A REGIONAL-SCALE, HYDROSTRATIGRAPHICALLY-BASED CHARACTERIZATION OF A HETEROGENEOUS SEDIMENTARY AQUIFER SYSTEM: CONSTRUCTION OF A NEW HYDROGEOLOGIC FRAMEWORK FOR PALEOZOIC STRATA IN THE NORTHERN MIDCONTINENT, NORTH AMERICA

Sedimentary aquifer characterizations used by environmental managers are commonly substantial simplifications of much more complex hydrogeologic systems. Heterogeneities routinely documented at scattered, site-specific investigations are rarely synthesized and integrated into the regional-scale hydrogeologic frameworks necessary for many ground-water management functions.

We have constructed a regional-scale characterization of hydrostratigraphic attributes, sensu stricto, for the relatively undeformed Lower Paleozoic sedimentary bedrock in southeastern Minnesota that shows the spatial distribution of both matrix and secondary porosity. Thousands of borehole geophysical logs and hundreds of laboratory analyses of matrix porosity and permeability were tied to detailed (largely 1:100,000 or greater), conventional, lithostratigraphic maps. We compiled and conducted a large number of hydraulic tests of discrete hydrostratigraphic components (packer tests and borehole flowmeter and video logging) and analyzed thousands of specific capacity tests to produce the hydrogeologic framework. Nearly the entire Paleozoic aquifer system has representative outcrops and we incorporated field observations of the distribution of secondary pores and their hydraulic significance. Borehole investigations demonstrated that many of these field observations can be extrapolated into the deeper subsurface because the development of secondary porosity is commonly controlled by stratigraphy.

The new hydrogeologic framework more accurately depicts this aquifer system's heterogeneities. It improves prediction of well yields and flow paths and speeds over the regional framework in use for the past few decades. The markedly higher definition of individual aquifers and confining units and quantification of their hydraulic properties will facilitate modeling at both regional and site-specific scales. This new hydrogeologic framework more closely resembles the first attempt (led by O.E. Meinzer) in the early 1900s at a regional hydrogeologic characterization than those published in later decades using what were presumed to be more advanced techniques. This early work, like ours, produced a regional framework based largely on a compilation of site-specific field and borehole observations.