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The primary objectives of this characterization activity were to: 1) determine the extent of uranium contamination in the sediments, 2) quantify the leachable (labile) concentration of uranium in the sediments, and 3) create a data set that could be used to correlate the present data to existing 300 Area data. In order to meet these objectives, sediments collected from wells 399-2-5 (C5708), 299-3-22 (C5706) and 299-4-14 (C5707) were analyzed for moisture content, 1:1 sediment:water extracts (which provide soil pH, electrical conductivity [EC], cation, and anion data), total carbon and inorganic carbon content, 8 M nitric acid extracts (which provide a measure of the total leachable sediment content of the contaminants), microwave-assisted digestion (which results in total digestion of the sediment), and carbonate leaches (which provide an assessment of the concentration of labile uranium present in the sediments). Additionally, pore waters present in select samples were extracted using ultracentrifugation. The mobility characteristics of uranium vary within the multiple subsurface zones that contain residual contaminant uranium. Principal subsurface zones include 1) the vadose zone, 2) a zone through which the water table rises and falls, 3) the aquifer, and 4) a zone where groundwater and river water interact beneath the river shoreline. Principal controls on mobilization include the form of the residual uranium (e.g., crystalline minerals, amorphous precipitates/coatings, sorbed onto sediment), the transporting medium (e.g., water infiltration from the land surface, groundwater), and the rate of exchange between the form and transporting medium. The bicarbonate content of aqueous media strongly influences the rate of exchange, with relatively higher content enhancing mobility. Groundwater has a higher bicarbonate content than river water or other freshwater sources, such as utility and potable water systems. The variety of processes affecting the mobility of uranium in the subsurface, along with the numerous potential compartments where residual contamination may be located, presents challenges for predicting uranium movement through environmental pathways. The processes responsible for the persistence of the plume may involve cycling of uranium between the aquifer and overlying zone through which the water table fluctuates. Contaminated groundwater is moved upward into the lower vadose zone, and when the water table subsequently falls, contaminated moisture is left behind. Some of the uranium in groundwater may become sorbed to sediment in that zone, to subsequently slowly release. Also, near the Columbia River in the zone of groundwater/river water interaction, where the bicarbonate content is lowered because of infiltrating river water, the tendency for uranium to adsorb onto sediment is enhanced, thus slowing dissipation via the groundwater pathway. Fluctuations in the Columbia River stage are the driving mechanism for the rise and fall of the water table beneath the 300 Area, and also for creating the dynamic hydraulic and geochemical environment found in the zone of interaction beneath the shoreline

Uranium Contamination in the 300 Area: Emergent Data and their Impact on the Source Term Conceptual Model