Aquitard Distribution in a Northern Reach of the Miami Valley Aquifer, Ohio, USA: Part 2. Interpretation of Facies and Geostatistical Results

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For a reach of the Miami Valley aquifer system centered on Dayton, Ohio, USA, lithologic information was compiled from outcrops, from the logs of 624 wells, and from a continuous core 15 cm in diameter. These data were analyzed using geostatistical and stratigraphic methodologies.

The aquifer system is composed of two contrasting facies, sand and gravel (aquifer) and diamicton (aquitard). The sand-and-gravel facies was deposited by glacial meltwater streams on valley-wide outwash plains, and the diamicton was deposited as till. Detailed analysis suggests that the till may represent several glacial advances and was initially heterogeneous, being locally interbedded with sand and gravel deposited by subglacial meltwater. Some combination of increased deposition and decreased erosion of till created an elevation interval with a high proportion of diamicton facies. As estimated from lithologic indicators, the diamicton facies accounts for less than half the volume of this interval, suggesting that erosion was a dominant process. Therefore, previous conceptual models of upper and lower aquifers separated by an aquitard are inappropriate.

Erosion is attributed to meltwater streams that dissected previous deposits more completely in the valley center. These streams flowed to the southwest, controlled by the orientation of the bedrock valley and the local topography of ice-contact deposits. Variogram ranges in the maximum and minimum principal directions give estimates of the typical distances over which facies are correlated, which are related to the preserved extent of individual channel segments and undissected, interchannel till. Combining the geostatistical results with an understanding of depositional processes and a conceptual model of facies distributions creates a quantitative hydrofacies model for this portion of the aquifer system. Such quantitative hydrofacies models are useful for modeling other portions of the aquifer system and other aquifer systems created by similar depositional processes.



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