Scale and Hydrogeologic Complexity in Models of Ground-Water Flow for Newark-Basin Aquifers (Goode and others, 2004)
D.J. Goode, J. Lewis-Brown, D.W. Risser and L.A. Senior
U.S. Geological Survey
Ground-water flow models are used for estimation of aquifer properties and simulation of regional or basin-scale flow, among other uses. These applications are difficult in Newark-Basin aquifers due to the highly heterogeneous flow properties of the fractured rocks. We discuss the application of different models for analysis of ground-water flow at different scales in these complex formations, and efforts to develop a unified modeling approach. Most ground-water flow in Newark-Basin aquifers occurs in fractures, joints and bedding-plane partings, not within the primary porosity of the rock matrix. These openings are typically constrained within sedimentary units or beds that have a regional geometric character, usually non-horizontal, and that may be faulted. Hydraulic properties vary dramatically between the openings and the rock matrix, between adjacent beds, and also vary, though less dramatically, in areal extent within beds. Despite these complexities, nearly all models of flow in these aquifers are based on porous-media concepts, and on assumptions of relatively uniform hydraulic properties, at least within major units. With sufficient discretization, and specification of spatially-variable hydraulic properties, such models have been shown to adequately model regional-scale flow, often invoking anisotropy in aquifer properties to mimic the effects of fracture and bed geometry. Improvements in computer power and improvements in site investigation, well-field and borehole data collection have allowed the construction of increasingly complex flow models. For example, recent site-scale models incorporating dipping, hydraulically-distinct beds have been used to analyze drawdown during short-duration aquifer tests, and to simulate advective flow paths for dissolved contaminants. In other cases, aquifer tests are analyzed using spatially-uniform porous media models, and the resulting aquifer properties are used to estimate spatially-variable hydraulic properties in a regional-scale model. Analysis of borehole packer tests of isolated intervals yields a degree of variability in hydraulic properties that is rarely, if ever, incorporated in the flow models. The result is a gap, or disconnect, between site-scale models used to interpret measurements at the well field or borehole scale, and models for regional flow. Ideally, the same model could be used both for interpretation of aquifer and borehole tests, and for simulation of regional flow. Nesting of grids is one approach used to link models at different scales. Recent attempts to merge the quantitative analyses of aquifer and borehole tests and the simulation of regional-scale flow in Newark-Basin aquifers have met with moderate success, and suggest future avenues for narrowing the gaps in scale and complexity.
Presented 12 November 2004 at Regional Workshop on Hydrogeology of the Newark Basin, Rutgers U., New Brunswick, NJ.
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