Scale and Hydrogeologic Complexity in Models of Ground-Water Flow

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Scale and Hydrogeologic Complexity in Models of Ground-Water Flow Powered By Docstoc
					Scale and Hydrogeologic Complexity in Models of Ground-Water Flow for Newark-
Basin Aquifers
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.

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