Hydrogeology of the Paleozoic bedrock in southeastern Minnesota, Report of Investigations

The Paleozoic bedrock of southeastern Minnesota contains some of the most heavily used aquifers in Minnesota. In this report we characterize the hydrogeologic attributes of these strata by compiling and interpreting a large volume of hydrostratigraphic and hydraulic data. The result is a hydrogeologic framework for southeastern Minnesota that can be used to formulate more effective ground-water management strategies, and in particular it improves our ability to predict aquifer productivity and contaminant transport paths. This report describes the hydrostratigraphic heterogeneity within individual Paleozoic lithostratigraphic units in detail for the first time. Our hydrostratigraphic analysis is based chiefly on plug tests of rock samples, outcrop and core observations of secondary pores, and a number of borehole geophysical techniques. Collectively, this information allows us to define "hydrostratigraphic units"—bodies of rock defined on the basis of their characteristic porosity and permeability—without regard for traditional lithostratigraphic boundaries (Seaber, 1988). Our hydrostratigraphic characterization provides a depiction of the spatial distribution of matrix and secondary porosity in a spectrum of geologic settings across southeastern Minnesota. Of particular importance is our effort to fully integrate the distribution and abundance of fractures and dissolution cavities into the hydrostratigraphic characterization. The Paleozoic bedrock of southeastern Minnesota can be divided into three principal matrix hydrostratigraphic components: coarse clastic rock of high porosity and permeability; fine clastic rock of low porosity and permeability; and carbonate rock, also of low porosity and permeability. All three of these matrix components contain secondary pores such as systematic fractures, dissolution features, and nonsystematic fractures, but they are most abundant in "shallow" bedrock conditions—areas where Paleozoic strata are within about 200 feet of the bedrock surface. In deeper bedrock conditions, secondary pores such as systematic and bedding-plane fractures are known to occur, but their distribution and abundance is poorly understood. They appear to be concentrated along a few discrete stratigraphic intervals, separated from one another by strata with few secondary pores. Hydraulic analyses of Paleozoic strata provide information on the manner in which ground water travels through matrix and secondary pores, and is evaluated in this report based chiefly on interpretation of pump tests, dye-trace studies, borehole flowmeter logs, water chemistry, and potentiometric data within the context of our hydrostratigraphic framework. The ground-water system appears to be relatively simple and predictable in conditions of deep burial by younger bedrock. Under these conditions, coarse clastic strata are of relatively high hydraulic conductivity, typically ranging from a few feet per day to a few tens of feet per day, presumably reflecting flow through large, well-connected intergranular pore spaces. In contrast, the matrix conductivity of the fine clastic and carbonate rock components is low enough in a vertical direction (10-7 to 10-3 foot per day) that intervals dominated by these components can provide hydraulic confinement. Intervals of carbonate rock containing abundant dissolution features have hydraulic conductivity values commonly as high as hundreds of feet per day, and in locally deep bedrock settings, have flow speeds so rapid that they are measured in miles per day along discrete intervals where well-developed conduit systems are present.[we do not have room to display the entire abstract]