SR AND U ISOTOPES AS INDICATORS OF INFILTRATION FLUX AND DISSOLUTION IN VADOSE ZONE SEDIMENTS; HANFORD, WA

The Sr isotope systematics of groundwaters and host soils can in theory be used to evaluate the ratio of fluid flux to dissolution rate. Where one or the other of these quantities can be independently estimated, the other can then be determined. The U series isotopes are much more sensitive to dissolution rate than fluid flux and hence combining U and Sr isotopes can yield estimates of both quantities. We have applied this approach to estimating the long-term average infiltration flux at a site on the Hanford/DOE complex in eastern Washington. Concern over the migration of contaminated groundwater towards the Columbia River requires accurate estimates of solute travel times through the vadose zone. The dense network of monitoring wells, the man-made disturbances to the hydrologic system, and the availability of related geologic and hydrologic data, also make the Hanford site a unique study area for basic hydrologic and geochemical processes. Sr isotopic analyses were performed on a 70 meter uncontaminated core proximal to several documented tank leaks. ⁸⁷Sr/⁸⁶Sr values were measured for pore water extractions, concentrated nitric acid leachates, and bulk sediment to determine the distribution and mobility of Sr with depth in the core. The ⁸⁷Sr/⁸⁶Sr values of the pore water rinses decrease systematically from 0.721 near the surface to 0.712 at a point just above the water table at ~70 meters depth. Similarly, the ⁸⁷Sr/⁸⁶Sr signatures of the acid leachates are generally lower than those of the pore water; 0.719 near the surface to 0.713 at depth. A simple mass transfer model was used to infer flow velocity and infiltration fluxes within the core. The infiltration flux is estimated to be 3 ±2 cm/yr using a bulk soil weathering rate based on studies of soils of similar age. The inferred infiltration flux is considerably greater than previously estimated , but also represents an average value for the past 100 to 1000 years. Measurements of [²³⁸U] and ²³⁴U/²³⁸U compositions are currently underway in order to better evaluate the bulk dissolution rate for the sediments.