Hydrogeology
The unconfined aquifer flows through the lower Hanford (U1) and upper Ringold formation sediments, and is in hydrologic continuity with the Columbia River which flows along the eastern margin of the site (see figure). The groundwater U plume exists primarily in the more permeable (Ksat = 980 to >5000 m/d) U1 formation paleo-flood deposits that unconformably overlay the less permeable (Ksat = 2 to 51 m/d) Ringold formation E/B Gravel. The Hanford sediments consist of unconsolidated pebble-to-boulder gravels and fine-to-coarse sands, with occasional fine-textured intercalations. The Hanford sediments in the area of the proposed IFRC are generally coarse grained, with sands and silts/clay packed in voids between rounded river cobble that exhibit frequent clay "skins". The void fill exhibits an apparent in situ structure in particle-size gradation that has not been well characterized. Preferential flowpaths are associated with partially or unfilled voids between gravel-textured cobble.
Major geohydrologic units and well placement in the 300 Area uranium plume;a) east-west cross section normal to the Columbia.
b) north-south cross section parallel to the Columbia.
The contaminant plume is constrained by the elevation of the Hanford-Ringold contact boundary and ranges from 1-15 m in thickness across the site. The Hanford-Ringold contact is an erosional discontinuity and reduced permeability boundary that varies in elevation throughout 300-FF-5. The topographic surface of the Ringold was created by high-energy paleo-flood and/or Columbia River flows eroding into the Ringold formation (Brown 1960). These channels with a northwest trending erosional axis, subsequently filled with younger, more permeable and unconsolidated Hanford formation sediments. The channel fill sediments constrain the uranium plume, and form the primary groundwater flow path through the plume region to the Columbia River. The vadose zone ranges from 1-15 m in thickness depending on land surface topography.
Unique aspects of the site are the hydrologic and geochemical conditions imposed by the boundary condition of the Columbia River on the east. During periods of low river flow, the hydrologic gradient across the site is 0.13 m/1.2 km toward the Columbia River. However, river stage in the Columbia can vary by 1 m daily as a result of dam operations and by 2 m seasonally as a result of snowmelt. The river acts as a line sink or source for groundwater, depending on river stage. Thus, the hydrologic gradient of the U plume can change by almost 180º between low and high river stage. At low stage, groundwater drains into the river, but at high stage chemically distinct river water flows into the river bank sediments and the aquifer. Generally, river stage is high in the spring and early summer, and low in late summer and fall, with superimposed daily variations (Lindberg and Bond, 1979). This hydrologic system causes seasonal and daily variations in well water levels throughout the 300 Area. It is responsible for variable groundwater flow trajectories in the permeable channel fill, and causes changes in water chemistry as a result of groundwater-river water mixing. The site groundwater has an ionic strength I = 0.03-0.06 with pH ~ 8.3 and is dominated by Na, Ca, Mg, and HCO3, SO4, and NO3. Columbia River water, in contrast, is dilute (I<0.001) with low alkalinity and near neutral pH, but can contain elevated concentrations of dissolved and particulate organic carbon that can potentially promote microbiologic activity.
Hanford formation sediments exposed by excavation in the 300 Area North Process Pond.
References
Brown DJ. 1960. Evaluation of Earth Samples from Churn-Drilled Wells. HW-67415, General Electric Company, Richland, Washington.
Lindberg JW and FW Bond. 1979. Geohydrology and Groundwater Quality Beneath the 300 Area, Hanford Site, Washington. PNL-2949, Pacific Northwest Laboratory, Richland, Washington.