Use of Sr isotopes to determine long-term average vadose zone
infiltration flux at hanford 200w area
Katharine Maher, Donald J. DePaolo, and Mark S. Conrad
Research Objectives
Infiltration rates are an important variable in assessing
contaminant transport at the Hanford Site, where the majority of
the contaminants are contained in the upper 30 m of a thick (~70
m) vadose zone. The transit time to the aquifer for a given contaminant
largely determines the remediation action, especially for mobile
radioactive contaminants. Efforts to quantify infiltration rates
at the Hanford Site have been hindered by the complexity of the
hydrogeologic setting, the thick vadose zone, and recent anthropogenic
disturbances. Independent estimates of infiltration rates range
from 0.01 mm/yr to 200 mm/yr, depending on the method and surface
cover.
Using variations in the natural strontium (Sr) isotopic compositions
of vadose zone pore waters and sediments, the infiltration flux
can be quantified using a simple one-dimensional reactive transport
model. The strontium isotope ratio (87Sr/86Sr)
of pore water is a very sensitive indicator of interaction with
the rock matrix. Because of the relatively low concentration of
Sr in the pore water relative to the sediments, even minor exchange
between the solid and fluid phases will quickly shift the pore-water
Sr isotope ratio towards that of the rock. The pore water 87Sr/86Sr
value is thus controlled by a balance between the infiltration flux
and weathering of the sediments.
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 Steady-state model best-fit trajectory and weathering rate profile: (a) the 87Sr/86Sr values for the pore waters decrease systematically with depth, from a high value of 0.721 near the surface towards the bulk sediment average value of 0.711; (b) weathering rates required to fit the data for various infiltration rates, and the range of estimated rates based on soils data from White et al. (1996). The models suggest that the infiltration flux for the site is 5 to 10 mm/yr. The method shows potential for providing long-term in situ estimates of infiltration rates for deep heterogeneous vadose zones.
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