John M. Zachara1 Calvin C. Ainsworth,1 Gordon E. Brown,
Jr.,2 and Jeffrey G. Catalano2
1Pacific Northwest National Laboratory, Richland, WA
2Stanford University, Stanford, CA
Chromate (hexavalent chromium as CrO42-) is a significant
groundwater contaminant at the U.S. Department of Energy (DOE) Hanford Site in
southeastern WA (Poston et al., 2001) where Pu was produced during WWII and the
cold war, and where DOE's largest inventory of legacy wastes remain. Chromate
in ground water is readily transported through the Pleistocene-age flood
deposits at Hanford, resulting in groundwater contamination plumes that
discharge CrO42- to the Columbia River, posing risk to
spawning salmon and downstream potable water supplies. The present work
suggests that ca 42% of the Cr known to be present in the plumes is immobile
due to precipitation of relatively insoluble Cr(III) phases, abating somewhat
the extent of the problem. The balance of chromium in the plumes occurs as
mobile chromate. Defining the chemical and physical forms of the contaminant,
such as is presented here, is the crucial first step to mitigating a
Waste Management Area (WMA) S-SX, the focus of this study, is one of 12
high-level waste tank farms at Hanford. The SX tank farm contains 15 massive,
single-shell, high-level waste tanks, each with 3,785 kL (1,000 kgal) capacity.
The SX waste tanks received self-boiling waste solutions from Hanford's
Reduction-Oxidation (REDOX) Plant in the mid 1950s. The REDOX process was used
to recover Pu from irradiated nuclear fuels. Hexavalent Cr [Cr(VI)] in the
form potassium dichromate was used as an oxidant in the REDOX process to
manipulate the valence states of Pu and U. The REDOX wastes were self-boiling,
and self-concentrated from the radioactive decay of short-lived isotopes.
During the 1960s and 1970s nine of the SX tanks developed leaks as a result of
the high thermal load from waste boiling and the caustic nature of the wastes.
One of the largest leaks was from SX-108 that discharged 57,760 L of hot,
highly concentrated REDOX waste to the vadose zone (i.e., the area of the
subsurface above the water table) in 1969. The leaked SX-108 tank waste (Table
1) contained high concentrations of salt, base, Cr,
137Cs+, 99Tc(VII), and
other radioactive and chemical constituents. Hanford personnel collected
subsurface samples beneath tank SX-108 in 2000 using a sophisticated slant
drilling device to assess the depth distribution and fate of tank waste
contaminants. Some of the retrieved core samples were the most radioactive
geomedia ever brought to the surface at Hanford.
Characterization measurements of the core samples indicated an unusual pattern
of chemical attenuation of CrO42- that required
explanation for risk assessment (Fig. 1). A well-defined vadose zone plume of
high concentration Na-NO3 (Fig. 1a) that was almost coincident with
Cr (Fig. 1b) was observed between depths of 23-32 m and 34-42 m in a region of
high residual heat (Fig. 1c). Model calculations indicate that temperatures at
25 m approached 100o C at the time of leakage in 1969. The
was freely soluble in water, but in contrast, sizable fractions of the total Cr
were sorbed (i.e. associated with solid sediments) and only extractable in
strong acid. The limited solubility of sorbed Cr, which was higher in the
upper regions of the core, implied that it existed in a precipitated or
strongly adsorbed state. Because there was little or no previous evidence for
adsorptive retardation of Cr(VI) in oxic Hanford vadose zone sediments, XAS
measurements were performed at SSRL to define Cr valence in sediment as a basis
for establishing a conceptual model of geochemical reaction.
Cr K-XANES spectra were measured on eight SX-108 samples (asterisk in Fig. 1)
using the Molecular Environmental Science Beam Line (BL 11-2) (Bargar et al., 2001) or the general XAFS beamline (BL 4-3) over the
energy range 5900 eV to 6350 eV. The samples were extremely radioactive and
contained up to 0.1 mCi/g of 137Cs+ (Liu et
al., 2003). The methods of Peterson et al. (1997) were used to analyze the
Cr(VI) content of the sediments from the intensity of the pre-edge feature.
The percent Cr(VI) was calculated from the sample specific pre-edge peak height
and positions using the following beamline-specific calibration curves:
BL 11-2: %Cr(VI) = 102.83(height) - 12.80
BL 4-3: %Cr(VI) = 128.64(height) - 12.77
The calibration curve correctly predicted the Cr(VI) percentages of three blind
samples that were used to test the calibration process. The normalized Cr
K-XANES spectra of the eight Hanford borehole samples all displayed the
presence of both Cr(VI) and Cr(III) (Fig. 2, Table 2). The largest Cr(III)
concentration [smallest Cr(VI)] was observed in SX-108 Sample 7A, which had the
highest pH and significant mineral alteration resulting from reaction of the
high pH waste with the sediment. In contrast, the highest Cr(VI)
concentrations were noted deep in the core where pH was ambient and where
mineral alteration was minimal. These data indicated that Cr(VI) present
initially in the REDOX waste was variably reduced by the Hanford sediments, and
that reduction extent correlated with sediment mineral alteration.
The XANES measurements and others not described here (Zachara et
allowed us to conclude that ferrous iron (Fe2+) was released from
the oxic Hanford sediment by base-induced, heat facilitated dissolution of
Fe(II)-containing minerals including biotite (iron-bearing aluminuosilicate
clay mineral), clinoclore (chlorite aluminosilicate), and ilmenite
(FeTiO3). The Fe(II) so liberated was an effective reductant of
Cr(VI), which then precipitated as an insoluble Cr(OH)3 phase or was
incorporated within iron-bearing precipitated phases. This reductive
precipitation reaction apparently occurred after the waste plume had reached
its current configuration and depth. The results demonstrate that a minimum of
42% of the total Cr inventory is effectively immobilized as Cr(III)
precipitates that are unlikely to dissolve appreciably under the low drainage
conditions of the Hanford vadose zone. The remaining Cr(VI), however, will be
free to migrate to groundwater unless surface infiltration at the tank farm
surface is controlled. These and other ongoing studies at SSRL on contaminated
sediments from different Hanford tank farms are providing key scientific
insights on the hazards posed by such extreme chemical and radioactive
materials and the most effective, long-term environmental management strategies
to deal with them.
Poston T. M., R. W. Hanf, R. L. Dirkes, and L. F. Morasch. 2001.
Hanford Site Environmental Report for Calendar Year 2000. Pacific Northwest
National Laboratory, Richland, WA.
- Jones, T. E., R.
A. Watrous, and G. T. Maclean. 2000. Inventory Estimates for Single-Shell
Tank Leaks in S, and SX Tank Farms, RPP-6285, CH2M HILL Hanford Group,
Inc., Richland, Washington.
- Serne R. J., H. T.
Schaef, B. N. Bjornstad, B. A. Williams, D. C. Lanigan, D. G. Horton, R. E.
Clayton, V. L. LeGore, M. J. O'Hara, C. F. Brown, K. E. Parker, I. V.
Kutnyakov, J. N. Serne, A. V. Mitroshkov, G. V. Last, S. C. Smith, C. W.
Lindenmeier, J. M. Zachara, and D. B. Burke. 2001a. Geologic and Geochemical
Data Collected From Vadose Zone Sediments from Borehole SX 41-09-39 in the S/SX
Waste Management Area and Preliminary Interpretations. Pacific Northwest
National Laboratory, Richland, WA.
Serne R. J., G. V. Last, G. W. Gee, H. T. Schaef, D. C. Lanigan, C. W.
Lindenmeier, R. E. Clayton, V. L. LeGore, R. D. Orr, M. J. O'Hara, C. F. Brown,
D. B. Burke, A. T. Owen, I. V. Kutnyakov, and T. C. Wilson. 2001b. Geologic
and Geochemical Data Collected From Vadose Zone Sediments From Borehole SX
41-09-39 in the S/SX Waste Management Area and Preliminary Interpretations.
Pacific Northwest National Laboratory, Richland, WA.
Bargar, J. R., G. E. Brown, Jr., I. Evans, T. Rabedeau, M. Rowen, and
J. Rogers. 2001. A new hard X-ray XAFS spectroscopy facility for environmental
samples, including actinides, at the Stanford Synchrotron Radiation Laboratory.
Proc. 2nd Euroconference and NEA Workshop on Speciation, Techniques, and
Facilities for Radioactive Materials at Synchrotron Light Sources.
Liu, C., J. M. Zachara, S. C. Smith, J. P. McKinley, and C. C.
Ainsworth. 2003. Desorption kinetics of radiocesium from subsurface sediments
at Hanford Site, USA. Geochim. Cosmochim. Acta 67(16), 2893-2912.
Peterson, M. L., A. F. White, G. E. Brown Jr., and G. A. Parks. 1997.
Surface passivation of magnetite by reaction with aqueous Cr(VI): XAFS and TEM
results. Environ. Sci. Technol. 31, 1573-1576.
Zachara, J. M., C. C. Ainsworth, G. E. Brown, Jr., J. G. Catalano, J.
P. McKinley, O. Qafoku, S. C. Smith, J. E. Szecsody, S. J. Traina, and J. A.
Warner. 2004. Chromium speciation and mobility in a high level nuclear waste
vadose zone plume. Geochim. Cosmochim. Acta 68(1), 13-20.