Projects & Programs
Conceptual Model Development of Contaminant Transport at Waste Management Area T
Researchers from PNNL are identifying environmental risks from past waste-disposal activities performed in the Central Plateau region of the Hanford Site.
Specifically, project 52716, which started in 2006, involves detailed characterization and testing of sediment collected from two Resource Conservation and Recovery Act (RCRA) groundwater monitoring wells (299-W11-48 and 299-W10-32) being installed within Waste Management Area T (WMA T). WMA T overlies the 200-ZP-1 groundwater operable unit, which has recently been the subject of increased scrutiny. Groundwater monitoring well 299-W11-25B was drilled during early 2005, and depth-discrete groundwater samples collected during drilling indicated that groundwater in the vicinity of this well was contaminated with technetium-99 (99Tc) in excess of 180,000 pCi/L (the current drinking water standard for 99Tc is 900 pCi/L). Additionally, the highest 99Tc activity was observed in a sample collected 10 m (32 ft) below the water table. This was quite remarkable because most of the RCRA groundwater monitoring wells in the vicinity of WMA T are screened within 5 m (16 ft) of the water table. As a result of this finding, several additional groundwater monitoring wells are being installed in WMA T including the two being characterized as part of this effort.
Sediment grab samples, with occasional split-spoon cores, are being collected vertically within WMA T every 1.5 m (5 ft) extending from the ground surface to the terminating depth of each borehole. PNNL chemists, geologists and hydrologists will conduct 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, the likely source(s) 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 be combined with results from previous characterization activities at WMA T to assist Fluor Hanford in its assessment of contamination and contaminant transport in the vadose zone and groundwater. At the conclusion of the project, PNNL researchers will develop a formal PNNL data package documenting results of the multi-tiered studies.