Permeability structure in fractured aquifers
Halihan, Todd, 1970-
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Understanding water movement through fractured and karstic aquifers is difficult, but it is important for those managing these resources. Determining which features contribute to flow in these aquifers is important because accurate predictions of flow and transport are most sensitive to variations in the permeability field. A continuum approach to these aquifers has led to two problems that are approached with two new techniques that quantify the permeability. First, continuum hydraulics does not allow an understanding of which individual features in an aquifer provide flow. This permeability structure problem is manifested in scale dependent permeability (larger permeability values for larger scale tests). Interpretations of these aquifers are limited by ignoring small-scale data in addressing larger scale problems. The Edwards aquifer of central Texas was used to determine if data sets of matrix permeability, fracture aperture, and conduit size from cores and outcrops can be effective utilized to interpret permeabilities measured at the small-, well-, and regional-scales. The results demonstrate that by quantifying permeability on the small-scale, larger scale interpretations of the aquifer are possible and have a stronger quantitative basis by utilizing geologic information from the aquifer. The second problem is that standard hydraulic measurement techniques are optimized for porous media. This approach does not allow individual features and their connections with other features to be easily evaluated in these aquifers. Fractured carbonate aquifers in Wisconsin and Australia were evaluated using asymmetric dipole-flow tests to determine if the structure of permeability could be determined more effectively. Dipole-flow testing, analogous to resistivity dipole testing, is a relatively new technique that was developed from the use of recirculation wells in contaminant remediation. This asymmetric technique may overcome many of the problems inherent in other testing strategies. Asymmetric dipole-flow tests provided rapid testing and demonstrated the ability to quantify heterogeneities. The results demonstrate that boreholes can be connected in complex geometries with drawdown occurring above and below areas of pressure buildup.