HYDROSTRATIGRAPHY OF A FRACTURED, URBAN AQUITARD

The Ordovician Platteville aquitard in the Twin Cities Metropolitan area of Minnesota, USA, is a shallowly buried, extensively fractured carbonate rock in an urban setting, and therefore vulnerable to contaminants. A large number and wide variety of geomechanical and hydrogeologic studies over the past few decades have yielded a wealth of data that combined with our own borehole geophysics and outcrop observations, has led to a more comprehensive understanding of the Platteville. Key information acquired includes borehole flowmeter logs in ambient and stressed conditions, discrete interval packer tests, multiple head measurements from individual boreholes, long duration “aquifer” tests, spring hydrostratigraphy, and vertical fracture characterization based on mechanical stratigraphic analysis of outcrops, and observations of subsurface fractures in underground excavations. These data are brought together within detailed stratigraphic context using natural gamma logs collected from both outcrops and boreholes.

The Platteville Formation is best considered a complex “hybrid” hydrogeologic unit. Under certain conditions, and from one perspective, it can serve as an important aquitard that limits vertical flow, while in other conditions, and from another perspective, it is best considered a karstic aquifer with bedding-plane parallel conduits of very high conductivity that permit rapid flow of large volumes of water. Despite this complexity there is predictability in both vertical and bedding-plane fracture patterns that in turn provides some degree of predictability of flow paths in three dimensions. For example, one or more relatively thin (<1m) stratigraphic intervals where vertical fractures preferentially terminate serve as key discrete aquitard(s), and a high conductivity bedding plane fracture network also has developed preferentially along a single, thin stratigraphic position. At least some of these relationships appear to be operative for the Platteville in other portions of the Upper Midwest where the Platteville is shallowly buried. Effective management of such complex, karst, "hybrid", hydrogeologic units requires a sophisticated, nuanced understanding of their heterogeneous behavior.