Hydrogeologic Model for the Gable Gap Area, Hanford Site

Bjornstad, Bruce N.; Thorne, Paul D.; Williams, Bruce A.; Thomas, Gregory S.; Thompson, Michael D.; Ludwig, Jami L.; Lanigan, David C.

doi:10.2172/992820

Cited by 5 publications

(2 citation statements)

References 21 publications

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“…Bedrock beneath the 200 East Area consists of Miocene basalt flows of the Columbia River Basalt Group (Myers et al 1979). These are overlain by fluvial sediments deposited and repeatedly scoured over the past 10.5 million years by the ancestral Columbia and Salmon‐Clearwater River systems (Lindsey and Gaylord 1990; Bjornstad et al 2010), and by Pleistocene cataclysmic flooding (U.S. DOE 2002). A northwest to southeast trending anticline occurs to the north and east of LLBG WMA‐1.…”

Section: Introductionmentioning

confidence: 99%

Measuring a Low Horizontal Hydraulic Gradient in a High Transmissivity Aquifer

McDonald¹

2021

Groundwater

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High transmissivity aquifers typically have low hydraulic gradients (i.e., a flat water table). Measuring low gradients using water levels can be problematic because measurement error may be greater than the true difference in water levels (i.e., a low signal‐to‐noise ratio). In this study, the feasibility of measuring a hydraulic gradient in the range of 10−6 to 10−5 m/m was demonstrated. The study was performed at a site where the depth to water from land surface ranged from 40.1 to 94.2 m and the aquifer transmissivity was estimated at 41,300 m2/d (hydraulic conductivity of 18,800 m/d). The goals of the study were to reduce measurement error as much as practicable and assess the importance of factors affecting water level measurement accuracy. Well verticality was the largest source of error (0.000 to 0.168 m; median of 0.014 m), and geodetic survey of casing elevations was the next most important source of error (0.002 to 0.013 m; median of 0.005 m). Variability due to barometric pressure fluctuations was not an important factor at the site. Hydraulic heads were measured to an accuracy of ±0.0065 m, and the average hydraulic gradient was estimated to be 8.0 × 10−6 (±0.9 × 10−6) m/m. The improvement in accuracy allowed for two reversals in the groundwater flow direction to be identified, after which the gradient averaged 2.5 × 10−5 (±0.4 × 10−5) m/m. This study showed it is possible to sufficiently control sources of error to measure hydraulic gradients in the 10−6 to 10−5 m/m range.

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Section: Introductionmentioning

confidence: 99%

Measuring a Low Horizontal Hydraulic Gradient in a High Transmissivity Aquifer

McDonald¹

2021

Groundwater

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“…This simplified layer-125 cake depositional model has been complicated by the depositional process, and subsequent removal, of Across the Hanford Site, these stratigraphic units have continuously been mapped to build a 3D geologic 140 framework model (GFM) by interpolating depths of different geologic contacts interpreted from approximately 1500 boreholes (CHPRC, 2018). Borehole datasets may include drillers' descriptions, geologists' descriptions, geophysical logs, grain-size analyses, and/or sediment photographs, and were also used to establish the presence of ancestral fluvial channels or paleochannels across the site (Bjornstad et al, 2010;Reidel and Chamness, 2007). The 3D GFM is expected to be accurate in areas with higher 145 borehole densities; however, it may have more uncertainty in areas with sparsely located boreholes or areas where subsurface heterogeneity exists at length scales smaller than the borehole separation.…”

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confidence: 99%

Stratigraphic Identification with Airborne Electromagnetic Methods at the Hanford Site, Washington

Jaysaval

Robinson

Johnson

2020

Preprint

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Abstract. Stratigraphic units can influence the fate and transport of subsurface contaminants within groundwater. Units having coarse-grained sediments act as preferential flow pathways, and therefore can accelerate the transport of contaminants to reach human and ecological receptors. At legacy waste sites, detailed knowledge of subsurface stratigraphy can be used for effective monitoring and remediation planning to help minimize risk to human health and the environment. Airborne electromagnetic (AEM) methods can non-invasively provide information on kilometer-scale or larger subsurface stratigraphic features and fill informational gaps in directly sampled data from sparsely located boreholes. In this paper, we present inversion results of a 412 line-km frequency-domain AEM survey to delineate subsurface stratigraphic features at the Hanford Site, located in southeastern Washington State. The inversion was performed using a massively parallel 3D electromagnetic modeling and inversion code, where the modeling is based on solving frequency-domain Maxwell’s equations using an unstructured-mesh finite-element method and the inversion employs a Gauss-Newton optimization scheme. The results are compared to an underlying geologic framework model (GFM), built by interpolating contact depths of stratigraphic units interpreted from site borehole datasets. In areas with good borehole coverage, the inversion results show a good match with the GFM to a depth of about 60 m. Outside of these areas, the inversion results exhibit inconsistencies from the assumptions made to create the GFM, demonstrating that the AEM survey results can be used to improve the understanding of the geological conceptual model.

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