(477c) pH-Dependent Transport of Energy By-Products in the Subsurface

pH-dependent transport of energy by-products in the subsurface
Valentina Prigiobbe^a, Marc A. Hesse^b,c, and Steven L. Bryant^a,c
aDepartment of Petroleum and Geosystems Engineering, ^bDepartment of Geological
Sciences, ^cInstitute for Computational Engineering and Sciences, The University of Texas
at Austin, Austin, Texas, U.S.A.

Abstract
The transport of solutes through reactive porous media can be enhanced by the effect of pH-dependent adsorption/desorption processes. This effect can be particularly critical when the solutes, such as toxic metals and radionuclides, have hazardous impact on the environment and the health.
To study the effect of pH-dependent adsorption/desorption on the toxic metal and radionuclide transport, we have developed an analytical and a numerical model coupling transport and surface complexation for a single-phase incompressible fluid. We have analyzed a one-dimensional (1D) system of conservation laws to account of the pH-dependence and longitudinal dispersion and to gain a physical intuition of the phenomenon as well as a 2D system to include the effect of transverse dispersion and to investigate more realistic scenarios.
An important feature of these reactive transport problems is that one of the conservation laws is for the total hydrogen (c_ht = c_h - c_oh), also called acidity, which gives rise to an additional nonlinearity to the systems distinguishing them from other competitive adsorption/desorption problems where hydrogen is not involved.

Here, we present the results for two cases. A first one of pH-dependent transport of naturally-occurring toxic metals in shallow aquifers mobilized through a brine acidied by carbon dioxide (CO₂) leaking from deep geological storage [1,2]. A second case of pH-dependent transport of adsorbing radionuclides accidentally discharged into the subsurface from temporally storage pools [3,4]. In both cases, we have analyzed the transport through a reactive porous medium composed of hydrophilic iron-oxide.
In the first case, the 1D solution of the intrusion of a CO₂-acidified brine into a shallow aquifer has two retarded fronts separated by a spike of concentration of desorbed metal with a maximum much larger than the initial value. This is a classical behavior of pH-dependent adsorption, which occurs when the adsorption of the proton is more favorable than of the metal. In a 2D system, the structure of the plume resembles the 1D solution which, migrating into the aquifer with a maximum concentration much larger than the initial one, may present a health hazard.
In the second case, the 1D solution of the injection of an alkaline brine containing radionuclides into an aquifer consists of a retarded classical front and an unretarded non-classical pulse traveling at the average fluid velocity. The suitable initial and boundary conditions associated with the interaction of pH-dependent adsorption and the longitudinal dispersion are responsible for this phenomenon. In a 2D system in addition to the retarded front and the fast pulse, a fast spreading plume forms due to mixing induced by transverse dispersion. The unretarded pulse is a new transport phenomenon due to the interaction of pH-dependent adsorption and hydrodynamic dispersion, it leads to the migration of a contaminant with a velocity much faster than expected and it should be taken into account in the design of contaminated site remediation.

References

[1] Apps, J.A., Zheng, L., Zhang, Y., Xu, T., Birkholzer, J.T. (2010), Evaluation of potential changes in groundwater quality in response to CO₂ leakage from deep geologic storage. Transp Porous Med 82, 215-246.
[2] Kharaka, Y.K., Cole, D.R., Hovorka, S.D., Gunter, W.D., Knauss, K.G., and Freifeld, B.M. (2010), Gas-water-rock interactions in Frio formation following CO₂ injection: Implications for the storage of greenhouse gases in sedimentary basins. Geology 34, 577-580.
[3] Prigiobbe, V., Hesse, M. A., and Bryant, S. L. (2012a), Fast strontium transport induced by hydrodynamic dispersion and pH-dependent sorption. Geophys Res Lett 39 L18401.
[4] Prigiobbe, V., Hesse, M. A., and Bryant, S. L. (2012b), Anomalous reactive transport in the framework of the theory of chromatography. Transp Porous Med 91, 127-145.