Deep Vadose Zone Characterization at the Hanford Site: Accomplishments from the Last Ten Years

The overall goal of the Tank Farm Vadose Zone Project, led by CH2M HILL Hanford Group, Inc., is to define risks from past and future single-shell tank farm activities at Hanford. To meet this goal, CH2M HILL Hanford Group, Inc. tasked scientists from Pacific Northwest National Laboratory (PNNL) to perform detailed analyses on vadose zone sediments collected within/adjacent to the twelve single-shell tank farms contained within Hanford’s Central Plateau region. This work has been performed under the Resource Conservation and Recovery Act (RCRA) Corrective Action Program and is associated with the Hanford Federal Facility Agreement and Consent Order. While there are many facets to the laboratory studies employed by PNNL, the four primary objectives of this work are to: identify the type and quantity of contamination present, understand the physical processes that affect the transport of contaminants in the vadose zone sediments, when practical, identify the source(s) of the contamination found in the sediment samples, and when practical, determine if a link can be made between the vadose zone contamination observed and any known groundwater contaminants in the vicinity. Since its inception in 1997, PNNL’s Vadose Zone Characterization Project has evolved to better meet these four key objectives. The single-largest adaptation of the Vadose Zone Characterization Project over its ten years of operation was the advent of a tiered sample analysis approach. Use of a tiered approach allows resources to be focused on those samples/tests that provide the largest amount of scientific information to best meet the four key project objectives within the budget available. Another significant, but more recent, adaptation has been the implementation of a rapid turnaround characterization process in which sediment samples are analyzed in near real-time to aid drilling activities within the tank farms. This paper highlights details of the characterization activities performed as well as significant findings from the last ten years.