Kisa Mwakanyamale

Use of Induced Polarization to Characterize the Hydrogeologic Framework of the Zone of Surface‐Water/Groundwater Exchange at the Hanford 300 Area, WA

An extensive continuous waterborne electrical imaging (CWEI) survey was conducted along the Columbia River corridor adjacent to the U.S. Department of Energy (DOE) Hanford 300 Area, WA, in order to improve the conceptual model for exchange between surface water and U-contaminated groundwater. The primary objective was to determine spatial variability in the depth to the HanfordRingold (H-R) contact, an important lithologic boundary that limits vertical transport of groundwater along the river corridor. Resistivity and induced polarization (IP) measurements were performed along six survey lines parallel to the shore (each greater than 2.5 km in length), with a measurement recorded every 0.5-3.0 m depending on survey speed, resulting in approximately 65,000 measurements. The H-R contact was clearly resolved in images of the normalized chargeability along the river corridor due to the large contrast in surface area (hence polarizability) of the granular material between the two lithologic units. Cross sections of the lithologic structure along the river corridor reveal a large variation in the thickness of the overlying Hanford unit (the aquifer through which contaminated groundwater discharges to the river) and clearly identify locations along the river corridor where the underlying Ringold unit is exposed to the riverbed. Knowing the distribution of the Hanford and Ringold units along the river corridor substantially improves the conceptual model for the hydrogeologic framework regulating U exchange between groundwater and Columbia River water relative to current models based on projections of data from boreholes on land into the river.

Use of Time‐Domain Induced Polarization for Lithology Identification: A Case Study from the Hanford 300‐Area

Use of the induced polarization (IP) method, as being applied to determine the hydrostratigraphic framework at the Hanford 300-Area, Washington, offers advantages over electrical resistivity (ER) in yielding information about the physicochemical characteristics of the subsurface. The IP response in soils results from surface polarization that is largely determined by lithology. For non-metallic soils, IP typically shows a weak dependence on fluid conductivity, but strong nearlinear relationships to textural parameters (surface area, grain size, etc.). Compared to the ER method, IP has smaller signal-to-noise ratio, requiring additional care during data acquisition and processing. In particular, appropriate data weight needs to be considered for successful inversion of IP datasets.