The fate and transport of 57Co(II)EDTA, 58Co(II)EDTA, 109CdEDTA, and 51Cr(VI) were investigated in undisturbed soil columns of fractured weathered shale from the Oak Ridge Reservation (Figure 1 and Figure 2). Multicomponent transport of Co(II)EDTA and CdEDTA revealed that sorption, redox reactions, and ligand dissociation reactions significantly influenced the geochemical behavior of the toxic metals and radionuclides in the soil (Figure 3). Co(II)EDTA was oxidized to Co(III)EDTA during transport through the soil (Figure 4). The environmental implications of this reaction are pronounced, since Co(III)EDTA is extremely stable and this enhances its mobility and persistence in subsurface environments.
Figure 1: Profile of the subsurface material used to obtain undisturbed columns on the Oak Ridge Reservation. Subsurface bedding planes dip 45° relative to ground surface. Alternating beds of weathered shale (tan color) and clay lenses (reddish color) makeup the saprolite of the C-horizon. The primary mineralogy of the soils is illite, with lesser quantities of 2:1 interstratified minerals. Clay minerals are heavily coated with Fe- and Mn-oxides, and the CEC and pH of the soils is on average 15 cmol kg-1 and 4.5, respectively.Subsurface Mn- and Fe- oxides catalyze the redox reaction (Figure 5 a,b). Cadmium is retained by the soil to a more significant extent relative to Co since its breakthrough is displaced farther to the right of nonreactive Br (Figure 3). Greater reactivity of Cd results from the dissociation of the CdEDTA complex by subsurface Fe-oxides, with subsequent formation of Fe(III)EDTA and free Cd2+ and CdCl+ (Figure 6). This is advantageous from a remedial prospective, since the transport of Cd will be significantly reduced due to electrostatic adsorption on the soil.
Flow interruption was imposed on the multicomponent experimental system in figure 3 at approximately 5 pore volume for a duration of 7 d in order to quantify the extent of physical and chemical nonequilibrium (see Reedy et al., 1996, Soil Sci. Soc. Am. J.). The observed concentration perturbations for nonreactive Br indicated that physical nonequilibrium (i.e. diffusion limitations) were prevalent within the system (Figure 3). Larger concentration perturbations for Co and Cd also suggested chemical nonequilibrium influenced the rate and extent CdEDTA dissociation and Co(II)EDTA oxidation. Since Co(II)EDTA is less stable than Co(III)EDTA in subsurface environments, it is desirable to reverse the undesirable effects caused by the soil and reduce Co(III)EDTA back to Co(II)EDTA. Experiments are being planned to incorporate metal-reducing bacteria into the soil columns so that Co(II) can be stabilized via bioreduction reactions.
Figure 3: Multicomponent displacement of
57Co(II)EDTA, 58Co(II)EDTA (not shown),
109CdEDTA, and Br- through an undisturbed column
of fractured weathered shale at a flux of 2 cm/d, I=0.03, and pH=3.8.
Flow interruption was initiated for 7 d at approximately 5 pore volumes
and the tracer input pulse was terminated at approximately 8.5 pore
volumes.
The fate and transport of 51Cr(VI) was quantified in undisturbed columns of the fractured shale media. The mobility of Cr(VI) was significantly influenced by interfacial sorption processes and redox reactions. The breakthrough of Cr(VI) was delayed relative to nonreactive Br, which reflected solid phase retardation via sorption and reduction to Cr(III) (Figure 7). Batch studies confirmed that Cr(VI) and Cr(III) were both strongly sorbed to the soil during transport, with retardation factors near 20 and 450, respectively (Figures 8 a,b). Cr(VI) sorption experiments were only 75% reversible using a Cl- matrix. X-ray Absorption Spectroscopy, performed at the Stanford Synchrotron Radiation Laboratory, revealed that the remaining surface bound Cr was as Cr(III). Thus, a significant portion of Cr(VI) was reduced to Cr(III) during transport in the subsurface media. The environmental implications of these reactions are pronounced since Cr(III) is less soluble and more reactive with the solid phase relative to Cr(VI). This is advantageous from a remediation perspective since the formation of Cr(III) decreases the magnitude of Cr(VI) mobility in subsurface environments, where Cr(VI) is a potent carcinogen and is quite toxic.
The state of Tennessee has granted us authorization to perform our groundwater radiotracer experiment at the WAG5 field facility. Site preparation is planned to begin in August and the multicomponent tracer experiment is scheduled for September 1997. A detailed description of the WAG5 field facility, showing instrumentation and previous characterization efforts, can be found at http://research.esd.ornl/hydrology/WAG5/.
To account for the multiscale physicochemical processes in structured geological formations and soils, we have incorporated a fracture flow module into the multiregion groundwater flow code 3DMURF (see progress report March, 1997 or the web site at http://www.csm.ornl.gov/~g4p/hpc97_2/hpc97_2.html). Two- and one-dimensional fractures such as plane fractures and pipes of various degrees of orientation in three-dimensional problem domains can be modeled in conjunction with multiple pore regions in the soil aggregates and rock mass. This additional capability allows a model user more degrees of freedom to conceptualize contaminant transport processes in natural, undisturbed porous media and to facilitate the modeling of these processes. Verification of the code is in progress.
Models of the Melton Branch Watershed Experimental Station (see http://research.esd.ornl.gov/hydrology/WATERSHD/INDEX.HTM) are currently being constructed to study the uncertainties of model predictions of off-site contaminant movement. We will use these models to conduct conditional simulations based upon field data previously collected at the site. The data collection effort was supported by the Subsurface Sciences Program of the Office of Health and Environmental Research, DOE. An abstract of this study was submitted to the joint Chapman/Outreach Conference sponsored by the American Geophysical Union and the Soil Science Society of America, and a conference proceeding is currently in preparation.
Having identified the primary visualization techniques for graphical display of raw and modeled data, the menu-based user interface was designed and continues to be refined. This user interface will allow scientists to quickly and easily display multi-dimensional data and to run the parallel computer model components. This interface will eventually communicate directly with the parallel model, accept user parameters, and display model results as desired by the user.
Work continues on the World Wide Web information for ETPI. The most recent information about the WAG5 field facility is now available at http://research.esd.ornl/hydrology/WAG5 /.