SR ISOTOPE COMPOSITIONS OF GROUNDWATER IN THE HANFORD SITE UNCONFINED AQUIFER AS A RECORD OF FLOW, RECHARGE, AND VADOSE ZONE INFILTRATION

Sr isotope compositions from more than 260 wells in the unconfined aquifer below the contaminated Hanford Site, WA provide a means to image groundwater recharge, infiltration, and perturbations from site activity.  Natural recharge to the unconfined aquifer primarily comes from the exposed basalt of the Yakima Ridge and Rattlesnake Hills at the western margin of the site, and thus groundwater in this area has lower ⁸⁷Sr/⁸⁶Sr (~0.707) than either Columbia River water (0.714±0.0004) or water that has equilibrated with the granitic sediments (~0.712).  Anthropogenic recharge of the unconfined aquifer from disposed Columbia River water has resulted in plumes of high ⁸⁷Sr/⁸⁶Sr groundwater down gradient from infiltration ponds.  Transient, high concentration Sr flushed out of the vadose zone by infiltrating disposal water at the 200 areas dominates the shift of Sr isotopes caused by weathering of aquifer sediments.  Low ⁸⁷Sr/⁸⁶Sr water south of Gable Mountain is evidence of upwelling from the upper confined aquifer, which may affect the shape of contamination plumes in this area.

Groundwater ⁸⁷Sr/⁸⁶Sr increases systematically from 0.707 to 0.712 along aquifer flow paths, generally running west to east across the Hanford Site.  This observed variation in groundwater Sr isotope composition is modeled as a process whereby lower ⁸⁷Sr/⁸⁶Sr recharge waters are shifted toward a ratio of 0.712, due to the addition of Sr from weathering of the aquifer sediments and infiltration through the vadose zone.  The degree to which groundwater ⁸⁷Sr/⁸⁶Sr approaches the dissolving or infiltrating Sr is governed by the groundwater velocity, infiltration flux, aquifer thickness, and by weathering rates in the aquifer sediments.  Infiltrating vadose zone pore waters are approximately an order of magnitude higher in Sr concentration than the groundwater, and therefore have a strong effect on the Sr isotope composition of groundwaters.  Combining measurements of hydraulic head and aquifer thickness with measurements of groundwater ⁸⁷Sr/⁸⁶Sr and Sr concentration allows for estimates of vadose zone infiltration flux.  The ⁸⁷Sr/⁸⁶Sr increase along a "background" profile across the Hanford site is consistent with an average vadose zone flux of 5-8 mm/yr, based on a steady-state model of Sr reactive transport and infiltration.