Earth Sciences Division Berkeley Lab
Environmental Remediation Technology Program
Annual Report 2000–2001
ISOTOPE TRACKING OF WATER INFILTRATION THROUGH THE UNSATURATED ZONE AT HANFORD Mark E. Conrad and Donald J. DePaolo Contact: Mark E. Conrad, 510/486-6141, msconrad@lbl.gov
RESEARCH OBJECTIVES
Significant amounts of radioactive waste were generated by the production of fuel for nuclear weapons at the Hanford site in Washington. High-level waste is currently stored in tanks, some of which are known to be leaking. In addition, large quantities of low-level waste were discharged directly to the ground. As a result of these practices, radionuclide contamination has been detected in the groundwater at the site and poses a significant threat to the nearby Columbia River. To examine the pathways of fluid and contaminant transport through the vadose zone, we undertook an isotopic and chemical study of pore-water samples from a borehole through the unsaturated zone.
APPROACH
The stable hydrogen and oxygen isotope compositions of water provide natural labels for different water sources. Further, evaporation modifies the isotopic signature of water, producing a distinctive signal that can be used to identify affected fluids. In the area of the borehole, the potential inputs to the vadose zone (local precipitation, process waters derived from the Columbia River) have isotopic compositions in the same range as the groundwater. However, evaporation appears to have significantly affected most of the vadose zone waters in the area. To quantify these effects, the isotopic compositions of 32 samples of pore water that were vacuumdistilled from vadose zone core samples have been analyzed. The chemical compositions of the pore waters were also measured by analyzing 1:1 de-ionized water leaches of the core material.
precipitation/Columbia River water. Beneath 2 m, however, most of the pore waters are shifted to higher δ18O values, indicating that they have also evaporated. Two pore-water samples from the deeper part of the core did not have an evaporated signal (the 44.7 and 71.8 m samples). The deeper sample is from the saturated zone and reflects the isotopic composition of the groundwater. The other sample is from a perched water zone above a caliche layer in the core. This perched water sample was the only pore-water sample with significantly elevated concentrations of several contaminants (Se, Mo, Cr, U, NO3–, SO42–, and possibly 99Tc).
SIGNIFICANCE OF FINDINGS
Two key aspects of these data have significant implications for fluid infiltration at Hanford: • The 2–4% shift in the oxygen isotope composition of most of the pore-water samples indicates that they have undergone 20–35% evaporation. This has important implications for natural infiltration rates at the site. No detailed model is currently available for translating the average oxygen isotope shift to an infiltration flux. However, to first order, it can be reasoned that the changes observed in these samples correspond to infiltration rates of approximately 2.5–6.0 cm/yr. • The isotopic composition of the perched water sample is clearly distinct Figure 1. Oxygen isotope compositions of from the pore water above and below this pore water samples distilled from a borehorizon, indicating that it was not derived hole drilled through the Hanford unsatufrom direct, vertical infiltration. Further, the rated zone elevated levels of contaminants in the perched water imply that this water was ACCOMPLISHMENTS partially derived from Hanford operations. The most likely Most of the measured isotopic compositions of the poresource of this water is a set of surface holding ponds that were water samples were shifted by evaporation. The oxygen isotope located ~50 m from the borehole, although it is also possible that compositions of the pore-water samples are plotted versus there could be some input from a tank farm further up gradient. depth on Figure 1. The δ18O value of the shallowest sample ACKNOWLEDGMENTS (taken at ~0.5 m depth) is much higher than either local precipThis work has been supported by the Assistant Secretary of itation or process water derived from the Columbia River (the the Office of Environmental Management, Office of Science and two potential sources of surface water), signifying that it is Technology, Environmental Management Science Program of strongly evaporated. This is typical of near-surface soil waters, the U.S. Department of Energy, under Contract No. DE-AC03especially in arid and semi-arid environments. At 2 m depth, 76SF00098. the δ18O value of the pore water is equal to the average value of
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