USING NATURAL ISOTOPIC TRACERS TO UNRAVEL THE HYDROLOGY OF THE HANFORD 300 AREA AND ITS IMPACT ON URANIUM MOBILITY AT THE SITE

The 300 Area of the Hanford Site was used for producing fuel rods for Hanford nuclear reactors and research. Waste materials from these activities were disposed of in trenches and ponds in the 300 Area. Following detection of a plume of uranium in the groundwater, the ponds were excavated. However, the groundwater uranium plume has persisted. The 300 Area is located on the banks of the Columbia River above its confluence with the Yakima River. The geology consists of Hanford formation flood deposits overlying the sedimentary Ringold Formation, above the Columbia Flood basalts. This results in four potential sources of groundwater in the 300 Area; local infiltration, upwelling from the underlying basalts, the Columbia River, and the Yakima River. It is possible to quantify the contributions of all four of these sources using a combination of δD, δ¹⁸O and ^87/86Sr of the groundwater. The groundwater in the upper part of the Hanford formation has high ^87/86Sr values (0.712 to 0.715) that could be attributed to either the Columbia River or local infiltration. However, local infiltration has higher δD and δ¹⁸O values (-110‰ to -120‰ and -14‰ to -15‰, respectively) than Columbia River water (-125‰ to -135‰ and -16.5‰ to ‑17.5‰, respectively). Based on this difference, the primary source of groundwater in the upper part of the Hanford formation is derived from local infiltration, with an increasing component of Columbia River water towards the river. The contribution from the Columbia fluctuates through the year, peaking at high river flow during June and July. The Columbia River component in the upper Hanford aquifer can be quantified based on the degree of isotopic shift. The groundwater from the lower part of the Hanford formation and the Ringold Formation has lower ^87/86Sr values (0.708 to 0.710), which are in the same range as groundwater from the basalts and the Yakima River. However, their δD and δ¹⁸O values (-130‰ to -140‰ and -17‰ to -18‰, respectively) are characteristic of the basalt groundwater and much lower than the Yakima River, indicating that the primary source of groundwater is upwelling from the basalts. These results indicate that the upper and lower sections of the Hanford formation are hydraulically isolated from one another.