Scientific Investigations Report 2006-5122
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5122
The primary objective of developing a conceptual model of ground-water flow beneath the INL and vicinity is to identify features of the flow system that most affect interpretations of contaminant transport in the aquifer. The conceptual model simplifies real-world conditions and interactions through time and in space, primarily because of scaling considerations, computational constraints, and data availability. Although hydrogeologic judgment is used in determining input to the model to supplement or substitute for the lack of data, simplification invariably decreases the realism of model results. This decrease is a necessary trade off so that the model can represent large-scale, complex systems and processes that include the effects of many small-scale features and interactions.
Two of the more significant simplifications in the conceptual model are (1) representation of individual hydrogeologic units as homogeneous anisotropic porous media, and (2) implicit representation of infiltration recharge. The first simplification largely precludes simulating the tortuous character of flow that occurs naturally in heterogeneous and anisotropic fractured porous media; and the second simplification precludes realistic simulations of residual contaminant movement through the 200- to 600-ft thick unsaturated zone, a source of contaminants that is likely to persist over a long period of time.
The conceptual model implies that most contaminant movement beneath the INL takes place in the thin, densely fractured basalts and interbedded sediment of hydrogeologic unit 1 that compose most of the upper 200 ft of the aquifer. This hypothesis is based on (1) definition of the hydraulic properties and the inferred distribution of the three hydrogeologic units composing the aquifer, and (2) interpretation of the generalized northeast-to-southwest cross section of ground-water flow across the model area. The hypothesis is consistent with conclusions reached by earlier investigators who studied contaminant migration and aquifer characteristics at the INL and noted that waste plumes “…generally remain as relatively thin lenses within about 250 feet of the water table” (Robertson, 1974, p. 6). The current conceptual model still needs to be verified through field testing, guided by the results of future flow-and-transport modeling.
The features of the flow system represented in the conceptual model that significantly affect contaminant transport in the aquifer at the INL and vicinity are (1) the implicit representation of infiltration recharge through the unsaturated zone that precludes realistic simulation of flow and future contaminant movement through the unsaturated zone, (2) preferential flow along highly conductive interflow zones, primarily in thin, densely fractured basalts of hydrogeologic unit 1, implying large horizontal to vertical anisotropy and rapid lateral movement of contaminants in the upper 200 ft of the aquifer, (3) the restricted downward movement of flow and contaminants in hydrogeologic unit 1 into the older, less conductive massive basalts of hydrogeologic unit 2 beneath the INL, implying limited dilution and dispersion of contaminants near their sources, (4) the inferred downward movement and deeper circulation of water upgradient of where the massive, less densely fractured basalt of hydrogeologic unit 2 intersects the water table southwest of the INL, implying enhanced dispersion and dilution of contaminants away from their sources and at deeper depths in the aquifer, and (5) enhanced dispersion of contaminants resulting from the spatial and temporal variability of streamflow-infiltration recharge that is in close proximity to contaminated ground water.
U.S.
Department of the Interior | U.S. Geological
Survey
Persistent URL: https://pubs.water.usgs.gov/sir20065122
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