Dependence of conductivities and anisotropies on geologic properties within the near-surface aquifier in Milwaukee, Wisconsin.

摘要

The Silurian-Devonian dolomite portion of the aquifer system in southeastern Wisconsin consists of a series of alternating high (0.5 to 10 m/day) and low (less than 0.1 m/day) hydraulic conductivity (K) dolomites. The construction of 10 m tunnels through this sequence of dolomite units has created a hydraulic stress on the regional system that has allowed it to be studied in ways previously impossible. Several major conclusions can be drawn from this study of the near-surface aquifer (combination of Silurian Devonian dolomite and overlying glacial tills). The vertical anisotropy of hydraulic conductivity (K _h/K_v) for the dolomite units is far larger than values from previous studies of this aquifer. The conceptual model that best explains hydraulic observations in low K units (aquitards) is a dual-porosity medium, while for high K units (aquifers) it is a porous medium. There is also a strong correlation between the hydraulic conductivity of the stratigraphic units and the density of discontinuities within these individual units. This may indicate that the aquifers are either behaving as equivalent porous media, or porous media and the discontinuity systems yield results that are indistinguishable.; Analysis of flux into the Milwaukee Metropolitan Sewerage District deep tunnels and potentiometric levels near these tunnels reveals that K_h /K_v for the dolomite units ranges from 25 to 2000, with K _h/K_v values of aquifers approximately 10 times larger than K_h/K_v values of aquitards. The calibration of three finite-difference groundwater models and the verification of two of these models confirms the large values of K_h/K_v calculated by analytical techniques. In addition, sensitivity analysis indicates that this study's large values are necessary for the simulation of this aquifer to yield both reasonable fluxes into the deep tunnels and surface water bodies as well as reasonable potentiometric values throughout the aquifer.; Azimuthal electrical resistivity surveys, multiple-well-aquifer tests, dye tracer tests, and single-well aquifer tests, among other sources, reveal the best conceptual model for portions of the dolomite. The K of the units studied shows a strong correlation to the total density of discontinuities within the rock units. For K_h the best correlation is with the density of horizontal breaks. For K_v, it is with density of vertical joints. Other commonly used measures of fractured rock properties (including bulk porosity, abundance of vugs) were only weakly correlated to observed K. As a final note, the azimuthal resistivity surveys appear to indicate that the flow of water through clay-rich tills overlying the dolomite may also be influenced by discontinuities.