US Department of Energy

 

Date of this Version

7-2005

Citation

Pacific Northwest National Laboratory, July 2005.

Abstract

In 1996, an interim record of decision was issued for the 300-FF-5 Operable Unit on the Hanford Site in southeast Washington State. The record of decision specified the interim remedy as (1) continued monitoring of groundwater and (2) institutional controls to restrict groundwater use. This record of decision was based on a remedial investigation that suggested levels of uranium would decrease with time because of natural geochemical and hydrologic processes. A prediction was made that concentrations of uranium would decrease to the proposed drinking water standard or lower (20 μg/L) in 3 to 10 years from 1993. This prediction has not been realized, prompting investigations and update of the conceptual model for uranium in the 300 Area.

In June 2002, the U.S. Department of Energy (DOE) asked scientists at Pacific Northwest National Laboratory’s (PNNL) Remediation and Closure Science (RCS) Project to begin research on the 300 Area uranium plume, in collaboration with a small team of investigators from the Environmental Management Science Program, currently funded through the U.S. Department of Energy Office of Biological and Environmental Research. The study completed by the RCS Project, and summarized in this report, will be used to improve the conceptual model of the 300 Area uranium plume and develop a final record of decision for the 300-FF-5 Operable Unit.

Significant progress has been made on planned research and is summarized in this report. The information in this report is of a technical nature and is intended to provide the scientific basis for re-evaluation of the 300-FF-5 record of decision. The following are important conclusions and implications:

1. Residual hexavalent uranium U(VI) concentrations observed beneath the north and south process ponds (NPP and SPP) are heterogeneous and display no marked trend with depth. An average of 37.5% of the residual, sorbed uranium appears accessible to dissolution/desorption, but variation in this number between sediments is large. Dissolution/desorption extent was found to decrease with decreasing water content and at 21% water saturation was only 1 to 3% of total uranium.

2. Both precipitated and adsorbed U(VI) exists in the sediments. No evidence was found for precipitated U(VI) or metallic uranium. A precise demarcation of precipitated and adsorbed forms, and their relative concentrations is difficult. Adsorbed U(VI) predominates in sediments with total uranium <25 mg/kg.

3. The vadose zone sediments beneath both SPP and NPP will remain as potential source terms to maintain groundwater U(VI) concentrations at or above the drinking water standard. Their ultimate impact will be controlled by moisture flux rates through the vadose zone and their bicarbonate concentrations. Increasing groundwater levels at high river stage will solubilize sorbed U(VI) from the capillary fringe and lower vadose zone.