge Reservation showed an unexpected fast-moving peak of Sr. The original simulations involved several contaminants, a full set of geochemical speciation reactions, and a sophisticated surface complexation model . The only mechanisms for transport in the model were advection and dispersion (hydrodynamic dispersion plus molecular diffusion); colloids were not considered. Further investigation showed that the fast wave of Sr persists in much simpler chemical systems, an example of which appears in Fig. 1. In this model system, strontium ions sorb on iron oxyhydroxides (hydrous ferric oxide, HFO) with displacement of a proton.
Figure 1. Aqueous concentration profiles during simulated injection of a pH 11 solution containing 10-4 M Sr and a conservative tracer into a porous medium initially at pH 6. A dispersion-induced wave of Sr arises and moves rapidly, almost at interstitial fluid velocity, as indicated by the location of the tracer front.
To date there has been no attempt aimed at empirically validating the fast wave. A laboratory study carried out prior to the fast-wave discovery  revealed one instance of unusual behavior that in retrospect can be identified as the fast wave. Our goal is to eludicate the fast wave phenomenon illustrated in Fig. 1 by a series of experiments tied to numerical and theoretical modeling.
The frameworks for wave behavior in gas dynamics and related areas of mathematical physics have not been applied to this phenomenon. The classical theories of advection/diffusion/adsorption do not anticipate this wave. In transport with mineral precipitation and dissolution, diffusion plays a key role in constraining the corresponding hyperbolic problem [3,4], but the fundamental differences between sorption and precipitation equilibria preclude any direct application of this theory. Thus our current understanding of the fast wave is too limited to determine whether it is directly implicated in the field examples of unexpectedly rapid migration described above.
Preliminary investigation has already revealed a qualitatively different model chemical system exhibiting this behavior. A basic but still open question is the asymptotic behavior of this wave. The essential role of dispersion in its formation suggests that it should decay as it propagates, but on the other hand its connection with other sharpening fronts in the system may allow for a traveling wave solution. We would also explore the intriguing analogies with the nonclassical "transitional shock" that arises in three-phase incompressible flow .
1. Toran, L.; Bryant, S.; Saunders, J.; Wheeler, M. A two-tiered approach to reactive transport: Application to Sr mobility under variable pH. Ground Water (1998) 36, 404-408.
2. Bryant, S., C. Dawson and C. van Duijn. Dispersion-induced Chromatographic Waves. Ind. Eng. Chem. Res. (2000) 39, 2682-2691.
3. Bryant, S., R. Schechter and L. Lake. Mineral sequences in precipitation/dissolution waves. AIChE J. (1987) 33, 1271-1287.
4. Helfferich, F. The theory of precipitation/dissolution waves. AIChE J. (1989) 35, 75-87.
5. Kohler, M.; Curtis, G.; Kent, D.; Davis, J. Experimental Investigation and Modeling of Uranium(VI) Transport Under Variable Chemical Conditions. Water Resources Research (1996) 32, 3539.
6. Marchesin, D.; Plohr, B. Wave structure in WAG recovery. SPE 56480, SPE Annual Tech. Conf. Exhib., Houston, TX, 3-6 Oct. 1999.
Steven L. Bryant
Center for Petroleum and Geosystems Engineering
1 University Station C0304
The University of Texas at Austin
Austin, Texas 78712-0228
Phone: (512) 471-3250 FAX: (512) 471-9605