Dissolution rates and vadose zone drainage from strontium isotope measurements of groundwater in the Pasco Basin, WA unconfined aquifer

Strontium isotope ratios (87Sr/86Sr) measured in groundwater samples from 273 wells in the Pasco Basin unconfined aquifer below the US Department Of Energy Hanford Site show large and systematic variations that provide constraints on groundwater recharge, weathering rates (for saturated zone dissolution) of the aquifer host rocks, communication between unconfined and deeper confined aquifers, and vadose zone–groundwater interaction. The impact of millions of cubic meters of wastewater discharged to the vadose zone (103–105 times higher than ambient drainage) shows up strikingly on maps of groundwater 87Sr/86Sr. Extensive access through the many groundwater monitoring wells at the site allows for an unprecedented opportunity to evaluate the strontium geochemistry of a major aquifer, hosted primarily in unconsolidated sediments, and relate it to both long term properties and recent disturbances. Groundwater 87Sr/86Sr increases systematically from 0.707 to 0.712 from west to east across the Hanford Site, in the general direction of groundwater flow, as a result of addition of Sr from the weathering of aquifer sediments and from diffuse drainage through the Pasco Basin vadose zone. The lower 87Sr/86Sr groundwater reflects recharge waters that have acquired Sr from Columbia River Basalts. Based on a steady-state model of Sr reactive transport and drainage, there is an average natural drainage flux of 0–1.4 mm/yr near the western margin of the Hanford Site, and ambient drainage may be up to 30 mm/yr in the center of the site assuming an average bulk rock weathering rate of 10−7.5 g/(g/yr). The weathering rates cannot be higher than those assumed by more than a factor of 2–3, in order to match Sr isotope data.