Projects & Programs
Conceptual Model Development of Contaminant Transport: 216-A-4 Crib
Researchers from PNNL are defining risks from past waste disposal activities performed in the Central Plateau region of the Hanford Site.
Specifically, project 52717, which started in 2006, involves detailed characterization and testing of sediment collected from the 216-A-4 Crib. The 216-A-4 Crib was operated as a specific retention crib between December 1955 and December 1958. During its operational period, the 216-A-4 Crib received 6.2 million liters (1.6 million gallons) of low-salt waste that was neutral to basic in pH. The inventory of contaminants discharged to the 216-A-4 Crib includes nearly 7 curies of cesium-137, more than 4 curies of strontium-90 and nearly 8 curies of plutonium-239.
Sediment grab samples, with occasional split spoon cores, have been collected vertically within the 216-A-4 Crib footprint every 0.7 m (2.5 ft) extending from approximately 6.7 m (22 ft) to more than 106 m (350 ft) below ground surface. PNNL chemists, geologists and hydrologists are conducting multi-tiered investigations on these sediment samples. The basic premise of a tiered sample analysis approach is to acquire information, through an incremental process, that can be used to best meet project objectives while conserving resources and limiting risk to workers. The first tier of tests (Tier I) includes those that 1) provide information paramount to identifying contaminants present in the samples, and 2) help determine the basic physicochemical properties of the sediments. Additionally, several of the Tier I analyses are nondestructive (i.e., they do not alter the properties of the samples so that the sediments can be used again in other tests). Upon completion of the Tier I tests, the data are evaluated and a decision is made as to the relevancy of Tier II testing. Tier II testing is focused on better resolving the nature and extent of contamination present in the samples. Tests employed during Tier II characterization activities include specialized extraction and analytical techniques in addition to tests focused on identifying differences in the physical properties of the sediments. The resulting data will be interpreted within the context of sediment types, the vertical extent of contamination, as well as the migration potential of the contaminants. These data will ultimately be used to develop conceptual transport models for key contaminants of concern.
This research will provide a foundation for Fluor Hanford to make remedial action decisions at the 216-A-4 Crib waste site. At the conclusion of this project, PNNL researchers will develop a formal PNNL data package documenting results of the multi-tiered studies.